CC BY-NC-ND 4.0 · Organic Materials 2021; 03(02): 155-167
DOI: 10.1055/s-0041-1726450
Focus Issue: Peter Bäuerle 65th Birthday
Review

S,N-Heteropentacenes – Syntheses of Electron-Rich Anellated Pentacycles

Henning R. V. Berens
a   Institut für Organische Chemie und Makromolekulare Chemie, Heinrich-Heine-Universität Düsseldorf, Universitätsstraße 1, D-40225 Düsseldorf, Germany
,
Thomas J. J. Müller
a   Institut für Organische Chemie und Makromolekulare Chemie, Heinrich-Heine-Universität Düsseldorf, Universitätsstraße 1, D-40225 Düsseldorf, Germany
› Author Affiliations
 


Abstract

This review summarizes syntheses of S,N-heteropentacenes, i.e. electron-rich sulfur and nitrogen-embedding pentacycles, and briefly highlights selected applications in molecular electronics. Depending on the anellation mode and the number of incorporated heteroatoms, electron density can be raised by increasing nitrogen incorporation and polarizability is manifested by the sulfur content. In comparison to triacene analogues, the conjugation pathways of S,N-heteropentacenes are increased and the favorable acene-typical crystallization behavior allows for diverse application in organic electronics. Furthermore, substitution patterns allow fine-tuning the electronic properties, extending the π-systems, and supplying structural elements for further application.

1 Introduction

2 Thiophene-Centered S,N-Heteropentacenes

2.1 Dipyrrolo-Fused Thiophenes

2.2 Diindolo-Fused Thiophenes

3 Pyrrole-Centered S,N-Heteropentacenes

3.1 Dithieno-Fused Pyrroles

3.2 Bis[1]benzothieno-Fused Pyrrole

4 Fused 1,4-Thiazines

4.1 Dinaphtho-Fused 1,4-Thiazines

4.2 Bis[1]benzothieno-Fused 1,4-Thiazines

5 Conclusions and Outlook


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Biosketches

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Henning R. V. Berens was born in Ankum, Germany, in 1992 and studied chemistry from 2011 to 2017. He obtained his B.Sc. from Heinrich Heine University of Düsseldorf in 2014 and his M.Sc. from the University of Hamburg in 2017. He commenced his PhD thesis in 2017 under the supervision of Prof. T. J. J. Müller at Heinrich Heine University of Düsseldorf. His research interests include the synthesis and characterization of novel emissive heterocyclic systems based on thiazines with diverse fusing modes and substitution patterns.

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Thomas J. J. Müller was born in Würzburg, Germany, in 1964 and studied chemistry at Ludwig-Maximilians-Universität München (LMU) from 1984 to 1989. He obtained his diploma in 1989 and completed his PhD in 1992 with Prof. R. Gompper on novel cyanine systems as models for optical switches and molecular metals. After a post-doctoral stay with Prof. B. M. Trost at Stanford University, USA, in 1993 and 1994 working on ruthenium-catalyzed Alder-Ene reactions, he returned to Germany. In 1994, as a Liebig scholar he began his independent research at Technical University Darmstadt, moved to LMU as a DFG scholar in 1997, to obtain his habilitation and was appointed to Privatdozent in 2000. From 2002 to 2006 he was an associate professor of organic chemistry at Ruprecht-Karls-Universität Heidelberg. Since 2006 he is a full professor and holds the chair of organic chemistry at Heinrich-Heine-Universität Düsseldorf. In 2016 he received a call to a chaired professorship at Ruprecht-Karls-Universität Heidelberg, which he declined. Since 2019 he is the spokesman of the DFG-funded Research Training Group 2482 (Modulation of Intersystem Crossing - ModISC). His research interests encompass synthetic heterocyclic chemistry, functional chromophores, and the design of novel multi-component and domino reactions. He is an author of more than 290 journal articles and book chapters.

