Heterocyclic Nanographenes and Other Polycyclic Heteroaromatic

Review pubs.acs.org/CR

Heterocyclic Nanographenes and Other Polycyclic Heteroaromatic Compounds: Synthetic Routes, Properties, and Applications Marcin Stępień,* Elzḃ ieta Gońka, Marika Ż yła, and Natasza Sprutta Wydział Chemii, Uniwersytet Wrocławski, ul. F. Joliot-Curie 14, 50-383 Wrocław, Poland ABSTRACT: Two-dimensionally extended, polycyclic heteroaromatic molecules (heterocyclic nanographenes) are a highly versatile class of organic materials, applicable as functional chromophores and organic semiconductors. In this Review, we discuss the rich chemistry of large heteroaromatics, focusing on their synthesis, electronic properties, and applications in materials science. This Review summarizes the historical development and current state of the art in this rapidly expanding field of research, which has become one of the key exploration areas of modern heterocyclic chemistry.

CONTENTS 1. Introduction 1.1. Background 1.2. Scope 1.2.1. Fusion Patterns 1.2.2. Molecular Size and Structure 1.2.3. Literature Scope 1.3. Classification and Nomenclature 1.4. Synthetic Strategies 1.4.1. Dehydrogenative (Oxidative) Annulations 1.4.2. Other Direct Annulations 1.4.3. Bay-Region Cyclizations 1.4.4. Electrophilic Condensations 2. Coronenoids 2.1. Edge-Doped Aza- and Oxacoronenes 2.1.1. Diazacoronenes 2.1.2. Dioxacoronenes 2.1.3. Triazacoronenes 2.1.4. Tetraazacoronenes 2.2. Pyrrole-Fused Azacoronenes 2.3. B- and BN-Doped Coronenes 2.3.1. Diboracoronenes 2.3.2. BN-Embedded Systems 2.4. peri-Condensed Coronenes 2.5. ortho-Condensed Coronenoids 2.5.1. Coronenoids Fused to Azaheterocycles 2.5.2. Thieno-Fused Coronenoids 3. Perylenoids 3.1. Heteraperylenoids 3.1.1. Monoheteraperylenoids 3.1.2. Diheteraperylenoids 3.1.3. Tetraheteraperylenes © 2016 American Chemical Society

3.2. [ghi]Heteroannulated Perylenoids: 5-Membered Rings 3.3. [ghi]Heteroannulated Perylenoids: 6-Membered Rings 3.4. [cd]Heteroannulated Perylenoids 3.5. ortho-Heteroannulated Perylenoids 4. Pyrenoids 4.1. Triangulenes 4.2. Azapyrenoids 4.2.1. Boron-Containing Azapyrenoids 4.3. Oxa- and Thiapyrenoids 4.4. [cd]-Heterofused Pyrenoids 4.5. [a]-Heterofused Pyrenoids 4.5.1. Direct Pyrrole Fusion 4.5.2. Other N-Containing Systems 4.5.3. Other Five-Membered Rings 4.6. Pyrazacenes 4.6.1. Pyrazine-Fused Systems 4.7. Other [e]Fused Pyrenoids 4.7.1. Pyrrole- and Indole-Containing Systems 4.7.2. Imidazole-Containing Systems 4.7.3. Other Nitrogen-Containing Systems 4.7.4. Oxygen-Containing Systems 4.7.5. Sulfur-Containing Systems 5. Phenalenoids 5.1. Monoheteraphenalenes 5.1.1. Heterahelicenes 5.1.2. 9a-Azaphenalene Ylides 5.1.3. Ceramidonines

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Special Issue: Expanded, Contracted, and Isomeric Porphyrins Received: January 30, 2016 Published: June 3, 2016 3479

DOI: 10.1021/acs.chemrev.6b00076 Chem. Rev. 2017, 117, 3479−3716

Chemical Reviews 5.1.4. Dibenzo[c,mn]acridin-8-ones 5.1.5. Monooxa- and Monothiaphenalenoids 5.1.6. Miscellaneous Azaphenalenoids 5.2. Diheteraphenalenes 5.2.1. Pyridoacridines 5.2.2. Other 1,6-Diheteraphenalenoids 5.2.3. 1,4-Diheteraphenalenoids 5.2.4. 1,3a-Diheteraphenalenoids 5.3. Tri- and Tetraheteraphenalenes 5.3.1. Tricycloquinazolines and Related Systems 5.4. Phenalenoids with Nonbenzenoid peri-Fusion 5.4.1. Cyclopenta[cd]phenalenes 5.4.2. Cyclohepta[cd]phenalenes 6. Nonbenzenoid Fusion 6.1. Circulenoids and Related Systems 6.1.1. Heterofused Circulenes 6.1.2. [5]Heteracirculenoids 6.1.3. [6]Heteracirculenoids 6.1.4. [7]Heteracirculenes 6.1.5. [8]Heteracirculenes 6.1.6. Larger Systems 6.2. Fused Acenaphthylene Derivatives 6.2.1. Hetero[a]fused Acenaphthylenes 6.2.2. Hetero[e]fused Acenaphthylenes 6.2.3. Hetero[d]fused Acenaphthylenes 6.2.4. Carbazole-Based and Related Systems 6.2.5. Carbonyl-Free Azafluoranthenes 6.2.6. Miscellaneous Azaacenaphthylenes and Azafluoranthenes 6.2.7. Pyracylene-Based Systems 6.2.8. Extended Thiaacenaphthylenes 6.2.9. Fused Oxaacenaphthylenes 6.3. Cyclopenta[cd]indene Systems 6.3.1. Fused Pyrrolo[3,2,1-hi]indoles 6.3.2. Fused Pyrrolo[2,1,5-cd]indolizines 6.4. peri-Fused Seven-Membered Rings 6.4.1. peri-Fused Cycloheptatrienes 6.4.2. Fused Azepines and Diazepines 6.4.3. Oxa- and Thia- 7-Membered Rings 7. Macrocyclic Systems 7.1. [b]-Fused (β−β-Fused) Porphyrinoids 7.2. Benzo[cd]-Fused Porphyrinoids 7.2.1. Benzo[cd]-Fusion via meso-Substituent Coupling 7.2.2. Other Benzannulations 7.2.3. Pyrido[cd]fused Systems 7.2.4. Pyrano- and Thiopyrano[cd]fused Systems 7.2.5. Benzo-Fused Porphyrin Oligomers 7.2.6. Oxonaphtho-Fused Porphyrins and Benzooxochlorins 7.2.7. Indole- and Carbazole-Based Porphyrinoids 7.3. Naphtho[2,1,8,7-cdef ]-Fused Porphyrinoids 7.3.1. Arene-Fused Systems 7.3.2. Porphyrin Tapes 7.4. [cd]-Fused Porphyrinoids with 5- and 7Membered Rings 7.4.1. Dehydropurpurins 7.4.2. Indeno[1,2,3-cd]porphyrins 7.4.3. Other Cyclopenta-Fused Systems

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7.4.4. Fused 7-Membered Rings 7.5. Porphyrinoids with Polycyclic Subunits 7.5.1. Porphyrinoids with Benzannulated Bipyrrole Units 7.5.2. Cyclooctatetraene-Fused Systems 7.5.3. Thiophene-Fused Systems 7.5.4. Systems with Acene and Heteroacene Subunits 7.5.5. Systems with Macrocyclic Subunits 7.6. Internally Fused Porphyrinoids 7.6.1. Regular Porphyrins 7.6.2. N-Confused Porphyrins 7.6.3. Pentaphyrins 7.6.4. Hexaphyrins 7.6.5. Heptaphyrins 7.6.6. Other Porphyrinoids 7.7. peri-Fused Cyclophanes 7.7.1. peri-Fused Pyridine Cyclophanes 7.7.2. peri-Fused Pyrrole Cyclophanes 7.7.3. Sulfur-Containing peri-Fused Cyclophanes 7.7.4. Oxygen-Containing Cyclophanes 7.7.5. Heteroatom-Bridged Cyclophanes 8. Conclusions and Outlook Author Information Corresponding Author Notes Biographies Acknowledgments Abbreviations References

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1. INTRODUCTION 1.1. Background

The chemistry of heteroaromatic compounds is a vast discipline, spanning nearly two centuries of research. The focus of heterocyclic chemistry has evolved over time, to encompass a multitude of topics in synthetic and physical organic chemistry, natural product research, molecular biology, and materials science. The interest in polycyclic heteroaromatic molecules (PHAs), which are the subject of the present review, can be traced back to the initial investigations of natural and synthetic dyes (Chart 1; note the difference between PHA and PAH, the latter denoting a polycyclic aromatic hydrocarbon).1,2 The elucidation of the structure of flavanthrone, accomplished by Roland Scholl in 1907,3 was among the most prominent early achievements, because of flavanthrone’s unique eight-ring structure and its industrial relevance. The initial development of the field was often hampered by the lack of appropriate analytical methods: it had taken decades of research until the structures of xylindein, tricycloquinazoline, and tetrabenzotetraoxa[8]circulene were unequivocally determined. In the following years, the field gradually expanded from dyestuff research, to include other areas of theoretical and practical importance. The interest in the mechanism of PAH carcinogenicity4 became one of the major research motivations in the 1950s. In the following two decades, a remarkable number of new PHA structures were reported, notably by the groups of Partridge and Buu-Hoı̈. New work on large polyheterocycles was subsequently inspired by the demands of supramolecular chemistry, by the growing interest in

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However, current production methods10,11 lead to random incorporation of the dopant, and fine structural details regarding heteroatom doping are often unavailable. There are also intrinsic limits regarding the level and pattern of doping achievable using high-temperature methods.12 At the same time, incorporation of heteroatoms in graphene can induce or enhance desirable properties, such as electrochemical activity, charge polarization, bandgap opening, and n- or p-type semiconductor characteristics.13−15 This principle is applicable to other carbon-rich materials, and heteroatom doping is employed as a means of bandgap tuning in π-conjugated polymers,16 and graphene nanoribbons,17 and for creating small-molecule n-type semiconductors.18 In this context, extensively fused heterocyclic systems are seen as doped nanographenes, combining structural uniformity with tunable electronic structure. Two-dimensional extension of the π-conjugated framework, which is the defining feature of nanographene structures, can provide faster reduction of the HOMO−LUMO gap than observed in structurally similar linear (i.e., 1D-extended) molecules.19 By modifying the topology and heteroatom content of the π-conjugated system, it is possible to control key features of the electronic structure, including the band gap, optical absorption spectra, photoluminescence, and redox behavior. Because these parameters can be tuned very flexibly, many families of PHAs are attractive objects of photophysical investigations, and have been explored as NIR-active dyes,20,21 two-photon absorbers,22 and fluorescence sensors. The presence of heteroatoms facilitates the design of stable π-aromatic cations,23 high-spin organic molecules,24 and ligands for transition metals. Depending on the number and placement of heteroatom donors, PHAs can offer monodentate, chelating, or macrocyclic coordination, leading to the formation of mono- and polynuclear complexes, which have been investigated for their photophysical properties, biological activity, and supramolecular behavior. Since the early days of PHA chemistry, nitrogen has been the primary “dopant” element, a choice dictated by the availability of synthetic methods and the stability of the N-containing systems. Considerable effort is now being directed toward the development of large heteroaromatics containing boron,25 phosphorus,26,27 or chalcogens.28 This expanding portfolio of usable heteroatoms provides an additional variable in the design of functional PHA molecules. Because of their anisotropic molecular shapes, structural rigidity, and extended π-surfaces, polycyclic heteroaromatics are of particular interest as self-assembling materials. Importantly, the presence of heteroatoms in PHAs introduces intrinsic dipole moments and edge functionalities, which can be used as additional means of controlling the molecular organization. In the following sections, we will present numerous examples of self-assembling PHAs, displaying a great variety of organization types, including crystals, liquid crystals, porous materials, nanofibers, and gels. In addition to bulk self-assembly, PHAs often provide efficient organization in solution and as monolayers. PHAs are also useful as building blocks for covalent assembly, and have been employed for constructing conjugated polymers, including donor−acceptor and laddertype materials, graphene nanoribbons, and finite-length oligomers. PHA molecules thus offer a unique combination of tunable electronic structure with excellent self-assembling properties, which is highly desirable in materials science. By judicious choice of ring fusion, heteroatom doping, and substitution,

Chart 1. Early Examples of Two-Dimensionally Fused Heterocycles

porphyrin analogues, and by advances in natural product chemistry, in particular by the discovery of pyridoacridine alkaloids. Figure 1 provides a pictorial summary of the most active areas of modern PHA research, which are discussed in the following sections of this Review. These developments have been made possible by considerable improvements in synthetic methodology, which largely eliminated the harsh reaction conditions prevalent in the early work on PHAs and enabled preparation of highly functionalized, solution-processable derivatives. Transition metal-catalyzed coupling reactions are arguably the most important recent addition to the synthetic toolbox, and have been extensively used in modern PHA chemistry. The tasks that can be accomplished with catalyzed couplings include not only annulation reactions and installation of functional groups, but also the elaboration of oligoaryl precursors, which can then be cyclized to PHA targets using, for example, oxidative coupling reactions.5 Advances in macrocyclic chemistry, most notably in the synthesis of porphyrin analogues, have also been instrumental in shaping the field of PHA chemistry, providing access to structures of exceptional complexity. At the turn of the 21st century, the research on large polycyclic heteroaromatics gained new impetus with the discovery of graphene and with the prospect of its application in molecular electronics. The use of graphene as a semiconductor for postsilicon electronics is limited by its problematic production and handling, because currently available technology does not provide straightforward access to structurally uniform material. Furthermore, the requirement of a nonzero bandgap makes large-area graphene unsuitable in some key applications, such as field-effect transistors.6 The bandgap can be opened by lateral or two-dimensional confinement in graphene nanoribbons (GNRs) and nanographenes (graphene quantum dots, GQDs), respectively. Bottom-up synthetic approaches can provide such materials with atomic-level uniformity, as has been demonstrated by numerous syntheses of extended PAHs using solution chemistry.7,8 In molecular electronics, heteroatom doping of graphene is of high importance, as it changes the electronic structure of the material, opening new possibilities for device applications.9 3481


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Figure 1. Research directions in modern PHA chemistry.

Figure 2. Citation timeline of this Review. The scope of the literature search is defined in the text. 3482


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The above restriction means that purely ortho- or macrofused systems, even of large sizes, will be excluded from this Review. This class of structures includes linearly and angularly fused oligoacene analogues, as well as a number of branched ortho-fused systems, the latter often having a distinct twodimensional look. Several important classes of organic materials belong here, including linear heteroacenes,18,30,31 heterahelicenes,32 thiophene-based systems,33 indolocarbazoles,34 BNdoped aromatics,25 phosphole-based systems,26,27 and several types of macrocyclic molecules. 1.2.2. Molecular Size and Structure. This Review is further limited to systems that contain a contiguously πconjugated framework with at least 20 sp2-hybridized atoms. This arbitrary limit, which typically corresponds to systems containing at least five or six fused rings, was selected through preliminary literature searches, to provide a reasonable number of references. We believe that through this choice, the focus of this Review is improved, by highlighting the chemistry that has the highest potential of creating particularly large structures. Nevertheless, smaller PHAs are occasionally included, whenever they provide a relevant context for discussion. Some aspects of the chemistry of smaller peri-annulated heterocycles were reviewed by Mezheritskii35 and by Aksenov et al.36 The majority of nanographene molecules have approximately planar geometries; however, even highly distorted π-systems will be included in this Review, provided that the distortion does not interrupt the continuity of π-conjugation. As a general rule, any ring containing a tetravalent center will not normally be regarded as part of the fused system. Rings containing hypervalent heteroatoms and metallacycles are also excluded. Otherwise, any array of planarly hybridized carbons and heteroatoms will be considered contiguously π-conjugated, regardless of the formal valence structure. It will thus be assumed that the conjugation is not interrupted by exocyclic double bonds emanating from the fused framework (such as vinylidene or carbonyl groups), carbocationic or radical centers, etc. Oxygen and sulfur atoms, both di- and trivalent, will normally be considered part of the π-conjugated structure. Fused systems containing triple bonds, usually embedded in macrocyclic rings, are discussed when their structures are conformationally rigid and approximately planar. It should be noted that a number of flexible ultralarge acetylene macrocycles, such as those developed by the groups of Anderson37 and Würthner,38 which formally meet topological criteria for inclusion, are not discussed here. As an exception to the above rules, the presence of a cyclic imide and anhydride moiety in a molecule will typically not be considered sufficient for inclusion in this Review. This decision serves to eliminate a large number of structurally similar molecules, mostly N-substituted derivatives of perylene- and naphthalenediimides (PDIs and NDIs, respectively), and related rylene derivatives. PDIs and NDIs will however be discussed whenever they contain another PHA system of interest. Various aspects of rylene chemistry have been reviewed by the groups of Langhals,39 Li,40 Marder,41 Müllen,42−45 Wang,46 Wudl,47 and Würthner.48−51 Lactone rings are typically omitted as well, whereas conjugated lactam substructures, such as 2-pyridone rings, are normally retained because of their possible tautomerism. 1.2.3. Literature Scope. This Review attempts to cover the relevant work published since the late 19th century to the end of 2015. Even with the above restrictions, this Review spans a broad range of topics, which were partly reviewed by other

PHAs can be tailored into both p- and n-type semiconductors and have found extensive use in organic field-effect transistors and organic light emitting diodes. Photovoltaic applications of PHAs encompass both bulk-heterojunction (BHJ) and dyesensitized (DS) solar cells, with many examples presented below. Driven by such a diversity of stimuli, the area of PHA chemistry has become very heterogeneous. While certain subfields of this research have been reviewed, as discussed below, it seems that no unified view of the field has been offered so far. This Review aims to fill this gap by exploring the topological diversity of existing PHA molecules and bringing together pieces of research that have been developed in different research contexts. 1.2. Scope

This Review is focused on heteroaromatic systems in which the fused ring framework forms a distinct two-dimensional structure. In hydrocarbon chemistry, such molecules are typically called “nanographenes”, and the term is conveniently applied also to heterocyclic and nonbenzenoid analogues. While the above definition seems intuitively clear, it has to be refined in terms of molecular size and fusion patterns, to keep the review within a manageable length. Our primary purpose is to document the explosive growth of the field, which has occurred during the last 15 years (Figure 2), while also providing a concise account of earlier developments. 1.2.1. Fusion Patterns. First, we restrict the scope to systems that are “ortho- and peri-fused”, according to the IUPAC definition29 (Chart 2). Such systems are characterized Chart 2. Examples of ortho-, peri-, and macro-Fusion

by the presence of three-ring junctions (red dots, Chart 2), which we will call “peri-fusion points” in the following discussion. In common use, the IUPAC term “ortho- and peri-fused” is often abbreviated to “peri-fused” or “periannulated”, and we will also follow this convention here. A fusion situation not covered by the ortho/peri nomenclature involves pairs of rings that share more than one edge (i.e., more than two atoms). As this type of fusion typically occurs in macrocyclic systems (e.g., in porphyrin), we will label it “macrofusion” for convenience. A range of highly interesting molecules exist in which a peri-fusion point results from a combination of ortho- and macro-fusion, as in the cyclopenta-fused porphyrin analogue, known as dehydropurpurin (cf., section 7.4). The scope of this work will be accordingly expanded to include such “ortho-, macro-, and peri-fused” systems. 3483


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Chart 3. Examples of peri-Condensed Frameworks Discussed in This Reviewa

a

Heteroatom placement and double bonds are not indicated.

authors. Related review articles are cited at the beginning of each major section. Patent literature is generally excluded,

unless the claims are of historical importance or have been verified in subsequent nonpatent work. Older reports with 3484


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Scheme 1. Typical Synthethic Transformations Based on ortho- and peri-Annulations (Red Bonds) and Oligomer Formation (Purple Bonds)a

a

Popular types of aromatic oligomers are shown in box B. Examples of linking multiple fused systems via annulation chemistry are shown in box C.

produced peri-fusion points or formed the heterocyclic substructure of interest. Nonsynthetic aspects of PHA chemistry, including spectroscopy, electrochemistry, device fabrication, theoretical investigations, etc., are presented in detail for the most recent advances in the field. Isolation and structural determination of natural products is beyond the scope of this work.

insufficient analytical data and no follow-up work are usually cited without an in-depth discussion. This Review is principally focused on synthetic methodology, with a particular emphasis on bottom-up, atomically precise syntheses of monodisperse systems. Nevertheless, relevant polymeric structures, such as heteroaromatic nanoribbons, are presented in the context of related nanographene chemistry. Heterofullerene derivatives52 and heterocycle-modified fullerenes53 are considered structurally and synthetically distinct from PHAs and will not be covered here. The majority of reactions discussed herein pertain to the realm of solution chemistry, with just a few examples of gasphase syntheses of some highly strained molecules. In addition to conventional bulk synthesis, we present pertinent advances in the on-surface preparation of heteroaromatic systems. With the submolecular resolution of the current AFM methodology, such reactions can be followed on surface in remarkable detail. Synthetic routes leading to PHA targets typically involve multistep reaction sequences. To avoid presenting routine synthetic work, we usually limit our discussion to the key transformations of the fused framework, particularly those that

1.3. Classification and Nomenclature

Polycyclic heteroaromatic molecules presented in this Review display considerable structural diversity in terms of such features as the number and connectivity of constituent rings, heteroatom pattern, and peripheral substitution. The classification used in this work orders the material according to the decreasing extent of benzenoid (graphene-like) ring fusion (Chart 3). The proposed classification has no chemical significance and was chosen to present related contributions in a coherent fashion. In the interest of clarity, we sometimes depart from the main classification scheme, as discussed below. Nevertheless, some splitting of associated work (sometimes within a single paper) could not be avoided. We compensate 3485


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Scheme 2. Selected peri-Annulation Methods

pyrrole instead of isoindole), and are usually motivated by the research context.

for this occasional inconvenience by providing relevant crossreferences. We begin with discussing “coronenoids”, molecules that contain a coronene substructure (section 2) that is either heteroatom-doped itself or fused to a heterocyclic subunit. Some of the largest heteroaromatic nanographenes are discussed here, which typically have a sheet-like rather than ribbon shape. By selecting systems with progressively smaller benzenoid substructures, the remaining material is further divided into perylenoids (section 3), pyrenoids (section 4), and phenalenoids (section 5). All of these systems possess at least one “benzenoid peri-fusion point”, that is, a junction common to three six-membered rings. It should be noted that the coronene and perylene motifs satisfy the size criterion without fusion of additional rings, whereas pyrene substructures need to be expanded with at least one additional cycle. Similarly, the section on phenalenoids covers systems containing at least 5 fused rings. The remaining systems, which do not possess a phenalene substructure, are classified as nonbenzenoid in this Review (section 6). Here, the ortho- and peri-fused substructures typically contain at least one five- or seven-membered ring, although a number of systems containing larger cycles are also classified in this section (e.g., circulenoids). This Review concludes with section 7, which covers macrocyclic systems containing ortho-, macro-, and peri-fused motifs. For convenience, these systems are further divided into porphyrinoids and cyclophanes, depending on the choice of the original authors. The nomenclature used in the original work is usually retained even when it does not strictly follow IUPAC rules. Departures from IUPAC naming recommendations typically involve a nonstandard choice of the parent component (e.g.,

1.4. Synthetic Strategies

As with any class of cyclic molecules, the major challenge in PHA chemistry is posed by the need to design effective annulation strategies. Because of the structural diversity of PHA systems and the complexity of existing ring fusion patterns, several distinct synthetic approaches have emerged, which are applicable, often in a rather limited fashion, to particular classes of compounds. The basic ring forming operations can be broadly classified as ortho- and peri-annulations (Scheme 1, box A). ortho-Annulations typically provide lateral or peripheral extensions of the existing fused framework. One-step reactions leading directly to a π-conjugated ring are preferred, although in rare cases aliphatic rings are constructed first and subsequently aromatized (e.g., 36.16). Alternatively, ortho-annulations can be used to establish a fused connection between two ring systems, thereby rapidly increasing structural complexity (Scheme 1, box C). The condensation of ortho-diamines with 1,2-diones is particularly useful for the latter purpose, yielding pyrazine (quinoxaline) connections (cf., Schemes 22, 28, and sections 4.6 and 6.2). Similar fused connections can be created by condensing aromatic diamines with cyclic anhydrides, to produce imidazole- or pyrimidine-containing junctions (cf., Schemes 59, 91, 110), or by other specific annulations leading to, for example, pyrrole (152.4) or thiophene rings (144.17). orthoAnnulations involving more than two polycyclic molecules can be used to create branching points in the fused structures; relevant examples discussed in the following sections include Heck-type trimerizations leading to benzene rings (155.2, 3486


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that is, it involves the formation of a new ring via intramolecular coupling of two aromatic subunits. Dehydrogenative annulations were originally developed in the early phase of aromatic chemistry (notably through the work of Scholl and coworkers), but they remain of high interest because of their utility in the synthesis of large fused systems such as HBC derivatives (section 2) or peripherally fused porphyrinoids (sections 7.2 and 7.3). The chemistry usually involves an oxidative coupling with stoichiometric oxidant (e.g., FeCl3, DDQ, etc.), Scholl conditions (AlCl3 or AlCl3/NaCl), or photocyclization (Mallory reaction), although several other approaches have also been explored, such as radical−anion coupling (Schemes 9, 10, 29, 32, 130; 44.18), catalytic dehydrogenation (44.19−21, 136.12), coupling of aryllithiums (164.3), or surface-assisted dehydrogenation (e.g., Schemes 14, 19). The latter method is not suitable for bulk synthesis but is of increasing importance because of its applications in nanoribbon chemistry. The prototypical dehydrogenative annulation couples two benzene rings, but direct couplings of heterocyclic rings can also be achieved, for example, of pyrrole, imidazole, triazole, thiophene, pyridine, pyrazine, and pyrimidine (cf., Schemes 4, 5, 11, 18, 20, 23, 25, 26, 27, 69, 102, 116, 122, 173, 206, 207, 208, 294). Ring−substituent dehydrogenative couplings are also possible (e.g., 3.7−8, 60.4; Scheme 199). The feasibility of these reactions strongly depends on the properties of the units being coupled, and electron-donating substituents on the rings are usually required to ensure sufficient reactivity and selectivity. Competing processes may involve intermolecular couplings (Scheme 20). Most typically, a C−C bond is formed, although other bond types can also be obtained (e.g., C−B, Scheme 17; C−N, 43.7; N−N, 43.4; C−O, 90.2, 98.7, Schemes 96, 97; C−S, 99.2). The new ring, which may be carbo- or heterocyclic, is usually six-membered, although five(196.4, 197.3; Charts 59, 60; Schemes 263, 264, 273, 275, 276), seven- (Schemes 13, 156, 157, 186), and eight-membered rings (Scheme 162; 158.8, 164.3, 206.8, 268.3) have also been prepared. Nevertheless, when rings other than 6-membered are targeted, rearrangements may occur (cf., 33.3). The feasibility of closing a particular ring size is at least in part determined by the internal strain of the resulting fused ring system. One of the principal advantages of the dehydrogenative annulation approach is the possibility of performing tandem annulations on aromatic oligomers, in which several (often six or more) bonds are created at one time. Usually, the individual annulations are topologically independent (i.e., they can in principle be performed in any sequence), although zipper-like multiannulations can be designed in appropriately large precursors (e.g., 32.12, 58.2). Couplings inside macrocycles are also possible (53.12, 124.8, 252.1, C70.3). Dehydrogenative annulations are typically intramolecular, because intermolecular variants usually suffer from poor regioselectivity (cf., Scheme 33). Nevertheless, fairly selective intermolecular couplings are occasionally reported, but the yields of such reactions are usually moderate (43.2, 44.23, 50.3, 61.2,4, 62.3, C14.1). Rings other than six-membered are rarely obtained in this way, a notable example being the porphyrin−acetylene coupling leading to dehydropurpurin (Scheme 255). 1.4.2. Other Direct Annulations. Dehydrohalogenative annulations (C−H + C−X, X = halogen, sulfonate) do not involve a net change of the oxidation state of the substrate. The coupling chemistry is typically based on metal-catalyzed Hecklike reactions (87.16, 131.2, 144.8; Schemes 152, 180; see

192.6) and numerous macrocyclization reactions (see section 7.1). Because of the involved complexity, building a PHA system by incremental annulation is rarely employed. Exceptions from this rule are found among total syntheses of naturally occurring PHAs, which nevertheless tend to rely on tailored tandem annulations (cf., Schemes 44, 131, 132, 133). The preferred approach, which is usually more step-efficient, involves the synthesis of an aromatic oligomer, usually directly linked (sp2 to sp2), which can have a linear or branched topology. The presence of “cove” and “fjord” regions in such oligomers (Scheme 1, box A) provides convenient sites of further orthoand peri-annulation, making them especially useful in the synthesis of very large PHAs. Diverse branched oligoaryls, structurally analogous to hexaphenylbenzene, have been used for the synthesis of coronenoid systems (section 2; for examples of related precursors built around five- and sevenmembered rings, see Schemes 152 and 185, respectively). The use of linear oligomers is of importance, for example, in the synthesis of rylene derivatives (section 3) and porphyrin tapes (sections 7.2 and 7.3). Because of the strategic role of macrocycles as reactive building blocks, macrocyclization reactions (section 7) are often performed at an early stage of the synthesis, and are followed by subsequent ortho- and peri-annulation steps. Nevertheless, macrocyclizations can result in branched orthofusion (e.g., Scheme 193) or actually create peri-fusion points (Schemes 259, 269, 270, 271; section 7.7). peri-Annulations are occasionally performed inside macrocycles, such as those involved in porphyrinoid N-fusion chemistry (Scheme 273) and in the fold-in syntheses (Scheme 166, see also 161.8, 124.6, 53.12, 252.1, 70.3). Typical macrocyclization reactions reviewed in sections 7 and 6.1 involve electrophilic substitution reactions (porphyrinoids), biaryl couplings (Suzuki, Schemes 227, 231, 235, 286; Stille, Scheme 232; Ullmann-type, Schemes 286, 287, 291, 292; Chart 69), and acetylene couplings (Sonogashira, 288.3; Scheme 289; Chart 69; Glaser coupling, Schemes 228, 230; 289.9; Chart 68). Glaser macrocyclizations are often followed by a Paal−Knorr conversion of the diacetylene bridges to pyrrole or thiophene rings. Specific classes of macrocyclic rings were also be constructed using oxidative couplings (Schemes 236, 267; C63.11), olefination reactions (McMurry, Schemes 265, 290; Chart 69; Wittig, Scheme 290), and various nucleophilic reactions (Schemes 165, 269, 270, 286, 294, 296; Chart 67). Interestingly, certain annulation reactions can be used for building macrocycles (Scheme 293; 286.14). The construction of peri-fusion points in PHA frameworks can be achieved using diverse annulation methods. periAnnulations can be classified on the basis of their location in the substrate molecule (zigzag, bay, cove, or fjord region) and with respect to cyclization topology (direct, i.e., ring-to-ring, ring-to-substituent, or intermolecular, Scheme 1, box A). Fjord and cove regions can be annulated directly, producing, respectively, six- and five-membered rings. For zigzag and bay regions, the inclusion of a substituent or an incoming reactant is necessary to produce a five-membered or larger ring. The most successful and versatile peri-annulations are discussed below and illustrated in Scheme 2. 1.4.1. Dehydrogenative (Oxidative) Annulations. This general class of reactions is among the most versatile and frequently used synthetic methods in PHA (and PAH) chemistry. The transformation is typically of the direct type; 3487


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40), sulfur, and selenium reagents (100.2, 107.11, 142.6,8, Scheme 154). In the case of S and Se, the cyclizations can be induced not only by using formally electrophilic reagents, but also by direct reaction with elemental chalcogens (Schemes 168, 176; Chart 53). Nucleophilic condensations are a diverse class of annulation reactions, especially useful for the synthesis of hetero rings. Such reactions were extensively used in the classical syntheses of nitrogen-rich carcinogenic PHAs (e.g., Schemes 139, 140), and in the later work on pyridoacridine alkaloids (Schemes 44, 131, 132) and heteratriangulenes (section 4.1). The most important peri-annulation strategies include Schiff-like condensations leading to pyridine (36.2,10, 87.3, 132.4, 211.3, 165.9, 287.7; Schemes 94, 140), pyridazine (Schemes 41, 42, 57, 94; 87.5), and pyrimidine rings (64.1; Schemes 92, 93, 139, 140). Pyridine or pyrylium rings can also be formed by cyclization of ethynyl-substituted precursors (36.14, 37.8−9, 131.9; Scheme 94; for pyrylium-to-pyridine conversions, see Schemes 56, 94). Seven-membered rings can also be closed in nucleophilic condensations (146.3, Scheme 277), and a range of bicyclic systems, such as quinoline (Schemes 65, 130, 201), quinolizine (Scheme 30), 2,7-naphthyridine (8.3,5, 44.16,22, 131.15, 140.3; Scheme 133), 2,6-naphthyridine (83.3), or 1,6naphthyridine (141.2) have been assembled by means of tandem bis-annulations. Oxygen-containing rings can also be peri-annulated using nucleophilic cyclizations. Acid-catalyzed condensations of 1,4naphtho- and 1,4-benzoquinones are instrumental in the synthesis of tetraoxa[8]circulenes and related systems (Schemes 159, 160). In cationic methoxyarenes, the OMe groups become sufficiently labile to allow their efficient nucleophilic displacement, which is a key step in the syntheses of heteratriangulenes and heteracirculenes (Schemes 77, 78, 79, 80, 101; 135.4). Substitution of phenol/quinone functions is also usable for the synthesis of 1,2-dithiole rings (Scheme 144) and annulated oxepins (Scheme 188; 189.16). In addition to the above condensations, which typically involve the release of water or alcohol, nucleophilic substitution of haloarenes and aryl sulfonates can be effected under either base-promoted or catalyzed (Pd, Cu) conditions. This reactivity enables the synthesis of 5-membered (61.9, C52.8, 279.5, 281.2; Schemes 163, 283), 6-membered (Schemes 76, 81, 132, 138, 140, 142, 163, 213; 147.2), and 7-membered heterocycles (186.8, 189.6,10, Schemes 283, 274; see also 272.5). Zigzag-region annulations create a single additional perifusion point and are often achieved using electrophilic or nucleophilic chemistry (for an additional example of a nucleophilic benzannulation, see 217.3). Further cyclization strategies specific to the zigzag region are represented by [3+2] and [3+3] cycloadditions of azomethine ylides (Schemes 126, 152; Chart 42) and other systems (Scheme 181), and by transformations involving cumulene or acetylene substituents (178.2, 179.4). Ring-to-ring transformations are occasionally used for elaborating fused PHA frameworks. Examples of ring rearrangements employed for this purpose include a transamidation used in the synthesis of triazasumanenes (155.3), formation of azepinone dyes from indigo (Scheme 184), conversion of 1azatriptycene into azepines (Scheme 185), Graebe−Ullmann synthesis of pyridine rings from triazoles (136.2−5), and an unusual rearrangement of spiro-porphodimethenes into peripherally fused porphyrins (264.1). Cycloaddition−cycloreversion sequences can be used for remodeling of existing rings (130.4).

137.10 for a tandem process involving an initial Heck-type step), and less frequently on radically induced (Schemes 31, 82), thermal/base promoted (Scheme 85), or electrochemical couplings (144.5). In addition to six-membered rings, which are most typically formed, closures of five- (180.2,17, 189.4, 217.4, 256.2, Schemes 257, 258, 262) and seven-membered rings (182.6,7, 183.6, 186.10) have been reported. Reductive annulations are typically performed using Ullmann-type couplings, notably the Ni0-based Yamamoto method, which is valuable for its ability to introduce internal strain to the fused ring systems. Examples of such reductive cyclizations include intramolecular formation of 6-membered (Schemes 166, 173) and larger rings (Schemes 286, 287, 291, 292; Chart 69), and an intermolecular benzannulation (61.6). An intermolecular annulation was induced under McMurry conditions (Scheme 35). 1.4.3. Bay-Region Cyclizations. Annulations in bay regions enable lateral expansion of perylenoids, providing access to coronene and benzo[ghi]perylene analogues (sections 2 and 3). The classical synthesis of Cibalackrot (175.2) can be seen as a double bay annulation of indigo. Even though bay regions in aromatic systems are typically unreactive as dienes in cycloaddition reactions, combining a (formal) Diels−Alder step with dehydrogenation is possible with some reactants, leading to benzene rings (from maleic anhydride: 3, 47.3, Scheme 157; see also 180.17) or pyridazine rings (from azodicarboxylates, Schemes 8, 9, 10; from 4-phenyl-1,2,4-triazoline-3,5-dione, Schemes 9, 10, 34, 57). Intermolecular benzannulations of bay regions were also achieved by using catalytic cyclization of acetylenes (Schemes 143, 161) or conversion of nitroarenes into 1,2-dithiines (Scheme 57). Bay regions can be alternatively annulated using various intramolecular substituent-to-ring cyclizations, such as the Pictet−Spengler reaction (Scheme 7) or Friedel−Crafts cyclizations (see below). Bay-region cyclizations of ethynyl (Pt-catalyzed, 45.3−4, 95.4; InCl3catalyzed, 143.7) and nitrile substituents (Scheme 55) have also been reported. An intramolecular cycloaddition of a ketenimine was used for a tandem assembly of a quinoline subunit (136.9). Five-membered rings, such as pyrrole or thiophene, can also be closed in bay regions, using various synthetic approaches (Schemes 21, 46, 54, 158; Chart 54; 53.6, 53.8, 174.2). 1.4.4. Electrophilic Condensations. peri-Annulations based on Friedel−Crafts alkylation or acylation are highly useful in the synthesis of six-membered rings (Schemes 56, 70, 72, 75, 82, 97, 107, 125, 127, 134, 137, 143, 144, 179, 202, 217, 218, 220, 224, 225, 226, 241), but are also applicable for closing five-membered (Scheme 50) and seven-membered rings (Schemes 12, 187), as well as macrocycles (notably porphyrin analogues). In alkylations, the direct cyclization product typically requires an aromatization step, such as dehydrogenation (e.g., in pyrrole−aldehyde condensations, section 7), dehydration (32.6, 88.3,8), or reductive dehydroxylation (37.2; Scheme 89), whereas acylations may lead to the aromatic product via spontaneous tautomerization (e.g., 36.8). Sixmembered (90.5, 97.7, 144.11, 180.2,12) and sevenmembered rings (185.2) can be obtained via electrophilic cyclization of diazonium salts. peri-Fused pyridine rings have been formed via cyclization of nitrene intermediates (209.2, 210.2, 136.10; Scheme 212). Heteroatom bridges can also be introduced using electrophilic substitutions of C−H, C−Si, or C−M bonds, with boron (Schemes 15, 16, 39, 45, 57, 95), phosphorus (Schemes 29, 3488


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containing nitrogen or oxygen atoms are discussed in section 2.1. Section 2.2 discusses the emerging chemistry of pyrrolefused coronene systems, which are rare examples of internally doped azacoronenes (for triangulene derivatives containing an internally aza-doped coronene framework, see Scheme 71 in section 4.1). Our own work on peripherally expanded azacoronenes is discussed here for consistency, even though these systems are strictly classified as perylenoids. In section 2.3, we summarize recent advances in the chemistry of B- and BN-doped heteracoronenes. Recent advances in the field of B and BN aromatics have been reviewed by the groups of Ingleson,25 Piers,60 and Liu.61 Sections 2.4 and 2.5 discuss heteroatom-free coronenes that contain, respectively, peri- and ortho-fused heteroaromatic moieties.

Rare examples of ring contractions, such as the extrusion of sulfur from thiepines (36.2, 87.2, 128.2, 189.6) or contraction of imides to lactams (61.7), have also been reported.

2. CORONENOIDS The term “coronenoid” will be used throughout this Review to describe all systems containing the seven-ring framework of coronene. The first heteroaromatic coronene derivatives were reported in the 1950s (compounds 22.3 by Zinke et al.54 and 6.4 by Clar et al.55). In recent years, the interest in nanographene chemistry has produced a range of extended πconjugated systems, many of which contain heterocyclic rings. Recent advances in nanographene synthesis were reviewed by Yan and Li,56 Tran-Van and Wegner,57 Wu and co-workers,58 and the Müllen group.8 An account of the chemistry of contorted polycyclic aromatics was published by Nuckolls and co-workers.59 While the above reviews are largely focused on carbocyclic structures, this section is focused strictly on heteroaromatic derivatives. For convenience, these systems are classified as follows (Chart 4). Edge-doped heteracoronenes

2.1. Edge-Doped Aza- and Oxacoronenes

2.1.1. Diazacoronenes. The first successful synthesis of the unsubstituted 1,2-diazacoronene 3.1 was carried out by Tokita et al. in 1982 (Scheme 3).62 In the first step, 1,2diazacoronene-7,8-dicarboxylic anhydride 3.3 was prepared in the reaction between the 1,2-diazabenzo[ghi]perylene derivative 3.2 and maleic anhydride in the presence of chloranil. In the next two steps, the hydrolysis of compound 3.3 was performed, and the resulting compound 3.4 was heated with soda lime to yield 3.1. 1,2-Diazacoronene, poorly soluble in common organic solvents, had a yellow color and an electronic spectrum similar to that of coronene. Substituted Schiff base diazacoronenes were synthesized in 2012 by Cheng, Xiao et al. (Scheme 3).63 Two isomeric derivatives, 1,7-diaza- 3.7 and 1,8-diaza- 3.8, were prepared in a photocyclization reaction between bisaminoperylene tetraesters and butyraldehyde and were modified further without separating them. In subsequent steps, these coronenes were converted into anhydride (3.9/3.10) and diimide (3.11/3.12) derivatives. The diimides 3.11/3.12a−b show absorption largely independent of substitution and are fluorescent, with emission maxima at around 477 nm and extremely small Stokes shifts of only 2−4 nm. Electrochemical data revealed that these compounds could accept up to two electrons and might be usable as n-type semiconductor materials. Because of the low solubility of 3.11b/3.12b, the self-assembly behavior and

Chart 4. Classification of Coronenoids Used in This Sectiona

π-Conjugation and possible additional fused carbocyclic rings are not indicated. a

Scheme 3. Synthesis of Diazacoronenes and Schiff-Base Diazacoronenesa

Reagents and conditions: (a)62 chloranil, maleic anhydride, 20 min, reflux; (b) benzyl alcohol, NaOH, 30 min, 100 °C; (c) soda lime, powdered, 1 h at 320 °C, then 2 h at 350 °C; (d)63 butyric aldehyde, cat. I2, 6 h, 62 °C; (e) chlorosulfonic acid, 4 h, rt; (f) amine, 4 h, 115 °C, Ar atmosphere. a

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Scheme 4. Synthetic Routes to Diimide Diazacoronenes and Thiophene Fused Azacoronenesa

a Reagents and conditions: (a)64 hν, 3 h; (b) hν, 6 h; (c)65 K2CO3, 18-crown-6 cat., 2-ethylhexyl bromide, DMF, overnight, 120 °C, N2 atm; (d) hν with a 300 W incandescent lamp, I2, 36 h; (e) NBS, chloroform, overnight; (f) Pd(PPh3)4, toluene, DMF, 24 h, 100 °C, N2 atm; (g) n-BuLi in hexanes, THF, 30 min, 0 °C, then 1 h, rt, then 0 °C, trimethyltin chloride in hexanes, overnight, rt; (h) Pd2(dba)3, P(o-Tol)3, toluene, overnight, 100 °C, then bromobenzene, 3 h, then tributylstannyl thiophene, 3 h.

photocyclization was used to produce thienoazacoronenes 4.7a and 4.7b, which were converted into the functionalized dibromo and distannyl derivatives 4.8a and 4.8b, respectively. The latter functionalized 4.8 were subjected to the Stille coupling reaction, leading to the thienyl monomer 4.9a and polymer 4.9b. The 4.7a derivative showed a high degree of selforganization in solution and in the bulk, both in the crystal and as spuncast thin films. Thin-film field-effect transistors exhibited an average and maximum hole mobility of 0.0013 and 0.028 cm2 V−1 s−1, respectively. These mobilities were lower than those of high-performing acenes and thienoacenes, but were nevertheless 1 order of magnitude higher than observed for hexathienocoronenes and were among the top field-effect mobilities for solution-processed discotic materials. Organic photovoltaic devices using a thieno-fused diazacoronenecontaining conjugated polymer as the donor material had a high open-circuit voltage of 0.89 V, indicating the potential use of this coronene analogue as a low-HOMO electron donor for tuning the energy levels in the hole transporting system. In 2015, Liu and co-workers reported the synthesis of benzoand thieno-fused azacoronene derivatives (Scheme 5).66 Dichlorodiazaperylenes 5.1 or 5.4 were diversely functionalized at the α-pyridine positions to produce alkoxy- (5.2a and 5.5a), thioalkyl- (5.2b and 5.5b,c), alkyl- (5.2c), or aryl-substituted (5.2d) derivatives, all of which underwent oxidative dehydro-

charge-carrier mobility were studied only for 3.11a/3.12a. Both derivatives showed self-assembly into ordered nanobelts. The charge-carried mobility was investigated using the steady-state space-charge-limited current (SCLC) technique. The mobility level for 3.11a/3.12a reaches 5.65 × 10−4 cm2 V−1 s−1 at an electric field of 0.3 MV cm−1, which is the average level of SCLC mobility observed for perylenediimides (PDIs). Structurally related diimide analogues containing fused imidazole (4.2) and 1,2,4-triazole rings (4.4) embedded as functional constituents were obtained in 2006 by Li at al.64 These compounds were obtained through photocyclization reactions of doubly substituted precursors 4.1 and 4.3, respectively (Scheme 4). The electron-donating and -accepting nature of the π-system in 4.2 and 4.4, respectively, was reflected in the self-assembly capabilities of these molecules. Compound 4.2 formed one-dimensional nanostructures stabilized by strong π−π stacking interactions, in contrast to the π-electron-poor 4.4. These observations indicated the potential to control the self-assembly of π-extended PDI cores, by appropriate selection of fused heterocyclic rings. Thieno-fused diazacoronenes (4.7−9), substituted with peripheral alkoxyl groups, were described by Liu and coworkers (Scheme 4).65 The critical step of the synthesis was the simultaneous O-alkylation/aromatization process leading to azaperylenes 4.6a and 4.6b. In the next step, an iodine-assisted 3490


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(red) and 6.3 (blue). The absorption spectra of 6.2, 6.3, and 6.1 revealed that consecutive ring closures had limited influence on absorption bands in the ultraviolet range, while causing strong red shifts of the lowest energy bands (λmax = 576, 660, and 732 nm, respectively). In 2014, Gryko, Cywiński et al. reported the synthesis of a structurally related π-expanded coumarin.67 This new system, dibenzo-1,7-dioxacoronene-2,8-dione 6.6, which can also be viewed as a π-expanded pentacene derivative, was obtained by a double intramolecular oxidative coupling of the 3,9-dioxaperylene-2,8-dione derivative 6.5 (Scheme 6). Two oxidation methods were employed: irradiation in the presence of air and iron(III)-mediated oxidation. Both methods were regioselective, leading to the product with lower steric hindrance, providing yields of 54% and 87%, respectively. Compound 6.6 shows the lowest-energy absorption at 520 nm, and the fluorescence emission at λemmax = 571 nm (ΦF = 0.90), which are both red-shifted and more intense in comparison with the incompletely fused coumarin analogues. 2.1.3. Triazacoronenes. In 2010, a range of substituted 1,5,9-triazacoronenes 7.2a−h were obtained by Wei et al. in a 3-fold Pictet−Spengler reaction followed by spontaneous oxidative aromatization.68 This tandem reaction results in the insertion of a methylidyne group between each nitrogen atom and the adjacent benzene ring in compound 7.1, to build three isoquinoline motifs (Scheme 7). The reaction proceeds with a range of aldehydes, both aromatic and aliphatic, in the presence of triflic acid. All 7.2a−h derivatives are soluble in common organic solvents and show high thermal stability (above 300 °C). Their absorption maxima are red-shifted in comparison with the all-carbon analogue, 1,2,5,6,9,10-hexamethoxycoronene, and have higher values of their molar extinction coefficients. The HOMO energy levels (−6.0 eV) are also lower than those of the all-carbon analogue (−5.5 eV), suggesting that 7.2a−h constitute intrinsic n-type semiconductors with good hole-blocking and electron transport properties. These 1,5,9-triazacoronenes produced strong electrogenerated chemiluminescence (ECL) emission for radical ion annihilation, by stepping from the first oxidation wave to the reduction wave. Subsequent extension of the above synthetic approach yielded derivatives 7.3 devoid of methoxy groups, in particular the parent unsubstituted 1,5,9-triazacoronene.69 2.1.4. Tetraazacoronenes. In 1972, Hurley, Dutt, and Marvel reported the synthesis of a 1,4,7,10-tetraazacoronene derivative 8.3 (Scheme 8).70 The initial acid-catalyzed

Scheme 5. Synthesis of Peripherally Functionalized Diazacoronenesa

a Reagents and conditions: (a)66 (1) NaH, N2 atm, 2-ethylhexan-1-ol, DMF, 0 °C, (2) overnight, rt; (b) K2CO3, 2-ethylhexyl-1-thiol, cat. 18crown-6, N2 atm, DMF, overnight, 120 °C; (c) (1) (2-ethylhexyl) magnesium bromide, Ni(dppp)Cl2, THF, N2 atm, 2 h, 0 °C, (2) overnight, reflux; (d) 2-(4-((2-octyldodecyl)oxy)phenyl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane, K2CO3, Pd(PPh3)4, Aliquat 336, toluene, 48 h, 120 °C; (e) cat. I2, 300 W incandescent lamp, chloroform, 24 h; (f) cat. I2, 300 W incandescent lamp, benzene, 24 h; (g) K2CO3, 2decyltetradecyl-1-thiol, cat. 18-crown-6, N2 atm, DMF, overnight, 120 °C.

genations, yielding compounds 5.3a−d and 5.6a−c. These azacoronenes displayed significant changes of their absorption and fluorescence spectra upon protonation, suggesting their potential use as “naked-eye” dual-mode probes for protons. 2.1.2. Dioxacoronenes. The extended 1,7-dioxacoronene system 6.4 (Scheme 6) was reported by Clar et al. in 1956.55 The green compound 6.4 was prepared by self-condensation of dinaphthyloxyanthraquinone 6.1 in a sodium chloride− aluminum chloride melt at 175−180 °C. By reacting the same starting material at lower temperatures, it was also possible to isolate the partially coupled dioxaperylenoids 6.2 Scheme 6. Synthetic Routes to Dioxacoronene Derivativesa

Reagents and conditions: (a)55 NaCl, AlCl3, 10 min, 180 °C, then 15 min, 145 °C; (b) NaCl, AlCl3, 10 min, 175 °C; (c) NaCl, AlCl3, 35 min, 175− 180 °C; (d)67 hν (365 nm), THF, 24 h, rt; (e) FeCl3, dry DCM, cat. BF3·Et2O, 30 min, rt. a

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1,2,4-triazoline-3,5-diones as dienophiles.74−79 The utility of this approach was tested with 3,9- and 3,10-dihexylperylene, to yield the isomeric tetraazacoronenes 9.5 and 9.11 (Scheme 9), and with N-substituted perylenedicarboxyimide (to yield 9.1278). The first approach to 9.5, proposed by Müllen, Spiess, and co-workers in 1993,74 and subsequently refined,79 started with the coupling reaction of 1-bromonaphthalene in the presence of thallium trifluoroacetate. Next, the resulting 4,4′dibromo-1,1′-binaphthyl 9.2 was converted into the dialkyl derivative 9.3, which was reductively fused and following oxidative rearomatization gave the substituted perylene 9.4. The final product 9.5 was obtained in the 2-fold Diels−Alder reaction with N-alkyltriazolinediones. An alternative method, which obviates the use of thallium salts but suffers from poor regioselectivity,77 starts with a 2-fold bromination of perylene, which leads to an inseparable 1:1 isomeric mixture of 3,9- and 3,10-dibromoperylenes. The isomer mixture was subjected to the Hagihara coupling reaction, and the alkyne derivatives were catalytically hydrogenated. The final product was obtained in the same manner as in the first method. Both methods allow one to obtain variously substituted analogues of 9.5 and 9.11. All of these systems are blue solids with an absorption maximum at 585 nm.79 The triazolinone rings in 9.5b could be cleaved by treatment with NaOH at temperatures above 90 °C, followed by acidification, yielding the readily soluble tetraazacoronene 9.6.74 Depending on the R1 and R2 chain lengths, the 9.5 systems form liquid-crystalline phases of either hexagonal discotic or smectic type, and a similar mesomorphic behavior is also observed for isomer mixtures.74,75,77,79 In subsequent work, oligomeric systems such as 10.3 and 10.6 were developed and shown to reproduce the general mesomorphic characteristics, while showing variations of the dynamic properties of the LC phase (Scheme 10).76 Strict hexagonal order was also observed in the discotic phases formed by the T-shaped mesogen 9.12 and related derivatives.78

Scheme 7. Synthesis of 1,5,9-Triazacoronenesa

a

Reagents and conditions: (a)68 aldehyde, DMF, 1% triflic acid, 100 °C (for aromatic aldehydes) or 40 °C (for aliphatic aldehydes).

condensation of 8.1 and 8.2 apparently produced an incompletely fused intermediate, which upon further heating in vacuum was converted into a highly insoluble product, to which the structure 8.3 was assigned on the basis of elemental analysis and IR spectroscopy. In an extension of the above synthesis, a condensation of 8.1 and the tetraketone 8.4 was attempted, again combining acid-catalyzed and thermal steps. While no structural proof was provided for the expected laddertype structure 8.5, the work is one of the first attempts at the controlled synthesis of heteroatom-doped graphitic nanoribbons (for a slightly earlier example, 110.7 by Stille and Mainen,71,72 see section 4.6). In 1982, Tokita and co-workers reported a synthesis of 1,2,7,8-tetraazacoronene 8.7, analogous to the preparation of 3.1 described above.73 The heptacyclic framework was constructed by a double Diels−Alder reaction between perylene and diethyl azodicarboxylate, to produce the tetraester 8.6. The ester groups in the latter species were then saponified and decarboxylated, with concomitant oxidation of the NN bonds, providing the bis(pyridazino)-fused perylene derivative 8.7. The Diels−Alder strategy of Tokita et al. was subsequently extended by the Müllen group, who employed substituted

2.2. Pyrrole-Fused Azacoronenes

Hexapyrrolohexaazacoronene (HPHAC, Scheme 11) is the prototypical example of an emerging class of nanographenes containing peri-fused pyrrole rings (Chart 5). In an initial study by Jouini et al. in 2003,80 the synthesis of 11.2a by oxidation of hexapyrrolylbenzene 11.1a was attempted in the presence of iron(III) perchlorate. Only products of partial coupling of 11.1a were observed using MALDI-TOF mass spectrometry,

Scheme 8. Synthetic Routes to Tetraazacoronene Derivativesa

Reagents and conditions: (a)70 (1) DMA, AcOH, 160 °C, 10 h; (2) vacuum, 200 °C, 10 h; (3) vacuum, 350 °C, 10 h; (b) (1) PPA; (2) vacuum, 200−250 °C, 20 h; (3) vacuum, 300 °C, 10 h; (4) vacuum, 350−400 °C, 10 h; (c)73 DEAD, neat, 160 °C, 6 h; (d) NaOH, benzyl alcohol, 90−100 °C, 30 min.

a

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Scheme 9. Synthetic Routes to 1,2,7,8-Tetraazacoronenes Containing N-Substituted 3,5-Dioxotriazoles Unitsa

a Reagents and conditions: (a)79 thallium trifluoroacetate, TfOH, 3 h, rt, Ar atm; (b) Ni(dppe)Cl2, butylmagnesium bromide, ether, 18 h, reflux, Ar atm; (c) K, dimethoxyethane, dark, 2 days, rt then oxygen atm, 1 day; (d)78,79 N-heptadecyltriazolinedione, m-xylene, reflux; (e)74 NaOH, benzyl alcohol, 90 °C; (f)77 Br2, AcOH, 40 °C, 1 h, then 50 °C, 5 h; (g) piperidine, 1-hexyne, Pd(PPh3)4, CuI, THF, 80 °C, 14 h; (h) Pd/C, THF, H2, rt.

Scheme 10. Synthetic Routes to 1,2,7,8-Tetrazazcoronene Polymersa

a Reagents and conditions: (a)76 magnesium turnings, 1,10-dibromodecane, ether, 1 h, reflux, then Ni(dppe)Cl2, 2 h, rt, then 48 h, reflux; (b) K, dimethoxyethane, dark, Ar atm, 5 days, rt, then 1:1 mixture oxygen−nitrogen, 1 h, rt; (c) α-deuterated N-heptadecyltriazolinedione, xylene, 3 min, reflux; (d) α-deuterated didodecyl perylene, N-(ethoxycarbonylmethylene)triazolinedione, xylene, 3 min, reflux; (e) 1,10-decanediol, xylene, 120 °C, Ti(O-i-Pr)4, 2 h, reflux, Ar atm, then solvent removed in vacuo, pressure 10−3 bar, 200 °C, 6 h.

Jouini et al.82 11.2a was obtained as a soft homogeneous thin film on the electrode surface after electrochemical oxidation in organic media. In situ electrochemical−ESR measurements revealed that this film underwent reversible oxidation processes and was highly paramagnetic. The film also showed a continuously tunable spin concentration from 0 to 1 per charge at room temperature by controlling the electrochemical potential. In a subsequent study by Müllen and co-workers, published in 2007, a β-aryl substituted HPHAC, 11.2d, was synthesized and structurally characterized in its neutral and oxidized

presumably because of the low solubility of 11.2a in organic solvents. In contrast, the oxidation of 11.1b, possessing solubilizing octyl groups, led to a soluble product, with a mass spectrum consistent with complete peripheral coupling. The expected product, 11.2b, which likely formed as a mixture of regioisomers, was not investigated further. The oxidation of hexapyrrolylbenzenes was shown to proceed according to the radical cation/substrate mechanism, in contrast to oxidative polymerizations of pyrroles, for which dimerization of radical cations normally occurs. The electrosynthesis and magnetism of 11.2a were subsequently explored by Lazerges et al.81 and 3493


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Chart 5. Pyrrole-Fused Azacoronene Derivatives84

Scheme 11. Synthesis of Annularly Fused Hexapyrrolohexaazacoronenesa

a

Reagents and conditions: (a)80 Fe(ClO4)3, MeCN; (b)83 (1) CH3NO2/FeCl3, DCM, 1 h, rt; (2) N2H4.

dicationic states.83 11.2d was obtained by cyclodehydrogenation of the appropriate hexapyrrolylbenzene in the presence of iron(III) chloride, followed by quenching with hydrazine. The unsubstituted 11.2a was identified using MALDI-TOF, in contrast to Jouini’s work, whereas the mass spectra of the expected dodecabromo derivative 11.2c were consistent with partial debromination of the oxidized product. The molecular structure of 11.2d was confirmed by single-crystal X-ray diffraction, showing the HPHAC core to be completely planar, with the centers of the peripheral phenyl groups forming a wavelike pattern above and below the molecular plane. In contrast, DFT calculations predicted a bowl-shaped structure for 11.2a, arising from the tight annular fusion of the fivemembered rings. Compound 11.2d exhibited fluorescence at 570, 617, and 673 nm, bathochromically shifted relative to HBC, which fluoresces at much shorter wavelengths (λmax = 465, 484, 492, 517, and 528 nm). Electrochemical measurements revealed four reversible one-electron oxidation steps for 11.2d. Chemical oxidation with antimony pentachloride produced consecutively the radical monocation 11.2d+ and dication 11.2d2+. The oxidation could be reversed by addition of tetrabutylammonium iodide to the cationic forms. Structural data obtained for the dication, as well as NICS calculations, were consistent with the overall aromaticity of the oxidized HPHAC core. In 2013, HBC−HPHAC hybrids were prepared by Takase, Müllen, Nishinaga et al., using analogous oxidative coupling reactions performed on mixed pyrrole−arene precursors.84 The introduction of two alkoxy groups at the meta positions of the peripheral phenyl substituents was found to effectively promote the oxidative coupling of precursors, leading to the complete closure of the peripheral circuit. In this way, pentaaza-, tetraaza-, and triazacoronene derivatives C5.1a−c, C5.2a−c, and C5.3a−c were obtained (Chart 5). Stepwise replacement of pyrroles with dialkoxybenzene rings was found to drastically alter the optical properties of the hybrids. As in the parent HPHAC system, lowest-energy electronic transitions are partially allowed for C5.1a−3a, with a 10-fold increase of extinction coefficients recorded for C5.3a in comparison with C5.1a−2a. All compounds exhibited colorful fluorescence emissions, the color changing from red through yellow, deep red, to green as the number of pyrrole rings decreased from 11.2d to C5.1a−3a. X-ray crystal structures obtained for the neutral states of C5.3b and C5.4a confirmed the planarization

of the azacoronene core. Observation of fluorescence and phosphorescence at both room temperature and 77 K allowed one to estimate the small S1−T1 gaps (ΔES−T < 0.36 eV). Dications of hexaazacoronene 11.2d and pentaazacoronene C5.1a exhibited a closed-shell character, whereas open-shell configurations were predicted for tetraazacoronene C5.2c and triazacoronene C5.3c dications on the basis of theoretical calculations. A family of expanded hexapyrrolohexaazacoronenes 12.3 and 12.6 containing up to two saturated bridges at the periphery were reported in 2014 by Step̨ ień et al.85 The expansion of the peripheral circuit of these nanographenes was achieved by introducing additional carbon bridges between the pyrrole subunits. 12.3 and 12.6 were synthesized from substituted hexapyrrolylbenzenes using a two-step condensation−aromatization procedure (Scheme 12). In the first step, compound 12.1 was reacted with p-nitrobenzaldehyde, to yield consecutively two bridged species: 12.2 and 12.4. DDQ-mediated oxidative coupling of 12.2, followed by reductive workup with aqueous hydrazine, led to the product 12.3. In the case of the more congested 12.4, oxidation with DDQ, followed by acidic workup, yielded a monocationic species isolated as the tetrafluoroborate salt, [12.5][BF4]. In contrast to 12.3, possessing a saturated benzylidene bridge between two pyrroles, cation [12.5]+ contained one fully conjugated sevenmembered ring. Steric effects had an influence on the nucleophilic additions to the 12.5 cation, enabling stereocontrolled syntheses of cis- and trans-12.6. The oxidative dehydrogenation of benzylidene bridges in 12.3, [12.5]+, and stereoisomers of 12.6 was found to be stereochemically controlled: it was kinetically hindered in the prevailing endo arrangement of the sp3 bridges but proceeded easily for the exooriented sp3 bridges. Despite the interrupted conjugation on the periphery, expanded azacoronenes had easily accessible higher oxidation levels. The electrochemical analysis of the bridged systems revealed up to six (for cis-12.6) oxidation levels and up to one (for [12.5]+) reversible one-electron reduction of the central π-conjugated system. For cis-12.6, a diamagnetic, quadruply charged species was chemically generated by reaction with SbCl5. Oxidized forms of expanded hexapyrrolohexaazacoronenes show extensive π-electron conjugation and are efficient UV−vis−NIR absorbers, active up to ca. 2400 nm. In 3494


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Scheme 12. Synthetic Routes to Expanded Hexapyrrolohexaazacoronenesa

a Reagents and conditions: (a)85 p-nitrobenzaldehyde, BF3·Et2O, DCM, rt, 1 h; (b) (1) DDQ, DCM, rt, 1 h, (2) H2O, (3) N2H4; (c) pnitrobenzaldehyde, BF3·Et2O, chloroform/EtOH, rt, 1 h; (d) (1) DDQ, DCM, rt, 2 h, (2) H2O, (3) 12% HBF4 (aq); (e) NaCN, THF, DMF, N2 atm, rt, 1 h; (f) Na2CO3, DCM, MeOH, rt, 1 h; (g) NaBH4, THF, N2 atm, rt, 1 h; (h) (1) N2H4, DCM, rt, 5 min; (2) CuSO4, DMF, rt, overnight.

Scheme 13. Synthesis of a Peripherally Conjugated 5-6-7 Nanographenea

Reagents and conditions: (a)86 Ag2CO3, Pd(OAc)2 cat., pentafluorobenzene, DMF, dimethylsulfoxide, 120 °C; (b) (1) NaH, diarylpyrrole, DMF, ice bath, (2) 13.2a or 13.2b, 50 °C; (c) BAHA, diethyl ether, THF, rt; (d) Zn amalgam or Zn powder, DCM or chloroform-d; (e) BAHA, [NO][SbF6] or 2,3-dichloro-5,6-dicyano-1,4-benzoquinone, DCM or MeCN; (f) BAHA, diethyl ether, THF, rt. a

the doubly oxidized species [12.6]2+, a tendency toward biradicaloid electron configuration was indicated by computational data. In 2015, Step̨ ień and co-workers prepared heteroaromatic “56-7” nanographenes, containing an assembly of five-, six-, and seven-membered rings (Scheme 13).86 The oxidative coupling of an indole-containing precursor 13.3a (Scheme 13), using 12 equiv of tri(4-bromophenyl)ammoniumyl hexachloroantimo-

nate (BAHA) as the oxidant, led directly to a dicationic 5-6-7 nanographene [13.4a]2+, which was isolated as a hexafluoroantimonate salt. Interestingly, the reaction with BAHA strongly favored the formation of [13.4a]2+, and small amounts of the salt could be isolated even when 2 equiv of the oxidant was used. [13.4a][SbCl6]2 was quantitatively converted to the neutral species 13.4a by reduction with zinc amalgam. When the benzyl-substituted derivative 13.3b was subjected to the 3495


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Scheme 14. Synthesis of B-Doped Nanographenes and GNRsa

Reagents and conditions: (a)87 (1) 9-bromo-bis(mesityloxy)anthracene, n-BuLi, Et2O, 1 h, from 0 to 25 °C, (2) 14.1, toluene, 12 h, from 0 to 25 °C; (b) FeCl3, CH3NO2, DCM, 40 min, 0 °C; (c)89 Au(111) surface, annealing at 180 °C; (d) Au(111) surface, annealing at 400 °C; (e) Au(111) surface, annealing at 510 °C.

a

Ullmann-type step, performed by annealing the monomer 14.4 on an Au surface at 180 °C, the corrugated polymeric precursor 14.5 was obtained. The latter species was subjected to surface-assisted dehydrogenation at 400 °C, to yield completely flat nanoribbons of the general structure 14.6 (ribbon width N = 7). Further annealing at 510 °C led to the fusion of the armchair edges of 14.6, producing nanoribbons with N = 14 (e.g., 14.7) and N = 21, characterized by variable alignment of boron-doped sites. The Lewis acidity of the Bdoped sites in the nanoribbons was demonstrated by selective adsorption of nitrous oxide (NO) molecules. 2.3.2. BN-Embedded Systems. Tetraheteracoronenes 15.3a,b containing two embedded B−N fragments were described by Cao, Wang, Pei et al.90 In addition to the B−N units, these ring systems contained four ortho-fused thiophene rings. They were synthesized in three steps, beginning with 6,13-dihydro-6,13-diazapentacene, which was converted into the tetrabromo derivative 15.1 and subjected to a Stille coupling reaction with tributyl(5-alkylthiophen-2-yl)stannanes. Next, electrophilic borylation was performed on the multiply lithiated 15.2a and 15.2b, yielding the BN-embedded heteracoronenes 15.3a and 15.3b (Scheme 15). The introduction of BN units was found to modulate the molecular geometries of the π-conjugated heteracoronene core. The assembly of two different conformers of 15.3b into sandwichlike 2:1 aggregates was observed by single-crystal X-ray diffraction, providing an example of a self-organization process in a one-component system. Compound 15.3b exhibited photoconductivity as the first BN-containing π-conjugated species. In 2015, Cao, Wang, Pei, and co-workers91 as well as Zhang et al.92 independently reported the synthesis of 3,4,7,8,11,12hexamethoxy-1,5,9-triaza-2,6,10-triphenylboracoronene (16.2, Scheme 16). In both reports, the ultimate BN ring closures were achieved by reacting 2,3,6,7,9,10-hexamethoxy-1,5,9triaminotriphenylene with dichlorophenylborane in dry 1,2dichlorobenzene. In the solid state, 16.2 has a fully planar geometry of the BN heteracoronene backbone. In comparison with its all-carbon analogue, 16.2 showed blue-shifted absorption and emission spectra. Compound 16.2 is unstable in the presence of water because the B−Ph bonds are easily

same BAHA-induced oxidation, the outcome of the reaction was different. The green precipitate that formed was found to contain mainly the partly coupled product [13.5b][SbCl6]2, which was cleanly reduced by zinc amalgam, to yield the corresponding neutral species 13.5b. Near-infrared absorption and emission properties of the nanographene core were enhanced by peripheral expansion and ring fusion at all oxidation levels. The dicationic states (13.4) showed distinct aromaticity originating from a peripheral π-conjugated circuit. The partially coupled intermediate [13.5b][SbCl6]2, trapped in the synthesis of the 5-6-7 nanographene, was explored as a reference system, showing an unexpected reduction of the optical band gap due to intramolecular charge transfer. 2.3. B- and BN-Doped Coronenes

2.3.1. Diboracoronenes. In 2012, Saito, Yamaguchi et al. synthesized a stable coronene derivative 14.3, containing two boron atoms in the core87 (Scheme 14; an incompletely conjugated diboracoronene 82.8 is discussed in section 4.1). Their synthesis started with the lithiation of 9-bromobis(mesityloxy)anthracene, followed by the addition of the resulting lithio derivative to dibromodiborapentacene 14.1. The intermediate thus obtained, 14.2, possessed bulky electrondonating mesityloxy substituents, which were introduced to secure regioselectivity of the subsequent oxidation and to prevent the strong aggregation in the final product. The cyclodehydrogenation of 14.2, performed with an excess of FeCl3, proceeded successfully to form 14.3 in a 51% yield. 14.3 was isolated as a deep-purple solid, sufficiently soluble in common organic solvents. 14.3 is a closed-shell system and is an example of a structurally uniform boron-doped nanographene. Its electronic spectrum in solution showed the presence of broad absorption up to ca. 700 nm, complemented by fluorescence emission reaching the NIR region of the spectrum (λem = 679 nm, ΦF = 0.04). The nanographene showed three reversible redox events at Eox1 = 0.62 V, Ered1 = −1.45 V, and Ered2 = −1.66 V (vs Fc/Fc+). The solid-state structure of 14.3 revealed deviations of the fused core from planarity, caused by steric overcrowding in the cove regions. Using an analogous structural paradigm and an on-surface synthetic strategy,88 Kawai et al. developed in 2015 a synthesis of boron-doped graphene nanoribbons.89 In the initial 3496


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Scheme 15. Synthesis of 6a,12a-Diaza-6a1,12a1diboracoronene Derivativesa

Scheme 17. Synthesis of BN-Doped Hexa-perihexabenzocoronenea

a

Reagents and conditions: (a)93 Ar atm, 6 h, 550 °C.

cyclodehydrogenation of 18.1 proceeded smoothly to 18.3 (Scheme 18). 18.3 showed good solubility in organic solvents and had excellent thermal stability in the solid state. The UV− vis spectrum of 18.3 contained absorptions characteristic of the basic HBC structure, with two additional weak bands at 450 and 490 nm. 18.3 exhibited a strong emission band at 545 nm, which was quenched by gradual addition of trifluoroacetic acid. Heteroleptic ruthenium and palladium complexes of 18.1 and 18.3 were obtained in reactions with appropriate bipyridyl or allyl complexes (Scheme 18).95 Compound 18.4 turned out to be unstable in solution or after irradiation with visible light. In 18.7, Pd(II) coordination caused a red-shift in the lowenergy absorptions, a decrease in the intensity of the n−π* absorptions, and a quenching of the fluorescence emission. Each of the RuII compounds 18.6a and 18.6b possessed two discernible 1MLCT bands covering the entire visible range, and was therefore classified as a “black” absorber. The lower energy band (λmax = 615 nm) showed an associated 3MLCT emission (λmax = 880 nm), due to the unprecedented electron delocalization and acceptor properties of the rigid aromatic ligand 18.3. Both RuII complexes are near-IR emitters with unusually protracted emission lifetimes of 320 ns at 77 K. Cyclic voltammograms of 18.6a showed two sequential oneelectron reductions associated with the 18.3 ligand, and two observable oxidation processes. These reduction potentials were anodically shifted when compared to the bpy-based reduction potentials, confirming the extensive delocalization and the presence of low-lying π*-orbitals in the 18.3 ligand. Temperature- and concentration-dependent NMR studies on 18.7 and 18.6a suggested that extensive π aggregation was occurring in solution. A partially coupled N-heterosuperbenzene, 18.2, was obtained alongside 18.3 as a product of oxidative cyclodehydrogenation performed with iron(III) chloride instead of AlCl3/CuCl2 used in the earlier report (Scheme 18).96

a Reagents and conditions: (a)90 Pd(dba)3, P(o-Tol)3, 1,2,4-trichlorobenzene, 12 h, 170 °C; (b) (1) n-BuLi, o-dichlorobenzene, 2 h, 0 °C, (2) BBr3, 24 h, 180 °C.

cleaved. The possibility of sequential replacement of substituents, yielding systems 16.3−5, was envisaged as a means of fine-tuning the electronic properties of these BN coronenes. In 2015, Bettinger and co-workers described the synthesis of a hexa-peri-hexabenzocoronene with a central borazine core 17.2 (Scheme 17).93 The desired compound was prepared by thermolyzing N,N′,N″-tris(2-biphenylyl)borazine 17.1 at 550 °C for 6 h. After sublimation, 17.2 was isolated as a poorly soluble yellow powder. The solid-state structure of 17.2 is isotypic with that of the parent hexa-peri-hexabenzocoronene, as shown by a powder X-ray diffraction analysis. Scanning tunneling microscopy enabled direct observation of 17.2 on the Au(111) surface, revealing preferential alignment of the symmetry axes of the molecule relative to the substrate. 2.4. peri-Condensed Coronenes

The first example of a coronene peri-fused to a heterocyclic ring was provided by tetra-peri-(tert-butyl-benzo)-di-peri-(pyrimidino)-coronene 18.3, which was reported in 2002 by Draper and co-workers.94 18.3, the first member of the “N-heterosuperbenzene” family, was synthesized in a two-step procedure, beginning with the Diels−Alder cycloaddition between tetraphenylcyclopentadienone and di(pyrimidin-3,5-yl)ethyne. This cycloaddition yielded the heteroaromatic hexaarylbenzene 18.1, containing two adjacent pyrimidine rings. Oxidative

Scheme 16. Synthetic Routes to BN-Embedded Coronene Analoguesa

Reagents and conditions: (a)91 dichlorophenylborane, Et3N, o-dichlorobenzene, N2 atm, 12 h, 180 °C; (b)92 dichlorophenylborane, Et3N, odichlorobenzene, Ar atm, 24 h, reflux; (c) chloroform, 1 week; (d) chloroform, 2 weeks. a

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Scheme 18. Synthetic Routes to N-Heterosuperbenzenes and Their Complexesa

a

Reagents and conditions: (a)96 FeCl3/CH3NO2, DCM, Ar atm, rt, 30 min; (b)94 AlCl3, CuCl2, CS2, rt, 72 h; (c)95 (1) [Ru(bpy)2Cl2]·2H2O, AgBF4, acetone, reflux, 1.5 h, (2) n-BuOH, acetone, (3) 18.1, DCM, n-BuOH, reflux, 72 h; (d)96 (1) [Ru(bpy)2Cl2]·2H2O, diethylene glycol ethyl ether, sonication, 10 min, (2) Ar purging, 30 min, (3) 20 h, 127 °C; (e)95 (1) [Ru(bpy)2Cl2]·2H2O or [Ru(bpy-d8)2Cl2]·2H2O, diethylene glycol ethyl ether, sonication, 10 min, (2) Ar purging, 30 min, (3) 20 h, 127 °C; (f) (1) bis(chloro-η3-allyl palladium), MeCN, DCM, AgNO3, 6 h, rt, (2) filtration over Celite, (3) 16.3 in toluene, 12 h, rt.

