Annulated Isomeric, Expanded, and Contracted Porphyrins - Chemical

Review pubs.acs.org/CR

Annulated Isomeric, Expanded, and Contracted Porphyrins Tridib Sarma†,§ and Pradeepta K. Panda*,‡ †

Department of Chemistry, The University of Texas at Austin, Austin, Texas 78712-1224, United States School of Chemistry, University of Hyderabad, Hyderabad 500046, India

‡

ABSTRACT: Compared to porphyrin, its isomeric, expanded, and contracted analogues are less well explored. This contrast is found to be even more drastic in the case of their peripherally annulated counterparts. Nevertheless, the chemistry of annulated isomeric, expanded, and contracted porphyrins started flourishing recently with considerable efforts over the past few years, as evidenced by an increasing number of publications. While keeping the essence of porphyrins, these annulated versions exhibit quite unique properties that have no precedence in their nonannulated counterparts. An in-depth update of research carried out so far in this emerging area will be presented in this review.

CONTENTS 1. Introduction 2. Annulated Isomeric Porphyrins 2.1. Porphycenes, [18]Porphyrin(2.0.2.0) 2.1.1. Annulation through β−β′ Pyrrolic Positions 2.1.2. β,β-Pyrrole Fusion 2.1.3. Annulation through the meso Carbon Atoms 2.2. Other Related Fused Porphycenes 3. Annulated Expanded Porphyrins 3.1. Systems Containing Five Pyrroles or Other Heterocyclic Rings 3.1.1. Sapphyrins, [22]Pentaphyrin(l.1.1.1.0) 3.2. Systems Containing Six Pyrroles or Other Heterocyclic Rings 3.2.1. [28]Hexaphyrin(1.1.1.1.1.1) 3.2.2. Rubyrins, [26]Hexaphyrin(1.1.0.1.1.0) 3.2.3. Rosarins, Hexaphyrin(1.0.1.0.1.0) 3.3. Systems Containing Seven, Eight, or Nine Pyrroles or Other Heterocyclic Rings 3.3.1. Heptaphyrins, Octaphyrins, and Nonaphyrins 3.4. Cyclo[n]pyrroles 3.4.1. Cyclo[8]pyrroles, [30]Octaphyrin(0.0.0.0.0.0.0.0) 3.4.2. Cyclo[10]pyrrole 3.5. Other Related Systems 4. Annulated Contracted Porphyrins 4.1. Corroles 4.1.1. Annulation through β,β-Pyrrolic Positions 4.2. Triphyrins 4.2.1. Subporphyrins, [14]Triphyrin(1.1.1) © XXXX American Chemical Society

4.2.2. [14]Triphyrin(2.1.1) 5. Related Porphyrin-like Annulated Compounds 6. Conclusions Author Information Corresponding Author ORCID Present Address Notes Biographies Acknowledgments Abbreviations References

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1. INTRODUCTION Porphyrins are tetrapyrrolic macrocycles connected through four methine bridges, leading to planar aromatic species that contain 18π electrons in their major conjugation pathway. These naturally occurring pigments play major roles in various biological processes (such as in chlorophyll, myoglobin, hemoglobin, etc.) and are hence referred to as the “pigments of life”.1 Therefore, porphyrins remain of fundamental interest to researchers and are arguably the most widely studied of all known macrocyclic systems.2 Annulated porphyrins, wherein additional rings are fused onto the porphyrin periphery, have received much attention recently.3,4 This is because it has been observed that the electronic properties of porphyrins are sensitive to peripheral fusion, which offers scope to tune their properties precisely for a particular application. Annulated porphyrins trace their origin to naturally occurring chlorophylls

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Special Issue: Expanded, Contracted, and Isomeric Porphyrins

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Received: June 28, 2016

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DOI: 10.1021/acs.chemrev.6b00411 Chem. Rev. XXXX, XXX, XXX−XXX

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to annulation with aromatic moieties is typically accompanied by multiple advantages. For instance, besides extending the πconjugation, it also plays an important role in regulating the structures of the resultant porphyrinoids. The properties of porphyrinoids, in particular expanded porphyrins, often depend upon their structures, which tend to adopt nonplanar structures as the ring size increases. Although that leads to an altogether diverse range of compounds with interesting structures and properties, it does not achieve one of the preliminary goals of expanding π-conjugation in porphyrinoids that maintain a flat, rigid porphyrin-like structure. On the other hand, annulation with alicyclic rings may play an important role in maintaining the topology of expanded porphyrins, such as to switch between Hückel and Möbius topologies. However, often this chemistry is much less obvious, and is not a generalized strategy that would lead to a large number of functional molecules. To aid this momentum, in this review we would like to highlight the overall developments in the fields of annulated isomeric, contracted, and expanded porphyrinoids to date. Apart from a tutorial review by Sessler and co-workers in 2013, no other literature is available providing an in-depth summary of the advances in this area of research.16 Annulated porphyrins, including porphyrin sheets and tapes, were recently reviewed by Osuka and co-workers and are not covered in this review.17 As N-fusion in porphyrinoids does not lead to the extension of aromaticity, such compounds are not included in this review. Interested readers may go through recent literature on the topic.18,19 Similarly, N-confused and core-modified porphyrins, including carbaporphyrins, belong to a unique class and are better correlated together; therefore they are not included in this review.20−25 On the other hand, examples of ring annulation in core-modified isomeric, expanded, and contracted porphyrins are few in number, but as they are important milestones in the development of this chemistry, they are included here. Herein, we will be concentrating mostly on the synthesis and the impact of ring annulation on the structures and photophysical properties of these porphyrinoids. In a broader perspective, we divide this review into six sections. Following the general introduction in section 1, we discuss annulation in isomeric porphyrins in section 2. Here, we summarize various annulation strategies in porphycenes as well as the impact of annulations on their properties. Annulated isomeric porphyrins other than porphycenes are yet to be realized. In section 3, annulated expanded porphyrins including all-aza and coremodified porphyrinoids are discussed. Subsequently, our emphasis will be on smaller congeners of porphyrins, i.e. contracted porphyrinoids with ring annulation, in section 4. These include corroles, subporphyrins and triphyrins. In section 5, all related porphyrin-like annulated compounds that incorporate nonpyrrolic precursors such as phenanthrene, phenanthroline, etc. will be discussed. This class of molecules is comparatively less well explored; however, they exhibit interesting aromatic properties. We conclude the review in section 6 and discuss future perspectives.

and bacteriochlorophylls, both being reduced Mg(II)− porphyrin complexes, wherein an additional ring is fused onto the porphyrin periphery. This fusion led to greater rigidity of the resultant porphyrin derivatives, increasing their radiative lifetime and hence their utility in light-harvesting systems.5 The syntheses of chlorins and bacteriochlorins and their derivatives are quite laborious and are beyond the scope of this review.6 However, in synthetic porphyrins, generally ring annulation is introduced with an objective to extend the conjugation via aromatization of the exocyclic ring(s). Extension of πconjugation through additional aromatic rings often leads to bathochromically shifted absorption spectra relative to their nonannulated counterparts.7 Thus, these materials are promising for various applications such as dyestuffs (including NIR dyes), dye-sensitized solar cells, optical materials, nonlinear optics, organic semiconductors, and photosensitizers for photodynamic therapy (PDT).8−12 Annulation in porphyrins is often achieved in two ways: (1) the use of annulated pyrroles as a precursor, and (2) functionalization of porphyrins at their periphery with appropriate precursors/moieties, which upon oxidative cyclization or other specific chemical transformation leads to the desired annulated system. The latter approach to extending the π-conjugation in porphyrins has paved the way for the synthesis of a plethora of remarkable functional materials.7 However, compared to parent porphyrins, their nextgeneration analogues, i.e. expanded, contracted, and isomeric porphyrinoids, are much less well explored.13 While expanded porphyrins can be prepared by addition/rearrangement of meso-methines with or without substituting one or more pyrrole unit in the porphyrin by bipyrrole or other suitable polypyrrolic substrates so that the inner core has more than 16 atoms, contracted analogues are often achieved via removal/rearrangement of meso-methines with or without a pyrrolic unit. On the other hand, isomeric porphyrins with N4 cores are obtained by reshuffling the meso-methine units. In general herein, the pyrrolic unit is freely exchanged with other heteroaromatics such as furan, thiophene, etc. In order to synthesize annulated versions of these porphyrinoids, the availability of suitable novel annulated pyrrolic/bipyrrolic building blocks is essential.14,15 The alternative method, i.e. postsynthetic modification, is rather challenging for these classes of porphyrinoids. This is because, unlike porphyrins, they are difficult to functionalize appropriately; in addition, most often the geometry of their periphery is not suitable to undergo oxidative cyclization with other aromatic moieties. Therefore, ring annulation in contracted, expanded, and isomeric porphyrins is essentially dependent on the development of novel fused pyrrolic/bipyrrolic precursors. This perhaps accounts for comparatively slow progress in this area, even though much research on porphyrins has already been accomplished. Nevertheless, recent efforts from several research groups have led to significant development in this area over the past few years. These outcomes clearly reveal the highly intriguing and unprecedented structures and properties of annulated porphyrinoids that are otherwise not possible in their nonannulated counterparts. In general, annulated porphyrinoids are referred to systems bearing fused alicyclic or/and aromatic rings upon the macrocyclic periphery. A thorough overview of the literature suggests that major efforts have been directed toward the synthesis of porphyrinoids annulated with aromatic moieties, which in turn are often derived from their corresponding annulated alicyclic counterparts. The particular attention paid

2. ANNULATED ISOMERIC PORPHYRINS 2.1. Porphycenes, [18]Porphyrin(2.0.2.0)

Porphycene 1 (Figure 1) is the first constitutional isomer of porphyrin to be reported, and was synthesized by Vogel in 1986.26 It can be formally derived from porphyrin by replacing two alternate meso methine moieties with direct pyrrole− B


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molecules are very similar. This in turn indicates that dibenzoporphycenes are rather floppy with respect to out-ofplane distortions, which probably aids their radiationless deactivation.32 In line with this annulation strategy involving the β−β′ pyrrolic positions, a rational pathway to synthesize dinaphthoporphycene 6a and its complexation was presented by Panda and Sarma at the Sixth International Conference on Porphyrins and Phthalocyanines (ICPP-6).37 Subsequently, the groups of Sessler and Panda independently published syntheses of dinaphthoporphycenes via McMurry coupling of alkylated naphthobipyrrole dialdehydes 5a−5d (Scheme 2).38,39 Dinaphthoporphycenes were found to display unique photophysical (both linear and nonlinear optical properties) and metalation properties. The absorption spectra of 6a−6d exhibited considerable red shifts compared to 1 because of their extended π-conjugation. These porphycenes show a well-defined Soret band near 400 nm and the lowest energy Q-band appears at ∼715 nm, whereas Vogel’s dibenzoporphycenes 4a−4b show a very broad Soret band around 370 nm and two comparatively well separated Q-type absorptions at 588 and 769 nm. Nonlinear optical properties of 6a, 6c, and 6d, and their corresponding Ni(II) complexes 7a and 7b, along with excited state dynamics, were studied in detail. Intensity-dependent 2PA and 3PA were observed for these molecules, an attribute not reported earlier for the porphyrin class of compounds.38,40,41 Despite its rigid and distinct rectangular core, dinaphthoporphycene exhibits unprecedented coordination properties. Initial complexation studies explored by the group of Panda revealed that Ni(II) and Cu(II) complexes 7a−7b and 8, respectively, adopt a rather unusual square-type coordination core (Figure 2b).38,42 Again, Sessler and co-workers reported πmetal complexes of 6d, containing a [MCp*]+ fragment wherein a metal ion M (M = Ru, Rh, or Ir) is sandwiched between a pentamethylcyclopentadiene ligand (Cp*) and the π-face of the annulated aromatic framework of dinaphthoporphycene.43 Interestingly, under these conditions in the presence of the [MCp*]+ fragment, no σ-type complex formation with the macrocyclic N4 core could be detected, thereby leaving the central porphycene core uncoordinated. This behavior is significantly different from that observed for nonannulated alkyl porphycenes.44 The electron-accepting properties were found to be enhanced upon complexation in these πmetalloporphycenes. Very recently, Sarma et al. reported an unprecedented cisbimetallic complex of dinaphthoporphycene 6d, namely [Pd2(μ-6d)(μ-OAc)2] 9 (Figure 3), wherein two palladium atoms coordinate to the core nitrogens of the macrocycle on the same side.45 They are closely held together (Pd−Pd, 2.67 Å) by two bridging acetate ligands, exhibiting significant metal−metal bonding interactions (bond order 0.18 evaluated

Figure 1. Structure of porphycene with atom numbering scheme and its possible annulation positions.

pyrrole linkages and inserting additional ones, each at the remaining two meso positions. The reduced symmetry of porphycene relative to porphyrin causes a bathochromic shift in the absorption spectrum, particularly Q-like bands with more intense lowest energy transitions (λ > 600 nm, ε ∼ 50 000 M−1 cm−1) than those of porphyrin. This in turn stimulated interest among researchers to explore its ability as a potential photosensitizer for photodynamic therapy (PDT) of cancer and photoinactivation of viruses and bacteria.27,28 Ideally there are three likely positions through which annulation can be achieved in porphycenes. They are mainly (1) ring fusion through β−β′-pyrrolic positions or bipyrrolic fusion, (2) β,β-pyrrole fusion, and (3) annulation through the meso carbon atoms. All three types of annulated porphycenes have been experimentally realized. These annulated porphycenes exhibit unique chemistry compared to the previously known porphycenes. 2.1.1. Annulation through β−β′ Pyrrolic Positions. The annulation strategy was first introduced to porphycene chemistry with Vogel’s synthesis of dibenzoporphycene, wherein adjacent β−β′-bipyrrolic positions were connected via ethene bridges, following a classic McMurry coupling of the corresponding benzobipyrrole dialdehydes 2a−2b (Scheme 1).29−31 Similar to that of meso-substituted porphycenes, nonaromatic intermediates 3a−3b were found to be air stable and needed an external oxidant such as DDQ to obtain the final stable aromatic porphycenes 4a−4b. Although details of the synthesis of this annulated porphycene have not been disclosed, its spectroscopic, photophysical, and metal complexation properties have been studied over the years.32−35 Like βoctaalkylporphycenes,36 benzoporphycenes 4a−4b were found to be nonfluorescent in nature.32 Interestingly, their X-ray crystal structures show that the methylated derivative 4a possesses a nonplanar structure, whereas the t-Bu analogue 4b adopts a planar conformation, despite the fact that the two Scheme 1. Synthesis of Dibenzoporphycenes 4a−4b

C


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Scheme 2. Synthesis of Dinaphthoporphycenes 6a−6d and Their Metallo Derivatives 7a, 7b, and 8

Figure 2. (a) Absorption spectra of 6d, 7b, and 8 in CHCl3. (b) X-ray structure of 6d and its Ni (7b) and Cu (8) complexes. Reprinted from ref 38. Copyright 2010 American Chemical Society. Reprinted with permission from ref 42. Copyright 2015 Indian Academy of Sciences.

