The Tetraarylpyrrolo[3,2-b]pyrrolesâFrom Serendipitous Discovery to

Article pubs.acs.org/accounts

The Tetraarylpyrrolo[3,2‑b]pyrrolesFrom Serendipitous Discovery to Promising Heterocyclic Optoelectronic Materials Maciej Krzeszewski, Dorota Gryko, and Daniel T. Gryko* Institute of Organic Chemistry, Polish Academy of Sciences, Kasprzaka 44-52, 01-224 Warsaw, Poland CONSPECTUS: Progress in organic optoelectronics requires compounds possessing a suitable combination of photophysical and electronic properties. Another key constraint encompasses the availability of feasible, and hopefully scalable, synthetic procedures for preparing the molecular scaffolds of interest. A multicomponent reaction of aromatic aldehydes, aromatic amines, and butane-2,3-dione that was discovered in 2013 gives straightforward access to previously unavailable 1,2,4,5-tetraarylpyrrolo[3,2-b]pyrroles. These dyes are examples of heteropentalenesa class of 10-π-electron aromatic compounds. The unsurpassed variety of aromatic aldehydes and primary aromatic amines, which are commercially available or easy to prepare, allows for potential routes to thousands of 1,2,4,5-tetraarylpyrrolo[3,2-b]pyrroles that are currently unknown. This synthetic procedure offers a means for preparing the pyrrolopyroles in gram quantities and isolating them by simple filtration. Typically, the construction of an aromatic core is merely the first phase in a long procedure toward multistep functionalization. Conversely, the synthesis of 1,2,4,5-tetraarylpyrrolo[3,2-b]pyrroles leads to preinstalled substituents in frames with C2 symmetry, which “opens Sesame” to a wealth of structural possibilities. In addition, steric hindrance of the aldehyde components, rather than presenting a problem, is beneficial for increasing the yields of the products. This feature provides invaluable routes for the synthesis of a broad range of π-extended systems possessing the pyrrolo[3,2-b]pyrrole core in just a few steps. Indeed, this approach has enabled the preparation of a large number of previously unknown ladder-type heteroacenes possessing additional rings based on carbon−carbon, carbon−nitrogen, and nitrogen−nitrogen double bonds as well as nitrogen−boron single bonds. This set of chromophores includes planar and curved structures bearing up to 14 conjugated rings. 1,2,4,5-Tetraarylpyrrolo[3,2b]pyrroles manifest broad absorption bands between about 300 and 450 nm, strong violet-blue or blue fluorescence with typical quantum yields of ∼60%, significant Stokes shifts ranging between 3000 and 5800 cm−1, and emission while in the solid state. Should the two peripheral aryl groups have an electron-deficient character, the two-photon absorption cross section also becomes pronounced, i.e., ∼400 GM. Perhaps the most important feature of these dyes is their strong solvatofluorochromism, which predestines their value as environment-sensitive probes. Extension of the π-conjugation of 1,2,4,5-tetraarylpyrrolo[3,2-b]pyrroles enables further modifications of their photophysical properties, such as shifting the emission bathochromically, increasing the Stokes shift beyond 10 000 cm−1, and attaining solvatofluorochromism for curved, butterfly-shaped analogues without a decrease in emission intensity when the solvent polarity is increased. Common features of these chromophores include a significant difference between the geometries of their relaxed ground and relaxed excited states as well as strong electronic coupling through their aromatic cores. Past and future intense exploration of the wide chemical space built around the pyrrolo[3,2-b]pyrrole skeleton offers unprecedented opportunities for comprehensive elucidation of how photoexcitation increases the electronic coupling through biaryl linkages.

■

discovered by Hemetsberger and Knittel in 19729 and further elaborated by Mukai,10 it was not until 2013 that this area of research gained momentum as a result of our discovery of a onepot method for transforming simple and readily available building blocks into an unsurpassed variety of 1,2,4,5tetraarylpyrrolo[3,2-b]pyrroles.11 In this Account, we summarize the design, synthesis, chemistry, and physicochemical properties of pyrrolo[3,2-b]pyrroles. We first introduce the synthetic methodology leading to the key building blocks. Subsequently,

INTRODUCTION The past decade has witnessed a significant increase in activity in the chemistry of aromatic compounds.1,2 Consequently, the synthesis of large π-conjugated systems has become the mainstream of research.3−5 This trend is a corollary of application-driven research encompassing the likes of field-effect transistors, organic photovoltaics, and organic light-emitting diodes.6 Among the various goals within this research area, perhaps the most intriguing is a hunt for extreme properties with electron-excessive and electron-deficient building blocks.7 In this context, pyrrolo[3,2-b]pyrrole is integral as the most electronrich (unsubstituted) small heterocycle.8 Although it was © 2017 American Chemical Society

Received: June 6, 2017 Published: August 10, 2017 2334

DOI: 10.1021/acs.accounts.7b00275 Acc. Chem. Res. 2017, 50, 2334−2345

Article

Accounts of Chemical Research we present an overview of structural possibilities that can be reached starting from pyrrolo[3,2-b]pyrroles. Finally, we discuss their optoelectronic properties.

Scheme 2. Plausible Mechanism for the Synthesis of TAPPs

■

SYNTHESIS OF 1,2,4,5-TETRAARYLPYRROLO[3,2-B]PYRROLES Our journey with 1,2,4,5-tetraaryl-1,4-dihydropyrrolo[3,2-b]pyrroles (from now on called tetraarylpyrrolo[3,2-b]pyrroles, or TAPPs) began with a serendipitous discovery regarding the wellknown Debus−Radziszewski imidazole synthesis.12,13 We encountered an unexpected outcome in a condensation between 4-cyanobenzaldehyde, p-toluidine, butane-2,3-dione, and ammonium acetate in acetic acid (Scheme 1). Quickly after addition of Scheme 1. Serendipitous Discovery of the Multicomponent Reaction Leading to Tetrarylpyrrolo[3,2-b]pyrrole 5