Chart 6. Methoxy-Functionalized97 and Terpyridine-Fused98 Coronene Derivatives

Compound 18.2 did not undergo further oxidative coupling in the presence of either reagent, indicating that it was thermodynamically stable and that it formed independently of 18.3. In analogy to 18.3, 18.2 produced a complex with RuII, 18.5 (Scheme 18). 18.2 exhibited photophysical properties consistent with a less conjugated π/π* framework, including an

increase in the energy of the LUMO, and a blue-shift of the solvent-dependent fluorescence (λem = 474 nm, ΦF = 0.55, toluene), as compared to 18.3. Complex 18.5 absorbed throughout the visible region, exhibiting a slightly blue-shifted 3 MLCT emission (868 nm, CH3CN) in comparison with 18.6a. Electrochemical analyses permitted further elucidation of 3498


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Scheme 19. Synthesis of Chevron-Shaped Graphene Nanoribbons Doped Nitrogen Atomsa

a

Reagents and conditions: (a)17 250 °C; (b) 440 °C.

pyridyl rings in the Scholl reaction. The absorption and emission properties of the RuII bis-terpy complex of ligand C6.5 were comparable to those of [Ru(terpy)2]2+. The new complex was nonemissive at room temperature but emitted at 77 K with excited-state lifetimes of 12 μs. In 2013, Bronner, Hecht, Tegeder et al. proposed a synthesis of chevron-shaped graphene nanoribbons consisting of fused 2aza- or 2,5-diaza-HBC subunits (19.3a,b, Scheme 19).17 These compounds were obtained from triphenylene monomers 19.1a,b, by means of an on-surface coupling strategy developed earlier for the analogous all-carbon nanoribbons.88 Adsorption of several layers of monomers (19.1) on Au(111) and heating at 250 °C led to desorption of the second and higher layers as well as halogen dissociation and coupling of the resulting activated biradical monomers, yielding a sterically crowded and hence twisted polyphenylene 19.2. In a second heating step at 440 °C, these polymers underwent a subsequent cyclodehydrogenation reaction to yield the completely fused nanoribbon 19.3. Using angle-resolved high-resolution electron energy loss spectroscopy (HREELS) in combination with photoelectron spectroscopy, the researchers showed that the band gap is linearly shifted relative to the electronic structure of the environment of the graphene nanoribbons but remains almost unchanged in magnitude, as expected for pyridine-like

the intermolecular interactions of 18.6a and 18.5. These results and the concentration- and temperature-dependent NMR spectra of 18.5 confirmed the complex to be nonaggregating, a direct result of the two uncyclized and rotatable phenyl rings in 18.2. Methoxy-substituted N-heterosuperbenzenes containing one (C6.3) or two pyrimidine rings (C6.1 and C6.2, Chart 6) were obtained similarly to 18.3.97 In the solid state, C6.1 and C6.3 showed a slight distortion of the aromatic core and two distinct types of columnar stacking. The absorption and emission spectra of C6.1, C6.2, C6.3, and C6.4 were comparable with those of other N-heterosuperbenzenes.94−96 For all compounds, broadening of emissions and solvatochromic shifts were observed in methanol, possibly associated with the formation of an intramolecular charge-transfer (ICT) excited state, which was stabilized by polar solvents. Cyclic voltammograms showed that each of the completely coupled compounds C6.1, C6.2, and C6.3 possessed a single irreversible oxidation wave under positive potential and one reversible reduction. Terpyridine-containing derivatives C6.5 and C6.6 were prepared by Draper et al. in 2012, by employing Scholl reaction conditions similar to those used for the synthesis of 18.3.98 Compounds C6.5 and C6.6 were obtained, respectively, in 34% and 5% yields, showing the lower reactivity of the 3499


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Scheme 20. Synthesis of S-Doped Hexabenzocoronenesa

a

Reagents and conditions: (a)101 FeCl3/CH3NO2, DCM, rt, 5 h.

nitrogen at the edges of armchair graphene nanoribbons. In 2014, using a similar procedure, Müllen, Fasel, and co-workers prepared another example of a nitrogen-doped graphene nanoribbon consisting of 1,3,4,6-tetraaza-HBC subunits (19.5) as well as hybrid nanoribbons combining undoped and nitrogen-doped segments (19.6).99 The heterostructure of the nanoribbon was confirmed by scanning probe methods, and it was shown to behave similarly to traditional p−n junctions. A band shift of 0.5 eV was determined for this material as well as an electric field of 2 × 108 V m−1 at the heterojunction. In the same year, Sinitskii et al. proposed a bulk synthesis of the 1,3diaza-HBC nanoribbon 19.4, obtained as a highly insoluble powder in a sequence of Yamamoto and Scholl coupling reactions.100 However, the unsymmetrical structure of the precursor resulted in a nonuniform distribution of N-doping sites in 19.4. Oxidative cyclodehydrogenation of the thienyl precursor 20.1, reported by the Draper group, yielded the thieno-fused coronene derivative 20.2 and its dimer 20.3 (Scheme 20).101 It was presumed that the intramolecular couplings involving the thiophene rings occurred prior to the intermolecular coupling of the 20.3 dimer. Photochemical and electrochemical properties of the monomer and dimer were similar to those reported in the literature for the all-carbon analogues of hexaalkyl-substituted coronenes. Nevertheless, the dimeric structure exhibited dual luminescence that varied with concentration. This behavior was explained in terms of a concentration-dependent conformational change involving the orientation of the two HBC-like subunits in 20.3, likely associated with the formation of excited-state aggregates. In 2015, a new class of peri-thiophene-fused coronenoids was developed by Müllen and co-workers.102 Triply annulated HBC derivatives 21.2a−c, accompanied by traces of the bisannulated 21.3a,c, were obtained by exhaustive thiolation of perchlorinated HBC (21.1, Scheme 21). The bay-selective sulfur annulation probably results from the minimization of strain in the aromatic core, incurred upon the initial thiolate substitution (for a related example, cf., Scheme 54, section 3.2). The densely functionalized π-electron system in 21.2a is characterized by an optical bandgap that is reduced by ca. 0.5 eV relative to the unsubstituted HBC. In 2015, Zheng, Tan et al. employed 21.2a as the hole-transport material in perovskite solar cells, yielding efficiencies of up to 12.8% in pristine form, and up to 14.0% when doped with graphene sheets.103

Scheme 21. Synthesis of Thiophene-Fused p-HBC Derivativesa

a

Reagents and conditions: (a)102 sodium thiophenolate, dry 1,3dimethyl-2-imidazolidinone, 0 °C to rt in 5 h.

employ oxidative annulations of heteroaromatic substituents attached to PAHs (in particular perylene) or direct assembly of a hetero ring on the periphery of a PAH. The latter method was employed in the early synthesis of the quinoxalino-fused coronene 22.3 described in 1952 by Zinke and co-workers.54 Compound 22.3 was obtained from coronene-1,2-dione and ortho-phenylenediamine in the presence of nitrobenzene and glacial acetic acid. The same approach was used in 2008 by Müllen and co-workers to obtain a triply quinoxalino-fused coronene derivative, 22.6.104 In 2009, Liu, Sanguinet et al. employed the same strategy to obtain the coronenetetrathiafulvalene hybrid 22.7, characterized by extended πconjugation.105 This triply TTF-fused compound was obtained via direct condensation of coronene-1,2,5,6,9,10-hexaone (22.5) with 5,6-diamino-2-[4,5-bis(hexylthio)-1,3-dithio-2ylidene]benzo[d]1,3-dithiole (Scheme 22). Compound 22.7 possessed strong ICT absorption in the visible part of optical spectrum (λmax = 580 nm) and yielded a NIR-absorbing cationic species upon oxidation with (NO)[SbF6], which was interpreted as the [22.7]3+ trication. Quinoxaline condensations were also used recently by Xiao et al. for the synthesis of ladder-

2.5. ortho-Condensed Coronenoids

2.5.1. Coronenoids Fused to Azaheterocycles. Several strategies are available for the synthesis of coronenoids containing ortho-fused heterocyclic rings. Typical approaches 3500


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Scheme 22. Synthetic Routes to Quinoxaline Derivatives of Coronenea

a

Reagents and conditions: (a)54 (1) nitrobenzene, glacial AcOH, (2) Na2Cr2O7, glacial AcOH; (b) nitrobenzene, glacial AcOH, ophenylenediamine; (c)104 (1) BBr3, DCM, overnight, rt, (2) HNO3, 1 min, rt; (d) o-phenylenediamine; (e)105 DMF/AcOH (7/1), 160 °C, microwave irradiation.

Scheme 23. Synthesis of Pyrido-Annulated Coronenesa

a

Reagents and conditions: (a)106 pyridin-4-ylboronic acid, Pd(PPh3)4, THF, 2M K2CO3, 4 h, reflux; (b) hν, DCM, 5 h, rt; (c) pyridin-4-ylboronic acid, Pd(PPh3)4, THF, 2M K2CO3, 24 h, reflux; (d) iodomethane, MeCN, 30 h, reflux; (e)107 hν (halogen lamp, 500 W), ethyl acetate, 10 h; (f) TFA, chloroform, overnight, rt.

like oligomers 22.8 and 22.9, comprising respectively dibenzo[a,j]coronene and naphtho[1,2,3,4-ghi]perylene subunits.

A bis-pyrido-fused coronene 23.3 was prepared by Wang et al. in 2010 (Scheme 23).106 Its synthesis, which applied the 3501


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Scheme 24. Synthesis of Ladder-Type Perylene Derivativesa

Reagents and conditions:108 (a) I2, toluene, hν, O2, rt, 3 h; (b) 4-methylbenzenesulfonic acid, 110 °C, 3 h; (c) dodecylamine, imidazole, 200 °C, 4 h.

a

Scheme 25. Synthesis of Thiophene-Fused Coronene Derivativesa

a Reagents and conditions: (a)110 cat. I2, hν, DCM, 2 h, rt; (b)111 FeCl3/CH3NO2, DCM, 20 min, rt, Ar atm; (c)112 FeCl3/CH3NO2, DCM, 30 min, rt, Ar atm.

noncyclized precursor 23.5. The t-butyl-protected compound 23.6 showed strong green fluorescence. The emission of the water-soluble derivative 23.7 was weaker and had a yellow-toorange color. Coronene derivatives containing two ortho-fused carbazole units were reported in 2011 by Xiao et al. (Scheme 24, for structurally related ladder PDI chromophores C11.6−7, see section 3.3).108 The target structures 24.4 and 24.6, containing the PDI motif, were obtained from isomeric bis-carbazolyl perylene precursors (e.g., 24.1 for 24.4) by means of a double photocyclization reaction, followed by conversion of the tetraester into the diimide. Compounds 24.4 and 24.6 have good solubility in common solvents and showed enhanced third-order nonlinear properties in comparison with the parent nonfused PDI.

substituent annulation strategy to the PDI precursor 23.1, could be performed as a sequence consisting of Suzuki crosscoupling and Mallory photocyclization, or in a one-pot fashion, by extending the reaction time of the Suzuki coupling step. The dicationic derivative 23.4, obtained by quaternization with methyl iodide, was characterized by a drastic reduction in the fluorescence intensity relative to 23.3. In 2011, Hirsch et al. reported on the synthesis of water-soluble triazole-fused coronenes.107 Hexaester 23.6 was obtained by UV irradiation of the bay-functionalized precursor 23.5. The double cyclization reaction was significantly slower than the analogous single cyclization leading to the triazole-fused benzoperylenes C11.3a−4a (section 3.3). Deprotection with trifluoroacetic acid led to the free hexacarboxylate 23.7. 23.6 and 23.7 had absorption spectra with sharper bands and higher extinction coefficients, and exhibited stronger fluorescence than the 3502


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Chart 7. Dithienocoronene Diimide-Based Conjugated Polymers113−115

2.5.2. Thieno-Fused Coronenoids. Coroneno[1,2-b:7,8b′]dithiophene 25.3a was prepared by Ko et al. in 2011 (Scheme 25).109 Its tetraester analogue 25.2 was reported in 2014 by Xiao et al., who similarly used the Mallory photocyclization in the final step.110 Estimated HOMO and LUMO levels in 25.2 were about 0.4 eV higher than in the diimide, indicating that the tetraester was less electron deficient and easier to be oxidized. 25.3a was subsequently functionalized with oligothienyl substituents to provide derivatives 25.3b,c, with bathochromically shifted absorptions. On the basis of these coronene derivatives, bulk-heterojunction organic solar cells were prepared, yielding power conversion efficiencies of up to 1.42%. Thiophene-annulated core-extended rylene diimides, terrylene 25.6 and quaterrylene 25.7, were obtained by Negri, Wang,111 et al. in 2012. Thiophene units were introduced regiospecifically using Stille cross-coupling of a brominated terrylene with 2-(tributylstannyl)thiophene, to yield the thienyl-substituted precursor 25.5. For the quaterrylene derivative, thienyl regioisomers could be easily separated by chromatography. The final annulations were performed under typical Scholl reaction conditions using iron(III) chloride (Scheme 25). These thiophene-annulated rylenes were poorly soluble in common organic solvents and exhibited hypsochromic shifts relative to the parent perylenediimide. Their fluorescence was characterized by small Stokes shifts (8−12 nm) and high fluorescence quantum yields (0.56 and 0.14 for 25.6 and 25.7, respectively). A range of dithienocoronene-containing polymers were developed by several research groups. Donor−acceptor (D− A) systems reported in 2012 by Usta, Facchetti,113 et al. consisted of dithienocoronene diimide acceptor units linked with thiophene, bithiophene, or 3,3′-dialkoxybithiophene donor moieties (Chart 7). Polymers were obtained from dibromodithienocoronenediimide 25.4, which was subjected to the Stille coupling reaction with an appropriate donor derivative. The near-linear linkage geometry of dithienocoronenediimide is crucial for achieving a high degree of regioregularity in the polymeric backbone, which is of importance in organic thinfilm transistor (OTFT) devices. Top-gate bottom-contact TFTs were produced by spin-coating solutions of C6.1a and C7.1c. Ambipolar behavior was observed in ambient, with electron and hole mobilities of up to 0.30 and 0.04 cm2 V−1 s−1,

respectively. A conjugated copolymer C7.2 combining planar dithienocoronene diimide acceptor units with strongly donating porphyrin moieties was prepared in 2012 by Duan, Zhan, et al.114 The polymer was synthesized through the Sonogashira coupling reaction of an appropriate Zn diethynylporphyrin with the dibromodithienocoronenediimide 25.4. Polymer C7.2 exhibited a low bandgap (1.44 eV) and good thermal stability (decomposition temperature of 323 °C). It also possessed strong absorption in the 300−900 nm range with a molar extinction coefficient per repeat unit of 1.05 × 105 M−1 cm−1 in the CHCl3 solution. As compared to a perylenediimide− porphyrin polymer,116 C7.2 exhibited up-shifted HOMO and LUMO levels and blue-shifted absorption, as a consequence of its weaker electron-withdrawing ability. High two-photon absorption (TPA) cross sections were measured for C7.2, with a maximum of 7809 GM per repeat unit at 1520 nm. Zhan115 et al. reported the synthesis of a series of n-type semiconducting polymers C7.3a−d by Pd-catalyzed Stille coupling polymerization. Those compounds were based on the dithienocoronenediimide core linked with oligothiophenes of varying lengths (m = 0−3 units, Chart 7). Compound C7.3d was insoluble in common organic solvents. The soluble polymers C7.3a−c exhibited good thermal stability, with decomposition temperatures of 340−390 °C, and showed progressively red-shifted absorption spectra with increasing m number. The optical band gap was estimated as 1.7−1.9 eV and was found to decrease with increasing m. Interestingly, the estimated HOMO values (−6.0 to −5.7 eV) were also increasing with increasing m, whereas the LUMO values (ca. −3.5 eV) remained virtually constant. Solution-processable BHJ all-polymer solar cells were fabricated using compounds C7.3a−c as acceptors and a polythiophene derivative PT5TPA117 as the donor. Measured power conversion efficiencies (up to 0.84%) were also observed to increase with the linker length. Tri(benzothiopheno)hexa-peri-hexabenzocoronene 26.2, the first synthesized example of large PAHs fused with electron-rich thiophene rings, was reported in 2007 by Müllen and coworkers.118 Compound 26.2 was obtained by means of a 6-fold intramolecular oxidative cyclodehydrogenation of the oligoaryl precursor 26.1 (Scheme 26). Compound 26.2, which may be described as a triply benzothiophene-fused HBC, is poorly 3503


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Scheme 26. Triply Thiophene-Fused Coronenoidsa

triphenylene derivative. 26.4 possesses a planar conformation in the solid state, due to the smaller size of thiophene ring, which eliminates steric congestion present in other similarly fused coronenes reported in the same work. 26.4 also exhibited lower HOMO energy levels, in line with the electron-donating properties of thiophene. Trithieno hexa-cata-hexabenzocoronene (c-HBC) analogues 27.3a−c were synthesized in 2014 by Wei and co-workers.120 These authors used a “covalent self-sorting assemble” (CSA) strategy, which was devised as a two-step reaction sequence (Scheme 27). In the first step, sym-tribenzylbenzenes 27.2a−c were obtained through the Suzuki cross coupling of 1,3,5tri(bromomethyl)-benzene with 3,4-dialkoxyphenylboronic acids. In the final, “self-sorting” step, c-HBC derivatives were obtained in a tandem Friedel−Crafts/oxidative coupling reaction between 27.2a−c and aryl aldehydes. The use of thiophene-3-carbaldehyde yielded the triply thieno-fused coronenes 27.3a−c. Dibenzotetrathienocoronenes 27.5a−g, first described in 2010 by Nuckolls and co-workers,121,122 are structurally and synthetically related to “contorted” hexabenzocoronenes (cHBCs).59 A variety of tetrasubstituted derivatives 27.5a−g were obtained by photocyclization of the pentacene precursors 27.4a−g. The latter compounds were prepared using Suzuki cross-coupling. In the solid state, 27.5a−g adopted either the “up-down” or “butterfly” conformation, depending on the substitution pattern and crystallization conditions. In particular, cocrystallization with small (TCNQ) and large electron acceptors (C60) stabilized the “up-down” or “butterfly” conformations, respectively.121 27.5c forms columnar stacks capable of hole transport in the solid state. Upon annealing, spuncast layers of 27.5c became organized into reticulated

a

Reagents and conditions: (a)118 FeCl3, CH3NO2, DCM, 40 min, rt; (b)112 FeCl3, DCM, 30 min, rt.

soluble in common organic solvents, and its absorption and fluorescence spectra exhibit a blue shift of 10 and 30 nm, respectively, as compared to HBC itself. 26.2 was successfully processed into thin layers using pulsed laser deposition (PLD).119 A smaller trithiophene system, coroneno[1,2-b:5,6b′:10,9-b″]trithiophene 26.4, was obtained by Wang et al. in 2009.112 The structure of the trithienyl precursor 26.3 was dictated by the availability of the unsymmetrically brominated Scheme 27. Synthetic Pathways to Thienocoronene Derivativesa

Reagents and conditions: (a)120 (1) Na2CO3, acetone/water (1/1), Ar atm, 10 mol % PdCl2, 1,3,5-tris(bromomethyl)benzene, 0 °C, (2) 12 h, rt, (3) 3 days, 37−38 °C; (b) (1) Ac2O, DCM, FeCl3, CH3NO2, Ar atm, 1 h, rt, (2) 27.2, DCM, FeCl3, CH3NO2, Ar atm, 1 h, rt; (c)121 I2, hν, propylene oxide, benzene, N2 atm, 4 h, rt. a

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Scheme 28. Quinoxaline Assemblies Based on Tetrabenzo[bc,ef,hi,uv]ovalene-1,2-dionea

a

Reagents and conditions:127 (a) 3,3′-diaminobenzidine tetrahydrochloride, AcOH, reflux, 3 d; (b) o-phenylenediamine dihydrochloride, AcOH, reflux, 3 d; (c) AcOH, 1,2-diamino-4,5-dicyanobenzene, reflux, 6 h; (d) 5,6-diamino-1,10-phenanthroline, pyridine, reflux, 16 h; (e) 1,2,4,5benzenetetraamine tetrahydrochloride, AcOH, reflux, 8 d; (f) DMF, cis-bis(2,2′-bipyridine)ruthenium(II) chloride, reflux, 3 days; precipitation from aqueous NH4PF6; (g) urea, CuCl2, quinoline, 220 °C, 16 h.

28.7. The presence of t-butyl groups in the bay regions of the ovalene block apparently led to out-of-plane distortions of the aromatic surface, which translated into increased solubility of the products, that could be purified chromatographically and produced well-resolved 1H NMR spectra. 28.6 was found to be a quadrupolar (donor−acceptor−donor) system, with the LUMO level partially localized in the central part of the molecule. The quadrupolar character of 28.6 led to a marked solvatochromic effect in the fluorescence emission spectra (λmaxem = 658 nm in cyclohexane and 740 nm in THF/MeCN), indicating the presence of a polar excited state. In contrast, the absorption spectra of 28.6 showed very weak solvent dependence. With 224 atoms in the aromatic core, 28.8 is apparently the largest monodisperse PHA system reported to date. It showed a considerably red-shifted absorption spectrum, with an optical bandgap of 1.34 eV (869 nm).

layers, which were found to provide excellent bulk heterojunctions for use in photovoltaics upon doping with C60.122 In subsequent work, a supramolecular complex was found to form in film blends containing 27.5c and phenyl-C70-butyric acid methyl ester (PC70BM).123 This complex formed the supramolecular heterojunction in the first fully solution-processed photovoltaic device with ball-and-socket motif, showing a solar efficiency of 2.7%. Structurally related hexathienocoronenes (HTCs) 27.7a,b and 27.8a−c were reported in 2012 by Müllen and coworkers.124 These compounds, obtained using dehydrogenative photocyclizations, revealed remarkable self-assembly behavior in solution, in the solid state, and at the solution−substrate interface. 27.8c forms a columnar mesophase over a wide temperature range close to room temperature, much lower than measured for the structurally related hexadodecylhexabenzocoronene. Hexathienocoronene 27.8d, reported in 2013 by Müllen, Chen, and co-workers,125 had a substitution pattern containing two alkyl chains and two oligoether groups, designed to provide a gemini-type amphiphilic structure of the type described earlier by Aida, Fukushima, et al.126 To achieve amphiphilic functionalization of the HTC core, the authors introduced two kinds of thiophene segments in a stepwise manner (as pinacol boronic esters), and completed the fused aromatic framework in 27.8d using a dehydrogenative photocyclization. The choice of solution-processing conditions allowed the tuning of supramolecular structures from fibers to helices and tubes. This property was probably associated with the slightly twisted skeleton of 27.8d and its amphiphilicity. In particular, self-assembly at the liquid−solid interface led to the formation of large domains of defect-free monolayers. An STM study of these monolayers showed a peculiar packing pattern, in which pairs of molecules were adsorbed very close to each other and arranged in the p2 plane group. 27.8d formed a stable radical cation upon oxidation with (NO)[BF4], and yielded a strong photocurrent in PCBM blends. In 2007, Müllen et al. reported the synthesis of substituted tetrabenzo[bc,ef,hi,uv]ovalene-1,2-dione 28.1 and demonstrated its use as a building block for the synthesis of a variety of orthofused quinoxaline systems 28.2−6 (Scheme 28).127 The dicyano compound 28.4 was subsequently converted into the copper(II) phthalocyanine 28.8, whereas the phenanthroline derivative 28.5 was transformed into the ruthenium complex

3. PERYLENOIDS The current interest in perylene derivatives is motivated by their importance for materials science, notably in the area of organic electronics. Recent advances in the field have been reviewed by several authors (see section 1.2 for references). The perylene chromophore can be modified in a number of ways, including extension into rylene ribbons, bay-region substitution and fusion, and terminal substitution and fusion. Perylene mono- (PMIs) and diimides (PDIs) are among the most studied derivatives because of their easy accessibility and commercial importance. As explained in section 1.2, PMI and PDI systems, containing no additional heteroannulated motif, will not be covered here. The remaining material is divided into five sections (Chart 8). Heteraperylenoids, discussed in section 3.1, contain at least one heteroatom in a fully conjugated perylene substructure. Perylenes containing five- and sixmembered rings fused to the bay region ([ghi]-fusion) are covered, respectively, in sections 3.2 and 3.3. peri-Fusion at the perylene “terminus” ([cd]-fusion) is discussed in section 3.4, whereas ortho-heteroannulated systems ([a]- and [b]-fused) are presented in section 3.5. In each section, perylene defines the minimum required extent of fusion, and we include any larger system not containing the coronene motif. 3505


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that, after the first Stille coupling between 29.5 and 29.6, an intramolecular Heck-type cyclization effectively competed with the second intermolecular coupling under typical reaction conditions. The first synthesis of a monoazaperylene, 1-azaperylene 29.15, was reported in 2010 by Gryko et al.131 29.15 was obtained in an anionic radical coupling reaction132 from each of the two isomeric naphthyl isoquinoline precursors 29.14 and 29.16. 1-Azaperylene could be directly functionalized with either the OH or the CN group, yielding, respectively, the substituted derivatives 29.17 and 29.18. 29.15 and 29.18 are strongly emitting fluorophores (ΦF > 80% in cyclohexane). The emission from 29.17 was weaker (17% in cyclohexane), but the chromophore itself showed a considerable bathochromic shift of 75 nm for the lowest energy absorption. While an ESIPT process was envisaged for 29.15, no conclusive experimental evidence could be offered, apparently because of the unfavorable overlap of emissions for the keto and phenol tautomers. 3a1-Azoniaperylene bromide 29.21, an extended cyclo[3.3.3]azine133 analogue, was reported in 1991 by Fourmigué, Eggert, and Bechgaard.134 The key precursor, quinolizinium bromide 29.20, was obtained by acid-catalyzed cyclization of the pyridine derivative 29.19. 29.20 was subsequently cyclized with 2,3-dihydroxy-1,4-dioxane, a masked glyoxal equivalent, to produce the azoniaperylene 29.21 in 55% yield. 29.21 showed charge delocalization and magnetic properties consistent with its formulation as a combination of weakly interacting naphthalene and quinolizinium units. It could be reversibly reduced (E1/2 = −1.08 V vs SCE) to a neutral radical isoelectronic with the perylene radical anion.

Chart 8. Classification of Perylenoids Used in This Sectiona

a π-Conjugation and possible additional fused carbocyclic rings are not indicated.

3.1. Heteraperylenoids

3.1.1. Monoheteraperylenoids. Early examples of perylenoid structures containing a single embedded heteroatom were provided in 1946 by Schönberg et al.128 and in 1957 by El-Shafei and Ismail,129 who synthesized the fused xanthene (29.3) and thioxanthene (29.4) derivatives in light-induced oxidative cyclizations of the respective ethylenes 29.1−2 (Scheme 29). Two isomeric monoazaperylene derivatives 29.7 and 29.8 were obtained unselectively as byproducts in the synthesis of the sterically congested 1,8-diarylnaphthalene 29.9, as reported in 2003 by Wolf and Mei.130 It was found Scheme 29. Monoheteraperylenesa

Reagents and conditions: (a)128,129 benzene, sunlight, 2−4 weeks; (b)130 Pd(PPh3)4, DMF, 100−140 °C, 5−18 h, up to 17% of 29.7 and 29.8; (c)135 AlCl3 (3 equiv), S8 (1.5 equiv), NEt(i-Pr)2 (2.0 equiv), toluene, 80 °C, 12−18 h; (d)135 (1) PEt3, chlorobenzene, 0 °C, (2) 120 °C, 12 h; (e)131 (1) K, toluene, (2) air; (f)131 Pd(OAc)2, PhI(OAc)2, n-BuOH; (g)131 Pd(OAc)2, CuCN, CuBr2; (h)134 HBr (48%), AcOH, reflux, 3 h; (i)134 EtOH, NEt3, reflux, 2 h. a

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A perylenoid substructure with an embedded phosphorus atom 29.13 was reported in 2011 by Hatakeyama, Nakamura, et al. (Scheme 29).135 In the initial synthetic step, performed on the dichloroarylphosphine 29.11, the fused framework was built around the P atom in a phospha-Friedel−Crafts reaction. The resulting phosphine sulfide 29.12 was subsequently oxidized to the respective oxide or reduced to the free phosphine 29.13. In all derivatives, the C−P bonds retain a pyramidal geometry. In solution, 29.13 shows a strong green fluorescence, which includes an excimer-type emission at 540 nm. The phosphine center in 29.13 is capable of metal coordination, and a gold(I) complex, (29.13)AuCl, was isolated and structurally characterized. Vinylogous NMI and PDI derivatives 30.2−4, developed in 2015 by the Müllen group, contain the 3a1-azaperylene substructure with a nitrogen atom at a peri-fusion point.136 The simplest system, 30.2, was obtained in a tandem condensation between naphthalene monoanhydride and the diacetylaniline 30.1. The analogous condensation leading to 30.4 was low-yielding (5%) and was replaced with a sequential route. 30.2 was further modified by double olefination of the enone moieties with malonodinitrile. 30.2−4 showed considerably red-shifted absorption spectra relative to the parent NMI and PDI systems (λmax = 451, 630, and 700 nm, respectively). 30.4 displayed strong fluorescence with a relatively small Stokes shift (Δλ = 26 nm, ΦF = 0.45).

Scheme 31. Synthesis of Monoheteraperylenes Doped with a Boron Atoma

a

Reagents and conditions: (a)137 1,1′-azobis(cyclohexanecarbonitrile), (Si(CH3)3)3SiH, dry toluene, 120 °C, 17 h; (b) pyridine-d5; (c) hν; (d) NBS, chlorobenzene, 120 °C, 16 h; (e) phenylboronic acid, Pd(PPh3)4, 2 M Na2CO3, toluene, EtOH, 90 °C, 8 h.

coupling leading to compound 31.5. Despite this stabilization, 31.2 shows sufficient Lewis acidity to form adducts with pyridine and its derivatives in solution (31.3). The B−N Lewis adducts exhibit unprecedented photodissociation behavior in the excited state, reminiscent of the photogeneration of carbenium ions from triarylmethane leuco dyes. Consequently, these B−N Lewis adducts exhibit dual fluorescence emission arising from the initial tetracoordinate B−N adducts and the photodissociated tricoordinate boranes. 3.1.2. Diheteraperylenoids. 1,12-Diazaperylene 32.2a was obtained in 2001 by Müllen, Mews, and co-workers in a potassium/air-induced anionic radical coupling of 1,1′bisisoquinoline 32.1a (Scheme 32).132 Interestingly, the 1,12diazaperylene framework could also be formed in 32.4, by subjecting the mixed ruthenium complex 32.3 to catalytic dehydrogenation, as shown in 2002 by Tor and Glazer.138 32.2a was then used to investigate heterosupramolecular structures consisting of the diazaperylene chromophore and CdSe or CdSe/ZnS core/shell nanocrystals. The observed binding ratio was between 3 and 20 dye molecules per nanocrystal. Nanocrystal fluorescence quenching was observed upon dye complexation, even for core/shell particles containing just a few monolayers of the high bandgap ZnS semiconductor. 32.2a, typically denoted “dap” in coordination chemistry work, was subsequently explored as a highly π-conjugated ligand for transition metals. Complexes of the general structure [M(dap)3]n+ were prepared for a range of metal ions, including RuII,139 OsII,139 NiII,140 and FeII.140 Heteroleptic complexes could also be prepared for RuII with the bpy ligand, and the complex [Ru(bpy)2(dap)]2+ was found to photocleave plasmid DNA.139 For sterically encumbered dap derivatives (32.2b−d, R = Me, Et, i-Pr), pseudotetrahedral [Cu(dap)2]+ complexes were reported by Holdt et al. in 2009,141 and their stability was subsequently explored in the gas phase using ESI mass spectrometry.142 Polynuclear CuI and AgI complexes containing dap and bridging phosphane ligands, exhibiting extended πstacking in the solid state, were synthesized by Scheer, Lescop, et al.143

Scheme 30. Vinylogous NMI and PDI Derivativesa

Reagents and conditions:136 (a) Zn(OAc)2, base, 140−180 °C; (b) AcOH, Ac2O, malononitrile, reflux.

a

In 2014, Yamaguchi and co-workers described the synthesis of 7b-borafluorantheno[1,10,9-abcd]perylene 31.2, a planarized trinaphthylborane with partially fused structure (Scheme 31).137 The fused framework of 31.2 was prepared in one step from tris(8-bromo-1-naphthyl)borane 31.1, which was treated with (Me3Si)3SiH in the presence of 1,1′-azobis(cyclohexanecarbonitrile) (ABCN) as a radical initiator. This reaction resulted in the formation of 31.2, isolated as a red solid in 26% yield. The fused framework of 31.2, which has a connectivity consistent with a rearrangement of one of the naphthyl groups, apparently confers high chemical and thermal stability on the system. For instance, the boron moiety in 31.2 remained intact during further functionalization reactions, for example, bromination and further Suzuki−Miyaura cross3507


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Scheme 32. 1,12- and 3,9-Diazaperylenoids and Related Systemsa

Reagents and conditions: (a)132,141 (1) 1,2-DME, potassium, rt, 12 h, (2) air; (b)138 Pd/C, ethylene glycol−acetone 10:1, 1 h, 175−190 °C; (c)144 70% H2SO4, reflux, 30 min; (d)145 (1) UV irradiation, benzene, 2 h, 10 °C, (2) UV irradiation, DCM, 1 h, rt, ca. 10% (two steps). a

Scheme 33. 2,8-Diazaperylenoids and Related Systemsa

Reagents and conditions: (a)157 Na2S, DMF, reflux, 3 h; (b) 2-methoxyethanol, KOH, 125 °C, 4 h; (c)158 (1) phenol, KOH, 230 °C, 30 min, (2) H2O, air, overnight; (d) (1) H2SO4, H2O, Na2Cr2O7, (2) NaOH, NaHSO3, (3) aerial oxidation.

a

The purple-red bisacridine 32.6, containing the 3,9diazaperylene motif, was prepared in 1980 by cyclodehydration of the diaminoanthraquinone derivative 32.5.144 This strategy, first employed in 1945 by Cook and Waddington,146 was later used by Gibson and co-workers,144 and more recently by Bock et al., who additionally prepared the extended tetraaza derivative 32.7.147 Related work by Tokita et al. showed that,

when a tertiary diamine analogous to 32.5 was subjected to cyclization in AlCl3−NaCl, the nonquinoidal product 32.8 was obtained (for related pyrenoid systems, see Scheme 89, section 4.2).148 To date, the only known fully conjugated nonfused 3,9diazaperylene is 32.10, first reported in 1983 by Tatke and Seshadri.149,150 However, a range of bis-lactams of the general structure 32.9 has been reported by several research 3508


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Scheme 34. Synthesis of Periazapyrenoidsa

Reagents and conditions: (a)159 DCM, 44 h, rt; (b) Br2, pyridine, DCM, 4 days, 8 °C; (c) 34.2, DCM, 4 days, rt; (d) hν (solidex glass, mercury medium-pressure lamp), DCM, 30 min, rt; (e) hν (solidex glass, mercury medium-pressure lamp), toluene, 3.5 h, rt.

a

groups151−155 (for a structurally related triaza system, see ref 156). π-Expanded terpyridine 32.12 was obtained in 2015 by Wang et al. in a photochemically induced oxidative cyclodehydrogenation of the nonfused precursor 32.11.145 The triple annulation occurred in a stepwise manner, starting in the vicinity of the ortho-phenylene linker, and all intermediates were isolated. 32.12 was fluorescent (λmaxem = 533 nm) and formed heteroleptic RuII complexes containing an additional nonfused terpyridine ligand. Diazaisoviolanthrone dyes, such as the 33.3 derivative originally described in 1972 by Boffa et al.,157 are the key examples of 2,8-diazaperylene systems (Scheme 33). 33.3 was obtained from the azabenzanthrone derivative 33.1, which was coupled into the thioether derivative 33.2 and rearranged into the product under basic conditions. Acid- or base-induced binary condensations of 7H-dibenzo[de,h]quinolin-7-one 33.5, investigated in 1984 by Iwashima, Ueda, et al., were found to produce mixtures of diazaperylenoids 33.6−10, which were separated chromatographically.158 The structures of these complex systems were elucidated from their oxidation products, UV−vis, IR, and mass spectra. Compounds 33.8−10 were converted into violanthrone derivatives 33.11−13 upon treatment with sodium dichromate in concentrated sulfuric acid. These violanthrones could be reduced back to the starting materials in the presence of zinc chloride and zinc dust. 34.5, a 2,3-azaperylene-containing system, was reported in 1988 by Polansky and co-workers (Scheme 34).159 The cycloaddition reaction between dibenzo[f,pqr]tetraphene 34.1 and 4-phenyl-1,2,4-triazoline-3,5-dione 34.2 produced a mixture of products, containing a small amount of 34.5. The noncyclic product 34.3 could be converted into 34.5, for example, by treatment with dibromine. UV irradiation of the original reactant mixture or 34.5 itself yielded the π-expanded diazepinone 34.6. The synthesis of 3,10-diaza-3,10-dihydroperylene 35.2 and related systems was achieved by Maeda and co-workers in the photochemically induced cyclization of the lucigenin-derived 9,9′-biacridinylidene 35.1.160 This method was later applied to the synthesis of other derivatives.161 A different approach was employed in 2007 by Fukai and Kimura, who observed that the reductive coupling of acridinone 35.3 yielded directly the fused product 35.4.162 35.4 showed red fluorescence, with an emission maximum at 609 nm and a quantum yield of 0.23. The prototypical 1,7-diazaperylenoid, 36.2, was first reported in 1951 by Braude and Fawcett, who performed a reductive

Scheme 35. 3,10-Diazaperylenoidsa

Reagents and conditions: (a)160 hν, benzene, 30 °C, 17 h; (b)162 Zn dust, TiCl4, dioxane, reflux, 48 h. a

double ring closure in the dinitro precursor 36.1 (Scheme 36).163 An alternative route to 36.2, involving a double ring contraction of the bisthiazepine 36.3, was proposed in 1958 by Galt et al.164 In 1998, Kobayashi et al.165,166 synthesized a related system, 36.6, as a secondary photoproduct in the photochemical cyclization of the diimine 36.4. The mechanism of the cyclization was shown to involve an excited-state intramolecular proton transfer (ESIPT) step. A π-extended 1,7-diazaperylenoid, 36.8, was obtained in 1988 by Kitahara and Nishi in a thermally induced selfcondensation of the diester 36.7.167 The initial structural uncertainty was resolved in 1996 by an X-ray structural analysis, which confirmed the double enol structure of 36.8.168 In films obtained by vapor deposition, the pigment exhibited a violet color, which changed to reddish-purple upon exposure to certain organic solvents, such as acetone. A 1997 study by Mizuguchi revealed the presence of two crystalline phases, corresponding to the two colors.169 The color difference was found to originate from changes in relative intensities of vibrational−electronic transitions, caused by variations of the molecular environment. In 2011, Gisslén and Scholz reported a theoretical investigation of the optical properties of 36.8 based on an exciton model.170 The study revealed that the charge transfer between stack neighbors increases the second moment of the optical response. This behavior resulted in a red shift of the neutral excitation energy. 3509


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Scheme 36. 1,7-Diaza- and 3a,9a-Diazoniaperylenesa

a

Reagents and conditions: (a)163 Na2S2O4, MeOH, 5% NaOH, reflux, 1 h; (b)164 copper bronze, diethyl phthalate, reflux, 5 min; (c)165 photochemical; (d)167 1-chloronaphthalene, reflux, 5 min; (e)171 NH3 (20%), Zn powder, rt, 2 h; (f) 2-chloropyrimidine, K2CO3, DMF, reflux, 2 h; (g) Raney Ni, cyclohexene, DMF, 110 °C, 1 h; (h)172 urea, DMF, reflux, 2 h; (i)173 reflux, Ar, 48 h; (j)174 HgO, AcOH, reflux, 48 h.

reported by Clar et al.55 (6.2−3, section 2.1). A conceptually related approach was used by Gryko et al. to develop “πexpanded” coumarins 37.5a−e as precursors to π-expanded pentacenes (see also Scheme 6, section 2.1).67 A synthetic route complementary to 37.2c, involving an acid-catalyzed cyclization of a disulfoxide 37.6, followed by demethylation, was reported in 2010 by Liu et al.186 The red dixanthene 37.2a is a photochromic compound, easily undergoing photooxidation to the colorless endoperoxide 37.3a.177 Heating the latter species above 120 °C regenerates the starting 37.2a quantitatively. Photocycloreversion of the 37.3a and 37.3c endoperoxides was investigated by Brauer and Schmidt187 and by Jesse and Comes.188 37.2c was employed as a red emitter in OLED devices, yielding current efficiencies of up to 4.4 cd A−1.186 In 2010, Nishihara et al. reported that the treatment of a diacetylene anthraquinone derivative 37.7 with the strong protic acid TFSIH results in the formation of a double pyrylium salt 37.8, containing the 1,7-dioxaperylene substructure.189 The protonation and rearrangement of 37.7 occurred in a stepwise manner, producing an unusual optical spectrum containing a broad NIR absorption (up to 2200 nm), described as an intervalence charge-transfer band. A biradicaloid contribution, corresponding to the bis(ammoniumyl) valence structure 37.8′, was determined on the basis of ESR and SQUID measurements. In cyclic voltammetry, 37.8 underwent two reversible one-electron reductions (−0.06 and −0.16 V vs Fc/Fc+), and could be chemically reduced to the highly unstable neutral species 37.9, which was characterized in the solid state.

The parent 1,7-diazaperylene, 36.12, was reported in 1990 by Naumann and Langhals.171 Their synthesis involved an ammonia-mediated double cyclization of the anthraquinone diacetate 36.9. The resulting dihydroxy intermediate 36.10 was dehydroxylated in two steps to yield the unsubstituted product 36.12. A similar condensation approach was employed in 1992 by Gorelik and co-workers, who reported several substituted derivatives of 36.12.175 A different strategy was employed in 2014 by Baranov et al., who cyclized ethynyl-substituted anthraquinones 36.13a−b in the presence of urea, obtaining diazaperylenes 36.14a−b in moderate yields.172 A uniquely short synthesis of the 3a,9a-diazaperylenium dication 36.16 was reported in 2002 by Sotiriou-Leventis, Leventis, et al.174 The hydrogenated p-phenylenediamine precursor 36.15, which is extremely easily oxidizable, was first obtained by a one-step quadruple cyclization of p-phenylenediamine with 1-bromo-3-chloropropane.173 Dehydrogenation of 36.15 using mercury(II) oxide led to the formation of the dication 36.16 and the monocation 36.17 in an approximately 1:3 molar ratio. 36.16 is a rare instance of a perylenoid internally doped with nitrogen (an earlier example of a 6b,12bdiaza system 33.14 was reported in 1961 by Ziegler et al.176). Benzo[1,2,3-kl:4,5,6-k′l′]dixanthene 37.2a177 and its analogues containing varying patterns of heteroatoms (37.2b−e) and substituents,148,177−184 as well as additional fused rings,185 were prepared from disubstituted anthraquinones 37.1a−e, following the strategy used for the synthesis of 32.6 (Scheme 37, for related earlier work, see ref 146 and references therein). Related systems containing additional fused benzene rings were 3510


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Scheme 37. O- and S-Containing 3,9- and 1,7-Diheteraperylenoidsa

Reagents and conditions: (a)148,177 AlCl3−NaCl, hydroquinone 150−180 °C, ∼40%; (b) hν, O2; (c) hν or heat; (d)67 K2CO3, DMSO, 100 °C; (e)186 (1) TFA, (2) pyridine, no yield given; (f)189 TFSIH, DCM, rt, 3 min; (g) Li[TCNQ], MeCN/DCM, rt, 2 h.

a

Scheme 38. O- and S-Containing 3,10-Diheteraperylenoidsa

Reagents and conditions: (a)190 ZnCl2−NaCl, rt to 300 °C, 20 + 3 min; (b)191 benzene, N2 atm, Hg lamp, 64 h; (c)194 photolysis; (d)196 ultraviolet irradiation, λ = 390 nm; (e)195 CHCl3, air, Xe lamp. a

Compound 38.2, an early example of an extended 3,10dioxaperylenoid, was reported in 1964 by Lang and Zander (Scheme 38).190 This ring system was obtained in one step from the chrysene derivative 38.1, which was subjected to a high-temperature acid-catalyzed condensation. In 1965, Schönberg and Junghans observed that photocyclizations of

bisxanthylenes 38.3a−c produced good yields of the respective benzodixanthenes (“helixanthenes”) 38.4a−c.191 The photophysics of this reaction were subsequently investigated.192,193 38.4c was also found to form in the photolysis of the bisxanthogenate 38.5.194 The regioselectivity of bisxanthylene photocyclization was explored in 2011 by Song and co-workers 3511


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Scheme 39. Synthesis of a Doubly Boron-Bridged Dibenzo[g,p]chrysenea

Reagents and conditions: (a)198 (1) n-BuLi, THF, −78 °C to rt, (2) 39.2, THF, −78 °C to rt, (3) rt, 16 h; (b) UV irradiation (Hg mediumpressure lamp; I2, propylene oxide), toluene, rt, 3 h; (c) (1) excess neat BBr3, 4 days, 200 °C, (2) MesMgBr, toluene/THF, 0 °C, rt, 16 h. a

Scheme 40. Synthesis of a P-Fused Double Helicenea

Reagents and conditions: (a)199 (1) 1-(2-methylnaphthyl) magnesium bromide, THF, 0 °C, (2) rt, 1 h, (3) 80 °C, 14 h, (40 I2, 0 °C, (5) rt, 30 min; (b) (1) n-BuLi, THF, Ar atm, −78 °C, 1 h, (2) 0 °C, 1 h, (3) bis(diethylamino)chlorophosphine, −78 °C, (4) rt, 1 h, (5) S8, chlorobenzene, rt, 8 h, (6) AlCl3, o-dichlorobenzene, 0 °C, (7) rt, 8 h, (8) DABCO, 150 °C, 12 h; (c) (1) m-chloroperoxybenzoic acid, DCM, 0 °C, (2) rt, 14 h, (3) mchloroperoxybenzoic acid, DCM, 0 °C, (4) rt, 10 h; (d) Et3P, o-dichlorobenzene, N2 atm, 60 °C, 12 h.

a

for a range of substituted derivatives 38.8.195 Only two regioisomers, 38.9 and 38.10, were observed among the reaction products, and 38.9 was formed exclusively for electronwithdrawing R groups, such as CN or COMe. The conversion of bisxanthylenes into helixanthenes is associated with a significant conformational change of the nonplanar aromatic core. Bisxanthylenes typically exhibit a thermally controlled equilibrium between an anti-folded and a twisted conformer, which gives rise to thermochromism. In contrast, helixanthenes are characterized by helicene-type geometry. The structural change caused by the coupling was employed in the design of a photoactive amphiphile 38.6, reported in 2011 by Feringa et al.196 The latter species selfassembles into bilayers, with interdigitating hydrophobic chains of the opposing amphiphiles. In the presence of phospholipids, the bilayer forms vesicle-capped nanotubes. Upon irradiation with ultraviolet light, which induces the photoconversion 38.6 into 38.7, these nanotubes are disassembled at a rate that is dependent on the irradiation wavelength and light intensity. A substituted helixanthene derivative, 38.11, synthesized in 2014 by Franceschin, Altieri, et al., revealed its potential for stabilizing telomeric G-quadruplex DNA.197 The improved selectivity of 38.11 was demonstrated using ESI−MS data, in vitro cancer screening, and specific immunofluorescence assays. A peripherally bis-B-doped bisanthene 39.5 was prepared in 2015 by Wagner and co-workers (Scheme 39).198 This species, containing the 3,10-diboraperylene substructure, can alternatively be described as a doubly boron-bridged dibenzo[g,p]-

chrysene. 39.5 was prepared in a modular synthesis sequence based on a Peterson olefination, followed by a stilbene-type photocyclization, and a Si−B exchange reaction. Unlike the parent bisanthene, which is a NIR chromophore with the absorption onset above 700 nm, 39.5 is an efficient blue luminophore (λem = 449, 475 nm, ΦF = 0.78). Additionally, the system was found to undergo reversible redox transitions both in the cathodic and in the anodic regimes. The silyl derivative 39.1 used in the synthesis of 39.5 is a versatile starting material, and was further employed for the preparation of smaller boroncontaining PAHs, 39.6−7.198 A doubly P-fused double helicene 40.3, structurally related to the monophospha species 29.13 (vide supra), was obtained in 2014 by Nakamura, Hatakeyama, et al. (Scheme 40).199 The key precursor, 2,3,5,6-tetraaryl-1,4-diiodobenzene 40.2, obtained by sequential arylation−iodination of hexabromobenzene, was transformed into a bisphosphine, sulfurized, and subjected to a tandem phospha-Friedel−Crafts reaction, providing 40.3 in a 10% overall yield. This compound easily underwent desulfurization or oxidation, leading to compounds 40.5 and 40.4, respectively. 40.3 was characterized in the solid state, showing a syn arrangement of the PS bonds and a highly distorted inner benzene ring (bending angle of 23°). Despite this distortion, which leads to an apparent reduction of aromaticity, the double helicene showed good thermal and chemical stability. 3.1.3. Tetraheteraperylenes. The synthesis of a 1,2,7,8tetraazaperylene derivative 41.4a was reported in 1932 by 3512


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Scholl et al.200 The diaroyl precursor 41.2a was synthesized by reacting the bis(acyl chloride) 41.1 with m-xylene under Friedel−Crafts conditions. The reaction was however not general, because of the competing formation of the dilactone 41.5a, which was typically the dominant product for other arene reactants. 41.4a was then obtained from 41.2a by reaction with hydrazine hydrate. An even larger product, 41.5, was reported by the same group in subsequent work,201 while a diaza system, 33.15 (Scheme 33), was similarly synthesized in 1980 by Gomes and Cabares.202 Pummerer and co-workers reported the formation of 41.6, a π-expanded 2,3,10,11tetraazaperylenoid, in a reaction of the corresponding hexaketone with hydrazine hydrate.203 In 2014, this synthetic strategy was rediscovered by Zhang, Feng, et al., who developed a more general and efficient route to the diaroyl precursors, for example, 41.2b−c (Scheme 41).204 It was found that the introduction of heteroatoms

Scheme 42. 7,8,15,16-Tetraazaterrylenea

Reagents and conditions: (a)205 lauric aldehyde, neat, 180 °C, 1 h; (b) TFA, reflux, overnight; (c) hydrazine, HCl (cat.), BuOH, reflux, 3 days. a

donor/acceptor interface by Barnes et al.208 In that application, they revealed high variability of emission properties, which was proven to result from packing defects. TAT crystals exceeding ca. 200 nm in width were observed to act as photoluminescence waveguides and optical microcavity resonators. Several tetraheteraperylene systems have been obtained by various dimerization reactions of diheteraacene precursors. Unexpected formation of pyridazino[4,3,2-kl:5,6,1-k′l′]diphenothiazine 43.2 was observed by Müller and co-workers in an attempted Buchwald−Hartwig arylation of phenothiazine (Scheme 43).209 A Ni0-mediated coupling of the substituted phenothiazine 43.1b was used by Yamamoto and Higashibayashi in the synthesis of the cyclodimer product 43.3b.210 The dimer was nonplanar, forming a congested, slit-like cavity, and could be oxidized to produce the butterfly shaped dimer 43.4b containing an inner N−N bond. An analogous reaction sequence was performed on the substituted acridone 43.1c, yielding the diazaperylene product 43.4c. In 2015, Sakamaki, Seki, et al. reported the synthesis of “N-heterohelicenes” 43.7a−c, each containing a tetraheteraperylene core.211 These systems were obtained from π-expanded phenazine and phenoxazine derivatives 43.5a−c, which were initially oxidized to cruciform dimers 43.6a−c, which were then cyclized to 43.7a−c using a stronger oxidant system. The double helicenes showed highly nonplanar structures in the solid state, and were found to be very stable toward racemization. The 1,6,7,12-tetraazaperylenoid eilatin (44.5), a bright yellow alkaloid first described by Kashman et al. in 1988, is a remarkable example of a natural product containing an extended benzenoid fusion pattern.212,213 The compound was isolated in minute amounts (up to 0.001% of dry weight) from the Red Sea tunicate Eudistoma sp. Eilatin, and its analogues or complexes have been intensely studied for their potent growth regulatory properties and ability to affect cell shape and adhesion,214 their anti-HIV activity and RNA binding,215,216 as well as antimalarial and antitrypanosomal potential.217,218 The first total synthesis of eilatin was published in 1993 by Kubo and co-workers,219 who compared two synthetic pathways, each starting from a 4-phenylquinoline-5,8-dione derivative bearing, respectively, a nitro or trifluoroacetylamino substituent (44.1 or 44.6, Scheme 44). Compound 44.1 was converted regioselectively into 6-(2-acetylanilino)-4-(2-nitrophenyl)quinoline5,8-dione (44.2). In the next two steps, tetracyclic (44.3) and pentacyclic (44.4) intermediates were obtained. In the last step, catalytic hydrogenation of the NO2 group followed by cyclization afforded eilatin. The second pathway started with

Scheme 41. 1,2,7,8-Tetraazaperylenes and Related Systemsa

a Reagents and conditions: (a)200 m-xylene, FeCl3, reflux; (b)204 CrO3, AcOH/water, 75 °C, 1.5 h; (c)200 hydrazine hydrate, toluene, reflux; (d)204 hydrazine hydrate, EtOH, 78 °C, 2 days.

results in a pronounced blue shift of the electronic spectra of the alkyl substituted tetraazaperylenes (such as 41.4b) in comparison with perylene, but this effect could be offset by means of aryl substitution (e.g., in 41.4c), to produce an overall red shift. All tetraazaperylene derivatives showed very weak fluorescence, typical of N-doped acenes, in contrast to the undoped perylene. 7,8,15,16-Tetraazaterrylene (TAT, 42.3, Scheme 42) was synthesized in 2012 by Wudl et al.205 Their three-step synthesis began with a thermally induced condensation of phenalene-1,3dione with lauric aldehyde. The resulting product, 42.1, was subjected to an acid-catalyzed dehydrative cyclization, to yield the xanthenedione system 42.2. The latter intermediate was condensed with excess hydrazine, undergoing an extrusion of the internal methylene bridge. In subsequent work, the electronic structure of TAT derivatives was investigated theoretically by Wang et al.206 The aggregation behavior of 42.3 was explored in solution and in the solid state by Spano et al.,207 showing a unique “HJ-aggregate” behavior. TAT single crystals coated with pentacene were used as a photovoltaic 3513


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showed that eilatin forms octahedral complexes with osmium, capable of dimerization via π−π stacking, and that the nonequivalence of binding sites allows the selective synthesis of both mono- and binuclear complexes.224,225 Investigations of mononuclear complexes of isoeilatin226−228 and dibenzoeilatin229,230 also revealed the formation of discrete dimers in solution and in the solid state, which were stabilized by weak π−π stacking interactions between ligands. In 2002, Tor and Glazer showed that ruthenium complexes of 1,6,7,12-tetraazaperylene and its two benzologues, including eilatin (44.19−44.21), could be prepared using the dehydrogenative approach described above for 32.4 (Scheme 32).138 The free 1,6,7,12-tetraazaperylene 44.18 was synthesized by Holdt et al. in 2012 by anionic radical coupling of 1,1′-bis-2,7naphthyridine (44.17).231 Mono- and binuclear complexes of 44.18 with ruthenium(II) were then obtained under mild reaction conditions. The syntheses of 44.18−44.21 represent two oxidative approaches available for the construction of heteraperylene cores, that is, anionic radical coupling and catalytic dehydrogenation, both of which involve intramolecular ring closures. A rare example of an intermolecular oxidative coupling was provided in 2007 by Cushman et al.,232 who showed that aerial oxidation of 2,7-naphthyridine-1,3,6,8-tetraol 44.22, available from a simple one-step synthesis,233 leads to the formation of the highly oxygenated 2,5,8,11-tetraazaperylenoid 44.23. The latter species was found to be a competitive inhibitor of lumazine synthase, significantly more potent than the monomeric 44.22. Recent research on BN-doped graphenoids provided two examples of tetraheteraperylenoid structures (for related pyrenoid systems, see Scheme 95, section 4.2). 5b,7b-Diaza3b,9b-diborabenzo[ghi]perylenes 45.4b−c with a “doubly buttressed” B2N2C2 core were obtained in 2010 by Piers and co-workers (Scheme 45).234 The assembly of the perylenoid core was accomplished by condensing a chelating Lewis acid, 2,2′-diborabiphenyl 45.2, with 2,6-bisalkynyl-substituted pyridazines 45.1a−c. This step, which led to the simultaneous formation of two B−N bonds and one C−C bond, yielding 45.3a−c, was followed by a Pt-catalyzed C−C bond closure, which however was only effective in the presence of terminal alkyne substitution. As part of their effort to make a borazineembedded HBC analogue (cf., Scheme 17, section 2.3), Bettinger and co-workers reported in 2014 the photochemical cyclization of 1,2:3,4:5,6-tris(o,o′-biphenylylene)borazine 45.5, which yielded the singly coupled product 45.6.235 45.6 underwent solvolysis with methanol, producing the B2N2 product 45.7, which was shown to have an essentially planar structure in an X-ray crystallographic analysis.

Scheme 43. Tetraheteraperylenoids via Dimerization of Diheteraacenesa

Reagents and conditions: (a)209 2-bromomesitylene, Pd2(dba)3·dba, P(t-Bu)3·HBF4, t-BuOK, THF, MW, 150 °C, 15 min; (b)210 Ni(cod)2, bpy, cod, THF, 50−80 °C; (c)210 (1) DDQ, (2) hydrazine; (d)210 tBuOK, rt, 4 h; (e)211 DDQ, DCM; (f)211 DDQ, Sc(OTf)3, DCM, 19% for all derivatives; (g)211 (1) DDQ, DCM, (2) DDQ, Sc(OTf)3, DCM, 14% for 43.7a, 10% for 43.7c. a

the synthesis of pyridoacridinone 44.7, which was cyclized to 44.8, and subsequently converted to eilatin by a one-pot annulation of ring G. Kashman et al. proposed a synthesis of eilatin using a biomimetic two-step procedure.220 Starting from catechol and protected kynuramine under oxidative conditions, 1,2-acridinedione derivative 44.9 was obtained in the first step. Next, upon treatment with boron trifluoride diethyl etherate, 44.9 was converted into 44.10. Eilatin was prepared from 44.10 or directly from 44.9 using basic conditions. In 2015, Plodek and Bracher proposed a synthesis of eilatin starting from the readily available 9-methyl-3,4-dihydroacridin1(2H)-one 44.11.221 This compound was converted into pyridoacrid-4-one (44.13), the key intermediate, which represents the common substructure of many pyridoacridine alkaloids. Starting from 44.11, it was also possible to prepare pyridoacrid-6-one isomeric to 44.13, which was used as a precursor of isoeilatin 44.14. Earlier, Kashman et al. proposed a synthesis of isoeilatin starting from an 1,4-acridinedione isomeric with the 1,2-dione 44.9.222 In the same work, dibenzoeilatin 44.16 was obtained by acidic treatment of compound 44.15. Eilatin and its analogues containing the 1,6,7,12-tetraazaperylene core can be viewed as potential bifacial ligands. In 1997, Kashman, Kol, et al. showed that 44.5 prefers to bind metals to its less hindered face, by preparing two ruthenium complexes.223 Further research carried out by Kol and co-workers

3.2. [ghi]Heteroannulated Perylenoids: 5-Membered Rings

Heteroannulation of one or two bay regions of the perylene core has been highly investigated, with synthetic efforts focusing mostly on perylene itself and on its mono- and diimides. These investigations are rationalized by the effect of such annulation on the photophysical properties of these materials (for a theoretical analysis, see a 2008 paper by Zhao and Han236). A practical strategy for the preparation of [ghi]annulated perylenes 46.2−4 in good yields was described by Wang et al. in 2008 (Scheme 46).237 The strategy was based on earlier work by Langhals and co-workers, who synthesized a range of variously substituted pyrrolo- and thieno-annulated PMI and PDI derivatives 46.7−10.238,239 The crucial step of Wang’s synthetic pathway was the efficient preparation of 13514


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Scheme 44. Synthesis of Eilatin and Related Systemsa

Reagents and conditions: (a)219,221 CeCl3·7H2O, 2-aminoacetophenone, EtOH, 3 h, rt; (b)220 10% H2SO4 in AcOH, 15 min, 60 °C; (c) (1) DMF diethyl acetal, DMF, 30 min, 120 °C, (2) NH4Cl, AcOH, 30 min, 115 °C; (d) H2 (1 atm), 10% Pd/C, 1.5 h; (e)220 BF3·Et2O, DCM, 24 h, rt; (f) NH3, MeOH; (g) NH3, MeOH, 4-dimethylaminopyridine (cat.); (h)221 (1) DMF diethyl acetal, DMF, 30 min, 55 °C, (2) NH2OH·HCl, AcOH, 1 h, 100 °C; (i) (1) acetic anhydride, 2 h, 50 °C, (2) 10% NaOH in H2O/MeOH, 2 h, 80 °C, (3) MnO2, toluene, reflux; (j) 10% H2SO4 in AcOH, 20 min, 90 °C; (k) (1) DMF diethyl acetal, DMF, 30 min, 120 °C, (2) NH4Cl, AcOH, 1 h, 120 °C; (l)222 AcOH:H2SO4:trifluoroacetic acid (45:10:45), 40 min, 60 °C; (m)231 (1) potassium, dimethoxyethane, N2 atm, 12 h, reflux, (2) air, THF, 6 h; (n)232 (1) EtOH, diethylamine, rt, 48 h, (2) H2SO4 (70%), 100 °C, 10 min; (o) DMSO, air, rt, 1 month. a

Zhu and co-workers studied semiconducting properties of the S-annulated perylene 46.2 (Scheme 46) as a potential organic field-effect transistor (OFET).243,244 The X-ray analysis of a 46.2 single crystal revealed stacking along the b axis with intermolecular distances of 3.47 Å, with short distances (3.51 Å) between sulfur atoms of neighboring columns related by an inversion center. This material exhibited a high mobility of up to 0.8 cm2 V−1 s−1 measured for transistors based on an individual single-crystal wire and 0.05 cm2 V−1 s−1 for spincoated thin films.243 On highly oriented polyethylene thin films, the S-annulated perylene 46.2 was processed into lathlike crystals using vapor phase epitaxy.244 Wang, Hu, and coworkers described single-crystalline microribbons of the Seannulated perylene 46.3, which were prepared by drop casting and physical vapor transport techniques.245 Single-crystal transistors, fabricated by the “gold layer glue” technique, showed mobilities approaching 2.66 cm2 V−1 s−1. Crystals of

nitroperylene 46.1, which formed alongside the 3-nitro isomer. Chalcogen-annulated perylenes 46.2 and 46.3 were then obtained by heating 46.1 with elemental sulfur and selenium, respectively. Perylenothiophene 46.2 had been prepared earlier by several groups, using harsh reaction conditions.240−242 An extension of the above synthesis, which included the benzannulation of the other bay region in 46.1, was used to produce the benzo derivative 46.6. Phenanthro[1,10,9,8cdefg]carbazole 46.4 was synthesized by Wang et al. using the Cadogan reaction with triethyl phosphite.237 When performed in the perylene bay region, the Cadogan reaction is likely associated with an increase in internal strain. Consequently, an attempted bis-annulation of the dinitro derivative 46.11 produced only the singly annulated product 46.12.238 In the solid state, perylenes 46.2−4 adopt planar conformations, but their crystal packing shows marked dependence on the identity of the annulated heteroatom. 3515


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Scheme 46. Synthesis of [ghi]Heteroannulated Perylenoidsa

Scheme 45. Synthesis of BN-Containing Tetraheteraperylenesa

a Reagents and conditions: (a)234 toluene, 25 °C, 48 h; (b)234 PtCl2, 100 °C, 24 h; (c)235 I2, hν, THF, dry toluene, 16 h; (d)235 DCM, MeOH, 70 h.

46.3 revealed a near planar conformation with regular π stacking. The observed Se···Se contacts of 3.49 Å were considered responsible for the observed efficient carrier transport. A different route to the benzo[6,7]peryleno[1,12-bcd]thiophene ring system 47.3 was developed in 1975 by Wynberg et al. in the course of their work on dehydrohelicenes (Scheme 47, for related circulene derivatives, see Scheme 157).246,247 The helicene 47.1 underwent a ring closure to yield 47.2, when heated at 140 °C in the NaCl−AlCl3 melt. Compound 47.2 was then converted into 47.3 via the Diels−Alder addition of maleic anhydride followed by anhydride cleavage. 47.3 was oxidized to sulfone 47.4 with m-chloroperoxybenzoic acid in methylene chloride. Phenanthrocarbazole 46.4, typically called “N-annulated perylene” (NAP), is a valuable heteroaromatic building block for materials chemistry applications because of its favorable electron-donating properties and the availability of a functionalizable nitrogen. The latter site is typically alkylated in the presence of a base, although a Ru-catalyzed dehydrogenative N-arylation was recently reported (Scheme 48).248 46.4 is readily brominated at positions 3 and 10, opening a convenient path for further functionalization. Wu et al. synthesized several N-annulated derivatives 49.1a− d (Scheme 49) possessing variable electron-deficient subunits, for use as acceptor−donor−acceptor dyes.249 These systems showed tunable absorptions in the visible range and efficient emission that extended into the NIR region. Wang and coworkers reported a series of diarylamino-functionalized Nannulated perylenes containing different electron-acceptor segments: cyanoacrylic acid (49.2a), benzothiadiazole−benzoic acid (49.2b), and pyridothiadiazole−benzoic acid (49.2c).250,251 Dye-sensitized solar cells employing 49.2a−c showed power conversion efficiencies (PCEs) of 5.0−8.8% at air mass 1.5 global (AM1.5G) conditions. Further increase in PCE values was achieved by introducing an alkoxyphenyl group in the donor part of the dye. In particular, the ethynylbenzothiadiazole-benzoic acid derivative 49.3b yielded a PCE of

a

Reagents and conditions: (a)237 fuming HNO3/H2SO4, 1,4-dioxane, 60 °C, 25−30 min; (b) sulfur powder, NMP, 180 °C, 5 h; (c) Se powder, NMP, 180 °C, 5 h; (d) triethyl phosphite, reflux, 2 h; (e) (1) diethyl maleate, 180 °C, 5 h, (2) chloranil, 180 °C, 24 h; (f) sulfur powder, NMP, 180 °C, 5 h; (g)238 triethyl phosphite, reflux, 1 h 40 min.

Scheme 47. Benzo[6,7]peryleno[1,12-bcd]thiophenea

Reagents and conditions: (a)246 NaCl, AlCl3, 15 min, 140 °C; (b)247 (1) maleic anhydride, chloranil, 2 h, reflux, (2) soda lime, N2 atm, 1 h, 370 °C; (c) m-chloroperoxybenzoic acid, DCM, 5 h. a

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tion, and enabled the successful preparation of the diimide derivative 50.7.255 A range of bis-N-annulated quaterrylenes were subsequently prepared using the above protocol,256−258 some of them showing self-assembly behavior as supramolecular nanotapes257 and columnar liquid crystals.256 BisN-annulated quaterrylenes are strongly absorbing, fluorescent dyes, with emission maxima in the 700−800 nm range. In 2014, Wu et al. reported the synthesis of an s-indacenebridged quinoidal perylene dimer 50.6 (Scheme 50).258 Compound 50.6 was obtained form diol 50.4, which was cyclized to the 50.5 intermediate in the presence of boron trifluoride and subsequently dehydrogenated with DDQ. Compound 50.6 exists as a closed-shell quinoid with an intramolecular charge-transfer character. It showed shorter singlet excited-state lifetimes, stronger two-photon-absorption, and a smaller energy gap than a corresponding bis-N-annulated quaterrylene derivative. The synthetic paradigm used to assemble quaterrylene 50.3 was refined in 2010 by Wang et al., to enable the synthesis of tri-N-annulated hexarylenes 50.8a−c.259 The terperylene precursors were assembled using Suzuki cross-coupling and subsequently cyclodehydrogenated using Sc(OTf)3/DDQ to yield 50.8a−c. Electronic absorptions of 50.8a−c are very intense and bathochromically shifted with respect to lower rylene homologues (to ca. 846 nm). Compounds 50.8a−c exhibit remarkably large dipole moments, which are likely responsible for stabilization of π-stacked H-aggregates. The aggregation of 50.8a−c was inferred from the concentration and substitution dependence of their electronic spectra, which could be rationalized using vibronically resolved TD-DFT calculations. In 2013, an iterative synthetic scheme based on Suzuki− Miyaura cross-coupling was employed by Wu and co-workers to assemble NAP oligomers 51.1b−f in a length-selective

Scheme 48. Ru-Catalyzed N-Arylation of Phenanthro[1,10,9,8-cdefg]carbazolea

a

Reagents and conditions: (a)248 4-methyl-N-phenylaniline, [{(pcymene)RuCl2}2], Cu(OAc)2, cumene/tetrachloroethylene/AcOH, O2, 150 °C, 24 h.