Figure 3. Synthetic path and molecular structures of 9 and 10. Reprinted with permission from ref 45. Copyright 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Scheme 3. Synthesis of Monobenzo-Fused Porphycene 14

ation with metal ion(s), as noticed from their solid-state structures. 2.1.2. β,β-Pyrrole Fusion. In 1994, Richert et al. reported monobenzo-fused porphycene 14 (Scheme 3). This was of particular interest because of its photosensitizing activity in PDT.46,47 Apparently this was the first report of a porphycene bearing a fused aromatic ring at the β,β-pyrrolic positions, which was achieved via functional group interconversion at a porphycene side chain. Removal of the methyl ether protection of methoxyporphycene 11 using BBr3 in the presence of boric acid provided bromo derivative 12. Furthermore, di-, tri-, or tetrabrominated products could also be isolated depending

by NBO analysis). The importance of this outcome could be gauged from the fact that this type of complexation mode had been neither sought-after (including computational studies) nor proposed in the literature. Interestingly, replacing acetate with acetylacetonate (acac) led to stabilization of an unusual monopalladium complex 10, wherein Pd coordinates unsymmetrically to two ring N’s above the macrocyclic plane, in addition to coordinating with two O’s of the acac ligand. Remarkably, the rigid core of 6d displays enhanced complexation-induced aromaticity (as per NICS and HOMA analysis), despite undergoing severe core deformation during complexD


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Scheme 4. Synthesis of BCOD-Fused Pyrrole 24

Scheme 5. Synthesis of β,β-Annulated Di- (32) and Tetrabenzo (29a−29c) Porphycenes

Figure 4. (a) Absorption spectra of 33, 29a, and 28a. (b) Emission spectra of 33 and 29a in CH2Cl2. Reprinted with permission from ref 54. Copyright 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

extended π-conjugation (Soret, 396 nm; lowest energy Q, 636 nm). BCOD-fused pyrrole derivatives were used as versatile precursors for the synthesis of a wide variety of π-extended porphyrinoids. Originally, BCOD-fused pyrrole derivatives were developed for the synthesis of tetrabenzo porphyrins, which led to a remarkable breakthrough in synthesis of annulated porphyrins, primarily due to its stability and

upon the amount of Lewis acid used. The monobromo derivative 12 was further converted to vinylporphycene 13 upon treatment with DBU. This undergoes Diels−Alder reaction with DMAD to provide the monobenzoporphycene dimethyl ester, which upon selective hydrolysis with LiOH yielded monoacid 14 (Scheme 3). Benzoporphycene 14 showed a red-shifted absorption spectrum owing to its E


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Figure 5. Chemical structures of β,β-annulated naphthoporphycenes 35−37 and precursor 34.

Scheme 6. Synthesis of Dibenzoporphycene 44

solubility in organic solvents.48,49 The initial synthesis of BCOD-fused pyrrole ester 23 reported by Ono and co-workers involved Diels−Alder reaction of 1,3-cyclohexadiene 15 with dienophile 16, followed by a Barton−Zard strategy for pyrrole synthesis with the resultant bicyclic compound 19 (Scheme 4).50 Later, facile syntheses were reported by Okujima and Ono et al. by reacting dienophiles such as 17 and 18 with 15 via the formation of bicyclic compounds 20 and 21.51,52 Upon decarboxylation under basic conditions, the BCOD-fused pyrrole ester 23 furnished pyrrole 24. Subsequently, the BCOD-fused bipyrrole ester 25 was synthesized via Ullmann coupling of the BCOD-fused pyrrole ester 23, as detailed later in Scheme 12.53 Decarboxylation of 25, followed by Vilsmeier−Haack formylation or acylation, yielded bipyrroles 27a−27c, useful for the synthesis of various annulated porphycenes. Yamada, Kobayashi, and co-workers reported the synthesis of di- and tetrabenzo-fused porphycenes that involved the use of BCOD-fused bipyrrole dialdehyde 27a (Scheme 5). McMurry coupling of the dialdehyde 27a provided the corresponding BCOD-fused porphycene 28a, which was subsequently converted to tetrabenzo-fused porphycene 29a under thermal conditions via a retro-Diels−Alder reaction of BCOD-fused bipyrroles.54,55 On the other hand, mixed

McMurry coupling56 of BCOD-fused bipyrrole dialdehyde 27a and alkylated bipyrrole dialdehyde 30 was carried out to generate dibenzoporphycene 32 along with two other possible derivatives. Annulation of the benzene ring causes significant bathochromically shifted absorptions for these tetra- and dibenzoporphycenes (Soret at 434 and 410 nm and lowest energy Q-bands at 670 and 647 nm, respectively) compared to tetrahexylporphycene 33 (Figure 4a), whereas the BCOD-fused porphycenes 28a and 31 exhibit moderately red-shifted absorption bands. Further, both the BCOD-fused porphycenes were found to be nonfluorescent, whereas their corresponding dibenzo- and tetrabenzoporphycenes were found to emit in the NIR region (654, 716 and 673, 752 nm, respectively). In addition, the meso-tetraalkylated versions of 28a and 29a, i.e. dodecasubstituted porphycenes 28b−28c and 29b−29c, were reported recently by the group of Yamada.57 Macrocycles 29b− 29c obtained from the retro-Diels−Alder reaction of corresponding BCOD-fused porphycenes 28b−28c display further red shifts in their absorption spectra (ca. 29b, B-band at ∼450 nm, Q-band at ∼741 nm) compared to 29a and 32. However, insertion of meso-alkyl substituents makes these molecules nonfluorescent, unlike 29a and 32. Interestingly, attempts to synthesize a dodecasubstituted porphycene with allF


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Scheme 7. Synthesis of meso-Benzo-Fused Porphycene 47 and Its Ni(II) Complex 48

Scheme 8. Synthesis of meso-Benzene-Fused Dioxadithiaporphycene 53

Scheme 9. Synthesis of meso-Monobenzoporphycene 59 and meso-Dibenzoporphycene 57a

a

(a) LTMP (lithium tetramethylpiperidide), CuCl2, THF, 65%; (b) LTMP, B(OEt)3, THF, 94%; (c) o-diiodobenzene, Pd(PPh3)4, K2CO3, H2O/ DMF, 82%; (d) POCl3, DMF, 93%; (e) 140 °C; (f) Zn, CuCl, TiCl4, THF, 48%; (g) 180 °C, ethylene glycol, 80%; (h) POCl3, 3dimethylaminoacrolein, CH2Cl2, 97%; (i) Zn, CuCl, TiCl4, THF; (j) p-chloranil, CH2Cl2.

alkyl substituents led to a nonplanar pyrrolocyclophene derivative, whose oxidation to the corresponding porphycene could not be realized. In a continuation of their efforts toward the synthesis of π-extended porphycenes, the group of Yamada utilized fused bipyrrole dialdehyde 34 in the synthesis of tetranaphtho- and dinaphthoporphycenes 35 and 37, respectively, adopting a similar retro-Diels−Alder approach (Figure 5). However, tetranaphthoporphycene 35 suffers from serious solubility problems and could not be characterized properly. Nonetheless, the photophysical properties of these annulated porphycenes were evaluated, along with those of their Zn(II) complexes. For instance 36, owing to its four fused naphthalene rings, shows a large bathochromically shifted absorption spectrum with a Soret band at 476 nm and a lowest energy Q-band at 685 nm, which are 27 and 33 nm red-shifted

compared to the corresponding Zn(II) complex of tetrabenzoporphycene 29a.58 Toward this goal, Jeong et al. recently reported an improved synthesis of BCOD-fused pyrrole 24 that leads to more facile access to tetrabenzoporphyrins than the existing methods (Scheme 6).59 Jux and co-workers recently utilized this method for the synthesis of meso-diphenyldibenzoporphycene 44.60 Unlike the Ullmann coupling method used by Kuzuhara et al. for the synthesis of key BCOD-fused bipyrrolic precursors, the new synthesis involved PIFA coupling and led to bipyrrole 26. However, the α-unsubstituted positions on bipyrrole make 26 sensitive toward aerial oxidation. Mixed McMurry coupling of 41 (obtained via Vilsmeier benzoylation of 26 with 4-tert-butylN,N-diethylbenzamide) with bipyrrole 42 resulted in 43 as a minor product (0.5%). While 2,7,12,17-tetra-n-propylporphycene forms as the major product (22.7%), no trace of G


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Figure 6. (a) UV−vis−NIR absorption spectra for 1 (black), 59 (blue), and 57 (red) recorded in CH2Cl2. Crystal structures of (b) 59 and (c) 57 with key bond lengths. Bottom: side view. Reprinted with permission from ref 65. Copyright 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Figure 7. Chemical structures and absorption (solid line) and emission (dashed line) spectral changes upon reacting 60 with primary and secondary amines. Spectral changes shown upon ring annulation correspond to 61b measured in acetone. This article is licensed under a Creative Commons Attribution 3.0 Unported License. Published by The Royal Society of Chemistry. Reprinted with permission from ref 68. Copyright 2015 Royal Society of Chemistry.

coupling followed by mono-McMurry coupling, unlike the general “2 + 2” coupling of bipyrrole dialdehydes in the final ring closure step (Scheme 8). Thus, diborylation of compound 49 produced 50, which upon Suzuki coupling with 5bromothiophene-2-carboxaldehyde 51 furnished the dialdehyde 52. Intramolecular McMurry coupling of 52 led to the synthesis of desired benzo-fused porphycene 53 in 18% yield. However, chemistry of this type of meso benzo-fused porphycene was not explored in great detail until very recently, probably owing to the associated synthetic difficulties. Very recently in 2015, Hasegawa and Hayashi’s group reported a successful synthetic strategy toward meso-fused diand monobenzoporphycenes (57 and 59, respectively) that involves Suzuki−Miyaura coupling and mono-McMurry coupling in the ring-closing step (Scheme 9).65 The key precursor 55 was synthesized by Suzuki−Miyaura coupling of borylated bipyrrole 54 and o-diiodobenzene. Vilsmeier−Haack reaction of 55 with 3-dimethylaminoacrolein or DMF produced dialdehydes 56 or 58, respectively. McMurry coupling of 56, followed by oxidation with p-chloranil, produced 57, whereas

corresponding tetrabenzoporphycene was observed, presumably due to steric hindrance caused by the meso-phenyl substituents. The absorption spectrum of 44 resembles that of meso-unsubstituted dibenzoporphycene 32, with slightly redshifted Soret and Q-bands (9 and 14 nm, respectively). 2.1.3. Annulation through the meso Carbon Atoms. Continuing their pioneering role in the development of novel porphycene synthesis, the group of Vogel synthesized mesobenzo-fused porphycene 47 employing a McMurry coupling of monovinylogous diformyl bipyrrole 45 that proceeded via formation of macrocycle 46 (Scheme 7).61,62 Fused porphycene 47 shows a large red-shifted Q-band at 762 nm (in toluene); however, no emission was observed.63 The corresponding Ni(II) complex 48 displayed a further red shift in absorption (Soret, 406 nm; Q, 817 nm, measured in CH2Cl2), which was also supported by a smaller HOMO−LUMO gap (ΔE1/2 = 1.64 V) compared to other Ni(II) complexes of alkylated porphycenes and Ni(II) dibenzoporphycene 4b.35 In 2004, Dai and co-workers disclosed the synthesis of the meso-benzo-fused dioxadithiaporphycene 53.64 The synthesis of this product is unique in the sense that it involves Suzuki H


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Figure 8. Chemical structures and UV−vis spectra of benzochloracene 62a−62b and benzochlorin 63. Reprinted with permission from ref 70. Copyright 2001 John Wiley & Sons, Ltd.