intermediate, tetrahydropyrrolo[3,2-b]pyrrole, is actually oxidized by the excess of Schiff base present in the reaction mixture. This hypothetical pathway is supported by a positive outcome of the reaction between the prior-synthesized Schiff base and butane-2,3-dione as well as isolation of tetrahydropyrrolo[3,2b]pyrrole from the reaction of 2,6-dichlorobenzaldehyde with 4tert-butylaniline and butane-2,3-dione.14 An alternative mechanism in which aldol condensation of the aldehyde with butane2,3-dione occurs first (and undergoes Michael addition to the amine) was ruled out by an experiment conducted with the corresponding α,β-unsaturated ketone and p-toluidine, which failed to provide TAPP.11 Low yields of TAPPs led to optimization of the conditions of this multicomponent reaction. The conducted study concluded that a catalytic amount of strong acid (either Lewis or Brønsted) increases the reaction yield. The most significant improvement (up to 49% yield) was seen when p-toluenesulfonic acid was added to the reaction mixture.17 The scope of this methodology (aldehyde/amine/butane-2,3-dione = 2:2:1, TsOH, AcOH, 90 °C, 3 h) proved to be general, with some limitations. A pivotal building block is butane-2,3-dione, which seems irreplaceable. Using analogous 1,4-dibromobutane-2,3-dione, hexane-3,4dione, or diethyl 3,4-dioxohexanedioate gave no expected products.11,14 All attempts with aliphatic aldehydes and aliphatic amines failed as well. Nevertheless, when aromatic aldehydes, aromatic amines, and butane-2,3-dione are used as the substrates, this reaction is a powerful tool for constructing elaborated TAPPs. This methodology accepts a very wide variety of benzaldehydes possessing both electron-withdrawing and electron-donating groups. However, with 1-formylnaphthalene, anthracene-9-carboxaldehyde,15 and some heterocyclic aldehydes, the yields are usually significantly lower (Figure 1). The range of applicable anilines is somehow limited, and typically their primary role is to improve the solubility of TAPPs (the series of TAPPs originating from aniline or p-toluidine have very limited solubility in common organic solvents). Interestingly, using 1-aminonaphthalene did not give rise to the formation of TAPP, whereas even more sterically congested compounds such as 2-bromoaniline and 2-aminobiphenyl proved to be viable substrates for this reaction.14 The scope of multicomponent reactions leading to TAPPs is comprehensively illustrated with examples in Table 1. The most important examples of primary

butane-2,3-dione, a strongly fluorescent yellow crystalline product started to precipitate from the reaction mixture. Whereas the expected imidazole 4 present in the supernatant was formed only in 2% yield, the main product (20% yield) was 1,2,4,5tetraarylpyrrolo[3,2-b]pyrrole 5. This structure assignment was confirmed by NMR spectroscopy and later by X-ray crystallography. The identification of this heterocycle prompted an investigation concerning the possible mechanism of this multicomponent reaction. As proven in early-stage experiments, the first step is the formation of the Schiff base from the aldehyde and amine, most likely followed by Mannich-type addition to butane-2,3-dione. The next stage of this process remains rather uncertain, with the most plausible order of steps shown in Scheme 2. The formed β-amino ketone cyclizes with the formation of a five-membered ring. Subsequent dehydration gives rise to the enamine, which reacts with a second molecule of the Schiff base, followed by a second cyclization and dehydration to form a bicyclic compound. The intriguing aspect that has not yet been elucidated thoroughly is the role of the oxidizing agent, since the described pathway inevit ably ends on tetrahydropyrrolo[3,2-b]pyrrole. All attempts to add oxidizing agents of different strength and nature (e.g., DDQ, DMSO, Ce(NH4)2(NO3)6, and nitrobenzene) at various points of this reaction (i.e., simultaneously with the substrates or a few hours after the addition of butane-2,3-dione) failed to increase the yield of TAPPs.11,14 The most probable explanation is that the 2335


Article

Accounts of Chemical Research

Figure 1. Exemplary TAPPs prepared from other aldehydes. Notes: aYields obtained with unoptimized procedure (without TsOH). bCompound obtained in a statistical reaction. cProduct obtained from 51 or 52 via Buchwald−Hartwig amination. dCorresponding Schiff base obtained and purified prior to reaction with butane-2,3-dione.

of (1) lack of reactivity of 3,6-diformyl-TAPPs in Knoevenagel reactions with malonitrile, diethyl malonate, and 4-methylpyridine oxide and (2) lack of reactivity of 3,6-dibromo-TAPPs in the Sonogashira reaction and Suzuki and Stille coupling.14 To resolve this latter problem, we resorted to direct arylation methodology.34 The only reaction conditions that proved to be feasible were those reported by Doucet for sterically congested pyrroles.35 Hexaarylpyrrolopyrroles (HAPPs) such as 59 were formed in reasonable yields for electron-poor bromides (Scheme 4). The reactivity of peripheral functional groups and moieties can also be utilized to bring about interesting transformations of TAPPs. Starting from 2,5-bis(pyridin-4-yl)-1,4-di-p-tolyl-1,4dihydropyrrolo[3,2-b]pyrrole (55) it was possible to obtain TAPP 60 possessing azulenyl moieties attached at positions 2 and 5 of the pyrrolopyrrole core.31 The same pyridinyl derivative has been utilized in the synthesis of 61, the first water-soluble analogue comprising the pyrrolo[3,2-b]pyrrole core (Scheme 5).32

aromatic amines and aromatic aldehydes that failed to give TAPPs are collected in Figure 2. However, this reaction is not clean, and TLC inspection reveals TAPP as one of many products. In a great majority of cases, TAPP is the only product that crystallizes out directly from the crude reaction mixture. The rationale is that TAPPs are the only products lacking a basic nitrogen atom, while the side products possessing this functionality form salts with acetic acid. This situation closely resembles the Adler−Longo porphyrin synthesis, where tetraarylporphyrins also precipitate directly from propionic acid in essentially pure form.33 In principle, the synthesized pyrrolo[3,2-b]pyrroles possess a symmetrical architecture. It is possible to obtain unsymmetrical TAPPs possessing a push−pull system in a statistical manner by using two different benzaldehydes (Figure 1).26,30 Fascinatingly, the highest yields to date were obtained for sterically congested ortho-substituted benzaldehydes. The intrinsically electron-rich nature of the pyrrolo[3,2b]pyrrole skeleton is responsible for the fast decomposition of the parent molecule under ambient conditions as reported by Mukai and co-workers.10 In this context, decomposition of 1,2,4,5-tetraarylpyrrolo[3,2-b]pyrroles is prevented by the presence of four benzene rings. Simultaneously, TAPPs are subject to the same rules as other aromatic compounds, i.e., the presence of electron-donating substituents decreases the first oxidation potential, so the presence of light and oxygen decomposition is very fast, especially in solution.