10.4%.252 An even higher value (12.5%) was reported for the N-annulated indenoperylene derivative 49.6.253 The latter compound was obtained in five steps from ester 49.4. The indene unit, installed to enforce coplanarity in the donor segment, was constructed by a double Grignard addition reaction with (4-hexylphenyl)magnesium bromide, followed by the acid-catalyzed cyclization of the resulting tertiary alcohol. In 2009, Li and Wang proposed an efficient synthetic method toward a processable bis-N-annulated quaterrylene 50.3 (Scheme 50).254 50.3 was obtained in one step by reacting compound 50.1 with 5 equiv of DDQ and scandium trifuoromethanesulfonate in 23% yield. Interestingly, a significantly higher yield was achieved in a two-step variant of this synthesis, involving sequential oxidation with 1 and then 5 equiv of the DDQ/Sc(OTf)3 reagent. Under these conditions, the terminal naphthyl substituents would not undergo oxidative dehydrogenation in either 50.3 or its monoperylene analogue.254 An alternative approach, developed almost concurrently by Wu and co-workers, combined Yamamoto homocoupling with a K2CO3/ethanolamine oxidative annulaScheme 49. N-Annulated Perylene Dyesa

a

Reagents and conditions: (a)253 (1) (4-hexylphenyl)magnesium bromide, THF, reflux, overnight, (2) Amberlyst 15, toluene, reflux, overnight. 3517


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Scheme 50. N-Annulated Bis-perylene Dyesa

Reagents and conditions: (a)254 (1 equiv) DDQ, Sc(OTf)3, toluene, 25 °C, 10 min; (b) (5 equiv) DDQ, Sc(OTf)3, toluene, 50 °C, 24 h; (c)258 BF3·Et2O, dry DCM, rt, 10 min; (d) (2 equiv) DDQ, dry toluene, 85 °C, overnight. a

manner (Scheme 51).260,261 51.1b−f and their parent monomer 51.1a were then converted into the extended tetracyanoquinodimethanes 51.2a−f by the application of the Takahashi coupling reaction with malononitrile, followed by either aerial or chloranil-mediated oxidation.260 In subsequent work, this transformation was applied to the fully condensed rylenes 51.3a−b, to yield the corresponding quinodimethanes 51.4a−b.261 Both classes of quinone oligomers showed a clear chain-length dependence of their photophysical and electrochemical properties. The nonfused series exhibited tunable ground-state structures: a closed-shell singlet for 51.2a, openshell singlet for 51.2b−d, and triplet biradical for 51.2e−f.260 Analogously, the ground-state electronic structures in the fully fused series changed from a closed-shell singlet for 51.4a to an open-shell singlet biradical for 51.4b. Both of the latter compounds revealed a very strong one-photon (OPA) and twophoton (TPA) absorption response in the NIR range, due to the extended π-conjugation, as well as a singlet biradical character.261 Thiophene-extended analogues 51.5a−b were shown to possess singlet biradical ground states with small singlet−triplet energy gaps.262 Similar reaction sequences were used in 2015 by Wu et al. to prepare another series of push− pull type quinoidal perylene oligomers, 51.8a−d.263 These compounds possessed a ground-state electronic structure with a balanced contribution from closed-shell quinoidal, open-shell diradical, and closed-shell zwitterionic resonance forms. As a result, their ground states and physical properties showed a clear chain length and solvent polarity dependence. For example, it was found that with the extension of the chain

length, the diradical character greatly increased, while the contribution of the zwitterionic form diminished. Solvent polarity played a major role in controlling the ground state and physical properties of the monomer, with a smaller effect on the dimer and trimer. In 2010, Wu and co-workers reported the synthesis of two Nannulated perylene systems containing one (52.1) or two (52.2) fused porphyrin units (Scheme 52). 264 These compounds were synthesized by means of Sc(OTf)3/DDQmediated oxidative cyclizations of appropriate meso-linked porphyrin precursors. These hybrid molecules are highly soluble in common organic solvents and exhibit intense NIR absorption, showing principal Q bands at 775 and 952 nm, respectively. Importantly, both compounds revealed detectable photoluminescence, with quantum yields of 5.6% and 0.8% for 52.1 and 52.2, respectively. Subsequent work on NAP− porphyrin hybrids as materials for dye-sensitized solar cells was reported by the groups of Wang and Wu.265 Compounds 52.5a and 52.5b exhibited power conversion efficiencies of 10.3% and 10.5%, respectively, comparing favorably with the efficiency of the YD2-o-C8 porphyrin benchmark (12.3%).266 Because of inefficient π-conjugation between the segments in compound 52.3 and the low-lying LUMO energy level and nondisjointed HOMO/LUMO profiles in 52.4, these compounds exhibited significantly lower power conversion efficiencies of 5.6% and 0.3%, respectively. Using an analogous oxidative coupling strategy, the Wu group synthesized peri-fused hybrids 52.7a−b consisting of NAP and boron dipyrromethene (BODIPY) subunits.267 52.7b has its principal absorption band at 670 nm 3518


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Scheme 51. Synthesis of Tetracyano Derivatives of N-Annulated Rylenesa

a

Reagents and conditions: (a)260 (1) malononitrile, NaH, Pd(PPh3)2Cl2, reflux, 48 h, (2) 2 M HCl, (3) p-chloranil, CHCl3, rt; (b) (1) malononitrile, NaH, Pd(PPh3)2Cl2, reflux, 48 h, (2) 2 M HCl, air; (c)261 (1) malononitrile, NaH, Pd(PPh3)2Cl2, reflux, (2) 2 M HCl, 0−5 °C, air.

and shows relatively weak, solvent-dependent photoluminescence in the 700−860 nm range. Related work on N-annulated perylene−porphyrin tape hybrids is discussed in section 7.3. Yu et al. synthesized a donor−acceptor copolymer C9.1, containing N-annulated perylene blocks, with the aim of extending the π-conjugation across the polymer main chain (Chart 9).268 Solution-processed field-effect transistors based on C9.1 exhibited a high hole mobility value of up to 0.30 cm2/ (V s) and a current on/off ratio above 105. Kim and co-workers prepared NAP copolymers C9.2 and C9.3 containing differently substituted dithienylquinoxaline subunits.269 Both polymers showed similar physical properties: broad absorption in the 400−700 nm range, optical bandgaps of ∼1.8 eV, and the appropriate frontier orbital energy levels for efficient charge transfer separation at polymer/PC71BM interfaces. When used in sensitized solar cells, these materials showed PCE values of up to 3.3%. Donor-linked NAP derivatives C10.1−2 containing a “doubled” polycyclic core were investigated by the groups of Fu and Wang (Chart 10).270 In contrast to the NAP systems described above, C10.1−2 were π-extended in the bay direction. In both molecules, the donor units (triphenylamine or tetraphenylporphyrin) were installed by bay annulation of the corresponding amines under Pd-catalyzed conditions.271,272 In these systems, fluorescence quenching due to intramolecular charge transfer was observed, with the rate of charge separation reaching 1012 s−1 in DCM. A PEGylated bis-NAP system C10.3 was used by Wang, Chen, et al. as a nonporphyrin PDT photosensitizer, showing a high efficiency of singlet oxygen generation (ΦΔ = 0.66).273

Chalcogen-annulated perylenes have been less studied than their N-annulated counterparts. An early example of a sulfurcondensed perylenoid was provided by the synthesis of dibenzo[5,6:7,8]pentapheno[13,14-bcd]thiophene 53.2 (Scheme 53), first reported by Steinkopf in 1935.274 53.2, named “flavophene” after its orange-yellow color, was obtained by cyclodehydrogenation of tetraphenylthiophene 53.1, a procedure subsequently refined by Sasse and co-workers.275 Desulfurization of 53.2 with Raney nickel produced dibenzoperylene 53.3 in moderate yield.275 The mechanism of potassium-induced desulfurization of 53.2 was investigated in 1999 by Rabinovitz et al.276 Zander and Franke reported an alternative route to 53.2 involving sequential thermal desulfurization−dehydrogenation of tetrabenzothianthrene 53.4.277 They additionally showed that the thiophene ring could be reintroduced to 53.3 using direct reaction with elemental sulfur. An analogous reaction, performed with Se powder, yielded the Se-annulated 53.6.278 A low-yielding synthesis of the furan analogue 53.8, reported by Zander, employed heating the dione 53.7 with elemental Cu.279 Compound 53.9, a dienone derivative of 53.8, was proposed by Clar as an oxidation product of 53.3,280 but its structure was not supported by later work.281,282 Benzo[6,7]peryleno[1,12-bcd]thiophene 53.12 (“thiacoronene”) was synthesized during the 1970s by Boekelheide and co-workers in the course of their work on bridged [18]annulenes.283,284 53.12 was obtained from cyclophane 53.11 by UV irradiation in the presence of air. The reaction proceeded through the cycloadduct 53.10, which could be observed using NMR upon irradiation of a degassed solution of 53.11. 3519


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Scheme 52. N-Annulated Perylene−Oligopyrrole Hybridsa

a

Reagents and conditions: (a)265 FeBr3, DCE, 29%; (b)267 FeCl3, DCM, 23% for 52.7b.

Chart 9. Copolymers Containing N-Annulated Perylene Units268,269

In 2012, Réau et al. reported the synthesis of P-annulated perylenes with tunable optical properties (Scheme 53).285 Compound 53.15b was obtained alongside the incompletely fused 53.14b by using photochemically induced cyclodehydrogenation of the dibenzo[e,g]isophosphindole sulfide derivative 53.13b. Both fused products were air-stable and soluble in common organic solvents. 53.15b was transformed into the air- and moisture-stable phospholium salt 53.16 by treatment with methyl triflate. Finally, a reaction with the nucleophilic phosphane P(NMe2)3 converted 53.16 into σ3,λ3-dibenzophosphapentaphene 53.17. Electronic absorptions of charged derivatives of 53.17, such as 53.16, showed marked bathochromic shifts, which were accordingly reflected in the emission properties of these systems. In subsequent work, Nyulászi, Hissler, et al. described an analogous synthesis of the corresponding P-oxides 53.14a and 53.15a, which however formed less efficiently. 286 The electronic structure of heteroannulated perylenoids such as 53.6, 53.8, and 53.17 was investigated theoretically by Hissler, Nyulászi, et al.287

A doubly S-annulated perylene diimide 54.2 was described in 2006 by Wang, Zhu, and co-workers.288 The two thiophene rings in 54.2 were obtained in a Stille-type coupling reaction between the tetrachloro PDI derivative 54.1 and bis(tributyltin)sulfide (Scheme 54). In contrast to 54.1, in which the PDI core is highly twisted because of steric repulsions between adjacent Cl atoms, 54.2 shows a planar conformation in the solid state. The reduction of strain may therefore be a contributing driving force in the synthesis of 54.2 (for a related example, cf., Scheme 21, section 2.4). The above bis-annulation was also applicable to bis-PDI system 54.3.289 In addition to the doubly S-annulated system 54.4b, analogous to 54.2, the Wang group prepared the bis-N-annulated species 54.4a by means of the Buchwald−Hartwig amination reaction. Both compounds exhibit nonplanar conformations, each with two bowl-shaped units of opposing curvatures, as shown by the X-ray structural analyses. The highest local curvature, corresponding to a POAV angle of 4.7°, was found in 54.4a. The absorption spectra revealed a marked red shift of the 3520


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Chart 10. Bay-Expanded NAP Derivatives270,273

Double S-annulation of an otherwise unmodified perylene was accomplished by Wang, Shuai, Pei, et al. in 2010.290 Peryleno[1,12-bcd:6,7-b′c′d′]dithiophene 54.7 was obtained in three steps with an 8% overall yield (Scheme 54). After the initial mononitration of 46.2 with an excess of HNO3/H2O, the resulting 54.5 was converted into the dithiine intermediate 54.6 in a reaction with sulfur powder. In the final step, 54.6 was subjected to copper-mediated partial dechalcogenation. 54.7 is a solution-processable semiconductor, from which highperformance 1D single-crystalline nanoribbon transistors were fabricated, with mobilities of up to 2.13 cm2 V−1 s−1 measured for individual nanoribbons. The integration of two sulfur atoms into the perylene skeleton induces a compressed, highly ordered packing mode directed by S···S interactions. In 2014, Li, Müllen, and co-workers described a synthetic pathway to the doubly S-annulated 3,4,9,10-tetrabromoperylene 54.11.291 Their reaction sequence involved the Stille S-coupling annulation, followed by cyclization to the respective anhydride 54.10 and conversion of the latter compound into the tetrabromo target 54.11 using the Hunsdiecker reaction. 3.3. [ghi]Heteroannulated Perylenoids: 6-Membered Rings

Phenanthro[1,10,9,8-klmna]phenanthridine 55.3, a pyridofused perylene derivative, was reported in 1972 by Shine and Ristagno.292 It was formed, in addition to 1-deuterioperylene 55.2, in a reaction of 1-cyanoperylene 55.1 with LiAlD4

second allowed electronic transition associated with the presence of five-membered rings in the fused system. Scheme 53. Heteroannulated Perylenoidsa

Reagents and conditions: (a)275 AlCl3, NaCl, 130 °C, dry air, 5 h; (b) Raney Ni, mesitylene, MeOH; (c)277 S powder, 320 °C, 30 min, 17%; (d) Cu, 430 °C, 35%; (e) Cu, 500 °C; (f)278 Se powder, 380 °C, 14 h; (g)279 Cu powder, heating; (h)284 degassed THF-d8, −80 °C, 253.7 nm; (i) THFd8, air, rt, 253.7 nm, 100%; (j)285 I2, hν, toluene, 20 h; (k) MeOTf, DCM; (l) P(NMe2)3, DCM. a

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Scheme 54. Doubly Bay-Annulated Perylenesa

a

Reagents and conditions: (a)288 Bu3SnSSnBu3, Pd(PPh3)4, toluene, reflux, 10 h, 72%; (b)289 Pd(OAc)2, PCy3, PhNH2, KOtBu, toluene, reflux, 5 h, 51%; (c)289 Bu3SnSSnBu3, Pd(PPh3)4, toluene, reflux, 12 h, 68%; (d)290 HNO3/H2O, 1,4-dioxane, 60 °C, 5−10 min, 15%; (e)290 sulfur powder, Nmethyl-2-pyrrolidone, 180 °C, 5 h, 61%; (f)290 copper nanopowder, 250−280 °C, 1 h, under argon, 88%; (g)291 Bu3SnSSnBu3, Pd(PPh3)4, toluene, under Ar, reflux, 24 h, 66%; (h)291 KOH, 2-propanol, water, reflux, overnight, 98%; (i)291 bromine, 1 M NaOH, water, 10 min, 30 °C, 89%.

56). The N-doped species 56.7a was obtained in 2006 by Benniston and Rewinska294 in a multistep synthesis, with an overall yield below 5%. Their work was based on a much earlier extensive report by Dilthey et al., published in 1939, which described the synthesis of ortho-fused precursors 56.6, 56.8, and 56.10, and their presumed conversion into singly coupled perylenoid products.297 In the first step, the three-component condensation of N-phenyl-2-naphthylamine, benzaldehyde, and 2-naphthol in glacial acetic acid produced 56.4.294 Oxidation of this intermediate with MnO2 yielded the light-sensitive carbinol 56.5, which was converted into the fully aromatized hexafluorophosphate salt 56.6a. In the presence of ambient light, 56.6a was found to convert to the doubly fused species 56.7a, rather than to the singly fused product proposed by Dilthey et al.297 In a modified synthetic route, described in 2008 by Müllen and co-workers, acridinium intermediates 56.6 were obtained by refluxing 56.8a or 56.8b with appropriate amines, and subsequently photocyclized to 56.7b−e.295 Derivatives 56.7b and 56.7e were found to self-assemble into nanoscale wire-like fibers and helical aggregates, respectively. Oxygen- and sulfur-containing cations, 56.9b−e and 56.11, were described in 2009 by Müllen and co-workers.296 The photocyclization of 56.8b−e under 300 nm UV light led to the fused xanthenium salts 56.9b−e, and a sulfur analogue, 56.10, was prepared in a similar fashion (Scheme 56). The work revealed that optoelectronic properties of these xanthylium and thioxanthylium salts are strongly dependent on the heteroatom incorporated into the core. Ordered columnar liquid crystalline phases were observed for di- and tridodecyl-substituted salts 56.9c−e. X-ray scattering data and molecular modeling indicated that in each mesophase, three molecules selfassembled into a disclike structure, and the resulting discs were further stacked into columns to ultimately yield hexagonal columnar phases.

(Scheme 55). A related structure, 55.4, was proposed in 1970 by Zander and Franke as a possible pyrolysis product of αScheme 55. Perylenes with Bay-Fused 6-Membered Hetero Ringsa

a

Reagents and conditions: (a)292 LiAlD4, THF, 4 h, 54%.

anthramine hydrochloride.293 A tetraester derivative 55.5 was synthesized in 2012 by Cheng, Xiao, and co-workers in the course of their work on Schiff-base diazacoronenes (Scheme 3, section 2.1).63 Similarly, the work of Wang et al., targeting bisimidazolo-fused coronenes, led to the synthesis of the singly fused perylenoid 55.6.64 A range of cationic 14-heteraphenanthro[2,3,4,5-pqrab]perylenes have been reported, containing nitrogen (56.7),294,295 oxygen (56.9),296 or sulfur296 (56.11, Scheme 3522


Chemical Reviews

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Scheme 56. Synthesis of 14-Heteraphenanthro[2,3,4,5-pqrab]perylenesa

a

Reagents and conditions: (a)294 glacial AcOH, reflux, overnight; (b) MnO2, glacial acetic anhydride, bubbled with HCl, protected from light, reflux, 1 h; (c) (1) conc. HCl, glacial AcOH, 5 min, (2) Et2O, (3) KPF6 (aq); (d) MeCN, air, hν, several hours; (e)295 aniline/amine, THF, argon atm, reflux, 6 h; (f) EtOH, hν, rt, 24 h; (g)296 AcOH, 300 nm, rt, 24 h; (h) DCM, UV irradiation (300 nm), rt, 24 h.

core, and prepared two types of nitrogen-doped derivatives, 58.2 and 58.3 (Scheme 58).305 The 58.2 polymer was synthesized by the Suzuki coupling reaction between a pyrazine linker and a 2,6-disubstituted naphthalene followed by cyclodehydrogenation. By using equimolar amounts of pyrazine and benzene linkers for polymerization, a nonregioregular polymer 58.3 with 50% N-doping was obtained. The electron mobility of the graphene nanoribbons increased while the hole mobility decreased, with the increasing amount of nitrogen doping. This relationship indicated that N-doping changed the charge-transport behavior of these graphene nanoribbons from ambipolar to an n-type semiconductor. The threshold voltage of the graphene nanoribbons shifted from 20 to −6 V upon increasing the amount of nitrogen doping. A range of systems containing a hetero ring fused to the 1,2 edge of benzo[ghi]perylene have been synthesized, typically by fusion of a heterocyclic substituent to the bay region of a PDI or a perylene tetraester (for a relevant phthalocyanine derivative, see Scheme 193, section 7.1). Pyridine-fused benzoperylenes C11.1−2 (Chart 11) were obtained by Wang and co-workers in a manner analogous to that of the corresponding bispyrido-fused coronenes (see Scheme 23, section 2.5).106 Triazole-fused benzoperylenes C11.3a−4a, bearing different generations of Newkome dendrimers, were assembled with the aid of the Huisgen reaction by Hirsch et al.107 In comparison with the bis-fused coronene analogues (section 2.5), the photochemical electrocyclization reaction leading to benzoperylene C11.3a occurred faster (30−90 min), apparently for steric reasons. The terminal t-butyl groups of the dendron appendages in C11.3a−4a were cleaved with TFA in nearly quantitative yields to produce water-soluble derivatives C11.3b−4b. The above strategy could be extended to larger fused heterocycles and to the formation of heterocycle-bridged bisperylene molecules. In 2010, Yuan, Xiao, and Qian reported a photochemical synthesis of octaester C11.5, with a thieno[3,2b]thiophene linker embedded between the two perylene

The fusion of a pyrazine ring in the bay region of perylene was first achieved in 1949 by Zinke, who obtained the dibenzoperylene derivative 57.2 by condensing the 57.1 dione with hydrazine (Scheme 57).298 57.2 was also prepared by photochemical cyclodehydrogenation from 3,4,5,6-tetraphenylpyridazine 57.3299 and by reaction of 53.3 with diazenedicarboxylates73 or with cyclic hydrazides.300 The oxidative cycloaddition approach was effective in preparing derivatives 57.4−5, with a varying extent of fusion. The oxidative cycloaddition of perylene with 4-phenyl-1,2,4-triazoline-3,5dione, reported in 1974 by Zander, yielded the urazole derivative 57.6, which was converted photochemically into the fused 1,3-diazepine-2-one 57.7.301 The seven-membered ring fused in the bay region of 57.7 is a rare feature, presumably for steric reasons (other examples include a diazepine− hypericin hybrid 57.14302 and a hypocrellin B derivative 57.15303). Zander’s bay-annulation approach was extended to PDI derivatives 57.9a−b by Langhals and Kirner.239 57.9a shows a bathochromically shifted absorption at 600 nm, and strong NIR fluorescence with an emission maximum beyond 750 nm. In the same paper, the closure of a 1,2-dithiine ring in the bay region of nitro PDI derivatives 57.10a−b was achieved by means of a reaction with elemental sulfur.239 57.11a is a brilliant green, nonfluorescent dye with significantly lower stability than other bay-annulated PDI systems. In 2015, Zhang et al. reported the first BN-annulated perylene bisimide derivative 57.13.304 This compound was obtained by reacting the 1-amino-PDI 57.12 with dichlorophenylborane. Compound 57.13 was explored as an OLED material and as a selective anion sensor for fluoride detection, discriminating among 12 anions (F−, Cl−, Br−, I−, SO42−, NO3−, CH3COO−, H2PO4−, CN−, BF4−, ClO4−, PF6−) in the chloroform solution. The detection limit was as low as 1.5 μM, and the observed effect was ascribed to N-deprotonation of the azaborine ring. In 2013, Jo and co-workers described an oxidative strategy for the synthesis of graphene nanoribbons based on a perylene 3523


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Scheme 57. [ghi]Heteroannulated Perylenoids Containing 6- and 7-Membered Ringsa

a

Reagents and conditions: (a)298 hydrazine hydrate, pyridine, reflux, 30 min; (b)299 UV irradiation, MeOH; (c)3001,4-dioxo-1,2,3,4tetrahydrophthalazine, Pb(OAc)4, DCM, under nitrogen, 0 °C, 3 h, then rt, 3 h; (d)301 4-phenyl-1,2,4-triazoline-3,5-dione, 3 h, rt with simultaneous addition of Pb(OAc)4, then rt, 1 h, DCM, 59%, (e)301 hν, mesitylene, under nitrogen, rt, 7 h, 24%; (f)239 4-phenyl-1,2,4-triazoline-3,5dione, chloranil, dry toluene, reflux, 24 h, 25% for a, 25% for b; (g)239 sulfur, NMP, Ar, 130 °C; (h)239 sulfur, DMF, Ar, 130 °C, 10 h; (i)304 dichlorophenylborane, triethylamine, toluene, Ar, reflux, 24 h.

Scheme 58. Nitrogen-Doped Graphene Nanoribbonsa

a

Reagents and conditions: (a)305 10 equiv of FeCl3, DCM/MeCN, 40 °C, 3 days.

units.306 Compound C11.5, highly insoluble in common solvents, could only be characterized by MALDI−TOF mass spectrometry and was converted into a potassium salt for further analysis. In subsequent work from the Xiao group, the concept was extended to carbazole derivatives C11.6−7, prepared by sunlight-induced intramolecular cyclization.108 C11.6−7 showed weak fluorescence emission, each with a maximum at ca. 640 nm, and enhanced nonlinear absorption in comparison with the parent PDI.

A different approach was adopted in 2000 by Langhals and Kirner, who obtained the perimidine-fused derivative 59.2 by condensing the bay-fused anhydride 59.1 with peri-diaminonaphthalene.239 Compound 59.2 exhibits extended absorption in the visible region but is nonfluorescent. A tetrathiafulvalenefused PDI derivative 59.4 was similarly synthesized by Liu et al. in 2010.307 Compound 59.4 is a π-conjugated molecular dyad, showing excited charge-transfer states at three different oxidation levels, leading to intense optical absorption over a wide spectral range. 3524


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Chart 11. Bay-Fused PDI Derivatives106−108,306

Scheme 59. Bay-Benzannulated PDI Systems with Extended Fusiona

a

Reagents and conditions: (a)239 1,8-diaminonaphthalene, diethylene glycol monomethyl ether, reflux, 5 h, then 2 N HCl, rt, 16 h, 71%; (b)307 5,6diamino-2-(4,5-bis(propylthio)-1,3-dithio-2-ylidene)benzo[d]-1,3-dithiole, under Ar, propionic acid, 90 °C, 2 h, then 135 °C, 2 h, 65%.

Scheme 60. Synthesis of a Fan-Shaped Thiophene-Fused PAHa

Reagents and conditions:308 (a) (1) Sm, 1,2-diiodoethane, HMPA, THF, 1.5 h, rt, N2, (2) reactants in THF, 0 °C, 20 min, then rt, 2−10 h; (b) pTsOH, benzene, reflux, 5−12 h; (c) DDQ, toluene, reflux, 7−18 h, overall yield (a−c) 44%; (d) FeCl3, DCM, CH3NO2, rt, 5 h, 81%, or excess AlCl3, anhydrous CuCl2 in CS2, air, 36 h, 91%. a

intermediate was submitted to the oxidative annulation in the presence of iron(III) chloride or by treatment with AlCl3/ CuCl2/O2.

A related fusion pattern was developed in 2002 by Fang and co-workers, who reported on the synthesis of the fan-shaped system 60.6 (Scheme 60).308 The synthesis consisted of a SmI2-induced coupling reaction of ethyl thiophene-2-carboxylate (60.1) with α-phenylacetophenone, followed by acidcatalyzed dehydration and DDQ oxidative cyclization, giving tetraarylbenzothiophene 60.5 in a good overall yield. The latter

3.4. [cd]Heteroannulated Perylenoids

Perylene mono- and diimides (PMIs and PDIs) constitute the largest class of [cd]heteroannulated perylenoids, and have been 3525


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Scheme 61. Synthesis of Biscyclopenta-Fused Perylenoidsa

Reagents and conditions: (a)311 (1) KOH, 250 °C, (2) O2; (b)312 t-BuOK, DBN, diglyme, 130 °C for 1 h, then addition of 61.1, 130 °C, 3 h, under N2, then workup in air, 78%; (c)313 Ni(PPh3)2Br2, Zn, Et4NI, THF, 50 °C, 20 h; (d)291 sulfur powder, N-methyl-2-pyrrolidone, 190 °C, 3 h, 97%; (e)316 on-surface dehydrogenation, Cu adatoms, annealing to 300 °C.

a

by the codeposited or substrate-supplied transition metal adatoms (Scheme 61). The dehydrogenation proceeded to 61.13 only in the presence of Cu adatoms and at elevated temperatures. The formation of azo bridges led to characteristic changes in the N 1s XPS signature and in NEXAFS spectra. The dehydrogenated 61.13 formed porous networks with the Cu adatoms with structures reflecting the symmetry of the underlying substrate. Peropyrene (dibenzo[cd,lm]perylene) is an extended PAH structure, which can be viewed (and synthesized317) as a union of two phenalene subunits. 2,9-Diazaperopyrene 62.3 was first synthesized in 1989 by Hélène, Lehn, et al. in the form of the dimethyl dichloride salt 62.1 (Scheme 62).318 62.1 was obtained by NBS aromatization of the diamine 62.2, which is available from perylene dianhydride in two steps. 62.1 is fluorescent and was observed to dimerize in an aqueous buffer solution. 62.1 strongly binds to certain small aromatic molecules, including nucleosides, and was investigated as a potential selective fluorescence probe and sequence-specific artificial nuclease.318 A dealkylating aromatization procedure for 62.2, developed by Stang et al., involved heating with a palladium catalyst at high temperatures and furnished the Nunsubstituted 62.3.319 The low yield of the latter reaction was apparently due to the low solubility of the product in common solvents. Modified procedures for both nondealkylating and dealkylating aromatizations were subsequently developed by Würthner and co-workers, enabling the synthesis of substituted derivatives of 62.1 and 62.3.320,321 A fairly low-yielding but remarkably simple synthesis of 62.3 was reported in 2009 by Aksenov et al. as part of their research on naphthalene acylation.322 The polycyclic framework was assembled in one step from naphthalene and 1,3,5-triazine, which were heated together in polyphosphoric acid (Scheme 62). Stang et al. employed diazaperopyrene ligands to prepare tetranuclear PtII- and PdII-based molecular squares 63.1a−b (Scheme 63).319 Each complex was prepared by reacting 62.3 with an equimolar amount of the square-planar cis-bis(phosphine) Pt or Pd bis(triflate) complex. Compounds 63.1a−b were more soluble than the free 62.3 ligand, but

intensely explored since the original synthesis of PDI by Kardos.309 PDI and PMI derivatives fall outside the scope of this Review unless they contain additional heteroatoms or fused hetero rings. Accordingly, this section focuses on other types of [cd]fused systems, notably on molecules containing fivemembered rings and on N-doped peropyrenes. In 1954, Bradley and Pexton reported a synthesis of the red bisquinoxaline dye 61.2, which was obtained by oxidative dimerization of acenaphtho[1,2-b]quinoxaline 61.1 (cf., section 6.2) under basic conditions (Scheme 61).310 A similar dimerization was observed for aceanthra[1,2-b]quinoxaline 61.3. The resulting blue dye was identified as the “trans” isomer 61.4 in a reinvestigation carried out in 1995 by Désilets et al.311 A refined synthesis of 61.2 was reported in 2001 by Sakamoto and Pac, who performed the oxidative coupling by base-induced dimerization of 61.1, followed by aerial oxidation.312 Both approaches could be applied to the synthesis of a range of PDI-related systems.311,312 A conceptually related strategy, based on a Ni-catalyzed reductive coupling, was employed in 2003 by Ono et al. in the synthesis of the bispyrrole fused derivative 61.6.313 The compound showed red fluorescence with an emission maximum of ca. 570 nm. The strategies used for the syntheses of [cd]heteroannulated perylenoids include, in addition to dimerizations discussed above, also ring contractions, exemplified by Langhals’ synthesis of 61.7 lactams from the corresponding PDIs,314 as well as direct heteroannulations. An example of the latter approach was provided by Müllen and co-workers, who described a highly selective reaction of 3,4,9,10-tetrabromo-1,6,7,12-tetrachloroperylene 61.8 with elemental sulfur, yielding bis-1,2-dithiole 61.9. The pink compound 61.9 showed an absorption maximum at 560 nm in dichloromethane, which corresponded to a bathochromic shift of 100 nm relative to 61.8.291 The chlorine-free parent system of 61.9 was prepared by Scholl-type dimerization of naphtho[1,8-cd][1,2]dithiole.315 In 2014, Shchyrba et al. described the on-surface reactivity of 4,9-diaminoperylene quinone-3,10-diimine 61.10.316 Compound 61.10 underwent a two-level dehydrogenation to 61.11 (−1 H2) and 61.13 (−3 H2), specifically determined 3526


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Scheme 62. Synthetic Routes to Diazaperopyrenesa

In 2001, Würthner and co-workers reported a series of diazadibenzoperylenes C13.1a−d bearing mesogenic dodecyloxybenzoyl substituents.324 The aza coordination sites present in these systems enabled additional control of dye packing by intermolecular interactions, such as in the carboxylic acid adducts C13.2a−d (Chart 13). Compounds C13.1a−d exhibited strong luminescence in solution and in the solid state, and were found to form thermotropic columnar liquid crystalline phases with and without the addition of benzoic acids. The formation of complex hydrogen-bonded superstructures between compounds C12.1a−b and a substituted isophthalic acid was also investigated.325 It was found that only the phenoxy-substituted diazadibenzoperylene C12.1a formed extended assemblies with a 1:1 stoichiometry (C13.3), whereas for the C12.1b analogue, no indications of superstructure formation were found. The different behavior was explained by the presence of additional π−π interactions, which were only observed for C13.3, as revealed by concentration-dependent optical absorption and fluorescence spectroscopy. In 2006, the first, second, and third generation dendronized diazadibenzoperylenes C13.1e−g were synthesized by DCC/DPTSactivated esterification of C12.1c with the respective benzoic acid dendrons.326 The yields of diazadibenzoperylene dendrimers C13.1e−g successively decreased from the first to the third generation (75% to 20%), in line with the increasing steric hindrance of the four dendritic wedges around the core. The first and second generation dendrimers afforded Ag-linked coordination polymers, while the third generation resisted polymerization, apparently as a result of the steric shielding of the aza coordination sites by dendritic wedges. The unsubstituted 1,3,8,10-tetraazaperopyrene (TAPP) 64.1 was first prepared in 2007 by Gade and co-workers in a reaction of 4,9-diamino-3,10-perylenequinone diimine 61.9 with triethyl orthoformate (Scheme 64).327 2,9-Disubstituted derivatives 64.2a−g were similarly obtained by reacting 61.9 with the corresponding carboxylic acid chlorides or anhydrides. A range of aryl-disubstituted TAPPs was subsequently synthesized using the same general method.328 Compounds 64.1 and 64.2a exhibit low solubility in common organic solvents due to efficient aromatic stacking between peropyrene cores.327 The UV−visible absorption spectra of TAPPs contain a characteristic visible π* ← π absorption band at 440 nm with a strong vibrational progression (Δν ≈ 1450 cm−1). This absorption

a

Reagents and conditions: (a)318 (1) NBS, AcOH, reflux, 30 min, (2) ion exchange resin; (b)319 neat, 10% Pd/C, 300−310 °C, 1 h; (c)322 PPA, 65−70 °C, 3 h, then 100−110 °C, 6 h.

their complete purification and characterization was not feasible due to their low solubility in common organic solvents. The Würthner group employed tetraaryloxy-substituted diazaperopyrene bridging ligands in the transition metal-directed selfassembly with Pd(II) and Pt(II) phosphane triflates, which resulted in complex dynamic equilibria between molecular triangles 63.2a−d and molecular squares 63.3a−d in solution.323 It was found that the equilibria depended on several factors, such as the metal ion (platinum complexes exhibited higher stability than their Pd analogues) or the solvent. The introduction of bulky p-(tert-butyl)phenoxy groups as the R 1 substituents shifted the equilibrium significantly in the direction of the molecular squares. The suitability of the diazaperopyrene derivative C12.1a as a photo- and redox active supramolecular building block was investigated by Würthner and co-workers, who performed complexation experiments with carboxylic acids (3,4,5trisdodecyloxy benzoic acid) and a range of metal ion sources: (trans-[Pd(PPh3)2(OTf)2]), AgOTf, and ZnTPP, leading to structurally diverse adducts, C12.3, C12.4, and C12.5a−b (Chart 12).320,321 Compounds C12.1a and C12.2 were characterized by optical absorption and fluorescence spectroscopy,321 exhibiting emission quantum yields of 75% and 50%, respectively. The C12.2 dication revealed reversible oxidation and reduction waves, whereas the oxidation of C12.1a−b underwent irreversible deposition of a conductive film on the electrode surface.

Scheme 63. Metallacycles with Diazadibenzoperylene Bridging Ligands319,323

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Chart 12. Tetraaryloxy-Substituted Diazadibenzoperylenes320,321

Chart 13. Diazadibenzoperylenes Bearing Mesogenic Substituents324−326

TAPPs can be halogenated on the perylene core, yielding tetrachloro332 and tetrabromo333 derivatives (64.5 and 64.6, respectively), when reacted under highly acidic conditions with either dichloroisocyanuric acid (DIC) or dibromine. Halogen substitution has a drastic influence on the optical and electrochemical properties of TAPPs. Members of the 64.5 series are strongly fluorescent and yield high-performance nchannel transistors with a field-effect mobility of up to 0.14 cm2 V−1 s−1, an on/off current ratio of >106, and good long-term stabilities. Current−voltage characteristics of the TFTs based on the bromo derivatives 64.6 established electron mobilities of up to μn = 0.032 cm2 V−1 s−1. Halogenated TAPPs were subjected to further functionalization using metal-catalyzed coupling reactions and nucleophilic halogen substitutions to yield a range of aryl-,334,335 alkynyl-,334 and aryloxysubstituted335 derivatives. Water-soluble, pyridinium-containing TAPP derivatives showed both high photostability and high fluorescence quantum yields (>80%) in aqueous solutions.335

exhibits a bathochromic shift upon protonation of the nitrogen atoms. The first and second as well as the third and fourth protonations occur concomitantly, which implies that the pKa values are very similar within each pair. TAPP derivatives undergo two electrochemically reversible one-electron reductions. In toluene solutions, TAPP dyes show weak fluorescence (Φ ≤ 0.02), which typically becomes much more intense upon protonation (Φ = 0.47 for 64.1 in TFA). 64.1 contains four potentially metal-ligating exodentate nitrogen donors. Furthermore, the C−H bond between each two nitrogen atoms offers the potential of tautomerization to an N-heterocyclic carbene isomer or may undergo metal-induced C−H activation at elevated temperatures. This last property was exploited in a study of TAPP metal−organic coordination networks self-assembled on the Cu(111) surface.329−331 Upon annealing at 250 °C, the initially formed Cu-coordinated 2D network was converted into covalently linked oligomer chains, for which the general structure 64.3 was proposed on the basis of STM data. 3528


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Scheme 64. Synthesis of 1,3,8,10-Tetraazaperopyrenesa

Reagents and conditions: (a)327 triethyl orthoformate, cat. formic acid, reflux, 48 h; (b)332 dichloroisocyanuric acid, concentrated H2SO4, 85 °C, 72 h, 44−87%; (c)333 Br2, I2 (cat.), fuming H2SO4, 80 °C, 10 h, 29−81%.

a

Scheme 65. Extended [cd]-Fused Rylene Dyesa

Reagents and conditions: (a)336 BINAP, Pd2(dba)3, NaOtBu, toluene, 80 °C, overnight; (b)336 K2CO3, ethanolamine, 130 °C, 5 d (for 65.5); NaOtBu, DBN, diglyme, 130 °C, 16 h (for 65.6); (c)136 Zn(OAc)2, quinoline, 180−200 °C; (d)136 imidazole, 140 °C.

a

2-acetylaniline as the nucleophilic component.136 The condensation performed on perylene anhydrides 65.8 and 65.11 showed remarkable chemoselectivity dependent on reaction conditions. Heating in quinoline in the presence of catalytic zinc acetate produced 4-oxoquinoline products 65.7 and 65.10. When the reaction was carried out in imidazole, the imidization−aldol condensation sequence was followed by a rearrangement step, which produced the thermodynamically more stable 4-hydroxyquinoline products 65.9 and 65.12. 65.9 and 65.12 can be viewed as “remotely” [cd]heteroannulated perylenoids because the hetero ring is not directly fused to the perylene substructure. Such systems are relatively rare, with two additional examples provided by heterofused isoviolanthrone dyes (Chart 14). Cyananthrene C14.1, reported in 1952 by Bradley and Sutcliffe, was obtained by fusing 13H-phenaleno[2,1-f ]quinolin-13-one with potassi-

These dyes showed selective staining of cell nuclei and low cytotoxicity with IC50 values of up to 67 μM. Extended rylene dyes 65.5−6, bearing a structural resemblance to Indanthrene Olive Green B (C22.2, section 4.2), were reported in 2002 by Müllen and co-workers.336 The synthesis involved a Buchwald-type double amination between 1,5-diaminoanthraquinone and each of the imides 65.1−2, followed by base-promoted cyclization of the intermediate oligomers 65.3−4. Dyes 65.5−6 showed extended chargetransfer absorptions in the NIR range (up to ca. 900 and 1400 nm, respectively), as well as excellent photostability. The NH groups in 65.5−6 are stabilized by intramolecular hydrogen bonding, leading to negligible solvatochromism. The tandem condensation method of Müllen et al., used for the construction of the 3a1-azaperylene substructure (Scheme 30, section 3.1), was also employed in a simpler version, using 3529


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Chart 14. Hetero-Fused Isoviolanthrone Dyes337,338

intramolecular cyclization of a bis-anthraquinone precursor carried out in the presence of Cu and H2SO4.341 Propellershaped indole-fused PDI derivatives 66.6a−b were obtained by Wang, Jiang, et al. in a 4-fold Heck-type arylation reaction.342 These systems showed reduced HOMO−LUMO gaps, and intense absorption in the 500−750 nm range. Pyrrole ring fusion in hypocrellin derivatives was reported by Zhang and coworkers.343,344 Their method involved heating hypocrellin B (67.1) with appropriate amines in pyridine, yielding fused systems 67.2a−d, characterized by enhanced optical absorption relative to the parent system (Scheme 67). Scheme 67. Synthesis of Amino-Bridged Hypocrellin Ba

um hydroxide and potassium acetate.337 In 1966, Nair and Shah proposed the structure C14.2 for a dye obtained by AlCl3catalyzed cyclization of a disubstituted isoviolanthrone precursor.338 3.5. ortho-Heteroannulated Perylenoids

Early examples of ortho-heterofused perylene derivatives, such as 66.1, date back to the work of Zinke and co-workers (Scheme 66).339 In 1934, they reported the synthesis of the bisquinoxalino system 66.3 in an acid-catalyzed condensation of 2,11-dihydroxyperylene-3,10-dione 66.2 with ortho-phenylenediamine.340 A structurally related indole-fused helianthrone derivative 66.4 was obtained in 1970 by Wick, as a product of

a Reagents and conditions: (a)344 amine, pyridine, darkness, 10−20 h, 50 °C.

In 2012, an imidazole-[a]fused PDI derivative 68.2 was described by Langhals and Hofer (Scheme 68).345 68.2, obtained by reacting PDI with sodium amide and benzonitrile, was subjected to a sequence of transformations leading ultimately to the bisperylene dyad 68.6. Heterodimer 68.7 was similarly assembled using the Suzuki coupling reaction. These two strongly absorbing dyes were characterized by dynamic reorganization of the “biaryl” torsional angle in the excited state, which led to considerably increased Stokes shifts. In 2013 Pei, Wang, et al. prepared an extended fused system 69.2, containing both perylene and pyrene substructures (Scheme 69).346 The four ortho-fused thiophene rings were installed by performing a quadruple Scholl-type cyclization on the substituted pyrene precursor 69.1. 69.2 was the first example of a pyrene derivative fused outside the K-regions and was described as a thiophene-fused “superpyrene”. In concentrated solutions, 69.2 was observed to undergo Haggregation, as evidenced by UV−vis spectroscopy, a feature that might be beneficial for charge transport in condensed phases. The self-assembly of 69.2 was investigated by SEM and TEM measurements, which revealed the formation of 1D nanowires with high aspect ratios.

Scheme 66. Bis-ortho-Fused Perylenoidsa

4. PYRENOIDS Both the parent hydrocarbon pyrene and its numerous derivatives are currently of great importance as aromatic building blocks for use in device applications.347 A variety of fused heterocyclic systems containing pyrenoid substructures were originally developed by the dyestuff industry. The most important of these derivatives, usually acridine and anthraquinone dyes, are highlighted in the following sections. For comprehensive reviews, the reader is referred to chapters by Tilak and Ayyangar,348 Venkataraman and Iyer,349 Hunger and Herbst,350 and to the book by Allen.1 This section focuses on heteroatom-containing pyrenoids containing at least five fused rings, although relevant nonfused (four-ring) heterapyrenes are

a

Reagents and conditions: (a)340 o-phenylenediamine, nitrobenzene, glacial AcOH, reflux; (b)342 [Pd2(dba)3], P(t-Bu)3, K3PO4, 1,4dioxane, reflux, argon. 3530


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Scheme 68. Synthesis of Imidazole[e]-Fused Perylene Derivativesa

a

Reagents and conditions:345 (a) sodium amide, benzonitrile; (b) KOH, tert-butyl alcohol; (c) Cu, 3-picoline, 48%; (d) Br2, chloroform, 88%; (e) Pd(OAc)2, (C4H9)4NBr, NEt3, toluene, 60%.

Scheme 69. Synthesis of a Thiophene-Fused “Superpyrene”a

Chart 15. Classification of Pyrenoids Used in This Sectiona

a Reagents and conditions: (a)346 FeCl3, CH3NO2, DCM, N2 atm, 8 min, 0 °C.

also discussed. The material is grouped into seven sections (Chart 15). Section 4.1 discusses the rich chemistry of heteratriangulene derivatives (for a recent account, see a review by Lacour et al.32). In sections 4.2 and 4.3, we summarize the chemistry of the remaining heterapyrenes. In particular, BN containing pyrenes are discussed along with other aza systems in section 4.2 (for a B-only pyrenoid system, 39.6, see Scheme 39, section 3.1). The peri- and ortho-heteroannulated pyrenes are discussed in sections 4.4, 4.5, and 4.6. The chemistry of pyrazacenes, recently reviewed by Mateo-Alonso,351 is a major topic in section 4.6. As in the preceding sections, the presence of a cyclic imide or anhydride functionality does not count toward the required number of rings and peri-fusion points. “Horizontally” expanded NDI derivatives, containing additional rings ortho-fused to the naphthalene fragment, such as the systems recently reported by the groups of Gao, 352 Takimiya,353 and Würthner,354 are accordingly not covered here. Such derivatives, which are of high interest as organic semiconductors, have been reviewed recently by Suraru and Würthner50 and by Zhang et al.355


shell molecule with a triplet ground state, the majority of existing heteratriangulenes have closed-shell configurations, which are achieved through appropriate placement of heteroatoms, exocyclic double bonds, and saturation points. In particular, a large number of azatriangulenes have been synthesized, containing the triply bridged, electron-deficient triphenylamine core. The first such system, 70.3, possessing three carbonyl bridges, was reported by Hellwinkel and Melan in 1971.357 It was originally obtained by heating trimethyl 2,2′,2″-tribenzoate 70.2 in the presence of either sulfuric or phosphoric acid. An improved procedure, reported by Field and Venkataraman, furnished 70.3 in 80% yield.358 70.3 is a planar molecule with D3h symmetry, yielding dense packing in the solid state (Kitaigorodskii packing coefficient = 0.72), with columnar π-stacks being formed along the a-axis. The absorption maximum of 70.3 was observed at 422 nm (10% trifluoroacetic acid in dichloromethane), with the corresponding green fluorescence at 499 nm. The solubility of 70.3 in

4.1. Triangulenes

Triangulene (70.1, Scheme 70), the simplest non-Kekulé PAH system,356 forms the structural basis of numerous heterocyclic analogues. In contrast to triangulene itself, which is an open3531


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Scheme 70. Synthetic Routes to Carbonyl-Bridged Azatriangulenesa

a Reagents and conditions: (a)357 MeOH, H2SO4, 24 h, 100−105 °C; (b) H3PO4, 16−20 h, 180 °C; (c)358 (1) NaOH, EtOH:water (1:1), 3 h, reflux, (2) HCl to pH 2−3, (3) SOCl2, DMF (cat.), dry DCM, 3 h, reflux, (4) SnCl4, reflux, 15 h, (5) NaOH, 0.5 h; (d)359 I2, Ag2SO4, EtOH, 12 h, rt; (e) (1) KOH, MeOH, water, overnight, 50 °C, (2) cooled, water, HCl to pH = 3, (3) SOCl2, DMF (cat.), dry DCM, 3 h, reflux, (4) SnCl4, reflux, 24 h, (5) NaOH, 1 h.

Chart 16. Carbonyl-Bridged Azatriangulenes

carbazole branches and the heteratriangulene core. The organic light-emitting diodes (OLEDs), fabricated with C16.1 and C16.2 as nondoping emitters, exhibited aggregation-induced luminescence, peaking at 600 and 630 nm, respectively. A maximum luminance of 1586 cd m−2 at current efficiency of 5.3 cd A−1 was measured for C16.1, whereas C16.2 yielded 827 cd m−2 at a current efficiency of 6.9 cd A−1. In the same year, Chen et al. described the synthesis of C16.3a by a palladium-catalyzed Sonogashira cross-coupling

common organic solvents was too low to permit electrochemical measurements. In 2009, Du, Chen, and co-workers used direct iodination of 70.2 to obtain the triiodo derivative 70.5 (Scheme 70).359 Compound 70.5, a versatile synthetic intermediate, was converted into two star-shaped donor−acceptor dendrimers C16.1 and C16.2 using Ullmann coupling.359 Both dendrimers showed good thermal stabilities and distinct ground-state intramolecular charge transfer (ICT) transitions between 3532


Chemical Reviews

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Scheme 71. Synthesis of Arylvinylidene-Bridged Heteratriangulenesa

Reagents and conditions: (a)366 CBr4, PPh3, toluene, 18 h, 110 °C; (b) phenylboronic acid or 2-naphthylboronic acid, Pd(PPh3)4, K2CO3, water, toluene, dimethoxyethane, 36 h, 100 °C; (c) FeCl3, CH3NO2, DCM, 30 min, 0 °C; (d) FeCl3, CH3NO2, DCM, air, 18 h, rt; (e)367 (1) Lawesson’s reagent, dry toluene, 10 min, rt, (2) 1−1.5 h, 90 °C; (f) (1) 9-diazo-9H-thioxanthene, dry THF, dry toluene, 1.5 h, 0 °C, (2) PPh3, dry xylene, 21 h, 135 °C; (g) (1) diphenyldiazomethane, dry THF, 2 h, rt, (2) PPh3, dry xylene, 16 h, 135 °C; (h) H2O2 (30 wt %), glacial AcOH, 3 days, rt.

a

reaction between 70.5 and 1-dodecyl-4-ethynylbenzene.360 Two-level self-assembly of C16.3a, from nanowires to microrods, was observed when the dichloromethane solution of C16.3a was mixed with methanol. This type of self-assembly was attributed to the strong π−π stacking interactions of heteratriangulene cores and the hydrophobic interactions of alkyl chains with solvent molecules. The same behavior was later observed for derivatives with shorter alkoxy chains (hexyl, octyl, and decyl).361 In 2011 Pisula, Müllen, et al. used the alkyl-substituted C16.4a, which is soluble in common organic solvents, to fabricate fibers characterized by very good long-range alignment.362 The fibers were prepared via simple dip-coating, by controlling the concentration of C16.4a. In the fiber, columnar stacks of molecules were oriented along the main axis, with one crystal plane within the fiber arranged preferentially in-plane to the substrate surface. Two years later, Kivala, Müllen, et al. synthesized a series of carbonyl-bridged heteratriangulenes with n-dodecyl chains attached through different spacers to the nitrogen-centered core.363 All compounds were obtained from 70.5 using different cross-coupling reactions. All ethynylated derivatives C16.3b−f were prepared in good yields by the Sonogashira reaction with the corresponding terminal alkyne.363,364 Compound C16.4a was prepared by the Pdcatalyzed Negishi reaction with n-dodecylzinc bromide, whereas C16.4b was obtained by using Suzuki−Miyaura coupling with 4-dodecylphenylboronic acid. The self-assembly behavior of these derivatives was investigated in solution, on surface, and in the bulk. It was found that additional phenylene moieties between the core and alkyl chains facilitated self-assembly by extending the π-conjugated polycyclic disc. The rod-like ethynylene spacers introduced additional flexibility, thus lowering the overall aggregation tendency. The combination

of both features in the phenylene−ethylene moieties was found to induce thermotropic liquid crystallinity. In 2014, multichromophoric carbonyl-bridged triarylamines were obtained by Hildner, Kivala, Schmidt, and co-workers.365 Naphthalimides C16.6 and C16.7 as well as the reference system C16.5 were prepared from an appropriate triamine derivative of 70.4 (Chart 16). Steady-state and time-resolved spectroscopy of C16.6−7 provided clear evidence for energy transfer in both multichromophoric compounds. In compound C16.6, the energy was forwarded from the peripheral naphthalimides to the carbonyl-bridged triarylamine core, while in compound C16.7 the same process proceeded in the opposite direction. Compound C16.7 is an efficient gelator for ortho-dichlorobenzene (at 0.7 mM concentration) and retained its energy-transfer and photoluminescent properties in the gel state. A strategy for expanding heteratriangulenes at the bridging positions was developed in 2014 by Yamaguchi and co-workers (Scheme 71).366 In the first step, compound 71.1a was converted into the hexabrominated derivative 71.2, which was further transformed by the Suzuki−Miyaura coupling with phenylboronic acid or 2-naphthylboronic acid into the corresponding 6-fold coupling products 71.3a−b. 71.3a then underwent partial oxidative coupling to yield the 10-ring-fused product 71.4. When the reaction was conducted in the presence of air, a dicarbonyl-substituted species 71.5 was obtained, resulting from the cleavage of two exocyclic double bonds. The dehydrogenative coupling of 71.3b smoothly took place at the naphthyl groups, to produce the dibenzo[c,g]fluorenylidene-substituted 71.6. Compound 71.6 has a highly twisted conformation due to the overcrowded alkene moieties, which impart a highly electron-accepting character to the naturally electron-rich heteratriangulene skeleton, thereby 3533


Chemical Reviews

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Scheme 72. Synthesis of Dimethylmethylene-Bridged Heteratriangulenesa

Reagents and conditions: (a)368 85% H3PO4, 0.5 h, rt; (b)369 (1) 85% H3PO4, 2 h, rt, (2) 2 M NaOH; (c)371 (1) POCl3, dry DMF, Ar atm, 0 °C, (2) 0.5 h, rt, (3) 15 h, 80 °C; (d) (1) PH3PCH3I, n-BuLi, anhydrous ether, −78 °C, (2) 1 h, rt, (3) 72.3a, 15 h, rt; (e) KI, KIO3, AcOH, 2 h, 80 °C; (f) (1) NBS, chloroform, 30 min, 0 °C, (2) 1 h, rt; (g) (1) 72.4a, K2CO3, n-Bu4NBr, DMF, water, Ar atm, 20 min, (2) Ph3P, 72.5a, 15 min, rt, (3) Pd(AcO)2, 15 h, 100 °C; (h) (1) n-BuLi in hexane, THF, Ar atm, 15 min, −78 °C, (2) 1 h, rt, (3) 2-isopropoxy-4,4,5,5-tetramethyl[1,3,2]dioxaborolane, 16 h, rt; (i) Pd(PPh3)4, Na2CO3, toluene, water, Ar atm, 30 h, 110 °C. a

Dimethylmethylene-bridged triarylamines are formally azatriangulene derivatives lacking the complete π-conjugation in the peri-fused framework. They will be discussed here because the synthetic methodology involved in their chemistry is of direct relevance to the synthesis of fully π-conjugated heteratriangulenes. The first dimethylmethylene species, 72.2a, was synthesized by Hellwinkel and Melan in 1974 by acid-catalyzed cyclization of the tricarbinol 72.1a (Scheme 72).368 Using 72.2a as the building block, Lai, Samoc, and coworkers developed star-shaped and dendritic acetylene-linked oligomers, such as C17.1a (Chart 17).369 Those compounds combined high fluorescence quantum yields with large twophoton absorption cross sections (1300−6100 GM). Bromosubstituted (C17.1b) and cationic (C17.1c) tetramers were obtained in the same year by Lai, Liu, and co-workers, using a similar synthetic strategy, showing TPA cross sections of 4340 GM (in toluene) and 4150 GM (in methanol), respectively.370 The approach developed by Lai et al. is illustrated by the synthesis of two starburst molecules with dimethylmethylenebridged triangulene moieties that are connected either directly (as in 72.10) or by vinylene links (72.7).371 The unsymmetrical substitution needed for the assembly was achieved either by preparing unsymmetrical arylamine precursors and their

inducing NIR absorptions and multiredox behavior with a low reduction potential. An alternative approach to vinylidene-bridged azatriangulenes was proposed in 2015 by Kivala et al.367 Instead of the dibromoolefination/Suzuki-coupling sequence of Yamaguchi et al., they employed the Barton−Kellogg olefination as the key bond-forming step (Scheme 71). Initially, azatriangulenes 71.1b−c were converted into thioketones 71.7b−c with Lawesson’s reagent and subjected to the 3-fold olefination with either diphenyldiazomethane or 9-diazo-9H-thioxanthane. The intermediate thioepoxides were used directly without purification for the subsequent reaction with triphenylphosphine because of their tendency to decompose during column chromatography. To investigate the effect of the electronwithdrawing sulfone units on the optoelectronic properties of the π-system, compound 71.8b was oxidized in the presence of H2O2 and acetic acid. Further attempts to cyclize compounds 71.8b−c and 71.9b−c using chemical or photochemical oxidation led to material decomposition or complex mixtures of unidentifiable products. 71.8b−c and 71.9b−c are fairly strong electron donors, susceptible to several reversible oxidation steps. In particular, compound 71.8b undergoes the first oxidation at almost the same potential as ferrocene. 3534


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Chart 17. Dimethylmethylene-Bridged Azatriangulenes369,370,372,374,375

making the system a good candidate for a spin-selective conductor. The half-metallicity was robust under external strain beyond the effect of a substrate. In 2013, Fasel and Müllen reported a comparative analysis of solution and surfacemediated syntheses of heteratriangulene macrocycles.374 Both synthethic methods started from the same precursor, a dibromo-substituted dimethylmethylene triphenylamine. The preference for macrocyclization (C17.4a) over the formation of the zigzag oligomers (C17.3) was revealed on surface as a result of two-dimensional confinement. Using solution chemistry, the C17.4a macrocycle was obtained on a several hundredmilligram scale and characterized by single-crystal X-ray analysis. C17.4a was brominated and subjected to the Suzuki−Miyaura cross-coupling to yield the more extended systems, C17.4b and C17.4c. In comparison to 72.2a, compounds C17.4a−c showed a deep blue emission in solution, together with a significant improvement in the photoluminescence quantum yield. These effects resulted from the extended π-conjugation along the cyclic backbone. C17.2, an air-stable, p-type semiconductor polymer, was synthesized in 2013 by List, Müllen, and co-workers and employed for the construction of air-stable OFET devices.375

subsequent derivatization (e.g., 72.1b) or by selective monosubstitution of 72.2a, the latter leading to the vinyl derivative 72.4a. Trihalogenation of 72.2a was achieved using either the KI/KIO3 mixture or the NBS. The vinylene target 72.7a was obtained from 72.4a and 72.5 using the Heck reaction conditions, whereas the directly linked 72.10b was prepared by means of the Suzuki−Miyaura coupling between 72.9b and 72.6. Both starburst molecules showed decreased oxidation potentials, resulting from the increased HOMO energy level. Double-layer sandwich electroluminescence devices were fabricated from both oligomers. Compound 72.10b showed significantly enhanced performance in comparison with 72.7a. In 2011, Fasel and co-workers reported the assembly of tribromo-substituted 72.6 on the Ag(111) surface.372 Depending on the activation temperature, two-dimensional porous metal-coordination or covalent networks were obtained, structurally similar to the C17.4 motif. One year later, Kan, Deng, Yang, and co-workers presented first-principles calculations demonstrating that the porous sheet obtained by Fasel et al. is a ferromagnetic half-metal.373 The computed band gap in the semiconducting channel was found to be roughly 1 eV, 3535


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Scheme 73. Synthesis of Thiophene-Functionalized Azatriangulenesa

a

Reagents and conditions: (a)376 Pd(PPh3)4, K2CO3, THF, water, 18 h, reflux; (b) TFA, THF, water, 6 h, rt; (c) cyanoacetic acid, MeCN, chloroform, piperidine, 6 h, reflux.

of 0.745 V, and a fill factor of 0.70, corresponding to an overall conversion efficiency of 8.71%. The triply substituted system 73.6, also reported by Ko et al., exhibited good p-type semiconducting performance in solution-processed OFETs.378 A power conversion efficiency of 4.16% was observed, with a hole mobility and on/off ratio of 7.6 × 10−3 cm−2 V−1 s−1. In 2013, Grät zel, Liu, and co-workers showed that the introduction of an alkene linkage between the triangulane core and the peripheral group in 73.7 and 73.8 improved electron delocalization and caused a large red shift of the absorption peaks.379 The power conversion efficiency for compounds 73.7 and 73.8 was 7.51% and 8.00%, respectively. In 2012, Perry, Müllen, and co-workers reported a study of the one- and two-photon absorption properties of azatriangulenes with one, two, or three diarylborane arms (74.2a−c, Scheme 74).380 Compounds 74.2a−c were obtained from one-, two-, or tribrominated dimethylmethylene bridged azatriangulenes 74.1a−c, in reaction with n-BuLi followed by addition of Mes2BF. These compounds were stable at ambient conditions because of the presence of two mesityl groups at each boron center, and their solubility increased with the increasing number of B-moieties. It was found that, on going from the single-arm to the two- and three-arm systems, the peak in twophoton absorption (TPA) cross-section was suppressed by factors of 3−11 for the lowest excitonic level associated with the electronic coupling of the arms, whereas it was enhanced by factors of 4−8 for the higher excitonic level. These results showed that the coupling of arms redistributed the TPA crosssection between the excitonic levels in a manner that strongly

Because of the deeper-lying HOMO level (EHOMO = 5.1 eV) and wider bandgap (Eg = 2.9 eV) of C17.2, bottom-gate/ bottom-contact FETs could be fabricated, characterized, and stored under ambient conditions, showing excellent stability over months. The optimization of electrode/− and dielectric/ polymer interfaces yielded mobilities of ∼4 × 10−3 cm2 V−1 s−1 and on/off current ratios of ∼105, showing that C17.2 is a viable alternative for the best performing amorphous air-stable semiconducting polymers. Polymer C17.2 underwent Nprotonation by hydrochloric acid in chloroform solutions, resulting in significant changes of the OFET characteristics. In 2011, Ko and co-workers developed a series of thiophenefunctionalized azatriangulenes for use as sensitizers in dyesensitized solar cells (73.4a and 73.5a−b, Scheme 73).376 The synthesis of 73.4a (Scheme 73) involved stepwise elaboration of the oligomer chain using the Suzuki−Miyaura coupling reaction and Knoevenagel condensation, and a similar approach was employed for the preparation of other derivatives. Under standard AM 1.5 sunlight, the sensitizer 73.5b yielded a shortcircuit photocurrent density of 15.32 mA cm−2, corresponding to an overall conversion efficiency of 7.86%. The 73.5b cell exhibited excellent stability, revealing only a slight change after 1000 h of light soaking at 60 °C. The cell showed a higher open-circuit voltage (Voc) as compared to that of 73.5a due to an increased electron lifetime (τC) in the conduction band of TiO 2 . Further work from the Ko group focused on azatriangulene derivatives 73.4b−d.377 Under standard global air mass 1.5 solar conditions, the 73.4d cell gave a short circuit photocurrent density of 16.78 mA cm−2, an open-circuit voltage 3536


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Azatriangulene derivatives containing oligofluorene arms, 75.6a−c, were obtained in 2012 by Yang, Wu, and coworkers.383 These materials showed glass transitions in the 123−129 °C range and a good film-forming ability. Doublelayer OLEDs fabricated from 75.6a−c showed high electroluminescence performance. In particular, the device based on 75.6c achieved a maximum current efficiency of 3.83 cd A−1 and a maximum external quantum efficiency of 4.19% with CIE coordinates of (0.16, 0.09). C18.1, the radical cation of dimethylmethylene-bridged heteratriangulene, a rare example of a stable triphenylamine radical cation without para-substitution, was obtained in 1975 by Bamberger, Hellwinkel, and Neugebauer (Chart 18).384 It was generated by oxidizing the parent 72.2a with lead tetraacetate in trifluoroacetic acid and characterized using ESR spectroscopy. In 2013, Wang and co-workers succeeded in obtaining the C18.1 radical cation in nonacidic solvents, using either Ag[Al(OC(CF3)3)4] or B(C6F5)3 as the oxidant.385 Additionally, it was shown that the reaction of C18.1 and its parent amine with trace amounts of Ag+ led to the formation of the dimeric C18.2a dication. The X-ray structural analysis and theoretical calculations indicated a significant singlet biradicaloid contribution (C18.2b), structurally analogous to Chichibabin’s hydrocarbon. In 2005, Okada and co-workers reported a new tetraheteratriangulene system, 2,2′:6′,2″:6″,6-trioxytriphenylamine 76.7 (Scheme 76).386 Compound 76.7 was obtained from 2,6difluoro-nitrobenzene 76.1 in a six-step sequence, involving nucleophilic substitutions and Buchwald−Hartwig aminations as the key bond-forming steps. The chemical oxidation of 76.7 to the stable, planar radical cation salt 76.8a was performed by using tris(p-bromophenyl)aminium hexafluorophosphate. In contrast to the radical cation, the neutral form 76.7 had a shallow bowl structure in the solid state. In 2007 Shiomi, Okada, et al. described the solid-state structures and magnetic properties of the tetrachloroferrate (76.8b) and tetrachlorogallate (76.8c) radical cation salts of 76.7.387 In the 76.8b crystals, the radical cations form π-stacked dimers surrounded by tetrachloroferrate anions, which also form dimers in the solid state. The magnetic properties of 76.8b were characterized by strong (2 J/kB = ∼ −1.3 × 103 K, H = −2JS1/2·S1/2) and weak (2 J/k B = ∼ −1.76 K, H = −2JS 5/2 ·S 5/2 ) antiferromagnetic interactions due, respectively, to the radical cation and [FeCl4]− dimers. In 2012, Okada et al. reported a

Scheme 74. Synthesis of Azatriangulenes with One, Two, or Three Diarylboryl Acceptorsa

a Reagents and conditions: (a)380 (1) n-BuLi, 20 min, −78 °C, (2) Mes2BF, −78 °C, (3) overnight, rt.

favored the higher-energy excitonic level. The data analysis revealed that, in compound 74.2b, a sizable increase in peak TPA cross-section for the lowest excitonic level was observed, whereas for compound 74.2c, the corresponding peak was weakened and shifted to lower energy. The diarylmethylene-bridged azatriangulene 75.2 (Scheme 75), reported in 2009 by Ma et al., possessed an almost planar skeleton and exhibited excellent thermal and morphological stability.381 Phosphorescent OLED devices with 75.2 as the host material and Ir(ppy)3 as the triplet emitter showed a maximum current efficiency of 83.5 cd/A and a maximum power efficiency of 71.4 Im/W for green electrophosphorescence. Extended derivatives 75.4 and 75.5, obtained from 75.2 using triple electrophilic bromination and Suzuki coupling, exhibited excellent thermal stabilities with high glass transition temperatures of 237 and 272 °C, respectively, as well as good solution-processability. 75.4 and 75.5 additionally displayed good hole mobility, combined with efficient hole injection and electron-blocking functions.382 Compound 75.4 showed significantly red-shifted absorption and emission, reflecting the reduced band gap in comparison to 75.5. Double-layer Alq3-emitting OLEDs with 75.5 as the transporter layer showed greatly improved performance relative to standard NPB-based devices prepared by vacuum evaporation. The optimized threelayer Alq3-emitting OLEDs, using 75.5 and NPB as a double hole transport layers, exhibited the maximum current efficiency of 6.83 cd/A.

Scheme 75. Synthesis of Diarylmethylene-Bridged Azatriangulenesa

Reagents and conditions: (a)381 (1) 4-bromotoluene, n-BuLi, −10 °C, (2) overnight, rt, (3) HCl, AcOH, 3 h, reflux; (b)382 (1) NBS, chloroform, 0 °C, (2) 12 h, rt; (c) Pd(PPh3)4, Na2CO3, toluene, water, 36 h, 90 °C; (d)383 Pd(PPh3)4, 2 M Na2CO3, THF, water, 48 h, reflux. a

3537


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Chart 18. Azatriangulene Radicals385

Scheme 76. Synthesis of Trioxytriphenylamine and Its Derivativesa

a Reagents and conditions: (a)386 (1) 2-bromo-3-methyoxyphenol, NaH, dimethylsulfoxide, 1 h, 65−70 °C, (2) 2,6-difluoronitrobenzene, 2.5 h, 130 °C; (b) (1) 2-bromo-3-fluorophenol, NaH, dimethylsulfoxide, 1 h, 65−70 °C, (2) 76.2, 3 h, 130 °C; (c) hydrazine hydrate, Pd/C, p-bromophenol, EtOH, 2 h, reflux; (d) NaO-t-Bu, Pd(dba)2, P(t-Bu)3, toluene, 3 h, reflux; (e) (1) BBr3, DCM, −78 °C to rt; (f) K2CO3, DMF, overnight, rt; (g) tris(p-bromophenyl)aminium hexafluorophosphate, DCM, 30 min, rt; (h)387 thianthrene+ FeCl4−, DCM, MeCN, 30 min, rt; (i) thianthrene+ GaCl4−, DCM, acetonitrile, 30 min, rt; (j)388 pyridinium tribromide, benzene, EtOH, 3 h, reflux; (k) (1) n-BuLi, benzene, diethyl ether, 15 min, rt, (2) (tBuNO)2, overnight, rt; (l) Ag2O, dry DCM, 2 h, rt; (m) tris(4-bromophenyl)aminium hexachloroantimonate, DCM, 30 min, rt.

CF3SO3D at room temperature, gradual deuteration of 77.3 takes place ortho to oxygen bridges (C19.3), indicating that this carbocation can react with positively charged electrophiles.395,396 The pertinent dication, C19.4, was generated by using 100% magic acid (1:1 FSO3H/SbF5) in SO2 at −60 °C. The origin of the stability of trioxatriangulenium cations can be traced to the extensive delocalization of positive charge, which was investigated by Wilbrandt et al. using vibrational spectroscopy and theoretical methods.397 Treatment of carbocation 77.3a with MeLi led to compound C19.5, which was further converted into rigid molecular cavities, C19.6a−b, by Siegel and co-workers.399−401 Related work from Freedman, Luk, et al. focused on the supramolecular properties of chiral bowls, such as C19.7.402 In 1997, Krebs et al.398 obtained the tert-butyl-substituted sesquixanthydryl cation 77.3b, which could be easily reduced using sodium borohydride or methylated with MeLi in the presence of a catalytic amount of copper(I) iodide. For compound 77.4a, a pyroelectric coefficient of 0.5 ± 0.4 μC m−1 K−2 was determined in the temperature interval 24−35 °C. An extensive crystallographic study of 77.3a−b salts with a broad range of anions, reported in 1999 by Krebs and co-workers, revealed planar structures of the cations and anion-dependent packing patterns.403 Photophysical and electron-transfer properties of 77.3a were

nitroxide-substituted neutral triradical 76.11, which was obtained from 76.7 in three steps.388 Because of strong antiferromagnetic exchange coupling, the 76.11 triradical had a doublet ground state, which was changed to a diradical cation triplet 76.12, upon one-electron oxidation of the π-conjugated trioxytriphenylamine skeleton using tris(4-bromophenyl)aminium hexachloroantimonate. “Sesquixanthydrol” 77.2a, first reported in 1964 by Martin and Smith, was originally obtained by heating carbinol 77.1a with pyridine hydrochloride, followed by treatment with base (Scheme 77).389 At pH 9.05, a 50% dissociation of 77.2a into the corresponding cation 77.3 was observed in the borate buffer. By reducing 77.3 with Cr2+, Martin, Gutowsky, et al.390 as well as Feldman and Bowie391 produced the covalent dimer C19.2, which showed no significant dissociation into radicals at room temperature, either in the solid state or in solution. At high temperatures in diglyme and xylene solutions, reversible dissociation into radicals was however observed by Müller and co-workers. 392 77.3 is considerably more stable than nonbridged triarylmethyl cations. For instance, Carey and Tremper were not able to reduce it by hydride transfer from Et3SiH at room temperature,393 although prolonged refluxing in acetic acid did produce the expected sesquixanthene 77.4a.394 Pagni and co-workers showed that, upon treatment with 3538


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Scheme 77. Synthesis of Sesquixanthydrol Derivativesa

a Reagents and conditions: (a)389 (1) pyridine hydrochloride, 1 h, 205 °C, (2) NaOH, H2O; (b)398 (1) HCl, H2O, 4 h, reflux, (2) pyridine hydrochloride, 2.5 h, 130 °C; (c) HPF6 (60% w/w), diethyl ether, 0−30 min; (d) (1) NaBH4, 1,2-dimethoxyethane, 45 min, (2) diethyl ether, 30 min; (e) NaBH4, 1,2-dimethoxyethane, 1 h, reflux; (f) CuI, CH3Li, Ar atm, THF, 1 h, −78 °C; (g) LiI, N-methyl-2-pyrrolidone, 4 h, 170 °C; (h) MeLi, THF, 20 h, rt.

investigated by Gopidas and Dileesh.404 The study revealed that both the singlet and the triplet excited states of 77.3a can accept an electron from donor molecules, leading to the formation of the donor radical cation and the radical C19.1. In aqueous solution, 77.3a was found to photo-oxidize DNA nucleosides such as guanosine or adenosine. Further investigation into the excited-state properties of trioxatriangulenium cations, made by Reynisson et al.,405 revealed a Jahn−Teller distortion in the S1 state, sizable fluorescence quantum yields, and low-temperature phosphorescence. 77.3a was also shown to intercalate and photooxidize duplex and quadruplex DNA.406−408 In 1998, Laursen et al. described a series of aminosubstituted carbenium ions 77.6a−c, obtained by ring closure of the cationic precursors 77.5a−c in the presence of LiI in Nmethyl-2-pyrrolidone.409 With a pKR+ of 19.7, the 77.6a cation is very stable, and its electrochemical reduction was found to lead to rapid dimerization. The X-ray crystal structure of 77.6b revealed a planar geometry of the cation, which formed infinite tilted stacks, segregated with PF6 anions. Compounds 77.3a, 77.4d, and 77.6a were used to form highly ordered adlayers on Au(111) surfaces.410 D3h symmetric C19.8a−d ions, prepared in 2005 by Laursen, Harrit, et al., showed photophysical properties resembling those of rhodamines with respect to band shape, absorption coefficient, fluorescence quantum yield, and excited-state lifetime.411 Distinct blue shifts in their absorption together with red shifts in the fluorescence and reduced emission quantum yields were observed for these systems with the decreasing polarity of the solvent. The observed effects were rationalized in terms of in-solution aggregation effects. The electronic properties of ion pairs of C19.8d with Cl−, BF4−, PF6−, and TRISPHAT anions were subsequently investigated using spectroscopic and theoretical methods.412 The absorption spectrum of free C19.8d contains one degenerate electronic transition in the visible region, which upon ion-pair formation

was split into two bands, whose separation depended on the size of the anion. The splitting of the electronic states in trioxatriangulenium dyes in nonpolar solvents could be fully explained by an internal Stark effect in the formed ion pairs. Laursen, Simonsen, et al. synthesized amphiphilic trioxatriangulenium salts carrying two (C19.9a−b) or four (C19.9c) n-decyl chains and the PF6 anion.413 In Langmuir−Blodgett (LB) films, they self-assembled into columnar aggregates, with the cations standing upright with respect to the water surface. Compounds C19.9a and C19.9b form well-ordered 2D crystalline monolayers with a repeat distance along the columnar aggregates of only ∼3.45 Å. For both compounds, a small tilt of 8−9° for the planar carbenium ions relative to the columnar axis was deduced. Multilayer LB films were prepared from C19.9a−b on lipophilic glass by standard vertical dipping.414 Grazing incidence X-ray diffraction (GIXD) measurements showed that the planar organic cores, despite their positive charge, form closely packed columns with a repeating distance of ∼3.45 Å reported earlier.413 Specular Xray reflectivity (SXR) revealed the LB multilayers to consist of Y-type bilayers with a thickness of 31 Å for C19.9a and 41 Å for C19.9b. GIXD and polarized UV−vis absorption and emission spectroscopy showed that the columnar aggregates in the LB films were oriented along the dipping direction. This alignment was attributed to shear effects during LB transfer. The main absorption band of the LB films was blue-shifted as compared to that in solution, while the fluorescence was redshifted by more than 100 nm. These findings suggested the presence of H-aggregates, in agreement with the cofacial packing derived from the X-ray measurements. Self-assembly of the amphiphilic π-conjugated carbenium ion C19.9a in aqueous solution selectively led to discrete and highly stable nanotubes or nanoribbons and nanorods, depending on the nature of the counterion (Cl− vs PF6−, respectively).415 The nanotubes formed by the Cl− salt in aqueous solution had an exceptionally well-defined (29 ± 2 nm 3539


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Chart 19. Derivatives of Heteratriangulenes with Oxygen Atoms390,395,396,399−401

Scheme 78. Synthesis of Trihydroxytrioxatriangulenea

in outer diameter), unilamellar tubular morphology. Compound C19.9c was aligned by simple spin-casting on substrates with friction-transferred PTFE layers.416 These fluorescent crystalline thin films showed near-perfect macroscopic alignment on centimeter-large areas. C19.9c organizes into a layered structure with columns of alternating C19.9c cations and PF6− anions. The highly anisotropic distribution of the PF6− counterions dictates the orientation and coupling of the optical transitions. By use of a fluorescent dopant, very efficient energy migration was demonstrated to take place in the thin film. In 2010, the synthesis of trioxatriangulenium salts C19.10a− d with one or two dialkylamino donor groups attached to the “resonant” peripheral positions was reported by Sørensen and Laursen (Chart 19).417 The dyes with two amino groups (C19.10c−d) were found to be strong fluorophores with blueshifted emissions relative to rhodamine B. The monosubstituted compounds C19.10a−b were only weakly fluorescent. In 2012, Laursen, Simonsen, et al. reported the formation of uniform and highly stable unilamellar vesicles (ULVs) consisting of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and 5 mol % of C19.9d.418 By extending the original Martin synthesis to the tris(2,4,6trimethoxyphenyl)methylium salt 78.1, Laursen et al. produced the trihydroxytrioxatriangulenium analogue, which was isolated in its neutral keto form 78.2 (Scheme 78).419 Compound 78.2 is slightly soluble in common organic solvents and can be converted into the water-soluble dianion 78.3 by treatment with base. 78.3 possesses a planar D3h symmetry. All protonation states of 78.3 are fluorescent. The species may be viewed as an extended fluorescein analogue with a blueshifted absorption, characterized by comparable extinction coefficient and fluorescence quantum yield. Aromatic nucleophilic substitution of methylenium cations 79.1 and 79.6 was shown by Laursen and Krebs to provide triazatriangulenium cations of the general structure 79.5 (Scheme 79).420 Partially bridged intermediates 79.2−4 could be isolated in the presence of a smaller amount of the amine. (The chemistry of diheterahelicene intermediates such as 79.4 is discussed in section 5.2.) Highly stable triazatriangulenium carbocations, synthesized by Nicolas and Lacour, were characterized by pKR+ values higher than 20 (79.5c−f) and could be employed as phase transfer catalysts.421 Maeda et al.

a

Reagents and conditions:419 (a) pyridinium hydrochloride, 2.5 h, 170 °C; (b) 1 M KOHaq, tetrabutylammonium bromide, 12 h, rt.

reported solid-state charge-by-charge assemblies consisting of planar 79.5b and 79.5g−h cations combined with supramolecular anions containing BF2 complexes of 1,3-dipyrrolyl1,3-propanediones.422,423 Laursen et al. used compound 79.5g to form highly ordered thin films by direct spin-casting onto rotating substrates.424 These homogeneous and crystalline thin films showed macroscopic order over centimeters. The crystal structures of the 79.5·BF4 salts were investigated for the propyl, 3methylpentyl, and octyl derivatives. In all cases, the molecules were unusually packed into hexagonally ordered bilayers, with the rigid discotic 79.5 cores organized coplanarly in sheets separated by perpendicularly oriented alkyl chains. The structure of the thin films was investigated by optical spectroscopy, X-ray reflectometry, and grazing incidence Xray diffraction. It was confirmed that the 15−30 nm thick films maintained the lamellar structure of the bulk crystals, showed good flatness on macroscopic length scales, and were completely ordered relative to the substrate. The triazatriangu3540


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Scheme 79. Triazatriangulenes and Their Mixed-Heteroatom Analoguesa

a

Reagents and conditions: (a)420 methylamine, N-methylpyrrolidone, 20 h, rt; (b) n-propylamine, N-methylpyrrolidone, 45 min, reflux; (c) benzoic acid, methylamine, N-methylpyrrolidone, Ar atm, 9−24 h, reflux; (d)425 NaH, aniline, 20 min, 200 °C; (e) (1) 79.7c, naphthalene-1-boronic acid, toluene, 80 °C, (2) K2CO3, (PPh3)2PdCl2, H2O, 12 h, reflux; (f)426 (1) molten pyridine hydrochloride, pyridine, 1 + 4 h, 200 °C, (2) KOH, water; (g)427 H2SO4.