Scheme 10. Synthesis of Annulated Sapphyrins 66a−66b

dialdehyde 58 underwent McMurry coupling to yield 59 directly via aerial oxidation. The effect of extended π-conjugation could be vividly noticed in the absorption spectra of the products. For instance, the lowest energy Q-bands of mono- and dibenzoporphycenes 59 and 57 were bathochromically shifted by 92 and 418 nm, respectively, in comparison to unsubstituted porphycene 1 (Figure 6a). Especially, Q-band absorptions of 57 were located in the NIR region at 648, 884, and 1047 nm. Strikingly, mesodibenzoporphycene 57 was found to exist in a cis-tautomeric form supported by XPS and theoretical studies. This phenomenon is very rare in porphycene chemistry.66,67 The X-ray crystal structures of both compounds exhibited a planar arrangement. The different bond lengths of the fused benzene ring suggested electronic involvement of the benzene ring with the porphycene macrocycle upon ring fusion (Figure 6b,c). Nonell and co-workers recently showed that porphycene isothiocyanate 60 reacts with primary and secondary amines, thereby producing thiazolo[4,5-c]porphycenes 61a−61g (Figure 7). These new annulated porphycenes were found to exhibit emission in the near-IR region (750−775 nm) with high fluorescence and singlet oxygen quantum yield. Interestingly, this annulation leads to significant red shifts (>70 nm) in the

emission spectra of these porphycenes compared to their corresponding isothiocyanate precursor. These properties make them promising as theranostic agents. Their potential applicability in biological systems was demonstrated by reacting porphycene isothiocyanate with bovine serum albumin (BSA) and amino-functionalized gold nanoclusters (AuNCs).68 2.2. Other Related Fused Porphycenes

Benzochloracene 62a, a porphycene analogue of benzochlorin,69 was reported by Robinson and co-workers in 2001.70 However, unlike benzochlorin, reduction of the porphycene macrocycle did not result in red-shifted Q-band absorption (Figure 8). This macrocycle was found to possess perturbed absorption spectral features with split Soret bands (394, 407, 429) and a lowest-energy Q-band at 629 nm. Benzochloracene 62a does not show any emission; however, its Zn complex 62b emits with a fluorescence quantum yield (ϕf) of 3.67% relative to TPP.71 This indicates that this mode of fusion in porphycene possibly led to deactivation of its singlet excited state. I


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Figure 9. UV−vis absorption spectra of sapphyrins 66a−66b in 1% TEA−CHCl3. Reprinted from ref 81. Copyright 2004 American Chemical Society.

Scheme 11. Synthesis of Linearly Annulated Benzosapphyrin 72

3.1.1.1. Sapphyrins with Annulated Pyrroles or Other Heterocyclic Rings. The first examples of π-extension in sapphyrin, namely phenanthrosapphyrin 66a and acenaphthosapphyrin 66b, were reported by Richter and Lash in 1998 by following a TFA-mediated [4 + 1] condensation of the corresponding fused pyrrole dialdehydes 65a−65b and tetrapyrrole 64, followed by oxidation with DDQ (Scheme 10).79 This strategy was also applied to the synthesis of carbasapphyrins such as 67. The same group later reported an improved synthesis of annulated sapphyrins by the use of aqueous FeCl3 as oxidant during the aromatization step of the [4 + 1] synthesis.80,81 The UV−vis spectrum of free base phenanthrosapphyrin 66a exhibits a Soret band at 473 nm, whereas that of acenaphthosapphyrin 66b displays further a red-shifted Soret band at 500 nm (Figure 9). Nonetheless, both compounds exhibit substantially red-shifted absorption spectra compared to the nonannulated β-alkylated sapphyrins.74 In 2004, Ono and co-workers reported the synthesis of benzo-fused sapphyrin 72, which involves their retro-Diels− Alder approach utilized for synthesizing various π-extended porphyrinoids.53 Condensation of bipyrrole dialdehyde 70 with

3. ANNULATED EXPANDED PORPHYRINS 3.1. Systems Containing Five Pyrroles or Other Heterocyclic Rings

Although several pentapyrrolic porphyrin systems have been synthesized thus far,72 ring annulated systems could only be realized in the case of sapphyrins, which incidentally happen to be the first expanded porphyrins reported in the literature.73,74 3.1.1. Sapphyrins, [22]Pentaphyrin(l.1.1.1.0). Sapphyrin is the first and one of the most widely studied expanded porphyrins so far. Owing to an increased number of pyrroles, sapphyrin possesses 22 π-electrons in its shortest conjugation pathway compared to the 18 π-electrons of porphyrin, and is aromatic in nature. Initial efforts in this area were mostly related to the development of efficient synthetic routes and the anionbinding abilities of these macrocycles.75,76 As the area of expanded porphyrins has evolved tremendously in the past two decades, concomitantly sapphyrins have also been investigated with respect to their potential applications as functional materials, as receptors for neutral molecules toward drug delivery, and as photosensitizers in photodynamic therapy.77,78 J


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Scheme 12. Synthesis of Linearly Annulated Dibenzosapphyrin 78

Scheme 13. Synthesis of Linearly Annulated Trithiadibenzosapphyrin 84 and Trithiapentabenzosapphyrin 85

BCOD-fused tripyrromethane diacid 69 led to the synthesis of mono-BCOD-fused sapphyrin 71, which was subsequently converted to 72 by heating at high temperature (200 °C) (Scheme 11). On the other hand, dibenzosapphyrin 78 was synthesized by condensation of BCOD-fused bipyrrole diacid 75 with tripyrromethane dialdehyde 76, followed by retro Diels− Alder reaction of the resulting macrocycle (Scheme 12). Synthesis of the precursor 75 involved Ullmann coupling of NBoc protected BCOD-fused iodopyrrole ester 74. In comparison to their BCOD-fused analogues (71, 77), the benzo-fused sapphyrins 72 and 78 show red shifts of 12 and 22 nm, respectively, in their corresponding absorption maxima. Compared to monobenzosapphyrin 72, dibenzosapphyrin 78 shows a red-shifted Soret band (465 nm for 72 and 476 nm for

78), while the lowest energy Q-band is significantly blue-shifted (683 nm for 77 and 640 nm for 78). In a continuation of their work, Okujima and Uno reported BCOD-fused trithiasapphyrin 83, following [3 + 2] condensation between diol 81 and thiatripyrrane diacid 82, in the presence of TFA. BCOD-fused trithiasapphyrin 83 could be selectively converted to trithiadibenzosapphyrin 84 or trithiapentabenzosapphyrin 85, depending upon the temperature used during the thermal conversion processes (Scheme 13).82 Significant red shifts in the absorption spectrum of trithiasapphyrin 85 were observed. For instance, the Soret band was observed at 551 nm and Q-bands appeared at 710, 766, 810, and 915 nm (Figure 10). The X-ray crystal structure analysis, UV, and theoretical calculations suggest that these macrocycles possess another 22π-electron conjugation pathway involving the inner 19 atoms, which includes all five K


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Figure 10. UV−vis−NIR absorption spectra of sapphyrins 83 (solid line), 84 (broken line), and 85 (bold line). Adapted with permission from ref 82. Copyright 2010 Elsevier.

Scheme 14. Synthesis of Benzosapphyrin 91

Scheme 15. Synthesis of Dioxabenzosapphyrin 96

heteroatoms.82 Another way to achieve peripheral annulation in sapphyrin was demonstrated by Sessler, Cavaleiro, and coworkers via Diels−Alder reaction of β-free sapphyrin with

pentacene, which took place at one of the pyrrole rings of the sapphyrin bipyrrolic unit. Nevertheless, this type of adduct can not lead to an extension in π-conjugation.83 L


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Figure 11. (a) UV−vis absorption spectra of dioxabenzosapphyrin 96 (black line) and 96·2TFA (green line) in CH2Cl2. Plots of change in absorption of 96·(HClO4)2 (b) at 466 nm vs concentration of phenol in CH2Cl2 (3.24 × 10−6 M) and (c) at 455 nm vs concentration of 4nitrophenol (4.86 × 10−6 M). Both plots are overlaid by the calculated 1:1 binding profile (as derived for use with a UV−vis titration). Reprinted from ref 86. Copyright 2008 American Chemical Society.

Scheme 16. Synthesis of Naphthosapphyrin 98 and Naphthorubyrin 99

Scheme 17. Protonation-Induced Ring Inversion in 98

3.1.1.2. Sapphyrins with Fused Bipyrroles or Their Heteroanalogues. In 2005, Lee and co-workers reported benzodipyrrole84-derived sapphyrin 91, wherein benzodipyrrole dialdehyde 89 was condensed with a tripyrromethane diacid 90 in a [3 + 2] MacDonald-type approach (Scheme 14).85 Initial formylation of benzodipyrrole 86 under Vilsmeier−Haack conditions produced compound 87, which upon reduction with LiAlH4 produced 3,6-dimethylbenzodipyrrole 88. Further, formylation of 88 led to the formation of the key precursor benzodipyrrole dialdehyde 89. The absorption spectrum of benzosapphyrin 91 was found to be red-shifted compared to decaalkylsapphyrin as a result of its extended π-conjugation. The Soret-type bands appeared at 466 and 469 nm in its free base and diprotonated forms, respectively.

Dioxabenzosapphyrin 96, an oxa analogue of benzosapphyrin 91, was also reported by Lee and Sessler following a similar approach employed in the synthesis of 91 (Scheme 15).86 Condensation of dialdehyde 94 with tripyrrane diacid 95 in the presence of TFA afforded the desired dioxabenzosapphyrin 96. Dialdehyde 94 in turn was synthesized from diester 92, following reduction of the ester groups to alcohols and subsequent oxidation of the resultant diol 93 with MnO2. The absorption spectrum of free base 96 was found to be very broad compared to 91 or decaalkylsapphyrin.74 Absorption maxima lie below 400 nm, along with very broad Q-bands centered at 650 and 750 nm. However, upon diprotonation, an intense Soret-type band appeared at 455 nm along with Qbands that were resolved to some extent compared to its free base form (Figure 11a). These authors also tested the anion M


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Figure 12. (a) 1H NMR spectral changes in 98 with addition of trifluoroacetic acid (TFA) in CDCl3. Proton-induced changes in the chemical shifts of the CH (Hb) and NH (Ha) protons of the pyrrole subunit undergoing inversion are shown by arrows. (b) UV−vis spectral changes of naphthosapphyrin 98 recorded in the absence and presence of increasing amounts of trifluoroacetic acid in CH2Cl2 ([98] = 8.46 × 10−6 M). Reprinted with permission from ref 87. Copyright 2011 Royal Society of Chemistry.

ring fusion led to marginal core expansion, this was not sufficient to affect pyrrole ring inversion in 101b. The effect of ring fusion on the structural and photophysical properties, including the NLO properties, of the resultant naphthosapphyrins and their various salts (obtained by washing with aqueous solution of different acids) were discussed.91 Similar to the meso-tetraaryl analogue 98, the absorption spectra of free bases 101a−101b also showed split Soret bands. For 101a, the absorption maxima was centered around 488 nm with a shoulder at 468 nm, whereas the lowest-energy Q-like transitions appeared at 765 nm. Further, in comparison to benzosapphyrin 91, the Soret and lowest-energy Q-bands exhibited red shifts of 22 and 37 nm, respectively. Interestingly, free base naphthosapphyrin 101b forms a unique sandwiched supramolecular aquo-bridged dimer in its solid state with strong π−π stacking between the two monomeric sapphyrin units (interplanar distance of approximately 3.48 Å) (Figure 13). Lee and co-workers recently reported core-modified naphthosapphyrins 105 and 106 obtained from the reaction of naphthobipyrrole dialdehyde 102 with core-modified tripyrranes 103 or 104, respectively (Scheme 19). Monothianaphthosapphyrin 105 was found to be planar in its neutral and protonated forms, whereas monooxanaphthosapphyrin 106 contains an inverted furan ring in both its neutral and protonated forms.92 The absorption spectrum of monothianaphthosapphyrin 105 showed split Soret-like intense bands at 484 and 504 nm along with a series of Q-bands in the range 570−800 nm. Monooxanaphthosapphyrin 106 displayed a similar absorption pattern with slightly red-shifted absorption bands. Intensities of the Soret-like bands in 106 were found to be lower than those of the thia analogue 105 (Figure 14). The fused precursor dithienothiophene (DTT) is known for its utility in the synthesis of various optoelectronic materials.93 Chandrashekar and co-workers have recently utilized DTT as a precursor for the synthesis of a variety of core-modified expanded porphyrins such as sapphyrins, rubyrins, heptaphyrins, octaphyrins, and nonaphyrins. The electron-rich character and structural rigidity of the DTT moiety lends attractive structural and electronic features to these core-modified porphyrinoids. Thereby, the syntheses of fused core-modified

binding ability of dioxabenzosapphyrin as well as benzosapphyrin. The study revealed that 96 in its diprotonated form binds only weakly with fluoride and chloride ions compared to diprotonated 91, presumably because of replacement of two N’s by O’s. However, diprotonated 96 binds with neutral phenolic derivatives in contrast to diprotonated 91 and decaalkylsapphyrin (Figure 11b,c).86 Recently, the group of Lee in collaboration with those of Sessler, Kim, and Panda reported a synthesis of mesotetraarylnaphthosapphyrin 98 derived from unsubstituted naphthobipyrrole 97.87 Reaction of naphthobipyrrole 9788 with pentafluorobenzaldehyde and pyrrole in the presence of TFA yields naphthosapphyrin 98, albeit in low yield, along with the formation of naphthorubyrin 99 (Scheme 16). Like meso-tetraaryl sapphyrin, naphthosapphyrin 98 exists in an inverted conformation in its free base form.89,90 However, monoprotonation causes a ring flip, producing an all N in planar conformation, which undergoes a second ring flip in its diprotonated form (Scheme 17). These conformational changes could be monitored by NMR and UV−vis spectroscopies and excited-state lifetime studies (Figure 12). In a continuation of these efforts, recently the group of Panda reported the rational synthesis of meso-free naphthosapphyrins 101a and 101b involving alkylated naphthobipyrrole 5d as a fused pyrrolic precursor, as depicted in Scheme 18.91 Although Scheme 18. Synthesis of meso-Free Naphthosapphyrins 101a−101b