■

π-EXPANSION OF TAPPS Linking a given chromophore with another aromatic unit via a carbon−carbon triple bond is the most popular strategy for synthesizing organic π-conjugated materials. As pointed out earlier, 3,6-dibromo-TAPPs do not undergo Sonogashira reaction.14 It was proven, however, that TAPPs 13 and 41 possessing TMS-ethynyl substituents at positions 2 and 5 can serve as substrates for the sila-Sonogashira reaction (Scheme 6).16 Even more exciting are transformations directly concerning the core. Free positions 3 and 6 of the intrinsically electronexcessive backbone of 1,2,4,5-tetraarylpyrrolo[3,2-b]pyrroles offer ideal reaction sites for a number of transformations that eventually extend the conjugated π-system. Indeed, a series of TAPPs bearing an additional aryl or heteroaryl ring at the ortho position with respect to the central core (24−29, 42, and 50) were obtained in moderate to high yields. These compounds smoothly underwent oxidative aromatic coupling to give π-expanded ladder-type dyes 69−76 bearing the indolo[3,2-b]indole skeleton (Scheme 7).19 During this study, we surprisingly learned the crucial role played by steric effects in this methodology.22 When steric congestion right next to the oxidation site is increased, attack of the formed radical cation on an already occupied position is kinetically and

■

REACTIVITY OF TAPPS One of the striking features of 1,2,4,5-tetraarylpyrrolo[3,2b]pyrroles is that they possess two free, strongly electron-rich positions, namely, positions 3 and 6. Their reactivity in electrophilic aromatic substitution can be compared to that of their analogues pyrrole and indole. Many classical electrophilic aromatic substitutions occur rather easily, such as bromination, iodination, nitration, formylation, and cyanation with the chlorosulfonyl isocyanate/DMF system (Scheme 3).14 These functional groups at positions 3 and 6 should enable further transformations, which to date have been limited in scope. The decreased reactivity of functional groups placed in these positions is due to their direct connectivity to the very electron-rich pyrrolo[3,2-b]pyrrole core (although steric congestion resulting from the presence of two flanking aryl substituents also contributes). The exemplary failures consist 2336


Article

Accounts of Chemical Research Table 1. Scope of the One-Pot Synthesis of Tetraarylpyrrolo[3,2-b]pyrroles11,15−29

TAPP

R1

5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42b 43 44 45

H H H H H H H H H Br H OMe H H NO2 NO2 NO2 NO2 NO2 X2 X2 X3 X4 X5 X6 H H X2 X6 X7 X8 NO2 NO2 NO2 NO2 X1 H X2 H H H

R2

R3

R4

R5

R6

yield (%)

H CN H CN H H H Me H H H H H Me H H OMe H O−CH2−O H H Br H H X1 H H H H H SF5 H H H H H CN H H SF5 H H H H H Br H H O−CH2−O H F H H CF3 H H H H H CN H H H H H H H H H H H H H H NO2 H H CF3 H H H H H H H H H H H H H H H H H H F OMe H H H H H H H H H X1 H H NO2 H H H H H Ph2N H H H H

H H H H H H H H H H H H H H H H H H H H H H H H H H H Br Br Br H H H H H H H H H H H

Me Me Br Me Me Me Me Me Me Me Me Me Br SF5 n-Dec n-Hex n-Hex n-Bu n-Bu t-Bu n-Oct t-Bu t-Bu t-Bu n-Oct n-Oct Me t-Bu t-Bu t-Bu t-Bu n-Oct n-Bu n-Hex OMe Me n-Oct n-Oct CO2Me H H

37 34 22 34 34 15a 13a 15a 15a 49 35 45 23 26 24 35 24 30 25 48 33 25 37 38 40 36 35 45 36 35 43 35 14 34 32 21 11 14c 16 26 15

Figure 2. Substrates that failed to produce the desired TAPPs.

Scheme 3. Electrophilic Aromatic Substitutions of TAPPs

thermodynamically favored to produce pyrrolopyrrolium salt 77 possessing a new spiro carbon atom (Scheme 7). An alternative approach toward the synthesis of π-expanded indolo[3,2-b]indoles 86−92 relies on the InCl3-mediated double cyclization of TAPPs possessing o-(arylethynyl)phenyl substituents formed in a sila-Sonogashira reaction from 78 (Scheme 8).25 Although indolo[3,2-b]indole has been known since 1884,8,36 new approaches leading to compounds bearing this skeleton have been proposed by Liu37 and Langer and co-workers.38 Even more recently, the groups of Jin39 and Du40 discovered new copper-mediated pathways toward these heterocycles (Scheme 9). A versatile library of π-expanded derivatives of nitrogen-doped polycyclic aromatic hydrocarbons have been synthesized utilizing the corresponding TAPPs 19−23 and 36−39 possessing orthosituated nitro groups (Table 2). Conducting a reaction between nitro-substituted pyrrolo[3,2-b]pyrroles 19−23 and triethyl phosphite (the Cadogan reaction) leads to the formation of unprecedented heteroacenes 97−101 comprising four consecutive fused pyrrole rings.18 Reduction of the nitro groups followed by reactions with tert-butyl nitrate, dichlorophenylborane, and aldehydes leads to the formation of dicinnolinopyrrolo-

a

Yield obtained with the unoptimized procedure (without TsOH). Compound obtained in a Suzuki reaction from the corresponding obromo derivative. cOverall yield starting from 2-bromo-4-nitrobenzaldehyde. b