Chart 20. Mixed-Heteroatom Triangulenium Derivatives428,436

In 2001, Laursen and Krebs synthesized a series of mixedheteroatom triangulenium 79.9, 79.10, and 79.12 (Scheme 79).426 These compounds were prepared from appropriate azabridged precursors using the pyridine hydrochloride method. pKR+ values of 14.5 and 19.4 were determined for compounds 79.9 and 79.10, respectively. In the synthesis of 79.12, spontaneous debenzylation occurred, leading to a neutral product. A structurally related species, 79.15, was obtained in 1980 by Reznichenko, Shapkin, and Popov via stepwise cyclization of the 1,8-disubstituted anthraquinone 79.13.427 Dileesh and Gopidas studied fluorescence quenching and laser flash photolysis of stable azatriangulenium ions 79.10 and C20.1−2 (Chart 20).428 Their results indicated that 79.10 acts as an excited-state electron acceptor, whereas C20.2 is an excited-state electron donor. C20.1 could act as an acceptor or

lenium molecules are electroactive, and showed intense fluorescence and efficient exciton transport in the films. N,N′,N″-Triaryl-substituted triazatriangulenium dyes were reported in 2012 by the Laursen group (79.7−8).425 The synthesis of 79.7 was achieved by heating the tris(2,6dimethoxyphenyl)methylium ion in various anilines in the presence of NaH. Aryl derivative 79.7c was further functionalized using Suzuki−Miyaura coupling, leading to 79.8. Singlecrystal structures, obtained for 79.7a and 79.7c, showed torsional angles larger than 80° between the cation core and the aryl substituents. The 79.7 dyes emitted in the red region of the spectrum (λfl = 560 nm) and showed a surprising 3-fold increase in fluorescence quantum yields (>50%) when compared to the alkyl-substituted salts. 3541


Chemical Reviews

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donor in photoinduced electron transfer (PET) reactions. The research revealed that the replacement of oxygen atoms by Nalkyl moieties leads to a gradual shift of the photoinduced electron transfer behavior from an acceptor to that of a donor. The photophysics of azaoxatriangulenium fluorophores were further characterized by Laursen et al.429 Their work showed that the optical properties of these triangulenium dyes were solvent-independent in both absorption and emission but were affected by the symmetry of the chromophore. The C2v symmetric dyes, azadioxatriangulenium 79.10 and diazaoxatriangulenium C20.1, have high emission anisotropies, fluorescence lifetimes around 20 ns, and fluorescence quantum yields of ∼50%. The trioxatriangulenium (77.3a) and triazatriangulenium (C20.2) dyes, nominally of D3h symmetry, have fluorescence lifetimes around 10 ns and fluorescence quantum yields of 10−15%. A amino-substituted azadioxatriangulenium ion C20.4 was shown to have very high stability, with a reported pKR+ value close to 25.430 This ion showed an intense absorption at 458 nm and a high fluorescence quantum yield. The chloride analogue of 79.10, C20.6a, exhibited an enhancement of fluorescence emisson induced by selfassembled silver nanoparticles on a gold film.431,432 In another application, the use of the substituted azadioxatriangulenium C20.6b in combination with time-correlated single-photon counting detection was used to eliminate autofluorescence background from fluorescence tissue images.433−435 By methylation of an unsymmetrically N-substituted diazaoxatriangulenium cation, Lacour et al. produced a chiral cup-shaped “diazaoxatricornan” derivative C20.3, which was isolated in the racemic form.436 The enantiomers of C20.3 were chromatographically separated on a chiral stationary phase, and the absolute configuration of (−)-(S)-C20.3 was determined by comparison of the experimental and theoretical vibrational circular dichroism (VCD) spectra. The isolation of (−)-(S)C20.3 and (+)-(R)-C20.3 constituted the first report of a nonracemic closed-capped chiral bowl molecule for which the chirality was due to the intrinsic dissymmetry of the central core of the structure only. Further work by Krebs on functionalized azadioxatriangulenium systems such as C20.5a437 enabled the synthesis of large amphiphiles C20.5c (MW ≈ 3000).438 These molecules consisted of a polar cationic chromophore (A-domain), which serves to organize the assembly at the surface, and a hydrophobic part, designed as a light-harvesting antenna (Jdomain). By compression of the molecules at the water−air interface in a Langmuir trough, regular pressure−area isotherms were obtained. The monolayers were transferred in the condensed state to a conductive substrate yielding closely packed Langmuir−Blodgett (LB) films with a thickness that could be varied between 4 and 8 nm depending on the molecular size. It was found that the lifetime and quantum yield for the light harvesting and energy transfer increased significantly in the JA-assembly as compared to mixtures of separate A- and J-domains. The emission from the A-domain was increased 20 times when covalently connected to the lightharvesting J-domain and a further 2−3 times when going from solution to the solid LB-film. Magnussen, Herges, et al. proposed a versatile concept for the preparation of well-defined functional adsorbate layers on metal surfaces using triazatriangulene units as pedestals to provide perpendicular orientation of the attached functions relative to the surface (Figure 3).439−442 To prove this concept, diversely substituted triazatriangulenes of the general structure

Figure 3. The platform approach to the construction of functional monolayers. Adapted from ref 439. Copyright 2009 American Chemical Society.

C21.2 were synthesized by nucleophilic attack at the central C atom of triazatriangulenium ions C21.1a−d (Chart 21). As Chart 21. Triazatriangulenes for Application in Photoswitchable Self-Assembled Monolayers439−442

shown for molecules with side chains of different lengths or substituents, this approach allowed the preparation of very stable, hexagonally ordered adlayers on Au(111). The intermolecular spacings in these adlayers were independent of the attached functions with the latter being oriented perpendicular to the Au surface. Several sulfur-bridged heterocyclic carbocations of the acridinium and triangulenium family were prepared by Lacour et al., who used 80.4 tetrafluoroborate as the key synthetic intermediate443 (Scheme 80, for an earlier report of a related structure, 80.9, see ref 444). Using simple aromatic nucleophilic substitution reactions, salts 80.5−7 were obtained in moderate to good yields. From the X-ray structural analysis of 80.3, bond lengths between sulfur and carbon atoms were confirmed to be much longer than those of C−O or C−N bonds in analogous frameworks. These larger bond lengths caused a distortion of the sulfur-containing six-membered ring, which was invoked to explain the relative lack of stabilization of the resulting carbenium ions. A comparison between electronic absorption spectra of the sulfur-bridged dyes and their aza analogues showed a ca. 50 nm shift toward the low-energy region upon introduction of the sulfur bridge. 3542


Chemical Reviews

Review

Scheme 80. Sulfur-Bridged Triangulenesa

a Reagents and conditions: (a)443 (1) lithium cyclohexyl(isopropyl)amide, THF, 10 min, −20 °C, (2) 1-bromo-2-methoxybenzene, 20 h, 25 °C; (b) (1) 1,3-dimethoxybenzene, n-BuLi, TMEDA, toluene, 1.5 h, 0 °C, (2) 24 h, rt; (c) HBF4, EtOH, 60 min, rt; (d) aniline, benzoic acid, 20 h, 110 °C; (e) aniline (neat), 22 h, 190 °C; (f) pyridine hydrochloride, 4 h, 200 °C; (g) MeLi, THF, 4 h, 0−25 °C.

Scheme 81. Synthesis of Phosphatriangulenesa

Reagents and conditions: (a)445 95% t-BuOK, N-methyl-2-pyrrolidone, 1 h, 200 °C; (b) 35% H2O2, chloroform, overnight, rt; (c)448 30% H2O2, chloroform, 15 h, rt; (d) CH3I, Cs2CO3, DMF, 15 h, rt; (e) Fe powder, Br2, DCM, 22 h, rt; (f) BBr3, DCM, 15 h, rt; (g) t-BuOK, DMF, 23 h, 140 °C; (h) CuI, Pd(PPh3)4, phenylacetylene, THF, Et3N, 62 h, 60 °C. a

column. Compound 81.9 possesses a chiral concave structure and exhibits chiroptical properties with a large anisotropy. In the cocrystal with C60, four molecules of the enantiopure 81.9 perfectly wrapped the surface of C60. The analysis of the induced CD signal of a guest fullerene derivative indicated the transfer of the chiroptical properties of 81.9 to the guest. A boron-containing triangulene 82.5, which can be viewed as a planarized triarylborane derivative, was synthesized in 2012 by Wakamiya, Yamaguchi, et al. (Scheme 82).449 Their synthesis started from the dihydroborinine derivative 82.1, and involved Friedel−Crafts cyclization as the key bond-forming step. An extension of the above strategy led to the diboracoronene derivative, 9,10-diphenyl-9,10-dihydro-9,10-diboraanthracene 82.8. These compounds were stable toward air, water, and amines, despite the absence of steric protection of the B atoms, and showed characteristic structural, electronic, and photophysical properties. In addition, upon treatment with the fluoride ion, these compounds underwent a plane-to-bowl conversion in a controlled manner. In subsequent work, a radical anion of 82.5 was generated by reduction with potassium, and characterized structurally and spectroscopically.450 It was found that the delocalization of spin density in

Phosphatriangulene 81.2 was obtained in 1997 by Krebs and co-workers, by heating triarylphosphine 81.1 with tBuOK.398,445 81.2 was oxidized to the respective P-oxide, 81.3, using hydrogen peroxide in chloroform. 81.2 showed permanent polarization in the solid state, and its pyroelectric coefficient was found to be −3 ± 2 μC m−2 K−1. A molecular dipole moment of 4.7 D was determined for compound 81.2 in the solid state, corresponding to a 42% dipole moment enhancement as compared to the dipole moment measured in a chloroform solution.446 Atomic-resolution images of 81.2 adsorbed on Ag(111) revealed three distinct binding configurations, corresponding to different orientations of the nonplanar triangulene structure relative to the substrate surface.447 An acetylene-substituted derivative 81.8 was reported in 2014 by Yamamura, Saito, and Nabeshima.448 In their approach, 81.1 was initially converted into the respective P-oxide and methylated, to permit regioselective bromination of the aryl rings. The tribromo derivative 81.6 was then deprotected and cyclized, to yield the prefunctionalized triangulene 81.8. In the final Sonogashira coupling step, compound 81.9 was obtained in 21% yield as a racemic mixture, and separated into enantiomers on a chiral HPLC 3543


Chemical Reviews

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Scheme 82. Synthesis of Boron-Doped Pyrenoids and Related Systemsa

Reagents and conditions: (a)449 (1) 2-bromo-1,3-di(propen-2-yl)benzene, n-BuLi, toluene, 0 °C, (2) 6 h, rt, (3) 82.1 or 82.6, 20 h, rt; (b) Sc(OTf)3, 1,2-dichloroethane, 24 h, reflux; (c) CrO3, AcOH, 1 h, reflux; (d) (1) TiCl4, Me2Zn, DCM, 30 min, −25 °C, (2) 82.4, 12 h, −25 °C; (e)451 (1) nBuLi, ether, 40 min, −78 °C, (2) 82.10, ether, rt; (f) 1,1′-azobis(cyclohexanecarbonitrile), tris(trimethylsilyl)silane, toluene, 15 h, 110 °C; (g) (1) FeCl3, CH3NO2, DCM, 0 °C; (h) (1) n-BuLi, ether, 45 min, −78 °C, (2) BF3·Et2O, (3) rt; (i) n-Bu3SnH, 1,1′azobis(cyclohexanecarbonitrile), toluene, 15 h, 120 °C. a

Scheme 83. Synthesis of Flavanthrone and Its Derivativesa

Reagents and conditions: (a)454 Cu, naphthalene, 45 min, 220−240 °C; (b) H2SO4, 50 °C; (c)456 30% NH3, CuSO4, 40 h, 230−240 °C; (d)457 molten KOH, 20 min, 295−305 °C (65%); (e)458 SbCl5, nitrobenzene, 1 h, reflux; (f)459 (1) H2SO4 (conc.), (2) H2O, (3) 2% lye, Na2S2O4, without air, 1 h, 75 °C, (4) caustic soda, air, 70 °C; (g)460 zinc dust, ZnCl2, NaCl, 10 min, 280−290 °C; (h) (1) zinc dust, pyridine, o-dichlorobenzene, 10 min, reflux, (2) benzoyl chloride, 30 min, reflux; (i) (1) 4% NaOH, 2% Na2S2O4, 20 min, 20−75 °C, (2) benzoyl chloride, 2 h, 20−75 °C; (j) (1) triisopropylsilylacetylene, dry THF, n-BuLi, 1 h, 0 °C, (2) 83.3a, overnight, rt, (3) NaI, NaH2PO2·H2O, AcOH, 4 h, reflux. a

with the electron-accepting boron center, yielding broad absorption bands over the entire visible region and a fluorescence in the visible/near-IR region. In addition, 82.11 showed a dramatic change of its properties upon formation of a tetracoordinated borate, leading, for instance, to a thermochromic behavior in the presence of pyridine. Compound 82.9 was also used for the preparation of a symmetrical precursor 82.14, which yielded a very unusual nonbenzenoid framework 82.15

this radical was more extensive than in its nonfused analogues, and that it adopted a bowl-shaped, rather than planar, conformation. A related pyrenoid molecule 82.13, in which the tricoordinated boron atom was embedded in a fully fused polycyclic π-conjugated skeleton, was prepared from the triaryl precursor 82.11 in a sequence of reductive and oxidative C−C couplings.451 This compound, which is as stable as 82.5 and 82.8, combines an electron-donating π-conjugated skeleton 3544


Chemical Reviews

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Chart 22. Selected Azapyrenoid Pigmentsa

when subjected to a radical-promoted intramolecular homocoupling. 4.2. Azapyrenoids

Flavanthrone 83.3a, a common yellow vat dye, known under a variety of commercial names, was first obtained by Bohn in 1901.452 The original procedure, which involved fusion of 2aminoanthraquinone with alkali, produced flavanthrone at temperatures of 300−350 °C, whereas the blue indanthrone was formed at 220−250 °C.1 Early industrial syntheses, which used strong Lewis acids, such as SbCl5 or TiCl4, for promoting the condensation, were replaced with a procedure involving 1chloro-2-aminoanthraquinone and phthalic anhydride. The extended fused-ring structure of 83.3a was first established by Scholl in 19073 and confirmed by a later X-ray crystallographic analysis.453 In their ultimate synthetic proof, Scholl and Dischendorfer prepared flavanthrone in two steps, by first converting 1-chloro-2-benzylidenaminoanthraquinone into the dianthraquinonyl derivative 83.2a, and dehydration of the latter intermediate to 83.3a.454 A systematic analysis of the dehydration reaction was carried out by Kratochvı ́l, Slavı ́k, and Nepraš.455 A range of substituted flavanthrones was subsequently prepared including the dibromo,456 di-tertbutyl,457 and dimethyl458 derivatives 83.3b−d. A different route to flavanthrones involves intramolecular Friedel−Crafts acylation of diazapyrene intermediates such as 83.5.459 Zinc reductions of flavanthrone, reported in 1968 by Aoki, furnished two leuco-type derivatives 83.4a−b.460 A TIPSethynyl leuco-derivative, 83.4c, was reported in 2013 by Briseno and co-workers.461 The latter compound was prepared by lithium acetylide addition to 83.3a followed by reductive aromatization with NaI/NaH2PO4. Along with related undoped PAH derivatives, compound 83.4c was tested as a semiconductor material in single-crystal FET devices, yielding the hole mobility of up to 0.14 cm2 V−1 s−1. “Dinylin” 84.2, a smaller fused system related to leucoflavanthrone derivatives, was reported in 1940 by Rieche, Rudolph, and Seifert (Scheme 84).462 Their synthesis involved

a

Azapyrene substructures are indicated in red.

lesser-known dyestuffs, see ref 464). One of the directions taken in the dyestuff research focused on increasing the number of heteroatom sites, leading to systems such as C22.4465 or C22.5.466 In the 1970s, Buu-Hoı̈ et al.467 and Jacquignon and coworkers468,469 synthesized a series of benzo- and naphtho-fused 2-azapyrenoids 85.2, 85.4, and 85.6 (Scheme 85). All compounds were obtained using a common strategy, by baseinduced thermal cyclization of appropriate 9-(2-chlorophenyl)acridine precursors. An isomer of 85.2a, with a different benzofusion pattern, was reported by Arbuzov and Grechkin.470 Cationic pyrenoids with the nitrogen embedded inside the fused core were reported in 1979 by Katritzky and co-workers (Scheme 86).471 A range of derivatives, including the monoaza and diaza systems 86.2a and 86.4, were prepared by oxidative photocyclization of the corresponding 1,2,6-triarylpyridinium cations.472 The planarity of the fused core in these systems was established on the basis of an X-ray structural analysis performed for 86.4a.471 By using the same synthetic strategy, other systems were subsequently obtained, such as 86.5472 or the dithieno derivative 86.6.473 The cationic benzo[1,2]quinolizino[3,4,5,6-def ]phenanthridin-15-ium motif, present in 86.2, was reported in 2007 by Zhi, Müllen, et al. to provide a controllable self-assembly into nanofibers and nanotubes.474 By drop-casting methanolic solutions of the chloride salts 86.2b and 86.2d onto a silicon wafer, it was possible to generate fibers with a cylindrical and ribbon-like shape, respectively. WAXS data revealed a different mode of packing in the two fiber types, with the ribbon fibers of 86.2d adopting a lamellar structure. The morphology of the ribbons could be further modified by changing the counteranion, and the tetrafluoroborate salt 86.2e yielded nanotube-like aggregates constructed by lamellar packing. In contrast, the effect of counteranion on fiber packing in 86.2c was insignificant. In subsequent work, the Müllen group explored the self-organization of cations in the series 86.2f−j as a function of the counteranion.475 Columnar mesophases were observed for 86.2i−j, with a staggered arrangement of consecutive cation molecules in the stacks, analogous to that observed in the crystal for 86.2f. Interestingly, the regioselectivity of photocyclization in hexaaryl-substituted pyridinium cation 86.7 is altered, and results in fusion of meta and para substituents, as shown in 2010 by Lainé, Campagna, et al.476 The resulting π-expanded

Scheme 84. Synthesis of “Dinylin”a

Reagents and conditions: (a)462 H2SO4 (conc.), H2O, 5 h, 130 °C; (b) aniline, 2 h, reflux.

a

heating bisacetamino derivative 84.1 with concentrated sulfuric acid. Refluxing 84.2 with aniline was proposed to result in the formation of the substituted product 84.3. Pyranthridone C22.1, synthesized by Scholl and Dischendorfer in 1918, as the intermediate member of the pyranthrone−flavanthrone series, is an early example of an extensively fused 4-azapyrene derivative (Chart 22).463 The anthra[2,1,9-mna]acridin-14-one motif, which contains the 1-azapyrene substructure, is found in a range of industrial vat dyes, exemplified by two indanthrene pigments, Olive Green B C22.2 and Olive T C22.3. The former of these dyes is synthesized by condensing 3bromobenzanthrone with 1-aminoanthraquinone1 (for a discussion of base-induced oxidations of this type and some 3545


Chemical Reviews

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Scheme 85. 2-Azapyrenoidsa

a

Reagents and conditions: (a)467−469 NaOH, benzo[h]quinoline, 4 h, 350−360 °C.

Scheme 86. Photobiscyclization of 1,2,6-Triarylpyridinium Cationsa

a

Reagents and conditions: (a)472 hν (300 W), MeOH, 24 h, rt; (b)472 hν (300−350 W), MeOH, 15−24 h, rt; (c)476 UV irradiation, MeOH/DCM, O2 gas; (d)477 (1) reactant as BF4 salt, MeOTf, MeCN, rt, (2) KPF6, H2O.

cation 86.8 had a saddle-like distortion caused by orthosubstituents, but nevertheless showed π-stacking dimerization in the solid state. The fusion caused a moderate red shift of fluorescence (λmaxem = 455 and 479 nm in 86.7 and 86.8, respectively), accompanied by a reduction of quantum yield (from 0.48 to 0.13). This result was in opposition to the effect of fusion in 86.2f, which showed a blue-shifted and considerably stronger luminescence in comparison with 86.1f. Subsequent work from the Lainé group showed that, in contrast to N-pyridine-substituted pyridinium cations 86.3, the Nmethylated derivative 86.9 was photostable and did not undergo oxidative cyclization to 86.10.477 The latter species could however be obtained by direct methylation of 86.4a. This dicationic product was characterized by two reversible oneelectron reductions, and a red-shifted electronic spectrum with a charge-transfer character. In 1940, Waldmann and Hindenburg described the synthesis of two N,N-dihydro-3,10-diazapyrene derivatives C23.1−2 (Chart 23).478 Both compounds were prepared via a condensation reaction between 7,10-dichlorotetracene-5,12dione and benzotriazole or naphthotriazole, respectively, followed by a double Graebe−Ullmann cyclization step. C23.4 was described in 1971 by Tsuchiya et al. as one of the products obtainable in the photocyclization of tetraphenylpyr-

Chart 23. Diazapyrene Derivatives

idazine 57.3 (Scheme 57, section 3.3).299 The yield of C23.4 was maximized (up to 60%) by performing the irradiation in methanol. The structure of 1,6-diazapyrene derivative C23.3 was determined crystallographically by Ammon and Reid.479 This compound was obtained in low yield from a fairly unusual 3546


Chemical Reviews

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Scheme 87. Synthethic Routes to Azapyrenesa

a Reagents and conditions: (a)164 copper bronze, diethyl phthalate, dioxane, 5−7 min, reflux; (b)481 benzylamine, pyridine, 1.5 h, reflux; (c)480 85% N2H4 hydrate, pyridine, 1 h, reflux; (d) Al, cyclohexanol, overnight, reflux; (e)482 (1) KOH, MeOH, 16 h, reflux, (2) K2CO3, 0.5 h, rt, (3) HCl, (4) P4O10, trichlorobenzene, 4 h, reflux; (f) benzaldehyde, 10% KOH in MeOH, pyridine, 1 h 15 min, reflux; (g) Pd/C; (h)483 250 °C; (i) 5% Pd/C, xylene; (j)485 CuI, dioxane, microwave irradiation (300 W), 1 h, 150 °C; (k) Pd(OAc)2, PPh3, K3PO4, DMF, 4 h, 130 °C; (l)484 NH4OAc, AcOH, 15 min, reflux; (m) N2H4·H2O, 2-propanol, AcOH, 2−4 h, reflux.

Scheme 88. Synthesis of 1,8-Diazapyrenesa

a

Reagents and conditions: (a)486 PPA, 3−24 h, 175−180 °C; (b) aniline, reflux.

dimerization reaction of 2-(4-methylphenyl)quinoline induced with phenyllithium. Nonbonded interactions in the fjord regions of C23.3 are apparently responsible for the substantial

out-of-plane distortions of the ring system, observed in the solid state. 3547


Chemical Reviews

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Scheme 89. Diheterapyrenoids from Anthraquinonesa

The thiazepine method, described in 1958 by Galt, Loudon, and Sloan, provided, in addition to the perylenoid derivative 36.2 (section 3.1), also the isomeric diazapyrenoid 87.2 (Scheme 87).164 The requisite bisthiazepine 87.1 was obtained from 1-chloro-4-nitroanthraquinone in a reaction with sodium o-aminophenyl sulfide, and subsequently heated with copper bronze to yield the product. Starting from pentaphene5,8,13,14-tetraone 87.4, Tilak, Unni, and Venkataraman synthesized the pyridazine fused system 87.6 as a potential carcinogenic agent.480 87.4 was refluxed with hydrazine hydrate, and the resulting quinone 87.5 was subjected to reduction with aluminum tricyclohexoxide, which produced the desired, lemon-yellow 87.6. A monoaza analogue, 87.3, was synthesized by the Scholl group in 1936, by heating 1,2phthaloylanthraquinone 87.4 with benzylamine in boiling pyridine (Scheme 87).481 Diketodiazapyrene derivatives 87.9−10, reported in 1962 by VanAllan, Reynolds, and Adel,482 were obtained by double electrophilic cyclization of the phenazine derivative 87.7, followed by a reaction with benzaldehyde, and the optional catalytic debenzylation. A structurally similar system, 87.13, was synthesized by Kappe, Linnau, and Stadlbauer via condensation of 1,2,3,4-tetrahydrophenazine 87.11 with a substituted malonate, followed by dehydrogenation.483 Interestingly, an analogous condensation performed on phenazine itself yielded a diazaperylene derivative 33.14176 (section 3.1), isomeric to 87.13. Di- and triaza- systems 87.18a−b were obtained by Deady and Smith by condensing the respective tetracyclic precursors 87.17a−b with ammonium acetate.484 Analogous condensations with hydrazine were found to yield the corresponding diazepines 87.19a−b. Highly fused perimidines, such as 87.16, were prepared by Fujii, Ohno, and co-workers in 2011.485 The reported two-step procedure consisted of a copper-catalyzed double annulation of 87.14 with 1,8-diaminonaphthalene, combining aldehyde condensation with hydroamination of the ortho-ethynyl substituent, followed by a palladium-catalyzed C−H arylation/dehydrogenation. Acid-catalyzed cyclizations of anthraquinone and anthrapyridone derivatives, reported by Kazankov, Bernadskii, and Mustafina, yielded benzannulated 1,8-diazapyrenes shown in Scheme 88.486 Specifically, compound 88.8 was prepared in two steps, by refluxing 88.6 in polyphosphoric acid and subsequent heating of the intermediate 88.7 in the presence of aniline. The overall yield of the latter sequence was improved to 64% when the order of steps was reversed. Compounds 88.3 and 88.5a−d exhibited intense fluorescence, in contrast to 88.8, which was an appreciable fluorescence quencher. In 1997, Tokita and co-workers used AlCl3-induced cyclizations of substituted anthraquinones to synthesize two diheterapyrenes 89.2 and 89.4 (Scheme 89) and a diazaperylenoid (32.8, section 3.1).148 89.2 and 89.4 exhibited absorption spectra similar to those of their dioxa and dithia analogues. The original low yields of these reactions were improved in subsequent work by Shiroishi, Kaneko, et al.487 It was also found that the endoperoxide 89.5 could be obtained by UV irradiation of 89.4 in the presence of air. Upon addition of trifluoroacetic acid, compound 89.4 underwent stepwise protonation to the mono- and dicationic forms. Gradual elimination of dioxygen from 89.5 was observed upon protonation. 89.2 is an example of a mixed-heteroatom heterapyrene. Other fused frameworks of this type include 90.2, reported in

a

Reagents and conditions: (a)148 hydroquinone, AlCl3, NaCl, 3 h, 150 °C; (b)487 hydroquinone, NaCl, AlCl3, 2 h, 180 °C; (c) hν (300 W tungsten lamp), air, toluene, 40 min; (d) TFA, 5 h 20 min.

1958 by Barrett and Buu-Hoı̈,488 and the sulfur-containing anthrone derivatives 90.5a−d obtained by Carlini et al. in 1982 (Scheme 90).489 The latter species were derived from Scheme 90. Synthetic Routes to Diheterapyrenoidsa

a Reagents and conditions: (a)488 zinc dust, distilled; (b)489 Na2CO3, DMF, 2 h, reflux; (c) (1) DMF, 30 min, 80 °C, (2) 36% HCl, NaNO2, H2O, 2 h, 0−5 °C, (3) CuSO4, 25 °C, (4) 3 h, 110 °C.

benzanthrones and 1-azabenzanthrones 90.3, which were functionalized via nucleophilic substitution with o-aminothiophenols, converted to diazonium salts, and cyclized using Pschorr reaction conditions. Bathochromic shifts were observed in the absorption spectra of 90.5a−d upon the introduction of nitrogen atoms and methoxy substituent. 1,4,5,8-Naphthalenetetracarboxylic dianhydride 91.1 is a potentially versatile precursor to heteroatom-containing pyrene derivatives (Scheme 91). Two isomeric condensation products of 91.1 with o-phenylenediamine, “perinones” 91.2a−b, 3548


Chemical Reviews

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pyrazole cyclization strategy by Peters and Saidi provided fused xanthene and thioxanthene systems 91.7−8.492 A benzo-fused 1,3b,4-triazapyrene 92.2, reported in 1964 by Partridge, Slorach, and Vipond, was obtained by heating the perimidine derivative 92.1a with sodium aluminum chloride (Scheme 92).493 In 1996, Dutt and Rao used a similar strategy for obtaining the hexacyclic analogue 92.4.494 The latter species was synthesized from 92.1a, which was cyclized with trifluoroacetyl ketene diethylacetal to yield 92.3. The final benzannulation could be achieved by heating 92.3 in diphenyl ether. Alternatively, both cyclizations could be performed in a single step, providing an improved overall yield. Several triazapyrenoids were prepared by Kazankov and Bernadskii in their work on pyridonoanthrapyrimidines (Scheme 93).156 93.1 was cyclized with aniline to form 93.2, which was in turn acetylated to yield the oxazole 93.4. The interaction of 93.4 with dimethylamine led to the amidine 93.5, which was reverted to the oxazole upon heating. Further examples of pyrenoid frameworks containing multiple nitrogen atoms include 93.8, obtained by the same authors using a twostep pyrimidine cyclization, a heptacyclic structure 93.9 reported by Ursenau and Teszler,495 and a tetraaza system 93.9 of Dutt and Rao.496 “Diagonal” π-expansion of naphthalenediimides (NDIs)50 is a versatile concept for elaboration of extended pyrenoid frameworks. In 2011, Luscombe et al. used a Stille-coupling polycondensation to create a functionalized NDI−phenylene polymer 94.1.497 Upon acid-catalyzed deprotection, 94.1 was converted into the nitrogen-rich, ladder-type polymer 94.2. This material was used for the fabrication of n-channel OFETs, providing electron mobilities up to 0.0026 cm2 V−1 s−1, corresponding to an improvement of 3 orders of magnitude in comparison with 94.1. The group of Marder used diacyl-NDIs 94.4a−b, synthesized via Stille acylation of the 94.3, to prepare the bis-pyridazino-fused NDI analogues 94.5a−b.498 Fusion of pyridine rings, achieved by means of a double TFSIH-induced cyclization, was reported in 2013 by Zhang, Zhang, et al.499 (for related cyclizations, see compounds 36.14 and 37.9 in section 3.1). Starting with diethynyl NDIs 94.6a−g, the authors initially obtained the bis-pyrylium salts 94.7a−g, which were directly converted into the fused products 94.8a−g. Derivatives 94.8b−f, containing no ferrocenyl appendages, were fluorescent in solution and in the solid state, with solution λmaxem ranging from 476 nm (94.8f, ΦF > 0.99) to 637 nm (94.8b, ΦF > 0.06). Thin films of 94.8c showed a p-type semiconductor behavior with hole mobilities reaching 0.0063 cm2 V−1 s−1. Polymers based on the 94.8 core were subsequently explored as OTFT materials by the same group.500

Scheme 91. 2,7-Diazapyrenoids Derived from 1,4,5,8Naphthalenetetracarboxylic Dianhydridea

a

Reagents and conditions: (a)491 o-phenylenediamine, glacial CH3COOH, overnight, reflux; (b) CH3COONa (anhydrous), butan2-ol, overnight, reflux; (c)492 DMF, KOAc, reflux.

separable by fractional precipitation, are valuable industrial pigments.350 They show a marked color difference, which is partly due to excitonic interactions between stacked molecules in the crystal.490 The use of 91.2a−b as high-performance polymerization photoinitiators was explored by Gigmes, Lalevée, and co-workers.491 In the latter work, a related pair of isomers 91.4a−b was reported, obtainable by the cyclization of a bis(acetylacetone hydrazone), 91.3. An earlier use of the Scheme 92. Synthetic Routes to Triazapyrenoidsa

Reagents and conditions: (a)493 sodium aluminum chloride, 1 h, 320 °C; (b)494 1,1,1-trifluoroacetyl ketene diethylacetal, toluene, 6 h, reflux; (c) diphenyl ether, 3 h, reflux; (d) 1,1,1-trifluoroacetyl ketene diethylacetal, diphenyl ether, 6 h, reflux. a

3549


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Scheme 93. Synthesis of Triazapyrenesa

Reagents and conditions: (a)156 aniline, 100 °C, 2 h; (b) acetic anhydride, AcOH, H2SO4, 20 min, reflux; (c) acetic anhydride, AcOH, H2SO4, 1 h, reflux; (d) 50% aq dioxane, 30% HCl, 20 min, reflux; (e) NHMe2, dioxane, 1.5 h, 20 °C; (f) heating in a capillary; (g) POCl3, DMF, 50 °C, 2 h; (h) NH4OAc, MeOH, 20 °C, 30 min.

a

Scheme 94. NDI-Derived Pyrenoidsa

see Scheme 45, section 3.1). In the initial work of Dewar and co-workers on BN-embedded PAHs, published in 1959−1960, two nonfused BN-embedded pyrenes were obtained, 95.8501 and 95.9.502 The interest in BN aromatics has been revived recently; however, the majority of available BN pyrenoids still lack extended ring fusion. In 2007, 3a1-aza-5a1-borapyrenes 95.4a−e were obtained by Piers and co-workers, by condensing boracyclohexadienes 95.2 with bis(ortho-alkynyl)-substituted pyridines 95.1.503 The products were obtained in two steps, with or without isolation of the intermediate 95.3, the final cyclization of the acetylene substituent being catalyzed with PtCl2. 95.4a−e had an orange-brown color and featured higher electron affinities and smaller band gaps than pyrene. The individual aromaticity of the pyridine and borabenzene fragments, assessed using NICS(1) values, was greater than in the flanking C4BN rings. The presence of internal BN units imparts a dipolar character to these molecules, leading to a head-to-tail π-stacking pattern in the solid state. In subsequent work, dimeric analogues of 95.4b were synthesized by the Piers group.504 A tandem intramolecular electrophilic arene borylation reaction was developed by Hatakeyama for the synthesis of BN-fused PAH analogues, such as 95.6 and 95.7.505 To obtain 95.6, the authors prepared the diamine 95.5, which was Nborylated with BCl3 and subjected to an intramolecular electrophilic aromatic borylation. The latter step required extensive optimization, which revealed that the right combination of Lewis acid and Brønsted base was necessary for effective cyclization. Both 95.6 and 95.7 adopt a twisted conformation, which results in a tight and offset face-to-face stacking array in the solid state. Because of its flexible molecular framework and dipole moment, 95.6 is moderately soluble in chlorinated organic solvents. Time-resolved microwave conductivity measurements confirmed that the substitution of the C−C units in 95.7 with isoelectronic B−N units dramatically enhanced hole mobility, due to the strong electronic coupling between neighboring molecules in the solid state. The results of CV experiments indicate that 95.6 showed an irreversible oxidation wave and a reversible reduction wave with peak potentials at +0.10 and −1.57 V, respectively (vs Fc/Fc+).

a

Reagents and conditions: (a)497 TFA, DCM, anisole, reflux, overnight, TEA, acetone; (b)498 CuI, Pd(PPh3)2Cl2, toluene, 100 °C; (c)498 hydrazine monohydrate, EtOH, 100 °C, pressure tube, 2 d; (d)499 TFSIH, DCM, 30 min; (e)499 MeOH, NH3(aq), reflux, 1 h.

4.2.1. Boron-Containing Azapyrenoids. B−N heterapyrene systems are of interest as potential organic semiconductors and fluorescent materials (Scheme 95, for related perylenoids, 3550


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Scheme 95. Synthesis of BN-Embedded Pyrenesa

both as a synthetic and as a fabrication method, resulted in a stepwise generation of the fully conjugated BN pyrenes 95.12a−b. The resulting devices showed only modest brightness and current efficiencies as a result of the relatively low photoluminescence of the BN pyrenes. 4.3. Oxa- and Thiapyrenoids

Benzo[c]phenanthro[2,1,10,9-klmn]xanthene 96.5a, containing the 1-oxapyrene substructure, was prepared by Clar and Kelly in 1957 starting from chrysene (Scheme 96).507 The most straightforward although unselective route involved the Elbs reaction performed on the acylation products obtained from chrysene and o-toluoyl chloride. In an alternative stepwise synthesis, proposed as a structural proof, dibenzo[a,f ]tetraphene 96.4 was assembled in three steps and oxidized to 96.5a by heating in the presence of air. In a reinvestigation of the Elbs route, Mabille and Buu-Hoı̈ additionally obtained the methyl derivative 96.5b, and suggested anthrols 96.7a−b as intermediates en route to 96.5a−b.508 Structurally related systems, 96.9 and 96.10, were proposed by Zander as fusion products of 96.8 on the basis of mass spectrometric analysis.509 A dithiapyrene system 96.11 was reported by Brauer and coworkers, without providing synthetic details.182 Self-sensitized photooxygenation of 96.11 in oxygen-saturated toluene, carried out in the presence of sunlight, yielded the endoperoxide 96.12 with a quantum yield of 0.02. Irradiation of 96.12 at 248 nm led to photocycloreversion accompanied by an unspecified rearrangement process. Quinone system 97.2, named cis-coerodioxonone by Cook and Waddington, was obtained by condensing compound 97.1 in sulfuric acid.146 97.4 was prepared by Cooke and Dagley as a model of the chromophore found in plant pigments isolated from members of the Hemodoraceae family.510 The nucleophilic cyclization of the dimethoxy precursor 97.3 proved to be more efficient than the oxidative coupling of 97.5. Interestingly, a similar chromophore can be found in the commercial fluorescent dye, Solvent Orange 63 97.7, containing the benzothioxanthene motif. The dye can be prepared by a thermal cyclization of a diazonium salt derived from the benzanthrone precursor 97.6.511 Xylindein (“wood-derived indigo-like dye”, 98.1, Scheme 98) is a natural blue-green pigment found in wood infected by the fungus Chlorociboria aeruginosa (Figure 4). This pigment was isolated in the 1860s,512,513 but its structure was only fully established in 2000, when Hashimoto and co-workers assigned the absolute configuration of the substituted derivative 98.2 using X-ray diffraction.514 While the total synthesis of 98.1 has still to be accomplished, a number of derivatives containing the xylindein chromophore have been reported. In particular, the chiral analogue 98.5, described in 1966 by Blackburn, Cameron, and Chan, was obtained by dimerization of the quinone precursor 98.3, followed by acetylation of the intermediate “xylaphin” 98.4.515 The unsubstituted chromophore, xantheno[2,1,9,8-klmna]xanthene-4,10-dione 98.8, was synthesized earlier by Pummerer and co-workers by means of Cu-mediated cyclization of 1,1′-bi-2-naphthol 98.6,516 followed by oxidation with peroxymonosulfuric acid.517 The intermediate “dinaphthylene dioxide” 98.7, also known as dioxaanthranthrene or peri-xanthenoxanthene (PXX),518,519 is an interesting fused system in itself, and a range of its extended analogues such as 98.11,520 98.12,521 or 98.13522 were subsequently prepared. The xylindein chromophore was also obtained in an unexpected rearrangement of the dinaphthofuran 98.9, which

a

Reagents and conditions: (a)503 (1) toluene, 3 days, rt, (2) PtCl2, toluene, 36 h, 95 °C; (b)503 toluene, 48 h, rt; (c)503 PtCl2, toluene, 24 h, 95 °C; (d)505 4 mol % Pd(dba)2, 4 mol % P(t-Bu)3, t-BuONa, toluene, 10 h, 100 °C; (e)505 (1) BuLi, toluene, 1 h, −78 °C, (2) 1 h, 0 °C, (2) BCl 3 , −78 °C, (3) 8 h, rt, (4) AlCl 3 , 2,2,6,6tetramethylpiperidine, 1,2-dichlorobenzene, 0 °C, (5) 12 h, 150 °C; (f)506 exciton-driven elimination.

While the electrochemical HOMO−LUMO gap (1.67 eV) in 95.6 was smaller than that in pentacene (2.09 eV), no obvious decomposition was observed, even at the melting point (358 °C) under atmospheric conditions. In 2015, Wang and co-workers reported a double arene elimination from the BN-pyrene precursors 95.10a−b, induced electrically within electroluminescent devices by means of exciton-driven elimination.506 This process, which is of interest 3551


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Scheme 96. Synthesis of Oxapyrenoidsa

Reagents and conditions: (a)507 AlCl3, phthalic anhydride, tetrachloroethane, 90 °C; (b) (1) benzyl chloride, reflux, (2) H2SO4 (conc.), (3) 10 min, reflux; (c) zinc dust, pyridine, 80% AcOH, 5 h, reflux; (d) heating above melting point; (e) (1) AlCl3, o-toluoyl chloride, tetrachloroethane, 9 min, 95 °C, (2) pyrolysis, 15 min, 400 °C; (f)508 (1) AlCl3, o-toluoyl chloride or 2,2-dimethylbenzoyl chloride, dry carbon disulfide, 10 min to 22 h, rt, (2) 1−3 h, reflux; (g) 30 min, reflux; (h) pyrolysis, 15 min, 400 °C; (i)509 Zn dust. a

Scheme 97. Oxa- and Thiapyrene Ketonesa

Reagents and conditions: (a)146 75% H2SO4, 6 h, 170 °C; (b)510 HBr, 45% AcOH, reflux; (c) VOF3, trifluoroacetic acid; (d)511 (1) nitrosylsulfuric acid, −10 to 0 °C, mixed pyridine solvent, (2) reflux, 3 h. a

In 2013, Bucher, Cronin, et al. reported the synthesis of the naphthoxanthenyl radical 101.5, which may be viewed as a resonance-stabilized phenalenyl species (Scheme 101).528 The radical could be obtained by NaI-mediated reduction of the naphtho[2,1,8-mna]xanthenium cation 101.2, which was itself prepared in two steps from phenalenone. Alternatively, the 101.5 radical could be photochemically generated from 9phenylphenalen-1-one 101.3 in the presence of TCNE as the oxidant. The radical showed an ESR spectrum with wellresolved couplings to 11 protons, and was found to absorb light up to ca. 700 nm. Electrochemical reduction of 101.2 to 101.5 was associated with a potential of −0.52 V (vs Fc/Fc+), whereas an additional reduction at −1.66 V corresponded to the formation of an anion. In subsequent work from the Bucher group, two isomeric extended cations 101.6−7 were prepared, along with the corresponding radicals 101.8−9.529 First reduction potentials of 101.6 and 101.7 were, respectively, higher (−0.43 V) and lower (−0.59 V) than the potential measured for 101.2, indicating a difference in π-conjugation effects. Unexpectedly, 101.7 showed the most red-shifted optical absorption in comparison with other systems. The strong yellow fluorescence of 101.7, combined with high

was converted into the tetramethoxy derivative 98.10 under the action of vanadyl trifluoride in TFA.523 The absorption spectrum of 98.10 is strikingly similar to that of xylindein. An example of a π-extended carbocyanine dye 99.4 containing 1,3-thiazine rings was reported in 1961 by Neunhoeffer and Weigel (Scheme 99).525 Their synthesis consisted of an oxidative cyclization of thioacetamide 99.1 followed by quaternization and condensation with triethyl orthoformate. The resulting 99.4 dye showed strong absorbance with λmax = 565 nm. A thioxanthene system 100.2 was synthesized in 2014 by Liu, Li, et al., using an acid-mediated ring closure followed by nucleophilic demethylation (Scheme 100).526 In comparison with its ortho-fused isomers (107.9 and 107.11, section 4.5), compound 100.2 exhibited the most pronounced spectral redshift and highest fluorescence quantum yield as well as the largest OFET mobilities. In the same year, Zhang and coworkers prepared a series of sulfur-bridged polycycles using oxidative double C(sp2)−H activation.527 The pyrene product 100.4 was subsequently reduced to 100.5, which contained a ring system isomeric to 100.2. Compound 100.5 showed a fluorescence quantum yield of 0.48, and exhibited good thermal and oxidative stability in air. 3552


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Scheme 98. Xylindein and Its Analoguesa

a

Reagents and conditions: (a)515 naphthalene-2,3-diol, phosphate buffer of pH 6.6, N2 atm, 12 h, rt; (b) CH3COONa, (CH3CO)2O, Zn dust, 2 h, reflux; (c)516 (CH3COO)2Cu, 8% caustic soda, H2O, 2 h, 280−290 °C; (d)517 (1) H2SO4, H2SO5, ice, H2O, 24 h, rt, (2) H2SO4, ice, 24 h, rt, (3) H2SO4, H2O; (e)523 TFA, VOF3, 15 h, rt.

Scheme 99. Pyrenothiazine−Carbocyanine Dyea

a

Reagents and conditions:525 (a) K3Fe(CN)6, KOH, MeOH, H2O; (b) TsOMe, 140−150 °C, 30 min; (c) HC(OEt)3, pyridine, reflux, 45 min.

chemical stability and good solubility in water, is of interest in the context of DNA intercalation studies. 4.4. [cd]-Heterofused Pyrenoids

Pyrenes containing a [cd]annulated six-membered hetero ring are also classifiable as heteraperylenes (which is our preferred choice) and have been discussed in the preceding section. [cd]Annulation of five- or seven-membered heterocycles is relatively rare, presumably because of the associated internal strain. Examples of such systems have already been discussed as

Figure 4. Chlorociboria aeruginosa and its growth on wood. Reprinted from ref 524. Public Domain.

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Scheme 100. Thiopyran-Fused Pyrenesa

Scheme 102. Synthesis of Thiophene-Fused Pyrene Derivativesa

a Reagents and conditions: (a)530 NBS, THF, chloroform, overnight; (b) (1) FeCl3, CH3NO2/dichloroethane (1:10), 2 h, (2) MeOH, 30 min; (c) (1) 1.6 M n-BuLi, THF, N2 atm, −78 °C, 2 h, (2) MeOH, −78 °C to rt.

a

Reagents and conditions: (a)526 (1) CF3SO3H, DCM, 24 h, rt, (2) water−pyridine (8:1), 30 min, reflux; (b)527 PdCl2, AgOAc, iodobenzene, AcOH, 1,1,2,2-tetrachloroethane, 48 h, 100 °C; (c) NiCl2·6H2O, Al powder, THF, 0 °C.

bromine substituents in 102.2b led to a markedly improved coupling yield. The resulting 102.3a−b were debrominated using n-BuLi, to yield the Br-free product 102.4. 102.4 showed a significantly red-shifted absorption spectrum (λmax = 484 nm), in line with its reduced HOMO−LUMO gap, but it showed relatively low hole mobility in OFET devices.

Scheme 101. Naphthoxanthenyl Radical and Related Systemsa

4.5. [a]-Heterofused Pyrenoids

4.5.1. Direct Pyrrole Fusion. The pyrrole ring can be fused to the [a] edge of pyrene in three distinct ways. First examples of simple pyrenopyrroles were provided by Saint-Ruf et al. in 1972. In that work, two phenaleno[1,9-fg]indoles 103.1a−b were prepared via the Möhlau−Bischler synthesis as potentially carcinogenic analogues of benzo[a]pyrene (Scheme 103).531 In 2004, Fang, Chou, et al. reported the synthesis of the isomeric phenaleno[1,9-ef ]indole 103.4 via the Hemetsberger−Knittel synthesis, followed by ester group removal.532 103.4 and related derivatives were fluorescent with λmaxem up to 438 nm and quantum yields reaching 0.96. 103.4 was further transformed into the bipyrrole 103.6 and the bis-phenalenefused indigo 103.7, using, respectively, Stille coupling and cumyl hydroperoxide-mediated coupling. 103.6 showed strong, bathochromically shifted fluorescence (λmaxem = 482, ΦF = 0.50). In the indigoid product 103.7, the red shift was considerably larger (λmaxabs = 687, λmaxem = 707), at an expense of considerable reduction of quantum yield (ΦF = 0.0076). In later work, it was shown by the same group that 103.6 acted as a selective fluoride sensor, switching from green to red fluorescence when treated with NBu 4 F in a DMSO solution.533,534 The effect was shown to be caused by the deprotonation of the bipyrrole caused by the basic fluoride anion, resulting in the formation of the highly colored anion 103.8. A pyrenopyrrole−naphthyridine guanine sensor 103.9, containing the 103.4 unit, was also developed by Fang et al., providing fluorescence detection in the visible region.535 In 2009, Castanheira, Ferreira, et al. showed that the Bocprotected enamine 103.10 underwent a Pd/Cu-mediated cyclization to the pyrrole-fused product 103.11 in moderate yield.536 The proposed mechanism of this reaction involves a double electrophilic attack of PdII on the 2 position of pyrene and the enamine nitrogen, followed by reductive elimination of Pd0 and reoxidation to PdII with the copper salt. A Ru-catalyzed synthesis of (Z)-3-methyleneisoindolin-1-ones from nitriles and vinyl sulfones or acrylates, described in 2014 by Reddy and

a Reagents and conditions:528 (a) (1) 2-methoxyphenylmagnesium bromide, THF, (2) DDQ, DCM; (b) (1) BBr3, DCM, (2) H2O, HBF4; (c) NaI, DMSO, or MeCN; (d) hν, TCNE.

intermediates in the synthesis of larger fused frameworks (e.g., 47.2 or 34.6). An unusual example of highly annulated pyrene derivative was provided in 2012 by Yu, Wong, Liu, et al., in their report on π-extended butterfly-shaped heteroarenes.530 Their work included the synthesis of the peri-fused molecule 102.4, containing the pyreno[1,2-b:8,7-b′:4,3-b″c″:5,6-b‴c‴]tetrathiophene ring system (Scheme 102). The key step involved oxidative coupling of brominated precursors 102.2a−b, which resulted in the formation of direct thiophene−thiophene bonds. Increasing the number of 3554


Chemical Reviews

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Scheme 103. Pyrenopyrrolesa

Reagents and conditions: (a)532 toluene, 110 °C, 4 h, 92%; (b)532 (1) KOH, THF, H2O, reflux, 2 h, 97%, (2) Cu, quinoline, 220 °C, 2 h, 91%; (c)532 Boc2O, cat. DMAP, THF, 25 °C, 5 h, 99%; (d)532 (1) t-BuLi, THF, (2) Bu3SnCl, −78 °C, 2 h; (e)532 (1) t-BuLi, THF, (2) I2, −78 °C, 2 h; (f)532 cat. Pd(PPh3)2Cl2, DMF, 110 °C, 5 h; (g)532 cumyl hydroperoxide, cumene, cat. Mo(CO)6, benzoic acid, 80 °C, 5 h; (h)533,534 [NBu4]F, DMSO; (i)536 Pd(OAc)2 (50 mol %), Cu(OAc)2 (3 equiv), DMF, 160 °C, 3−4 h, Ar; (j)537 [{RuCl2(p-cymene)}2], AgSbF6, Cu(OAc)2·H2O, AcOH, 120 °C, 60 h; (k)538 [Cp*RhCl2]2, Ag2CO3, DMF, 70 °C, 40 h. a

Scheme 104. Synthesis of Phenaleno[1,9-gh]quinolinea

Jeganmohan, was successfully used for the preparation of more extended fused frameworks, notably the highly fluorescent pyrene derivative 103.13.537 Substituted phenaleno[1,9-fg]indole 103.15 was prepared in 2014 by Jin et al., using the newly developed oxidative annulation of alkynes to N-aryl carbamates.538 π-Extended carbazole derivatives, including pyrene-containing systems 103.16−17, were obtained via FeCl3-mediated oxidative coupling by Kumar and Tao.539 The coupling was efficient (yields >60% for 103.17), even though the activating methoxy substituents were located meta (rather than para or ortho) to the expected coupling sites. 4.5.2. Other N-Containing Systems. Phenaleno[1,9gh]quinoline 104.2 (“pyrenoline”) was synthesized by Jahoda as early as 1887, by using the Skraup reaction (Scheme 104).540 In 1937, Vollmann et al. described a moderately selective oxidation of pyrenoline to the corresponding dione 104.3.541 Much of the subsequent interest in [a]fused pyrene derivatives was motivated by research on aromatic carcinogens and potential tumor inhibitors. Some of these systems, such as 105.3, were made with fused heterocyclic subunits. The synthesis of 105.3, reported in 1958 by Saint-Ruf, Buu-Hoı̈, and Jacquignon, consisted of Fischer indolization performed on the quinone 105.1, followed by dehydrogenation with

a

Reagents and conditions: (a)540 nitrobenzene, glycerin, H2SO4, reflux; (b)541 CrO3/AcOH.

chloranil.542 Thummel and co-workers used a similar strategy for the preparation of 105.4 and 105.5.543 These two compounds, which were not dehydrogenated further, were used as cyclometalating ligands for ruthenium(II). A different route to pyrene [a]fusion was taken in 2008 by Klumpp and coworkers in their exploration of superelectrophilic cyclizations.544 The pyridine-substituted precursor 105.7, available in two steps from pyren-1-ylboronic acid 105.6, was treated with trifluoromethanesulfonic acid, yielding the isoquinoxaline product 105.8, in which the electrophilic ring closure occurred with the elimination of a benzene molecule. In their work on multiply arylated pyrenes, reported in 2015, He and Zhong 3555


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Khan et al.547). The Martinet reaction was used to prepare the pyrenoisatin derivative 106.5, which was further reacted with ophenylenediamine to yield 106.6. Compound 106.7 was obtained from 106.1 in the Conrad−Limpach reaction. In subsequent work, a thiophene-fused system 106.8 was reported.548 4.5.3. Other Five-Membered Rings. [a]Fusion of other five-membered rings was explored to a limited extent. Pyreno[1,2-c]furan 107.1 was synthesized in 1988 by Moursounidis and Wege,549 using their cycloaddition/cycloreversion method, described in more detail below (Scheme 119, section 4.7). The other two isomeric pyrenofurans 107.2−3 were obtained in 1985 by Royer and co-workers via intramolecular aldol-type condensations.550 Dipyreno[1,2b:2′,1′-d]furan 107.6 and its quinone derivatives were reported by Chesnovskii et al., as products of acid-catalyzed condensation of 1,6-pyrenequinone.551 A four-step synthesis of pyreno[2,1-d]isoxazole 107.4 from pyren-1-ol was described by the Rebek group.552 The closure of the isoxazole ring was achieved using a newly discovered TsCl/i-Pr2NEt cyclization. 8Methylpyreno[2,1-d]thiazole 107.5 was obtained in 1961 by Neunhoeffer and Weigel by oxidative cyclization of 2thioacetaminopyrene, and used for the synthesis of carbocyanine dyes 107.7.553 Two isomeric benzothiophene-fused pyrenes 107.9 and 107.11 were synthesized by Liu, Li, and co-workers in 2014 (Scheme 107).526 These compounds were obtained from sulfoxides 107.8 and 107.10, respectively, which were subjected to the cyclization−demethylation sequence described earlier for 100.2. Pyreno[1,2-b]thiophene 107.14 was first reported in 1958 by Pandya and Tilak as a product of acid-catalyzed cyclization of (2,2-dimethoxyethyl)(pyren-1-yl)sulfane 107.15.554 A two-step synthesis of “thiophene end-capped” aromatics, developed in 2012 by Thongpanchang et al., provided an alternative route to 107.14.555 The compound was prepared by condensing pyren-1-ol 107.12 with 2mercaptoethanol, and subsequent dehydrogenation with chloranil. The isomeric 107.23 was obtained in 1981 by Castle et al.556 This synthesis was considerably more complex, requiring consecutive closure of two benzene rings.

synthesized the disubstituted derivative 105.9, which was subjected to a range of oxidative cyclization conditions.545 While FeCl3- and DDQ-mediated oxidations were ineffective, potassium metal was found to induce efficient coupling, leading to the isoquinoline product 105.10 (Scheme 105). Scheme 105. Synthesis of Nitrogen-Containing [a]Fused Pyrenoidsa

4.6. Pyrazacenes a

4.6.1. Pyrazine-Fused Systems. The availability of pyrene-4,5-dione and pyrene-4,5,9,10-tetraone building blocks (108.1 and 108.3, Scheme 108) provides access to diverse pyrazine-fused pyrenoids. Large molecular structures of this kind (“pyrazacenes”) have been developed as potential organic materials, as recently reviewed by Mateo-Alonso.351 First examples of this chemistry were reported in the comprehensive 1937 paper by Vollmann and co-workers, who synthesized the quinoxaline derivatives 108.2 and 108.4, which can be seen respectively as benzo-fused 5,12-diazatetracene and 5,8,13,16tetraazahexacene.541 These two systems illustrate the underlying structural paradigm, which enables the assembly of 2nazaacenes of considerable length and remarkable chemical stability. The diketone−diamine reactivity was soon found to be general and usable with variously substituted diamines557 and pyrenoquinones.558 Similar reactions with diaminomaleonitrile, reported in 1975 by Wöhrle et al., yielded cyano-substituted C24.1 and C24.2a (Chart 24).559 The latter species was used to show the dependence of nitrile group fragmentation in mass spectrometry on the aromaticity of the cyclic system.560

542

Reagents and conditions: (a) phenylhydrazine, HCl in CH3COOH; (b) chloranil, xylene; (c)543 2-aminobenzaldehyde, KOH, absolute EtOH, under Ar atm, 12 h, reflux; (d)544 (1) 3bromoisonicotinaldehyde, K3PO4, 5 mol % Pd(OAc)2, 1,4-dioxane, overnight, reflux, (2) benzyltriphenylphosphonium bromide, n-BuLi, THF, 2 h, −78 °C, (3) aldehyde obtained in step 1, 4 h, 25 °C; (e) CF3SO3H, chloroform, 4 h, rt; (f)545 K, dry toluene, 24 h, 125 °C.

Using 1-aminoperylene 106.1 as the starting material, the group of Buu-Hoı̈ prepared a series of pyrenes with ortho-fused heteroaromatic rings (Scheme 106).546 Benzoquinoline-fused derivatives 106.3a and 106.4a were obtained by refluxing 106.1 with paraformaldehyde and α- or β-naphthol, respectively. 106.2a or 106.2b, synthesized from 106.1 and α- or βnaphthol, were converted into the methyl analogues 106.3b and 106.4b by condensation in acetic acid. Alternatively, condensation reactions of 106.2a or 106.2b with arsenic trichloride or sulfur and iodine produced, respectively, arsazines 106.9−10 and thiaazines 106.11−12 (for another sulfurcontaining derivative, thioxanthenone 106.13, see the work by 3556


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Scheme 106. Synthesis of Heteroaromatic [a]Fused Pyrenoids from 1-Aminoperylenea

a Reagents and conditions: (a)546 α-naphthol, iodine, 36 h, 250 °C; (b) β-naphthol, iodine, 36 h, 250 °C; (c) α-naphthol, paraformaldehyde, reflux; (d) β-naphthol, paraformaldehyde, reflux; (e) ZnCl2 (anhydrous), acetic anhydride, 24 h, reflux; (f) diethyl mesoxalate, CH3COOH, 1 h, reflux; (g) o-phenylenediamine; (h) (1) ethyl acetoactate, piperidine, EtOH, 24 h, reflux, (2) Zn powder, distillation; (i)548 benzo[b]thiophen-3-ol, paraformaldehyde.

Scheme 107. Thiophene-Fused Pyrenesa

a Reagents and conditions: (a)526 (1) CF3SO3H, 24 h, rt, (2) water−pyridine, 30 min, reflux; (b)555 2-mercaptoethanol, TFA, chlorobenzene, 3 h, reflux; (c)555 chloranil; (d)554 PPA; (e)556 NaH, DME; (f)556 I2, cyclohexane, medium pressure Hg lamp; (g)556 NBS, benzoyl peroxide, benzene, reflux; (h)556 KCN, benzene, water, Aliquat-336, reflux; (i)556 DIBAL-H, toluene, benzene, rt; (j)556 PPA, steam bath.

bright blue fluorescence and three-step reversible reduction potentials,562 whereas two reversible reduction waves, with a LUMO level of −3.6 eV, were observed for C24.2c.563 C24.4, reported by Bunz, Hamburger, and co-workers, showed

Alternative reaction conditions were reported in 2012 by Wang, Zhi, et al., who used C24.2a for the synthesis of twodimensional zinc phthalocyanine polymers.561 The tert-butyl derivative C24.2b, synthesized in a similar manner, exhibited 3557


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absorption spectra and theoretical calculations. A pyrene-fused phenazinothiadiazole C24.7 with enhanced electron mobility in FET devices (0.016 cm2 V−1 s−1) was described in 2015 by Mateo-Alonso, Sun, et al.568 Pyrene−pyrazine hybrids were explored as ligands by several groups. Ishow and Gourdon prepared and characterized a mixed-ligand ruthenium complex C25.1 (Chart 25) along with its dinuclear analogue (Chart 30).569 Compound C25.1 was obtained by heating {[Ru(bpy)2(5,6-diamino-1,10-phenanthroline)](PF6)2·2H2O} with tetraketone 108.1 in acetonitrile.570 Gao, Chao, Ji, et al. prepared closely related systems C25.2a−b by reacting bis(bipyridyl) Ru complexes with a preassembled phenazine ligand.571 Both complexes intercalated into DNA base pairs with very high affinity even at high salt concentrations. The complexes were able to effectively inhibit DNA transcription activity by blocking the binding of T7 RNA polymerase to the template DNA. Barton and co-workers used C25.2a for targeting DNA defects, investigating the luminescent response to base-pair mismatches and/or abasic sites.572 A related structural design, C25.3, was used by Ruan and coworkers as a fluorescent sensor.573 Compound C25.3 exhibited a selective fluorescence-quenching response to Hg2+ in aqueous acetonitrile solution, and its detection limit was lower than the toxic level of Hg2+. The crown ether C25.4b574 and its smaller analogue C25.4a were explored as cation sensors by Al-Sayah, Kaafarani, et al.575 NMR investigations showed that C25.4a formed 1:1 complexes with small ions (sodium and calcium) and 1:2 complexes with large ions (potassium, rubidium, cesium, and barium) as evidenced by the changes of chemical shifts. Sensor C25.4b showed weak binding with small monovalent ions (Li and Na), but it formed 1:1 complexes with Ca2+, K+, and Ba2+. Similar investigations were performed on the bis(crown ethers) C25.5b576 and C25.5a,575 revealing strong binding to Ba and Ca ions. Diazapentacene systems containing a single pyrene substructure were reported by several research groups (Scheme 109). Compound 109.1a, prepared in 2007 by Mateo-Alonso, Guldi, Prato, and co-workers, was shown to promote dispersion of SWCNTs in THF.577 Photophysical experiments indicated that the dispersion was induced solely by π−π interactions. The TIPS-ethynyl-substituted derivative 109.1b, investigated by Bunz, Hamburger, et al., exhibited a pyrene-induced red-shift

Scheme 108. Synthesis of Pyrazine-Fused Perylenes from Perylenoquinonesa

a

Reagents and conditions: (a)541 o-phenylenediamine, AcOH, reflux.

markedly red-shifted absorption (λmax = 458 nm) and emission (λmax = 461 nm) relative to the less extensively fused analogues, but its OLED performance was not tested.564 Compounds C24.3 (Chart 24) and 110.2b (Scheme 110) also prepared by direct condensations of appropriate amines, were used as anion sensors by Al-Sayah, Kaafarani, et al.565 C24.3 exhibited high affinity to acetate, benzoate, cyanide, and fluoride ions, yielding pronounced changes in absorption spectra and 1:1 sensor-toanion binding ratios. A similar binding capability was observed for 110.2b, which yielded a sensor-to-anion ratio of 1:2. In 2014, Lin, Cui, and co-workers prepared a series of fused 5,12-diazatetracenes C24.5a−h containing different terminal aromatic units.566 Compounds C24.5a−b were prepared using the conventional condensation protocol, and the aromatic units were attached to C24.5b using Suzuki and Sonogashira reactions. C24.5a−h displayed moderate to strong fluorescence, with quantum yields of 0.10−0.60 and emission maxima occurring typically in the 470−480 range (DCM solution, 640 nm for C24.5h). A TTF-containing donor−acceptor system C24.6 was developed in 2011 by Jia and co-workers.567 The observed increase in the oxidation potentials of C24.6 relative to the TTF reference suggested a good electronic communication between the donor and acceptor. Evidence for intramolecular charge transfer (ICT) in compound C24.6, arising from a HOMO−LUMO transition, was obtained from Chart 24. Pyrazine-Fused Pyrenoids

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Chart 25. Pyrene−Pyrazine Hybrids as Ligands

similar to that of its smaller analogue C24.4.564 The nitrilesubstituted molecule 109.2 was synthesized by the Lee group as a reference system for comparison with electron-deficient pyrazine−acenes (see Chart 26).578 109.3 was obtained as an intermediate used for the synthesis of the extended acene C31.8 (Chart 31).579 The synthesis of the diazapentacene 109.4, reported in 2012 by Sun, Zhang, et al., relied on a dione−diamine conversion sequence, consisting of dioxime formation and subsequent reduction (109.5 to 109.7, Scheme 109).580 109.5 was then condensed with diamine 109.7, producing 109.4 in 30% yield. 109.4 possessed two absorption peaks centered at 350 and 460 nm with a shoulder at 434 nm, corresponding to an optical bandgap of 2.69 eV. The compound exhibited bright green fluorescence with an emission maximum at 481 nm. The UV/ vis absorption and the solution color of 109.4 could be changed by adding a Lewis acid or a strong protic acid. Following the synthesis of the parent tetraazahexacene system 108.4, discussed above, a number of its substituted derivatives have subsequently been reported, such as 110.1a581 and 110.1b582 (Scheme 110). The latter species was shown by Gigmes, Lalevée, and co-workers to be an efficient polymerization photoinitiator.582 108.4 and 110.1b were employed in bottom-gate p-channel organic thin-film transistors, showing field-effect mobilities of 2.5 × 10−3 and 7.5 × 10−5 cm2/V·s in ambient air.583 By condensing tetraone 108.3 with benzene-1,2,4,5-tetraamine, Stille and Mainen synthesized the ladder polyquinoxaline polymer 110.7.71,72 Interestingly, this polymer (with an estimated molecular weight of ca. 7000) showed partial solubility in HMPA, and its thermal stability under nitrogen (up to 700 °C) was improved relative to nonladder analogues, such as the pyrene-containing 110.8. The work on 110.7, published in the 1960s, apparently constitutes the first attempt to synthesize heteroatom-doped graphitic nanoribbons (for a related coronenoid nanoribbon, see Scheme 8, section 2.1). Following Stille’s reports, a related modular approach was explored by Arnold584,585 and Hedberg et al.586 Compound 110.2a, obtained by reacting 108.3 with 1,2-diamino-4,5-(ptoluenesulfonamido)benzene, was detosylated to provide 110.2c. The latter tetraamine was condensed with naphthalene-1,4,5,8-tetracarboxylic acid, pyromellitic anhydride, or the

Scheme 109. Diazapentacenesa

a

Reagents and conditions: (a)580 NH2OH·HCl, pyridine, EtOH, 24 h, reflux; (b) (1) SnCl2, HCl conc., EtOH, 0 °C, (2) overnight, reflux; (c) 109.5, AcOH, trifluoroacetic acid, 2 days, reflux.

double benzil 110.6, yielding polymers 110.3−5, respectively. The formation of imidazole or pyrazine linkages in these polymers was most likely nonregioregular. The thermal properties of the ladder-type polymers 110.3−4 were comparable to those of 110.7. Functionalized tetraazahexacenes bearing mesogenic alkoxy and alkylthio substituents were explored by several groups (Scheme 111). A series of tetraalkoxy derivatives, including systems 111.4d−f, were reported in 2004 by Harris and coworkers.587 111.4d was shown to form a liquid crystalline phase 3559


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Scheme 110. Pyrene-Containing Tetraazahexacenes and Related Polymersa

a

Reagents and conditions: (a)71,72 (1) conc. H2SO4, rt, (2) 20% ammonium carbonate, 65%; (b) PPA, N2, 180 °C; (c)586 m-cresol, 120 °C, 18 h.

Scheme 111. Synthetic Route to Pyrene [e]Fused Hexaacenesa

a

Reagents and conditions: (a)592 4,5-dichloro-1,2-phenylenediamine, EtOH:AcOH (1:1), 24 h, reflux; (b)589,592 K2CO3, alkylthiol, DMF, Ar atm, 1 week, 80 °C; (c)587,594 4,5-dialkoxy-1,2-phenylenediamine, EtOH:AcOH 1:1, Ar atm, 24 h, reflux.

in the broadest temperature range (213−268 °C), in contrast to 111.4e−f, which were nonmesomorphic. The roomtemperature structure of 111.4d was determined to be a crystalline phase with orthogonal symmetry, in which the πstacked cores were tilted relative to the columnar axis.588 At 235 °C, the LC phase of 111.4d was found to be of the Colr type, with C2/m planar symmetry. Columnar phases were also identified for other substituted derivatives, such as 111.4 g.588 Alkylthio derivatives 111.3a−d were prepared in 2007 by Kaafarani, Jabbour, et al. in a two-step synthetic procedure, consisting of pyrazine cyclization and thiolate substitution.589 These systems turned out to be nonmesogenic, although

multiple crystal-to-crystal transitions were observed for 111.3a.590 With estimated HOMO and LUMO energy levels of 5.57 and 2.97 eV, respectively, these compounds were investigated as potential semiconductor materials for organic electronics. An OFET device based on 111.3a showed pchannel characteristics, with an average saturation hole mobility of ∼10−3 cm2 V−1 s−1.589 In thin films, the 111.3a molecules are arranged within layers, and the aromatic cores are separated from the alkane side chains.591 Both the core long axes and the side chains were found to be highly tilted with respect to the surface normal, with cofacial packing of the core units. The columnar mesomorphism of 111.3e−h was subsequently 3560


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Chart 26. Unsymmetrically Substituted Hexacenes578,596−598

investigated by Eichhorn, Kaafarani, et al.592 They showed that lateral extension of the core to cruciform structures (as in 111.3g−h) was essential for the formation of hexagonal columnar mesophases, while the incorporation of phytanyl side-chains, although not mesogenic by itself, is most effective in the control of phase transition temperatures. In 2010, Valentini, Kaafarani, and co-workers described the morphological and electronic properties of 111.3d combined with butylamine-modified graphene sheets.593 The film deposited from the butylamine-modified graphene sheets/ 111.3d blend showed a photoelectrical response higher than those prepared with neat 111.3d or with unmodified graphene blends. Fused tetraazahexacenes showed photoluminescent behavior resembling that of pyrene. Concentration-dependent fluorescence measurements performed for 111.3a−b and 111.4c−d showed the formation of excimers even at nanomolar concentrations.594 The observed variation of the Stokes shift, fluorescence quantum yield, and fluorescence lifetime was found to correlate with the number of carbons in the alkyl chain.595 Stepwise condensations provide access to unsymmetrically substituted hexacenes. In 2008, Lee and co-workers used this strategy to prepare compounds C26.1a,e,g (Chart 26).596 All of these materials were observed to form organogels through the growth of nanofibers via a cooperative interplay of π−π stacking, hydrogen bonding, and van der Waals interactions. Compound C26.1g exhibited acid-sensing abilities in the xerogel film. In subsequent work from the Lee group, compounds C26.1a,b,e−g and C26.2 were investigated as self-assembling n-type organic semiconductors, with substituent-dependent properties.597 Among the substituents studied, the nitro group (C26.1f and C26.2) gave the most pronounced lowering of the LUMO energy level (by ∼0.6 eV), and the narrowest HOMO−LUMO gap, when compared to C26.1a. Significant influence of these peripheral substituents was also observed in the morphology of self-assembled structures. Even with different types of peripheral substituents, the ability to produce 1D nanostructures was largely maintained. In addition, the length of alkoxy group, assembly method, and assembly conditions influenced the fiber morphology significantly. In a subsequent report, compounds C26.1a−d,f and C26.3 containing F, Cl, Br, and NO2 substituents were described.598 It was found that LUMO levels were lowered more significantly in compounds containing Cl and Br than in the F-substituted derivatives.