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Figure 13. Molecular structure of a supramolecular dimer of free base 101b showing the interplanar distance between the two monomers. Reprinted with permission from ref 91. Copyright 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Scheme 19. Synthesis of meso-Free Core-Modified Naphthosapphyrins 105 and 106

enhanced ε-values. Q-like bands appeared in the range 500− 900 nm (Figure 15). While 109a displays red shifts of both the Soret (29 nm) and Q-like bands (43 nm) compared to that of Lee’s benzosapphyrin 91, on the other hand it undergoes a small blue shift of the Soret band (12 nm) and a red shift of the Q-band (39 nm), with a substantial decrease in their ε-values, as compared to nonfused sapphyrin 110a.95

sapphyrins containing a DTT moiety were reported recently by the Chandrashekar group via an acid-catalyzed condensation reaction of DTT-diol 107 and core-modified (S or Se) tripyrranes 108a−108b (Scheme 20a).94 In the case of sapphyrin 109a, the thiophene ring was found to be inverted, whereas selenophene derivative 109b exhibits structural diversity and exists in both normal and inverted forms in the free base. Upon protonation, 109b solely displays the inverted form. Surprisingly, fusion in 109a leads to reduced aromaticity in comparison to its nonfused analogue 110a.95 The absorption spectra of 109a−109b show intense Soretlike bands at 495 and 502 nm, respectively, which upon protonation undergo substantial red shifts accompanied by

3.2. Systems Containing Six Pyrroles or Other Heterocyclic Rings

3.2.1. [28]Hexaphyrin(1.1.1.1.1.1). The first example of a β-benzo-expanded porphyrin, namely doubly-N-fused βbenzo[28]hexaphyrin(1.1.1.1.1.1) 111, was synthesized by Osuka and co-workers,96 utilizing 4,7-dihydro-4,7-ethano-2HO


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Figure 14. UV−vis absorption spectra of monothianaphthosapphyrin 105 (solid line) and monooxanaphthosapphyrin 106 (dashed line) recorded in CH2Cl2 (2.4 × 10−6 M). Inset shows Q-band regions measured at higher concentration (2.4 × 10−5 M). Adapted with permission from ref 92. Copyright 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Scheme 20. (a) Synthesis of Core-Modified DTT-Fused Sapphyrins 109a−109b, and (b) Chemical Structures of Nonannulated Analogues 110a−110b

isoindole 24 (Scheme 21). Interestingly, condensation of isoindole 24 with pentafluorobenzaldehyde leads to different porphyrinoids (as judged by the MALDI mass spectrum of the reaction mixture), including BCOD-fused hexaphyrin, which under retro-Diels−Alder conditions yield a highly ruffled nonaromatic N-fused β-benzo[28]hexaphyrin(1.1.1.1.1.1) 111 in 47% yield (Figure 16). 1H NMR studies revealed the nonaromatic nature of this molecule. Nonannulated hexaphyrin under identical conditions gives only a trace amount of the corresponding N-fused hexaphyrin 112. DDQ oxidation of 111 leads to an unusual rearranged product 113. Interestingly, 113 exhibits solvent-dependent emission in the NIR region (718 nm in CH2Cl2), the intensity of which decreases with solvent polarity, while the parent molecule 111 was virtually nonfluorescent. BCOD-fused pyrrole 24 was recently used for the synthesis of opp-dibenzohexaphyrin(1.1.1.1.1.1) via condensation with dipyrromethanedicarbinol 114 (Scheme 22).97 The resultant BCOD-fused [26]hexaphyrin 115 displays a rigid rectangular structure, irrespective of solvent polarity. However, its reduced congener 116 and dibenzohexaphyrin 117 (obtained via retro-

Figure 15. UV−vis absorption spectra of 109a−109b along with its protonated salts in CH2Cl2. Reprinted from ref 94. Copyright 2014 American Chemical Society.

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Scheme 21. Synthesis of β-Benzo[28]hexaphyrin(1.1.1.1.1.1) 111

Figure 16. X-ray crystal structure of benzohexaphyrin 111. (a) Top and (b) side views. Reprinted with permission from ref 96. Copyright 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Scheme 22. Synthesis of opp-Dibenzohexaphyrins 117 and 118

Q


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Figure 17. UV−vis−NIR absorption spectra of fused hexaphyrins (a) 116 and (b) 117 in acetone (solid line) and toluene (dotted line) (∗, solvent peaks). Reprinted with permission from ref 97. Copyright 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Scheme 23. Synthesis of Naphthalene- (121−122) and Anthracene-Fused (124) Hexaphyrins

In another report, the group of Osuka demonstrated the first cycloaddition reaction at the peripheral double bond(s) of an expanded porphyrin, leading to peripherally annulated expanded porphyrins. Thus, [26]hexaphyrin(1.1.1.1.1.1) 119 was subjected to a Diels−Alder reaction with benzosultine 120, followed by DDQ oxidation, producing naphthohexaphyrin 121 (55%) and bisnaphthohexaphyrin 122 (4.6%) (Scheme 23).98 On the other hand, use of benzodisultine 123 as a bis-oxylylene equivalent provided the anthracene-bridged doubledecker hexaphyrin 124 along with naphthosulfolene-fused hexaphyrin 125 in 20 and 24% yields, respectively (Scheme 23).98 The π-extended hexaphyrins 121 and 122 show broad and red-shifted Soret bands (565 and 589 nm, respectively) compared to the precursor 123, owing to naphthalene fusion. In contrast, dimeric structure 124 displays a relatively broader absorption profile with a slightly blue-shifted Soret band (545 nm) with respect to that of 119, which was attributed to exciton coupling between the face-to-face oriented hexaphyrins (Figure 18).

Diels−Alder reaction of 116), possessing 28 π electrons, display solvent-dependent conformational changes such as a figureeight-type motif in nonpolar solvents and aromatic Möbius topology in polar solvents (Figure 17). Unlike [26]hexaphyrin 115, benzo derivative 118 did not exist as a single rectangular isomer, but as a mixture of conformers with a predominant rectangular conformer. The synthesis of 118 via a retro-Diels−Alder reaction of 115 was complicated by incomplete conversion followed by a challenging purification procedure owing to the poor solubility of 118. However, 118 was obtained in its pure form by MnO2 oxidation of the corresponding reduced derivative 117. The UV−vis absorption spectrum of [26]dibenzohexaphyrin 118 shows a broad Soret-type band at 584 nm (578 nm for BCODfused derivative 115). The dirhodium(I) complexes of 117 and 118 exhibited antiaromatic and aromatic character, respectively. Notably, the absorption spectra of these complexes were found to be red-shifted compared to the corresponding metal complexes of nonannulated hexaphyrins. R


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Figure 18. Comparison of UV−vis−NIR absorption spectra of (a) 119 (black line), 121 (dotted line), 122 (dashed line) and (b) 119 (black line), 124 (dotted line). Reprinted with permission from ref 98. Copyright 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Figure 19. X-ray crystal structures of (a) 124 and (b) 125. meso-Pentafluorophenyl groups are omitted for clarity. Reprinted with permission from ref 98. Copyright 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

The X-ray crystal structure of 124 exhibits a zigzag structure where the bridging anthracene unit makes a dihedral angle of 42.7° with the two hexaphyrin units (Figure 19). The interplanar distance between the two hexaphyrin units was determined to be 6.34 Å. The aromatic character was preserved in this annulated hexaphyrins, similarly to its precursor 119. In 2007, Suzuki and Osuka reported intramolecular [3 + 2] annulation between the meso aryl and ethynyl groups of [26]hexaphyrin(1.1.1.1.1.1) 126a−126e by taking the advantage of dynamic conformational changes in these molecules, which exist in rectangular or all N in spectacle-shaped conformations (Scheme 24).99,100 This annulation resulted in disruption of the cyclic conjugation pathway because of the formation of a spiro sp3 carbon center, as revealed by their solid-state X-ray structures. Interestingly, indenylene-bridged macrocycles 127a−127e behave as twin tripyrrolic monoanionic and dianionic ligands. Similar annulated products were also obtained with mesothienyl and -naphthyl precursors. Their usefulness as preorganized ligands was demonstrated by synthesizing a mixed-valence Cu complex such as 128a and Zn(II) complexes 129a and 130 (Figure 20).

Peripheral modification of [26]hexaphyrin(1.1.1.1.1.1) 119 to its benzopyran-fused [28]hexaphyrin(1.1.1.1.1.1) 131 was achieved via heating in acetic acid at 130 °C, in 27% isolated yield, along with the corresponding two-electron reduced hexaphyrin 132 (Scheme 25).101 Notably, annulation via fusion of benzopyran ring induces a twisted Möbius topology and hence Möbius aromaticity in 131 over a wide range of temperatures. This was further confirmed by 1H NMR spectroscopy as well as X-ray crystal structure analysis of 131 (Figure 21a). Interestingly, electrochemical analysis of 131 showed very low oxidation potentials, including negative oxidation potential (−0.04 and 0.22 V vs the ferrocene/ferrocenium ion couple in CH2Cl2). However, chemical oxidation of 131 with DDQ resulted in planar fused [26]hexaphyrin derivative 133a, which exhibited Hückel aromaticity. Further, 133a slowly isomerized to 133b under ambient conditions to exist in a 4:1 mixture of 133a and 133b. Treating 133a−133b with NaBH4 gave 131 in quantitative yield, exhibiting a topological switch via interconversion between Mö bius and Hü ckel aromaticity through twoelectron-oxidation and -reduction processes. S


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Scheme 24. Synthesis of 1,3-Indenylene-Bridged Macrocycles 127a−127e via Intramolecular [3 + 2] Annulation of 5-Aryl-20ethynyl-Substituted [26]Hexaphyrins 126a−126e

Figure 20. X-ray crystal structures of 128 and 130. The meso aryl groups and hydrogen atoms are omitted for clarity. Reprinted with permission from ref 99. Copyright 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Scheme 25. Synthesis of Benzopyrane-Fused [28]Hexaphyrin(1.1.1.1.1.1) 131

T


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Figure 21. (a) X-ray crystal structure of 131. The meso-aryl groups have been omitted for clarity. (b) UV−vis−NIR absorption (black) and fluorescence (red) spectra of 131 in THF. Reprinted from ref 101. Copyright 2009 American Chemical Society.

Scheme 26. Synthesis of Thienyl-Fused Hexaphyrins 136a and 138

Figure 22. Conformational dynamics of 138.102

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Scheme 27. Synthesis of Bis-anthracene-Fused Hexaphyrin 143

2H2O gave bis-Au(III) complex 142.106 142 underwent oxidative fusion to yield the doubly anthracene-fused hexaphyrin 143. Notably, hexaphyrins without any electron-donating group on the anthracene moiety did not undergo this type of oxidative fusion. This was also observed for anthracene-fused porphyrins reported earlier by Anderson and co-workers.107−109 Owing to its elongated π-conjugation, 143 exhibits a markedly red-shifted absorption spectrum and two-photon absorption cross section (σ(2) value of 7600 GM; photoexcitation at 1700 nm). Of particular interest is its intense absorption in the NIR region (1467 nm) with a high extinction coefficient (ε [M−1 cm−1] = 108 500) (Figure 23). The X-ray crystal structure of hexaphyrin 143 was found to display a nearplanar structure with substantially elongated and rectangular πconjugation (Figure 24). Sessler and co-workers recently reported the first TTFannulated expanded porphyrin, namely meso-(pentafluorophenyl)-substituted [28]hexaphyrin(1.1.1.1.1.1) 144. This electron-