1 Introduction

Over many years, organic heterocycles have attracted considerable interest as functional molecules in academia and industry. Besides numerous medicinal applications, as impressively emphasized by recent reports on antibiotic, anti-viral, or anti-cancer drugs,[1] they found increasing entry as functional molecular components in optoelectronics. Often heterocyclic structures ideally feature high polarizability and high propensity for charge separation, a prerequisite in organic photovoltaics (OPVs).[2] In addition, increased polarizability and favorable redox properties enable hole mobility, a crucial feature for application as semiconductive functional molecules in organic field effect transistors (OFETs)[3] or organic light emitting diodes (OLEDs).[4] Hole mobility relies on crystalline packing and morphology of the electron-rich molecular entities with short intermolecular distances.[5] This calls for planar fused systems with improved crystallinity of folded structures.[6] Sulfur-containing π-systems inherently gain electron density and substantial polarizability.[7] Among these sulfur heterocycles, thiophenes and congeners are extensively explored due to their rich reactivity patterns and stability.[8] The increase of electron density by sulfur atoms can be even enhanced by combination with nitrogen atoms, where lone pairs can more effectively overlap with orbitals in conjugated π-systems, thereby enhancing the delocalization.[9] Therefore, integration of both heteroatoms increases both polarizability and electron density, making S,N‑heterocyclic systems ideal candidates for applications in organic electronics. Many tricyclic S,N‑heterocycles are reported in the literature and applications in OPV are well documented.[10] While five-membered heterocycles such as thiophenes and pyrroles and their congeners tend to crystallize easily, 1,4-thiazine-based structures typically adopt butterfly-like structures that can hamper efficient crystallization.[11] Tetra- and hexacyclic fused systems are more challenging synthetic targets and symmetry-based synthetic approaches are often difficult.[12] Higher homologues like octa- and decacenes were efficiently synthesized by Bäuerle and coworkers.[13] For the synthesis of pentacyclic fused structures, symmetry-based approaches enable broader accessibility. While this synthesis-based reasoning also holds true for heptacyclic systems, synthetic effort has considerably increased even for highly symmetrical systems.[14] Conceptually challenging remains minimization of the molecular size with simultaneously meeting optoelectronic requirements, such as excitation and emission within the visible spectrum at shortest possible conjugation length. Interestingly, pentacyclic fused systems present a good tradeoff between molecular size for crystallization, photophysical properties, and synthetic feasibility. They additionally possess comparably low molecular weights and open avenues for functionalization and fine-tuning of important physical parameters, such as crystallization, solubility, and electronic properties. In addition, syntheses can take advantage of the tunable reactivity of easily accessible building blocks such as thiophenes. For instance, regioselective α-lithiation opens a broad array of selective functionalization at 2-position.[8] Furthermore, by regioselective electrophilic bromination, the adjacent β-position can be addressed, opening all avenues for unsymmetrical functionalization and intermediates en route to pentacyclic systems. Ultimately, thiophenes as starting materials prove to be almost abundant, commercially available ideal building blocks. Otherwise, pyrroles and their scaffolds as starting materials are significantly more electron-rich, yet, syntheses, workups, and handlings might become complicated.[15] Therefore, pyrroles are generally better formed in the final synthetic steps. Cadogan-type cyclizations of nitrobiaryls[16] or transition-metal-catalyzed C–N-couplings of dibromides with amines have become general solutions.[17] The latter approach is particularly attractive for generating substance libraries in a modular fashion. Starting from privileged building blocks (thiophenes and benzothiophenes) and versatile methodologies, a broad range of novel S,N-pentacyclic condensed systems became feasible. This review exclusively focuses on the syntheses of symmetrical pentacyclic fused S,N-topologies based upon the central cores: thiophene, pyrrole, and thiazine ([Scheme 1]). Therefore, the synthetic concepts are developed from bottom-up anellation starting from the central heterocycle (thiophenes) or leading to late-stage anellation furnishing the central heterocycle (pyrroles, thiazines). Some representative electronic properties are briefly presented and highlighted.

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Scheme 1 Conceptual structure of this review based on the central heterocyclic cores thiophene, pyrrole, and thiazine, where various symmetrical pentacyclic fused S,N-topologies are accessible.