Terminal aryl substitution in linear pyrazacenes was reported in 2008 by Wang and co-workers, enabling the preparation of hexacenes C27.1a−c (Chart 27), as well as more extended Chart 27. Aryl-Substituted Fused Tetraazahexacenes

oligomeric tapes (C33.1−4, Chart 33).599 Further examples were provided in 2013 by Kaafarani, Eichhorn, et al., who prepared octaalkoxy derivatives C27.2 and C27.3, by direct condensations of respective ortho-terphenyl and triphenylenediamines.600 Attempted oxidative coupling of compound C27.2 to C27.3 was unsuccessful. Both compounds displayed several types of fluorescent columnar mesophases over wide temperature ranges. Self-assembly and electronic properties of ethynyl-substituted systems C28.2−3 were explored by Lee and co-workers (Chart 28).601 All of these systems were derived from the dibromosubstituted C28.1 by using Sonogashira reactions. Placing different functional groups (OCH3, H, CN) on the ethynyl arms of C28.3a−c progressively reduced LUMO energies, as predicted by theoretical calculations and experimentally verified by cyclic voltammetry (CV). Furthermore, the morphologies of 1D assembly were greatly influenced by the substituents. While the conformational flexibility of OCH3 groups hampered successful assembly, hydrogen and cyano substitution induced 3561


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Chart 28. Asymmetrically Substituted Pyrene-[e]Fused Hexaacenes

Chart 29. Tetraazahexacene-Based Polymers

formation of the nanofibers, while π−π interactions are dominant for the straight strands. Another class of self-assembling derivatives, C28.4a−e, showed the effect of the position, type, and number of peripheral substituents on the optical and electrochemical properties of the entire system.603 For these systems, organogelation was also observed, including a thixotropic behavior, which originated from extensive three-dimensional entanglement of the fibers. The substitution showed a

the formation of rigid microstrands and flexible nanofibers, respectively. Self-assembly of C28.3c using a phase transfer method showed morphological transformation from straight strands to flexible nanofibers with significant bundling and coiling, and finally flat nanofibers with less bundling as solution concentration increased.602 X-ray diffraction and FT-IR spectroscopy of the assembled structures showed that intermolecular cyano interactions are the major driving force for the 3562


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significant influence on the morphology of the fibers formed through organogelation, with dodecyl chains providing higher gelation temperatures than hexadecyl chains. The position of the peripheral substituents influenced the fiber morphology by modulating the intermolecular CN (or CHO) interactions and π−π stacking. In contrast to CN-substituted derivatives, compounds with CHO groups formed islands of fiber aggregates. Tuning of HOMO and LUMO energies via terminal fusion of aromatic “π-extender” rings to pyrene−pyrazine hybrids was explored in 2013 by Lee and co-workers.578 π-Extended derivatives containing pyridine (C26.4), pyrazine (C26.5), and benzothiadiazole (C28.5a and C28.5b) were prepared, and their properties were compared to those of the dicyano reference 109.2. It was found that the imine function was comparable to the nitrile substituent in its LUMO-lowering ability, with the strongest effect being observed for the dibromobenzothiadiazole-fused C28.5a. In all cases, the HOMO energy was negligibly affected. Thiophene-containing donor−acceptor systems C28.5c and C28.6 were also prepared, with the former showing a HOMO−LUMO gap of 1.55 eV.604 This compound self-assembled into fibers with a 100−200 nm width, composed of nanotapes ca. 10 nm wide. Conjugated copolymers C29.2a−d and the reference molecule C29.1, all based on 9,9-dioctylfluorene and arylsubstituted hexacene subunits, described above, were synthesized in 2010 by Wang and co-workers using Suzuki coupling (Chart 29).605 A range of polymer compositions were obtained, wherein the molar fraction x of the hexacene component varied from 0.01 to 0.20. Energy transfer from the conjugated main chain to the hexacene moieties was observed using fluorescence spectroscopy. Full energy transfer was achieved when the molar content of the hexacene was 0.20 (C29.2d, toluene solution). In the thin film, complete energy transfer was observed even for the C29.2a copolymer. The LUMO energy levels of the copolymers (−3.06 eV) were lower than those of the polyfluorene homopolymer (−2.65 eV), indicating that the introduction of the hexacene unit was beneficial for electron injection. Single-layer electroluminescent devices (ITO/PEDOT:PSS/polymer/LiF/Al) were fabricated, and showed better maximum brightness and current efficiency than the homopolymer. The best single-layer device was based on C29.2b, with a maximum brightness of 1532 cd/m2 and current efficiency of 1.09 cd/A. Much higher efficiency could be achieved for multilayer EL devices of C29.2b (ITO/ PEDOT:PSS/PVK/polymer/TPBI/LiF/Al), which showed a maximum current efficiency of 10.01 cd/A. In subsequent work from Wang and co-workers, analogous copolymers C29.4a−d were prepared, with fluorene chains attached to the pyrene core.606 Compound C29.3a was used to synthesize the copolymers and the reference system C29.3b, using Suzuki coupling. A single-layer PLED device based on C29.4c provided a maximum brightness of 485 cd/m2, a current efficiency of 0.29 cd/A, and an external quantum efficiency of 0.10%. These parameters could be considerably improved in multilayer devices. In 2015, highly porous imidazole-linked polymers C29.5 and C29.6 were reported by Arachchige, Kaafarani, El-Kaderi, et al.607 Their synthesis involved condensations of the tetraamine 110.2d with appropriate branched tetraaldehydes, which produced the highly crosslinked structures in 74−79% yields. These materials had a high surface area (SABET = ∼950 m2 g−1), revealing bandgaps in the semiconducting range (2.02 and 2.16 eV) and luminescence

emission in the orange−red region of the spectrum (600 and 630 nm). In 1998, Gourdon et al. prepared a dinuclear ruthenium complex C30.1 containing a pyrazacene bridging ligand.569 Compound C30.1 was obtained in a reaction between tetraketopyrene 108.3 and {[Ru(bpy)2(5,6-diamino-1,10phenanthroline)](PF6)2·2H2O} with a 64% yield. C30.1 formed dimers in solution maintained only by π−π stacking of the bridging ligand, stable enough to be observed not only by 1 H NMR spectroscopy but also by electrospray mass spectrometry at low accelerating cone voltage. Subsequent work by Gourdon, Chiorboli, and co-workers indicated a relatively weak interaction between the RuII units and the bridging ligand.570 A related ligand, C30.2a, reported in 2014 by Ruan, Li, Chang, and co-workers, exhibited selective off−on fluorescence response to Cd2+ over other metal ions, and the detection limit was as low as 0.02 μM.608 The Cd2+ sensing of C30.2a had a high water tolerance and could be carried out in aqueous media with the water content of up to 70%. C30.2a was also successfully applied to the in vivo imaging of intracellular Cd2+ in living HeLa cells, showing low cytotoxicity and cell membrane permeability in these experiments. A differently substituted analogue, C30.2b, was employed by Ruan et al. as a Hg2+ chemosensor.573 Chart 30. Pyrazacene-Based Ligands

Acetylene substitution was explored as a means of tuning the electronic and geometrical properties of pyrazacene oligomers by Mateo-Alonso and co-workers.609 Hexacenes C31.1a−c, C31.2, C31.3a (Chart 31) and octacenes C31.6a−c were originally synthesized by direct condensations of prefunctionalized amines and ketones. It was subsequently found that increasing the bulk of silyl substituents in the C31.3a−c series of compounds results in twisting of the aromatic backbone, with a 24° quinoxaline−pyrene twist angle observed for C31.3c.610 The twist was found to affect the energy ordering of LUMO and LUMO+1 levels. Ethynyl-substituted C31.4 and C31.5, bearing electron-withdrawing terminal CN and NO2 groups, exhibited significant lowering of the LUMO levels, with an ELUMO value of −4.3 eV reported for C31.5.563 A similar synthetic approach was used by Miao and co-workers for the preparation of pyrene-fused tetraazaheptacene (C31.8) and tetraazaoctacene (C31.7).579 An additional advantage of using bulky silyl groups was the enhanced solubility of the above materials in organic solvents. An alternative strategy of making 3563


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Chart 31. Ethynyl- and Alkyl-Substituted Pyrazacenes563,609,610

Chart 32. Pyrene-[e]Fused Azaacenes with Terminally Condensed 5-Membered Rings

containing C32.1a and PCBM showed an absorption red shift of 26 nm. NIR electrochromism was observed for C32.1a, with absorption being turned on at 1034 nm when electrochemically switched (at 1000 mV) from its neutral state to a radical cation state. C32.1a−b emit above 1000 nm with quantum yields below 2% and large Stokes shifts (256−318 nm). Variabletemperature photoluminescence data obtained for the C32.1a film suggested that the nonradiative decay was diminished by restriction of bond rotations, leading to luminescence enhancement. A related class of molecules, C32.2a−d, containing

soluble pyrazacenes is the use of linear alkyl chains on the pyrene core, as demonstrated by Mateo-Alonso and coworkers.611 In the hexaoctyl system C31.9, the four alkyl chains were installed by quadruple Sonogashira coupling followed by catalytic hydrogenation. The product displayed a solubility of more than 20 mg/mL in chloroform. In 2009, Wang and co-workers prepared a series of filmforming, low-bandgap chromophores C32.1a−b (Chart 32).612 Terminal fusion of 1,2,5-thiadiazole rings to the hexacene core resulted in bandgaps of 1.27−1.22 eV. Two-component films 3564


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Scheme 112. Synthetic Routes to Pyrene-Fused Azaacenesa

Reagents and conditions: (a)616 benzyltrimethylammonium tribromide, DCM, MeOH, 12 h, rt; (b) NaIO4, H2O, RuCl3·xH2O, DCM, dioxane, Ar atm, 18 h, rt; (c) 1,2-diaminobenzene, AcOH, Ar atm, 4 h, reflux; (d) boronic acid, 2 M K2CO3, Pd(PPh3)4, toluene, EtOH, Ar atm, 24 h, 90 °C.

a

Chart 33. Pyrene-[e]Fused Acene Derivatives588,599,617

spins were respectively ferromagnetically and antiferromagnetically coupled, indicating a change in the exchange coupling pathways. The more extended biradical C32.5 was used to show that the parameter-free extraction of distances and relative orientations of spin centers in nucleic acids can be achieved by orientation-selective PELDOR experiments using common Xband frequencies.615 In the majority of reported pyrazacene systems, the pyrene cores are either unsubstituted or bear identical substituents at positions 2 and 7. Unsymmetrical substitution patterns were developed in 2013 by Hu, Yamato, and co-workers, starting from 1,3-dibromo-7-(tert-butyl)pyrene 112.2, which was found

terminal cyclic imides as acceptor functionalities, was reported in 2012 by Chi et al.613 These systems displayed LUMO energies in the range from −3.4 to −3.5 eV, with bandgaps of ca. 2.8 eV. As part of their work on spin labels for use in pulsed electron−electron double resonance (PELDOR), Prisner, Sigurdsson, Schiemann, et al. prepared isomeric biradicals C32.3 and C32.4, which were characterized by favorable rigidity, while providing the necessary difference in the intramolecular spin−spin distances.614 The two species were obtained by oxidation of the respective diamines assembled in diamine−diketone condensations. In C32.3 and C32.4, the two 3565


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Chart 34. Heteroacenes618−620

to form in good yield in direct bromination of 112.1 with benzyltrimethylammonium tribromide (Scheme 112).616 The bromines then served as attachment points for introducing aryl substituents via Suzuki coupling. These arylations could be performed directly on 112.2 or on the preassembled hexacene 112.4. In either route, the oxidation to the tetraketone was achieved in moderate yields using the NaIO4/RuCl3 reagent. Stepwise dione−diamine condensations were used in 2008 by Wang and co-workers in their synthesis of extended acenetype molecular ribbons.599 The shortest systems, hexacenes C27.1a−c (Chart 27), served as the prototypes for the longer ribbons C33.1−4, containing up to 6 pyrazine units, and up to 16 rectilinearly arranged fused benzene rings (Chart 33). The energy gaps of these molecules, estimated from the absorption onsets, decreased with the increase in molecular length, covering the range from 2.66 to 2.18 eV in the C27.1c, C33.1b−4b series. Similarly, the onset of the first reduction wave was shifted from −1.16 to −0.62 V (vs Ag/AgCl), corresponding to the LUMO energy levels of −3.24 and −3.78 eV, respectively, indicating a length dependence of the potential n-type semiconductor properties. With the increasing molecular length, the ribbons tend to aggregate in solution more readily, as evidenced by 1H NMR and UV−vis absorption spectra. It was however found that the introduction of t-butyl substituents noticeably suppressed aggregation. A liquid-crystalline derivative C33.1c, reported subsequently by Cheng and co-workers, revealed a three-dimensionally ordered columnar phase stable over a temperature range of ca. 145−215 °C.588 Dodecacene derivatives C33.5a−b containing a single pyrene subunit and 12 nitrogen atoms were reported in 2012 by Chan, Chi, et al.617 These compounds were obtained by a condensation reaction between tetraketopyrene and appropriately substituted 5a,6,11,11a-tetrahydroquinoxalino[2,3-b]quinoxaline-2,3-diamines. These expanded pyrazinacenes showed good solubility in chlorinated solvents, but their strong tendency to aggregate in solution led to extreme broadening of their NMR spectra. The latter effect could however be suppressed by protonation. A linear pyrazacene C34.1 with terminal pyrene units was reported in 2010 by Mateo−Alonso, Paolucci, Prato, et al. (Chart 34).618 This poorly soluble, unsubstituted system, prepared alongside the tetraazaoctacene C31.6d, showed strong aggregation in solution, which was however minimized by dissolution in acetic acid. C34.1 displayed an oxidation wave and three reduction waves, which was considered unusual for an azaacene containing fewer than 11 linearly fused rings. A phototransistor device based on a C34.1 single crystal demonstrated good performance in signal amplification under the photoconductive effect and could be functionalized as photocontrolled switches.619 Higher analogues of C34.1, nonacenes C34.2 and C34.3, were prepared in 2014 by Hu, Zhang, et al.620 UV−vis spectra and electrochemical data indicated that compound C34.2 possessed a smaller bandgap than that of C34.3, indicating that the conjugation was disrupted by the inclusion of oxygen atoms. A condensation of tetraketone 111.1a with the triangular hexamine 113.1 was reported by Jiang and co-workers to yield a chemically stable, electronically conjugated organic framework 113.2 (Scheme 113).621 In 113.2, the π-conjugated twodimensional sheets are slip-stacked, forming nanochannels with a diameter of 1 nm, which were shown to incorporate C60 molecules and enable H2 adsorption. 113.2 demonstrated a very high hole mobility of 4.2 cm2 V−1 s−1, and was shown to be

a useful material for high on−off ratio photoswitches and photovoltaic cells. In 2014, Mastalerz and co-workers developed a family of πextended D3h-symmetric triptycene derivatives 114.3a−e, which were obtained by condensing the triptycene hexaketone 114.2 with appropriate diamines (Scheme 114).622 114.3a−e are fluorescent and exhibit high internal molecular free volumes (IMFVs). They were shown to form micropores in the solid state, with high surface areas and narrow pore-size distribution. In subsequent work, the Mastalerz group reported the linear bis-triptycene system 114.4, which combined good solubility with a high tendency to crystallize.623 In the solid state, 114.4 showed noticeable bending of the aromatic core, which was rationalized in terms of dispersive interactions. Despite the presence of the undecacene substructure, 114.4 is a stable compound, although it undergoes a slow photooxidation to the quinone 114.5 under ambient conditions. 4.7. Other [e]Fused Pyrenoids

4.7.1. Pyrrole- and Indole-Containing Systems. Two examples of indole-fused pyrenoquinones 115.1−2 were described by Vollmann et al. in their extensive account of pyrene chemistry (Scheme 115).541 These systems were obtained by cyclization of the respective p-tolylaminosubstituted pyrenoquinones using either sulfuric acid, AlCl3 in pyridine, or the AlCl3−NaCl melt. Indole-fused pyrenes 115.4−6 were reported in 2015 by Lee and Park.624 115.4 was synthesized by Cadogan cyclization of 115.3 and further functionalized using C−C and C−N coupling reactions. 115.6 was used as a nondoped emitting layer in OLED devices, yielding a turn-on voltage of 3.1 V and current efficiency of 3.99 cd/A. Pyreno[4,5-c]pyrrole 116.5 (more correctly, phenanthro[4,5efg]isoindole) and its ethyl ester 116.4 were synthesized by Lash et al. using the Zard−Barton reaction followed by ester cleavage (Scheme 116).625 4-Nitropyrene 116.3, required as the starting material, cannot be obtained by direct nitration and was prepared via the hexahydro intermediate 116.1. The pyrenopyrroles are key intermediates for the synthesis of [b]fused pyrenoporphyrins, which are discussed in section 7.1 3566


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Scheme 113. A Pyrazacene-Based Covalent Organic Frameworka

a

Reagents and conditions: (a)621 (1) 113.1 chloride, ethylene glycol, 3 M AcOH, ultrasound, 15 min, 25 °C, (2) in autoclave, 3 days, 120 °C.

Scheme 114. Synthesis of Pyrazacene−Triptycene Hybridsa

Reagents and conditions: (a)622 KOAc, chloroform, EtOH, glacial AcOH, Ar atm, 16 h, 85 °C; (b) KOAc, DCM, glacial AcOH, 15 h, 70 °C; (c)623 CDCl3, spontaneous photooxidation under ambient lighting. a

monoimidazoles were synthesized in 1971 by Sakaino and shown to form charge-transfer complexes with chloranil.627 The interest in such derivatives was revived recently, and functionalized pyreno[4,5-d]imidazoles are now being explored as dyes for dye-sensitized solar cells628,629 (e.g., C35.1628), ligands 630−634 (e.g., C35.2, 630 and C35.3 631 ), fluorophores,635−637 and mechanochromic materials (e.g., C35.4636) (Chart 35). A NiII-catalyzed variant of the Debus reaction was reported in 2014 by Maiti and co-workers.638 The bisimidazole-fused pyrene 117.1, prepared from the tetraone 111.1a via a double Debus annulation (Scheme 117), was used by Lu and co-workers as a dicarboxylate building block for the synthesis of a CuII-based MOF material.639 The resulting structure was highly porous, consisting of noninterpenetrating square grids, with a size of 25.5 × 25.5 Å2, and was additionally stabilized by layer-to-layer NH···N interactions forming a unique hydrogen-bonded network with a 64·82-nbo

along with structurally related pyrenophthalocyanines (Chart 56 and Scheme 193). Highly photostable, anthracene-fused BODIPY dyes 116.7− 8 were developed in 2011 by Wu et al.626 The synthesis involved FeCl3-promoted oxidative coupling of the nonfused precursor 116.6, which produced a mixture of monomeric and dimeric fused BODIPY products. Both dyes showed similarly small optical bandgaps, indicating a relatively weak interaction of the subunits in the dimer, with absorptions extending up to ca. 950 nm, and weak NIR emission above 900 nm. 4.7.2. Imidazole-Containing Systems. In analogy to quinoxaline condensation, which is the synthetic basis of pyrazacene chemistry, the Debus−Radziszewski reaction provides a straightforward access to functionalized pyreno[4,5-d]imidazoles and 4,10-dihydropyreno[4,5-d:9,10-d′]diimidazoles, which are prepared, respectively, from pyrene4,5-diones and pyrene-4,5,9,10-tetraones. Apparently, the first 3567


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Scheme 115. Indole-Fused Pyrene Derivativesa

Chart 35. Pyreno[4,5-d]imidazoles

a

Reagents and conditions:624 (a) triethyl phosphite, o-DCB, reflux, 24 h; (b) Pd(OAc)2, 2 M, K2CO3, THF, reflux, 6 h; (c) CuI, 18-crown-6, K2CO3, DMA, reflux, 24 h.

depend on the relative configurations of N-substituents. In 2014, Peris and co-workers prepared a series of pyrene-based bisazolium salts starting from 4,5,9,10-tetrabromo-2,7-di-tertbutylpyrene 117.3.643 Subsequently, multiple Buchwald− Hartwig aminations were employed to convert 117.3 into tetraamines 117.4a−b. Interestingly, the bisannulations performed with trialkyl orthoformates yielded the imidazolium systems 117.5a−c, which unexpectedly contained alkyl groups introduced by the orthoformate. This result allowed the preparation of derivatives with linear alkyl groups, which could not be prepared otherwise, because of the restriction of the Buchwald−Hartwig coupling to amines with no βhydrogens. A still different behavior was observed, when tri-nbutyl orthoformate was used. This reaction yielded the dealkylated product 117.8, which was converted into 117.5d in two steps. 117.5a−d were all fluorescent, with emissions in the range of 370−420 nm and quantum yields of 0.29−0.41. Several bis-Rh and bis-Ir complexes were subsequently prepared, such as 117.6−7 in which 117.5a−c acted as a bridging bis(N-heterocyclic carbene) ligand. Electrochemical data indicated that, despite the conjugated character of the bridging ligand, those dimetallic complexes consisted of two essentially decoupled metal fragments. In 2014, Baitalik et al. employed the 4,10-dihydropyreno[4,5d:9,10-d′]diimidazole motif for the construction of a terpyridine-containing ligand C36.1 (Chart 36).644 Light-harvesting bimetallic RuII complexes C36.2a−b and C36.3, containing a C36.1 bridge, exhibited room-temperature emissions in the range of 657−703 nm with lifetimes between 5.8 and 67.0 ns. Detailed temperature-dependent emission spectroscopic studies showed that in all three cases, decreasing the temperature led to an increase of emission intensity, quantum yield, and lifetime. The electronic communication between the two ruthenium centers was evidenced by the presence of intervalence charge transfer transition (IVCT) bands in the NIR region of the spectrum when the mixed-valence species (RuII/RuIII) were electrochemically generated. An analysis of these IVCT bands indicated that couplings in these complexes were considerable, despite the large metal−metal distances (∼30 Å). 4.7.3. Other Nitrogen-Containing Systems. A stable, twisted heteroacene 118.3, containing terminal pyrene and pyridinone moieties, was reported in 2012 by Wudl, Zhang, and

a

Scheme 116. Pyreno[4,5-c]pyrroles

a Reagents and conditions:625 (a) Na, n-pentanol, reflux; (b) Cu(NO3)2, Ac2O, rt, 8 h; (c) DDQ, toluene, reflux, overnight; (d) phosphazene base 172.2, ethyl isocyanoacetate, THF, rt, overnight; (e) KOH, ethylene glycol, hydrazine, reflux, 1 h; (f)626 FeCl3, MeNO2.

topology. Related derivatives 117.2a−b, exhibiting deep-blue fluorescence and high thermal stability, were subsequently reported by Lu et al.640 Subsequent work from the groups of Lu641 and Huang642 showed that the emission properties 3568


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Scheme 117. Synthetic Routes to Pyrene−Imidazole Hybridsa

a

Reagents and conditions: (a)639 4-carboxybenzaldehyde, NH4OAc, glacial acetic acid, 2 h, reflux; (b)640 aniline, NH4OAc, benzaldehyde for 117.2a or 9H-carbazole-9-carbaldehyde for 117.2b, 2 h, reflux; (c)643 NaOtBu, Pd(OAc)2, 1,3-bis(2,6-diisopropylphenyl)imidazolium chloride, tertbutylamine for 117.4a or tert-amylamine for 117.4b, toluene, overnight, reflux; (d) HBF4·OEt2, trimethyl orthoformate for 117.5a or triethyl orthoformate for 117.5b or tripropyl orthoformate for 117.5c, 16 h, reflux for 117.5a,b or 24 h, 150 °C for 117.5c; (e) [IrCl(cod)]2, KOtBu, THF, overnight, rt; (f) CO gas, DCM, 20 min, 0 °C; (g) tributyl orthoformate, HBF4·OEt2, 24 h, 150 °C; (h) (1) NaOH, dimethylsulfoxide, 2 h, rt, (2) Nbutyl bromide, 30 min, rt, (3) overnight, 37 °C, (4) N-butyl iodide, 24 h, 100 °C; (i) [Et3O][BF4], DCM, 30 min, rt.

Scheme 118. Synthetic Route to a Twisted Heteroacenea

Chart 36. Bimetallic RuII Complexes Containing a Pyrenodiimidazole Bridge644

a Reagents and conditions: (a)645 isoamyl nitrite, dichloroethane, 2 h, reflux; (b) vacuum, 4 h, 220 °C; (c) tetrahydronaphthalene, 4 h, 220 °C; (d)646 isoamyl nitrite, dichloroethane, reflux, overnight.

Compound 118.3 is green in dichloromethane solutions, with an optical band gap of 1.86 eV, comparable to that of hexacene. Upon addition of a strong Lewis acid, such as anhydrous ZnCl2, a considerable blue shift was observed in the spectrum of 118.3, which was assumed to result from an interaction with the carbonyl oxygen. Analogous reactions performed on 118.1

co-workers (Scheme 118).645 Its synthesis involved a [4+2] cycloaddition between the aryne dienophile generated from 118.1 and a mesoionic pyrimidine as the diene. The intermediate adduct 118.2 was subjected to thermal cycloreversion, yielding the lactam 118.3 in excellent yields. 3569


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Scheme 119. Pyrenofuransa

Reagents and conditions: (a)647 (1) furan, 1.6 M n-BuLi, 2 h, −70 °C, (2) overnight, rt; (b) dipyridyltetrazine, chloroform, 18.5 h, reflux; (c) (1) dipyridyltetrazine, chloroform, overnight, rt, (2) N,N′-1,5-bis(maleimido)propane or N,N′-1,5-bis(maleimido)pentane, 1 h, rt. a

Scheme 120. Synthetic Pathways to [e]Fused Furanopyrenoidsa

a Reagents and conditions: (a)652 (1) cyclic ketone, 5 min, rt, (2) FeCl3, 12 h, rt; (b)651 CF3SO3H, m-cresol, N2 atm, 24 h, 180 °C; (c)652 allyl bromide, In powder, NaI, DMF, 4 h, rt; (d) (1) NaBH4, MeOH, 10 min, 0 °C, (2) 8 h, rt; (e) I2, THF, 12 h, rt; (f) DBU, DMF, 4 h, 50 °C; (g)653 (1) benzylphosphonium chloride or p-bromophenylmethylphosphonium bromide, potassium t-butoxide, dimethyl sulfoxide, 30 min, rt, (2) dihydrofuran, 2,3-dichloro-5,6-dicyano-1,4-benzoquinone, toluene, 4 h, 70 °C; (h)648 alkyl vinyl ketone, TiCl4, DCM, rt, 3 h; (i)650 4methoxyphenol, TFA, o-DCB, reflux.

The 1H NMR spectrum of 119.4a indicated that the short aliphatic tether was conformationally restricted within the cyclophane. Dione 108.1 is a convenient starting material for the synthesis of [e]fused furan derivatives (Scheme 120). By subjecting 108.1 to a tandem condensation, on the basis of the Baylis−Hillman reaction, Basavaiah et al. obtained a range of formyl substituted furans 120.11,648 as well as a range of alicyclic analogues such as 120.13.649 Electrophilic cyclization of 108.1 with a variety of phenols, explored in 2015 by Xiao, Ba, et al.,650 was shown to produce fused furans, such as 120.12, 120.14, and 120.15 in moderate yields. These furans were strongly fluorescent and exhibited self-assembly into nanostructures, tunable by reprecipitation conditions.

using diaryl-1,2,4,5-tetrazines as the diene reagent produced the “diazatwistpentacenes” 118.4a−c.646 4.7.4. Oxygen-Containing Systems. In 2009, Dibble et al. reported the synthesis of pyreno[4,5-c:9,10-c′]difuran 119.3 and its transformation into pyrenophanes 119.4 (Scheme 119).647 119.3 was obtained by means of a cycloaddition− cycloreversion procedure, developed in 1988 by Moursounidis and Wege for the synthesis of the monofuran analogue 119.5.549 In the first step, arynes generated sequentially from 119.1 were subjected to a double furan cycloaddition. The resulting bisepoxide intermediate 119.2, which formed as a mixture of syn and anti isomers, was reacted with dipyridyltetrazine to give 119.3. This difuran was reacted with tethered bis(maleimide) dienophiles, yielding pyrenophanes 119.4a−b, which were characterized in the solid state. 3570


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An acid-catalyzed condensation of 108.1 with m-cresol was reported in 2011 by Zhang and co-workers to yield the benzofuran system 120.1c in moderate yield.651 Nanowirebased films, obtained by reprecipitation of 120.1c in THF/ water mixtures, showed enhanced performance in organic lightemitting diodes in comparison with thin films of 120.1c deposited by direct drop-coating of the THF solution. An alternative route to benzofuran-fused systems 120.1a−d was developed by Rao and Vijjapu, who condensed 108.1 with cyclohexanone and its derivatives under oxidative conditions.652 Absorption and emission spectroscopic studies revealed the bathochromic effect of benzofuran annulation on the photophysical properties of the pyrene chromophore, and an associated increase of fluorescence quantum yields. In the same work, 10-methylpyreno[4,5-b]furan 120.5 was prepared in four steps starting from 108.1. Indium metal-mediated allylation of 108.1 first yielded the keto−alcohol 120.2, which was stereoselectively reduced at the carbonyl group, furnishing the trans-diol 120.3. The iodine-mediated cyclization−elimination of 120.3 produced the dihydrofuran derivative 120.4, which underwent final dehydroiodination to 120.5. In 2014, Kawase et al. reported the synthesis of highly fluorescent pyreno[4,5-b]furans 120.6a−b and the related isomeric pyrenodifurans 120.7 and 120.8. In these systems, the furan ring closure was achieved by reacting the 1,2 dione fragment with 2 equiv of an appropriate benzylidenephosphorane.653 An alternative, more complex approach to [e]furano-fused pyrenoids was described in 1985 by Royer et al.654 Instead of 108.1, they used as the starting material the aldehyde 120.9, which is available from pyrene in six steps. By condensing the latter aldehyde with bromonitromethane or ethyl bromoacetate, substituted furans 120.10a−b could be obtained in good yields. Pyreno[4,5-b]oxepine 121.3 was obtained by Boyd and coworkers as one of the products of base-induced eliminations performed on both racemic and enantiopure bromoacetates 121.1 (Scheme 121).655 121.3 formed alongside the isomeric epoxide 121.2, which was produced as a racemic mixture regardless of the enantiopurity of the starting 121.1.

Scheme 122. Benzo[b]benzo[10,11]chryseno[6,5d]thiophenea

a

Reagents and conditions: (a)556 (1) phenyllithium, (2) 2,2,2′,2′tetraethoxydiethyldisulfide; (b)556 PPA, chlorobenzene, reflux; (c)656 HBr/H2O/AcOH, pressure tube, 188 °C; (d)656 AlCl3, benzene, 10 min, reflux.

this approach was described in 2011 by Müllen and co-workers, who prepared the substituted benzo[8,9]triphenyleno[1,2b:4,3-b′]dithiophene 123.3 using a sequence of Stille coupling and photochemical dehydrocyclization (Scheme 123).657 The latter ring-closure turned out to be crucial for utilization in solution-processed OFET devices, and 123.3 showed an exceptional hole mobility for a perylene-based system. Related work from Duan, Nishioka, et al.658,659 and from the Müllen group660 described the preparation of the tetrathiophene system 123.5a−c. These molecules were similarly obtained, by using either photochemical or chemical oxidation in the ring-forming step (123.5a could not be obtained via chemical oxidation). In the solid state, 123.5b has a “saddle”-shaped πconjugated 1D supramolecular structure, which favors highly ordered self-assembly by π−π interactions, as evidenced by its concentration-dependent 1H NMR spectra in solution. 123.5b showed a relatively large band gap (2.86 eV), low EHOMO level (−5.64 eV), indicative of its potential use as a stable holetransporting material. An approach to the synthesis of pyreno[4,5-c][1,2,5]thiadiazole 124.6 and its oxides was developed in 2010 by Sutherland, Baumgartner, and co-workers (Scheme 124).661 The initial ring closure was performed on the bis-TMS derivative 124.1, yielding the S-oxide 124.3, which was reduced to 124.6 as well as oxidized to the S-dioxide 124.4. An alternative pathway to 124.6 was reported in 2013 by Xiao, Jiang, et al.662 Their synthesis involved direct condensation of pyrene-4,5-diamine 124.5 with thionyl chloride. Additionally, pyreno[4,5-c][1,2,5]selenadiazole 124.7 was obtained from the same diamine in a reaction with SeO2. A very different synthesis of 124.6, based on oxidative cyclization of the cyclophane precursor 124.8, was reported in 1991 by Mataka and coworkers.663 Increasing the number of S-bound oxygens in the series 124.6, 124.3, 124.4 resulted in a gradual shift of the first reduction potential (−2.08, −0.88, −0.66 V vs Fc/Fc+), providing access to building blocks for n-type semiconducting polymers.661 Upon reprecipitation from THF, 124.6 produced nanowires 160−400 nm in diameter, whereas 124.7 yielded quadrangle-type nanostructures with the morphology dependent on the precipitation conditions.662

Scheme 121. Synthesis of Pyreno[4,5-b]oxepina

(a)655 NaOCH3, THF, 2 h, 0 °C, 121.3 was the major product; (b) NaOCD3, THF-d8, NMR control, 121.2 (70%), 121.3 (30%).

a

4.7.5. Sulfur-Containing Systems. In 1981, Castle et al. reported a synthesis of pyreno[4,5-b]thiophene 122.3.556 Their low-yielding procedure involved the initial formation of a thioether 122.2, which was subjected to acid-catalyzed cyclization under fairly harsh conditions. Benzo[b]benzo[10,11]chryseno[6,5-d]thiophene 122.6 was prepared in 1966 by Henson and Vingiello (Scheme 122).656 This ring system was obtained by oxidative cyclization of the benzo[b]thiophene precursor 122.2 under Scholl conditions. 122.5 and related derivatives were obtainable in a highly efficient, acid-catalyzed cyclization of benzyl phenyl ketones, such as 122.4. A more recent approach to thiophene-fused pyrenoids relies on oxidative coupling of 2-thienyl substituents. An example of 3571


Chemical Reviews

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Scheme 123. Thiophene-Fused Pyrenesa

a

Reagents and conditions: (a)657 2-(tributylstannyl)thiophene, Pd(PPh3)4, toluene, overnight, reflux; (b) I2, hν at 300 nm, toluene, overnight; (c)658 Pd(PPh3)4, DMF, 100 °C; (d)660 Pd(PPh3)4, toluene, Ar atm, 110 °C, 7−16 h; (e) I2, hν at 300 nm, toluene, Ar atm, overnight, rt; (f)658,659 FeCl3, CH3NO2, DCM, 0 °C, 15 min; (g)660 FeCl3, CH3NO2, DCM, Ar atm, rt, 30 min.

Scheme 124. Pyreno[4,5-c][1,2,5]thiadiazole and Related Systemsa

Chart 37. Classification of Phenalenoids Used in This Sectiona


to phenalene results in the pyrene substructure). Interestingly, among the systems presented in this section, the choice of heteroatoms is virtually limited to nitrogen, oxygen, and sulfur. An isolated example of a boron-containing phenalenoid (39.7, Scheme 39) is shown in section 3.1. Large phenalenoid structures with ortho-fused heterocycles have been explored to a limited extent. A range of phenalene-1one derivatives have nevertheless been reported, including heterofused benzo[de]anthracen-3-ones C38.1665 and C38.2666 (Chart 38), and benzanthrones (benzo[de]anthracen-7-ones), containing pyridine (C38.3−4,667 C38.5668), pyridinone (C38.6−7669), imidazole (C38.8,670 C38.9,671 C38.11− 12672), oxazole (C38.10673), and indole (C38.13478) moieties. Fluorescent anthra[9,1-gh]quinoline dyes C38.5, obtained in a Skraup synthesis, were shown to have a very good orientation parameter in nematic liquid crystal (SA = 0.64−0.67), indicating a high potential for applications in display technology.668

a

Reagents and conditions: (a)661 SOCl2, toluene; (b)661 m-CPBA; (c)662 SnCl2·H2O, HCl (conc.), anhydrous EtOH, 70 °C, 5 h; (d)661 PPh3, CCl4; (e)662 SOCl2, TEA, DCM, reflux, 4 h; (f)662 SeO2, EtOH; (g)663 NBS, benzoyl peroxide, CCl4.

5. PHENALENOIDS The tricyclic phenalene ring system is the smallest graphene substructure containing a peri-fusion point. The phenalene framework contains an odd number of carbon atoms and has to be further modified to yield neutral, fully conjugated molecules with a closed-shell character. Simultaneously, this structural feature provides for the stability of phenalene-based radicals, which have been intensely explored.664 In line with the scope of this Review, this section is mostly restricted to phenalenoids containing at least 20 atoms in the ring system, a number that corresponds to at least two additional ortho-fused rings. As in previous sections, we exclude from our discussion derivatives of naphthalene anhydrides, imides, and their simple condensation products. Heteraphenalenes containing an increasing number of heteroatoms are discussed in sections 5.1, 5.2, and 5.3 (Chart 37). Phenalenes containing peri-fused 5- and 7-memebred rings are presented in section 5.4 (peri-fusion of a six-membered ring

5.1. Monoheteraphenalenes

5.1.1. Heterahelicenes. In the course of their investigations of heteratriangulenes (section 4.1) and following earlier work by Hellwinkel and Melan,357 Venkataraman et al. reported the synthesis of several quinolino[3,2,1-de]acridine5,9-diones358 (Scheme 125). The prototypical systems, containing five fused rings, were obtained by double intramolecular acylation of appropriately substituted triarylamines,358 exemplified by the conversion of 125.1 to 125.2.674 125.2 can be viewed as a [4]helicene derivative, and the work was extended to include higher helicenes, 125.3 and 125.4.674 Similar to carbocyclic helicenes, these systems 3572


Chemical Reviews

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Chart 38. Hetero-Fused Phenalene-1-one Derivatives

Scheme 125. Triarylamine-Derived Heterahelicenesa

5.1.2. 9a-Azaphenalene Ylides. The chemistry of 9aazaphenalene ylides was independently disclosed in 2014 by the group of Müllen680 and by Ito, Tokimaru, and Nozaki.681 Both groups presented a similar synthetic approach, in which a functionalized m-terphenyl 126.1 was cyclized and oxidized to the phenanthridinium salt 126.2 in either one680 or two681 steps. 126.2 could be easily deprotonated with triethylamine, but the resulting ylide 126.3 was too unstable to be isolated. 126.3 is ESR-silent and yielded a complex but sufficiently resolved 1H NMR spectrum at −60 °C,680 in line with the closed-shell structure predicted for the ylide using DFT calculations.681 The extended π conjugation in 126.3 is however evident from the electronic spectrum, which shows a broad band at 580 nm. 126.3 was shown to be a reactive dipole in the [3+2] cycloaddition, yielding a range of adducts, such as 126.4a680 or 126.9,681 and a dimerization product, 126.8.680,681 Acetylene adducts were occasionally observed to undergo spontaneous dehydrogenation (as in 126.5b681), or could be dehydrogenated with DDQ (126.5a680 and C42.1−4, Chart 42, section 5.4682). A phenyl-substituted salt 126.7 was obtained from 126.3, by using Grignard addition followed by hydride abstraction, and was further deprotonated to yield the corresponding ylide 126.6.680 The double ylide 126.12, the higher homologue of 126.3, was synthesized as a model of an N-doped zigzag graphene nanoribbon.680 The synthesis of the precursor salt 126.11 followed the strategy established for 126.2 and 126.7, with a separate hydride removal step. 126.12 could only be generated in solution, revealing an extended optical absorption with maxima at 685 and 751 nm. DFT calculations performed for 126.3, 126.12, and their predicted higher homologues indicated the zwitterionic nature of the azodimethine moieties and the retention of Clar sextets in the ribbon framework. A singlet ground state was predicted for all systems up to the “tetramer”, with a gradual reduction in the singlet−triplet gap with increasing ribbon length. 5.1.3. Ceramidonines. Naphtho[3,2,1-kl]acridin-9-ones, known as ceramidonines or ceramidones, are obtained by dehydration of 1-arylaminoanthraquinones under acidic conditions.683 Syntheses of hydroxy-substituted ceramidonines such as 127.2 were reported in 1945 by Cook and Waddington146 (Scheme 127). In these reactions, the

a Reagents and conditions: (a)674 Cu or CuI, K2CO3, Ph2O or (nBu)2O, 150−190 °C, 2−5 days; (b)674 (1) NaOH, H2O/EtOH (1:1), reflux for 1−3 days, then HCl, (2) (COCl)2, CH2Cl2, reflux, 0.5 h, then SnCl4, reflux 2−3 h.

show helically chiral, nonplanar conformations in the solid state, but their racemization barriers were not determined. 125.2−4 showed strong fluorescence in the visible range (460− 490 nm). Subsequent work led to camphanate-substituted enantiopure helicenes 125.5−6, which produced circularly polarized luminescence with dissymmetry factors of ∼| 0.001|.675 Acetylene linked homochiral bishelicene 125.7 was obtained by subjecting a Br-substituted helicene to double Stille coupling.676 125.5−7 were explored using fluorescence excitation circular dichroism on a single-molecule level.677−679 3573


Chemical Reviews

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Scheme 126. 9a-Azaphenalene Ylidesa

a Reagents and conditions: (a)680 HCl in dioxane (4 M), microwave, O2; (b)680 TEA (not isolated); (c) R = COOMe:680 DMAD, TEA, DCM, 25 °C; R = Ph:681 diphenylacetylene, toluene, 100 °C; (d)682 DDQ, toluene, 25 °C; (e)680 PhMgBr, THF, 0 °C; (f)680 [Ph3C][BF4], toluene, MeCN, 90 °C; (g)680 DMSO, NBu3, 190 °C; (h)681 C60, TEA, o-dichlorobenzene, 40 °C, 18 h.

extending up to 650 nm, and exhibited pinkish-violet fluorescence with maximum emission at 665 nm.688 Bock and co-workers extended the classical ceramidonine synthesis to obtain mono- and bis-ceramidone derivatives 127.15−17.147 127.16 and 127.17 showed continuous absorption up to ca. 500 nm. 5.1.4. Dibenzo[c,mn]acridin-8-ones. The parent system 128.2 (“benzo-semiflavanthrene”, Scheme 128), isomeric with ceramidone, was apparently first obtained by Koelsch in 1936,720 although related structures were postulated in earlier work on flavanthrone (Scheme 83, section 4.2) by Scholl and co-workers.721,722 128.2 and its derivatives were obtained by cyclization of appropriate phenanthridine,720 anthraquinone,163,723 and imine724 precursors and by extrusion of the thiazepine derivative 128.1.164 Larger fused derivatives 128.3725 and 128.4726 were reported by Wick. 5.1.5. Monooxa- and Monothiaphenalenoids. The majority of pyran- and thiopyran-containing phenalenoids, are, respectively, derivatives of benzo[kl]xanthene and benzo[kl]thioxanthene. In particular, ceroxene683,727 (naphtho[3,2,1kl]xanthene, C39.2a, Chart 39) and cerothiene derivatives (C39.2b)728,729 were investigated in the context of dyestuff research (see ref 146 for a brief summary). These compounds were named after von Baeyer’s “cöruleı̈n” C39.1, a classical mordant dye combining xanthene and anthraquinone motifs.730 Naphtho[1,2,3-kl]xanthenes and thioxanthenes of the general structure C39.3 were obtained via photocyclization of the corresponding benzylidene precursors731 or as side products in photoadditions of aromatic ketones to arylacetylenes.732 Quinone-fused bis-xanthenes were developed by Sakla et al. (e.g., C39.4), 733 whereas a range of phenazine-based derivatives, such as C39.5, were reported by Ooyama, Yoshida,

cyclization was accompanied by demethylation of methoxy substituents. 70% H2SO4 was the most typical condensation reagent, providing access to various substitution and fusion patterns144,146,147,478,684−688 and enabling an analysis of the condensation kinetics.689 A range of other acidic catalysts were also used, including AlCl3 melts478,690−693 and H3PO4.694,695 The reaction is generally regioselective, although the formation of carbazole byproducts was occasionally observed.690 More recently, ceramidonines (e.g., 127.11 and 127.13) were obtained by direct benzyne cycloadditions to 1-aminoanthraquinone 127.9. The requisite benzyne intermediates were generated from o-(trimethylsilyl)phenyl triflate 127.10 by Rogness and Larock696 and from 5-arylthianthrenium perchlorates (e.g., 127.12) by Kim et al.697 The reactivity of ceramidonines in electrophilic698 and nucleophilic699 substitutions was investigated, with a range of reagents being explored, such as amines,700−710 active methylene compounds,711 arylsulfinic acids,712 alkoxides,713 potassium cyanide,714 and thiophenols.715 Functionalized ceramidonines containing striazine716 and sulfonamide717 groups were developed for use in dyeing. Ceramidine derivatives of the general structure 146.14 were obtained by Wittig olefination of ceramidonine 127.11 by Lee et al.693 Sokolyuk and Pisulina reported a reversible photoarylotropic rearrangement of the ceramidonine derivative 127.7, in which the phenyl group could be induced to migrate between two adjacent oxygen atoms by alternating irradiation with UV and visible light.692 The use of more elaborate precursors was explored as a means of obtaining extended fused systems, such as 127.3−4718 and 127.5.719 The carbazole-linked bisceramidonine 127.6, reported by Müllen and co-workers, had a yellowish color in solution, despite electronic absorptions 3574


Chemical Reviews

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Scheme 127. Ceramidoninesa

Reagents and conditions: (a)146 75% H2SO4, 160 °C, 3 h; (b)692 visible light irradiation (λ > 450 nm); (c)692 UV light irradiation (λ = 300−380 nm); (d)696 CsF, MeCN, 65 °C, 24 h; (e)697 LDA, THF, reflux; (f)693 t-BuOK, THF, 10−43%. a

Scheme 128. Dibenzo[c,mn]acridin-8-onesa

Chart 39. Monooxa- and Monothiaphenalenoids

a

Reagents and conditions: (a)164 diethyl phthalate, copper bronze, reflux.

et al.734,735 The Pd-catalyzed dehydrogenative cyclization developed by the group of Zhang (cf., Scheme 100, section 4.3) was used for the construction of various fused sulfoxides, including the dibenzo[b,mn]thioxanthene 8-oxide C39.6.527 A smaller fused system of interest is the ferrocenyl-conjugated pyrylium cation C39.7, obtained in 2009 by Nishihara et al., by means of the acid-catalyzed acetylene cyclization described

above (e.g., 37.7, section 3.1).736 Unusually, this system shows an anion-dependent valence tautomerism, the TFSI and PF6 3575


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salts gradually switching from the FeII structure C39.7a to the FeIII species C39.7b, as the temperature was increased. The interaction between ferrocene and pyrylium subunits could be switched off by nucleophile addition to the pyrylium fragment of C39.7.737 Expanded iminocoumarins of the general structure 129.2, and their N-protected analogues, were reported in 2015 by Gryko, Cywiński, et al. (Scheme 129).738 These systems, which

Miserez described the synthesis of two isomeric derivatives C40.1−2 in the course of their work on indigo yellow 3G.739 Related oxazole derivatives, such as C40.3, were obtained by Kazankov et al.740,741 Gerasimenko et al. described naphthacenopyridones capable of photoinduced aryl group migration (e.g., C40.4a−b).742−744 Pyrazole-fused systems C40.5 were obtained by Shechter et al. by alkyne cycloaddition to 2diazoaceanthrenone, followed by rearrangement.745 An interesting example of an azaphenalenoid found in dyestuff chemistry2 is compound C40.6, known as C.I. Disperse Yellow 77, obtainable by condensing 1-aminoanthraquinone with anthranilic acid. Various derivatives of C40.6 were synthesized,495,746−748 including its π-extended analogues such as C40.7−8. The hexacyclic system C40.9 was obtained in 1984 by Katritzky and co-workers,473 using the photocyclization approach presented in Scheme 86, section 4.2. C40.10, a 1azaphenalene derivative, was synthesized using the Graebe− Ullmann reaction by Waldmann and Hindenburg.478 A synthesis of “vertically expanded” imidazo[1,2-a]pyridines, which contain the 2-azaphenalene motif, was recently reported by Gryko and co-workers (Scheme 130).749 Using Heck-type coupling, the unsubstituted 130.1 was converted into 3-aryl derivatives 130.2, which were subjected to the anion−radical coupling with potassium metal to yield the fused products 130.3a−e. A sequential cycloaddition−cycloreversion performed with DMAD was used for further modification of the ring system, producing fused indolizines such as 130.4. Müllen and co-workers reported a concise method of expanding naphthalene and perylene anhydrides via condensation with acetylanilines (for perylenoid derivatives described in this work, see Scheme 30, section 3.1, and Scheme 65, section 3.5).136 The condensation of naphthalene monoanhydride (NMA) with 2-acetylaniline was found to produce either of two isomeric systems: the 4-hydroxyquinoline-fused 130.5 or the 4oxoquinoline derivative 130.6, depending on the reaction conditions. 130.5 was the thermodynamic product of the reaction and was available through rearrangement of the initially forming 130.6. The latter species was further expanded by condensation with malonodinitrile, yielding 130.7.

Scheme 129. Expanded Iminocoumarinsa

a

Reagents and conditions: (a)738 K2CO3, DMSO, 60 °C.

display polarity-sensitive fluorescence, were obtained in moderate yields via a double annulation reaction between the anthraquinone derivative 129.1 and substituted acetonitriles. Interestingly, the 2 equiv of the acetonitrile reagent produced two rings of different sizes, leading to products that combined benzofuran and iminocoumarin fragments. 5.1.6. Miscellaneous Azaphenalenoids. Syntheses of other extended azaphenalene systems were occasionally reported in the literature (Chart 40). De Diesebach and Chart 40. Miscellaneous Monoazaphenalenoids

5.2. Diheteraphenalenes

Out of many possible heteroatom patterns, 1,6-diheteraphenalene is encountered most frequently as the key structural motif of pyridoacridine alkaloids and heterahelicenes. π-Extended 1,4and 1,3a-diheteraphenalenes, represented mostly by quinolino[4,3,2-kl]acridines and phenoxazine derivatives, respectively, are discussed at the end of this section. For reported examples of 1,3- and 1,9-diheteraphenalenoid systems, see refs 750−753. 5.2.1. Pyridoacridines. Pyridoacridines are a family of alkaloids isolated from marine invertebrates (tunicates, sponges, and bryozoans) sharing the common fused motif of pyrido[2,3,4-kl]acridine (131.1).754−758 The interest in these alkaloids stems from their strong cytotoxicity and DNA binding.756 The following discussion is restricted to reported syntheses of pyridoacridines containing at least 5 fused rings, and showcases key peri-annulation steps. The chemistry of the perylenecontaining alkaloid, eilatin 44.5, is discussed in section 3.1. Ascididemin 131.2, one of the most prominent examples of pyridoacridine alkaloids, was first synthesized in 1989 by Bracher, who performed self-condensation of enamine 131.6 to close ring E in the final step (Scheme 131).759 This approach enabled the synthesis of related alkaloids, such as 2bromoleptoclinidone,760 neocalliactine acetate,761 and kuanoni3576


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Scheme 130. π-Expanded 2-Azaphenalenesa

Reagents and conditions: (a)749 ArBr or ArI, Pd(OAc)2, KOAc, DMA, 150 °C, 50−83%; (b)749 K, toluene, air, 95 °C; (c)749 dimethyl acetylenedicarboxylate, mesitylene, 150 °C; (d)136 imidazole, 140−160 °C; (e)136 Zn(OAc)2, quinoline, 180 °C; (f)136 AcOH, Ac2O, malononitrile, reflux, 31%. a

Scheme 131. peri-Annulations in the Syntheses of Ascididemina

a Reagents and conditions: (a)759 NH4Cl, AcOH, 1 h; (b)775 H2SO4, hν, 2 h; (c)776 NaN(CHO)2, DMF, reflux; (d) BBr3, DCM (−78)−0 °C; (e) NBS, DMF, 0 °C; (f) 10 kbar, 80 °C, MeCN; (g)780 NaH, DMF, MW, 95 °C, 25 min; (h) O2, rt, 3 h; (i)774 CeCl3·7H2O, air, EtOH, reflux, 9 h; (j) 25% NH3 (aq), MeOH, rt, 7 days.

amine A762,763 (131.3−5), as well as several synthetic analogues.764−767 A refinement of the Bracher synthesis, in which the final ring closure was achieved by reaction with paraformaldehyde, was reported by Copp and co-work-

ers.768−771 Mechanistically related syntheses of 131.2 were reported by the groups of Kashman772 and Echavarren.773 The Kashman route222,772,774 is particularly efficient because it closes two rings in a single reaction step, and enables the 3577


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Scheme 132. peri-Annulations in the Syntheses of Pyridoacridine Alkaloidsa

a Reagents and conditions: (a)782 (1) aq HCl/THF, 70−80 °C, 3 h, 86%, (2) Me2SO4, K2CO3, DMF, 23 °C, 3 h, 96%; (b)789 (1) H2, Pd/C, MeOH, 91%, (2) 57% aq HI, 88%; (c)795 diphenyl ether, reflux, 5 min, 74%; (d)797 chlorobenzene, acetophenone, hν; (e)798 KF/Al2O3, 18-crown-6, DMSO, 120 °C, 2 h; (f)802 TMPMgCl·LiCl (2.2 equiv).

synthesis of analogues with extended fusion, such as benzoascididemin222 or 131.15.774 Moody et al. synthesized ascididemin by photochemical cyclization of the iodo derivative 131.7.775 The approach of Á lvarez et al.776 involved an early closure of the E ring, performed on the acetylene intermediate 131.8, followed by the construction of ring A via a heteroDiels−Alder reaction. Kristensen et al. developed an anionic cascade reaction providing a step-efficient route to various pentacyclic systems.777−779 In a particular application of this method, the nitrile precursor 131.12 was transformed into the unstable deoxy intermediate 131.13, which was oxidized in situ to ascididemin.780 In late 1980s, three synthetic approaches to amphimedine781 (132.2) were disclosed almost simultaneously by Echavarren and Stille,782 Kubo and Nakahara,783,784 and Prager et al.785,786 In particular, Echavarren’s route involved imine bond formation on a preassembled quinone derivative 132.1 (Scheme 132). A similar scheme was employed in the syntheses of isoascididemin by Gómez-Bengoa and Echavarren,787 meridine 132.4 by Kubo et al. (from 132.3),788,789 sebestianine A by Delfourne et al.,790 and related systems.791−794 Meridine could alternatively be constructed from the peri-fused precursor 132.5, which was cyclized by brief heating in diphenyl ether.795 An alternative peri-fusion strategy, based on intramolecular photocyclization of aryl azides, was developed by Ciufolini et al., yielding sulfurcontaining alkaloids, such as nordercitin 132.9, kuanoniamine D 132.10, and dercitin 132.12.796,797 Arnoamine A (132.15), a unique pyridoacridine with a peri-condensed pyrrole ring, was synthesized by Delfourne et al., who used intramolecular nucleophilic substitution to simultaneously create two perifusion points.798 A range of amphimedine analogues (e.g., 133.2a−b, Scheme 133) was obtained by Wang and co-workers by means of CuII-799 or AuI-catalyzed800 tandem bis-annulations of N-propargylaminoquinones, such as 133.1. A Brønsted acid-

Scheme 133. Syntheses of Pyridoacridines via Tandem BisAnnulationsa

Reagents and conditions: (a)799 1 equiv of CuCl2, MeNO2, 80 °C, 4 h; (b)800 Ph3PAuCl (5 mol %), AgOTf (5 mol %), TFA, rt, 1 h; (c)801 Fe2(SO4)3 (10 mol %), H2SO4/HOAc, 100 °C, 2 h. a

promoted bisannulation of quinone−ethynyl precursors (e.g., 133.3) was subsequently developed as a route to ascididemintype alkaloids.801 Another distinct cyclization strategy, relying on a directed remote ring metalation of benzo[c][2,7]naphthyridine 132.16 with Knochel−Hauser base followed by intramolecular trapping of an ester group, was employed by Bracher et al. in the synthesis of demethyldeoxyamphimedine 132.17.802 5.2.2. Other 1,6-Diheteraphenalenoids. Outside the field of pyridoacridine alkaloids, the 1,6-diheteraphenalene unit is found in several classes of heterocycles, such as the 3578


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chromeno[2,3,4-kl]acridines 134.1803 and 134.3a804 and their heterologues 134.3b−d (Scheme 134).804 134.3a−d were

The efficiency of the SNAr synthesis of 135.3 is somewhat dependent on the identity of the amine, and the competing formation of a neutral acridine side product was observed when benzylamine was used as the nucleophile.809 Subsequent detailed analysis by Laursen, Lacour, et al. showed that the 135.3 cations could be resolved into enantiomers, via the formation of diastereomeric binphat salts.810 These enantiomers were configurationally stable, but slow racemization could be induced above 200 °C. The 135.3 cations underwent nucleophilic addition to the central carbon. The resulting adducts (e.g., 135.10810 and 135.11811) retained the sense of helicity present in the cation. The addition of enantiopure sulfoxide anions yielded diastereomeric products 135.11, which could be resolved and converted back to the diazahelicenium cations in an unusual type of Pummerer rearrangement.811−813 Diazahelicenium cations unsymmetrically substituted with two different R′ groups reacted with hydride and organolithium nucleophiles in a highly diastereoselective manner (dr > 49).814 Reduction of 135.3b with sodium borohydride yielded the neutral helicene 135.14.808 The central CH bond in 135.14 was found to be susceptible to deprotonation with strong bases, and the resulting carbanion could be trapped as thioamide 135.15 by reaction with benzyl isothiocyanate. The 135.3 helicenes are highly stable carbocations (pKR+ ≈ 19), showing strong UV−vis absorption (up to ca. 680 nm, εmax > 104 M−1 cm−1), which is considerably red-shifted in

Scheme 134. Other 1,6-Diheteraphenalenoidsa

a

Reagents and conditions: (a)804 94% H2SO4, 130 °C, 3 h.

prepared by acid-catalyzed cyclization−demethylation of xanthenone precursors 134.2a−d.804 The most versatile route to 1,6-diheteraphenalenoids was disclosed by Laursen and Krebs in their synthesis of triheteratriangulenium salts (section 4.1).32,420,426 When tris(2,6-dimethoxyphenyl)carbenium cations 135.2 are reacted with a primary amine, the methoxy groups are displaced in a sequence of SNAr reactions, leading, among other products, to a range of N-substituted 1,13dimethoxyquinacridinium (“diazahelicenium”) cations 135.3 (Scheme 135).805 Scheme 135. Diheterahelicenesa

a Reagents and conditions: (a)805 (1) n-BuLi, Et2O, reflux, (2) (EtO)2CO, Et2O, reflux, (3) aq HBF4, Et2O, rt, 85%; (b)805 PrNH2, NMP, 45 min, 110 °C, 85%; (c)805 N,N-dimethylethylenediamine, N-methylpyrrolidone, 110 °C, 60%; (d)805 (1) MeI, MeOH, 110 °C, (2) t-BuOK, DMF, room temperature, (3) aq HBF4, room temperature, 100%; (e)805 (1) aq HBF4, MeOH, 70 °C, (2) aq Na2CO3, CH2Cl2, room temperature, 94%; (f)805 PbO2, benzene, room temperature, 99%; (g)806 (1) MeNH2 (2.5 equiv), MeCN, 0.5 h, (2) HBF4, 95%; (h)806 (1) N2H4 (2.5 equiv), MeCN, 0.5 h, (2) HBF4, 95%; (i)806 N2H4 (aq, 25 equiv), DMF, 90 °C, 8−14 h, darkness, 97%; (j)806 MeNH2 (25 equiv), DMF, 90 °C, 10−14 h, darkness, 91%; (k)807 (1) BBr3 (2.5 equiv), DCM, (2) HBF4 (aq), (3) 100 °C, neat, (4) NaBH4, EtOH, 15%; (l)807 I2, Et2O, quant.; (m)807 DMSO-d6, 120 °C; (n)808 NaBH4, EtOH, 95%; (o)808 (1) n-BuLi (2 equiv), Et2O, 40 min, −78 to 0 °C, (2) BnNCS (10 equiv), 20 min, 0 °C, 85%.

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comparison with the completely fused triazatriangulenium cations. The helicene cations show NIR fluorescence with quantum yields in the 2−20% range and lifetimes of 1−12 ns.815 The interactions of 135.3 with DNA were consequently investigated using absorption, fluorescence, and circular dichroism spectroscopies, showing that the binding is dependent on the substituents and the chirality of the helicene core.816 A diazahelicene radical was reported in 2012 by Takui, Morita, et al.805 In their approach, the 135.3c cation was deprotected by using a multistep sequence consisting of quaternization, Hoffmann elimination, and acidolysis of the vinyl substituents. The resulting neutral species 135.5 was resolved into enantiomers on a chiral HPLC column, and oxidized with PbO2 to yield the chiral, configurationally stable radical 135.6. The enantiomers of 135.6 were investigated in detail using ESR, ENDOR, and CD spectroscopic methods. The synthesis of singly N-substituted helicene cations (e.g., 135.9) was subsequently explored by Lacour et al., who developed two alternative routes, each involving a quinacridinium intermediate (135.7 or 135.8) and a hydrazine-mediated cyclization in either the first or the second step.806 In each approach, an excess of hydrazine or amine was used to effect the cleavage of the N−N bond. The synthesis of the dioxa analogues of 135.3, chromenoxanthene cations 135.13, was achieved in 2010 by Lacour and co-workers.807 In the optimized reaction sequence, the reduced neutral species 135.12 was first obtained by partial demethylation of 135.2a, followed by thermal cyclization of the resulting mixture of xanthen-9-ylium cations and hydrogenation with NaBH4. 135.12, which could be separated into enantiomers by means of chiral HPLC, was quantitatively converted into 135.13 by oxidation with diiodine. 135.13 was unstable and was gradually converted into the trioxatriangulenium cation 77.3a upon heating. 5.2.3. 1,4-Diheteraphenalenoids. The quinolino[4,3,2kl]acridine system 136.2, structurally related to the pyridoacridines described above, was synthesized by Mitchell and Rees817 and Stevens et al.818 from the benzotriazole 136.1, using, respectively, photochemically and thermally induced variants of the Graebe−Ullmann synthesis (Scheme 136). The reaction is quite general, and a range of substituted derivatives could be obtained. 136.2 and its derivatives were explored for their antitumor properties by the Stevens group, who developed alternative synthetic protocols based on radical cyclizations and Pd-catalyzed couplings.819−825 Related pentacyclic systems, 136.3−5, were obtained by Katritzky et al. in a modified synthesis involving corresponding dihydroacridines, xanthenes, and thioxanthenes (136.6).826 136.4 was also obtained in low yield in a direct coupling reaction between benzotriazole and xanthene.827 Alajarı ́n et al. reported the synthesis of the hexacyclic system 136.10, which was obtained from 136.7 in an attempted bisannulation of the transient ketenimine 136.8.828 The desired bisannulated product 136.9, which formed alongside 136.10, could be reductively cyclized to yield 136.10. A structurally distinct 1,4-diheteraphenalenoid, komarovidinine 136.12, was obtained by Tulyaganov et al. as a dehydrogenation product of alkaloid isokomarovine, and was synthesized independently by oxidative cyclization of the βcarboline 136.11.829 In later work, an N-oxide of 136.12 was isolated and characterized.830 5.2.4. 1,3a-Diheteraphenalenoids. This heteroatom pattern is found in a range of fused phenoxazine and phenothiazine derivatives. In a 1944 work of Gilman and co-

Scheme 136. Syntheses of Quinolino[4,3,2-kl]acridines and Related Systemsa

a Reagents and conditions: (a)817 UV irradiation, 254 nm, MeCN, 39%; (b)818 Ph2O, reflux, 2 h, 84%; (c)826 (1) n-BuLi, THF, (2) CuI, reflux; (d)827 DDQ, toluene, reflux; (e)828 (1) PPh3, toluene, rt, 1 h, (2) Ph2CCO, toluene, rt, 30 min; (f) H2, Pd/C, EtOH, 60 °C, 14 h, 65%; (g) P(OEt)3, o-xylene, reflux, 16 h, 30%; (h)829 Pd black, 180−200 °C, 40 min.

workers, 9H-quinolino[3,2,1-kl]phenothiazin-9-one 137.2 was synthesized by intramolecular cyclization of two isomeric acyl chlorides 137.1 and 137.3.831 Fused phenoxazine 137.6a and its sulfur analogue 137.6b were obtained in 1962 by VanAllan and Reynolds in a two-step sequence involving intramolecular acylation of the corresponding nitriles 137.4a−b, followed by dehydrogenation.482 In subsequent work, the carbonyl reactivity of 137.6a−b was explored by the same authors.832 More recently, the group of Lautens described a domino bisannulation leading to the phenoxazine derivative 137.10.833 The reaction combined two bifunctional reactants, 137.7 and 137.8, which were assembled catalytically using norbornene as a template. It was proposed that the templating effect was achieved through the formation of a cyclic intermediate 137.9, which directed the first Heck-like coupling ortho to the iodo substituent. The antitumor activity of quinobenzoxazine834 (A-62176, 138.4, Scheme 138) spurred an interest in fused 3Hpyrido[3,2,1-kl]phenoxazin-3-one derivatives. A general synthetic strategy to these systems (138.3) involves a basecatalyzed double condensation of enamino-ketoacid 138.1 followed by regioselective nucleophilic substitution of one of the remaining fluorines. By employing a range of fused oaminophenols, the research groups of Hurley835,836 and Na837 3580


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Scheme 137. 1,3a-Diheteraphenalenesa

of the 20th century, can be prepared with variable efficiency from a range of anthranilic acid derivatives,838−844 or from indazole838,840 (Scheme 139). A higher yield was achieved in Scheme 139. Syntheses of Tricycloquinazolinea

a

Reagents and conditions: (a)841 R = COO(NH4), heat, 0.08%; (b)841 R = COOMe, ammonium benzenosulfonate, heat, 5−16%; (c)841 R = CONH2, p-TsOH, heat, 20−25%; (d)840,841 R = CN, p-TsOH, 210− 230 °C, 26%; (e)839 R = CHO, NH4Cl, 17−22%; (f)838,840 Cu, 270− 290 °C, 17%; (g)845 SnCl4, P4O10, xylene, reflux, 66%; (h)846 for 139.1: NH4OAc, sulfolane, AcOH, 170 °C, 10 h.

a Reagents and conditions: (a)831 (1) PCl5, xylene, 10 min, rt, (2) SnCl4, xylene, 45 min, rt; (b)482 HCl, AcOH, reflux; (c) Pd/C, pcymene, reflux; (d)833 Pd(OAc)2, P(m-chlorophenyl)3, norbornene, K2CO3, MeCN, 135 °C, 18 h.

1959 by Butler and Partridge, who prepared 139.1 by dehydration of the oligoamide 139.2, thus providing an initial structural proof.845 Tricycloquinazoline is a thermally stable compound and easily undergoes electrophilic substitution reactions.839,845,846 It is highly carcinogenic,841 and a large number of its substituted derivatives were prepared by Partridge, Baldwin, et al., with the aim of studying the tumorinducing properties.847−851 An improved synthesis of tricycloquinazolines, developed in 1973 by Yoneda and Mera,846 involves heating of substituted anthranils with ammonium acetate in a mixture of sulfolane and acetic acid. Substituted derivatives 139.3−6 obtained in this way were subsequently functionalized with alkoxy- and alkylthio chains by the groups of Keinan and Kumar, to yield a new family of discotic liquid crystals with electron-deficient cores.852−856 These compounds are highly fluorescent with an emission maximum at ca. ∼570 nm. An oligoether-substituted TCQ was found to form a Colh phase that could be doped with up to 10 mol % of potassium, yielding electron mobilities of ∼10−4 cm2 V−1 s−1 at 150 °C.857 A class of TCQ-containing materials was obtained in 2004 by Faul et al., using ionic self-assembly between the hexahydroxy derivative of 139.1 and tetraalkylammonium salts.858 The hexaanionic TCQ tecton was found to produce several types of self-assembled structures, including a lamellar mesophase with an interlayer spacing of 2.98 nm. In the course of their investigations on tricycloquinazoline and its carcinogenic properties, published mostly in the 1960s, Partridge and co-workers synthesized a number of related cyclic amidine derivatives, including isomers of 139.1 (140.7,859 140.9,860 and 140.15861), triaza systems (140.12,493 140.13,493 140.17,861 140.19861), and diazaoxa systems 140.3859,862 and 140.22861,863 (Scheme 140). The synthetic strategies mostly relied on classical dehydrative condensations and nucleophilic substitutions, and were generally concise and high-yielding. In the case of 140.12, the structural ambiguity associated with the

Scheme 138. 3H-Pyrido[3,2,1-kl]phenoxazin-3-onesa

Reagents and conditions: (a)836 NaHCO3, DMF, 100 °C, 2.5 h; (b) amine, pyridine, reflux, 36 h.

a

successfully prepared extended derivatives of quinobenzoxazine 138.5−9. 5.3. Tri- and Tetraheteraphenalenes

5.3.1. Tricycloquinazolines and Related Systems. Tricycloquinazoline (TCQ) 139.1, known since the beginning 3581


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Scheme 140. Syntheses of Tricycloquinazoline Analoguesa

Reagents and conditions: (a)859 AcOH, 180−190 °C, 90 min; (b)860 (1) acetone, 30 min, (2) NH3/EtOH; (c) neat, 210 °C, 1 h; (d) (1) NaCl, AlCl3, 100−320 °C, 1 h, (2) NaNO3, HNO3 (76% as nitrate), (3) NEt3; (e) (1) POCl3, PCl5, 120−140 °C, 3 h, (2) 2-aminopyridine; (f)493 NaCl, AlCl3, 320 °C, 1 h; (g)861 neat, 320 °C, 15 min; (h) N2H4, Raney Ni, EtOH, 70 °C; (i) aniline, reflux, 30 min; (j) DMF, aq NaOH, reflux, 15 min; (k)864 H2, Pd/C, EtOH. a

final Scholl-type coupling was resolved by a parallel synthesis of the methyl substituted derivative 140.13.493 In related later work by Martı ́nez et al., the formation of N-oxides 140.24 and 140.26 was postulated on the basis of partial spectroscopic data.864 A different approach to tetraazaphenalenoid systems was developed by Shaw et al.865−867 By condensing a substituted 1,3,6,9a1-tetraazaphenalene with various bidentate nucleophiles, a range of extended nitrogen-rich frameworks were obtained (C41.1−3, Chart 41). In 2014, a pentacyclic phenalenoid 141.3 was obtained in a uniquely concise synthesis by Rajanarendar et al.868 The initial step involved a CAN-mediated multicomponent condensation of isoxazolyl cyanoacetamide 141.1 with malonodinitrile, benzaldehyde, and 2-hydroxyacetophe-

none, and was followed by the closure of the pyrimidinone ring with acetic anhydride (Scheme 141). A route to heterahelicene derivatives 142.2 and 142.3 was developed in 2004 by Okada et al. (Scheme 142).869 Their strategy involved double intramolecular N-arylations of appropriate linear precursors, 142.1 and 142.4, which were performed, respectively, under Pd- and Cu-catalyzed coupling Scheme 141. Multicomponent Synthesis of a Pentacyclic Phenalenoida

Chart 41. Extended 1,3,6,9a1-Tetraazaphenalenes

Reagents and conditions: (a)868 CAN, EtOH, 60 °C; (b) Ac2O, reflux, 5 h. a

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Metzger.873 In 2005, Takahashi and co-workers employed a chromium-mediated benzannulation to convert N-phenylpyrrole into ullazines 143.5a−d in a sequence of two cyclization steps.874 A concise approach to ullazine derivatives was reported in 2013 by the Grätzel group,875 who transformed bisalkynes 143.6 into substituted ullazines (e.g., 143.7) using the Fürstner cyclization/hydride shift reaction.876 Interestingly, 143.7 undergoes Vilsmeier−Haack formylation with good regioselectivity, providing convenient access to functionalized derivatives, such as 143.9a−b. The latter two species as well as a range of their differently substituted analogues were used as sensitizers in DSSC devices, yielding power conversion efficiencies of up to 8.4% (for 143.9a). A number of larger cyclopenta[cd]phenalenes are known, containing a heterocyclic five-membered ring. However, no general synthetic route to such systems has been developed. POCl3-induced dehydration of two different dicarboxylic acids 144.1 and 144.2, carried out in 1943 by Gilman and Stuckwisch,877 yielded benzo[8,1]indolizino[2,3,4,5,6-defg]acridine-7,11-dione 144.3 (Scheme 144, for an extension of this approach, see ref 444). In 1983, Grimshaw and Hewitt reported the formation of the fused triazole 144.5, which could be induced electrochemically or photochemically.878 The fused thiophene 144.8 was prepared in 2004 by Höger et al. in a twostep sequence involving the condensation between sodium 3thienylacetate and pyrylium salt 144.6, followed by double intramolecular Heck coupling.879,880 144.8 was subsequently elaborated into the highly functionalized rod-like structure 144.9, which aggregated in solution and on solid substrates. Interestingly, a similar thiophene−phenalene fusion is found in a classical dye, Indanthrene Blue Green FFB 145.2 (Scheme 145), obtainable via cyclization and dimerization of benzanthrone-3-sulfinylacetic acid 145.1.1,464 Ren et al. used the Lewis-acid-activated Friedel−Crafts arylation881 to convert the bis(triazene) precursor 144.10 into the dibenzoullazine derivative 144.11.882 Extended dibenzoullazines C42.1−4 were also obtained in a [3+2] cycloaddition− dehydrogenation sequence performed on azaphenalene ylide 126.3 (Chart 42, see also Scheme 126, section 5.1).682 Other systems containing a hetero cyclopenta[cd]phenalene unit include arnoamine A (132.15, section 5.2) and the fused phosphole system 53.14285 described in section 3.2. Each of the phenalene derivatives discussed above has a closed-shell configuration in the neutral state. A number of heteroaromatic phenalene derivatives were developed with the aim of stabilizing open-shell electronic structures. The prime example of such a structure is 1,3-diazaphenalenyl, a persistent neutral radical prepared in 2002 by Morita et al., which displayed thermally controlled σ-dimerization in the solid state.883−885 Tetrathio-886 and hexathiophenalenylium887 cations [160.13]+ and [160.15]+ were prepared by Haddon et al. by multiple substitutions of the respective ketones 144.12 and 144.14 using various sulfur sources. Phenalenyl 160.13 was obtained by one-electron reduction of the monocation dimerizes in the solid state with concomitant opening on one of the disulfur bridges,886 whereas the 160.15 radical has so far only been observed in solution.884 A thiophene-based bis(phenalene) structure 144.19 was reported in 2004 by Kubo and co-workers.888 Its synthesis started with a one-step formation of the central thiophene ring from acenaphthylene 144.16. The resulting intermediate, 144.17, which formed as a mixture of regioisomers, was elaborated into the bis(phenalene) precursor 144.18 using a

conditions. An extension of this strategy was later used by the same group in the synthesis of 2,2′:6′,2″:6″,6-trioxytriphenylamine 76.7 (section 4.1).386 A different approach to the synthesis of substituted heterohelicenes, such as 142.6, was proposed in 2008 by Menichetti and co-workers.870 142.6 was obtained by direct electrophilic insertion of two sulfur bridges to the electron rich arylamine 142.5, in a reaction with phthalimidesulfenyl chloride 142.9. In an analogous reaction, the benzo[a]phenothiazine derivative 142.7, itself prepared by electrophilic bridging of an appropriate triarylamine, was converted into the extended helicene 142.8 in 75% yield. 142.6 and 142.8 were separated into enantiomers by HPLC using a column packed with an amylose-based chiral stationary phase. Radical cations of 142.3 and its substituted derivatives were investigated in 2015 by Menichetti, Viglianisi, et al.871 These radicals could be generated in nearly quantitative yields using AgSbF6 as the oxidant, and the oxidation process was electrochemically reversible. Scheme 142. Triarylamine Heterohelicenesa

a

Reagents and conditions: (a)869 Pd(dba)2, (t-Bu)3P, t-BuONa, toluene, reflux 2 h; (b)869 CuI, KI, HMPA, 160 °C, 16 h; (c)870 2.3 equiv of 142.9, CHCl3, 60 °C, 5 h; (d)870 1.5 equiv of 142.9, CHCl3, 60 °C, 16 h.