The steady-state absorption spectrum of 131 displayed attributes comparable to that of an aromatic expanded porphyrin with an intense, sharp B-type band at 613 nm and distinct Q-type bands at 760, 844, and 968 nm, while exhibiting a fluorescence emission band at 1058 nm (Figure 21b). In another similar approach, upon heating in toluene meso5,20-bis(3-thienyl)-substituted [26]hexaphyrin 134 produced doubly spiroannulated product 135 and thienyl-fused [28]hexaphyrin 136a (Scheme 26). The latter exhibits distinct Möbius aromaticity.102 Interestingly, 136a exists in a thermal equilibrium with its Hückel antiaromatic counterpart 136b, in a 10:1 ratio at room temperature, as revealed by NMR and femtosecond transient absorption spectroscopies. In order to suppress the formation of nonaromatic spiro-type product 135, one of the thienyl groups was replaced with pentafluorobenzaldehyde, i.e. 137, which upon heating resulted in the formation of 138 that showed stable Möbius aromaticity at room temperature. This was further confirmed by the diatropic ring current of 138, along with a large HOMA value, a large negative NICS value, and a large TPA cross-section value, even at room temperature. Interestingly, 138 exists as a racemic mixture at room temperature. Temperature-dependent conformational dynamics were observed in the mixture. Slower exchanges were observed between the two enantiomers upon lowering the temperature (ca. 223 K), whereas increasing the temperature led to a faster racemization, in addition to a contribution from a higher-energy antiaromatic isomer (Figure 22). Peripheral annulation in hexaphyrin allowed control of its dynamic conformational features by rigidifying its structure in a particular conformation, thereby controlling its aromaticity properties; however, that leading to a rigid planar π-extended structure is rare. Separately, hexaphyrin−porphyrin hybrid tapes and arrays have been extensively studied by the group of Osuka.103−105 In relation to these studies, the Osuka group recently reported an anthracene-fused hexaphyrin 143 (Scheme 27). Condensation of tripyrrane 139 and anthracene aldehyde 140 in the presence of methanesulfonic acid afforded hexaphyrin 141, which upon metalation with Na[AuCl4]·

Figure 23. UV−vis−NIR absorption spectra of 142 and 143 recorded in CH2Cl2. Reprinted with permission from ref 106. Copyright 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. V


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145a−145c under modified Lindsey conditions (Scheme 28).111,112 Annulated rubyrin 147 displayed significant red-shifted absorptions (Soret, 596 nm; lowest energy Q, 1076 nm) and emission (1118 nm) because of its extended π-conjugation. The X-ray crystal structure of 147 reveals a bowl-shaped geometry that arises from steric congestion between the mesoaryl substituents and the phenanthrene rings. This in turn reduces self-aggregation in this molecule. Although 147 presents a dominant 26π conjugation pathway, its spectroscopic and redox behaviors suggest the possible existence of an additional 52π cross-conjugated pathway owing to fusion of the phenanthrene rings. Interestingly, rubyrin 147 could be utilized for selective detection of Hg2+ ion in aqueous solution, by embedding into a polyurethane membrane.111 Rubyrin 148a, containing a biselenophene unit, acts as a pHactivatable NIR-photosensitizing agent together with a tumorselective nanoparticle folate (FA). In addition, it possesses absorption and emission in the NIR region (Figure 26). The presence of Se enhances ISC due to the heavy atom effect, thereby increasing its singlet oxygen generation efficiency. Further, functionalization at the para-positions of the meso-aryl substituents with N(Me)2 moieties provides control over singlet oxygen generation upon irradiation, depending on the pH of the medium. Thus, NIR irradiation of 148a (at 635 nm) under acidic conditions results in a high singlet oxygen quantum yield (ΦΔ) (pH 5.0, ΦΔ = 0.69), which was quenched drastically under physiological pH (pH 7.4, ΦΔ = 0.06) (Figure 26). Analogous derivative 148b did not exhibit such attributes, thereby confirming the role of the NMe2 groups in this process. Further, nanoparticles containing 148a exhibited high efficiency for in vivo PDT treatment with no observable side effects (Figure 27).112 Continuing the application of TTF-annulated pyrrole for the synthesis of expanded porphyrins, very recently the group of Lee reported core-modified rubyrin 149 and porphyrin analogue 150. These macrocycles showed intramolecular charge transfer from the TTF moiety to the macrocyclic core as studied by spectroscopic and electrochemical measurements.113 As expected, protonation enhances the electronaccepting capacity of the macrocyclic core from the TTF moiety, leading to internal charge transfer that occurs at room temperature, which was further confirmed by EPR spectroscopy (Figure 28). The dithienylethene (DTE) moiety 151, which is known to exhibit switchable properties upon external stimulation, has

Figure 24. X-ray crystal structure of 143. (a) Top view. (b) side view. Solvent molecules and pentafluorophenyl groups are omitted for clarity. Reprinted with permission from ref 106. Copyright 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

rich expanded porphyrin showed solvent-dependent conformational change, which regulates its physical properties (Figure 25).110 The X-ray crystal structure analysis of 144 showed that this molecule adopts a figure-eight-type structure, as usually observed for β-tetraphenyl-substituted [28]hexaphyrins. The figure-eight structure was also seen to be predominant in CH2Cl2 solution. The 1H NMR spectrum of 144H recorded in CD2Cl2 shows weak antiaromatic character in accordance with its 28π electron Hückel pathway. However, changing solvent polarity (e.g., with the use of acetonitrile) or lowering the temperature induced a structural change that leads to a Möbius topology, displaying aromaticity as in 144M. This conformational change was also accompanied by intramolecular charge transfer in the case of the latter having Möbius conformation, and this observation was further supported by theoretical calculations as well as femtosecond transient absorption studies. 3.2.2. Rubyrins, [26]Hexaphyrin(1.1.0.1.1.0). Shen and co-workers synthesized a core-modified, expanded porphyrin with polycyclic aromatic units, representing the first annulated rubyrin. Thus, core-modified rubyrins 147 and 148a−148b were synthesized from a reaction between phenanthreneannulated pyrrole 146a and bithiophene/biselenophene diols

Figure 25. (a) Reversible interconversion of hexaphyrin 144, between the limiting figure eight (144H) and twisted conformers (144M). (b) UV− vis−NIR absorption spectra of 144 in CH2Cl2 (black) and CH3CN (red). Reprinted with permission from ref 110. Copyright 2013 The Royal Society of Chemistry. W


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Scheme 28. Synthesis of Phenanthrene-Fused Hexaphyrins 147 and 148a−148b

Figure 26. Top: pH-controlled singlet oxygen generation in 148a. Bottom: (a) UV−vis−NIR spectrum of 148a in DMF. (b) Fluorescence spectra of 148a (2 μM citrate buffer solutions containing 10% DMF) in pH 8.0, 7.4, 6.5, 6.0, 5.0, 4.0, and 3.0 (V/V) (λex = 635 nm). Reprinted from ref 112. Copyright 2013 American Chemical Society.

Figure 27. (a) Schematic diagram of a mouse bearing a Hela tumor cell, intravenously injected with FA-148a NPs. (b) Changes in relative tumor volume (V/V0) after intravenous injection of mice with PBS, 148a NPs, or FA-148a NPs, followed by irradiation with a 808 nm laser at 100 mV cm−2 for 30 min. Reprinted from ref 112. Copyright 2013 American Chemical Society.

DTE subunit was synthesized via condensation of diol 151 and tetrapyrrane 153 under Lindsey conditions. Furthermore, 154 shows characteristics of a 26π aromatic macrocycle and notably

been utilized recently by the groups of Kobayashi and Shen for the synthesis of core-modified expanded porphyrins (Scheme 29). Thus, rubyrin derivative 154 containing a single closed X


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Figure 28. Top: Chemical structure of tetrathiafulvalene-annulated rubyrin 149 and its porphyrin analogue 150. Bottom: (a) UV−vis−NIR spectra of free base 149 (black) and its TFA salt H21492+ (red) recorded in CH2Cl2. (inset) Magnified NIR region. (b) EPR spectrum of H21492+ in CH2Cl2−TFA mixture. Reproduced from ref 113 with permission from the PCCP Owner Societies. Copyright 2015 the Owner Societies.

Scheme 29. Synthesis of DTE-Fused Rubyrins 152 and 154

Scheme 30. Synthesis of Core-Modified Naphthorubyrins 156a−156c

naphthobipyrrole 97 with diols 155a−155c, followed by DDQ oxidation, led to the formation of naphthorubyrins 156a−156c. Rubyrins 156a−156c showed NMR spectra corresponding to a 26π aromatic macrocycle. Both the thiarubyrin analogues displayed no conformational changes owing to the fused naphthobipyrrole units. Interestingly, dioxa derivative 156c was found to be NMR-silent in its free base form, which upon protonation showed a spectrum similar to those of diprotonated 156a and 156b. All-aza naphthorubyrin 99 also displayed

remains in the closed form even in the dark owing to its aromatic stability. In contrast, rubyrin 152, containing two DTE moieties with one open form and one closed form, proved to be nonaromatic.114 The aromatic and nonaromatic natures of these macrocycles were further analyzed by MCD spectroscopy, TD-DFT calculations, and AICD calculations. Unsubstituted naphthobipyrrole 97 was again used by the group of Lee for the synthesis of core-modified rubyrins 156a− 156c (Scheme 30).92 BF3·Et2O-catalyzed condensation of Y


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These fused rubyrins show remarkably flat structures compared to other rubyrins reported earlier, as confirmed by their X-ray crystal structure analysis, due to their increased conformational restriction.116 Fused rubyrins 158a and 159a display Soret bands at 517 and 513 nm respectively, which are slightly blue-shifted (6−10 nm) compared to their nonfused analogues. However, fusion leads to a 4-fold increase in their molar extinction coefficients. 3.2.3. Rosarins, Hexaphyrin(1.0.1.0.1.0). In an important finding, Sessler and collaborators recently reported a rosarin [hexaphyrin(1.0.1.0.1.0)] derived from the unsubstituted naphthobipyrrole 97, namely naphthorosarin 162, which displays a planar structure and hence exhibits distinct antiaromatic character (Figure 30).117 This 24π antiaromatic species undergoes proton-coupled two-electron reduction with reducing agents such as sodium dithionite, to yield 26π aromatic species H3162+. This species was characterized by NMR, X-ray, and various steady-state and time-resolved spectroscopies, as well as theoretical assessment. Interestingly, formation of an unprecedented one-electron-reduced product of 162, i.e. a 25π dication radical species H3162•2+, was observed upon addition of acids such as HCl or HBr. This could be transformed into the 26π species H3162+ by treatment with another one-electron-reducing agent such as decamethylferrocene. On the other hand, owing to its favorable reduction potentials, HI could reduce both 162 and H3162•2+ to the 26π species H3162+. Other protic acids, such as trifluoroacetic acid or methanesulfonic acid, were unable to produce the radical species and instead formed the triprotonated H31623+. This redox reaction cycle is depicted in Figure 31. Notably, all three forms, namely antiaromatic, radical, and aromatic species, are totally reversible. This in turn indicates the role of counteranion in the reduction process. In contrast, rosarin 161, derived from nonfused bipyrrole, was found to be conformationally flexible and for this reason does not undergo such an electron transfer process and instead exhibits nonaromatic features.118 Very recently, Sessler and collaborators further unraveled the triprotonated annulated rosarins (H31623+ and H31633+) via treatment with redox inactive acids such as TFA, leading to triplet diradical formation at low temperature (4 K) as revealed by EPR analysis (Figure 32a).119 The intensity of the EPR

no conformational changes in its free base form, as well as its mono- and diprotonated forms.87 All three forms of 99 showed substantially red-shifted absorptions and emission bands (λmax emission = 888, 908, and 948 nm for free base, monoprotonated, and diprotonated forms, respectively). Among these core-modified rubyrins, only the dioxa derivative 156c was found to be very weakly emissive (λmax = 613 nm). Although the Soret band of all-aza naphthorubyrin 99 is comparable to the core-modified analogues 156a−156c, its lowest energy Qband was found to be markedly red-shifted (868 nm) (Figure 29).

Figure 29. UV−vis absorption spectra of dithianaphthorubyrins 156a (solid line), 156b (dashed line), and dioxanaphthorubyrin 156c (dotted line) in CH2Cl2. Adapted with permission from ref 92. Copyright 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Chandrashekar and co-workers have recently reported the first singly fused rubyrins 158a−158c, following a [4 + 2] McDonald-type condensation of DTT-diols 107a−107b and modified tetrapyrranes 157a−157c. Similarly, doubly fused rubyrins 159a−159b were synthesized by using the same diols with 2 equiv of pyrrole or in the presence of TFA (Scheme 31).115 Further, they demonstrated an alternate route to doubly fused rubyrins by synthesizing 159a via acid-catalyzed condensation of modified tetrapyrrane 160a and diol 107a.

Scheme 31. Synthesis of Core-Modified Annulated Rubyrins 158a−158c and 159a−159b

Z


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Figure 30. Nonannulated vs annulated rosarin: control over conformation.

Figure 31. Proton-coupled redox reaction cycle of naphthorosarin 162, leading to antiaromatic−aromatic transformation via an intermediate radical species.

signal increases with TFA concentration to reach a constant value after addition of 3 equiv (Figure 32a). The triplet yield was found to decrease upon increasing temperature. This was further supported by theoretical calculations, which explain that triprotonation increases the electronegativity of these nitrogen atoms and thus stabilizes a LUMO orbital to produce degenerate HOMOs, leading to a triplet ground state.