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2 Thiophene-Centered S,N-Heteropentacenes

In the context of photonic or electronic molecular materials, fused S-heterocycles can be either five- or six-membered. Although five-membered rings in contrast to six-membered systems cause an angular kink, they are equally regarded as linear topologies ([Figure 1]). Therefore, dipyrrolo-fused and diindolo-fused thiophenes are considered.

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Figure 1 Doubly fused thiophenes.

2.1 Dipyrrolo-Fused Thiophenes

For dipyrrolo-fused thiophenes (DPTP) and their pentacenes, the electron density as well as the conjugation considerably increases in comparison to all-thieno-fused systems. The synthesis is feasible from thiophene (1) in five steps ([Scheme 2]). Starting with the synthesis of tetrabromothiophene (2) from thiophene and with in situ prepared thienylzinc chloride 3, two required building blocks are obtained to give tetrabrominated terthiophene 4 upon Negishi cross-coupling. By cyclizing through Buchwald–Hartwig coupling with aniline (5a), the fused system 6a is finally formed ([Scheme 2]).[18] Employing various amines 5 allows a broad variation of the N-substituent pattern.

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Scheme 2 Synthesis of a generic parent building block 6 for various dipyrrolo-fused thiophene compounds, depicted for the synthesis of 6a.

Most advantageously, terminal thiophene moieties can be easily functionalized by α-lithiation as well as electrophilic substitution reactions such as Vilsmeier–Haack formylation. Thus, difunctionalized (via 7, see [Scheme 3]) and even unsymmetrically substituted structures (via 9, see [Scheme 4]) are feasible. Employing this strategy in a symmetrical approach from aliphatic substrate 6b gives dialdehyde 7. Concluding Knoevenagel condensation furnishes the symmetrically acceptor-substituted derivative 8 was successfully employed in DSSC (dye-sensitized solar cell) dyes ([Scheme 3]).[19]

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Scheme 3 Synthesis of an acceptor-substituted dipyrrolo-fused thiophene 8 by twofold Vilsmeier–Haack formylations followed by Knoevenagel condensation reactions.
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Scheme 4 Synthesis of the unsymmetrical counterparts 13a and 13b by single Vilsmeier–Haack formylation, followed by electrophilic bromination and subsequent Suzuki cross couplings and Knoevenagel condensations.

Compound 8 features an overall incident photon-to-current efficiency (IPCE) of 5.01%, which is quite high for these relatively small molecules. The photocurrent production of this material amounts to J SC = 12.9 mA/cm2 with an open-circuit voltage of V OC = 580 mV. By employing malononitrile as the condensation partner, Bäuerle and coworkers furthermore reported bulk-heterojunction type solar cell dyes with efficiencies between 4 and 6%. Interestingly, the solar cell efficiencies were influenced by the chain lengths of the aliphatic N-alkyl substituents with even numbered chain lengths exhibiting the best performances.[20]

By a desymmetrization approach, efficiencies of this structure can be further enhanced ([Scheme 4]). Starting from the base structures 6 the unsymmetrical aldehydes 7 can be obtained by singular Vilsmeier–Haack formylation. Having only one side of the molecule 9 decorated, the other counterpart region can be addressed by electrophilic bromination with NBS to give 10, which opens reactivity towards cross-coupling reactions. Exploiting this with strong donor fragments like 11 to give 12 and final Knoevenagel condensation leads to 13 as target structures. Depending on the employed amine 5 in the cyclization reaction towards 6, two differently substituted structures 13a and 13b were obtained in the literature. Interestingly, the nature of the pyrrole substituent exerts a significant effect on the dye's performance. Aromatic substituents are apparently more favorable than purely aliphatic side chains. The p-hexyloxyphenyl-substituted donor–acceptor type DSSC dye 13b ([Scheme 4]) reaches an IPCE of 6.6% with J SC = 11.6 mA/cm2 and V OC = 800 mV. The hexyl-substituted dye 13a, however, does not exceed 4.4% with J SC = 8.6 mA/cm2 and V OC = 710 mV.[21]

Decoration of compound 6a with amino donor substituents via the brominated derivative 14 by twofold Buchwald–Hartwig coupling with secondary amines 15 furnishes electron-enriched systems 16 that can be applied as a hole-transport material in electroluminescent devices like OLED ([Scheme 5]).