5.4. Phenalenoids with Nonbenzenoid peri-Fusion

5.4.1. Cyclopenta[cd]phenalenes. The synthesis of ullazine (indolizino[6,5,4,3-ija]quinoline), a four-ring cyclopenta[cd]phenalene isoelectronic with pyrene, has been achieved by means of several distinct strategies (Scheme 143). In the original report by Balli and Zeller, published in 1983, substituted ullazines 143.2a−d were obtained by in situ deprotection and condensation of the double dithianes 143.1a−d.872 A charge-separated valence structure corresponding to an outer [13]annulene anion (143.10) can be drawn for ullazine, indicating a potential for stabilizing charged states. Radical cations and anions of 143.1a−d were generated and studied using ESR and ENDOR spectroscopies by Gerson and 3583


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Scheme 143. Ullazinesa

Reagents and conditions: (a)872 HgCl2, MeOH, H2O, reflux, 24 h; (b)874 (1) n-BuLi, TMEDA, hexane, 0 °C, 1 h, (2) CrCl3, 3-hexyne, 50 °C, 12 h; (c)874 (1) n-BuLi, THF, −78 °C, 1 h, (2) CrCl3, (R)2-acetylene, 50 °C; (d)875 60% InCl3, toluene, 100 °C, 24 h; (e)875 (1) POCl3, DMF, DCE, rt, 2 h, (2) 1 M NaOH, rt, 1.5 h; (f)875 cyanoacetic acid, piperidine, CHCl3, 4 h, 80 °C. a

Scheme 144. Syntheses of Heterocyclic Cyclopenta[cd]phenalenesa

Reagents and conditions: (a)877 POCl3, xylene, rt, 20 min; (b)878 cathode reduction, −2.25 V vs SCE (20%) or hν, MeOH (29%); (c)879 Ac2O, 150 °C, 2 h; (d)879 PdCl2(PPh3)2, DBU, DMA, 160 °C, 12 h; (e)882 BF3·Et2O, DCM, rt, 30 min; (f)886 (1) P2S5, toluene, reflux, 16 h, (2) HCl (4 M), reflux, 1 h; (g)887 (1) KSAc, DMF, rt, 72 h, 70%, (2) Lawesson’s reagent, chlorobenzene, 135 °C, 36 h, (3) TfOH, 8% (two steps); (h)888 sulfur, DMF, reflux, 2 h, 90%; (i)888 p-chloranil, benzene, reflux, 5 min. a

1.00 V. The latter feature was reflected in the electronic spectrum of 144.19, revealing an extended NIR absorption in the 800−2000 nm range. The redox amphotericity of 144.19

multistep annulation sequence. 144.18 was oxidized with chloranil to the fully conjugated target 144.19. 144.19 had an extraordinarily small electrochemical HOMO−LUMO gap of 3584


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Scheme 145. Indanthrene Blue Green FFBa

a

6. NONBENZENOID FUSION All fused ring systems discussed in the preceding sections contained at least one “benzenoid peri-fusion point”, that is, one combining three conjugated six-membered rings. Such an arrangement can be denoted “6-6-6 fusion” for simplicity. A large number of nonbenzenoid PHAs exist; however, because of their considerable structural diversity, they are difficult to classify. Heteroaromatic circulenes constitute one well-defined and consistently explored group of molecules, which are discussed in section 6.1 (Chart 43). The remaining material is

Reagents and conditions: (a)1,464 KOH, NaOCl.

Chart 42. Dibenzoullazines

Chart 43. Classification of Nonbenzenoid Systems Used in This Sectiona

was manifested in the easy accessibility of reduced and oxidized species, including dianion, anion radical, cation radical, and dication. The neutral species had a pronounced biradicaloid character, with the experimentally determined singlet−triplet gap of ca. 5 kJ/mol. 5.4.2. Cyclohepta[cd]phenalenes. peri-Fusion of a 7membered heterocyclic ring to phenalene was reported by Franz (Scheme 146).889 By condensing 9-butoxy-1H-phenalen-

a

Heteroatom placement and π-conjugation are not shown.

divided according to the type of peri-fusion present, that is, according to the sizes of individual fused rings. By far the most common motif is the 5-6-6 fusion (section 6.2), corresponding to the parent acenaphthylene hydrocarbon. The 5-5-6-fused systems and structures containing 7-membered rings are presented, respectively, in sections 6.3 and 6.4.

Scheme 146. Phenalene-Fused Diazepinea

6.1. Circulenoids and Related Systems

In hydrocarbon chemistry, [n]circulene is a general name for a ring system in which n angularly fused benzene rings fully surround an inner n-membered ring. The two most ubiquitous motifs are those of coronene ([6]circulene) and corannulene ([5]circulene). Circulene hydrocarbons with n = 7,890−892 8,893−895 and 4896 are also known, but they are significantly more difficult to prepare and, consequently, much less explored. In a broader sense, which follows from the original usage of the term by Dopper and Wynberg,897 the name “circulene” refers to a variety of structurally related molecules containing such additional features, as heteroatoms, nonbenzenoid peripheral rings, or nonconvex central polygons, and has also been applied to systems with incompletely fused periphery (quasicirculenes898). Regardless of these differences, the central ring in circulenoid molecules is normally close to planarity. Systems discussed below have been selected according to this broader definition of circulene, with the exception of [6]circulenes containing at least one benzenoid fusion point, which were discussed in the preceding sections. A number of theoretical studies on heteroaromatic circulenoids have been reported, concerning the structure and dynamics of bowl-like systems,899−901 cation−π interactions,902 and boron-containing circulenes.903 The following discussion is focused on relevant

a Reagents and conditions:889 (a) o-diphenylamine, 40% HBF4, xylene, reflux, 4 h; (b) toluene, aq NaOH.

1-one 146.1 with o-diphenylamine, the fused 1,4-diazepinium salt 146.3 was obtained, which was subsequently deprotonated to the free base 146.4. 3585


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relative to the nonfused dipyrrolyldiketone−boron complex. Compound 149.13 revealed the formation of three mesophases on cooling from the isotropic phase, with Colr, lamellar, and Colh structures, respectively. 149.13 was also found to exhibit increased electrical conductivity, and an ability to form anionresponsive supramolecular gels. In 2015, Cao and co-workers reported the synthesis of two isomeric thiophene-fused corannulenes 150.2 and 150.4, which were prepared, respectively, from 2- and 3-thienyl-substituted precursors 150.1 and 150.3, using different oxidative coupling reactions (Scheme 150).914 The OFET performance of 150.2 and 150.4 was tested in the bottom gate/top contact architecture, showing no field-effect properties for the 150.4 isomer. In contrast, 150.2 revealed p-type transport properties with a hole mobility of 0.06 cm2 V−1 s−1, threshold voltage of −38 V, and a current on/off ratio of 103. 6.1.2. [5]Heteracirculenoids. The synthesis of [5]circulenes with embedded heteroatoms is made difficult by the need to introduce curvature into the π-conjugated framework, and by potential side reactivity. These points are illustrated in the attempted synthesis of 1,2-diazadibenzo[d,m]corannulene 151.2 by Bodwell, Scott, et al. (Scheme 151).915 The synthetic plan relied on the use of the pyridazine precursor 151.1, following the effective paths established for dibenzo[d,m]corannulene, the carbocyclic parent system of 151.2. However, 151.1 was found unreactive under Pd-catalyzed coupling conditions, possibly as a result of intramolecular coordination of pyridazine to palladium. Interestingly, attempts to effect the required ring closures using flash vacuum pyrolysis led to a rearranged product of N2 elimination, 151.3. In one mechanistic hypothesis, the diaza product was assumed to form as an unstable intermediate and subsequently collapse into 151.3 under the harsh conditions of FVP. In 2015, Ito, Tokimaru, and Nozaki reported the synthesis of quintuply benzo-fused 6b2-azacorannulene 152.2 (Scheme 152).916 Their strategy relied on the [3+2] cycloaddition of a substituted diarylacetylene to the fused 9a-azaphenalene ylide 126.3 (section 5.1), generated in situ from the respective 126.2 salt. The resulting fused pyrrole derivative 152.1 was subjected to a Heck-type triple ring closure, producing 152.2 in moderate yield. The bowl structure of 152.2 is slightly skewed because of the presence of the nitrogen atom and the distance of the outer rim atoms the pyrrole plane varies from 1.38 to 1.73 Å. The large size of the bowl enhances π stacking interactions, and convex-to-concave π aggregation was observed in the solid-state packing pattern. Evidence for aggregation in solution was obtained using 1H NMR spectroscopy, yielding an aggregation constant of ca. 5.0 M−1. 152.2 has a bright yellow color in solution, with the lowest-energy absorption maximum at 466

experimental work and is organized according to the placement of heteroatoms and the size of the inner circulene ring. 6.1.1. Heterofused Circulenes. Carbocyclic circulene motifs with fused peripheral heterocyclic rings are currently limited to corannulene derivatives (for corannulene-fused porphyrinoids, see section 7.1). Pentakis(1,4-benzodithiino)corannulene 147.2 was obtained in 2004 by Scott et al. by exhaustive nucleophilic substitution of decachlorocorannulene 147.1 with 1,2-benzendithiolate (Scheme 147).904 147.2 Scheme 147. Synthesis of Pentakis(1,4benzodithiino)corannulenea

a

Reagents and conditions:904 (a) 1,2-benzendithiol, NaHCO3, DMF, 3 h, rt.

possesses five flexible flaps, which extend the corannulene cavity, and an assembly of two molecules of 147.2 was found to encapsulate a benzene molecule in the solid state. On the basis of this finding, 147.2 was considered as a potential receptor for C60 fullerene. A convenient route for corannulene functionalization leads through the unstable corannulyne 148.2,905 which can be converted into a variety of Diels−Alder adducts, such as the 148.3 endoxide (Scheme 148). The latter compound was converted into isocorannulenofuran 148.4, using the “stetrazine approach”,906 as reported in 2006 by Sygula et al.907 148.4 is moderately stable and can be stored for weeks under inert conditions. It is however a reactive Diels−Alder diene and was successfully applied to the synthesis of benzannulated corannulene derivatives (e.g., 148.6) as well as corannulenebased molecular clips and tweezers.908−911 Corannuleno[1,2-c]pyrrole 149.3 was prepared from nitrocorannulene by Lash and co-workers912 (Scheme 149), as a precursor to corannulene-fused porphyrins (section 7.1). Pyrrole 149.3 was subsequently used by Seki, Maeda, et al. to construct diketone−boron complexes containing one (149.10− 13) or two (149.5) corannulene fragments.913 The presence of the corannulene unit decreases the HOMO−LUMO gaps and significantly increases chloride and bromide binding constants, Scheme 148. Chemistry of Isocorannulenofurana

Reagents and conditions:907 (a) NaNH2, t-BuOK, THF; (b) CHCl3, 55 °C, 15 min; (c) benzenediazonium-2-carboxylate, 1,2-dichloroethane, reflux; (d) Fe2(CO)9.

a

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Scheme 149. Chemistry of Corannuleno[1,2-c]pyrrolea

a

Reagents and conditions: (a)912 [NO2][BF4], MeCN/DCM; (b)912 ethyl isocyanoacetate, THF, 149.14; (c)912 KOH, N2H4, ethylene glycol, 180− 190 °C; (d)913 malonyl chloride, DCM, 2 h, rt; (e)913 2-R-substituted pyrrole, malonyl chloride, DCM, 2 h, rt; (f)913 BF3·Et2O, DCM, rt.

Scheme 150. Thiophene-Fused Corannulenesa

contributions from carbazole-like resonance contributions. The bowl inversion barrier in 152.2 was not determined experimentally, but an estimate of 17 kcal/mol was provided by DFT calculations. An alternative approach to 6b2-azapentabenzo[bc,ef,hi,kl,no]corannulene was subsequently reported by the group of Shinokubo.917 Their synthesis started with an oxidative selfcoupling of phenanthrenoamine 152.3, yielding the bis(phenanthro)-fused pyrrole intermediate 152.4. The azacorannulene framework was subsequently completed in three steps, with the Heck cyclization used in all C−C bond-forming transformations. Finally, in the resulting azacorannulene 152.7, the 2,4,6-triisopropylphenyl group was installed in two steps, yielding the fully substituted 152.9. The presence of the bulky aryl group in 152.9 enabled the experimental determination of the bowl-inversion barrier using EXSY spectroscopy (23.3 kcal/ mol at 393 K in 1,2-dichlorobenzene-d4). 152.9 easily forms a NIR-absorbing radical cation in the presence of excess trifluoroacetic acid or upon direct chemical or electrochemical oxidation. 152.9 was found to interact with C60 in solution, with an association constant of 3800 M−1. This interaction resulted in partial fluorescence quenching in 152.9, and the appearance of a weak charge-transfer band in the absorption spectrum at ca. 800 nm. The convex−concave π interaction between 152.9 and C60 was additionally observed in the solid state, with the shortest van der Waals distance between the π surfaces of 3.29 Å. The solid-state complex 152.9 ⊃ C60 exhibited a large local charge mobility of 0.17 cm2 V−1 s−1, which was attributed to partial charge transfer and favorable alignment of molecules in the solid state. 6.1.3. [6]Heteracirculenoids. Systems containing a benzenoid peri-fusion point (i.e., at least a phenalene substructure) that would formally fit the definition of a [6]circulene are discussed in the preceding sections. The remaining structural motifs contain the triphenylene substructure and have been mostly developed as heteroaromatic analogues of sumanene. In an initial attempt to make trithiasumanene 153.7, Klemm and co-workers used a catalyzed

Reagents and conditions: (a)914 DDQ, MSA, CH2Cl2, 0 °C, 10 min; (b)914 FeCl3, Et2O, DCM, rt, 30 min. a

Scheme 151. Attempted Synthesis of 1,2Diazadibenzo[d,m]corannulenea

Reagents and conditions: (a) Pd-mediated coupling; (b) flash vacuum pyrolysis, 15−20%. a

nm, and shows blue-green fluorescence (λmax = 490 nm, ΦF = 0.24). On the basis of NICS calculations, it was proposed that the conjugation in 152.2 is principally described by assigning Clar sextets to all of the outermost benzene rings, with minor 3587


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Scheme 152. 6b2-Azapentabenzo[bc,ef,hi,kl,no]corannulenea

Reagents and conditions: (a)916 DMSO, N,N-diisopropylethylamine, 100 °C, 17 h; (b)916 Pd(PCy3)2Cl2, Cs2CO3, DMA; (c)917 DDQ, TFA, toluene, rt, 1 h; (d)917 Pd(OAc), PCy3·HBF4, K2CO3, DMA, 130 °C, 43 h; (e)917 Br2, CCl4, 70 °C, 12.5 h; (f)917 Pd(OAc)2, PCy3·HBF4, K2CO3, DMA, 130 °C, 16 h; (g)917 bis(pinacolato)diboron, [Ir(OMe)cod]2, 4,4′-di-tert-butyl-2,2′-bipyridyl, octane, 10.5 h, 110 °C, 80% yield; (h)917 2bromo-1,3,5-triisopropylbenzene, PdCl2(dppf)·CH2Cl2, Cs2CO3, 1,4-dioxane, 13 h, 100 °C. a

Scheme 153. Synthesis of Trithiasumanenea

Reagents and conditions: (a)918 H2S, Harshaw C4-0101 T catalyst, 500 °C; (b)919 NBS (3 equiv), DMF, rt; (c)919 trimethylsilylacetylene, Pd(PPh3)4, CuI, NEt3, reflux; (d)919 conc. HCl, AcOH, 80 °C; (e)919 1000 °C, 0.005 Torr, N2 flow. a

SnMe2, and GeMe2 units were prepared by Saito et al. using stepwise lithiation and electrophilic bridging of triphenylene, but trithiasumanene was not accessible using such an approach.922−924 A successful route to substituted trithia- and triselenasumanenes (154.4 and 154.6, Scheme 154) was developed in 2014 by Shao et al.925,926 These compounds were obtained from hexabutoxytriphenylene 154.1, which was directly lithiated and reacted with elemental sulfur or selenium, to yield “expanded” sumanenes containing one or two dichalcogen bridges (154.2 and 154.5). The latter compounds were contracted to the desired sumanenes by means of a Cumediated chalcogen extrusion. This reaction, which was run without the use of a solvent, is an efficient means of strain induction and provides multigram amounts of 154.4 and 154.6. Triselenasumanene 154.6 has a smaller bowl depth (0.47 Å) than its trithia congener and shows efficient columnar stacking in the solid state. The presence of S and Se heteroatoms in 154.4 and 154.6 leads to a narrowing of HOMO−LUMO gaps

gas-phase reaction between triphenylene and hydrogen sulfide (Scheme 153), which however only yielded trace amounts of triphenyleno[1,12-bcd:4,5-b′c′d′]dithiophene 153.2.918 The first successful synthesis of trithiasumanene, reported by Otsubo et al. in 1999,919 started with benzotrithiophene 153.3, which was converted into a regioisomeric mixture of tris(1-chlorovinyl) derivatives 153.6. The latter mixture was subjected to flash vacuum pyrolysis (FVP), producing 153.7 in 35% yield. Interestingly, FVP was not useful for the synthesis of the parent hydrocarbon sumanene attempted in 1993 by Mehta et al.920 Sumanene was obtained in 2003 by Sakurai, Daiko, and Hirao by means of a nonpyrolytic method.921 The bowl-shaped molecules of trithiasumanene 153.7 have a depth of ca. 0.79 Å and show efficient convex−concave stacking in the solid state. The principal difficulty in synthesizing 153.7 lies in the high internal strain of the bowl, which must be energetically compensated during the cyclization process. Strain-free sumanene derivatives containing combinations of S, SiMe2, 3588


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Scheme 154. Chemistry of Substituted Trithia- and Triselenasumanenesa

a Reagents and conditions: (a)925 n-BuLi, TMEDA, 60 °C; (b)925 sulfur, −78 °C to rt; (c)925 Cu powder (10 equiv), 200 °C; (d)925 Se powder, −78 °C to rt; (e)927 Oxone, THF/H2O, rt, 2 h; (f)927 (1) NaOH, EtOH/H2O, reflux, 2 h, (2) aq HCl, (3) Ac2O, reflux; (g)927 2-aminopyridine, DCC, THF, reflux.

Scheme 155. Synthesis of Triazasumanenesa

relative to the all-carbon sumanene. The electron-rich sumanenes 154.4 and 154.6 undergo oxidative opening of one of the benzene rings, to yield diesters 154.7 and 154.8 (Scheme 154). 927 The diesters were cyclized to the corresponding anhydrides 154.9 and 154.10, which were converted into imides 154.11 and 154.12. The presence of the expanded, 7-membered ring in the latter imides results in completely planar structures, which show tight π−π stacking in the solid state. In comparison with 154.4 and 154.6, the [5-67]-fused systems are distinguished by red-shifted absorptions and emissions, and high fluorescence quantum yields. Even though not fully conjugated, the triazasumanenes 155.5 and 155.6 (Scheme 155), reported by Higashibayashi, Sakurai, et al., are of interest for preparative and structural reasons.928 The synthetic route follows the strategy established in the original sumanene work,921 and includes Pd-catalyzed cyclotrimerization, ring metathesis, and aromatization stages. The metathesis involves hydrolytic opening of lactam bonds in 155.2, followed by cyclization induced by pentafluorophenyl diphenylphosphinate. The bowl-shaped lactam 155.3 could not be directly dehydrogenated and had to be transformed into the thioimidate 155.4 prior to aromatization. The thioimidate was converted into 155.5 using triphenylmethyl fluoroborate as an oxidant. The unique feature of the triazasumanene synthesis is the enantiospecificity of the entire reaction sequence, which translates the point chirality of the enantiopure starting material, 155.1, into the “bowl chirality” of the sumanene product. Because of the greater depth of the bowl, caused by the shorter C−N bond distances, the barrier to inversion in 155.5 (over 40 kcal/mol) is much higher than in all-carbon sumanene, making the triaza structure configurationally stable at room temperature. 6.1.4. [7]Heteracirculenes. The first example of a heteroaromatic quasi-[7]circulene was provided in 1969 by Zander and Franke,929 who cyclized 5,10-dihydrocarbazolo[3,4c]carbazole 156.1, using the chloroaluminate method, to obtain the cycloheptanaphthodiindole 156.2 (Scheme 156). In the 1970s, Wynberg and co-workers used an analogous strategy to prepare a series of thiophene-containing quasi-[7]circulenes 157.2, 157.3, 157.6−9 (Scheme 157).246,930 The chloroaluminate method was found to be more efficient than dehydrogen-

a

Reagents and conditions:928 (a) Pd(OAc)2, PPh3, [Bu4N][OAc], Na2CO3, molecular sieves 4 Å, 1,4-dioxane, 100 °C, 2 h, 57%; (b) (1) 12 M HCl, AcOH, 60 °C, 3 h, (2) C6F5OP(O)Ph2, N,Ndiisopropylethylamine, DMF, 0−60 °C, 59% (two steps); (c) Lawesson’s reagent, 1,2-dichloroethane, microwave, 160 °C, 40 min, 92%; (d) trifluoroacetic acid, microwave, 100 °C, 2 h, 88%; (e) MeI, K2CO3, DMF, 30 °C, 3 h, 79%; (f) [Ph3C][BF4], DTBMP, CH2Cl2, 25 °C, 8 h, 73%; (g) m-CPBA, CH2Cl2, 0 °C then 15 °C, 2 h, 90%, PMB, p-MeOC6H4CH2; DTBMP, 2,6-di-tert-butyl-4-methyl-pyridine; Lawesson’s reagent, 2,4-bis(4-methoxyphenyl)-1,3,2,4-dithiadiphosphetane-2,4-disulfide; m-CPBA, 3-chloroperoxybenzoic acid.

Scheme 156. Synthesis of 5,10-Dihydrocarbazolo[3,4c]carbazole929a

a

Reagents and conditions: (a) phenylhydrazine, aq NaHSO3, reflux; (b) NaCl·AlCl3, 140−150 °C. 3589


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Scheme 158. Synthesis of “Sulflower” and Related Systemsa

Scheme 157. Thiophene-Containing [7]Circulenes and Quasi-[7]circulenesa

a Reagents and conditions: (a)932 LDA (16 equiv), sulfur (16 equiv), rt, 24 h; (b)932 aq HCl; (c)932 vacuum pyrolysis (80% yield of 158.3 over three steps); (d)933,934 LDA (16 equiv), selenium (16 equiv), rt, 24 h (50% yield of 158.4 over three steps).

cm2 V−1 s−1.933 STM investigations performed at the solvent− highly oriented pyrolytic graphite (HOPG) interface revealed the ability of these heteracirculenes to stabilize the “chickenwire” 2D polymorph of trimesic acid, and their propensity for self-aggregation.935 2D crystallization of 158.3, 158.4, and C16S5Se3 on a Au(111) surface revealed a considerable dependence of the self-assembly behavior on molecular symmetry.936 1,4-Naphthoquinone treated with strong acids undergoes self-condensation into a number of products including tetrabenzotetraoxa[8]circulene 159.1 (Scheme 159). Such condensations were already investigated in 1881,937 but 159.1 was first isolated in 1933 by Erdtman,938 who proposed a structure of the corresponding [6]circulene analogue (tribenzotrioxasumanene). The correct structure of 159.1 was elucidated 35 years later by Erdtman and Högberg.939 p-Benzoquinones reacted similarly (159.2−4, Scheme 159),940−942 but practicable yields were only obtained for substituted reactants (R ≠ H). In the solid state, the tetraoxa[8]circulene core is planar and relatively strain-free,943 as shown in an X-ray study by Berg et al. The acid-catalyzed condensation of quinones involves isolable helicene-like intermediates, such as 159.5, which were successfully used in stepwise syntheses of unsymmetrically fused circulenes such as 159.6944 (Scheme 159). In a related synthesis, naphthoquinone 159.7 yielded an incompletely characterized circulene, with a structure of 159.8 or 159.8′.945 The interest in tetraoxa[8]circulenes was renewed with the development of their soluble derivatives. In 2000, Christensen et al. synthesized a series of octasubstituted circulenes (C44.1− 5, Chart 44), containing long alkyl chains (C7−C11).947 These compounds were made in 11−56% yields by using BF3·Et2O in dichloromethane. C44.1−4 showed mesomorphic behavior, but the mesophases were not structurally characterized. In a related study, the same group prepared a family of indanonebased tetraoxa[8]circulenes.948 Using an analogous method, Rathore and Abdelwahed synthesized bicycloalkane-annulated derivatives C44.6 and C44.7, which formed isolable cation− radical salts upon oxidation.949 The respective cations showed NIR absorptions extending beyond 1100 nm. C44.5, and hybrid benzo-fused derivatives C44.8−11, obtained by cross-

a

Bonds formed by cyclodehydrogenation or cycloaddition are indicated in red. Reagents and conditions: (a)246,930 AlCl3, benzene; (b)247,897 NaCl·AlCl3, 140 °C; (c)247,897 maleic anhydride, chloranil; (d)247,897 Cu, quinoline.

ation by means of aluminum chloride in benzene. These systems served as precursors to completely fused thiacirculenes 157.5, 157.10−12, which were prepared by oxidative cycloaddition of maleimide followed by decarboxylation.247,897 In a related study, compound 157.1 was found to undergo an electrophilic cyclization to a [8]circulenoid structure containing an sp3 carbon in the inner ring.931 6.1.5. [8]Heteracirculenes. Octathia[8]circulene 158.3, synthesized in 2006 by Nenajdenko et al.,932 is the most heteroatom-abundant circulene reported to date (Scheme 158). With its elemental formula C16S8, the compound is a binary carbon−sulfur compound, and it was nicknamed “sulflower”, to highlight its high sulfur content and unique molecular geometry. 158.3 was efficiently synthesized by pyrolyzing the octathiol precursor 158.2, which was prepared from the tetrathiophene 158.1 by exhaustive lithiation of all α positions, followed by sulfurization. Sulflower 158.3 is a dark red solid, insoluble in common organic solvents, and was characterized by a variety of solid-state techniques, including X-ray crystal analysis. The mixed selenium−sulfur analogue 158.4 was prepared using an analogous procedure, in which elemental sulfur was replaced with selenium.933,934 In the latter synthesis, partial scrambling of heteroatoms occurred, yielding C16S3Se5 and C16S5Se3 circulenes as side products. 158.3 and 158.4 were explored as p-type semiconductors in organic field-effect transistors, showing limited hole mobilities of up to 9 × 10−3 3590


Chemical Reviews

Review

Scheme 159. Synthesis of Tetraoxa[8]circulenesa

Reagents and conditions: (a) AlCl3, nitrobenzene; (b)945 benzenesulfonic acid melt, 60 °C; (c)946 BF3·Et2O, DCM, 50%, mixture of isomers; (d)946 AlCl3, benzene. a

Chart 44. Tetraoxa[8]circulenes

Scheme 160. Synthesis of Azatrioxa- and Diazadioxa[8]circulenesa

Reagents and conditions: (a)951 chloranil, BF3·Et2O, DCM; (b)952 BF3·Et2O, DCM. a

employed for the synthesis of azatrioxa[8]circulenes (160.3− 4),952 and was subsequently extended into a stepwise method enabling the synthesis of unsymmetrically substituted derivatives.953 The introduction of nitrogens was found to produce red shifts in the absorption spectra and was also reflected in the lowering of the first oxidation potential. The effect is opposite to that induced by peripheral benzo fusion, which was observed to yield blue-shifted electronic absorptions. For the azacontaining circulenes, as well as for its tetraoxa parent, an antiaromatic contribution was postulated for the central cyclooctatetraene ring, on the basis of the center NICS(0) value, which was not in line with the predicted additivity scheme.951,952 Tetrathia- and tetraselena[8]circulenes 161.2 and 161.3 were obtained by Wong et al.,954 using a strategy reminiscent of Nenajdenko’s route932 to octathia[8]circulene 158.3. Compounds 161.2 and 161.3 were synthesized from tetraphenylene 161.1 by lithium−bromine exchange followed by reaction with elemental sulfur and selenium, respectively. The resulting polychalcogen intermediates were thermally dechalcogenated

condensation of the corresponding benzo- and naphthoquinone, were investigated as fluorescent components for OLEDs by Pittelkow and co-workers.950 These systems showed moderate to high fluorescence quantum yields, dual fluorescence decays, and multiple electrochemical reduction and oxidation events. Blue-emitting OLEDs fabricated using these materials showed light intensities up to 276 cd m−2 at 10 V (C44.9). Condensation of tert-butyl-p-benzoquinone yielded a mixture of regioisomeric tetra(tert-butyl) circulenes, from which the two major isomers, 159.9 and 159.10, could be isolated (Scheme 159).946 The mixture could be easily dealkylated to provide the parent 159.2 in excellent yield. Unexpectedly, both 159.9 and 159.10 revealed a strong, cooperative aggregation in solution. This effect was proposed to be driven by CH−π rather than π−π interactions. Diazadioxa[8]circulenes (160.1−2) were prepared in 2013 by Pittelkow et al. by oxidative condensation of 3,6dihydroxycarbazoles (Scheme 160).951 A similar approach was 3591


Chemical Reviews

Review

cycles. In a 1973 report by Högberg, tetraphenylenotrifuran 162.2 was obtained by fusing the helicene precursor 162.1 with sodium chloroaluminate at 140 °C (Scheme 162).958 Rajca et al. explored analogous ring closures in a number of [7]helicenes, which yielded the corresponding quasi[8]circulenes (162.4, 162.6, and 162.8, Scheme 162).898 In addition to tinand palladium-mediated couplings, compounds 162.6 and 162.8 could be efficiently obtained by pyrolysis carried out in a differential scanning calorimeter. Interestingly, an attempt to deprotect compound 162.9 using trifluoroacetic acid produced small amounts of the respective quasicirculene 162.10.959 A class of heteroatom-bridged tetraphenylenes, structurally related to [8]circulenes, was recently developed by Wong et al.960,961 Starting from tetrasubstituted tetraphenylene derivatives 163.1 and 163.3, these authors successfully prepared systems containing O, N, S, and Se bridges (163.2, 163.4−9), using a variety of metal-mediated coupling strategies (Scheme 163). As a consequence of bridging, the puckering of the central cyclooctatetraene ring in these systems is reduced relative to the bridge-free tetraphenylene, but the geometries remain saddle-shaped. The uniquely high planarization of the 163.4 derivative was explained by the steric effect of the phenyl substituent. In all cases, the heteroatom bridges contribute to the π-electron conjugation, leading to observable reductions of HOMO−LUMO gaps. With an estimated HOMO energy of −5.78 eV and favorable packing in the crystal, compound 163.4 is a candidate for a p-type semiconductor. The lowest-energy electronic absorptions are shifted by up to 16 nm relative to the heteroaromatic monomers (e.g., dibenzofuran, etc.), indicating a moderate effect of inner cyclic conjugation on the electronic structure of these bridged systems. An [8]circulenoid containing six thiophene rings (164.4) was constructed by Cu-mediated coupling of dithieno[3,4-b:3′,4′d]thiophene (164.1) by Nishinaga, Iyoda, et al. (Scheme 164).962 Sulfone and silane analogues, 164.5 and 164.6, were assembled using the same strategy. Similarly to other [8]circulenoids, these systems contain a highly planarized cyclooctatetraene (COT) ring, with the deviation from planarity increasing in the order 164.6 > 164.5 > 164.4. The authors showed that the increased planarity of the COT ring leads to a narrowing of the HOMO−LUMO gap and to a concomitant enhancement of paratropicity. Amphoteric redox behavior was demonstrated for 164.3 and 164.6, both of which could be chemically converted into the corresponding radical cations, radical anions, and dianions. A related system, 164.7, containing only four fused rings, is also planar and was shown to exhibit enhanced antiaromatic characteristics in comparison with 164.4.963 A derivative of 164.4 (with R = TIPS-ethynyl) was successfully tested as a single-crystal field-effect transistor, showing hole and electron mobilities of up to 0.40 and 0.18 cm2 V−1 s−1, respectively.964 6.1.6. Larger Systems. Circulenoids containing an inner ring larger than 8-membered are currently unknown, and this limitation may be due to steric reasons. (An unusual, and apparently highly strained, cyclic helicene derivative containing an incompletely conjugated inner 9-membered ring was reported by Maiorana et al.965) The circulene structural paradigm can be thought to encompass systems in which the fused ring periphery encloses a nonconvex polygon, typified by the hydrocarbon kekulene (165.1, Scheme 165). The successful synthesis of 165.1 by Staab and Diederich966 spurred an interest in its heterocyclic analogues,967,968 but the majority of structures reported to date, such as Bell’s torands 165.2 and

using copper powder to yield the desired products in moderate yields. The aromatic core of 161.2 is completely planar in the solid state, whereas the tetraselenium analogue 161.3 reveals a slight saddle-like distortion from planarity. A complementary method was developed in 2015 by Miyake et al., in which variously substituted tetrathia[8]circulenes 161.6 were obtained by quadruple benzannulation of a borylated cyclothiophene precursor 161.5.955 161.6 showed a strong dependence of intrinsic hole mobilities, measured using TRMC, on peripheral substitution. Highest values (up to 2.66 × 10−3 cm2 V−1 s−1) were observed for alkyl-substituted derivatives, which were thought to promote π-stacking in the solid state. A different strategy was employed by Tanaka, Osuka, et al. in their synthesis of tetrabenzotetraaza[8]circulene 161.8.956 The latter species was obtained from a cyclophane precursor 161.7, which was subjected to a fold-in-type957 oxidative coupling. Despite the planar geometry of 161.8, the central cyclooctatetraene ring showed a significantly reduced antiaromatic character, consistent with the radialene-like valence formulation of the circulene. 161.8 has a sharp absoprtion spectrum with the lowest energy band at 413 nm, and reveals strong fluorescence emission (ΦF = 0.55 in THF) with a very small Stokes shift (175 cm−1), consistent with insignificant reorganization in the excited state. A number of incompletely fused [8]heteracirculenes are known. The dehydrogenative cyclization of helicenes, described above for [7]circulenes, is also effective in closing 8-membered Scheme 161. Synthesis of Tetrathia- and Tetraselena[8]circulenesa

a Reagents and conditions: (a)954 n-BuLi, sulfur; (b)954 n-BuLi, selenium; (c)954 Cu powder, 250 °C; (d)955 HBpin, [Ir(OMe) (cod)]2, dtbpy, hexane, 10 °C, 90 h; (e)955 alkyne, [Cp*RhCl2]2, Cu(OAc)2·H2O, LiOAc, DMF, air, 100 °C, 2 h; (f)956 DDQ, Sc(OTf)3, toluene, reflux, 3 h.

3592


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Scheme 162. Synthesis of Quasi-[8]heteracirculenesa

Reagents and conditions: (a)958 NaCl·AlCl3, 140 °C; (b)898 Pd[P(t-Bu)3]2, K3PO4, toluene, 80 °C; (c)898 pyrolysis (DSC scan); (d)898 (nBu)3SnH, AIBN, toluene, reflux; (e)959 TFA, CHCl3, 45 min, 0 °C.

a

Scheme 163. Synthesis of Heteroatom-Bridged Tetraphenylenesa

Scheme 164. Synthesis of Thiophene-Containing [8]Circulenoidsa

a

Reagents and conditions:962 (a) (1) LDA, (2) TMSCl; (b) (1) nBuLi, (2) CuCl2; (c) TBAF.

In 2013, Myśliwiec and Step̨ ień reported the synthesis of chrysaorole 166.2 (Scheme 166), a jellyfish-shaped molecule named after the Chrysaora genus, which was synthesized via triple intramolecular Yamamoto coupling performed on a halogenated carbazolophane 166.1 (see section 7.7).971 The synthesis of 166.2 was designed as a model of the fold-in strategy, in which the aromatic cyclophane subunits were folded toward the interior of the macrocycle (Scheme 166, green arrows), yielding a belt structure with bowl-like curvature. More generally, the fold-in reaction can be defined as a transformation of a macrocyclic precursor, which results in the formation of an additional macrocyclic circuit that is smaller than that originally present in the precursor. The fold-in concept957 is a versatile synthetic approach that works with various reaction types and enables the construction of bowlshaped,971−973 planar,973 and contorted894 systems, including carbocyclic894,973 and incompletely conjugated972 aromatics. 166.2 is an unusual bowl-like structure, containing an inner opening, and has a bowl depth of 1.96 Å. For the unsubstituted 166.2 (R = H), the internal strain energy of 53.4 kcal/mol was

a Reagents and conditions:960,961 (a) Pd(PPh3)4, K3PO4, DMF, 140 °C, 68%; (b) PhNH2, Pd2(dba)3, t-BuONa, (t-Bu)3P, toluene, 120 °C, 85%; (c) CuI, NaN3, DMEDA, DMSO, 120 °C, 45%; (d) NaH, BnBr, THF, 0−20 °C, 85%; (e) (1) n-BuLi, I2, THF, (2) CuI, K2S, DMEDA, MeCN, 140 °C, 69%; (f) n-BuLi, Se, THF, 67%; (g) Cu powder, 250 °C, 100%.

165.3,969 were not completely conjugated. Two notable examples of fully conjugated torands include compound 165.4, mentioned in Bell’s PAC review,969 and 3,9,15,19,21,23hexaazakekulene 165.9, reported by Demeunuyck et al.970 The latter compound was assembled from proflavines 165.5 and 165.8 using a stepwise acid-catalyzed condensation protocol. 165.9 is insoluble in common organic solvents, but it is partly solubilized by strong acids, such as TFA or MSA. 3593


Chemical Reviews

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Scheme 165. Chemistry of Large Circulenoidsa

a

Reagents and conditions:970 (a) paraformaldehyde (0.5 equiv), 12 N HCl, 3 weeks; (b) NaOH, EtOH; (c) paraformaldehyde (2 equiv), 12 N HCl, 1 week.

Scheme 166. Chrysaorolesa

Scheme 167. Reactions of Acenaphthoquinones with Aminesa

a

Reagents and conditions: (a)974 AcOH, reflux, 3 h; (b)975 AcOH or EtOH, reflux.

derivative of aceanthra[1,2-b]quinoxaline 167.4, reported in 1911 by Liebermann and Zsuffa.975 Further examples were provided by Buu-Hoı̈ and Jacquignon976 (C45.1, Chart 45), Klingsberg and Lewis977 (C45.2−4), and Amer et al.978,979 (C45.6−13 and several related 1,2,4-triazine-fused systems). Acenaphtho[1,2-b]quinoxaline derivatives C45.14−16 were synthesized in 2014 by Nurulla et al. and subjected to electrochemical polymerization.980,981 As a result, low-bandgap polymers were obtained, which showed electrochromic switching in the NIR range. Compounds C45.17−18 were reported in 2011 by Kwon, Kim, et al. as red light emitting materials for use in OLED devices.982 These materials, obtained by Pd-catalyzed amination of C45.4, showed good thermal stability with glass transition temperatures above 140 °C. The dipolar characteristic of the quinoxaline derivatives resulted in prominent solvatochromic effects. OLED devices based on C45.17 showed maximum

a

Reagents and conditions:971 (a) [Ni(cod)2], 1,5-cyclooctadiene, 2,2′bipyridyl, DMF/toluene, 85 °C, 18 h; (b) H2, PtO2, CH2Cl2, 20 °C.

estimated by DFT calculations on the basis of a homodesmotic reaction. By extending the fold-in synthesis to a nonconjugated cyclophane 166.3, hexahydrochrysaorole 166.4 was synthesized, revealing configurational stability of the bowl in solution. 6.2. Fused Acenaphthylene Derivatives

6.2.1. Hetero[a]fused Acenaphthylenes. Straightforward access to heterofused acenaphthylenes is provided by condensation reactions of acenaphthoquinone and aceanthrylene-1,2-dione with aromatic diamines (Scheme 167). Early examples include acenaphtho[1,2-b]quinoxaline (167.2), prepared in 1899 by Ampola and Recchi,974 and a methyl 3594


Chemical Reviews

Review

Chart 45. Acenaphthylene-Based Quinoxalines

Chart 46. Acenaphtho[1,2-b]quinoxaline Ligands and Complexes

band at 500−950 and 600−1300 nm, respectively. Thin films of C45.21a−b yielded n-channel OFETs with electron mobilities of up to 0.12 cm2 V−1 s−1, and BHJ solar cells with power conversion efficiencies reaching 1.8%. Polymers C45.22a−b showed crystalline ordering in thin films and yielded considerably improved electron mobilities of up to 0.3 cm2 V−1 s−1. Acenaphtho[1,2-b]quinoxalines have found use as πextended ligands for transition metals. In 2000, Ji et al. reported the π-expanded phenanthroline C46.1 and its ruthenium complexes C46.4−6 (Chart 46).985 In contrast to C46.6, which was nonemissive, complexes C46.4 and C46.5

brightness of 17 523 cd/m2 and current efficiency of 4.55 cd/A (CIE coordinates x, y = 0.56, 0.43). Further examples of acenaphtho[1,2-b]quinoxaline structures, developed as potential OLED emitting layers (C45.19−20), were provided in 2013 by Bunz, Hamburger, et al.564 Also in 2013, Jenekhe and co-workers reported the use of tetraazabenzodifluoranthene diimides C45.21a−c and the corresponding polymers C45.22a−b as n-type semiconductor materials.983,984 The introduction of electron-donating thienyl R groups in C45.21a−b was found to produce red-shifted charge-transfer bands in the region of 460−700 nm. This effect was particularly remarkable in polymers C45.22a−b, which displayed the CT 3595


Chemical Reviews

Review

Scheme 169. Acenaphthylene-Based Cavitanda

displayed bright luminescence in acetonitrile but were only weakly luminescent in water solution. C46.4 and C46.5 were found to bind to calf thymus DNA (CT-DNA), with intrinsic binding constants of 7.6 × 104 and 8.8 × 104 M−1, respectively. These relatively low affinities were explained in terms of the geometric requirements of the C46.1 ligand. In the case of C46.6, the interaction had a nonintercalative character. Analogous work was subsequently performed on ruthenium(II) complexes C46.8−10 of the benzologous ligand C46.2.986,987 Further luminescent complexes containing the C46.1 ligand are the copper(I) species C46.7, reported in 2009 by Zhang et al.,988 and a ReI(CO)3Br complex of Li and co-workers.989 An asymmetric pyridine−imidazole ligand C46.3 and its ruthenium(II) complexes C46.11−12 were synthesized in 2013 by Liu and Zhang.990 Both complexes were found to intercalate into DNA and induce singlet-oxygen photocleavage under irradiation at 365 nm. These systems were further observed to be efficient inhibitors of DNA topoisomerase I. Alkoxy-substituted benzopyrazine-fused tetracenes 168.4−5 (TBPy) and their disulfides 168.6−7 (TBPyS) were designed by Matsuo et al. in 2013 (Scheme 168).991 The introduction of

Reagents and conditions:992 (a) H2, Raney Ni, toluene, 45 °C; (b) acenaphthenequinone, THF, AcOH, reflux, 6 h.

a

π-Expanded fluoranthene derivatives, reported by Buu-Hoı̈ and co-workers (Chart 47), include the carbazole system Chart 47. [k]Fused Fluoranthenes

Scheme 168. Benzopyrazine-Fused Tetracenesa

C47.1, obtained from fluoranthene via Haworth-type annulation, Fischer indolization, and aromatization,993 and the three fused systems C47.2−4,994 all derived from 8-aminofluoranthene. [k]Fused pyrylium salts C47.5 were reported by Shenbor and Azarov in 1979.995 Furans and oxazoles, such as C47.6−7, were obtained in 1982 by Shechter et al. via thermal and photochemical additions of acetylenes and nitriles, respectively, to diazoacenaphthenone and 2-diazoaceanthrenone.745 Diacenaphtho[1,2-b:1′,2′-d]thiophene C47.8 is a red-colored solid obtained surprisingly simply by fusion of acenaphthene and sulfur. The reaction, which also works with substituted acenaphthenes (see Scheme 144, section 5.4), was originally reported in 1903 by Dziewoński,996 although it was apparently discovered 10 years earlier, as subsequently claimed by Rehländer.997 C47.8 can alternatively be prepared by refluxing 167.1 with P4S10 in toluene.998 The compound is of interest because of its diverse reactivity in cycloaddition reactions.999−1001 The pyrrole analogue of C47.8, N-substituted diacenaphtho[1,2-b:1′,2′-d]pyrrole C47.9, was synthesized in 2011 by Kawase et al. via a two-step annulation procedure, starting from 1,2-dibromoacenaphthylene1002 (an earlier synthesis of a substituted variant of C47.9 is shown in Scheme 144,

a

Reagents and conditions: (a)991 AcOH/EtOH, reflux, 13 h; (b) S8, excess, >200 °C, 6−59 h.

the disulfide group was achieved by heating with an excess of elemental sulfur above 200 °C. These derivatives featured NIR absorptions (λonset up to 820 nm), improved photooxidative stability relative to tetracene, and solution processability. OFET devices fabricated from 168.4−7 showed performance characteristics dependent on the substitution pattern and fabrication method. Hole mobilities up to 1.7 × 10−2 cm2/V· s (with Vth = −30 V and ION/IOFF = 104) were obtained for thin-film OFET devices based on 168.7. A deepened acenaphthylene-fused cavitand 169.3, with a cavity ca. 14 Å in length, was reported by Rebek et al. (Scheme 169).992 In their synthesis, the octanitro cavitand 169.1 was reduced to the octaamine 169.2, which was then condensed with acenaphthoquinone to yield 169.3. While the binding constant of 169.3 toward C60 was relatively low (900 ± 250 M−1 in toluene), the cavitand showed good selectivity, because C70 showed no detectable binding. This discrimination was attributed to the relatively rigid structure of 169.3. 3596


Chemical Reviews

Review

Scheme 170. Acenaphtho[1,2-b]benzoquinolinesa

section 5.4). Phospholes C47.10a−b and several of their derivatives were reported in 2011 by Matano and coworkers.1003 These molecules were characterized by high electron affinity, yielding stable radical anions. Thin film of the P-sulfide of C47.10b showed a high electron mobility of 2.4 × 10−3 cm2 V−1 s−1 at E = 4.3 × 105 V cm−1. Annulations with appropriately designed nucleophiles, performed on diones 167.1 and 167.3, can proceed via a combination of imine and aldol-type condensations, typically resulting in the formation of a pyridazine ring. For instance, fusion of the 9-oxo-1,9a,10-triaza-9-hydroanthracene motif to acenaphthylene yields C48.1 (Chart 48), a yellow dye known Chart 48. Pyridazine[a]-Fused Acenaphthylenes and Related Systems

a Reagents and conditions: (a)1010 2-naphthylamine, EtOH, HCl, reflux, 4−5 h; (b) 1-naphthylamine, EtOH, HCl, reflux, 4−5 h; (c)1011 alkyllithium (R = Me, n-Bu, s-Bu), THF, TMEDA, 0 °C or rt, 1.5−1.75 h, 50−82%.

(171.211017 and 171.221018), and pyrylium (171.23995) moieties. In 1973, Zander et al. reported the synthesis of a π-expanded phenothiazinyl radical 171.18.1019 Initially, amine 171.6 was thermally condensed to provide di(fluoranthen-3-yl)amine 171.17, accompanied by traces of the π-expanded phenazine 171.16. Cyclization of 171.17 with elemental sulfur followed by oxidation with PbO2 produced the radical species 171.18. This stable radical, which was deep-violet in the solid state, displayed electronic spectrum with absorptions extending beyond 600 nm. The first synthesis of fluorantheno[3,2-b]thiophene 171.27, involving cyclization of a fluoranthene thioether, was reported in 1969 by Shenbor and Tsaberyabyi.1020 A different approach, which interestingly did not start from a fluoranthene derivative, was described in 1990 by Lee-Ruff and Chung.1021 In the initial step, the cyclobutane precursor 171.24 was assembled by a [2+2] cycloaddition of an in situ-generated ketene. 171.24 was then subjected to acid-catalyzed ring opening followed by electrophilic annulation. The resulting 171.26 was dehydrogenated with DDQ to yield 171.27. Pyrrole-fused fluoranthene 172.3, later used as a precursor to peripherally fused porphyrins1022 (Chart 56, section 7.1), was synthesized by Lash et al. in a modified Zard−Barton reaction (Scheme 172).1023 The inadequate reactivity of the starting 3nitrofluoranthene 172.1 toward isocyanoacetates was overcome by using the strong phosphazene base 172.2 and reflux conditions. A differently fused pyrrole 172.5 was efficiently synthesized through the RhIII-catalyzed annulation of Jin et al.538 (cf., Scheme 103, section 4.5). 6.2.3. Hetero[d]fused Acenaphthylenes. The hexacyclic system 171.28, reported in 1988 by Saalfrank, Schütz, et al., is a rare example of a π-expanded hetero[d]fused acenaphthylene (Scheme 171).1024 The compound was obtained in a thermally induced double cyclization of a fluorenylidene-terminated allene precursor. 6.2.4. Carbazole-Based and Related Systems. The 8Hindolo[3,2,1-de]phenanthridin-8-one (C49.1, Chart 49) and 8H-indolo[3,2,1-de]acridin-8-one (C49.2) ring systems are of interest because of their structural relationship to alkaloids of the canthine family.1025 Several syntheses of the parent C49.1

from patent literature,1004 the color of which can be tuned by substitution of the terminally fused benzene ring and other modifications.1004−1007 Structurally related systems C48.2 and C48.3 were developed by Alvarez-Builla et al.,1008,1009 and showed enhaced binding to DNA via combination of a cationic heterocycle with a PAH moiety. Aceanthrylene systems C48.4−7 were reported by Amer and co-workers.979 A one-pot synthesis of two isomeric acenaphtho[1,2b]benzoquinolines 170.1−2 was reported in 1996 by Ray et al. (Scheme 170).1010 The reaction involved acid-catalyzed condensation of 2-chloroacenaphthylene-1-carbaldehyde with appropriate naphthylamines. These compounds were later shown to undergo regioselective addition of alkyl-1011 and aryllithiums1012 (170.3−4). The selectivity was explained in terms of Li coordination to the quinoline nitrogen. 6.2.2. Hetero[e]fused Acenaphthylenes. Straightforward functionalization of the [b] edge of fluoranthene (corresponding to the [e] edge of the acenaphthylene substructure), and the availability of key derivatives, such as 171.1, 171.3, and 171.6, provided initial access to hetero[b]fused fluoranthenes. First examples of these systems, published mostly in the 1950s, were provided by Buu-Hoı̈ and Jacquignon976 (171.2), Kloetzel et al.1013 (171.4), Campbell and Temple1014 (171.5), and Barret and Buu-Hoı̈488 (171.7−8). In the following decade, the BuuHoı̈ group synthesized a range of other fluoranthene derivatives, 171.9, 993 171.10−13, 994 171.14, 548 and 171.15,1015 which were explored as potential carcinogens. Shenbor and co-workers reported several further [b]fused derivatives containing oxazole (171.19−201016), imidazole 3597


Chemical Reviews

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Scheme 171. Hetero[b]- and Hetero[d]fused Fluoranthenesa

Reagents and conditions: (a)1019 320 °C, CO2 atmosphere, 1 h; (b)1019 (1) sulfur, 250 °C, CO2 atmosphere, 10 min, (2) 1,2,4-trichlorobenzene, PbO2, reflux, 5 min; (c)1021 NEt3, DCM, 0 °C to rt; (d)1021 TFA, DCM, rt, 20 min; (e)1021 PPA, 100 °C, 4 min; (f)1021 DDQ, benzene, reflux, 1 h. a

and its substituted derivatives were described, starting from the 1930s work of Plant and Tomlinson,1026,1027 followed by contributions from Teitei,1028 and Ghosh et al.1029 More recent work from Gómez-Lor and Echavarren,1030 Bracher et al.,1031 Wiczk et al.,1032 and Xu, Yao, et al.1033 explored several synthetic approaches to C49.1, employing different disconnections of the fused framework. Various aza-analogues of C49.1, containing additional embedded nitrogens and, occasionally, additional fused rings, were reported by Berti et al.,1034 Shipchandler and Mitscher,1035 Desarbre and Merour1036 Markgraf et al.,1037 Poupon et al.,1038 Dang and Stadlbauer,1039 Huet et al.,1040,1041 Sarragiotto et al.,1042and Bracher et al.1031,1043 Related lactam derivatives were synthesized by Ziegler et al. (C49.5),1044 Wolfbeis (C49.6), 1045 and Stadlbauer et al. (C49.7).1046 Compound C49.2 was first reported by Eckert et al. in 1922,1047 and its chemistry was subsequently investigated by Hayashi,1048,1049 Gilman et al.,1050 Hellwinkel and Melan,368 and Brown, Eastwood, et al.1051 Azaanalogues C49.3 and C49.4 were reported, respectively, by Molina et al.1052 and Puzik and Bracher.1053

In 2015, Higashibayashi et al. reported the synthesis of a doubly linked carbazole dimer 173.5 (Scheme 173).1054 This system was made from a simple monocarbazole derivative 173.1 using three different routes, each including an oxidative N−N bond formation and either a reductive or an oxidative C− C coupling. When treated with trifluoroacetic acid, 173.5 was found to nonreversibly disproportionate to its radical cation [173.5]+ and the ring-opened bicarbazole monocation [173.4H]+. A somewhat disguised heteraacenaphthylene motif is found in alpkinidine C50.1 (Chart 50) and similar pyrroloacridine alkaloids, closely related to pyridoacridines discussed in sections 3.1 and 5.2. To date, two syntheses of alpkinidine analogues have been accomplished, by Kitahara et al. in 20041055 and Piggott et al. in 2013,1056 yielding pentacyclic systems C50.2 and C50.3, respectively. A distantly related bislactam system C50.4 and its several substituted derivatives were obtained by Yamaguchi et al. via multistep functionalization of enantiopure helicenes.1057 3598


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Scheme 172. Pyrrole-Fused Fluoranthenesa

Scheme 173. Synthesis of a Pyridazino[4,3,2-jk:5,6,1j′k′]dicarbazolea

a Reagents and conditions: (a)1023 CNCH2CO2Et, 172.2 (cat.), THF, reflux, overnight; (b)538 [Cp*RhCl2]2, AgSbF6, Cu(OAc)2·H2O, DCE, 110 °C, 36 h.

Chart 49. Carbazole-Fused Systems

a

Reagents and conditions:1054 (a) N-bromosuccinimide (110 mol %), SiO2, CH2Cl2, rt, 4 h; (b) KMnO4 (250 mol %), acetone, 60 °C, 4 h; (c) Ni(cod)2 (150 mol %), cod (150 mol %), 2,2′-bipyridyl (150 mol %), THF, 45 °C, 6 h; (d) Ni(cod)2 (300 mol %), cod (300 mol %), 2,2′-bipyridyl (300 mol %), THF, 80 °C, 6 h; (e) FeCl3 (200 mol %), CH2Cl2, rt, 15 min; (f) Bu4NMnO4 (200 mol %), pyridine, 70 °C, 24 h; (g) TFA.

Chart 50. Benzo[cd]indol-2-ones and Related Systems

6.2.5. Carbonyl-Free Azafluoranthenes. Formation of indeno[1,2,3-kl]acridine C51.1a as a product of photorearrangement of azatriptycene was observed in 1964 by Wittig and Steinhoff, who confirmed the identity of this species by an independent synthesis (Chart 51).1058 The rearrangement was initially proposed to proceed via a biradicaloid cleavage of a CN bond of azatriptycene. In the 1980s, the mechanism was investigated for C51.1a−b by Iwamura et al., who demonstrated nitrene formation as an additional cleavage route1059−1061 (cf., Scheme 185, section 6.4). A new synthesis of C51.1a, involving benzyne addition to 1-aminofluorenone, was reported in 2010 by Rogness and Larock.696 Using various annulation methods, several isomeric and expanded analogues of C51.1a were synthesized, including C51.2,1062−1064 C51.3− 4,1065 C51.5−6,468,469 and C51.7469 (for azafluoranthene systems developed by Prostakov et al., see refs 1066−1071). Expanded fluoranthenes containing more than one heteroatom have also been reported, notably C51.8,1072 C51.9,859,1073,862 C51.10,862,1073 C51.11,1074 and C51.12.1075,1076 The latter species formed in a Pd-catalyzed cyclization of the corresponding pyrazolo[3,4-b]quinoline (cf., Scheme 182, section 6.4). An interesting species, “dicarbazolylene” C51.13, was described in 1950 by Plant and Tomlinson.1077 This system, which is a

carbazole cyclodimer, can also be viewed as a diaza analogue of rubicene. 6.2.6. Miscellaneous Azaacenaphthylenes and Azafluoranthenes. Several nitrogen-rich π-extended acenaphthylenes have been obtained, characterized by the presence of one or two N atoms at the junction of five- and six-membered rings. Such systems, which typically contain ketone or lactam functionalities, were reported by Schefczik (C52.1, Chart 52),1078 Kappe and co-workers (C52.2,1079 C52.3,1080 C52.51081), Möhrle and Seidel (C52.4),1082 Gharagozloo et al. (C52.6),1083 Kovtunenko et al. (C52.7),1084 and Koutentis et al. (C52.8).1085 In contrast to C52.1−7, which were 3599


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conversion yields up to 0.42% under simulated AM 1.5 solar irradiation. In the systems described above, the acenaphthylene substructure was typically expanded by ortho-fusion of additional, mostly six-membered rings. Large frameworks containing an additional peri-fusion point, for example, one embedded in the cyclopenta[def ]phenanthrene substructure, are rarely encountered (Scheme 174). A peri-fused pyrrole diketone

Chart 51. Carbonyl-Free Azafluoranthenes

Scheme 174. Heteroaromatics Containing Cyclopenta[def ]phenanthrene Substructuresa

synthesized by classical condensation methods, compound C52.8 was obtained via AgI-mediated Pd-catalyzed cyclizations, of either oxidative or nonoxidative character. Compound C52.10, analogous to the substituent-free system C52.9 described in earlier work,1062,1086 was synthesized in 1981 by Křepelka et al. as a potential antineoplastic agent.1087 Dyes C52.11−12 were synthesized in 2011 by Wang et al. via condensation of N-substituted quinacridones with cyanoacetic acid.1088 For each R group, a mixture of single and double condensation products was obtained, each containing a single five-membered ring. C52.11 and C52.12 showed absorption in the entire visible range, extending, respectively, to ca. 700 and 750 nm. When employed as acceptor materials in heterojunciton solar cells, these systems provided power

a

Reagents and conditions: (a)481 benzylamine, reflux, 0.5 h; (b)1089 420−430 °C, CO2 atmosphere.

174.2 was described in 1936 by Scholl and co-workers as a product of condensation between pentaphene-5,8,13,14tetraone 174.1 and benzylamine.481 Zander and Franke reported the synthesis of benzo[def ]naphtho[2,3-b]carbazole 174.4, which could be obtained from each of the substituted

Chart 52. Miscellaneous Azaacenaphthylenes and Azafluoranthenes

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Scheme 175. Pyracylene-Based Heterocyclesa

benzocarbazoles 174.3a−b by means of the Elbs reaction (174.3b apparently underwent deacylation).1089 Unexpected lactamizations of dibenzophenothiazinecarboxylic acids were observed by Shirley and Gilmer, yielding the highly fused products 174.5−6.1090 This behavior was ascribed to steric effects. Interestingly, the highly fused indazolo[4,3,2-hij]quinoline motif is found in two commercial Indanthrene dyes, Navy Blue R (174.7) and Gray M (174.8).348 174.7 is prepared by condensation of 3-bromobenzanthrone with anthrapyrazole, followed by base-induced cyclization. The synthetic proof for the structure of 174.7 was provided by Bradley and Shah in 1959.1091 6.2.7. Pyracylene-Based Systems. The condensation of indigo 175.1 with ethyl phenylacetate or phenylacetyl chloride furnishes the fused 1,5-naphthyridine 175.2 known as Cibalackrot (Ciba Lake Red B), which is an industrial dye with photophysical properties very different from those of indigo (Scheme 175).1092 The compound was described in the 1910s by Engi (see ref 1093) and subsequently by Posner and Kemper.1094 The condensation was shown to work with other indigo derivatives, for example, with β-naphthindigo, yielding 175.3,1093 as well as with other arylacetyl derivatives. The fused ring system of 175.2 is otherwise rarely encountered; a structurally related molecule, 175.4, was reported in 1992 by Somei and Kodama, as a product of double cycloaddition of dimethyl acetylenedicarboxylate to 2,2′-biindole.1095 In 2014, a bisthienyl analogue of 175.2, now renamed “bayannulated indigo” was used by Liu et al. as an acceptor unit for the synthesis of donor−acceptor semiconducting polymers 175.5−6.1096 Thin-film OFETs fabricated with both polymers showed ambipolar transport properties with hole and electron mobilities reaching 1.5 and 0.41 cm2 V−1 s−1, respectively. Bronstein et al. reported a similarly constructed polymer based on the Cibalackrot core, which was successfully used as an ambipolar OFET material.1097 In contrast to indigo, which undergoes rapid internal conversion upon excitation, the polymer-embedded 175.2 core was found to be an effective photovolatic unit, providing power-conversion efficiencies of up to 2.35% in inverted BHJ devices. Tetrakis(arylimino)pyracene 175.8, first reported in 2008 by Cowley and co-workers as a bifunctional ligand,1101 has been converted to several heterocyclic derivatives. The bisimidazolium salt 175.7 obtained by Prades, Peris, and Alcarazo by bisannulation with chloromethyl methyl ether1098 was used as a bridging carbene ligand for Rh, Ir, and Pd.1098,1102 Cowley et al. reported the formation of bis-phosphenium dication salt 175.9 and the bis([1,3,2]diazaborole) radical cation salt 175.10 by annulations with appropriate heteroatom sources.1099 These two systems, and a related bis(TeI2) complex, demonstrated differences in electron transfer from each p-block element into the 175.8 core, and their dependence on the character of bonding interactions. A different placement of nitrogens in the pyracylene substructure is found in diindeno[1,2,3-de:1′,2′,3′ij]phthalazine 175.11 reported in 1989 by Bethell et al.1100 This species, which was only partially characterized, was obtained by thermal decomposition of 11,12-bis(diazo)-11,12-dihydroindeno[2,1-a]fluorene 175.11. 6.2.8. Extended Thiaacenaphthylenes. Dithiole-fused acenes and their Se and Te analogues, which are briefly discussed below, are an important class of organic materials because of their remarkable conducting properties (C53.1−14, Chart 53). Their chemistry, solid-state structures, and device performance were reviewed in 2011 by Briseno and co-

Reagents and conditions: (a)1094 reflux; (b)1098 MeOCH2Cl, 100 °C, 20 h; (c)1099 PI3, DCM, 12 h; (d)1099 BI3, DCM, 12 h; (e)1100 mesitylene, 100 °C, 3.5 h. a

workers.28 Fused dithiole rings are commonly synthesized by direct reactions of acenes with elemental chalcogens. Such reactions are often unselective and require fairly high temperatures, which are typically achieved by refluxing in DMF or 1,2,4-trichlorobenzene. Large, multiply substituted systems, such as the “hexasulfide” C53.12,1103 can be obtained using this approach. With larger molecules the solubility becomes very low, as in the hexacene substitution product, for which two alternative structures C53.13−14 were considered.1103 Nucleophilic displacement of halogens by means of dichalcogenides can provide certain substituted derivatives, such as C53.10−11 (for a related synthesis, see the perylene derivative 61.9, section 3.4). Similarly, C53.7 can be 3601


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Scheme 177. Se- and Te-Bridged Triphenylenesa

Chart 53. Edge-Fused Chalcogenoacenes

a

Reagents and conditions: (a)1108 Na2Se2, THF, 24 h, rt; (b) Na2Te2, THF, 24 h, rt; (c) DDQ, PhCl, reflux 10 h.

the oxidative step, and the resulting product mixtures could not be separated without decomposition. A different type of thia-containing 5-6-6 system was reported in 1986 by Nakasuji, Murata, et al. (Scheme 178).1109 1,2Scheme 178. Pentalene-Fused Thioxanthenea

synthesized from tetrachloroanthraquinone and sodium sulfide (see refs 1104−1106 and earlier work cited therein). A highly stable trithiapentaleno-fused pentacene-7-one 176.4 was reported in 2014 by Kintigh et al.1107 The compound is deep-blue when dissolved in chloroform, with an optical bandgap of 1.90 eV, and forms a densely stacked structure in the solid state, stabilized by a combination of π−π and CH−π interactions. It showed activity in bilayer photovoltaic devices, which improved considerably upon increasing the temperature.

a

Reagents and conditions: (a)1109 AlCl3, NaCl, 240−250 °C, 30 min.

Di(thioxanthen-9-ylidene)ethene 178.1 was subjected to oxidative double cyclization under Scholl conditions, providing a moderate yield of the dark-violet product 178.2. Despite the low solubility in organic solvents, the first oxidation potential of 178.2 was determined at ca. +0.72 V vs SCE. 178.2 formed charge-transfer complexes with DDQ, TCNQF4, and TCNQ as acceptors, characterized with a variable degree of CT character. 6.2.9. Fused Oxaacenaphthylenes. Extended 5-6-6-fused structures containing oxygen (either in a 5- or a 6-membered ring) are occasionally formed in rather exotic, and not necessarily related, systems. Grinev et al. reported the conversion of 179.1 into 179.2 via an intramolecular Friedel−Crafts acylation.1110,1111 As part of their research on bis(alkyne) cycloaromatizations, Domı ́nguez et al. reported in 2004 a complex transformation of the substituted xanthenol 179.3 into the indenoxanthenone 179.4, which was crystallographically characterized.1112 The reaction was proposed to proceed via alkyne−allene rearrangement, followed by Schmittel cyclization, radical acetylation, and aldol-type cyclization. When treated with perchloric acid, the extended “starphene” ketone C54.1 undergoes intramolecular cyclization, to form the extensively fused, emerald-green furan derivative C54.2a (Chart 54). The formation of such extended furans and their reactivity were investigated in the 1930s by Pummerer and coworkers,203,1113,1114 and subsequently by Fierz-David et al.1115 and Laatsch.1116 In particular, C54.2a can be acetylated to the singly and doubly substituted derivatives C54.2b−c, and reductively acetylated to produce C54.3.1116 Even more extended furans, red-violet C54.4 and orange C54.5, were

Scheme 176. Trithiapentaleno-Fused Pentacene-7-onea

a

Reagents and conditions: (a)1107 N2, HI/HOAc, reflux, 6 h; (b) N2, NaBH4, THF:H2O (5:1), reflux, 3 h; (c) N2, DMF, S8, reflux, 24 h; (d) N2, TCB (1,2,4-trichlorobenzene), S8, reflux, 4 h.

Bay-annulated triphenylenes containing Se and Te bridges (177.3a−4a and 177.3b−4b) were described by Singh et al. in 2014 (Scheme 177).1108 These systems were obtained by means of nucleophilic bridging of the saturated hexabromo derivative 177.1, followed by oxidative aromatization/ring contraction of the triply annulated intermediates 177.2a−b. Unfortunately, the bridges were unselectively cleaved during 3602


Chemical Reviews

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Scheme 179. Miscellaneous Oxaacenaphthylenesa

of nitro precursors such as 180.5, providing access to 180.2 and a range of its substituted derivatives,1122,1123 and aza analogues 180.5−7.1124 Compound 180.2 shows strong fluorescence emission (378 nm, quantum yield 0.41).1122 180.2 undergoes electrochemical and chemical dimerization, yielding compounds 180.8−10,1122 and has been shown to form conducting films upon oxidation.1122,1123 Liu et al. developed Heck cyclizations of halogenated precursors (yielding, for instance, 180.6),1125 which were subsequently used to prepare materials for dye-sensitized solar cells1126 and to synthesize a new range of planarized donors for PHOLED applications.1127 By extending the synthetic strategies described above, Khodorkovsky et al. succeeded in preparing dibenzo[2,3:5,6]pyrrolizino[1,7-bc]indolo[1,2,3-lm]carbazole 180.12 and a range of its substituted derivatives. 180.12 contains a “doubled” pyrrolo[3,2,1-hi]indole motif, which was assembled using either the diazonium method (from 180.11) or the Ullmann amination route (from 180.13).1128 The low yield of the diazonium route resulted from its imperfect regioselectivity. Indolocarbazole 180.13 was prepared from bis(indolyl)methane 180.141128,1129 or the dimethylbarbituric derivative 180.15.1130 180.12 is a fluorescent compound, characterized by uniquely small Stokes shifts and quantum yields close to 100%, and shows moderate TPA cross sections (ca. 20 GM at 850 nm). Zhang et al. reported a synthesis of benzo[c]pyrrolo[1,2,3lm]carbazoles (e.g., 180.18, Scheme 180).1131 The initial step involved a double photocyclization of a tethered indole− styrene precursor 180.16, which was followed by the DDQmediated aromatization of the pentacyclic intermediate 180.17. A cross-conjugated diaza system 180.21, containing the pyrrolo[3,2,1-hi]indole substructure and three exocyclic double bonds, was reported by Kovtunenko et al.1132 6.3.2. Fused Pyrrolo[2,1,5-cd]indolizines. The pyrrolo[2,1,5-cd]indolizine motif, isomeric with pyrrolo[3,2,1-hi]indole, constitutes the basis of a range of fused derivatives. The most important five-ring system, benzo[1,2]indolizino[3,4,5-ab]isoindole 181.3a, was first reported by Matsumoto and co-workers. 1133,1134 Pyridinium dicyanomethylides 181.1a−d underwent dipolar cycloadditions with benzyne, providing moderate yields of 181.3a−d and the monocycloaddition intermediate 181.2a−d (Scheme 181). Further refinements of this method enabled the synthesis of other fused systems, such as 181.3,1135 181.4,1136 and 181.5.1137 The peculiar conjugated structure of 181.3a was the subject of some interest. A peripheral conjugation pathway encircling the internal nitrogen was proposed by Matsumoto et al. on the basis of 14N NMR and redox data.1138 Wudl and co-workers reported another cycloaddition synthesis of 181.3a, starting from pyrido[2,1-a]isoindole 181.6.1139 The latter compound was also used for the preparation of pyridazine-fused derivatives 181.8−10. The introduction of the pyridazine ring caused a marked increase in fluorescence quantum yields (82% for 181.10, 40% for 181.3a in DMSO). The observed Stokes shifts were generally below 20 nm.

a Reagents and conditions: (a)1110,1111 (1) SOCl2, CCl4, (2) AlCl3; (b)1112 toluene, AcOH, 115 °C.