The Chandrashekar group recently reported the serendipitous formation of core-modified heptaphyrins containing DTT as a fused precursor. For instance, their targeted synthesis of sapphyrins, via a [3 + 2] condensation of DTT-diol 107a with core modified tripyrranes 108a−108b using 1 equiv of TFA, surprisingly resulted in the formation of heptaphyrins 167a− 167b as the major products (6 and 4% yields), along with the expected sapphyrin derivatives 109a−109b (4 and 3%) (Scheme 33).121 Notably, when the same reaction was performed in the presence of p-TSA (0.5 equiv) as the acid catalyst rather than TFA (1 equiv), sapphyrins 109a−109b were furnished as the sole products, with no traces of heptaphyrins. This dependence of the product distributions on the nature and concentration of the acid catalysts was attributed to the acidolysis of the coremodified tripyrranes 108a−108b, which increases with increasing acid concentration. The resultant heptaphyrins 167a−167b, containing six meso bridges, display planar

3.3. Systems Containing Seven, Eight, or Nine Pyrroles or Other Heterocyclic Rings

3.3.1. Heptaphyrins, Octaphyrins, and Nonaphyrins. Chandrashekar and co-workers reported the first peripherally annulated heptaphyrin 165 possessing a fused DTT unit following reaction between tetrapyrrane 160a and terthiophene 164 in the presence of TFA (Scheme 32).115 1H NMR analysis suggested that heptaphyrin 165 does not exhibit an inverted structure, unlike its nonannulated analogue 166, where one of the thiophene units undergoes ring inversion.120 AA


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Figure 32. Chemical structure of triprotonated naphthorosarin H31623+ and benzorosarin H31633+. (a) EPR spectrum H31633+ (1.0 × 10−4 M) in CH2Cl2 in the presence of TFA (3.0 × 10−4 M) measured at 4 K. (b) Plot of EPR intensity of H31633+ vs concentration of TFA measured at 4 K. Reprinted from ref 119. Copyright 2015 American Chemical Society.

Scheme 32. Synthesis of Core-Modified Fused Heptaphyrin 165

Scheme 33. Synthesis of Core-Modified Fused Heptaphyrins 167a−167b

32π conjugation pathway, and hence retains its antiaromatic character (Figure 33). The absorption spectra of 167a−167b display a broad band centered around 500−524 nm, typical of antiaromatic porphyrinoids, which upon addition of TFA led only to bathochromically shifted spectra with analogous spectral patterns. The X-ray crystal structure analysis of 167a reveals that the DTT unit is deviated from the plane by 28.8°, in an otherwise planar structure (Figure 34).

conformations and antiaromatic behavior as revealed from NMR and absorption spectroscopic analysis, along with NICS calculations (δ = +24.6 and +22.26 ppm, for 167a and 167b, respectively). Further, structural analysis of 167a−167b using XRD and NMR shows that, in the free base state, both the DTT unit and the pyrrole ring opposite to it adopt inverted conformations. However, upon protonation with TFA, the macrocycle undergoes a drastic structural change (as revealed by NMR and UV−vis spectroscopies) without having any impact on its AB


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Scheme 34. Rational Synthesis of DTT-Fused Heptaphyrins 168a−168b

Figure 33. Structural change of 167a upon protonation with TFA.

Figure 34. (a) UV−vis spectra of 167a−167b and their TFA salts. (b) X-ray structure of 167a. meso-Mesityl groups are omitted for clarity. Reprinted with permission from ref 121. Copyright 2014 The Royal Society of Chemistry. Figure 35. UV−vis−NIR absorption spectra of (a) 168a and (b) 168b and their TFA salts recorded in CH2Cl2. Reprinted with permission from ref 122. Copyright 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Very recently, in 2016, the group of Chandrashekar reported a rational synthetic approach toward 32π heptaphyrins 168a− 168b via condensation between DTT-fused tetrapyrrane 160a and core-modified tripyrranes 108a−108b, in the presence of 2 equiv of pentafluorobenzaldehyde and 0.3 equiv of p-TSA (Scheme 34). Thereby, heptaphyrins 168a−168b were formed in 8−10% yields.122 1 H NMR analysis of these heptaphyrins revealed their Möbius aromatic character, which was also supported by X-ray structural analysis and UV−vis spectroscopy (Figure 35), as well as by theoretical calculations such as NICS (−8.1 ppm at the center) and AICD plots. Unlike 167a−167b, which display near-planar arrangements, 168a−168b exhibit twisted figure-eight arrangements suitable for the overall π-conjugation required for Möbius aromatic

stabilization in these molecules. Protonation with TFA did not affect the aromaticity in 168a−168b. Again, core-modified doubly fused 36π octaphyrins (1.1.1.0.1.1.1.0) 169a−169b, involving DTT as the source of fused precursor, were obtained by reactions of 160a−160b in the presence of pentafluorobenzaldehyde (Scheme 35).123 The X-ray crystal structure of 169a revealed that the free base form adopts a figure-eight conformation with a twisted double-sided Hückel topology with nonaromatic character (Figure 36a). However, upon protonation, 169a−169b undergo dramatic structural changes and exhibit antiaromatic character. For AC


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Scheme 35. Synthesis of Core-Modified Fused Octaphyrins 169a−169b

Figure 36. X-ray crystal structures of (a) 169a and (b) diprotonated salt 169b·2H+ (counteranion: TFA). Top: front view. Bottom: side view. mesoAryl groups are omitted for clarity. Reprinted with permission from ref 123. Copyright 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

example, the crystal structure of diprotonated 169b·2H+ shows an open conformation, wherein one pyrrole unit of each dipyrrin unit is inverted (Figure 36b). These structural changes upon protonation can be followed by their absorption (Figure 37), as well as 1H NMR spectra. The antiaromatic nature of the diprotonated species was further supported by theoretical calculations. In addition, moderate increases in TPA cross sections (1600 vs 2700 GM), and enhanced excited-state lifetimes were seen upon protonation. Again in 2016, the group of Chandrashekar reported the first annulated nonaphyrins 171a−171b (Scheme 36) via condensation between precursors 160a−160b and pentapyrromethanes 170a−170b in the presence of pentafluorobenzaldehyde.124 Like the octaphyrins 169a−169b reported by the same group, nonaphyrins 171a−171b also displayed twisted figureeight conformations with nonaromatic behavior in their free base forms, while protonation led to open extended conformations corresponding to 40π Hückel antiaromatic macrocycles. The structural changes that took place upon protonation could be followed by 1H NMR studies, as well as by absorption spectroscopy (Figure 38a,b). The twisted conformation of free base 171b was further confirmed via single-crystal X-ray structural analysis (Figure 38c,d). Similar behavior was also observed for the nonannulated analogue 172.

Figure 37. UV−vis−NIR spectral changes of 169b in CH2Cl2 upon addition of TFA. Adapted with permission from ref 123. Copyright 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

3.4. Cyclo[n]pyrroles

3.4.1. Cyclo[8]pyrroles, [30]Octaphyrin(0.0.0.0.0.0.0.0). Cyclo[8]pyrroles, or [30]octaphyrin(0.0.0.0.0.0.0.0), is a class of expanded porphyrin, AD


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Scheme 36. Synthesis of Core-Modified Fused (171a−171b) and Nonfused (172) Nonaphyrins

Figure 38. UV−vis spectra of (a) 171a and its diprotonated salt 171a·2H+ and (b) 172 and its diprotonated salt 172·2H+ in CH2Cl2. X-ray crystal structure of free base 171b: (c) top view and (d) side view (meso-aryl groups are omitted for clarity). Reprinted with permission from ref 124. Copyright 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

wherein eight pyrrole units are directly linked through their αcarbon atoms resulting in a planar, 30π aromatic system in its diprotonated state.125 The most striking feature about this macrocycle is that it possesses a very intense near-infrared (NIR) absorption band at ∼1100 nm (designated as an L band) compared to a weaker Soret-type near-UV band at ∼430 nm (B band), unlike the other porphyrinoids wherein the lowest energy bands have low intensities. However, the

difficulties associated with the synthesis of appropriate building blocks have led to the synthesis of only a very few cyclo[8]pyrroles following a biphasic, acid-mediated (H2SO4), FeCl3-catalyzed oxidative coupling of alkylated bipyrroles. Therefore, annulated cyclo[8]pyrroles did not appear in the literature until very recently. The cyclo[8]pyrroles were isolated as their dihydrogen sulfate salts, wherein a sulfate ion coordinates strongly with the pyrrolic NHs at the core of the AE


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Figure 39. Chemical structure of cyclo[8]pyrroles.

macrocycle, a feature that persists in both solid and solution states. It is noteworthy to mention that the sulfate ion plays a very important templating role in the formation of cyclo[8]pyrrole, which probably also leads to comparatively higher yields for these macrocycles. Okujima et al. reported the synthesis of cyclo[8]isoindole 175, the first example of a π-extended cyclo[8]pyrrole, based on the retro-Diels−Alder reaction of BCOD-fused cyclo[8]pyrrole 174 (Figure 39).126 BCOD-fused cyclo[8]pyrrole 174 in turn was synthesized from BCOD-fused bipyrrole 26. These authors also explored different oxidizing agents, such as CAN, AgO, NaNO2, and Ce(SO4)2, in addition to FeCl3. The X-ray crystal structure of 174 exhibits a near-planar structure like βalkyl-substituted cyclo[8]pyrroles 173, although there is a significant saddling of the π-system of 175. Interestingly, fusion of BCOD moieties upon the cyclo[8]pyrrole periphery (in 174) led to a red-shifted B-band and a blue-shifted L-band compared to β-alkylated cyclo[8]pyrroles, although it retained a similar spectral pattern. On the other hand, the extension of πconjugation achieved via fusion of benzene (in 175) led to a markedly red-shifted and intensified B-band (627 nm) along with a less intense and blue-shifted L-band centered at 1078 nm, which arises from stabilization of the LUMO + 1 as inferred from MCD spectroscopy studies, owing to its structural distortion (Figure 40).

Figure 41. (top) Chemical structure of cyclo[8]pyrroles 166 and (bottom) synthetic scheme for its precursor 183.

characterization techniques. The less-polar and lower-symmetry isomer could be thermally converted into the other through a thermal ring flip, which persists even after cooling to room temperature. These authors reported that utilizing the FeCl3mediated biphasic coupling protocol developed by Sessler and co-workers led to poor yields (approximately 10%), but using Ce(SO4)2 as the oxidant under reflux conditions in CHCl3 improved the yield up to 55%. Interestingly, unlike cyclo[8]isoindole 175, the Soret band of 176 (at 512 nm) is less intense than the near-IR L-band (at 1482 nm), thereby displaying a trend similar to that observed in the case of alkylated cyclo[8]pyrroles. In addition, Okujima et al. reported cyclo[8]pyrrole 185 with eight 9,10-dihydroanthracene units via oxidative coupling of annulated bipyrrole 184 (Figure 42a). This cyclo[8]pyrrole, having a three-dimensional structure, possesses a cavity with a diameter of ca. 15 Å generated by 9,10-dihydro-9,10anthraceno moieties fused at the β-pyrrolic positions, as observed from the X-ray crystal structure.128 The group of Panda reported cyclo[4]naphthobipyrroles 186a−186c, cyclo[8]pyrroles containing four alkylated naphthobipyrrole units (Figure 42b).129 One of these derivatives,

Figure 40. UV−vis−NIR absorption spectra of 174 and 175. Reprinted with permission from ref 126. Copyright 2011Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Very recently, in a continuation of their efforts directed toward the synthesis of annulated porphyrinoids, Okujima et al. reported an acenaphthylene-fused cyclo[8]pyrrole, namely cyclo[8]acenaphthopyrrole 176, utilizing bipyrrole 183 (Figure 41).127 The synthesis of the key precursor 183 is depicted in Figure 41. Two conformational isomers of 176 could be isolated and characterized crystallographically, along with other AF


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Figure 43. Changes in absorption of 186b in toluene upon addition of NaOH and H3PO4. Absorption spectra of 186b in toluene (a), after washing with NaOH (b), and reprotonation with H3PO4 (c). Reprinted from ref 132. Copyright 2014 American Chemical Society.

Scheme 37. Synthesis of Cyclo[10]pyrrole 187

Figure 42. (a) Synthesis of 185. (b) Chemical structure of cyclo[4]naphthobipyrroles 186a−186c.