2337


Article

Accounts of Chemical Research Scheme 4. Exemplary Synthesis of Penta- and Hexaaryl Pyrrolopyrroles by Means of Direct Arylation

Scheme 6. One-Pot Sila-Sonogashira Reaction toward Peripherally π-Expanded TAPPs

Scheme 5. Transformations of TAPP 55 Possessing Pyridine Moieties

[3,2-b]pyrroles 102−105,24 pyrrolo[3,2-b]pyrrole-based BNheteroacenes 106−109,24 and diquinolinopyrrolo[3,2-b]pyrroles 110−116, respectively (Table 2).23 In 2016, Mo and co-workers described another approach relying on the Cadogan reaction as a key step, also leading to πconjugated heterocycles possessing more than two pyrrole rings (Scheme 10).41 By a combination of intramolecular direct arylation and oxidative aromatic coupling, we were able to synthesize highly conjugated derivatives 120, which adopted helicene-like architectures as a result of the steric repulsion between spatially adjacent aryl rings.21 The successful preparation of TAPPs 32− 34 in high yields (35−45%) starting from the corresponding 2bromo-6-(hetero)arylbenzaldehydes yet again proved that sterically congested benzaldehydes serve as perfect substrates in this multicomponent reaction (Scheme 11). The synthesized dye 120 exhibits three different isomeric forms because of its helical structure: two twisted enantiomers, (P,P) and (M,M), and one folded meso form, (P,M). These structures were confirmed by X-ray crystallography (Figure 3). TD-NMR experiments and DFT calculations showed miniscule energy barriers, making separation based on isomers impossible. 2338


Article

Accounts of Chemical Research Scheme 8. InCl3-Mediated Cyclization Leading to πExpanded Indolo[3,2-b]indoles 86−92

Scheme 7. Oxidative Aromatic Coupling of TAPPs

Scheme 9. Copper-Mediated Annulations toward Indolo[3,2b]indole Derivatives

■

OPTOELECTRONIC PROPERTIES It is worth emphasizing that 1,4-dihydropyrrolo[3,2-b]pyrrole was predicted to be the most efficient electron donor among the 10-π-electron systems. 42 This was confirmed by cyclic voltammetry data, as shown in Figure 4. The emission properties of TAPPs are strong, with solution fluorescence quantum yields (Φfl) typically well above 60% and intense solid-state emission (Figures 5−8).11,15,17 Depending on the type of substituents, λem in TAPPs shifts from 400 to 552 nm. A comparative study of the photophysical properties of various tetraarylpyrrolo[3,2-b]pyrroles clearly proves the efficient electronic communication across the skeleton of these molecules. The presence of electron-withdrawing substituents on the arene rings located at positions 2 and 5 has a particularly profound influence on both the absorption and emission maxima, causing substantial red shifts. The origin of this effect is associated with moderate dihedral angles between the C-aryl substituents and the pyrrolo[3,2-b]pyrrole core (35.6° as probed by X-ray crystallography),20 which ensures certain electronic communication in the ground state. Computational studies revealed that in the excited state this dihedral angle decreases to ∼26°.26 This fundamental difference in orbital overlap is particularly visible with TAPPs possessing two arylethynylaryl substituents at positions 2 and 5 (Figure 6). Their absorption is only slightly bathochromically shifted versus classical TAPPs bearing the same peripheral groups, whereas the emission is

gradually red-shifted with increasing strength of the electronwithdrawing groups, reaching 523 nm for SF5 (63) or even 621 nm in the case of CHO (67).16,26 This strong red shift proves the existence of electronic communication in the excited state throughout the whole molecule. This effect is also visible, in spite of the bent Z-like conformation, for regioisomeric TAPPs possessing arylethynyl substituents at position 2 of the 2,5-aryl rings (Figure 5).25 π-Expansion of the TAPP chromophore, realized in various ways,18,19,21,23−26 leads to significant changes in the photophysical properties (Figures 7 and 8). For dyes 69−76 lacking 2339


Article

Accounts of Chemical Research Table 2. Various Approaches toward π-Expanded Aza Analogues of Pyrrolo[3,2-b]pyrroles

TAPP

R1

R2

DIPP

yield (%)

DCPP

yield (%)

BNPP

yield (%)

QPPQ

R3

yield (%)

19 20 21 22 23 36 37 38 38 38 39

H 4-Br 4,5-OCH2O 4-F 4-CF3 H 5-F 3-OMe 3-OMe 3-OMe H

n-Dec n-Hex n-Hex n-Bu n-Bu n-Oct n-Bu n-Hex n-Hex n-Hex OMe

97 98 99 100 101 − − − − − −

34 42 61 34 50 − − − − − −

− − − − 102 103 104 105 − − −

− − − − 45 37 71 57 − − −

− − − − 106 107 108 109 − − −

− − − − 80 71 64 44 − − −

− − − − 110 111 112 113 114 115 116

− − − − CN CN OMe CN Me OMe CN

− − − − 63 38 88 54 65 42 60

presence of additional aryl substituents can be a game changer, however, as in the case of dyes 86−92.25 While dyes 69−76 and 86−92 have almost identical absorption maxima, the emission of the latter is markedly bathochromically shifted (465−530 nm vs 420−460 nm). This outcome points to negligible conjugation between the benzo[g]benzo[6,7]indolo[3,2-b]indole core and phenyl substituents in the ground state, whereas there is a significant change of geometry in the excited state. The stronger analogous effect is visible with 7,14-dihydroquinolino-[3″,4″:4′,5′]pyrrolo[2′,3′:4,5]pyrrolo[3,2-c]quinolines 110−116 (QPPQs).23 All of the QPPQs absorb in the UV region with a single strong band at 305−322 nm and a shoulder at around 360 nm (Figure 7). Their fluorescence maxima are in the 453−527 nm region, resulting in large Stokes shifts (of almost 14 000 cm−1 for 112). A relationship between the character of the substituents and the absorption maxima was not observed. This means that the twists of the phenyl substituents (the dihedral angles are ca. 55−72°) suppress the electronic communication between these fragments and the core of the molecule in the ground state.