Chart 54. peri-Fused π-Extended Furans

reported in 1992 by Laatsch and co-workers.1117 The reactivity involved in the formation of C54.2−5 is closely related to the chemistry of tetrabenzotetraoxa[8]circulene 159.1 (Scheme 159, section 6.1). 6.3. Cyclopenta[cd]indene Systems

The 5-5-6 peri-fused ring system (cyclopenta[cd]indene, cf., Chart 43) is quite highly strained, and therefore rarely encountered in large π-conjugated frameworks. The two most prevalent heterocyclic motifs, pyrrolo[3,2,1-hi]indoles and pyrrolo[2,1,5-cd]indolizines, are discussed below. For a dynamic 2a-selenacyclopenta[hi]indene system and related molecules, see papers by Reich et al.1118,1119 6.3.1. Fused Pyrrolo[3,2,1-hi]indoles. Indolo[3,2,1-jk]carbazole 180.2 was first reported by Dunlop and Tucker, who performed a thermal cyclization of the diazonium salt derived from 180.1 (Scheme 180).1120 The strained ring system in 180.2 was initially presumed to contain a pyramidal nitrogen center, an assumption that was shown to be incorrect in subsequent structural studies. 180.2 was later synthesized from the anhydride 180.4 by Brown, Eastwood, et al.1051 Their reaction, which involved pyrolytic generation of the aryne intermediate 180.3, gave a superior yield of 83%. A related FVP approach developed by McNab et al.1121 involved the pyrolysis

6.4. peri-Fused Seven-Membered Rings

6.4.1. peri-Fused Cycloheptatrienes. Heteroaromatic systems in which six or more rings are peri-fused to a central cycloheptatriene ring have been classified as [7]circulenoids and are discussed in section 6.1. Apart from circulenoids, large heterofused cycloheptatrienes are modestly represented in the literature, with a notable early example provided by Fieser’s 3603


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Scheme 180. Fused Pyrrolo[3,2,1-hi]indolesa

Reagents and conditions: (a)1120 (1) NaNO2, H2SO4, H2O, (2) reflux; (b)1051 900 °C, 0.03 Torr; (c)1122 875 °C, 0.02 Torr; (d)1125 Pd(OAc)2, PPh3, BnEt3NCl, K2CO3, DMA, reflux, 4 h; (e)1128 activated Cu, H2SO4, H2O, 9%; (f) solid, exposure to sunlight, 12%; (g) CuI, [NBu4]OH, DMF, 74%; (h)1128,1129 I2, MeCN, reflux, 25%; (i)1130 AcOH, reflux, ca. 40%; (j)1131 500 W medium pressure Hg lamp, acetone, pyridine, 12 h; (k), DDQ, DCM, rt, 12 h; (l)1132 NEt3, DMF, reflux; (m) p-dimethylaminobenzaldehyde, Ac2O, reflux, 30 min. a

Scheme 181. Fused Pyrrolo[2,1,5-cd]indolizinesa

a Reagents and conditions: (a)1133 i-AmONO, anthranilic acid, DCE, DME; (b)1139 2,2,6,6-tetramethylpiperidine, n-BuLi, bromobenzene, THF, −78 °C, 1 h; (c)1139 dimethyl maleate, CH2Cl2, 0 °C, 1 h; (d)1139 hydrazine, EtOH, chlorobenzene, reflux, 5 days; (e)1139 PCl5, POCl3, 130 °C, 4 h; (f)1139 phenol, K2CO3, DMF, reflux, 3 days.

pleiadenoisoxazolones (e.g., 182.2, Scheme 182), which were obtained in 1933 by cyclization of appropriate oximes.1140 A fused cyclohepta[cd]benzofuran derivative 182.4 was synthesized in 2010 by Mehta and Larock, who used a potentially highly general Pd-catalyzed alkyne annulation strategy.1141 A

versatile method of synthesizing fused 1,2,3-triazabenzo[cd]azulenes 182.61075,1076,1142−1144 and 182.71145 was developed by Danel and co-workers. Their strategy involves a threecomponent assembly of substituted pyrazolo[3,4-b]quinolines 182.5, which were subsequently cyclized to 182.6, possibly via 3604


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Scheme 182. peri-Fused Cycloheptatrienesa

a (a)1140 acid or base; (b)1141 Pd(OAc)2, NaOAc, TBAC, DMF, 100 °C, 24 h; (c)1075,1076,1142−1144 ethylene glycol, reflux, 3 h; (d) KOH, isoquinoline, reflux, 2 h, 30−70%.

Scheme 183. Synthetic Chemistry of Arcyriacyanin A and Radermachola

Reagents and conditions: (a)1150 (1) AcOH, 80 °C, MW heating, 10 min, (2) t-BuOK, THF, 25 °C, 1 h; (b)1147 (1) EtMgBr, THF, rt, 2 h, (2) 3,4dibromomaleimide, toluene, 110 °C, 2 h; (c) Pd(OAc)2, dppp, NEt3, DMF, 110 °C, 18 h; (d) Pd(OAc)2, PPh3, NEt3, MeCN, 80 °C, 3 h; (e)1151 (1) NaBH3CN, THF, 0 °C, (2) 180 °C, 5 min; (f)1152,1153 (1) Me3SiI, CDCl3, rt, 27 h, (2) MeOH, (3) p-TsOH, benzene, reflux, 5 h; (g)1154 (1) NaH, THF, (2) BuLi, −78 °C to rt. a

Joshi, Pelletier, et al.1152,1153 Their method relied on the closure of the peri-fused furan ring in the final step, a strategy that was retained in a modified synthesis of 183.11 described by Hauser and Yin.1154 A four-step total synthesis of radermachol, involving final cyclization of the 7-membered ring in 183.12, was reported in 2014 by Buccini and Piggot.1155 6.4.2. Fused Azepines and Diazepines. An early example of a heteroaromatic benzo[cd]azulene was provided by the synthesis of the 184.3 dye, known as Ciba Yellow 3G, which was obtained by heating indigo with benzoyl chloride and copper, as first reported in 1914 by Engi (Scheme 184).1093,1156 184.3 can also be obtained in two steps by thermal decomposition of 184.2 (Dessoulavy’s compound). When 184.2 or its hydroxy analogue (184.1, Höchst Yellow R) are heated with sulfuric acid, they are converted into Höchst Yellow U (184.4), a dye isomeric to 184.3. The correct structures of 184.3 and 184.4 dyes were established in 1949 by de Diesbach and co-workers,1157 and the structure of 184.3 was confirmed crystallographically.1158 Extensively fused azepin-4ones were also described by Niume et al. (184.5),1159 Palmisano et al. (184.6),1160 and by Chardonnens and

a benzyne intermediate. If the C-substituent on the pyrazole ring is nonaromatic, the cyclization reaction leads to azafluoranthene products C51.12 (section 6.2). 182.6 derivatives exhibiting stable glassy states were explored as OLED materials, emitting greenish-yellow light in ITO/PEDOT:PSS/ azulene/Ca/Al devices.1146 Two systems of particular interest, to which a considerable synthetic effort was directed, are the natural products arcyriacyanin A 183.4 and radermachol 183.10 (Scheme 183). Three routes to 183.4 (isolated from the slime mold Arcyria obvelata), starting from 183.3, 183.6, or 183.7, and based on organomagnesium or Heck-type couplings, were reported in 1997 by Steglich et al.1147 The route involving 183.3, which was also explored by Tobinaga et al.1148 and Murase et al.,1149 was subsequently extended by Kraus and Guo, who synthesized the 183.3 precursor in one step, using the 2-aminobenzyl phosphonium salt 183.1.1150 An alternative route to 183.5, based on the final indolization of aminonitrile 183.9, was also reported by the Steglich group.1151 The red pigment radermachol 183.11, isolated from the bignonia plant Radermachera xylocarpa, was first synthesized in the 1990s by 3605


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Gamba (184.7−9).1161 The latter three systems were obtained using either imine or oxime cyclizations in the final ringforming step. Fused azepines 185.2 were prepared in 1972 by Hellwinkel and Seifert, in an electrophilic cyclization of a diazonium salt generated from 185.1.1162 Iwamura et al. reported the partly conjugated azepine 185.10, which was obtained by photorearrangement of 1-azatriptynecene 185.61163 (cf., compound C51.1a, section 6.2). The mechanism of this reaction begins with the formation of nitrene 185.7, which is transformed into azanorcaradiene 185.8. The latter species rearranges into 185.10 under basic conditions. Hydride abstraction from 185.10 produced the fully conjugated cation 185.11. A similar type of reactivity could also be induced for nitrenes generated by photolysis of appropriate azides.1061 In 2013, a method of synthesizing densely substituted benzazepines (e.g., 185.4) through Pd-catalyzed oxidative cycloadditions of acetylenes to isatins was reported by Jiang, Wang, et al.1164 To demonstrate the utility of such derivatives, compound 185.4 was oxidatively coupled to the highly fused benzazepine 185.5. An unusual tandem bis-annulation of a “doubly N-confused” dipyrrin 185.13 to isomeric azaheptalenes 185.14 and 185.15 was observed by Anand and co-workers.1165 Oxidation of methylene-3,3′-diindolizine 186.1 with chloranil followed by anion exchange, in 1966 reported by Reid et al., yielded the green pentacyclic diazepine cation 186.2.1166 The cation could be cleanly reduced with sodium borohydride in methanol to the red neutral product 186.3. Benzo[b]fluoreno[1,9-ef ][1,4]diazepines 186.8 were synthesized by Baxendale et al. using flow techniques.1167 Further examples of related systems include carbazole-derived pentacyclic diazepines 186.4 reported by Voelter et al.1168 and carbolinediazepines 186.5 of Beugelmans−Verrier and Potier,1169 and the hexacyclic purine 186.6 obtained by Taylor and Yoneda in a reaction of 1,3-dimethyl-4,5-diaminouracil with anthranilic

Scheme 184. Indigo Yellow Dyes and Other Azepinone Systemsa

a

Reagents and conditions:1156 (a) PhCOCl, reflux; (b) PhCOCl, Cu powder, nitrobenzene, reflux; (c) heat, loss of PhCOCl; (d) conc. H2SO4, 130 °C.

Scheme 185. Azepines with Extended peri-Fusiona

a

Reagents and conditions: (a)1162 (1) NaNO2, AcOH, H2O, sulfamic acid, (2) heating, (3) Al2O3 chromatography; (b)1164 Pd(OAc)2, AgOAc, MeCN/dioxane, 100 °C, 24 h; (c) Cu(OTf)2, AlCl3, CS2, rt, 24 h; (d)1163 hν, MeOH, MeONa; (e) [Ph3C][BF4], ether/chloroform; (f)1165 DDQ; (g) Fe(acac)3; (h) Cu(OAc)2. 3606


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acid.1170 A hexacyclic urea derivative 186.10 was obtained by Lautens et al. using a new general strategy, which involves

Thiourea derivatives of 187.3 showed activity as inhibitors of bacterial and human protein kinases.1174 6.4.3. Oxa- and Thia- 7-Membered Rings. A naphthalene-annulated oxepin 188.2, structurally related to [7]circulenes, was prepared by König et al.1176 The central ring was efficiently closed by acid-catalyzed intramolecular condensation of the corresponding binaphthalene precursor 188.1 (Scheme 188). Oxepin 188.2 was subjected to a sequence of

Scheme 186. Fused Diazepinesa

Scheme 188. Chemistry of Annulated Oxepinsa

a Reagents and conditions: (a)1176 p-TsOH, toluene, reflux; (b) NBS, DMF, 40 °C; (c) PhB(OH)2, K3PO4, Pd(OAc)2, P(o-Tol)3, toluene/ dioxane/water, 70 °C; (d) NBS, DMF, 80 °C.

brominations and Suzuki couplings (188.3−6), which established a straightforward route to aryl-functionalized derivatives. Compounds 188.2, 188.4, and 188.6 are fluorescent in solution, showing emission maxima in the 427−465 nm range. As part of their 2004 study on the synthesis of fused polycycles by 1,4-palladium migration chemistry, benzo[f ]fluoreno[1,9-bc]oxepin 189.4 was synthesized by Larock and co-workers (Scheme 189).1177 The reaction was proposed to proceed via intramolecular migration of Pd between the palladated intermediates 189.2 and 189.3. A route toward benzannulated oxazepinocarbazoles 189.6, using Pd-catalyzed intramolecular C−O cross-coupling, was reported by Prasad et al.1178 A pentacyclic dibenzoxepin lactam 189.10 was obtained by Heo et al. using a Cu-catalyzed one-pot etherification/aldol condensation cascade reaction.1179 Irradiation of dibenzothiophene precursors 189.11 produced, in addition to phenanthro[4,5-bcd]thiophenes, also the corresponding pentacyclic thiepines 189.12, as reported by Furukawa et al.1180 Fused 1,4-thiazepines, such as 189.13 and its analogues containing two 7-membered rings, were obtained by Galt et al. in reactions

a

Reagents and conditions: (a)1166 (1) chloranil, MeOH, (2) HClO4, MeOH, reflux; (b)1167 NaBH4, MeOH; (c)1168 (1) n-BuLi, MW heating, (2) PtO2, flow hydrogenation, 37−51%; (d)1171 186.11, K2CO3, CsOPiv, toluene, 100 °C.

domino Heck coupling of 2,2-dibromovinylarenes, catalyzed by palladacycle 186.11.1171 Diazepine 187.3, containing the [1,4]diazepino[1,7-a:6,5,4h′i′]diindole framework isomeric to that found in arcyriacyanin A (183.4), was synthesized by a cyclization−aromatization sequence by Preobrazhenskaya et al. (Scheme 187).1172 This strategy enabled the synthesis of substituted derivatives of 187.3,1173,1174 as well as other related ring systems, such as the benzologue 187.41172 and the monoazepine isomer 187.5.1175 Scheme 187. Diazepinodiindolesa

a

Reagents and conditions:1172 (a) TFA, DCM, rt, 2 h; (b) DDQ, toluene, reflux, 1 h. 3607


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Scheme 189. 5-6-7-Fused Oxepins and Thiepinesa

Reagents and conditions: (a)1177 Pd(OAc)2, dppm, CsO2CCMe3, DMF, 110 °C; (b)1178 Pd(OAc)2, Cs2CO3, TBAB, DMF, 100 °C, 2 h; (c)1179 Cu, Cs2CO3, pyridine, 150 °C, 24 h; (d) photochemical, benzene; (e) NaOH, dioxane/EtOH/water, reflux, 30 min; (f) Cu bronze diethyl phthalate, 94% as picrate; (g)1181 PCl3, 1,2-DCE, 80 °C, 12 h. a

Chart 55. Classification of Macrocyclic Systems Used in This Sectiona

a

The macrocyclic fragment is shown in blue; π-conjugation and positions of optional heteroatoms are not indicated.

of substituted anthraquinones with 2-aminobenzenethiol.164 Subsequent Cu-mediated extrusion of sulfur from 189.13 yielded the dibenzoacridine derivative 189.14. A class of V-shaped acene analogues 189.16 containing a combination of oxepin and pyrrole rings was reported in 2016 by Juršeṅ as and co-workers.1181 These molecules were obtained from the dicarboxylic precursors 189.15 in a single step consisting of a double Friedel−Crafts acylation and dehydrative formation of the oxepin. Compounds 189.16 were fluorescent, with λmaxem at ca. 545 nm, and quantum yields in the 0.03−0.05 range. Hole drift mobilities of up to 8 × 10−4 cm2 V−1 s−1 were measured for wet-casted films of these materials under ambient conditions.

7. MACROCYCLIC SYSTEMS At first glance, macrocyclic systems may seem quite distinct from other polycyclic heteroaromatics. However, extensively fused macrocycles can be viewed as “punctured nanographenes”, and bear structural relationships to many classes of PHAs discussed above. At the same time, macrocyclic systems are distinguished by the unique repertoire of methods used to synthesize them. They additionally exhibit extraordinary reactivity patterns, and unusual electronic features. In line with the scope of this Review, our principal focus is on systems that are peri-fused and contiguously π-conjugated, although not necessarily planar. Acetylene-based macrocycles are excluded, except for shape-persistent, planar molecules. 3608


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Scheme 190. Syntheses of Acenaphthoporphyrinsa

a

Reagents and conditions:1201 (a) (1) ethyl isocyanoacetate, DBU, THF, rt, 16 h; (2) KOH, ethylene glycol, reflux; (b) 7% AcOH−EtOH, N2, reflux, 16 h; (c) (1) TFA, N2, 10 min, (2) CH2Cl2, N2, 25 °C, 2 h, (3) Et3N, 1 equiv of DDQ, 1 h.

systems have been included in the preceding sections, but the majority is grouped here for consistency. With the exception of section 7.1, all macrocycles presented in this chapter are “ortho-, peri-, and macro-fused,” as defined in the Introduction (section 1.2). This type of fusion can be achieved in porphyrinoids by condensing an aromatic ring to adjacent meso and β positions (sections 7.2, 7.3, and 7.4), by incorporating a polycyclic subunit in the macrocycle (section 7.5), or by introducing internal bridges into an existing structure (section 7.6). Polycyclic building blocks are typically assembled prior to the synthesis of the macrocycle, which is designed to span more than one ring of the subunit. A catch-all taxon “peri-fused cyclophanes” is created for the remaining systems, which are discussed in section 7.7. Many of the relevant cyclophane families contain members with highly variable geometries, ranging from planar, through ruffled, to highly distorted, which makes the selection process quite difficult. Thus, even though our principal focus is on planar, well-conjugated molecules, some very contorted cyclophanes are also included for consistency. Macrocyclic aromaticity is an important feature of many systems discussed below. Where necessary, we indicate conjugation circuits with bold bonds, especially in sections dealing with porphyrin analogues (for a more detailed discussion of π-conjugation in porphyrinoids, see refs 1193 and 1182). As in preceding sections, bold bonds are also used

In the current literature, heteroaromatic macrocycles containing peri-fusion points, which are the subject of the following sections, are typically classified as either porphyrinoids or (hetera)cyclophanes. The term “porphyrinoid” has no precise definition and currently encompasses a large group of porphyrin-like macrocycles, including some hydrocarbon systems.1182 Formally, all porphyrinoids meet the current IUPAC definition of cyclophane,1183 but it is nevertheless convenient to retain the former term for practical reasons. In the following discussion, the structures will generally be classified as porphyrinoids if they were considered as such by the original authors. Various aspects of porphyrinoid chemistry, relevant to the present chapter, have been reviewed by several groups.1182,1184−1190 A few more general reviews on heteroaromatic macrocycles have also been published, notably the yearly chapters in Progress in Heterocyclic Chemistry by Newkome (since 1989)1191 and a chapter on heteracyclophanes by Vögtle et al.1192 The largest circulene systems, which constitute a small class of macrocycles different from both porphyrinoids and cyclophanes, have been reviewed in section 6.1. The material is classified according to the topological features of the macrocycle and underlying synthetic chemistry (Chart 55). In section 7.1, we discuss molecules containing an orthoand peri-fused ring system that is itself ortho-fused to a pyrrolic β−β bond of a porphyrinoid macrocycle. A few of those 3609


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Chart 56. [b]-Fused Porphyrins Containing Fluoranthene, Pyrene, Corannulene, and Dibenzo[fg,op]tetracene Subunits

Scheme 191. Synthesis of Acenaphthylene-Fused Cyclo[8]pyrrolesa

a

Reagents and conditions:1212 (a) CNCH2CO2Et, t-BuOK; (b) (1) benzyltrimethylammonium dichloroiodate (BTMS·ICl2), CaCO3, CH2Cl2, MeOH, (2) (Boc)2O, 4-dimethylaminopyridine (DMAP), CH2Cl2; (c) (1) Cu powder, DMF, 110 °C, (2) conc. HCl, AcOEt; (d) NaOH, (CH2OH)2, 170 °C; (e) Ce(SO4)2, conc. H2SO4, Na2SO4, N(n-Bu)4HSO4, CHCl3, reflux, 27 h.

method 1.1197,1198 [b]-Fusion has been extensively applied as a method of expanding the π-conjugated system of the porphyrin, and the field was reviewed by several authors.1199,1200 Here, we focus on systems containing peri-fused subunits on the periphery of the macrocycle. In 1996, Lash and Chandrasekar reported the first examples of porphyrins containing β−β-fused acenaphthylene units.1201 The requisite monopyrrole 190.2 was obtained in moderate yield from 1-nitroacenaphthylene 190.1, by using the Zard− Barton method (Scheme 190). Fused porphyrins 190.6−8 were then synthesized in two steps using tripyrrane intermediates 190.4a−c. The UV−vis spectra of compounds 190.6a−c showed large bathochromic shifts relative to octaalkylporphyrins, larger than in the related naphtho-, phenanthro-, and phenanthrolino-fused derivatives. In subsequent work, the groups of Lash1202 and Ono1203 presented the syntheses of tetraacenaphthoporphyrins 190.9−10, characterized by even larger red shifts. For instance, in 0.5% TFA−chloroform, the

to indicate out-of-plane orientation of aromatic subunits. These two drawing conventions are nevertheless easily discerned. 7.1. [b]-Fused (β−β-Fused) Porphyrinoids

The synthesis of [b]annulated porphyrins is typically achieved using one of the following approaches:1194 (1) condensation of fully conjugated [c]fused monopyrroles, (2) condensation of nonconjugated [c]fused monopyrroles followed by aromatization of peripheral rings, and (3) condensation of β-aryl monopyrroles followed by coupling of β-substituents. In method 1, the starting monopyrroles can have a varying degree of isoindole character, affecting their reactivity, a problem that is not present in methods 2 and 3. In all cases, the monopyrrole precursors are conveniently prepared by means of the Zard− Barton nitroalkene−isocyanoacetate condensation1195,1196 and analogous reactions. In particular, nitroaromatics that have a certain degree of nitroalkene character are able to condense under similar conditions, providing precursors for use in 3610


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Scheme 192. [b]-Condensed Porphyrins Containing “Remote” peri-Fusion Pointsa

Reagents and conditions: (a)1214 270 °C, reduced pressure, 3 h, quantitative; (b)1217 Pd(OAc)2, PPh3, K2CO3, DMF, xylene, reflux, 60 h; (c)1217 veratrole, FeCl3 in MeNO2, CH2Cl2, rt. a

dication 190.9-H22+ showed a Soret band at 565 nm and several Q absorptions at longer wavelengths (up to over 750 nm). By varying meso substituents and metal ions, it was possible to increase the red shift even further, and a Soret band at 642 nm was recorded for the ethynyl-substituted lead(II) complex 190.14.1204 Apparently for steric reasons, a corrole analogue of 190.9 (R = Mes) was obtained instead of the porphyrin when mesitaldehyde was used in the condensation.1204 Further systems developed by Lash and co-workers include adjdiacenaphthoporphyrins 190.11a−b,1205 and analogues containing six-membered rings 190.12−13.1206 Kobayashi and Stillman investigated magnetic circular dichroism (MCD) spectra of tetraphenyltetraacenaphthoporphyrin complexes with zinc1207 and other metals,1208 revealing the effect of the saddle distortion of the chromophore on the electronic structure of these systems. Because of their bathochromically shifted spectra, acenaphtho-fused porphyrinoids are of interest in various applications, for example, as sensitizers in photodynamic therapy1209,1210 and as dyes for organic solar cells (190.15).1211 The combination of the Zard− Barton pyrrole synthesis with stepwise porphyrin macrocyclizations enabled the synthesis of further β−β-fused porphyrins containing fluoranthene1022 (56.1−3), pyrene625 (56.4−7), and corannulene subunits912 (56.8−9, Chart 56), all characterized by red-shifted optical spectra. The syntheses of the respective monopyrroles are discussed in preceding sections. Acenaphthylene-fused cyclo[8]pyrrole 191.6 was described in 2013 by Kobayashi, Okujima, et al.1212 This expanded macrocycle was synthesized by using an oxidative coupling reaction of the corresponding 2,2′-bipyrrole 191.5, which is a strategy typically employed for the synthesis of cyclopyrroles1213 (Scheme 191). The bipyrrole precursor 191.5

was prepared by using a Barton−Zard reaction of compound 191.1, followed by iodination and Boc protection of the NH moiety. Next, Ullmann coupling of monopyrrole 191.3 was performed, and the de-esterification of the resulting 191.4 yielded 191.5. In the synthesis of 191.6, different macrocyclization conditions were tested, and the best yield were observed when utilizing Ce(SO4)2 as the oxidant in the presence of concentrated H2SO4, Na2SO4, and N(nBu)4HSO4 in refluxing chloroform. Two conformational isomers 191.6a and 191.6b were isolated. The less polar and lower-symmetry isomer 191.6b could be converted into the 191.6a isomer through a thermally induced flip of one of the constituent pyrroles. 191.6 showed a markedly red-shifted electronic spectrum, with the lowest energy band at 1482 nm. Application of Michl’s perimeter model to magnetic circular dichroism spectroscopic data and TD-DFT calculations demonstrated a marked intensification of the L bands in the near-IR region relative to a benzo-fused cyclopyrrole analogue, due to significant stabilization of the LUMO level in the acenaphthylene-fused 191.6. Benzo[k]fluoranthenoporphyrin 192.2, reported in 2006 by Okujima, Ono, et al.,1214 is an example of a system with a “remotely” condensed peri-fused element (Scheme 192). The synthesis of 192.2, employing a retro-Diels−Alder reaction performed on the bicyclo[2.2.2]octadiene precursor 192.1, was later refined by the same group, to provide the quadruply fused derivative 192.3.1215 192.3 had the Soret band at 480 nm and a very intense low-energy Q-band at 739 nm. The compound had a measurable fluorescence above 750 nm (ΦF = 0.08). An even more extended assembly of rings is found in porphyrincontaining dihydrofulvalene pincers, such as 192.4, developed in 2006 by Johnston and co-workers.1216 192.4 consists of a planar aromatic part (blue, Scheme 192), which is linked to the 3611


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Scheme 193. Perylenophthalocyaninesa

phenanthroline unit (red) via a polycyclic linker (black). The linker introduces a sharp bend into the molecule, leading to an interplanar angle between the porphyrin and phenanthroline parts of ca. 95°, according to an AM1 calculation. A different type of remotely fused system, the dibenzo[fg,op]tetracenoporphyrin 192.8, was synthesized by Wang and co-workers.1217 Their approach was based on a tandem Heck coupling/electrocyclization/aromatization reaction,1218 which provided the benzo-fused porphyrin 192.6. The dibenzo[fg,op]tetracene ring system of 192.8 was then elaborated in a single step, which involved an oxidative cyclization of the two 3,4dimethoxyphenyl substituents, followed by intermolecular coupling with a veratrole molecule. 192.8 showed a Soret band at 457 nm and red-shifted Q bands with increased intensity. DFT calculations indicated that the dibenzo[fg,op]tetracene system in 192.8 is not completely planar, because of the steric congestion caused by the OMe groups. [b]Fused phthalocyanines were reported by several groups. Their preparation relies on the availability of appropriate dinitrile precursors, which are usually synthesized through multistep routes. A notable example of a giant p-HBC-fused phthalocyanine, reported in 2007 by the Müllen group,127 is discussed in section 2.5 (Scheme 28). Perylenophthalocyanines 193.2a−b, containing four perylene subunits bay-fused to the phthalocyanine benzene rings, were reported in 2006 by Cammidge and Gopee (Scheme 193).1219 These macrocycles were obtained by Zn-templated macrocyclization of the corresponding dinitriles 193.1a−b. The syntheses were lowyielding, and because of the low solubility, the phthalocyanine products were difficult to purify and characterize. Both derivatives were non-mesomorphic, with melting points above 300 °C. As a consequence of peripheral pyrene fusion, the electronic spectrum of 193.2a was strongly red-shifted, relative to the corresponding phthalocyanine, with the lowest-energy band at 797 nm. A phthalocyanine analogue of 193.2a, containing one fused perylene subunit, was prepared in a 5% yield using a mixed nitrile cyclization. In 2011, Shimizu, Kobayashi, et al. reported the synthesis of a boron subphthalocyanine 193.5b containing a single fused pyrene unit (a phenanthrene unit relative to the subpthalocyanine skeleton).1220 The compound was obtained in a 0.9% yield in a mixed condensation between the pyrene tetranitrile 193.3 and tetrafluorophthalonitrile 193.4, followed by ligand exchange. The new phthalocyanine showed moderately strong fluorescence (ΦF = 0.07) with the maximum intensity at 601 nm and a Stokes shift of 224 cm−1. Pure 193.5b formed πstacked dimers in the solid state, whereas in cocrystals with C60, it was observed to produce a 2:1 complex, with two subpthalocyanines embracing one fullerene molecule. Several derivatives of “4,5-pyrenocyanine” 193.7a−e, containing different peripheral substituents and metal centers, were prepared in 2012 by Müllen et al.1221 Their synthetic method resembled that used by Cammidge et al., but the isolated yields were considerably higher in Müllen’s work. The position of the lowest energy Q-band was dependent on the metal center (reaching 727 nm for 193.7d), and the absorption spectra showed considerable temperature dependence in toluene solutions. Self-assembly of pyrenocyanines was examined at the liquid/HOPG interface as well as in the bulk, showing features consistent with the disc-like shape of the molecules. A number of crystalline and LC phases were observed for 193.7a−e, characterized by hexagonal and rectangular packing symmetries and varying degrees of molecular tilt.

a

Reagents and conditions:1219 (a) hexanol, DBU, Zn(OAc)2; (b)1220 BCl3, 1-chloronaphthalene, 230 °C; (c)1220 phenol, 0.9% over two steps; (d)1221 MCl2 (except for 193.7b), DBU, 1-pentanol, reflux, overnight.

7.2. Benzo[cd]-Fused Porphyrinoids

7.2.1. Benzo[cd]-Fusion via meso-Substituent Coupling. Coupling of a polycyclic meso-substituent to an adjacent β-pyrrolic position is one of the simplest general methods of constructing benzo[cd]fused porphyrinoids. The scope of the reaction is easily extended to encompass naphtho[2,1,8,7-bcde]fused systems, discussed in the following section. An initial example of β-fusion involving a nonporphyrinic substituent was reported in 2005 by Cammidge and co-workers (Scheme 194).1222 Compound 194.3 was obtained as an intramolecular Heck-type side product in the attempted Suzuki reaction between porphyrin triflate 194.1 and ferrocene bis(boronic acid) 194.2 in 14% yield. Surprisingly, only trace quantities of 194.3 were observed when the reaction was performed under identical conditions in the absence of 194.2. An oxidative variant of such a fusion was described in 2007 by Imahori et al., who synthesized naphthalene-fused zinc porphyrin carboxylic acid 194.7 via oxidative dehydrogenation of the meso-naphthyl precursor 194.4.1223 In comparison to the 3612


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Scheme 194. Intramolecular Benzo[cd]-fusion via meso-Substituent Couplinga

a

Reagents and conditions: (a)1222 PdCl2(dppf), Ba(OH)2, DME, H2O, reflux, 3 days; (b)1223 (1) FeCl3, CH2Cl2, CH3NO2, rt, 30 min, (2) TFA, H2SO4; (c)1223 (1) KOH, H2O, THF, EtOH, reflux, 4 h, (2) Zn(OAc)2, CHCl3, rt, 4 h; (d)1224 Fe(ClO4)3, MeNO2, DCM, 20 °C; (e)1234 Fe(ClO4)3·2H2O, MeNO2, CH2Cl2.

only with meso substituents but also with activated β-aryl groups.1194,1231 Its efficiency is however controlled by structural and electronic factors. Attempts to fuse nonactivated benzenoid PAH substituents are generally ineffective, although a pyrene− bisporphyrin hybrid 248.5a1232 and bis-perylene porphyrins anti-198.5 and syn-198.51233 were successfully prepared by FeCl3 oxidation of the corresponding hexaarylbisporphyrin and tetraarylporphyrin (Scheme 248, section 7.3, and Scheme 198). The latter result was explained in terms of the low oxidation potential of the perylene moiety (0.61 V vs Fc+/Fc).1233 Effective coupling was also achieved with an indole-substituted porphyrin 194.10 by Gryko and Lewtak, who obtained the heterofused system 194.11 by oxidation with Fe(ClO4)3· 2H2O.1234 Dye-sensitized TiO2 solar cells based on 194.7 and 194.12, each obtained using the oxidative coupling route, showed power conversion efficiencies (η) of 4.1% and 1.1%, respectively, under standard AM 1.5 conditions, which were improved by 50% relative to a nonfused porphyrin reference cell.1235 To further improve the cell performance, 5-(4carboxylphenyl)-10,15,20-tris(2,4,6-trimethylphenyl)porphyrinatozinc(II), possessing different light-harvesting properties, was coadsorbed with fused 194.7 onto an TiO2 electrode. The cosensitized cell exhibited an improved power conversion efficiency of 5.0%. Nonbenzenoid PAHs provide an attractive class of oxidationsusceptible peripheral substituents. A family of azulene-fused porphyrins 195.2−4 was synthesized by Osuka and co-workers from the respective azulenyl precursors using an analogous oxidative coupling strategy (Scheme 195).1236 Attempted oxidations of the monoazulenyl derivative 195.1 with AgPF6 did not cause any conversion, whereas the use of a stronger oxidant, DDQ/Sc(OTf)3, gave rise to a complicated product mixture. It was found that the treatment of 195.1 with FeCl3

nonfused 194.5, the UV−visible absorption spectrum of 194.7 was red-shifted and broadened (λmax = 482 and 682 nm for 194.7, and 422 and 551 nm for 194.5, for the Soret and most red-shifted Q-band, respectively). A dye-sensitized TiO2 solar cell based on 194.7 exhibited a power conversion efficiency of 4.1%, which was improved by 50% relative to the reference cell containing the nonfused 194.5. Oxidative aromatic coupling of porphyrins containing a naphthyl substituent at a meso position was examined by Gryko et al.1224 Unwanted halogenation of electron-rich precursors on attempted oxidation with FeCl3 had been observed before by Osuka1225 and Anderson.1226−1228 To avoid chlorination of naphthyl porphyrins 194.8, various oxidative systems were tested. Utilizing oxidants such as PIFA/BF3·Et2O, DDQ/ Sc(OTf)3, Mn(CH3COO)3, and Fe(OTf)3 led to the formation of the desired product in poor yields, while reactions with other transition metal salts (CuCl2,O2/[iPrMgCl·LiCl+TMP], Fe(acac)3, K3[Fe(CN)6], Cu(ClO4)2, molybdenum 2-etylhexanoate) proved to be absolutely ineffective. Finally, it was found that treatment of precursor 194.8 with Fe(ClO4)3 gave the πextended porphyrins 194.9 with improved yields, and additionally no chlorinated byproducts were observed. The absorption spectrum of the free-base analogue 194.9e possessed five well-resolved peaks with the lowest energy Qband extending up to 750 nm. 194.9e displayed fluorescence in the near-infrared region (λmax = 710 and 760 nm, Φfl = 10%). Electrosynthetic preparation of related fused derivatives was demonstrated by Gryko, Kadish, et al.1229,1230 The electrochemical coupling could be used to directly obtain fused products in the dicationic form. In this manner, even porphyrins with relatively high oxidation potentials were successfully coupled to produce the targeted fusion pattern. Oxidative coupling of substituents is a general method of extending peripheral conjugation in porphyrins and works not 3613


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Scheme 195. Synthesis of Azulene-Fused Porphyrinsa

tigated using DFT methods as a potential panchromatic organic chromophore.1238 In 2014, Osuka, Tanaka, et al. reported that the FeCl3mediated oxidative fusion of β-linked dyads 196.1a−b provided systems 196.2a−b, in which the porphyrin ring and dibenzo[a,g]corannulene unit are fused to a common six-membered ring (Scheme 196).1239 The oxidation of 196.3a−b, containing Scheme 196. Synthesis of meso-β Dibenzo[a,g]corannuleneFused Porphyrinsa

a

Reagents and conditions:1236 (a) FeCl3, MeNO2, CH2Cl2.

under ambient conditions afforded the fused 195.2 in good yield. Double and 4-fold ring-closing reactions, yielding respectively 195.3 and 195.4, were achieved under similar conditions. Curiously, the syn regioisomer of 195.3 was not detected at all. The absorption spectra of azulenyl precursors exhibited slightly broadened Soret and Q bands, with small red shifts correlated with the number of azulenyl substituents (422, 426, and 437 nm for the Soret bands in 195.1, and the di- and tetraazulenylporphyrins, respectively). The azulene fusion makes a prominent impact on the Q bands, which are broadened and exhibit large bathochromic shifts (1000 nm for 195.2, 1014 nm for 195.3, and 1136 with shoulder up to 1200 nm for 195.4). The TPA cross-section values were measured by an open-aperture Z-scan method, and the maximum values σ(2) were 2050 GM at 1200 nm for 195.3, and 7170 GM at 1380 nm for 195.4. The electronic structure of azulene-fused porphyrins was examined by means of magnetic circular dichroism (MCD), electronic absorption spectroscopy, and time-dependent density functional theory (TD-DFT).1237 The MCD data suggested that the ΔLUMO value was only slightly larger than ΔHOMO, in the case of porphyrin 195.2, while in the case of the diazulene-fused 195.3, ΔHOMO < ΔLUMO (where the ΔHOMO is the magnitude of the energy gap between the HOMO and the HOMO−1 and ΔLUMO is the magnitude of the energy gap between the LUMO and LUMO+1). The presence of a pseudo-A Faraday term in the Q 00 band region of 195.4 demonstrated that the associated excited states were accidentally near degenerate. As compared to 195.2 and 195.3, the HOMO was destabilized because of the antibonding interaction between the porphyrin and azulene moieties, while the HOMO−1 and the LUMO were stabilized due to a bonding-type interaction. As a result, the Q-band of 195.4 was intensified and shifted to longer wavelengths. The tetraazulene core present in 195.4 was subsequently inves-

a

Reagents and conditions:1239 (a) FeCl3, CH2Cl2/MeNO2, rt, 10 min.

a corannulenyl substituent at a β-position, produced isomeric products 196.4a−b, containing five-membered-ring junctions. The use of DDQ−Sc(OTf)3 as the oxidant gave inferior results. The fused Zn-complex 196.2a displayed a Soret band at 494 nm and Q-bands at 669 and 691 nm, and showed fluorescence emission at 732 nm with an enhanced quantum yield (ΦF = 0.049). Dyad 196.2b exhibited a similar absorption spectrum, with no observable fluorescence. 196.4a was also nonfluorescent, with NIR absorptions tailing up to around 1050 nm. That optical signature was thought to reflect the contribution of the pseudo-20π electronic circuit of dehydropurpurin. The electrochemical HOMO−LUMO gaps of 196.2a and 196.4a were 1.69 and 1.50 eV, respectively. The smaller gap determined for 196.4a was rationalized in terms of a partially antiaromatic character of this system. Perylene anhydride-fused porphyrins 197.4 and 197.5 were synthesized by Wu et al.1240 (Scheme 197, for related systems containing NAP units,264 see section 3.2). The synthesis of 197.4 commenced with the preparation of the precursor 197.1, utilizing Suzuki coupling of porphyrin boronic ester and Brsubstituted perylene monoimide. In the key step, the sixmembered ring formation was promoted by FeCl3-mediated oxidative coupling. Preparation of 4-(dimethylamino)3614


Chemical Reviews

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Scheme 197. Preparation of Perylene Anhydride-Fused Porphyrinsa

Reagents and conditions: (a)1240 FeCl3, MeNO2, CH2Cl2, rt; (b)1240 (1) KOH, t-BuOH, reflux, (2) HOAc, 40 °C; (c)1241 FeCl3, CH2Cl2, MeNO2, reflux.

a

Scheme 198. Fused Porphyrins via Thermal Cyclodehydrogenationa

Reagents and conditions: (a)1233 530 °C, N2 atmosphere, preheated furnace 12 min. Redox potentials of substituent PAHs (in V) are given relative to Fc+/Fc. a

further red shift of the last Q-band (897 nm) relative to 197.2 (803 nm). A high-temperature cyclodehydrogenation, obviating the need for chemical oxidants, was used by Thompson and coworkers for the preparation of a bis-naphthalene-fused porphyrin (Scheme 198).1233 This alternative coupling method was particularly effective for low-potential PAH substituents that could not be chemically coupled. Compound anti-198.2 and its syn isomer syn-198.3 were obtained in a total 7% yield from the porphyrin 198.1 bearing unsubstituted naphthyl

phenylethynyl-substituted compound 197.5 employed a similar synthetic route, which included a Sonogashira−Hagihara coupling step needed to install the meso-alkynyl substituent. When used in dye-sensitized solar cells, both compounds showed broad incident monochromatic photon-to-current conversion efficiency spectra covering the entire visible spectral range and part of the NIR region up to 1000 nm. Interestingly, oxidation of 197.1 under more forceful conditions yielded a mixture of 197.2 and the doubly annulated product 197.3.1241 The more extended conjugation in the latter species caused a 3615


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Scheme 199. Annulation of 2,3-Dialkynylporphyrinsa

Reagents and conditions: (a)1245 5% cyclohexadiene in chlorobenzene, 190 °C; (b)1251 PtCl2, toluene, 115 °C; (c)1252 PtCl2, toluene, 90 °C, 8 h; (d)1252 toluene, 115 °C, 1.5 h. a

Scheme 200. meso-Substituent Fusion in β-Extended Porphyrinsa

Reagents and conditions: (a)1253 1,4-naphthoquinone, toluene, reflux, 36 h; (b)1253 10% TFA/CHCl3; (c)1255 NaI, 1,2,4-trichlorobenzene, 214 °C, 7 h.

a

pyrenyl- (198.6, λmax = 815 nm), and perylenyl-annulated derivatives (198.5, λmax = 900 nm). Pyrenyl-fused porphyrins exhibited strong fluorescence in the NIR spectral region, with a progressive improvement of quantum yields (up to 13% with λmax = 829 nm) with increasing degree of fusion. Analogous porphyrins containing a single fused pyrene substituent were

groups. A range of pyrene-, perylene-, and coronene-substituted porphyrins were successfully fused under similar conditions, providing synthetically acceptable yields (>30%). The absorption spectra of bis-fused porphyrins revealed a progressive bathochromic shift in the series: naphthyl(198.2, λmax = 730 nm), coronenyl- (198.4, λmax = 780 nm), 3616


Chemical Reviews

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Scheme 201. Synthesis of Quinolino-Annulated Porphyrinsa

a

Reagents and conditions: (a)1256 trichloroacetic acid, 175 °C, 2 min.

peri-Fusion in 200.4 leads to a small bathochromic shift of Soret and Q bands relative to the ortho-fused analogue 200.2. A similar type of reactivity was observed in a Diels−Alder cycloaddition reaction between meso-tetraarylporphyrins and a pyrazine o-quinodimethane derivative 200.5.1255 The latter species was generated in situ from the corresponding 2,3bis(bromomethyl)pyrazine derivative. In the reaction with porphyrins 200.6a−c, a mixture of four product types, 200.7− 10, was observed, including the peri-condensed systems 200.8a−c and 200.10a−c. Mesoporphyrin dimethyl ester was used by Lugtenburg as a starting material for the synthesis of peri-condensed quinoline− porphyrin hybrids (Scheme 201).1256 After conversion to its nickel complex, the porphyrin was formylated via the Vilsmeier method. Four monoformyl isomers were formed as a mixture and subsequently converted into the corresponding acrylonitriles 201.1a−d under Wadsworth−Emmons conditions. In the key step, pure isomer 201.1a was treated with hot trichloroacetic acid to form the corresponding 201.2a. The same reaction performed on a mixture of 201.1b and 201.1d produced quinoline-fused systems 201.2a, 201.2d, and 201.2d′. A β-Me group adjacent to the acrylonitrile substituent is required for effective annulation, and, consequently, no cyclization product was obtained for 201.1c. The quantum yield of 0.77 was determined for singlet oxygen generation by 201.2a, corresponding to an improved efficiency relative to meso-tetraphenylporphyrin. Additionally, biological effects of 201.2a were tested, and lack of toxicity toward Chinese hamster ovary cell was established, indicating a potential use of this compound as a sensitizer in far-red phototherapy. The (2-formylvinyl)-substituted porphyrin 202.1, obtained by Boyle and Dolphin by reacting 5,15-diphenylporphyrin with N,N-dimethylaminoacrolein−phosphorus oxychloride, was found to produce two products in a Lewis acid-catalyzed cyclization reaction (Scheme 202).1257 The oxobenzochlorin product 202.3 showed a long-wavelength absorption at 730 nm and yielded an unstable carbinol 202.4 when reduced with NaBH4. The other cyclization product formed via the cyclization of the adjacent meso-phenyl substituent to the

obtained via chemical cyclodehydrogenation in earlier work by Sugiura, Yamashita, et al.1242 peri-Condensation can also be achieved by coupling a mesoaryl substituent with an isoindole fragment in [b]benzoporphyrin derivatives. In 2001, Bergman cyclization1243,1244 of β-dialkynyl-meso-tetraarylporphyrins 199.1a−d was investigated by Smith et al. as a route to fused porphyrin derivatives.1245 Refluxing 199.1b−d in a chlorobenzene solution containing 5% 1,4-cyclohexadiene (CHD) gave picenoporphyrins 199.2b−d in moderate to very good yields (Scheme 199). The absence of CHD, which acted as an intermediate hydrogen donor, favored intermolecular reactions, leading to a complex mixture of oligoporphyrins. Alternative conditions, involving simple room-temperature oxidation with DDQ, were subsequently proposed by Zaleski and coworkers.1246,1247 The Bergman cyclization was studied by the same group at higher temperatures (up to 210 °C)1248,1249 and utilizing photochemical conditions.1250 Thermolysis of 2,3dialkynylporphyrins in the presence of a stoichiometric or even catalytic amount of PtCl2 yielded phenanthroporphyrins 199.3 accompanied by only small amounts of the corresponding picenoporphyrins or other side products.1251 The PtCl2mediated pathway enabled subsequent synthesis of nickel(II) bisphenanthroporphyrins 199.7 and 199.8 and picenophenanthroporphyrin 199.4.1252 The electronic absorption spectra of these compounds display a stepwise red shift of both Soret and Q-bands correlated with the increasing π delocalization. The low energetic Q-band of porphyrin 199.5 was shifted bathochromically by 35 nm from the tetraalkynyl starting material 199.6 (λmax = 609 and 635 nm, for 199.6 and 199.5, respectively). These shifts were increasingly larger for 199.7 (λmax = 683 nm) and 199.4 (λmax = 717 nm). Cavaleiro and co-workers reported using the Diels−Alder reaction to synthesize porphyrin−quinone derivatives with an extended π-system (Scheme 200).1253 2-Vinylporphyrin 200.1 was reacted with 5 equiv of 1,4-naphthoquinone to give three different products 200.2−4. The peri-fused product 200.4 could be obtained by treating 200.3 with 10% trifluoroacetic acid in CHCl3, and subjected to further functionalization.1254 3617


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spectrum of 203.2, the Soret-like band was observed at 520 nm and the longest λmax in the Q-band region occurred at 765 nm. Cyclic voltammograms of complex 203.2 exhibited a single one-electron oxidation wave and two reversible one-electron reduction waves. The electrochemical band gap of 203.2 was approximately 1.67 eV, considerably smaller than those of nonfused NCP rhenium complexes (1.75−1.97 eV). 7.2.3. Pyrido[cd]fused Systems. Pyridine annulations in the [cd] region of porphyrin are typically achieved using mesoor β-amino-substituted precursors. Unexpected formation of condensed chlorins C57.1−3 (Chart 57), with incomplete π-

Scheme 202. Synthesis of 5,15-Diphenyl-7oxobenzochlorin1257a

Chart 57. Pyrido-Fused Chlorins1259

a

Reagents and conditions: (a) BF3·Et2O; (b) NaBH4; (c) air.

activated β-pyrrolic position, and was shown to contain two fused five-membered rings sharing a tetravalent carbon. 7.2.2. Other Benzannulations. Unusual rhenium-mediated reactions of N-confused porphyrin (NCP) with 2-methyl azaarenes, yielding peripherally π-extended NCP derivatives, were developed by Furuta and co-workers (Scheme 203).1258 By heating 203.1 with Re(CO)5Br in the presence of 2,6lutidine, a rhenium(V) complex 203.2 was obtained in 49% yield. Compound 203.2 was also synthesized from 203.3 in an improved 75% yield under similar conditions. The peripheral ring fusion of 203.3 was examined with a variety of methyl arene reactants. In the proposed reaction mechanism, the involvement of pyridine-bound rhenium was implied in the activation of the ortho methyl group. The rhenium-mediated enlargement of NCP π system was also observed in the reaction with the dimethyl derivative 203.4, in which it led to the formation of an intramacrocyclic rhenium bound methine bridge (203.6, cf., Scheme 276, section 7.6). In the absorption

conjugation around the porphyrinic peri-fusion point, was observed by Gold et al. in the course of metalation and demetalation reactions of corresponding meso-N-arylamino and meso-N-arylacetamidoporphyrins.1259 The resulting fusion patterns lead to intensity enhancement and red shifts of lowest-energy Q bands, which were observed in the 686−725 nm range. In 2009, Ruppert et al. reported the synthesis of the πextended metalloporphyrin 204.2 via the Cadogan reaction (Scheme 204).1260 Subsequent Vilsmeier−Haack formylation, followed by Friedel−Crafts cyclization, produced the bis-fused

Scheme 203. Synthesis of Rhenium Complexes of Peripherally Fused π-Extended N-Confused Porphyrinsa

a

Reagent and conditions:1258 (a) Re2(CO)10 or Re(CO)5Br, K2CO3; (b) Re(CO)5Br, R−Me; (c) Re(CO)5Br, K2CO3; (d) Re(CO)5Br, 2,6-lutidine. 3618


Chemical Reviews

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Scheme 204. Quinoline-Fused Porphyrins via Cadogan Cyclizationa

a

Reagents and conditions: (a)1260 P(OEt)3; (b) DMF/POCl3, 90%; (c) TsOH, toluene, Δ.

Scheme 205. Synthesis of Quinoline-Annulated Porphyrinsa

a

Reagents and conditions:1262 (a) NH2OH·HCl, pyridine, rt; (b) p-TSA, toluene, reflux.

Scheme 206. Oxidative Fusion Reactions of meso-(Diarylamino)porphyrinsa

a

Reagents and conditions: (a)1265 10 equiv of FeCl3, 10 equiv of DDQ, CH2Cl2/MeNO2, 15 min; (b) (1) TFA, H2SO4, rt, 2 h, (2) excess of Zn(OAc)2·2H2O, CH2Cl2, 1 h; (c) 20 equiv of FeCl3, 20 equiv of DDQ, CH2Cl2/MeNO2, 4 h; (d) HCOOH, NEt3, Pd(OAc)2/SPhos, toluene, 120 °C, 5 h; (e) 20 equiv of FeCl3, 20 equiv of DDQ, CH2Cl2/MeNO2, 1.5 h.

system 204.3 accompanied by a small amount of 204.2 formed by retro-formylation. 204.3 and its corresponding free base

both showed high absorbance in the therapeutic window (700− 800 nm), whereas the corresponding Pd(II) complex was able 3619


Chemical Reviews

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to generate singlet oxygen efficiently, thus demonstrating its potential utility in photodynamic therapy. A synthesis of bis-quinoline-annulated porphyrins, starting from the generally useful porphyrin-2,3-dione 205.1,1261 was reported by Brückner and co-workers (Scheme 205).1262 The reaction of 205.1 with a ca. 100-fold excess of NH2OH·HCl formed the monoxime 205.2 as the major product and a small amount of bisoxime 205.3. Treatment of 205.2 with a strong acid produced the quinoline−oxochlorin framework 205.4, which gave the corresponding oxime 205.5 in a subsequent reaction with NH2OH·HCl. The final acid-catalyzed annulation yielded the bis-annulated 205.6. Bisoxime 205.3 could also be directly converted into the bisquinoline 205.6, albeit in a significantly lower yield (12%) in comparison to the stepwise protocol (∼40% in three steps). It was determined that in the direct bis-annulation, oxadiazole 205.7 formed as the major product. The UV−vis spectrum of 205.6 was bathochromically shifted with an optical bandgap of 775 nm. Recently, Brückner et al. utilized the same synthetic pathway to the synthesis of monoquinolino annulated metaloporphyrins.1263 Metalation of 205.6 had to be performed after the annulation step because the reaction of 205.8 and 205.9 complexes with p-TSA led preferentially to the expansion of the oxidized pyrrole ring via Beckmann rearrangement, as had been reported previously.1264 The UV−vis spectra of the nickel and palladium complexes of 205.6 are qualitatively similar to the free base spectrum, showing a broadened Q-band region with no detectable splitting of the Soret band. The possibility of developing fused porphyrins from mesoamino-substituted precursors was explored in 2013 by Osuka, Yorimitsu, Kim, et al.1265 The reaction of the noncyclic arylamine 206.1 with FeCl3 and DDQ led to the formation of the aminophenylene-fused derivative 206.2 in good yield (Scheme 206). Double fusion could only be achieved under more forceful conditions, which yielded a dichlorinated doubly fused porphyrin 206.4. Dechlorination of 206.4 was carried out by reaction with HCOOH and NEt3 in the presence of the Pd(OAc)2/SPhos catalytic system. The Soret and Q-bands of 206.3 and 206.6 are red-shifted, with the low-energy maxima at 640 and 629 nm, respectively. TPA values determined by openaperture Z-scan measurements at 1200 nm were 170, 250, and 430 GM for 206.1, 206.2, and 206.5, respectively. The electrochemical HOMO−LUMO gaps of 206.2 and 206.5 were 1.98 and 1.93 eV, respectively, smaller than measured for the parent 206.1 (2.17 eV). In contrast to 206.1, the oxidative coupling of 206.7a−b with FeCl3 and DDQ afforded unexpected fusion products 206.8a−b with two peri-condensed meso-3,5-di-tert-butylphenyl groups. The fusion of the phenoxazine/phenothiazine substituent could not be accomplished. The difficulties associated with coupling of meso-amino groups were further demonstrated in related work by Gryko and Nowak-Król (Scheme 207).1266 Under typical conditions (FeCl3, CH2Cl2/MeNO2), the attempted coupling of 207.1 led to a complex mixture of products including chlorinated porphyrins, whereas the use of DDQ/Sc(OTf)3 resulted only in the decomposition of the starting material. An alternative oxidant, Fe(ClO4)3·H2O, provided moderate yields of the monoconjugated derivative 207.2, not contaminated by chlorinated side products. Bis(3,5-dimethoxyphenyl)aminesubstituted 207.3 underwent oxidation into the fused product 207.4 under milder conditions. The electrochemical HOMO− LUMO gap of 207.2 was 1.90 eV, that is, slightly smaller than the value for parent 207.1 (2.08 eV), corresponding to a

Scheme 207. Oxidative Aromatic Coupling of mesoArylamino-porphyrinsa

Reagents and conditions:1266 (a) Fe(ClO4)3·H2O, MeNO2, DCM, rt, 72 h; (b) FeCl3, MeNO2, DCM, rt, 3 min. a

bathochromic shift of the lowest energy absorption band of ca. 35 nm. Recently, Osuka, Yorimitsu, et al. showed that meso,βoligohaloporphyrins are viable precursors for the synthesis of fused meso-aminoporphyrins (Scheme 208).1267 The reaction of Scheme 208. Synthesis of Diphenylamine-Fused Porphyrina

a

Reagents and conditions:1267 (a) diphenylamine, NaO-t-Bu, DMF, 100 °C, 7 h.

208.1 with diphenylamine in the presence of NaO-t-Bu afforded the fused porphyrin 208.2 in one step in 81% yield. The expected SNAr substitution product was not detected under these conditions. The precise mechanism of this transformation remains unclear, although the formation of an intermediate porphyrinyl radical via electron transfer to iodide and subsequent dissociation of I− was presumed, on the basis of previous reports.1268−1270 The preparation of porphyrins bearing chelating peripheral groups fully conjugated with the macrocyclic π-system was described by Callot, Ruppert, et al.1271 The initially tested 3620


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Scheme 209. Porphyrins with Peripheral Conjugated Chelates and Their Pd Complexesa

Reagents and conditions:1271 (a) P(OEt)3, 1,2-dichlorobenzene, 155 °C; (b) POCl3, DMF, CH2Cl2; (c) Lawesson’s reagent, benzene, 75 °C; (d) Pd(OAc)2, 1,2-dichloroethane.

a

Scheme 210. Quinolino-Fused Porphyrins from βAzidotetraarylporphyrinsa

Cadogan reaction on a meso-ortho-nitroaryl substituted porphyrin did not yield satisfactory results; however, clean cyclization was achieved with β-nitroporphyrins 209.1 (Scheme 209). When the nickel derivative 209.1 was treated by excess of triethyl phosphite at 155 °C, enamine 209.2 was obtained as the major product (75%). Under Vilsmeier−Haack conditions, the formylation of enamine 209.2 proceeded smoothly to give enaminoaldehyde 209.3 in 90% yield. Lawesson’s reagent was then used to convert the carbonyl group of 209.3 into the thiocarbonyl. The reaction of the resulting enaminothioaldehyde 209.4 with Pd(OAc)2 led to the formation of two isomeric complexes, 209.5a−b and 209.6a−b, characterized, respectively, by the trans and cis configuration at Pd. In contrast, the reaction of enaminothioaldehyde 209.4 under the same conditions led exclusively to the cis isomer 209.6c. The electronic spectra of isomers 209.5a−b and 209.6a−b were almost superimposable, with broadened and split Soret bands and bathochromic shifts of the lower wavelength bands relative to the free 209.3 and 209.4. The UV−vis absorption spectrum of 209.6c exhibited an even larger red shift up to 714 nm. In a related approach, reported by Chen et al. in 2007, thermal decomposition of β-azidotetraarylporphyrins similarly led to the formation of fused six-membered rings (Scheme 210).1272 In this work, a variety of azido derivatives 210.1a−d were conveniently synthesized by the classical conversion of the amino groups through diazotization and subsequent treatment with sodium azide. Depending on the nature of the substituents, the yields for various N-containing fused porphyrins 210.2a−d varied from 60% to 85%. In 2009, Osuka, Shinokubo, and Shinmori reported the synthesis of pyridine-fused oxobenzochlorins 211.3a−c (Scheme 211).1273 The zinc(II) dehydropurpurin 211.1 (discussed in more detail in section 7.4) was efficiently converted into 3,5-dibenzoylporphyrin 211.2a by photochemical oxidative cleavage of the outer double bond. The treatment of 211.2a with ammonium acetate at 130 °C in toluene/AcOH for 2 days furnished a chlorin derivative 211.3a in 60% yield. In a similar manner, Ni-pyridochlorin and the free base, 211.3c and 211.3b, were also synthesized in 65% and

a

Reagents and conditions:1272 (a) toluene, reflux, 2 h.

40%, respectively. The absorption spectra of oxopyridochlorins were strikingly altered in comparison with the typical features of porphyrins. The metal complexes 211.3a and 211.3c exhibited structured Q-bands at 753 and 767 nm, respectively. In each case, the Soret band was split (e.g., 403 and 477 nm for 211.3a). Free base 211.3b showed a split Soret band at 414 and 448 nm, and differently structured Q-bands at 690 and 785 nm. The absorption spectra of oxopyridoporphyrins 211.3 thus covered a considerable part of the PDT therapeutic window, and the compounds were shown to enable singlet oxygen generation. A doubly fused porphyrin was obtained by Jeandon and Ruppert by sequential application of two Cadogan cyclizations.1274 The first supplementary ring was closed by reacting 2nitro-TPP 212.1 with triethyl phosphite, in a manner described above (Scheme 212). The second ring closure required the protection of the outer nitrogen of 212.2 with the Boc group, prior to the Cadogan reaction. The resulting bis-quinolinofused porphyrin 212.5, bearing an internal and external coordination site, was easily demetalated, and free base 212.6 3621


Chemical Reviews

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Scheme 211. Synthesis of Oxopyridochlorina

a Reagents and conditions:1273 (a) air, light; (b) HCl; (c) Ni(acac)2; (d) NH4OAc, toluene/AcOH, 130 °C, 2 days; (e) NaBH4, CH2Cl2/MeOH; (f) air.

Scheme 212. Bis-quinolino-Fused Porphyrinsa

a

Reagents and conditions:1274 (a) P(OEt)3; (b) (1) NaH, THF, rt, 30 min, (2) isobutyl chloroformate, rt, 1 h; (c) LiNO3, Ac2O, AcOH, CHCl3, rt, 90 min; (d) 1,2-dichlorobenzene, reflux, 30 min; (e) TFA, H2SO4, rt, 3 h.

was isolated in an almost quantitative yield. Both nickel porphyrins 212.5 and 212.6 absorbed in the 650−800 nm window with very high extinction coefficients (from 15 000 to 22 000 M−1 cm−1 in the case of 212.5), which made them potentially useful in PDT applications. 7.2.4. Pyrano- and Thiopyrano[cd]fused Systems. Reactions of nickel(II) meso-tetraalkylporphyrins with elemental sulfur were performed in 1997 by Callot and co-workers to mimic geochemical processes.1275 Fusing the reactants at 200 °C resulted in the formation of product mixtures, from which thiopyran-fused macrocycles C58.1 were isolated chromatographically (Chart 58). The reaction of tetra(sulfophenyl)porphyrin with fuming H2SO4 was reported by Dixon et al. to produce the cyclic sulfone C58.2, a potential precursor to conjugated thiopyran derivatives.1276 A thiopyran-fused subporphyrin C58.3 was recently described by Osuka, Kim, and co-workers.1277

Heating of [26]hexaphyrin 213.1 in acetic acid led to the formation of the benzopyran-fused [28]hexaphyrin 213.2 (Scheme 213), accompanied by the corresponding nonfused [28]hexaphyrin.1278 Compound 213.2 had a stable Möbius conformation, and exhibited an aromatic character over a wide temperature range. The oxidation of 213.2 with DDQ Scheme 213. Benzopyran-Fused Hexaphyrinsa

Chart 58. Thiopyran-Fused Porphyrins and Their Derivatives

Reagents and conditions:1278 (a) AcOH, 130 °C, 6 h; (b) DDQ; (c) NaBH4.

a

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Scheme 214. Synthesis of Doubly meso-β-Linked Diporphyrinsa

a

Reagents and conditions: (a)1279 TeCl4, DCM, rt; (b)1280 1 equiv of BAHA, CHCl3, rt; (c) 1.2 equiv of BAHA, CHCl3, rt; (d)1281 BAHA, CHCl3, rt.

Scheme 215. Synthesis of meso-β Doubly Linked Porphyrin Tapesa

a Reagents and conditions: (a)1282 2 equiv of DDQ, 2 equiv of Sc(OTf)3, toluene, 50 °C; (b)1225 AuCl3, AgOTf, 1,2-dichloroethane, rt, 2−3 min; (c) HCO2H, Et3N, Pd(PPh3)4, toluene, reflux.

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Scheme 216. Synthesis of meso-β Doubly Linked Zn(II) Porphyrin Trimersa

a

Reagents and conditions:1284 (a) 2 equiv of DDQ, 0.5 equiv of Sc(OTf)3, toluene, 30 °C, 72 h.

214.1 with the DDQ−scandium(III) triflate oxidant system.1282 The reaction mixture was purified, and the following products were separated: singly linked diporphyrin 215.1 (27%), meso-β doubly linked dimer 215.2 (8%), trimer 215.3 (6%), tetramer 215.4 (5%), and pentamer 215.5 (4%, Scheme 215). The lowest energy bands in the absorption spectra were progressively red-shifted upon increase of the number of porphyrin units: 741 nm for 215.2, 892 nm for 215.3, 996 nm for 215.4, and 1075 nm for 215.5. The one-electron oxidation potential of 215.2 was determined to be 0.56 V, which is lower than the values for parent monomer 214.1 (0.74 V) and singly linked compound 215.1 (0.76 V) but higher than that measured for the corresponding triply bonded dimer (0.46 V, cf., section 7.3). These results suggested that the energy level of the HOMO orbital was lifted upon expansion of the π-system of the porphyrin, a hypothesis that was confirmed by oxidation potentials measured for longer oligomers (from 0.44 V for 215.3 to 0.35 V for 215.5). TPA cross sections for the lowestenergetic bands of 215.2−5 were found to increase from 8000 (215.2) to 41400 GM (215.5) with the increasing length of the array.1283 Coupling of the monobromoporphyrin 215.6, mediated by the AuCl3−AgOTf combination, was reported by Osuka and co-workers to provide high yields of the porphyrin dimer 215.7.1225 This reagent was found to be effective at ambient conditions, and with short reaction times. Additionally, an efficient Pd-catalyzed debromination of the meso position was developed, which enabled using bromine as a meso-protecting group in porphyrins. Zn(II) porphyrin trimer 216.1 and its syn isomer 216.2 were synthesized by DDQ−Sc(OTf)3 oxidation of the corresponding meso-β singly linked porphyrin precursors 216.3 and 216.4 (Scheme 216).1284 The UV−vis absorption spectrum of anti isomer exhibits peaks at 427, 852, and 963 nm. The spectrum of 216.2 is considerably different, showing six major bands at 422, 522, 604, 734, 927, and 982 nm, with molar extinction coefficients that are apparently smaller than those of 216.1. Interestingly, one of the isomers, 216.2, showed detectable

proceeded to exclusively give the planar Hückel aromatic [26]hexaphyrin 213.3, which slowly isomerized under ambient conditions to reach the stationary 4:1 mixture of 213.3 and 213.4. Interestingly, the reduction of this mixture with NaBH4 gave 213.2 quantitatively. 7.2.5. Benzo-Fused Porphyrin Oligomers. The first example of a peri-condensed porphyrin dimer, named [2]porphyracene, was reported in 1999 by Sakata, Sugiura, et al.1279 Compound 214.2 was obtained in an unusual reaction between [5,15-bis(3,5-di-tert-butylphenyl)porphyrinato]nickel 214.1 and tellurium(IV) tetrachloride (Scheme 214). The electronic spectrum of 214.2 showed a considerable increase in the intensity of Q-bands in comparison with 214.1, with the lowest-energy absorption at 743 nm, and a weaker Soret band. A similar type of coupling was achieved almost simultaneously by the Osuka group with the aid of one-electron oxidants.1280 The reaction of 214.1 with an equivalent amount of tris(4bromophenyl)aminium hexachloroantimonate (BAHA) produced a mixture of dimeric, trimeric, and tetrameric porphyrins, of which only the dichlorodiporphyrin 214.2a was successfully isolated. In the reaction of 214.1a with the same oxidant, compound 214.2a was also obtained in 68% yield. Similar doubly meso-β-linked diporphyrin products 214.2b−d were obtained in the reaction of Pd-porphyrin 214.1b, with the product ratio dependent on the reaction time and amount of BAHA used. The one-electron oxidation potentials of 214.2b, 214.2c, and 214.2d (0.57, 0.59, and 0.62 V, respectively) were significantly lower than that of monomer 214.1b (0.82 V). The bis-Ni analogue of 214.2b revealed a ruffled conformation in the solid state. In contrast to the Ni and Pd complexes, the Znand free base derivatives 214.1c−d produced exclusively the singly linked porphyrin 214.3b, whereas the oxidation of the Cu complex 214.1e provided a mixture of meso-β doubly linked diporphyrin 214.2f (25%), meso−meso linked dimer 214.3c (33%), and the meso-chlorinated monomer 214.4 (24%).1281 In subsequent work from the Osuka group, meso-β doubly linked porphyrin oligomers were produced by oxidation of Ni(II)-complex 5,15-bis(3,5-di-tert-butylphenyl)porphyrin 3624


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Chart 59. Indene-Fused Analogues of meso-β Doubly Linked Zn(II) Porphyrin Arrays

Scheme 217. Synthesis of para-Phenylene-Bridged Porphyrin Tapesa

a

(a)1286 BBr3, CH2Cl2, 20 °C, then Zn(OAc)2; (b) CH2(CN)2 (1000 equiv), TiCl4, CH2Cl2; (c) Pd(PPh3)4, K3PO4, DMF, 155 °C.

Scheme 218. Synthesis of a p-Quinodimethane-Bridged Porphyrin Dimera

a

Reagents and conditions: (a)1287 triisopropylsilylethynylmagnesium bromide, THF, rt, 48 h; (b) 4-tert-butylphenylmagnesium bromide, THF, rt, 48 h; (c) (1) excess BF3·Et2O, CH2Cl2, 10 min, (2) air oxidation.

fluorescence in the NIR region (λmax = 1048 nm), upon excitation at 420 nm. TPA cross-section values at 1850 nm

were measured to be 16 800 and 7900 GM, for 216.1 and 216.2, respectively. 3625


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Scheme 219. Benzo-Fused Corrole Dimersa

In 2010, Osuka, Kim, et al. synthesized meso-β doubly linked Zn(II) porphyrin arrays fused with indene moieties C59.1−3 via intramolecular oxidative DDQ−Sc(OTf)3-mediated coupling of singly bonded precursors (Chart 59).1285 The fusion of indene moieties at the porphyrin unit, which occurred concomitantly with the formation of interporphyrin bridges, caused steric repulsions and further expansion of the elongated π-system, leading to decreased HOMO−LUMO energy gaps. The S1-state lifetimes for the C59.1−3 arrays showed double exponential decays with a common value of the short time constant (2.5 ps) and the second decay time constant of 10.5, 8.4, and 10.3 ps, respectively. In comparison to the values measured under the same conditions for analogues possessing no indene units, excited-state lifetimes of the C59.1−3 arrays are shorter. TPA cross-section measurements yielded values of 11 600, 23 600, and 17 100 GM, for C59.1, C59.2, and C59.3, respectively. The anthracene-1,5-dione-fused bisporphyrin 217.2 was obtained by Anderson and co-workers using a double intramolecular Friedel−Crafts acylation of 217.1a (Scheme 217).1286 Interestingly, subsequent reaction with malononitrile gave the unexpected bisannulated compound 217.3. In the same work, an intramolecular Heck-type coupling performed on 217.1b was utilized for the synthesis of the cyclopenta[cd]fused dimer 217.4 in 20% yield. The longest wavelength absorption Q-band of dimer 217.4 was observed at 1077 nm, whereas the cross-conjugated dimer 217.2 exhibited the corresponding absorption maximum at 907 nm. Compound 217.2 exhibits measurable fluorescence, mirroring the lowest Q-band (960 nm, 1.7% quantum yield). In contrast, luminescence from 217.4 was undetectable. Surprisingly, both dimers had essentially identical electrochemical HOMO− LUMO gaps. In subsequent work by Wu et al., analogously designed diketobisporphyrins 218.2a−b were envisaged as precursors to p-quinodimethane-bridged porphyrin dimers (Scheme 218).1287 Interestingly, however, addition of either aryl or alkynyl Grignard reagents resulted in Michael addition to the pyrrole β-positions adjacent to the carbonyl groups (218.3). The quinodimethane target 218.1 was successfully prepared by intramolecular Friedel−Crafts alkylation of the diol-linked dimer 218.4 with concomitant oxidation in air. 218.1 showed intense one-photon absorption (OPA, λmax = 955 nm, ε = 45 400 M−1cm−1) and a large TPA cross-section (2080 GM at 1800 nm). In CV and DPV measurements, porphyrin 218.1 showed four quasi-reversible oxidation waves with half-wave potentials (Eox1/2) at 0.01, 0.14, 0.6, and 0.93 V and two quasireversible reduction waves with half-wave potentials (Ered1/2) at −1.14 and −1.36 V (vs Fc+/Fc). Compound 218.1 exhibited a high-lying HOMO energy level (−4.70 eV) and a small energy gap (0.86 eV). A benzo-fused corrole dimer 219.2 was obtained by oxidative coupling of the meso−meso-linked precursor 219.1 (Scheme 219).1288 219.2 was treated with sodium borohydride to provide the reduced dimer 219.3, wherein each macrocyclic subunit has the typical corrole-like oxidation level. 219.3 was oxidatively labile and reversed to 219.2 in the presence of air. Compounds 219.2 and 219.3 were described, respectively, as nonaromatic and aromatic, on the basis of their optical spectra and magnetic properties. The unusual stability of 219.2 was ascribed to the presence of two direct connections between the individual corrole units.

a

Reagents and conditions:1288 (a) DDQ, CHCl3, reflux, 1 h; (b) NaBH4, THF/MeOH, 0 °C, 15 min; (c) air oxidation.