186b, was also reported independently by Sessler and coworkers.130 These molecules showed an intense absorption in the NIR region compared to their nonannulated counterpart.125 Further, they observed that cyclo[4]naphthobipyrroles are sensitive to the nature of the substituents, which was reflected in their photophysical properties as well as their synthesis. Ring fusion led to a marked red shift in the L-type band (1339 nm for 186b), whereas the other two derivatives, possessing linear alkyl substituents, exhibited blue-shifted L-bands compared to 186b (1273 and 1276 nm for 186a and 186c, respectively). The groups of Rao and Panda separately reported large TPA cross sections for these cyclo[4]naphthobipyrroles, along with their excited state dynamics.131 Subsequently, the group of Waluk and Panda reported detailed structures, electronic states, and anion binding properties of these macrocycles.132 Detailed theoretical studies, MCD spectroscopy, and experimental analysis indicated that the sulfate ion is possibly playing a major role in determining the structure and, as a consequence, the electronic transitions in cyclo[4]naphthobipyrroles. For example, the characteristic intense NIR absorption of cyclo[4]naphthobipyrrole 186b disappears after removal of sulfate ion, thereby indicating a major structural change in the macrocycle (Figure 43). 3.4.2. Cyclo[10]pyrrole. Okujima et al. very recently showed that the croconate ion could be used as a template in synthesis of larger cyclo[n]pyrroles such as cyclo[10]pyrroles.133 Reaction of bipyrrole 183 possessing a fused acenaphthylene unit led to the largest cyclo[n]pyrrole 187 in 66−46% yields, following oxidative coupling reactions in the presence of the croconate ion (Scheme 37). This macrocycle displays further modulation of its optical properties compared to the cyclo[8]acenaphthopyrrole 176, which is comprised of the same bipyrrolic building block. Thus, addition of two acenaphthopyrroles resulted in a remarkable

red shift of the L-band (1982 nm vs 1482 nm in 176). The Xray crystal structure shows a saddle-type structure owing to steric hindrance from the peripheral tert-butyl group (Figure 44). 3.5. Other Related Systems

In 2014, Rath and co-workers utilized thieno[3,2-b]thiophene diol 188 as a precursor for the synthesis of various coremodified expanded porphyrins (189a−189c) along with pyrrole, and pyrroles annulated with phenanthrene and acenaphthylene (146a−146c), respectively.134 The resultant thieno[3,2-b]thiophene-embedded135 22π aromatic macrocycles revealed NIR absorptions (Figure 45). For instance, 189c displayed an intense Soret band at 503 nm and a lowest energy Q-band at 957 nm, whereas annulation at the β-pyrrolic positions resulted in significant red shifts of the Soret band in their absorption spectra of 189a and 189b (56 and 60 nm, respectively). The aromatic nature of these macrocycles was inferred from 1H NMR spectroscopic analysis as well as from their negative NICS(0) values, and HOMA and DFT calculations. Very recently, the same group has utilized the fused thieno[3,2-b]thiophene135 moiety for the synthesis of aromatic-fused [30] heteroannulenes 191a−191b, as shown in Figure 46.136 Macrocycles 191a−191b displayed rigid planar structures possessing extended π-conjugation, leading to 30π AG


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their lack of stability. Originally, such systems were synthesized via cyclization of the linear tetrapyrrole a,c-biladiene developed by Johnson and co-workers.138,139 However, corrole chemistry began to expand more significantly in 1999 with independent twin reports on the facile synthesis of meso-triarylcorroles from the research groups of Gross and Paolesse.140,141 Consequently, these facile synthetic methods, along with the stability of corroles endowed with electron-deficient substituents, aided in exploring the coordination chemistry of corroles. In particular, being trianionic ligands, corroles help stabilize metal ions in high oxidation states, which also accounts for the significant research into metallocorroles. Needless to say, the π-annulation of corroles is much less well-explored. Among others, an alternate synthetic approach to new corroles that stabilizes the corrole unit though the extended π-conjugation achieved via annulation with suitable aromatic moieties would be of particular interest.142 The limited methods to synthesize functionalized corroles have led researchers to explore reactions on the periphery of corroles by using them as substrates toward more complex functionalized corroles.143−145 Thus, corroles undergo bromination,146,147 nitration,148 amination,149 and cycloaddition150,151 reactions at their meso and β-pyrrolic positions. However, their lower symmetry often results in poor regioselectivity in the resultant products. Given the scope of this review, we will focus only on annulated corroles, although they are limited in number. Interested readers looking for further details of corrole chemistry are recommended to go through some of the important reviews in the area dealing with their synthesis,152−154 coordination chemistry,155−159 and applications.160−163 4.1.1. Annulation through β,β-Pyrrolic Positions. In 2011, the groups of Vicente and Paolesse reported two synthetic routes toward the synthesis of 5,10,15-triaryltetrabenzocorroles as their Cu(II) complexes.164 The first approach involves condensation of tetrahydroisoindole 194 with aryl aldehydes 195a−195c (Scheme 38). It has been observed that the use of BF3·Et2O as an acid catalyst provides better yields than TFA (8 vs 60°). In 2008, Kobayashi, Luk’yanets, and co-workers reported the synthesis of fully substituted subporphyrin derivatives 235a− 235c, namely, meso-triaryltribenzosubporphyrins, using isoindolinone 236 or phthalimide 237 as precursors and boric acid as a template (Scheme 45).183 The X-ray crystal structure displayed a similar bowl-shape geometry as was observed for benzosubporphine 231a, as well as similar optical properties. The lack of electronic effects owing

Scheme 45. Synthesis of meso-Aryltribenzosubporphyrins 235a−235c

AO


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Scheme 46. Synthesis of Thiopyrane-Fused Subporphyrin 241

Scheme 47. Synthesis of meso-Aryl Tribenzotriphyrins 243a−243c

nonannulated triphyrins, along with metal complexation studies of benzo-fused triphyrins.189 Interestingly, their investigations revealed that the higher concentrations of the acid catalyst used in the reaction are responsible for the formation of [14]triphyrin(2.1.1) over the expected porphyrin product. Tribenzotriphyrins show unique metal complexation ability with a range of metals, such as Pt(II), Pt(IV), Mn(I), Re(I), Ru(II), Pd(II), and Ir(III), as well as boron-containing fragments.190−194 Yamada and co-workers reported Fe(II) sandwich complexes 247a−247b, wherein the central Fe is sandwiched between the triphyrin ligand and one Cp ring, as shown in Scheme 48.195

of porphyrins. The resultant BCOD-fused triphyrins 242a− 242c could be converted to the corresponding benzo derivatives 243a−243c under retro-Diels−Alder conditions (Scheme 47). Absorption spectra of these triphyrins showed an intense band in the visible region and weaker Q-type absorptions in the NIR region, as is usually observed for aromatic porphyrinoids. The effect of π-extension in 243a−243c was reflected in their absorption properties. For instance, the Soret band of 243a is red-shifted by 44 nm (414 vs 370 nm) compared to BCODfused analogue 242a.187 Similarly, the lowest-energy Q-band of 243a was centered around 578 nm, while that of 242a was found at 569 nm. Similar red-shifted absorptions were also observed for 243a−243c, in line with those of nonannulated meso-tetraaryltriphyrin (Soret, 373 nm; Q, 564 nm).188 Continuing their efforts, recently the research groups of Yamada, Kobayashi, and Shen reported naphthalene-annulated triphyrin 245 following a similar retro Diels−Alder approach (Figure 56). The absorption spectrum of 245 exhibits a Soret band at 448 nm, while a Q-band appears at 612 nm. These bands are red-shifted by 34 and 44 nm, respectively, compared to tribenzotriphyrin 243. The same report also discussed a rational approach to the synthesis of a series of annulated and

Scheme 48. Synthesis of η5-Cyclopentadienyliron(II)− [14]Triphyrin(2.1.1) 247a−247b

Electrochemical analysis of these Fe(II) triphyrins revealed that their first oxidation potential corresponds to the Fe(II) center and is lower than that of ferrocene (−0.50 and −0.39 V for 247a and 247b, respectively). The second and third oxidation potentials correspond to the triphyrin ligands, which are slightly higher than those of the free base triphyrin. Chemical oxidation with AgPF6 and chloranil led to the formation of 247a+, which could be identified as containing a Fe(III) metal center by NMR, HR-ESI-MS, and EPR analysis. Notably, this Fe(III) center could again be reverted back to Fe(II) upon reduction,

Figure 56. Chemical structure of trinaphthotriphyrin 245 and its precursor 244. AP


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as revealed by controlled electrochemical oxidation and reduction of 247. This overall process could be monitored by absorption spectra, where the Fe(II) and Fe(III) species show distinct spectral changes (Figure 57). Interestingly, spectral

Figure 58. Catalytic cycle showing two-electron reduction of dioxygen by triphyrin 246b.196

deprotection and condensation with 2,5-bis(ptolylhydroxymethyl)furan 251 provided the two isomeric triphyrins 252 and 257 in their monoprotonated forms (Scheme 49). Triphyrin 252 exhibits aromatic character corresponding to a 14π conjugation path as revealed by 1H NMR, UV−vis absorption, and NICS calculations (δ = −11.4 ppm). However, isomer 257 possibly possesses an 18π conjugation path (the major contributing form among the possible resonance structures) and hence displays weak aromatic character compared to that of 252. Electrochemical analysis revealed facile reduction of these macrocycles (for 252, − 718 and −1186 mV; for 257, − 698 and −1128 mV (versus Fc/Fc+ in CH2Cl2)). Chemical reduction tested with zinc amalgam produced antiaromatic triphyrins having paratropic ring currents. The latter species were stabilized in the form of their boron complexes 253 and 258, wherein both of the macrocycles act as dianionic ligands. Distinct antiaromaticity was observed for 253, which possesses a 16π conjugation pathway. This is in contrast to 258, wherein antiaromaticity is less obvious, thereby indicating the 20π pathway (having an external S atom) is less contributing. Absorption spectra of 252 and 257 show Soret-type bands at 411 and 409 nm, respectively, along with broad Q-bands in the region of 480− 650 nm (Figure 59). Further, the compounds emit at 602 (ϕf = 3.8%) and 660 nm (ϕf = 1.2%), respectively. Following their report on the synthesis of oxatriphyrins 252 and 257, the same group synthesized oxatriphyrin(2.1.1) 259, incorporating an o-phenylene motif in place of the thiophene moiety.198 This macrocycle shows 14/18π aromatic character (contribution from both possible resonance pathways) as supported by NMR and UV−vis spectroscopies (Figure 60). The X-ray structure of 259 as its dichlorodicyano-hydroquinone dianion salt exhibited a planar structure, where the inner NH is strongly hydrogen bonded between two nitrogen atoms, accounting for its unusual downfield shift (δ ∼ 14.2 ppm), despite the presence of a diatropic ring current. Again, chemical reduction as well as complexation with boron led to a 16/20π antiaromatic product (260).

Figure 57. Spectrophotometric titration of 247a with (a) TFA and (b) with DBU in CH2Cl2 recorded at 298 K. Adapted with permission from ref 195. Copyright 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

changes similar to those observed for Fe(III) species were also achieved when 247a was titrated with TFA. This was attributed to proton-coupled reduction of dioxygen in the presence of acid, producing a Fe(III) species. Addition of a base such as DBU induced reversion back to the spectral change corresponding to the initial Fe(II). This perhaps could be utilized for the reduction of dioxygen to H2O2. Very recently, in a continuation of this study, Fukuzumi and Yamada disclosed that metal-free tribenzotriphyrin 246b could catalyze dioxygen reduction to form H2O2 by octamethylferrocene (Me8Fc) in the presence of perchloric acid in PhCN.196 The catalytic cycle involves proton-coupled electron transfer from Me8Fc to triphyrin H246b forming H3246b•+, which subsequently undergoes another electron transfer to form H3246b. In the presence of dioxygen, H3246b was oxidized to form complex H3246b/O2, which subsequently produced H2O2 and free base H246b upon transfer of two electrons and two hydrogens from H3246b. Detection of the H3246b•+/HO2• intermediate radical pair via EPR spectroscopy further supported this catalytic cycle (Figure 58). In 2014, Latos-Grażyński and co-workers synthesized coremodified oxatriphyrins(2.1.1) 252 and 257, containing a fused thiophene ring.197 The synthesis of these hybrid triphyrins utilized Suzuki−Miyaura coupling involving dibromothiophene 248 and 254 and Boc-protected pyrrole, which upon AQ


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Scheme 49. Synthesis of Oxatriphyrins(2.1.1) 252 and 257, and Their Reduced Analogues 253 and 258, Each Containing a Fused Thiophene Ringa

(a) N-Boc-2-pyrroleboronic acid, Pd(OAc)2, SPhos, K3PO4, n-BuOH, 100 °C, 4 h; (b) HOCH2CH2OH, reflux, inert atmosphere, 1 h; (c) BF3· Et2O; (d) PhBCl2, TEA, toluene, reflux.

a

5. RELATED PORPHYRIN-LIKE ANNULATED COMPOUNDS Recently a number of interesting macrocycles resembling porphyrins have appeared in the literature. In these macrocycles, 1,10-phenanthroline, phenanthrene, and triphenylene moieties are used in place of a bipyrrole unit. Porphyrinoids with embedded polyaromatic hydrocarbons, or other fused heterocyclic rings, resulted in a new class of hybrid molecules with unprecedented properties such as stable organic radicals, cation sensing, or the ability to stabilize unusual metal oxidation states. In 2010, Ishida et al. reported the synthesis of porphyrin analogue 264 following a [2 + 2] condensation of the 1,10phenanthroline vinyl derivative 262 with meso-phenyldipyrromethane 263, in relatively good yield (Scheme 50). The key precursor 262 in turn was synthesized from Knoevenagel condensation of 1,10-phenanthroline carbaldehyde 261 with diethylmalonate. The two exocyclic double bonds interrupt the macrocyclic conjugation in 264.199 The X-ray crystal structure of 264 revealed a gable-type nonplanar structure with a dihedral angle of 120° between the phenanthroline moiety and the dipyrromethane unit. However, this nonplanar geometry and the smaller core size of 264 were found to be ideal for coordination of Mg2+ and its ratiometric detection by its far-red fluorescent emission above 600 nm. Notably, free base 264

Figure 59. Absorption and emission spectra (inset) for (a) 252−HCl (solid line) and 253 (dashed line) and (b) 257−HCl (solid line) and 258 (dashed line) recorded in CH2Cl2 at 298 K. Adapted with permission from ref 197. Copyright 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Figure 60. (a) Delocalization pathway in 259 and its boron complex 260 (b). AR