Scheme 10. Synthesis of Diindole[3,2-b:4,5-b′]pyrrole

additional heteroatoms, extension of the conjugation results in a bathochromic shift of the absorption but a hypsochromic shift of the emission.19 The origin of this unexpected phenomenon probably lies in the fact that the dihedral angle between main chromophore and the N-aryl substituent is close to 90° (on the basis of the X-ray structure), which translates to a lack of conjugation with the core (unlike in the case of TAPPs, where the dihedral angle between N-aryl substituents and the pyrrolopyrrole moiety is 45.8°).20 The absorption maxima of dyes 69−76 and 86−92 do not significantly differ from those of indolo[3,2-b]indoles.37,38 This suggests that angularly fused benzene rings have little effect on the HOMO−LUMO gap of the pyrrolo[3,2-b]pyrrole core. The 2340


Article

Accounts of Chemical Research Scheme 11. Exemplary Synthetic Pathway toward Curved πExpanded Pyrrolo[3,2-b]pyrroles

Figure 4. Comparison of the HOMO levels of pyrrolo[3,2-b]pyrrole and related molecules based on cyclic voltammetry.8,17,18

Figure 5. Absorption (solid) and emission (dotted) spectra of compounds 17 (green) and 16 (blue) in DCM and 85 (orange) and 30 (red) in toluene.

Figure 6. Absorption (solid) and emission (dotted) spectra of compound 97 (blue) in toluene and compounds 59 (green), 70 (orange), and 63 (red) in DCM.

are in the 395−426 nm region, which results in Stokes shifts of less than 900 cm−1. This result suggests similar static geometry for the S1 excited state and the S0 ground state. All of the BNPPs were strongly fluorescent in both solution and the solid state, with Φfl reaching 78% in toluene (Figure 7). Perhaps the most intriguing was the discovery of symmetrybreaking in the S1 excited state for these quadrupolar, centrosymmetric molecules possessing strong electron-withdrawing groups at positions 4 of the C-aryl substituents. This effect was probed by Vauthey and co-workers for dye 5 possessing two CN groups.43,44 That study revealed that the symmetry-breaking could be visualized using ultrafast timeresolved infrared spectroscopy by monitoring the CN stretching modes. The investigation eventually led to the development of a simple model describing the symmetry-breaking of the electronic distribution of centrosymmetric molecules in the excited state.45

Figure 3. Molecular structures of two forms of the aza analogue of double helicene (hydrogen atoms and tert-butyl groups have been omitted for clarity).

The intramolecular charge redistribution followed by nuclear reorganization upon excitation is not universal, though. For BNPPs 106−109, the absorption bands were at almost the same wavelengths as those of their isoelectronic analogues 86−92 (Figure 7). Furthermore, aryl rings attached both to the nitrogen atoms and to the boron atoms were similarly twisted by ca. 56− 60° and 60°, respectively.24 However, their fluorescence maxima 2341


Article


Figure 7. Optical properties of selected π-expanded analogues comprising the pyrrolo[3,2-b]pyrrole skeleton.

whereas 2,5-bis(3-nitrophenyl)-TAPP behaved like “write-readerase-read” (typical for FLASH memory devices). The strong electronic coupling between the electron-rich core and the substituents can also be utilized if substituents have electron-donating character. Leung and co-workers discovered that 2,5-bis(triphenylamine)-substituted pyrrolo[3,2-b]pyrrole 44 reacts with halocarbons to form deeply colored species.28 This enables quick detection of CHCl3 with the naked eye. Since the photochromic responses differ in color, the mechanism of photodecomposition must follow electron transfer from the electron-rich dye to the chlorinated compound followed by chlorination, bromination, or iodination at positions 3 and 6 followed by further oxidation of these products to highly colored species. Another mode of chloroform detection was published by Dong and co-workers, who observed a change in the emission of thin films of 1,2,4,5-tetraphenylpyrrolo[3,2-b]pyrrole 45 in the presence of chloroform and light.29 Interestingly, the authors also noticed that the overall decrease in luminescence intensity accompanied by a bathochromic shift in the emission maximum is reversible, indicating that a different mechanism operates in this case. A TAPP possessing more complex substituents on the nitrogen atoms was found to exist in three different polymorphic forms that differ in emission maximum and can be interconverted by exposure to CHCl3 vapor.27 In this case the process is based on crystalline-induced emission (CIE). Langa and co-workers studied TAPPs possessing strongly electron-withdrawing groups at positions 2 and 5 as electrondonors in bulk-heterojunction solar cells, but the overall efficiency was limited because of the low charge mobility (10−9 cm2 V−1 s−1) measured for holes.47 Equally interesting are the two-photon absorption (2PA) properties of TAPPs. Given their intrinsically quadrupolar nature and the presence of the electron-excessive central unit, TAPPs

Figure 8. Fluorescence in solution and the solid state of selected dyes: from left to right 100 and 106 in toluene and 5 and 53 in DCM.

Similar striking phenomena were observed for its analogue 30 possessing two NO2 groups: the fluorescence was extremely strong in very unpolar solvents (Φfl = 96% in cyclohexane), poor (and bathochromically shifted) in THF, and undetectable in DCM.20 Similarly, a strong solvatofluorochromic effect has also been observed for various analogues of dye 30, varying in the level of rigidity and different dihedral angles.26 Such a solvatofluorochromic effect is very weak if NO2 is replaced with CN or CF3. The strong charge-transfer characteristics of acceptor− donor−acceptor (A−D−A)-type TAPPs have also been exploited in other areas of research. Basak and co-workers studied the performance of TAPPs possessing 4-nitrophenyl and 3-nitrophenyl substituents at positions 2 and 5.46 They found that after fabrication of organic resistive memory devices (ORMs), there is a significant difference in behavior of the two molecules. The 2,5-bis(4-nitrophenyl)-TAPP fitted characteristics of a “write-once-read-many” type of memory device, 2342



■

are positioned excellently to display significant 2PA cross sections (σ2) as long as the molecules possess two electronwithdrawing units at positions 2 and 5. Indeed, comprehensive studies led to the observation that 2PA spectra show a dominant band with maximum wavelengths in the range 640−800 nm that originates from the only nondark transitions in the two-photon spectra i.e. HOMO → LUMO+1 and HOMO → LUMO +3.17,20,26 In such cases, σ2 reaches 200−600 GM and is even larger (up to 1400 GM) for TAPPs bearing arylethynyl units at positions 2 and 5.16 These are not large values per se, but they are among the largest for chromophores possessing biaryl linkages as the tether between the donor and acceptor.

Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Dorota Gryko: 0000-0002-5197-4222 Daniel T. Gryko: 0000-0002-2146-1282 Notes

The authors declare no competing financial interest. Biographies

■

Maciej Krzeszewski graduated with honors from the Faculty of Chemistry of the Warsaw University of Technology in 2012. He obtained his Ph.D. with distinction from the Institute of Organic Chemistry of the Polish Academy of Sciences in 2017 under the supervision of Prof. D. T. Gryko. His Ph.D. thesis as well as current research are devoted to the synthesis and studies of the optical properties of π-expanded analogues of pyrrolo[3,2-b]pyrroles with a special emphasis on the curved architectures.

SUMMARY AND OUTLOOK Until recently, all of the synthetic strategies leading to 1,4dihydropyrrolo[3,2-b]pyrroles were elaborate and narrow in scope, and the overall yields were very low.8−10 As a result, these dyes were overshadowed by thieno[3,2-b]thiophene and its analogues. The discovery of the multicomponent reaction between aldehydes, amines, and butane-2,3-dione has altered the picture entirely. Although the yields of TAPPs are moderate to good (30−50%), this strategy possesses numerous advantages compared with previously developed methods: (1) the desired product is typically isolated by means of a chromatography-free procedure; (2) the scope is broad; (3) over 500 varieties of both key starting materials are commercially available; and (4) it is one-pot transformation. Judicious design of tetra-arylated pyrrolopyrroles possessing substituents such as bromine atoms, aryl rings, nitro groups, and triple bonds at the ortho position with respect to the central core facilitates further transformations that lead to π-expansion of the system. A combination of two factors is responsible for the wealth of π-expansion pathways realized to date: (1) the presence of two free electron-rich positions on the core of TAPPs and (2) high yields of TAPPs from sterically hindered aldehydes. The presence of two 2-nitrophenyl groups at positions 2 and 5 enables a broad range of condensations effectively closing another five- or six-membered ring, leading to heretoforeunknown heterocyclic skeletons. Sparkling, typically blue fluorescence that is strong in both solution and the solid state is accompanied by other beneficial photophysical properties such as large Stokes shifts and strong two-photon response. In many cases, the combination of an unusually strong donor and very strong acceptor make it possible to break the symmetry of the excited state. Most importantly, all of the evidence collected points out that pyrrolo[3,2-b]pyrrole is an efficient linker allowing the conjugation of peripheral benzene rings, which in turn enables broad manipulation of the photophysical properties. While four years of development has brought a number of new ladder-type chromophores possessing the pyrrolo[3,2-b]pyrrole backbone, multiple challenges still remain, such as the synthesis of (1) heteroacenes comprising more than four conjugated pyrrole rings; (2) heteroacenes possessing heteroatoms such as Si, S, and P; (3) NH-free pyrrolo[3,2-b]pyrroles, and (4) grossly curved architectures comprising this core. Harnessing the reactivity of functional groups located at positions 3 and 6 and elucidation of the mechanistic details of TAPPs’ formation represent other challenges for organic synthesis. In only a few years of research in this field, the tremendous library of scaffolds and their application appear limitless.

Dorota Gryko obtained her Ph.D. from the Institute of Organic Chemistry of the Polish Academy of Sciences in 1997 under the supervision of J. Jurczak. After a postdoctoral stay with J. Lindsey at NCSU (1998−2000), she started her independent career in Poland. In 2009 she received the prestigious TEAM Grant from the Foundation for Polish Science. Her current research interests are focused on photoredox catalysis and vitamin B12 chemistry. Daniel T. Gryko was born in Białystok in 1970. He obtained his Ph.D. from the Institute of Organic Chemistry of the Polish Academy of Sciences in 1997 under the supervision of J. Jurczak. After postdoctoral studies with J. Lindsey at NCSU (1998−2000), he began his independent career in Poland. His current research interests are focused on the synthesis of corroles, diketopyrrolopyrroles, and pyrrolo[3,2b]pyrroles as well as on two-photon absorption, solvatofluorochromism, excited-state intramolecular proton transfer, and fluorescence imaging.

■

ACKNOWLEDGMENTS D.T.G. thanks all of the past and present group members who worked on pyrrolo[3,2-b]pyrroles: Anita Janiga, Maciej Krzeszewski, Yevgen M. Poronik, Mariusz Tasior, Rafał Stez̨ ẏ cki, Bartłomiej Sadowski, Olena Vakuliuk, and Rafał Orłowski. The authors thank the National Science Centre, Poland (Grants MAESTRO-2012/06/A/ST5/00216, PRELUDIUM 2015/19/ N/ST5/00826, and SYMFONIA 2014/12/W/ST5/00589) for financial support. M.K. thanks the Foundation for Polish Science for a START Scholarship.