7.2.6. Oxonaphtho-Fused Porphyrins and Benzooxochlorins. During the attempted demetalation of tetraphenylporphyrins bearing a formyl group at the β position (e.g., 220.1), the cyclization of the adjacent meso and β substituents was observed by Henrick et al., and the resulting verdin 220.2 was characterized crystallographically (Scheme 220).1289 The Scheme 220. Synthesis of Oxonaphtho-Fused Porphyrinsa

a

Reagents and conditions: (a)1289 acidic conditions (TFA); (b)1294 pchloranil, 70% TFA in water, benzene; (c)1294 AlCl3, LiAlH4, ether.

formation of similar compounds was also reported by Buchler et al. in their investigations of the Vilsmeier reaction,1290 and the chemistry was further explored by Callot and coworkers,1291 Dolphin et al.1292 (cf., Chart 60), and Ishkov et al.1293 The latter group proposed a more efficient route to 220.2, involving the use of p-chloranil. Further studies by Ishkov revealed that the treatment of 220.1 with a saturated solution of p-chloranil in boiling benzene in the presence of 70% aqueous trifluoroacetic acid provided as the major product 3626


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Scheme 221. Naphtho-Fused Porphyrins Bearing Peripheral Chelating Groupsa

a

Reagents and conditions:1271 (a) (1) 4-H-4-amino-1,2,4-triazole, NaOH, (2) CF3COOH; (b) excess Pd(acac)2.

the isomeric system 220.3, with the carbonyl group located at the adjacent β-position of the macrocycle (71% yield).1294 Both 220.2 and 220.3 underwent reduction with a 1:1 AlCl3−LiAlH4 system. Enaminoketones 221.2a−b bearing a carbonyl group at the macrocyclic periphery can act as both internal and external ligands (Scheme 221).1271 The starting ketones 220.3, 221.1 were obtained from β-formyltetraarylporphyrins according to the procedure described by Ishkov.1294 Treatment of complexes 221.1 with 4-amino-4H-1,2,4-triazole gave the desired ligands, which however could not be converted into the thio derivatives by using Lawesson’s reagent. Metalation of enaminoketones 221.2 yielded monomeric and dimeric complexes, 221.3 and 221.4, respectively, characterized by hypsochromically shifted electronic spectra. Porphyrins acting as a fully conjugated aromatic system, capable of simultaneous external and internal ligation, were described by Callot, Ruppert, et al.1295 Such ligands were obtained from ketone derivatives 222.1, which were reacted with various nitrogen nucleophiles (Scheme 222). Under mild acid catalysis, reagents of the general composition H2NX (X = OH, NHTs, OSO3H) gave the enaminoketone 222.2 in variable yields. The highest yield (90%) was observed in the reaction with the hydroxylamine-O-sulfonic acid−AcOH− NaOAc system in refluxing dichloromethane. External metalation of 222.2a with an excess of nickel acetyloacetonate gave the trinuclear complex 222.3. Similarly, several trimetallic bisporphyrins were prepared (Ni/Cu/Ni; Ni/VO/Ni; Ni/ Pd/Ni). In the electronic absorption spectrum, the lowest energy band of 222.3 was shifted to 700 nm (ε = 33 000 dm3 mol−1 cm−1) in comparison to the band observed at 649 nm (ε = 18 500 dm3 mol−1 cm−1) for Ni-222.2a. In further investigations by the same group, it was found that the pyrrolic position next to the acyl group could be efficiently functionalized, yielding products with peripheral coordinating sites.1296 A thioketone analogue of 222.2 was also obtained and used for the preparation of the Ni2Pd complex 222.4.1271 Porphyrin enaminoketones were also used as ancillary ligands in ruthenium complexes 222.5a−c.1297 The coordination of ruthenium to the porphyrins induced a blue shift of the Soret band maxima (especially for 222.5b, Δλ = 20 nm) as well as a decrease of its intensity, while new absorption bands appeared in the 500−600 nm range. A significant red shift of the absorption bands was observed for the enaminothioketones and 222.6a−c.1298 Porphyrins bearing two divergent external coordination sites were subsequently developed with the prospect of building

Scheme 222. Preparation of Conjugated Trimetallic Dimeric Porphyrinsa

a Reagents and conditions: (a)1295 hydroxylamine-O-sulfonic acid, AcOH, AcONa, CH2Cl2, reflux; (b)1295 excess of Ni(acac)2, toluene, reflux.

higher metal-connected oligomers.1299 These bis-functionalized porphyrins, such as the centrosymmetric 223.5 (Scheme 223), were prepared using an alternative route, which involved cyclization of two meso-o-carbomethoxyphenyl substituents, followed by enamine formation. This synthetic route was also used for the preparation of monoketones in high yields (e.g., 223.4).1300 Oligonuclear complexes 223.6−9 were prepared from bis-amine ligands in reactions with palladium(II) acetyloacetonate.1299 The electronic spectra of compounds 3627


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Scheme 223. Porphyrin Oligomers Linked by Metal Ionsa

a Reagents and conditions: (a)1300 LiOH·H2O, water, dioxane, reflux, 24 h; (b)1300 (1) oxalyl chloride, benzene, rt, 2 h, (2) SnCl4, rt, 1 h; (c)1300 (1) TFA, rt, 45 min, 97%, (2) 4-amino-4H-1,2,4-triazole, toluene, EtOH, NaOH, reflux, 1 h, (3) Ni(acac)2, toluene, 80 °C, 24 h, 92%.

Scheme 224. Preparation of Isomeric Fused Bis-acylporphyrinsa

a

Reagents and conditions:1302 (a) (1) LiOH, (2) (COCl)2, (3) SnCl4; (b) (1) H2SO4, (2) Ni(acac)2, (3) (COCl)2, (4) SnCl4.

producing 7-membered rings, was reported by Jux, Röder, et al.1301 The method described above was systematically investigated in subsequent work of Callot, Ruppert, et al., who described the preparation of six cyclic diketones derived from meso-

223.6−8 showed large red shifts as compared to parent porphyrin 223.5. The most significant shift was observed on the two lowest-energy bands in the case of trimer 223.8 (690 nm vs 789 nm, and 758 nm vs 881 nm, in 223.5 and 223.8, respectively). An extension of the above cyclization approach, 3628


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Scheme 225. Synthetic Route to Porphyrinic Spiro Dimersa

a

Reagents and conditions: (a)1304 acetic anhydride, SnCl4, CH2Cl2, rt, 5 min; (b)1304 air, pyridine; (c)1304 pivalic anhydride, SnCl4, CH2Cl2, rt, 4 min.

tetraarylporphyrins.1302 The nickel complexes of diketones 224.3, 224.4, and 224.5 were prepared from the porphyrin 224.1 in 86% total yield, according to the previously reported procedure (Scheme 224).1299,1300 Reacting isomer 224.2 under the same conditions gave compounds 224.6 and 224.7 in 36% and 40% yields, respectively. Cu analogues of diketones 224.3 and 224.7 were also accessible from β-cyanoporphyrins via acid-catalyzed condensations. Moreover, the phenalenedionefused diketone 224.9 was prepared from porphyrin 224.8 in a 37% yield. Demetalation of the Ni complexes could not be accomplished, which prompted the attempts to perform carboxylic cyclizations on free base porphyrins. The latter approach produced the free-base analogues of 224.4 and 224.7 in moderate yields (11% and 20%, respectively). All diketones showed Soret bands in the 472−524 nm range and strongly red-shifted Q bands (up to 826 nm). Subsequent derivatization of diketones 224.4−7 and 224.9 enabled the preparation of the corresponding enaminoketones and enaminothioketones.1303 In these systems, the thionation was observed to reduce the HOMO−LUMO gaps of the porphyrins. Attempted acylation of nickel(II) meso-tetraarylporphyrins under Friedel−Crafts conditions was shown by Callot et al. to provide the porphyrinic spiro dimer 225.3 (Scheme 225).1304 In contrast, the reaction of nickel porphyrin 225.1 with acetic anhydride and SnCl4 as a Lewis acid gave compound 225.2 as the major product. When exposed to air in solution, 225.2 was converted into a stable spiro dimer 225.3 possessing three additional six-membered rings. This reaction proceeded faster in pyridine and could be run on the crude product from the acylation step, to give 225.3 in 45−58% yield. When pivalic anhydride was used in the acylation of 225.1, tertiary alcohol 225.4 was isolated (15%), along with the ketone 225.5 (5%) and recovered starting material (13%). To suppress the formation of dimer 225.3, anhydrides containing no α-hydrogens were explored as acylating agents.1305 The use of benzoic anhydride induced a complete change in the course of the reaction and led to the formation of the green porphyrin 226.2 and the dark blue corrole 226.3, in addition to several minor products (Scheme 226). The initial low yield of 226.3 (ca. 3−6%) was optimized by changing the reaction conditions and structure of the starting material (24% for R1 = OEt and M = Ni). The use of the NiII complex proved to be crucial, because the copper(II) species 226.1 did not

Scheme 226. Acylation-Induced Benzannulation and Macrocycle Contractiona

a

Reagents and conditions:1305 (a) (1) (PhCO)2O, SnCl4, (2) H2O, NaHCO3, (3) (PhCO)2O, pyridine, DMAP, air.

undergo contraction to produce the corrole, yielding only the fused porphyrin 226.2. Further optimization of this reaction was performed, revealing the formation of small amounts of lactonic seco-porphyrins under certain conditions.1306 Formation of oligomeric corrole derivatives during attempted nitration of 226.3 was subsequently observed by Callot et al.1307 In contrast to the Ni complex, which was photochemically inactive, the Pd complex 226.2 was shown to be an efficient photosensitizer for singlet oxygen generation.1308 7.2.7. Indole- and Carbazole-Based Porphyrinoids. Benzo[cd]fusion at a porphyrin ring formally results in the formation of an indole substructure, whereas two fusions of this kind performed at a single pyrrole ring produce a carbazole motif. An alternative synthetic approach to benzo[cd]porphyrinoids can thus be envisaged, relying on the assembly of indole or carbazole components. In 2010, the first example of a porphyrin-related macrocycle 227.2, consisting of directly linked pyridine and carbazole moieties, was reported by Müllen and Norouzi-Arasi et al. (Scheme 227).1309 The macrocycle was obtained via Suzuki−Miyaura cross-coupling reaction of 3629


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ingly, these systems are highly fluorescent (ΦF = 0.78−0.61), apparently because of their conformational rigidity. In contrast, the absorption spectrum of the porphyrin-like 228.8 displays intense Q-like bands with very large bathochromic shifts (845, 934, and 1049 nm). The macrocyclic conjugation in 228.8 can be described using the usual porphyrin like [18]annulenoid conjugation pathway, with contributions of larger paths involving the peri-fused benzene rings. The addition of ethynyl substituent groups to the periphery of carbazole-based core-modified porphyrins was seen as a means of modifying their optical and electronic properties.1312 Treatment of 228.7 with NBS gave the tetrabrominated compound 229.1 (Scheme 229) in 88% yield. The Stille coupling reaction of 229.1 with tributyl(trimethylsilylethynyl)tin provided the tetrakis-substituted product 229.2. In the final step, compound 229.2 was oxidized to 229.3 using MnO2. In the related derivative 229.7, bearing ethynyl chains on the carbazole moieties, these substituents were introduced prior to macrocycle formation. The ethynyl substituents in 229.3 and 229.7 led to marked reduction of the optical and electrochemical HOMO−LUMO gaps relative to the unsubstituted analogues. Even longer oligoethynyl substituents were introduced in subsequent work, to produce systems 229.8 and 229.9 with distinctly altered optical signatures.1313 Maeda and Yoshioka described the synthesis of porphyrinoids consisting of carbazole, indolone, and thiophene moieties.1314 In the first synthetic step, functionalized carbazole 230.1 was coupled with an aniline derivative via the Sonogashira reaction (Scheme 230). The subsequent InCl3catalyzed cyclization of 230.2 produced the bis(7-iodoindol-2yl)-substituted carbazole 230.3, which was converted into the diethynyl species 230.4 in the following step. The subsequent Glaser cyclization yielded the aromatic dione 230.5 instead of the expected 230.6. Finally, the Stille coupling between 230.3 and (tributylstannyl)thiophene afforded the core-modified system 230.7. Interestingly, it was found that 230.7 could be converted into the dioxygenated product 230.8 through MnO2mediated oxidation. The UV−vis absorption spectrum of 230.7 exhibited absorption primarily in the ultraviolet range, while the spectra of 230.5 and 230.8 extended into the visible region. The complexation reaction of 230.8 with palladium(II) acetate followed by ligand exchange produced 230.9 in an almost quantitative yield. Compounds 230.5 and 230.7−9 were considered weakly aromatic. The electronic absorptions of the diketo systems showed extended absorption in the vis and NIR regions of the spectrum (up to 800 nm for 230.9). In 2012, Müllen et al. reported the successful extension of their original approach1309 to the synthesis of the pyrrole− carbazole system 231.3 containing a complete porphyrin substructure (Scheme 231).1315 In contrast to the Osuka group’s strategy,1311 this route did not require protection of the pyrrole NHs, relying instead on a 4-fold Suzuki−Miyaura crosscoupling reaction between carbazole 231.1 and pyrrole 231.2. When performed at high dilution, this reaction produced the macrocycle 231.3 in 10% yield. Compound 231.3 featured two local maxima at 307 and 400 nm in its absorption spectrum and blue fluorescence emission (427 nm, ΦF = 0.68). Macrocycle 231.3 was subjected to MnO2 oxidation, yielding an unstable product 231.4, displaying bathochromically shifted bands in the absorption spectrum (846, 945, and 1076 nm), indicative of macrocyclic aromaticity. In 2013, bis-thiophene-bridged carbazole dimer 232.3 and trimer 232.4 were obtained by Masuda and Maeda through the

carbazole 227.1 with 2,6-dibromopyridine. The molecular design retains the inner circuit of the porphyrin ring, and the number of inner NH’s, providing a similarly sized macrocyclic cavity as well as the ability to bind metal ions. The coordination properties of 227.2 were demonstrated by the formation of a cobalt complex 227.3. Compound 227.2 showed two absorption maxima at 313 and 372 nm and blue fluorescence emission with a maximum at 402 nm (ΦF = 0.36). The emission of the free ligand was quenched upon complexation with cobalt(II). The UV−vis spectrum of 227.3 exhibited a bathochromic shift (331 and 425 nm) in comparison to the free base. In subsequent work, 227.3 was sublimed under ultrahigh vacuum onto a ferromagnetic Ni(001) thin film grown on a Cu(001) single crystal.1310 The adsorbed molecules were randomly distributed on the surface with their macrocyclic cores parallel to the substrate. The adsorbed complex was found to possess an exchange-stabilized magnetic moment. Both experimental and theoretical studies suggested that this induced magnetic moment is a general effect, not limited to the previously observed cases of metalloporphyrins and metallophthalocyanines. Scheme 227. Synthesis of a Hybrid Pyridine−Carbazole Porphyrinoida

a

Reagents and conditions:1309 (a) 2,6-dibromopyridine, Pd(PPh3)4; (b) Co(OAc)2, reflux, DMF.

A multiple annulation strategy enabling the synthesis of porphyrinoids from 1,3-butadiyne-bridged cyclic carbazole oligomers 228.2-3 was developed by Maeda and co-workers (Scheme 228).1311 The Glaser coupling of carbazole 228.1 provided three isolable oligomeric products: dimer 228.2 (35%), trimer 228.3 (4.9%), and tetramer 228.4 (7.6%). A CuCl-catalyzed annulation of 228.2 with anilines afforded Narylpyrrolylene-bridged carbazole dimers 228.5a−c. These dimers, which can be regarded as cross-conjugated isophlorin analogues, could not be oxidized to the porphyrin-like state characterized by macrocyclic aromaticity. An analogous aniline cyclization performed on the trimer 228.3 yielded the hexaphyrin-like macrocycle 228.6. Similar annulation reactions of 228.2 and 228.3 with sodium sulfide produced porphyrinoids 228.7 and 228.9, respectively. The oxidation of 228.7 with MnO2 furnished the corresponding aromatic porphyrin analogue consisting of thiophene−carbazole moieties 228.8. The absorption spectra of 228.5 and 228.6 were similar, with the most red-shifted bands in the 401−424 nm range, reflecting the nonconjugated character of these macrocycles. Interest3630


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Scheme 228. Synthesis of Carbazole-Containing Porphyrinoids by a Multiple Annulation Strategya

a

Reagents and conditions:1311 (a) Cu(OAc)2, air, pyridine, toluene, rt, 3 d; (b) RNH2, CuCl, mesitylene, reflux, 24 h; (c) Na2S·9H2O, THF, reflux, 24 h; (d) MnO2, CH2Cl2; (e) Na2S·9H2O, p-xylene, 2-methoxyethanol, reflux, 40 h.

the presence of hydrazine monohydrate provided the selenophene-bridged carbazole dimer 233.3 in 87% yield. The UV−vis spectra of the isophlorins 233.2 and 233.3 were similar and exhibited absorption bands at 310 and 400 nm, reflecting the nonaromatic character of these macrocycles. Both 233.2 (ΦF = 0.20) and 233.3 exhibited fluorescence, although the quantum yield of the latter system was lower than 0.01, due to the heavy atom effect of selenium. By treatment with MnO2, compound 233.3 was converted into the aromatic bis(selenophene) macrocycle 233.4, characterized by a bathochromically shifted porphyrin-like spectrum. Nonaromatic, mixed-heteroatom systems 234.2 and 234.3 were obtained by subjecting the thiophene−butadiyne macrocycle 234.1 to cyclizations with aniline or selenide (Scheme 234).1318 234.3 could be oxidized with MnO2, yielding the aromatic 234.4 with an optical HOMO−LUMO gap of 1.17 eV. The cyclic tetraindole 235.3 can be regarded as a π-extended porphyrinogen analogue, potentially oxidizable to a planar porphyrin-like structure (Scheme 235). The synthetic route to 235.3, established by Nakamura, Hiroto, and Shinokubo in 2012, employed the dimerization of borylated bisindole 235.2, which was synthesized from functionalized aniline 235.1 via a

Stille coupling of carbazole 232.1 and bithiophene 232.2 (Scheme 232).1316 The higher homologues, trithiophenebridged carbazoles 232.5 and 232.6, were prepared in low yield via Glaser coupling and subsequent annulation with Na2S. Significantly better yields were achieved for carbazole dimers 232.7 and 232.8, incorporating both thiophene and bithiophene or trithiophene linkages. The UV−vis absorptions of compounds 232.3−8 reached up to 455 nm, in line with the lack of macrocyclic aromaticity of these systems. While they were not susceptible to MnO2 oxidation, dicarbazoles 232.3, 232.5, 232.7, and 232.8 produced dramatic changes in their absorption spectra upon treatment with NOSbF6, displaying NIR absorption bands in the 1000−2000 nm region. These spectra apparently corresponded to the formation of radical cation species, as evidenced by the ESR spectroscopy measurements. An attempted synthesis of the furan-bridged carbazole 233.2 via Cu(I)-catalyzed annulation of 228.2 produced the bis(methoxyethoxy) adduct 233.1.1317 Compound 233.1 was nevertheless transformed into the desired furan derivative, 233.2, using an acid-catalyzed cyclization (Scheme 233). Moreover, the annulation reaction of 228.2 with selenium in 3631


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Scheme 229. Preparation of Peripherally Ethynylated Carbazole-Based Core-Modified Porphyrinsa

a Reagents and conditions:1312 (a) NBS, CH2Cl2; (b) tributyl(trimethylsilylethynyl)tin, Pd(PPh3)4, toluene; (c) MnO2, CH2Cl2; (d) tributyl(triisopropylsilylethynyl)tin, Pd(PPh3)4, toluene; (e) tetrabutylammonium fluoride, CH2Cl2; (f) (1) Cu(OAc)2·H2O, pyridine, toluene, (2) Na2S·9H2O, p-xylene, 2-methoxyethanol, (3) MnO2, CH2Cl2.

Sonogashira coupling and cycloisomerization (Scheme 235).1319 To prevent potential protodeborylation, 235.2 was used for the subsequent Suzuki−Miyaura without purification. The oxidized species 235.4 could be obtained from 235.3 either directly or with the intermediacy of the tetrabromo species 235.5, through the action of dibromine in the presence of tetrabutylammonium fluoride. Macrocycle 235.4 has a porphyrin-like N4 inner cavity and formed metal complexes with divalent Cu, Zn, and Ni ion. The free base 235.4 showed visible absorption extending up to ca. 700 nm, which underwent considerable red shifts upon metal insertion (up to ca. 1000 nm for 235.6c). The stepwise method of synthesizing 235.3 was later elaborated into a general macrocyclization strategy enabling the construction of mixedheteroatom derivatives 235.7−10.1320 In the same year, an oxidative coupling route to porphyrinlike cycloindoles was disclosed by Black and co-workers.1321 A variety of β-substituted indoles 236.1a−e were employed for the preparation of 2,7′-biindolyls 236.2a−d; however, the

coupling was inhibited by strongly electron-withdrawing groups such as NO2 (Scheme 236). Oxidative coupling of 236.2a in the presence of p-benzoquinone and acidified dichloromethane successfully afforded the cyclic tetraindole derivative 236.3a in 50% yield. It was proposed that the head-to-head-coupled compound 236.3a was preferentially produced due to the deactivation of the indole C2 position by electron-withdrawing group at C3. In line with this reasoning, when the bisindole 236.2c was treated with p-benzoquinone under the same conditions, two macrocyclic products were obtained, the headto-head product 236.3c and the fully symmetrical head-to-tail 236.4c in 18% and 25% yields, respectively. 7.3. Naphtho[2,1,8,7-cdef ]-Fused Porphyrinoids

7.3.1. Arene-Fused Systems. Lateral fusion of appropriately functionalized meso-9-anthryl substituents to the porphyrin ring was explored by Anderson and co-workers. In their initial work, published in 2008, a single substituent in 237.1 was oxidatively fused to the macrocycle using DDQ/Sc(OTf)3, to yield the annulated product 237.2 (Scheme 237).1226 An 3632


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Scheme 230. Synthesis of Fused Porphyrinoids Based on Cyclic Carbazole[2]indolonesa

a Reagents and conditions:1314 (a) 2,6-diiodo-4-tert-butylaniline, CuI, PdCl2(PPh3)2, K2CO3, THF/Et3N/MeOH, reflux, 15 h; (b) InCl3, toluene, reflux, 24 h; (c) (1) trimethylsilylacetylene, CuI, PdCl2(PPh3)2, THF/Et3N, 50 °C, 13 h, (2) K2CO3, CH2Cl2/MeOH, rt, 30 min; (d) air, Cu(OAc)2· H2O, pyridine, rt, 18 h; (e) 2,5-bis(tributylstannyl)thiophene, Pd(PPh3)4, toluene, reflux, 2 d; (f) MnO2, CH2Cl2, rt; (g) Pd(OAc)2, NaOAc; then brine, CH2Cl2/MeOH, rt, 24 h.

Scheme 231. Synthesis of a Carbazole-Containing Porphyrinoida

Scheme 232. Carbazole-Based Expanded Thiaporphyrinoidsa

a

Reagents and conditions:1315 (a) Pd(PPh3)4, K2CO3, EtOH, toluene, 85 °C; (b) MnO2, CH2Cl2, rt.

isomeric 1-anthryl derivative was converted to the corresponding 237.4 product with a benzo[cd] fusion pattern. The success of the coupling relied on the presence of activating alkoxy groups on the anthracene substituent. Porphyrins 237.2 and 237.4 display significant bathochromic shifts with longestwavelength absorption maxima at 855 and 725 nm, respectively. Subsequent attempts to prepare bis-anthracene hybrids revealed the dependence of oxidative coupling on metal coordination.1227 Treating the zinc complex 237.6 with Sc(OTf)3/DDQ only gave partially fused porphyrin 237.5, which could not be oxidized further with an additional amount of Sc(OTf)3/DDQ. Upon replacing the central ion with Ni(II), the Sc(OTf)3/DDQ reagent yielded the completely fused product 237.8. Combined intra- and intermolecular oxidative coupling of the meso-unsubstituted precursor 237.9 produced the fused bis-anthracene porphyrin dimer 237.10, characterized by a NIR absorption at 1495 nm. A porphyrin 237.12 fused to four anthracenes was eventually reported by the Anderson group. In their synthesis, all four anthryl groups were coupled

a

Reagents and conditions:1316 (a) Pd(PPh3)4, toluene, reflux, 12 h.

to the Ni(II) porphyrin core in a single-step FeCl 3 oxidation.1228 237.12 showed a very intense Q-band absorption at 1417 nm (ε = 1.2 × 105 M−1 cm−1, line width of 284 cm−1), corresponding to an optical band gap of only 0.87 eV. Functionalized anthracene-fused porphyrins were successfully used as sensitizers in solar cells exhibiting photocurrent collection at wavelengths up to ca. 1100 nm, albeit with low efficiency.1322 Photophysical properties of anthracene-fused 3633


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Scheme 235. Cyclic Tetraindoles and Related Systemsa

Scheme 233. Carbazole-Based Porphyrinoids Containing Selenophene and Furan Ringsa

a

Reagents and conditions:1317 (a) CuI, 1,10-phenanthroline, KOH, 2methoxyethanol, p-xylene, reflux, 16 h; (b) H2SO4, CHCl3, rt, 2 h; (c) Se, H2NNH2·H2O, KOH, DMSO, 100 °C, 20 h; (d) MnO2, CH2Cl2, rt, 3 d.

Scheme 234. Mixed-Heteroatom Carbazole-Based Porphyrinoidsa

a

Reagents and conditions:1319 (a) (1) t-BuOK, NMP, rt, (2) 2-bromo4-tert-butyl-6-iodoaniline, PdCl2(PPh3)2, CuI, THF−Et3N, rt, (3) ZnI2, toluene, reflux, (4) bis(pinacolato)diboron, [Ir(OMe)cod]2, 4,4′di-tert-butyl-2,2′-bipyridyl (dtbpy), 60 °C; (b) Pd2dba3·CHCl3, SPhos, CsF, Cs2CO3, toluene−DMF, 100 °C; (c) (1) TBAF, 0 °C, (2) Br2 (10 equiv), THF, 0 °C; (d) (1) TBAF, 0 °C, (2) Br2 (excess), THF, 0 °C; (e) (1) Br2 (excess), THF, rt, (2) NaHSO3−H2O; (f) Cu(OAc)2, CHCl3−MeOH, reflux for 235.6a; Zn(OAc)2, NaH, DMF, rt for 235.6b; Ni(OAc)2·4H2O, NaH, DMF, rt for 235.6c.

with DDQ. For easier separation, 238.1a−b were reduced to the [28] state and subsequently reoxidized after purification. Intramolecular cyclizations were attempted with DDQ/Sc(OTf)3 and FeCl3, but neither experiment was successful. The bis(AuIII) complex 238.2b, which was more conformationally rigid than the free base, proved to be more suitable for the oxidative fusion reaction, producing the fused 238.3b in acceptable yield. Because of the flat and elongated rectangular conjugated network, 238.3b displayed a remarkably red-shifted and sharp Q-type band at 1467 nm, seven reversible redox potentials between 1.02 and −1.76 V implying multicharge storage ability, and a large TPA cross-section (7600 GM at 1700 nm). Efficient cyclization of the porphyrin quinodimethane 239.1 was achieved by Wu et al. by using the DDQ/CH3SO3H oxidant system, affording the target fusion product 239.2

a

Reagents and conditions:1318 (a) RNH2, CuCl, mesitylene, reflux, 1 d; (b) Se, N2H4·H2O, KOH, DMSO, 100 °C, 20 h; (c) MnO2, CH2Cl2, rt, 3 d.

systems were investigated theoretically using DFT and coupledcluster methods.1323,1324 Osuka et al. reported the synthesis of 238.3, a [26]hexaphyrin fused to two anthracenes (Scheme 238).1325 Anthryl-substituted macrocycles 238.1a−b were prepared by the condensation of tripyrrane and 9-formylanthracene in the presence of methanesulfonic acid at 0 °C, followed by oxidation 3634


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existed as the triplet biradical, confirmed by ESR measurements, and its UV−vis−NIR spectra showed a broad redshifted absorption extending beyond 1200 nm. DFT calculations (UB3LYP/6-31G*) suggested that porphyrin 241.2 has an open-shell triplet ground state (⟨s2⟩ = 2.1194) with a singlet−triplet energy gap (ΔES−T) of +6.98 kcal mol−1. The molecular orbital (MO) characteristics and spin density maps indicated a nondisjoint character of nonbonding orbitals and negligible spin density on the Ni atom. A closed-shell ground state was identified for compound 241.1 on the basis of the sharp NMR spectrum observed even at elevated temperatures. This observation was supported by DFT calculations, which indicated a small singlet biradical contribution (y = 0.06; ΔES−T = −4.28 kcal mol−1). A different type of phenalene fusion was explored reported in 2016 by Thompson et al.1329 Starting with the bis(2,3-dihydro1H-phenalen-6-yl)-substituted porphyrin 241.8, they obtained the fully conjugated system 241.11 in a two-step synthesis, consisting of FeIII-mediated oxidative coupling of one substituent followed by thermal annulation−dehydrogenation. 241.11 was shown to be a diamagnetic system with a strongly red-shifted absorption spectrum (λmax = 796 nm) and weak NIR emission (λmaxem = 835 nm, ΦF = 0.003). A partial chargetransfer character was ascribed to the HOMO−LUMO transition on the basis of the partially disjoint spatial distribution of frontier orbitals. Broken-symmetry DFT calculations indicated an insignificant biradicaloid character. 7.3.2. Porphyrin Tapes. Conjugated porphyrin oligomers, in which consecutive macrocyclic units are triply linked via adjacent β, meso, and β positions (porphyrin tapes), have been the subject of intense research because of their unique electronic features and potential use as components for organic electronics with nanometer-scale dimensions.1330 A feasible and general synthesis of these systems, known as oxidative doublering closure (ODRC1331), was developed by Osuka and coworkers in 2000,1332 elaborating on the earlier discovery of direct meso−meso oxidative coupling.1333 In the original work, diporphyrin Cu complex 242.1 was treated with 2 equiv of tris(4-bromophenyl)aminium hexachloroantimonate (BAHA) in C6F6 at room temperature for 2 days, to provide triply linked dimers 242.3a−b in 62% and 6% yields, respectively (Scheme 242).1332 The oxidation of 242.1 carried out in CHCl3 instead of C6F6 led to extensive β-chlorination, indicating that the abstraction of chlorine was solvent-dependent. Triporphyrin 242.2 was oxidized in a similar manner to give the fused trimer 242.4 in a 33% yield along with the recovered starting material (30%). In contrast to singly linked oligomers 242.1 and 242.2, in which pairs of adjacent porphyrin subunits are perpendicular to one another, the triply linked arrays 242.3 and 242.4 contain a contiguous, completely planarized π-conjugated system. Oligoporphyrins 242.3 and 242.4 showed red-shifted UV− vis−NIR spectra with absorption maxima at 411, 576, and 996 nm and at 413, 657, and 1251 nm, respectively. The effect of the central metal on the electronic interaction was preliminarily examined by comparing CuII-diporphyrin 242.3a and ZnIIdiporphyrin 242.3c. The most red-shifted Q-band is observed at 1068 nm in 242.3c, being red-shifted by 72 nm more than 242.3a. The one-electron oxidation potentials of 242.3a and 3c are 0.39 and 0.11 V, respectively. Diverse analogues of 242.3, containing mixed substituents or metal ions, were subsequently reported by several groups.1334−1337 The majority of these systems contained meso-aryl groups, which were introduced to

Scheme 236. Synthesis of Macrocyclic Tetraindoles via Oxidative Coupling Reactionsa

a

Reagents and conditions:1321 (a) SnCl4, CH2Cl2, MeNO2, MeCN, reflux, 12 h; (b) HCl, p-benzoquinone, CH2Cl2, rt, 3 h.

(Scheme 239).1326 Compound 239.1 showed two distinct absorption bands at 450 and 635 nm in CH2Cl2, while 239.2 displayed a red-shifted absorption spectrum with two major bands at 533 and 780 nm together with two weak bands at 669 and 897 nm due to extended π-conjugation. The corresponding optical energy gaps were determined to be 1.69 and 1.33 eV for 239.1 and 239.2, respectively, from the onset of the lowest energy absorption band. An unusual negative shift of the first oxidation potential was observed for the highly contorted molecule 239.1. Similarly designed, nonquinoidal BODIPY−porphyrin hybrids were subsequently reported by the Wu group.1327 FeCl3 was utilized to promote intramolecular ring fusion of 240.1 yielding the fully condensed product 240.2 (Scheme 240). Further extension of the π-conjugation length was achieved by fusing two BODIPY subunits to the porphyrin ring. Complete oxidation of 240.3 to 240.4 required forceful conditions, affording the product in only 15% yield. Compounds 240.2 and 240.4 showed intense NIR absorption maxima at 890 and 1040 nm, respectively, and excellent photostability. Wide spectral coverage, as well as the amphoteric redox behavior of these systems, indicated their potential use in photovoltaic devices. Phenalenyl−porphyrin hybrids 241.1−2 were designed by the Wu group as potentially stable biradicaloid systems.1328 The fusion of one phenalenyl unit to the porphyrin core was successfully achieved by an intramolecular Friedel−Crafts alkylation of 241.3 promoted by BF3·Et2O, affording the dihydro precursor 241.4 in 67% yield (Scheme 241). Oxidative dehydrogenation of 241.4 with N-iodosuccinimide (NIS) resulted in the desired π-extended porphyrin 241.1. A similar strategy was used for the synthesis of 241.2, except that in the final step, NIS was replaced by p-chloranil as the dehydrogenative agent. Porphyrin 241.2 could not be isolated in pure form because of its air-sensitivity. Upon exposure to air for 3 h, it was transformed into two dioxo-porphyrin isomers 241.7a−b. Compound 241.1 showed higher stability but decomposed in protic solvents during crystallization. The unstable 241.2 3635


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Scheme 237. Synthesis of Anthracene-Fused Porphyrinsa

Reagents and conditions: (a)1226 Sc(OTf)3, DDQ, DCM, 20 °C; (b) Sc(OTf)3, DDQ, DCM, 40 °C; (c)1227 Sc(OTf)3/DDQ; (d) (1) TFA, DCM, (2) Ni(acac)2, xylene, 130 °C; (e) (1) FeCl3, CH3NO2, DCM, (2) FeCl3, AgOTf, CH3NO2, toluene; (f) Sc(OTf)3, DDQ, DCM; (g)1228 FeCl3, DCM. a

Scheme 238. Synthesis of a Hexaphyrin Fused with Two Anthracene Moietiesa

a

Reagents and conditions:1325 (a) Na[AuCl4]·2H2O, NaOAc, Ag2CO3, CH2Cl2/MeOH; (b) DDQ, Sc(OTf)3.

suppress π-stacking of the extended conjugated surfaces. For

absorbers and emitters with very large TPA cross sections,1343−1345 and as semiconducting liquid crystals.1346−1348 Direct oxidative coupling of 5,10,15-triaryl metalloporphyrins containing different metals (243.1a and 214.1b,c,e, Scheme 243) using the DDQ−Sc(OTf)3 couple revealed strong metalion-dependent regioselectivity.1349 Oxidation of the zinc complex 214.1c yielded exclusively the triply linked derivative

comparison, the n-hexyl-substituted dimer 242.5, obtained using the DDQ/Sc(OTf)3 reagent, displayed strong aggregation in noncoordinating solvents.1338 Functionalized triply linked dimeric and trimeric oligomers were explored as elements of covalent supramolecular assemblies,1339−1342 NIR 3636


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bearing electron-rich 3,5-di-tert-butylphenyl groups.1350 PIFA oxidation was instrumental in the synthesis of 244.7, a derivative containing six thioglucose units appended to perfluoroaryl substituents, which could not be obtained with the DDQ/Sc(OTf)3 oxidant.1354 The amphiphilic compound 244.7, which is chemically and photochemically stable, was taken up by breast cancer cells and caused cell death upon exposure to light. Further photophysical studies of 244.7 revealed absorption bands in the near IR region (the lowest energy at 1090 nm in DMSO), and photosensitized formation of singlet oxygen with high quantum yield (0.78 ± 0.03 in DMSO). Anderson, Armaroli, et al. investigated a deuterated dimeric porphyrin array 245.1 to test whether deuteration influences the rate of nonradiative S1−S0 deactivation.1355 Their synthetic strategy involved initial deuteration of dipyrromethane 245.2 in the two-phase D2O−CH2Cl2 system (Scheme 245). In the target 245.1, obtained from 245.2 using previously reported methods, the β-pyrrolic positions were approximately 91% deuterium-enriched and the aryl substituents were also partially deuterated. The absorption spectra for both the deuterated and the nondeuterated dimers were identical, indicating that the electronic structure was not affected by deuteration. The fluorescence quantum yield was completely unaffected by deuteration (φfD/φfH = 1.00 ± 0.05), demonstrating that C−H vibrations do not contribute to the radiationless deactivation of the S1 state (i.e., internal conversion). It was proposed that a specific deactivation pathway was active via an accessible intersection of the S1 and S0 potential energy surfaces. Osuka and co-workers reported that triply linked diporphyrin 246.1 underwent site-selective cycloaddition reactions with thermally generated o-xylylene 246.2 to provide a mixture of [4+2] and [4+4] cycloadducts, 246.3 and 246.4 (Scheme 246).1356 Oxidation of the latter product with DDQ provided a triply linked diporphyrin 246.5 fused with a benzocyclooctatriene moiety. Further dehydrogenation of 246.5, aimed at obtaining the corresponding cyclooctatetraene-fused derivative, was ineffective under a variety of conditions. The absorption spectrum of 246.5 was quite similar to that of the parent 246.1, with red-shifted Q-like bands (above 1000 nm). In contrast, the absorption spectra of 246.3 and 246.4 were ill-defined, and blue-shifted, exhibiting the lowest-energy bands at 820 and 729 nm, respectively. In related work from the Osuka group, 246.1a−b were shown to undergo a rare [3+4] cycloaddition reaction with azomethine ylide 246.6, prepared in situ from sarcosine and para-formaldehyde.1357 The resulting bischlorin adduct 246.7a had a low oxidation potential of only 0.05 V and proved to be unstable in air, gradually reverting to the parent dimer 246.1. In contrast, the Ni complex 246.7b was thermally stable, and it was possible to obtain it in a higher yield (42%). Attempted syntheses of the corresponding bisporphyrins via oxidation of 246.7a and 246.7b with DDQ, chloranil, and manganese dioxide failed, producing instead the parent dimers 246.1. The absorption spectral features of 246.7a and 246.7b were similar to those of the 246.3 and 246.4 bischlorins,1356 displaying a split Soret-like band and complicated Q-like bands. Triply fused porphyrin dimers were subjected to a range of peripheral extensions involving carbo- and heterocyclic units. Osuka, Kim, et al. employed the retro-Diels−Alder strategy in the synthesis of the tetrabenzo-fused triply linked diporphyrin 247.5 (Scheme 247).1358 Initially, bicyclo[2.2.2]-octadienecondensed porphyrin 247.1 was obtained by TFA-catalyzed

Scheme 239. Synthesis of Fused Quinoidal Porphyrin 239.2a

a

Reagents and conditions:1326 (a) DDQ, MeSO3H, CH2Cl2, rt, 20 min.

Scheme 240. Synthesis of BODIPY-Fused Porphyrinsa

a

Reagents and conditions:1327 (a) FeCl3, MeNO2, CH2Cl2, rt; (b) FeCl3, MeNO2, CH2Cl2, reflux.

242.3c in 86% yield, whereas 214.1b selectively produced the meso−β doubly linked dimer 214.2b in 74% yield. Both types of coupling product were produced simultaneously in oxidations of Cu and Ni complexes. The observed coupling regioselectivities are analogous to those found for oxidation with BAHA. Oxidation of triarylmetalloporphyrins bearing a sterically hindered phenolic substituent at position R 1 produced, in addition to bisporphyrins, also quinoidal monomers, such as 243.4. As shown above, DDQ/Sc(OTf)3 is a particularly versatile reagent for the synthesis of triply linked dimers and works even with relatively electron-poor systems, for example, containing perfluorophenyl meso substituents.1336 However, the coupling can also be effected with a range of other oxidants such as PIFA, which enables precise control of oxidation stoichiometry,1337,1350 PIFA−BF3·Et2O,1351 Cu(II) salts,1352 and Fe(OTf)3.1353 In the latter case, coupling regioselectivity was shown to depend on the oxidation potential, with the lowpotential reactants favoring triply linked products (Scheme 244). With PIFA, a regioselectivity dependent on mesosubstituents was achieved in the synthesis of 244.5, in which the ODRC reaction occurred only between porphyrin subunits 3637


Chemical Reviews

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Scheme 241. Synthesis of Phenalenyl-Fused Porphyrinsa

a

Reagents and conditions: (a)1328 KOAc, CH3CN, THF, reflux, 1 day; (b)1328 LiOH·2H2O, dioxane, H2O, reflux, 2 days; (c)1328 (1) oxalyl chloride, DMSO, DCM, Et3N, (2) Ni(acac)2, toluene, reflux, 24 h; (d)1328 (1) mesitylmagnesium bromide, THF, rt, 24 h, (2) excess BF3·Et2O, DCM, 10 min; (e)1328 NIS, DCM; (f)1328 p-chloranil, DCM; (g)1328 air; (h)1329 FeCl3, CH2Cl2; (i)1329 500 °C.

Scheme 242. Synthesis of Completely Fused Diporphyrins and Triporphyrina

a

Reagents and conditions:1332 (a) BAHA, C6F6, 2 days.

the tetrabenzo-fused compound 247.4. The meso-bonded diporphyrin 247.4 was oxidized with the DDQ−Sc(OTf)3 reagent system, yielding the 247.5 target in good yield. An

condensation followed by oxidation. In the next step, 247.1 was converted into 247.2 via AgPF6-promoted meso−meso coupling. Heating to 210 °C under reduced pressure provided 3638


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Scheme 243. Oxidative Coupling of 5,10,15-Triarylporphyrinsa

a

Reagent and conditions: (a)1349 DDQ, Sc(OTf)3, toluene, 50 °C.

Scheme 244. Oxidative Coupling of Porphyrins with PIFA and Fe(OTf)3a

a

Reagents and conditions: (a)1353 5.0 equiv of Fe(OTf)3, CH2Cl2/CH3NO2; (b)1350 0.5 equiv of PIFA, CH2Cl2; (c) 1.5 equiv of PIFA, CH2Cl2.

Scheme 245. Synthesis of a Deuterated Conjugated Porphyrin Dimera

a Reagents and conditions:1355 (a) d-TFA, D2O, CH2Cl2; (b) (1) 3,5-di-tert-butylbenzaldehyde, TFA, CH2Cl2, (2) DDQ; (c) Ni(OAc)2, DMF, reflux; (d) (1) 3,5-di-tert-butylphenyllithium, THF, 0 °C to rt, (2) D2O, (3) DDQ; (e) (1) D2SO4, CH2Cl2, (2) Zn(OAc)2·2H2O, MeOH, CH2Cl2; (f) Sc(OTf)3, DDQ, PhMe.

alternative route, involving initial ODRC to produce 247.3, was less efficient because of the low solubility of the latter intermediate. Compound 247.5 exhibits a perturbed absorption

spectrum and a large two-photon absorption cross section (15 400 GM at 1260 nm). 3639


Chemical Reviews

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Scheme 246. Bay-Region Cycloadditions to a Triply Linked Diporphyrina

a Reagents and conditions: (a)1356 benzosultine (7 equiv, gradual addition), toluene, reflux; (b) DDQ, CHCl3; (c)1357 sarcosine, paraformaldehyde, toluene, reflux; (d) oxidant (DDQ, chloranil, or MnO2).

Scheme 247. Synthesis of a Benzo-Fused Porphyrin Dimera

a

Reagent and conditions:1358 (a) AgPF6−CHCl3; (b) DDQ−Sc(OTf)3; (c) 210 °C, 0.1 mmHg for 30 min.

annulated perylene, 249.3a−b showed a very intense absorption beyond 1250 nm. The maximum TPA cross-section values (σ(2)) of 249.2a and 249.2b were measured at 2100 nm as 890 and 1100 GM, respectively. Fused compounds 249.3a and 249.3b exhibited larger σ(2) values of 1400 GM (at 2200 nm) and 1500 GM (at 2400 nm), respectively. In 2001, the Osuka group showed that triply linked porphyrin n-mers with n = 2−12 (250.2−n) could be efficiently obtained in the oxidative double-ring closure (ODRC) reaction of meso−meso-linked porphyrin oligomers (Scheme 250).1331 The ODRC reaction was conducted by refluxing a toluene solution of 250.1−n in the presence of DDQ/Sc(OTf)3, providing yields from 60% to 91%. The ODRC results in a complete flattening of the oligomer chains, which was visualized using STM for differently substituted analogues of 250.1−6 and 250.2−6 adsorbed on Cu(100).1361 Fused tape-shaped porphyrin arrays 250.2−n display spectacular red shifts of their absorption bands, in comparison with their singly linked precursors. The wavelength of the lowest-energy band increased linearly with the oligomer length, each porphyrin subunit introducing an average red shift of 175 nm. Consequently, these absorptions spanned a remarkable range from 9400 cm−1 for 250.2−2 to 3500 cm−1 for 250.2−12. In fact, for the longest oligomers, the low-energy absorptions

In 2010, the Thompson group described the synthesis and properties of β-meso-β′ bonded porphyrin dimers fused with two pyrene units (Scheme 248).1232 The meso-pyrenesubstituted intermediate 248.3 was obtained via oxidative cyclodehydrogenation of 248.2. Fusion of the pyrene substituents did not occur under the latter conditions and was therefore effected by means of FeCl3 oxidation, yielding 248.4a. The position of the lowest-energy Q-band varied considerably with metal coordination, ranging from 1269 nm in the free base, through 1323 nm in the Zn complex 248.4b, to 1459 nm in the Pb complex 248.4c, the latter value being comparable with that of the related bis-anthracene diporphyrin (237.4, vide supra). Compound 248.4b showed promising properties for use in NIR photodetector applications.1359 Porphyrin dimers fused with N-annulated perylene (NAP) units were reported by Wu, Kim, and co-workers1360 (for other NAP−porphyrin hybrids, see Scheme 52, section 3.2). Oxidative couplings and cyclodehydrogenations of 249.1 were attempted using PIFA and Sc(OTf)3/DDQ (Scheme 249). By carefully controlling the reaction temperature and amount of oxidant, both the partially fused 249.2 and the fully fused 249.3 porphyrin tapes could be obtained in reasonable yields. Because the fusion direction coincides with the S0−S1 transition moment axes of both the fused porphyrin dimer and the N3640


Chemical Reviews

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Scheme 248. Fully Fused Pyrene−Diporphyrin Hybridsa

Scheme 250. Synthesis of Fully Conjugated Porphyrin Tapesa

a

Reagents and conditions:1331 (a) DDQ, Sc(OTf)3, toluene, reflux.

to enclose each porphyrin subunit.1362 This modification resulted in the suppression of π−π stacking and in improved chemical stabilities of the longest tapes. A considerable amount of theoretical and experimental work has been devoted to the study of electronic structure of porphyrin tapes. Individual porphyrin macrocycles in fully conjugated oligoporphyrins exhibit different degrees of macrocyclic aromaticity, and a theoretical method for estimating these properties was proposed by Aihara and Makino.1363 Kim’s group explored the role of the naphthalene junctions in the electronic delocalization in linear and branched Zn porphyrin tapes.1364 It was proposed on the basis of GIAO and AICD calculations that the unique π-conjugation behavior of triply linked tapes is induced by their intrinsic molecular orbital interactions and subsequently by antiaromatic naphthalene and COT junctions, in one- and two-dimensional systems, respectively. Consequently, linear tapes were viewed as a superposition of alternating aromatic (porphyrin-centered) and antiaromatic (naphthalene-centered) units. The structural deformation by triple linkages was observed to have a negative

a Reagents and conditions:1232 (a) 4,4,5,5-tetramethyl-2-(pyren-1-yl)1,3,2-dioxaborolane, [Pd(PPh3)4], Cs2CO3, toluene, 110 °C; (b) Sc(OTf)3, DDQ, toluene, 110 °C; (c) FeCl3, CH2Cl2, 20 °C, then aq HCl (M = 2H); (d) Zn(OAc)2, MeOH, CH2Cl2; (e) Pb(OAc)2, pyridine, CH2Cl2.

overlapped with the vibrational part of the spectrum, leading to a decrease of intensity of C−H stretching vibrations. The oneelectron oxidation potentials also decreased progressively upon the increase in the number of porphyrins unit. Longer arrays (most notably 250.2−12) exhibited low solubility due to aggregation. This problem was partly alleviated in later work, in which double 1,10-dioxydecamethylene straps were introduced Scheme 249. Oxidative Fusion of meso-Substituted Porphyrina

a

Reagents and conditions:1360 (a) 5 equiv of Sc(OTf)3, DDQ, toluene, 90 °C; (b) 5 equiv of Sc(OTf)3, DDQ, toluene, 50 °C. 3641


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Scheme 251. π-Extended Porphyrin Tapes with Mixed Substitutiona

a

Reagents and conditions:1373 (a) (1) Pd2(dba)3, tri-2-furylphosphine, Cs2CO3, THF, DMF, H2O, (2) Zn(OAc)2, MeOH, CHCl3; (b) AgPF6, CH3CN, CHCl3, rt, 12 h; (c) AgPF6, DMA, CH3CN, CHCl3, reflux, 72 h; (d) DDQ, Sc(OTf)3, toluene, 80 °C, 2 h.

effect on the electronic delocalization between the inner and outer porphyrin units. Porphyrin tapes exhibited a rapid formation of the lowest excited states and an acceleration of relaxation dynamics of the lowest excited state from ∼4.5 ps for the dimer up to ∼0.3 ps for the hexamer.1365 Further examination of longer arrays (up to 8-mer) showed that excited-state dynamics changed systematically as the number of porphyrin moieties increased, revealing the occurrence of two simultaneous relaxation processes.1366 The electronic transition process, for which the recovery time constant becomes shorter as the number of the porphyrin units increases, competes with vibrational relaxation processes, strongly affected by structural and environmental inhomogeneities. TPA cross-section values measured in solution for triply linked Zn oligomers at 1200 nm showed a continuous increase as the array became longer (11 900, 33 100, and 93 600 GM for the dimer, trimer, and tetramer, respectively).1367 Second hyperpolarizabilities (γ) of doubly and triply linked porphyrin tapes were evaluated using the Pariser−Parr−Pople theory.1368 It was found that triply linked arrays exhibit remarkable evolution of γ/n with the increase in the number of units, and the γ is 3 orders of magnitude larger than in butadiyne arrays in the limit of infinite n. Magnetic circular dichroism (MCD) spectra of doubly and triply linked fused bisporphyrins and triply linked higher oligomers (up to four units) were measured, showing no dependence on the central metal atom or solvent.1369 Observed spectral patterns were considered to result from the strong perturbation of the porphyrin skeleton caused by direct linkages between chromophores. It was also demonstrated that the MO’s of the investigated arrays could be constructed as a linear

combination of the constituent monomeric MO’s, and that the effect of lowering the symmetry was always larger on the LUMO than on the HOMO orbital. Electrical conduction measurements were performed on a singly linked oligomer consisting of 48 porphyrin units, which showed a diode-like behavior, and on an 8-membered triply linked array.1370 I−V curves recorded for the latter system showed that the stronger π-electron conjugation led to higher conductivity and a smaller band gap. Measurements of electrical transport through singly and triply linked thiol-terminated porphyrin arrays showed a slower than expected decrease of conductance with the molecular length.1371 Using the I(s) implementation of the STM technique, 4-pyridyl-terminated triply linked porphyrin dimer and trimer were shown to have higher conductance in the Au|oligoporphyrin|Au junctions than their singly linked and butadiyne-bridged analogues.1372 The synthesis of long porphyrin tapes is made difficult by limited length-selectivity of porphyrin meso−meso couplings, aggregation of triply linked tapes, and the consequent solubility problems. Osuka and co-workers addressed these issues by devising a stepwise oligomerization protocol and introduction of a mixed substitution pattern (Scheme 251).1373 The synthesis began with the Suzuki−Miyaura coupling of differently substituted monomers 251.1 and 251.2, followed by zinc(II) insertion, which produced the meso−meso-linked hybrid diporphyrin 251.3−2. In the next step, 251.3−2 underwent an Ag-promoted oxidation coupling to produce longer arrays 251.3−4 and 251.3−6. The presence of bulky groups (R1) on one of the porphyrin units in 251.3−2 makes the coupling highly regioselective, yielding only head-to-tail coupled products. By using the Ag oxidant under more forceful 3642


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The ODRC strategy, used to synthesize triply linked oligomers, was also employed by Osuka, Kobayashi, Kim, et al. for the synthesis of the tetrameric porphyrin sheet 252.1 (Scheme 252).1375 It was obtained by direct DDQ/Sc(OTf)3

conditions, 251.3−4 was oligomerized further, to yield singly linked chains 251.3−n with lengths up to n = 24. The transformation of available 251.3−n oligomers into the corresponding porphyrin tapes 251.4−n was achieved with the DDQ/Sc(OTf)3 reagent system in good yields (38−68%). The remarkable length-dependent red shift of the lowestenergy Q-band was again observed in the 251.4−n series, but it was found to become saturated at n = 16. The effective conjugation length of porphyrin tapes was therefore estimated to be around 14−16 units, corresponding to the physical length reaching about 12 nm. In 2014, temperature-induced covalent dehydrogenative coupling between unsubstituted free-base porphine molecules was reported by Auwärter et al. to yield fused dimers and larger oligomers (exceeding 90 porphine units) directly on a Ag(111) support under ultrahigh-vacuum conditions.1374 The reaction provides simultaneously three fusion motifs: (1) β−β, meso− meso, β−β characteristic of porphyrin tapes, (2) meso−β, β− meso, and (3) β−β, meso−β (Figure 5). The percentage of the triply fused motif 1 decreased with increasing substrate temperature from 81 ± 5% (TS = 533 K) to 67 ± 6% (TS = 613 K). Binding motif 2 showed no pronounced temperature dependence, while motif 3 became more likely with increasing temperature.

Scheme 252. Synthesis of Porphyrin Sheetsa

Reagents and conditions: (a)1375 DDQ, Sc(OTf), toluene, 55 °C; (b)1377 TFA, H2SO4, CHCl3, 0 °C to rt; (c) Cu(OAc)2 in MeOH, CHCl3, reflux. a

oxidation of the cyclotetrameric singly linked porphyrin array 252.2, which was synthesized in a stepwise manner from the 252.3 building block utilizing a previously reported procedure.1376 A more convenient, although lower-yielding, synthesis of 252.1 was subsequently reported, involving noncyclic angularly meso-linked tetramers.1377 UV−vis absorption measurements showed considerably broadened absorption bands, with the most red-shifted region occurring between 1000 and 1500 nm. In comparison to the one-dimensional tetrameric porphyrin tape, 252.1 exhibited a slightly longer-lived S1-state (1.1 ps) and a smaller TPA cross section (2750 GM). The 252.1 sheet formed stable 1:2 and 2:4 complexes with specially designed bisimidazolyl1375 and bispyridyl1378 derivatives, which acted as bidentate apical ligands coordinating to the Zn centers. Chemical shifts observed for the bound ligands and NICS calculations were used to demonstrate a considerable paratropic ring-current effect around the planar cyclooctatetraene ring embedded in the center of 252.1.1375,1378 The 1H NMR spectrum of the free base 252.4, obtained by demetalation of 252.1, showed that the porphyrin rings exhibit no diatropic ring currents and that the NH tautomerism is completely frozen.1377 Tetranuclear Cu(II) complex 252.5 exhibited antiferromagnetic interaction among the CuII ions with J = −1.16 cm−1. First-principles calculations were performed to investigate the electron spin-polarization and magnetic ordering in infinite 2D polyporphyrin sheets based on the motif found in 252.1.1379 The magnetic coupling between the local magnetic moments in the free base sheet was very weak, exhibiting paramagnetic features. The chromium complex array had a ferromagnetic ground state, while other transition metal

Figure 5. STM observations of porphine homocoupling. (a) Random distribution of individual porphine molecules on Ag(111) after deposition at a sample temperature TS of 345 K (13.0 × 19.8 nm2, −0.8 V, 0.2 nA). (b) Overview image after annealing a porphine multilayer, grown at room temperature, to TS = 573 K. The colored circles highlight distinct species: monomer (blue), dimer (green), and trimer (yellow). The bright features (dashed circle) are assigned to porphines interacting with Ag adatoms (28.0 × 19.8 nm2, −0.2 V, 0.1 nA). (c) Porphine monomer with structural model and first symmetry axis (red dotted line). Directions of the Ag(111) substrate are indicated in white. (d−f) Three different binding motifs for oligomers with corresponding structural models and symmetry axes (2.5 × 2.5 nm2, −0.15 V, 0.12 nA). C, N, and H atoms are indicated in cyan, blue, and white, respectively. (d) Binding motif 1 of triply fused dimer: β−β, meso−meso, β−β (positions are labeled and marked by arrows in (c)). (e) Binding motif 2 of doubly fused dimer: meso-β, β-meso. (f) Binding motif 3: β−β, meso-β. Adapted with permission from ref 1374. Copyright 2014 American Chemical Society. 3643


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Scheme 253. Synthesis of Two-Dimensionally Extended Porphyrin Tapesa

a

Reagents and conditions:1380 (a) DDQ, Sc(OTf)3, toluene.

Scheme 254. Synthesis of Fused Hybrid Porphyrin−Hexaphyrin Tapesa

Reagents and conditions: (a)1381 BF3·Et2O, CH2Cl2, 30 min, rt, then DDQ, 1 h; (b)1381 DDQ, Sc(OTf)3, 80 °C, 5 h; (c)1381 NaBH4; (d)1381 MnO2; (e)1385 DDQ, Sc(OTf)3, 80 °C, toluene. a

Hybrid porphyrin−hexaphyrin tapes were synthesized by applying the ODRC reaction to mixed meso-linked oligomers (Scheme 254). Dimer 254.3 was obtained in a crosscondensation of meso-porphyrinyl-dipyrromethane 254.1, dipyrromethane 254.2, and perfluorobenzaldehyde, to be converted into the fused target 254.4 using the usual DDQ/ Sc(OTf)3 oxidation.1381 The NIR spectrum of 254.4 shows the lowest-energy maximum at 1333 nm with a tail reaching to ca. 1600 nm. In analogy to nonfused hexaphyrins,1382,1383 the [26]hexaphyrin unit in 254.4 could be quantitatively reduced with NaBH4 to yield the [28]hexaphyrin tape 254.5. The

complexes showed either antiferromagnetic or paramagnetic ground states. Two structurally related angular porphyrin tapes, the Lshaped trimer 253.1 and T-shaped tetramer 253.2, were elaborated by Osuka and co-workers using the ODRC strategy (Scheme 253).1380 The use of nonyl substituents was dictated by the steric demands of the branched tape design. Branching of the fused framework in 253.1 and 253.2 resulted in a marked decrease of TPA cross sections relative to their linearly fused analogues. 3644


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Scheme 255. Synthesis of Dehydropurpurin−Porphyrin Dyads and Dehydropurpurin Dimersa

Reagents and conditions:1386,1387 (a) AgPF6, CHCl3, rt, 6 h; (b) 1,4-diphenylbutadiyne, (o-Tol)3P, [Pd2(dba)3], NEt3, toluene, 110 °C; (c) mesobromoporphyrin (ca. 3.5 equiv), (o-Tol)3P (2 equiv), [Pd2(dba)3] (50 mol %), NEt3 (5 equiv), DMF, 110 °C; (d) meso-bromoporphyrin (5 equiv), (o-Tol)3P (2 equiv), [Pd2(dba)3] (50 mol %), NEt3 (5 equiv), DMF, 110 °C; (e) NBS, pyridine, CHCl3/EtOH, 0 °C (for a); NBS, pyridine, CHCl3/EtOH, 0 °C, then Zn(OAc)2·2H2O, CH2Cl2/MeOH (for c).

a

(5,15-diaryl-10-phenylethynylporphyrinato)zinc(II) complexes 255.1 gave directly linked 12,13-dehydropurpurin−porphyrin dyads 255.2a−b, and no product of meso-coupling was observed (Scheme 255).1386 The direct connection between the two macrocycles led to a strong electronic interaction and caused broadening of absorption bands. Compound 255.2a possessed a split Soret band with maxima at 421 and 471 nm, while a broad Q-like band was observed at 645 nm. In subsequent work, a Pd-catalyzed [3+2] annulation of bromoporphyrin 255.3 with 1,4-diphenylbutadiyne gave alkynyl-7,8-dehydropurpurins 255.4 and 255.5.1387 In the next step, the [3+2] annulation of 255.5 with 255.3 was achieved under similar conditions, providing dimers 255.6 and 255.7. The reaction of 255.3 and 255.4 was also attempted, furnishing 255.8 with a 42% recovery of 255.4. The UV−vis− NIR absorption spectra of 255.6 contained well-defined nearinfrared bands at 939 and 1056 nm, in contrast to 255.7 and 255.8, which have extremely broad bands reaching to ca. 1050 nm. The electrochemical HOMO−LUMO gap of 255.6 was 1.26 eV, distinctly smaller than those of dimers 255.7 and 255.8 (1.52 and 1.62 eV, respectively). Additionally, 255.6 reacted with N-bromosuccinimide to produce β-to-β vinylenebridged porphyrin dimers anti- and syn-255.9.

electronic spectrum of 254.5 is qualitatively similar to that of 254.4, although it reveals a blue shift in the NIR range as well as marked dependence on solvent polarity, possibly associated with a solvent-induced conformational change. In opposition to the optical parameters, the electrochemical HOMO−LUMO gaps were determined to be 0.87 and 0.63 eV for 254.4 and 254.5, respectively. This apparent discrepancy was explained by assuming that the HOMO−LUMO transition in the antiaromatic 254.5 is dipole-forbidden. In subsequent work from the Osuka group, bis-rhodium(I) complexes of 254.4 and 254.5 were reported.1384 The methodology used to make 254.4 was extended to synthesize the hybrid tapes 254.7a−b, consisting of two porphyrin and hexaphyrin moieties.1385 254.7a and 254.7b showed remarkably shifted Q-bands (at 1657 and 1912 nm, respectively), six to seven reversible electrochemical events, and narrow HOMO−LUMO band gaps (0.52 and 0.67 eV for 254.7a and 254.7b, respectively). 7.4. [cd]-Fused Porphyrinoids with 5- and 7-Membered Rings

7.4.1. Dehydropurpurins. The Ag-mediated oxidations of meso,β-free porphyrins generally produce doubly and triply linked porphyrin tapes described above. However, it was found by the Osuka group that an AgPF6-promoted oxidation of 3645


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Scheme 256. Pd-Catalyzed [3+2] Annulation of Porphyrins with Alkynesa

a

Reagents and conditions:1388 (a) (o-Tol)3P (20 mol %), [Pd2(dba)3] (5 mol %), R1CCR1 (1.5 equiv), Cy2NMe, toluene, reflux, 24 h; (b) air and light, 3 h.

In Pd-catalyzed [3+2] alkyne annulations, developed by Osuka, Shinokubo, et al., meso-bromo metalloporphyrins 256.1a−c reacted with a variety of nonterminal alkynes, yielding 7,8-dehydropurpurin derivatives 256.2a−j in very good yields (69−87%, Scheme 256).1388 The reaction proceeded cleanly without byproducts, other than the debrominated porphyrin. Cyclic voltammetry measurements showed a small separation between the first oxidation and the first reduction potential (ΔE = 1.66 V) for 256.2a relative to that of 5,15-bis(3,5-di-tert-butylphenyl)porphyrin nickel(II) (ΔE = 2.29 V). The UV−vis absorption spectra of these dehydropurpurins reached into the near-infrared. Quantitative conversion of compound 256.2h into the meso,β-dibenzoylporphyrin 256.3h was observed after exposure of the solution to air under ambient light. A related annulation, involving the use of silylacetylenes, was later developed by the Osuka group, providing access to cyclopentadiene-unsubstituted dehydropurpurin derivatives.1389 7.4.2. Indeno[1,2,3-cd]porphyrins. As discussed in the preceding section, treatment of metal complexes of 2-formylmeso-tetraarylporphyrins with strong acids results in an intramolecular cyclization leading to naphthoporphyrin derivatives. In 1994, Dolphin et al. observed that in the case of porphyrins bearing electron-donating m-methoxy groups on the meso-aryl substituents, an additional cyclization occurs at the same pyrrole ring, producing cyclopenta-fused products C60.1 and C60.2 (or C60.3) in a nonregioselective manner (Chart 60).1292 Absorption spectra of these fused compounds were measured, and the most red-shifted band was observed at 768 nm for C60.1b. In 2004, Boyle and co-workers reported Pd-catalyzed ring closure reactions of iodinated, meso-arylporphyrins, providing access to indeno-fused porphyrins 257.2a−d (Scheme 257).1390 In these cyclizations, Pd(PPh3)4 and K3PO4 were used, respectively, as the source of palladium and the base. The UV−vis spectra of compounds 257.2a and 257.2b exhibited bathochromically shifted Soret bands (425 and 453 nm, respectively) in comparison to the parent porphyrins 257.1a−b. Under similar conditions, double cyclization on the diphenyl derivative 257.3 was successfully conducted, yielding two isomeric fused porphyrins 257.4 and 257.5. The UV−vis spectrum for the mixture of isomers showed the Soret band at 424 nm. Under the above coupling conditions, analogous bromoporphyrins did not produce annulated products, indicating a requirement for the more reactive iodo species. Successful meso-aryl annulations of β-brominated porphyrins were subsequently reported by Chen and co-workers.1391 Porphyrins 258.2a−d were obtained in moderate yields by

Chart 60. Products of Double Intramolecular Couplings of 2-Formylporphyrinsa

a

Structures C60.2 and C60.3 are proposed as alternatives.1292

treating compound 258.1 with 50 equiv of metallic zinc in DMSO (Scheme 258). However, debromination of the parent porphyrins, producing 258.2a−d, was the dominant reaction in all cases. The annulations could be completely inhibited in the presence of 20 mol % of p-dinitrobenzene or hydroquinone, indicating a radical nature of the process. In the following report from the Chen group, the selectivity of the annulation was considerably improved by using Pd catalysis.1392 A similar approach was also used by Imahori et al. in their work on cyclopenta[cd]porphyrin-based dye-sensitized solar cells.1393 Both the Zn-promoted1391 and the Pd-catalyzed1392 approaches could be applied to the synthesis of doubly fused systems 258.5a and 258.6a, which formed from 258.4a with partial debromination. The quadruply fused target 258.7a was obtained in 2013 by Ishizuka, Kojima, et al., when Pd nanoclusters derived from [Pd(η3-C3H5)Cl]2 were employed as the catalyst.1394 This new procedure enabled the exploration of substituent effects in a range of derivatives 258.7b−f.1395 The lowest energy Q-band was observed for 258.7a below 1000 nm, and the fused porphyrin absorbed efficiently across the entire visible range. In the series 258.2a, 258.6, 258.8, and 258.7a, electrochemically determined HOMO−LUMO gaps became narrower with the increasing number of fused rings. However, the lowering of the LUMO level by ring-fusion was stronger than the correspond3646


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Scheme 257. Single- and Double-Intramolecular Pd(0)-Catalyzed Cyclization of Metalloporphyrins Bearing Iodo Substituentsa

a

Reagents and conditions:1390 (a) Pd(PPh3)4, K3PO4 (10.0 equiv).

Scheme 258. Cyclopenta[cd]fused Porphyrins via Heck-Type Cyclizationsa

Reagents and conditions: (a)1391 Zn (50.0 equiv), DMSO, 85 °C; (b)1392 Pd2(dba)3·CHCl3 (0.05 equiv), K2CO3 (10.0 equiv), DMF, 130 °C; (c)1394 [Pd(η3-C3H5)Cl]2, PPh3, (n-Bu)4N(OAc), K2CO3, 1,4-dioxane.

a

Scheme 259. Synthesis of Indenoporphyrinsa

a Reagents and conditions:1397 (a) Zn, propionic acid, CH3CH2CO2Na, >150 °C; (b) NCS, CHCl3, overnight, rt; (c) (1) p-TSA, AcOH, 2 h, rt, (2) H2, Pd/C, acetone, MeOH, Et3N; (d) TFA, CH(OMe)3, rt to 40 °C; (e) p-TSA, DCM, MeOH, overnight, rt.

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Scheme 260. Surface-Assisted Cyclodehydrogenation of Tetraphenylporphyrinsa

a

Reagents and conditions:1399 (a) annealing, Ag(111) surface.

Scheme 261. Coverage- and Temperature-Dependent Metalation and Dehydrogenation of Tetraphenylporphyrin on Cu(111)a

a

Reagents and conditions:1400 (a) ΔT, Cu(111). Green H atoms are removed in the following dehydrogenation step.

reduced, as judged by the chemical shifts of meso-protons (ca. 9.3 ppm). Indeno-fusion was also achieved by means of surface chemistry. Temperature-induced chemical modification of tetraphenylporphyrin on Ag(111) produced structures with entirely flat conformations in which the meso substituents were coplanar with the macrocycle.1398 The possibility of dehydrogenative coupling of substituents was confirmed by DFT calculations. Room-temperature deposition of either 260.1a or 260.1b molecules onto a Ag(111) substrate led to the formation of extended islands.1399 Annealing the free base 260.1a molecules to temperatures between 530 and 620 K for 10 min induced intramolecular dehydrogenation resulting in planar derivatives 260.2a−5a (Scheme 260). Upon annealing the Ru complex 260.1b to 620 K, the same ring-closing reactions took place; however, the occurrence of the four reaction products is very different for both molecules. It was found that the 2-fold symmetry of the 260.1a core, imposed by the two central hydrogens, drives the selectivity of the reaction at the periphery of the molecule toward 260.2a, as the major product. Such selectivity could not be observed when triggering the same annulation reaction in 260.1b or the corresponding cobalt complex, wherein the macrocycle core has a 4-fold symmetry. Annealing to temperatures above 670 K led to polymerization reactions between molecules by C−H activation and formation of new intermolecular C−C bonds similar to the homocoupling of porphine on Ag(111) (Figure 5, section 7.3).1374 Using temperature-programmed desorption (TPD), three main reactions of 5,10,15,20-tetraphenyl-21H,23H-porphyrin 260.1a with Cu(111), metalation, stepwise partial dehydrogenation to 261.3, and complete dehydrogenation of 261.3, were identified using STM (Scheme 261).1400 At low coverage

ing rise of the HOMO level. The magnetic properties of these cyclopenta-fused porphyrins indicated a contribution of antiaromatic resonance forms resulting from the presence of the additional double bond on the periphery. The zinc complex 258.9, which in contrast to the planar free base has a concave πconjugated surface, was reported by Ishizuka, Kojima, et al. to bind fullerenes C60 and C70.1396 Solid-state structures of these complexes revealed a 2:1 stoichiometry, with two molecules of 258.9 enclosing one fullerene molecule, the latter being fully covered by the concave porphyrin surfaces. In addition, a crystal structure of the 1:1 complex with C60 was obtained, in which the exposed part of the fullerene molecule formed a π−π interaction with C60 in the neighboring complex. In 2011, Lash and co-workers developed an alternative approach to indeno[1,2,3-cd]porphyrins, in which the indene substructure was preassembled at the monopyrrole stage.1397 A Knorr-type reaction of oxime 259.2 with 2-indanone 259.1 and zinc dust in propionic acid gave indenopyrrole 259.3, which was subsequently chlorinated to form 259.4 (Scheme 259). Dipyrrolic intermediates were prepared in a reaction between 259.4 and α-unsubstituted pyrroles 259.5a−b in the presence of p-toluenesulfonic acid, followed by hydrogenolysis of the benzyl ester protective groups. The resulting compound 259.6 was converted into the corresponding dialdehyde 259.7 with TFA−trimethyl orthoformate and then condensed with a dipyrromethane dicarboxylic acid 259.8, providing indenoporphyrin 259.9 in 26% yield. The porphyrin gave a highly modified UV−vis absorption spectrum with three strong bands showing up in the Soret region and a series of bathochromically shifted Q bands that extended beyond 700 nm. The proton NMR spectra of indenoporphyrins showed that the diamagnetic ring current for this porphyrin system was significantly 3648


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(5000 Å3) nanobox structures. Chem. Commun. 2002, No. 21, 2566−2567. (1546) Kalsani, V.; Ammon, H.; Jäckel, F.; Rabe, J. P.; Schmittel, M. Synthesis and Self-Assembly of a Rigid Exotopic Bisphenanthroline Macrocycle: Surface Patterning and a Supramolecular Nanobasket. Chem. - Eur. J. 2004, 10 (21), 5481−5492. (1547) Schmittel, M.; Kalsani, V.; Michel, C.; Mal, P.; Ammon, H.; Jäckel, F.; Rabe, J. P. Towards Nanotubular Structures with Large Voids: Dynamic Heteroleptic Oligophenanthroline Metallonanoscaffolds and their Solution-State Properties. Chem. - Eur. J. 2007, 13 (21), 6223−6237. (1548) Ammann, M.; Rang, A.; Schalley, C. A.; Bäuerle, P. A Synthetic Approach Towards Interlocked π-Conjugated Macrocycles. Eur. J. Org. Chem. 2006, 2006 (8), 1940−1948. (1549) Bäuerle, P.; Ammann, M.; Wilde, M.; Götz, G.; MenaOsteritz, E.; Rang, A.; Schalley, C. A. Oligothiophene-Based Catenanes: Synthesis and Electronic Properties of a Novel Conjugated Topological Structure. Angew. Chem., Int. Ed. 2007, 46 (3), 363−368. (1550) Jiang, H.; Léger, J.-M.; Guionneau, P.; Huc, I. Strained Aromatic Oligoamide Macrocycles as New Molecular Clips. Org. Lett. 2004, 6 (17), 2985−2988. (1551) Shirude, P. S.; Gillies, E. R.; Ladame, S.; Godde, F.; Shin-ya, K.; Huc, I.; Balasubramanian, S. Macrocyclic and Helical Oligoamides as a New Class of G-Quadruplex Ligands. J. Am. Chem. Soc. 2007, 129 (39), 11890−11891. (1552) Katz, J. L.; Geller, B. J.; Foster, P. D. Oxacalixarenes and oxacyclophanes containing 1,8-naphthyridines: a new class of molecular tweezers with concave-surface functionality. Chem. Commun. 2007, No. 10, 1026−1028. (1553) Bhalla, V.; Tejpal, R.; Kumar, M. Terphenyl-phenanthroline conjugate as a Zn2+ sensor: H2PO4− induced tuning of emission wavelength. Dalton Trans. 2012, 41 (34), 10182−10188. (1554) Maruyama, S.; Hokari, H.; Tao, X.; Gunji, A.; Wada, T.; Sasabe, H. Synthesis of Cyclic Oligomer Having a Low Ionization Potential. Chem. Lett. 1999, 28 (8), 731−732. (1555) Maruyama, S.; Hokari, H.; Wada, T.; Sasabe, H. Syntheses of Novel Carbazolylacetylene-Derived Macrocycles. Synthesis 2001, 2001 (12), 1794−1799. (1556) Zhang, W.; Moore, J. S. Arylene Ethynylene Macrocycles Prepared by Precipitation-Driven Alkyne Metathesis. J. Am. Chem. Soc. 2004, 126 (40), 12796−12796. (1557) Gross, D. E.; Moore, J. S. Arylene−Ethynylene Macrocycles via Depolymerization−Macrocyclization. Macromolecules 2011, 44 (10), 3685−3687. (1558) Cho, H. M.; Weissman, H.; Wilson, S. R.; Moore, J. S. A Mo(VI) Alkylidyne Complex with Polyhedral Oligomeric Silsesquiox3714


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Chemical Reviews

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(1614) Zieba, R.; Desroches, C.; Chaput, F.; Sigala, C.; Jeanneau, E.; Parola, S. The first approach to a new family of macrocycles: synthesis and characterization of thiacalix[2]thianthrenes. Tetrahedron Lett. 2007, 48 (31), 5401−5405. (1615) Zieba, R.; Desroches, C.; Jeanneau, E.; Parola, S. Insights into the reactivity of thiacalix[2]thianthrenes: synthesis and structural studies of sulfoxide and sulfone derivatives. Tetrahedron 2007, 63 (44), 10809−10816. (1616) Thabet, W.; Baklouti, L.; Zieba, R.; Parola, S. Cation binding by thiacalixthianthrenes. J. Inclusion Phenom. Mol. Recognit. Chem. 2012, 73 (1−4), 135−139.

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