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Scheme 5 Electronically enriched derivatives 16 synthesized by electrophilic bromination of 6a and Buchwald–Hartwig amination employing various arylamines 15.

The synthesis is eased by the reactivity of the terminal thiophene moieties of 6 towards even mild reagents like NBS to afford 14.

A broad substituent pattern has been reported in the patent literature of this motif with phenyl, naphthyl, and other aromatic groups.[18]

Molecules 16 exceed current efficiencies η L of 20–30 cd/A for some red-light emitting dyes with luminances L V of 3000–3500 cd/m2 for representative blue-light emitters. Four generic syntheses of both symmetrical and unsymmetrical substances are reported. The substitution patterns and reactions yields of the amination step are depicted in [Figure 2]. More distinctly substituted compounds are described as applied in devices however, demonstrating the diversity of the reactions.[18]

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Figure 2 Representative bis(thienopyrrolo)thiophenes 16 for OLED applications (BP = [1,1'-biphenyl]-4-yl).

Furthermore, DPTP can be employed as useful building blocks in synthetic strategies towards higher ladder-type heteroacenes as reported by Bäuerle ([Scheme 6]).[22] The selectivity of reactions including lithiation at any step could here be addressed by silylation of one of the two opposite facing α-thienyl regions. By single deprotonation of 17 and nucleophilic substitution with triisopropylsilylic chloride, monosilylated DPTP 18 was feasible. Bromination with NBS on the remaining thienyl moiety leads to α-brominated 19, which is employed in a “halogen dance” rearrangement reaction[23] to warrant the correct regioisomers to be formed in the following sequence of reactions. De novo α-deprotonation of 20 and oxidative homo-coupling with copper(II)chloride, followed by a Pd-catalyzed C–S-coupling with potassium thioacetate, forms a focal thieno-ring with two heteropentaceno-anellands. Final fluoride-mediated desilylation furnishes S,N-heteroundecacene 21. Its pronounced absorption with extinction coefficient of ε = 98000 L mol−1 cm−1 combined with a red-shifted absorption at λ max,abs = 453 nm and a HOMO energy level of as high as E HOMO = −4.84 eV presents S,N-heteroundecacene 21 and its congeners as very promising candidates for optoelectronic applications.[22]

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Scheme 6 Synthesis of higher ladder-type heteroacenes by utilization of DPTP-type heteropentacene 20 as the functional building block.

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2.2 Diindolo-Fused Thiophenes

Compared to their thieno-fused congeners bis[1]benzothieno-thiophenes, indolo-fused systems feature higher electron density, similarly to thienopyrrolothiophenes 6 ([Scheme 5]), and open application in electroluminescent devices, and here some derivatives have already been patented.[24] Diindolothiophenes can be designed as three regioisomers, depending on the mode of fusing. Here, only anti,anti- or syn,anti-derivatives are considered ([Schemes 7] and [8]). A synthetic approach to this class of compounds is achieved with relatively simple reactants. For instance, dibromothiophene 22 and 2-nitrophenylboronic acid (23) react by a Suzuki-coupling and dinitro product 24 is then transformed in a Cadogan-type cyclization[16] reaction to give the pivotal anti,anti-diindolothiophene 25. Subsequent copper-catalyzed Ullmann-type couplings of compound 25 form the N-phenyl derivative 26, which is transformed by nucleophilic aromatic substitution with chlorotriazine 27 to finally furnish product 28a ([Scheme 7]). This and related compounds have been published in the patent literature as red-light-emitting dyes for use in OLED devices. Likewise five novel compounds 28 were synthesized according to this route and their OLED performances were reported ([Figure 3]).[24]

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Scheme 7 Synthesis of pivotal diindolothiophene 25 from 2,5-dibromothiophene 22 and subsequent cyclization, followed by Ullmann-type arylation and SNAr furnishing an unsymmetrically bis(N-arylated) diindolothiophene 28a.
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Scheme 8 Synthesis of sickle-shaped NH-diindolothiophene 35 by Suzuki coupling, Cadogan-type cyclization, Buchwald–Hartwig amination, and CH-activation reaction.
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Figure 3 Synthetic examples of diindolothiophenes 28.