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Scheme 50. Synthesis of 1,10-Phenanthroline-Embedded Macrocycle 264

thereby making 264 as a promising ratiometric fluorescent Mg2+ ion sensor that could possibly allow determination of intracellular magnesium concentrations. On the other hand, a very similar triarylbipyricorrole endowed with a bipyridine moiety, reported by Srinivasan and co-workers, binds selectively to Zn2+ ions while exhibiting similar nonaromatic features.200 Ishida et al. subsequently reported a modified sapphyrin analogue 266 derived from 1,10-phenanthroline following a “4 + 1” condensation strategy of the 1,10-phenanthrolinecontaining tetrapyrrane analogue 265 with thiophene dicarbinol 155a (Scheme 51).201 This molecule does not exhibit any macrocyclic conjugation as inferred from its 1H NMR spectrum. However, interesting spectral changes were observed upon addition of TFA (Figure 62).

exhibits extremely weak fluorescence (λmax = 572 nm, ϕH = 0.003) that shows intense red-shifted emission (λmax = 639 nm, ϕMg = 0.015) upon complexation with Mg2+. Similar red shifts and intensity enhancement were also observed in its absorption spectra upon treatment with MgCl2, leading to a visible color change from reddish orange to purple (Figure 61). These

Figure 61. (a) UV−vis absorption and (b) emission spectral changes of 264 (2 × 10−5 M) upon titration with MgCl2 in MeCN. The isosbestic point in the UV−vis spectra is at λex = 520 nm. (c) UV−vis absorption and (d) emission spectral changes of 264 (2 × 10−5 M) upon titration with MgCl2 in 0.1 M HEPES buffer (pH 7.4, KNO3 (l = 0.1)) and DMSO solution (7:3 v/v). The isosbestic point in the UV− vis spectra is at λex = 535 nm. Insets show (a) solution colors and (b) fluorescent images of 264 (λex = 365 nm) in the absence and presence of MgCl2. Adapted with permission from ref 199. Copyright 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Figure 62. UV−vis−NIR absorption spectral changes of 266 upon titration with TFA (up to [TFA] = 0.2 M) in CHCl3 ([266] = 4 × 10−5 M). Inset: solid-state ESR spectrum of 266·3TFA at room temperature. Adapted with permission from ref 201. Copyright 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

spectral changes were not affected by the presence of other physiologically relevant cations such as Na+, K+, and Ca2+, Scheme 51. Synthesis of 1,10-Phenanthroline-Embedded Modified Sapphyrin 266

AS


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Scheme 52. Synthesis of Phenanthriporphyrin 271

The resultant tricationic oxidized species [266·3H]3+ exhibits singlet biradical character as confirmed by spectroscopic, electrochemical, and magnetic susceptibility measurements. Very recently, the Latos-Grażyński group utilized phenanthrene as a building block for the synthesis of porphyrin-like macrocycle 271, as detailed in Scheme 52.202 Unlike the 1,10phenanthroline-embedded porphyrin 264, this macrocycle does not possess exocyclic double bonds and is fully conjugated, exhibiting antiaromatic character. The X-ray crystal structure of 271 shows a bowl-shape deformation of the phenanthrene moiety. The UV−vis absorption spectrum exhibits an intense band at λ = 364 nm along with a weak and broad absorption band in the range 500−1000 nm with a maximum at 816 nm, suggesting delocalization of π-electrons in 271 (Figure 63).

precursor 270 (e.g., 10.41 vs 8.26 ppm). In contrast, peripheral protons appear in the range of nonaromatic or borderlinearomatic aceneporphyrinoids. The X-ray crystal structure analysis of 273a−273c revealed highly folded conformations that lend further credence to the observed weak π-conjugation in these macrocycles. Furthermore, UV−vis spectral analysis showed a wide and comparatively intense Soret-type band, as well as a broad Q-like transition. For instance, 273a exhibits an absorption at 389 nm (log ε = 4.6) and a broad peak centered around 606 nm (log ε = 3.8) with absorption tails extending up to 800 nm. All three derivatives were found to show similar optical properties. Protonation leads to a color change from dark green to orange, accompanied by red-shifted absorption spectra (Figure 64). However, it did not affect the aromaticity of the macrocycles, as revealed by 1H NMR analysis. Conversion of the 9,10-dimethoxyphenanthrene moiety of 273a to 9,10-phenanthrenequinone prevents macrocyclic πconjugation, which accounts for the nonaromatic character of this compound, as suggested by 1H NMR analysis. The aromaticity of these macrocycles was also studied by NICS calculations, which support the observed residual antiaromaticity in 273a−273c and nonaromaticity in 275. The group of Cammidge recently utilized the triphenylene moiety for the synthesis of expanded porphyrin-like structures. Two interesting new classes of macrocyclic chromophores containing thiophene or pyrrole rings along with triphenylenes were reported (Figure 65). Macrocycles 276a−276c were found to exist in an aromatic state in their fully oxidized forms, as inferred from UV−vis−NIR spectra. However, no NMR characterizations were possible for these macrocycles in their conjugated forms. On the other hand, macrocycles 277a−277d exhibit antiaromaticity as revealed by their 1H NMR spectra.205 Very recently, Osuka and co-workers developed a protocol for the synthesis of tetrabenzotetraaza[8]circulene 279 (Scheme 54).206 The key precursor [24]porphyrin(2.2.2.2) 278 required for this synthesis was prepared by utilizing Pdmediated chemistry, and was found to possess a twisted structure in the solid state. 1H NMR and UV−vis spectroscopies further revealed its nonaromatic nature, despite having an extended porphyrin-like structure. Macrocycle 278 upon “fold-in” oxidative fusion with DDQ/Sc(OTf)3 led to the desired product 279 in 96% yield. The fused porphyrinoid 279 displayed weak antiaromatic character at the central cyclooctatetraene (COT) moiety, which is in sharp contrast to analogous porphyrin sheets containing COT rings (NICS + 6.82 vs 35.71 ppm).207 Cyclic tetrapyrrole 278 exhibits a broad absorption at 326 nm and similarly broad emission in the range 450−700 nm (ϕf = 0.27) along with solid-state emission (ϕf = 0.15), whereas the rigid and planar 279 absorbs at 390 and 413 nm with mirror image emission bands at 416 and 441 nm. Notably, 279 exhibits quite a high emission quantum yield (ϕf = 0.55) with a single exponential decay (τf = 3.8 ns). These

Figure 63. UV−vis absorption spectra of 271 (solid black line), 271· HCl (dotted line), and 272 (solid gray line) in CH2Cl2 recorded at 298 K. Adapted with permission from ref 202. Copyright 2015 WileyVCH Verlag GmbH & Co. KGaA, Weinheim.

The trianionic {CCNN} core undergoes complexation with PCl3 to yield a hypervalent organophosphorus(V) derivative 272.202 Interestingly, an analogous triarylbiphenylcorrole of the group of Srinivasan, which incorporates a biphenyl moiety instead of the phenanthrene unit in 271 (and hence lacks the πextension pathway) shows nonaromatic behavior, while stabilizing Cu3+ inside its trianionic core.203 In another very recent report, the group of Latos-Grażyński reported core-modified phenanthrisapphyrins 273a−273c synthesized by the reaction of phenanthritripyrrane 270 and diols 155a/155d/155e in the presence of BF3·Et2O (Scheme 53).204 Unlike phenanthriporphyrin 271, these expanded analogues display limited macrocyclic π-delocalization, thereby demonstrating features of weakly antiaromatic compounds. This observation was further supported by NMR, UV, and Xray structural analyses of 273a−273c. For instance, the 1H NMR spectrum of 273a shows downfield chemical shifts of the inner phenanthrene CH protons when compared to its acyclic AT


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Scheme 53. Synthesis of Phenanthrisapphyrins 273a−273c

Scheme 54. Synthesis of Tetrabenzotetraaza[8]circulene 279

6. CONCLUSIONS The chemistry of annulated isomeric, contracted, and expanded porphyrins remains relatively unfamiliar compared to that of their porphyrin counterparts. However, encouraging research efforts have been observed in this area over the past few years as evidenced by a significant number of publications within a very short span of time. These efforts reveal that many annulated porphyrinoids share unique properties not previously observed in porphyrin chemistry. However, given the vast and ever-expanding domain of porphyrin chemistry, exploration of related new annulated isomeric, expanded, and contracted

Figure 64. UV−vis absorption spectra of 273a (black line), 273a·2H+ (2% TFA/CH2Cl2), gray line), and 275 (dashed line) recorded in CH2Cl2 at 298 K. Adapted with permission from ref 204. Copyright 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

spectroscopic features suggest that 279 possesses local aromaticity around the central COT unit.

Figure 65. Chemical structures of macrocycles 276a−276c and 277a−277d. AU


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ACKNOWLEDGMENTS

porphyrins remains a significant challenge. For example, except porphycenes, there have been no serious efforts toward the annulation of other isomeric porphyrins. The same is true for contracted isomers, and only a handful of examples are known. Of course, one cannot deny the underlying synthetic difficulties associated with these porphyrinoids. In this direction, the development of new organic synthetic strategies will definitely drive the development of new fused precursors, leading to novel annulated porphyrinoids with unprecedented structural and photophysical properties. In addition, fused pyrrolic precursors or their heteroanalogues, such as naphthobipyrrole, DTT, and benzopyrrole, could also be utilized toward the design and synthesis of a variety of new annulated porphyrinoids. Notably, recent progress in annulated porphyrin chemistry has revealed the usefulness of organometallic coupling reactions; in particular, Pd-catalyzed cross-coupling chemistry has been successfully implemented in order to attach/fuse aromatic rings onto the periphery of a number of porphyrinoid systems. We believe that this review will stimulate considerable interest among researchers to unveil new chemistry in this area in the near future.

This work was supported by the Science & Engineering Research Board (SERB), India (SR/S1/IC-56/2012 to P.K.P.).

ABBREVIATIONS Ac acetyl acac acetylacetonate AICD anisotropy of the induced current density BCOD bicyclo[2.2.2]octadiene BSA bovine serum albumin BTMA·ICl2 benzyltrimethylammonium dichloroiodate CAN ceric ammonium nitrate COT cyclooctatetraene Cp cyclopentadienyl DBU 1,8-diazabicyclo[5.4.0]undec-7-ene DCE 1,2-dichloroethane DDQ 2,3-dichloro-5,6-dicyano-1,4-benzoquinone DFT density functional theory DMAP 4-dimethylaminopyridine DMF N,N-dimethylformamide DMSO dimethyl sulfoxide DMAD dimethylacetylenedicarboxylate DTE dithienylethene DTT dithienothiophene ESR electron spin resonance EPR electron paramagnetic resonance FA folate Fc ferrocene GM Goeppert-Mayer HEPES 2-[4-(2-hydroxyethyl)piperazin-1-yl]ethanesulfonic acid HR-ESI-MS high-resolution electrospray ionization mass spectrometry HOMA harmonic oscillator model of aromaticity HOMO highest occupied molecular orbital ISC intersystem crossing LUMO lowest unoccupied molecular orbital LTMP lithium tetramethylpiperidide MALDI matrix-assisted laser desorption ionization MCD magnetic circular dichroism m-CPBA m-chloroperoxybenzoic acid Mes mesityl NICS nucleus-independent chemical shift NIR near-infrared NLO nonlinear optics NMR nuclear magnetic resonance NP nanoparticle o-DCB o-dichlorobenzene PBS phosphate-buffered saline PDT photodynamic therapy p-Tol p-tolyl PIFA phenyliodine bis(trifluoroacetate) p-TSA p-toluenesulfonic acid Py pyridine RDS rate-determining step rt room temperature p-TSA p-toluenesulfonic acid TD-DFT time-dependent DFT TEA triethylamine TFA trifluoroacetic acid THF tetrahydrofuran TIPS triisopropylsilyl

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected] or [email protected]. in. ORCID

Pradeepta K. Panda: 0000-0002-3243-5071 Present Address §

T.S.: Institute for Supramolecular Chemistry and Catalysis, Shanghai University, Shanghai, 200444, China. Notes

The authors declare no competing financial interest. Biographies Tridib Sarma was born and raised in Bhogpur, Assam, India. He obtained his B.Sc. degree in chemistry from B. Borooah College, Guwahati, Assam. Following his M.Sc. in chemistry with specialization in organic chemistry from Gauhati University, he joined the Panda group at the University of Hyderabad, India, in July 2007 and obtained his Ph.D. degree in chemistry in January 2014. He then joined The University of Texas at Austin, USA, and worked as a postdoctoral fellow with Prof. Jonathan L. Sessler. His research interests include design of novel porphyrinoids for application in photon therapy, nonlinear optics, artificial photosynthesis, and aromaticity. Pradeepta K. Panda was born in Odisha, India, in 1971. He received his B.Sc. from Utkal University, Bhubaneswar, and M.Sc. from the Indian Institute of Technology Kanpur in 1992. He obtained his Ph.D. from the Indian Institute of Science Bangalore in 2002 (supervisor, Prof. V. Krishnan). After postdoctoral research stints (including a JSPS fellowship) with Prof. Chang-Hee Lee (Kangwon National University, Korea) and Prof. Jun-ichiro Setsune (Kobe University, Japan), he joined the University of Hyderabad as an assistant professor in 2007 to start his independent research career, where he has been associate professor since 2012. His research interests include design of novel porphyrinoids in search of potential applications as therapeutics, materials for optoelectronics, sensing, diagnostics, and energy harvesting. AV


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tetramethylethylenediamine trimethylsilyl bromide two-photon absorption tetraphenylporphyrin tosyl two-photon absorption three-photon absorption tetrathiafulvalene ultraviolet X-ray photoelectron spectroscopy X-ray diffraction

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