■

REFERENCES

(1) Gulevich, A. V.; Dudnik, A. S.; Chernyak, N.; Gevorgyan, V. Transition Metal-Mediated Synthesis of Monocyclic Aromatic Heterocycles. Chem. Rev. 2013, 113, 3084−3213. (2) Fukazawa, A.; Yamaguchi, S. Ladder π-Conjugated Materials Containing Main-Group Elements. Chem. - Asian J. 2009, 4, 1386− 1400. (3) Segawa, Y.; Ito, H.; Itami, K. Structurally uniform and atomically precise carbon nanostructures. Nat. Rev. Mater. 2016, 1, 15002. (4) Narita, A.; Wang, X.-Y.; Feng, X.; Müllen, K. New advances in nanographene chemistry. Chem. Soc. Rev. 2015, 44, 6616−6643. (5) Stępień, M.; Gońka, E.; Ż yła, M.; Sprutta, N. Heterocyclic Nanographenes and Other Polycyclic Heteroaromatic Compounds: Synthetic Routes, Properties, and Applications. Chem. Rev. 2017, 117, 3479−3716. 2343


Article

Accounts of Chemical Research (6) Hu, W. Organic Optoelectronics; Wiley-VCH: Weinheim, Germany, 2013. (7) Po, R.; Bianchi, G.; Carbonera, C.; Pellegrino, A. All That Glisters Is Not Gold”: An Analysis of the Synthetic Complexity of Efficient Polymer Donors for Polymer Solar Cells. Macromolecules 2015, 48, 453−461. (8) Janiga, A.; Gryko, D. T. 1,4-Dihydropyrrolo[3,2-b]pyrrole and Its π-Expanded Analogues. Chem. - Asian J. 2014, 9, 3036−3045. (9) Hemetsberger, H.; Knittel, D. Synthese und Thermolyse von αAzidoacrylestern. Monatsh. Chem. 1972, 103, 194−204. (10) Kumagai, T.; Tanaka, S.; Mukai, T. Synthesis of 1,4dihydropyrrolo[3,2-b]pyrrole. Tetrahedron Lett. 1984, 25, 5669−5672. (11) Janiga, A.; Glodkowska-Mrowka, E.; Stoklosa, T.; Gryko, D. T. Tetraaryl-1,4-dihydropyrrolo[3,2-b]pyrroles − synthesis and optical properties. Asian J. Org. Chem. 2013, 2, 411−415. (12) Debus, H. Ueber die Einwirkung des Ammoniaks auf Glyoxal. Justus Liebigs Ann. Chem. 1858, 107, 199−208. (13) Radziszewski, B. Ueber die Constitution des Lophins und verwandter Verbindungen. Ber. Dtsch. Chem. Ges. 1882, 15, 1493−1496. (14) Janiga, A.; Krzeszewski, M.; Poronik, Y. M.; Tasior, M.; Vakuliuk, O.; Sadowski, B.; Gryko, D. T. Unpublished results. (15) Gryko, D. T.; Janiga, A.; Krzeszewski, M. Strongly fluorescent heterocycles and a method for their synthesis. WO2014070029A8, 2013. (16) Janiga, A.; Bednarska, D.; Thorsted, B.; Brewer, J.; Gryko, D. T. Quadrupolar, emission-tunable π-expanded 1,4-dihydropyrrolo[3,2b]pyrroles − synthesis and optical properties. Org. Biomol. Chem. 2014, 12, 2874−2881. (17) Krzeszewski, M.; Thorsted, B.; Brewer, J.; Gryko, D. T. Tetraaryl-, Pentaaryl-, and Hexaaryl-1,4-dihydropyrrolo[3,2-b]pyrroles: Synthesis and Optical Properties. J. Org. Chem. 2014, 79, 3119−3128. (18) Janiga, A.; Krzeszewski, M.; Gryko, D. T. Diindolo[2,3-b:2′,3′f ]pyrrolo[3,2-b]pyrroles as Electron-Rich, Ladder-Type Fluorophores: Synthesis and Optical Properties. Chem. - Asian J. 2015, 10, 212−218. (19) Krzeszewski, M.; Gryko, D. T. χ-Shaped Bis(areno)-1,4dihydropyrrolo[3,2-b]pyrroles Generated by Oxidative Aromatic Coupling. J. Org. Chem. 2015, 80, 2893−2899. (20) Friese, D. H.; Mikhaylov, A.; Krzeszewski, M.; Poronik, Y. M.; Rebane, A.; Ruud, K.; Gryko, D. T. Pyrrolo[3,2-b]pyrroles − from unprecedented solvatofluorochromism to two-photon absorption. Chem. - Eur. J. 2015, 21, 18364−18374. (21) Krzeszewski, M.; Kodama, T.; Espinoza, E. M.; Vullev, V. I.; Kubo, T.; Gryko, D. T. Non-planar butterfly-shaped π-expanded pyrrolopyrroles. Chem. - Eur. J. 2016, 22, 16478−16488. ́ (22) Krzeszewski, M.; Swider, P.; Dobrzycki, Ł.; Cyrański, M. K.; Danikiewicz, W.; Gryko, D. T. The role of steric hindrance in intramolecular oxidative aromatic coupling of pyrrolo[3,2-b]pyrroles. Chem. Commun. 2016, 52, 11539−11542. (23) Tasior, M.; Chotkowski, M.; Gryko, D. T. Extension of pyrrolopyrrole π-system: approach to constructing hexacyclic nitrogen-containing aromatic systems. Org. Lett. 2015, 17, 6106−6109. (24) Tasior, M.; Gryko, D. T. Synthesis and Properties of Ladder-Type BN-Heteroacenes and Diazabenzoindoles Built on a Pyrrolopyrrole Scaffold. J. Org. Chem. 2016, 81, 6580−6586. (25) Stężycki, R.; Grzybowski, M.; Clermont, G.; Blanchard-Desce, M.; Gryko, D. T. Z-Shaped Pyrrolo[3,2-b]pyrroles and Their Transformation into π-Expanded Indolo[3,2-b]indoles. Chem. - Eur. J. 2016, 22, 5198−5203. (26) Łukasiewicz, Ł.; Ryu, H. G.; Mikhaylov, A.; Azarias, C.; Banasiewicz, M.; Kozankiewicz, B.; Ahn, K. H.; Jacquemin, D.; Rebane, A.; Gryko, D. T. Symmetry Breaking in Pyrrolo[3,2-b]pyrroles: Synthesis, Solvatofluorochromism, and Two-photon Absorption. Chem. - Asian J. 2017, 12, 1736−1748. (27) Ji, Y.; Peng, Z.; Tong, B.; Shi, J.; Zhi, J.; Dong, Y. Polymorphismdependent aggregation-induced emission of pyrrolopyrrole-based derivative and its multi-stimuli response behaviors. Dyes Pigm. 2017, 139, 664−671.

(28) Wu, J.-Y.; Yu, C.-H.; Wen, J.-J.; Chang, C.-L.; Leung, M.-k. Pyrrolo-[3,2-b]pyrroles for Photochromic Analysis of Halocarbons. Anal. Chem. 2016, 88, 1195−1201. (29) Peng, Z.; Feng, X.; Tong, B.; Chen, D.; Shi, J.; Zhi, J.; Dong, Y. The selective detection of chloroform using an organic molecule with aggregation-induced emission properties in the solid state as a fluorescent sensor. Sens. Actuators, B 2016, 232, 264−268. (30) Orłowski, R.; Banasiewicz, M.; Clermont, G.; Castet, F.; Nazir, R.; Blanchard-Desce, M.; Gryko, D. T. Strong solvent dependence of linear and non-linear optical properties of donor-acceptor type pyrrolo[3,2b]pyrroles. Phys. Chem. Chem. Phys. 2015, 17, 23724−23731. (31) Poronik, Y. M.; Mazur, L. M.; Samoć, M.; Jacquemin, D.; Gryko, D. T. 2,5-Bis(azulenyl)pyrrolo[3,2-b]pyrroles − the key influence of the linkage position on the linear and non-linear optical properties. J. Mater. Chem. C 2017, 5, 2620−2628. (32) Santra, M.; Jun, Y. W.; Bae, J.; Sarkar, S.; Choi, W.; Gryko, D. T.; Ahn, K. H. Water-Soluble Pyrrolo[3,2-b]pyrroles: Synthesis, Luminescence and Two-Photon Cellular Imaging Properties. Asian J. Org. Chem. 2017, 6, 278−281. (33) Adler, A. D.; Longo, F. R.; Finarelli, J. D.; Goldmacher, J.; Assour, J.; Korsakoff, L. A simplified synthesis for meso-tetraphenylporphine. J. Org. Chem. 1967, 32, 476. (34) Alberico, D.; Scott, M. E.; Lautens, M. Aryl−Aryl Bond Formation by Transition-Metal-Catalyzed Direct Arylation. Chem. Rev. 2007, 107, 174−238. (35) Xu, Y.; Zhao, L.; Li, Y.; Doucet, H. Palladium-Catalysed Regioselective Sequential C-5 and C-2 Direct Arylations of 3Acetylpyrroles with Aryl Bromides. Adv. Synth. Catal. 2013, 355, 1423−1432. (36) Golubev, P. Ueber die Reduktion des Isodinitrobenzils. Ber. Dtsch. Chem. Ges. 1884, 17, A581−A582. (37) Qiu, L.; Yu, C.; Zhao, N.; Chen, W.; Guo, Y.; Wan, X.; Yang, R.; Liu, Y. An expedient synthesis of fused heteroacenes bearing a pyrrolo[3,2-b]pyrrole core. Chem. Commun. 2012, 48, 12225−12227. (38) Hung, T. Q.; Hancker, S.; Villinger, A.; Lochbrunner, S.; Dang, T. T.; Friedrich, A.; Breitsprecher, W.; Langer, P. Novel synthesis of 5methyl-5,10-dihydroindolo[3,2-b]indoles by Pd-catalyzed C−C and two-fold C−N coupling reactions. Org. Biomol. Chem. 2015, 13, 583− 591. (39) Ho, H. E.; Oniwa, K.; Yamamoto, Y.; Jin, T. N-Methyl Transfer Induced Copper-Mediated Oxidative Diamination of Alkynes. Org. Lett. 2016, 18, 2487−2490. (40) Yu, J.; Zhang-Negrerie, D.; Du, Y. Cu(OAc)2-Mediated Cascade Annulation of Diarylalkyne Sulfonamides through Dual C−N Bond Formation: Synthesis of 5,10-Dihydroindolo[3,2-b]indoles. Org. Lett. 2016, 18, 3322−3325. (41) Jiang, M.-J.; Xiao, W.-J.; Huang, J.-C.; Li, W.-S.; Mo, Y.-Q. Diindole[3,2-b:4,5-b′]pyrrole as a chromophore containing three successively fused pyrroles: synthesis, optoelectronic properties and πfunctionalization. Tetrahedron 2016, 72, 979−984. (42) Tanaka, S.; Kumagai, T.; Mukai, T.; Kobayashi, T. Bull. Chem. Soc. Jpn. 1987, 60, 1981. (43) Dereka, B.; Rosspeintner, A.; Krzeszewski, M.; Gryko, D. T.; Vauthey, E. Symmetry-Breaking Charge Transfer and Hydrogen Bonding: Toward Asymmetrical Photochemistry. Angew. Chem., Int. Ed. 2016, 55, 15624−15628. (44) Dereka, B.; Vauthey, E. Direct local solvent probing by transient infrared spectroscopy reveals the mechanism of hydrogen-bond induced nonradiative deactivation. Chem. Sci. 2017, 8, 5057−5066. (45) Ivanov, A. I.; Dereka, B.; Vauthey, E. A simple model of solventinduced symmetry-breaking charge transfer in excited quadrupolar molecules. J. Chem. Phys. 2017, 146, 164306. (46) Balasubramanyam, R. K. C.; Kumar, R.; Ippolito, S. J.; Bhargava, S. K.; Periasamy, S. R.; Narayan, R.; Basak, P. Quadrupolar (A-π-D-π-A) Tetra-aryl 1,4-Dihydropyrrolo[3,2-b]pyrroles as Single Molecular Resistive Memory Devices: Substituent Triggered Amphoteric Redox Performance and Electrical Bistability. J. Phys. Chem. C 2016, 120, 11313−11323. 2344


Article

Accounts of Chemical Research (47) Domínguez, R.; Montcada, N. F.; de la Cruz, P.; Palomares, E.; Langa, F. Pyrrolo[3,2-b]pyrrole as the Central Core of the Electron Donor for Solution-Processed Organic Solar Cells. ChemPlusChem 2017, 82, 1096−1104.

2345


The Tetraarylpyrrolo[3,2-b]pyrrolesâFrom Serendipitous Discovery to

Recommend Documents

The Tetraarylpyrrolo[3,2-b]pyrrolesâFrom Serendipitous Discovery to