Not only linear anti,anti-fused systems based on 28, but also sickle-shaped syn,anti-derivatives like 35 ([Scheme 8]) can be employed as OLED emitters. Likewise, starting from Suzuki-coupling of thiophene 29 and o-nitroarene 30 to form 31, Cadogan-type cyclization gives thienoindole 32, which is then coupled with aniline 33 under Buchwald conditions ([Scheme 8]). Finally, intramolecular CH-activation of intermediate 34 forms the pivotal structure 35. Buchwald–Hartwig amination of compound 35 leads to an immense variety of more than 30 sickle-shaped derivatives.[25] Performance measurements with these compounds at a luminance of 1000 cd/m2 show operating voltages of 5.3–5.6 V with efficiencies of 13–22 cd/A.


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3 Pyrrole-Centered S,N-Heteropentacenes

Azoles possess a higher electron density and, thus, lower redox potentials than their sulfur analogues. It is not surprising that nitrogen-containing heterocycles have therefore received particular attention in technical applications. The most dominant structure with this respect is pyrrole ([Figure 4]) with only few publications on pyrazines, however, none of them as S,N-heteropentacenes. In this context, dithieno-fused and bis[1]benzothieno-fused pyrroles are considered.

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Figure 4 Doubly fused pyrroles.

3.1 Dithieno-Fused Pyrroles

Multiple thieno-fused pyrroles have been reported and applications range from OLED emitters to DSSC dyes, or, if incorporated into polymer chains, as organic semiconductors. Syntheses mostly aim to assemble the basic structure, followed by subsequent functionalization, by either bromination or lithiation reactions. Unlike for thiophene congeners, the central pyrrole unit does not appear as a main building block for anellation. A major reason is the sensitivity of pyrroles against oxidation. Therefore, the central pyrrole unit is rather formed in the terminal step, exploiting the relative electroneutrality and low reactivity as well as the good functionalizability of thiophene-based building blocks. Syntheses thus start from brominated thienothiophene 39, which can be obtained from dibromothiophene 36 by monolithiation and reaction with an acetal-decorated disulfide 37 to form thienyl alkylacetalsulfide 38 ([Scheme 9]). Under acidic conditions, this intermediate forms thienothiophene 39 by cyclizing condensation using the acidic polymeric catalyst amberlyst 15®.

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Scheme 9 Two-step synthesis of 3-bromothieno[3,2-b]thiophene (39) starting from 3,4-dibromothiophene (36).

Compound 39 is then oxidatively dimerized[26] to intermediate 40 prior to conclusion by cyclizing Buchwald–Hartwig amination to form 41a ([Scheme 10]).[26] [27]

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Scheme 10 Synthesis of the generic building block 41a by oxidative dimerization of 39 and ring-closing Buchwald–Hartwig amination of 40.

Compound 41a can be further transformed via Pd-catalyzed CH-activation at the α-position by coupling an arylbromide, e.g., 42, to open entry to large, expanded π-systems ([Scheme 11]). In 2017 scientists of the Huaiyin Institute of Technology patented a tetrathienopyrrole-based hole-transport material 43 for use in perovskite solar cells, decorated with expanded carbazole substituents.[28] This system reaches a V OC = 1.118 V with a current production of J SC = 23.9 mA/cm2. The overall conversion efficiency was determined to be 21%.

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Scheme 11 Synthesis of indenocarbazole-substituted derivative 43 by CH-activation.

Besides perovskite-based solar cells, DSSC dyes were studied as well. The general donor–acceptor architectures can be either symmetrical or unsymmetrical. For instance, an unsymmetrical approach furnishes donor–acceptor dye 47 ([Scheme 12]).[27a] Starting from bis-hexyl-substituted tetrathienopyrrole 41b, formyl derivative 44 was obtained by single Vilsmeier formylation. After α-bromination, Suzuki coupling with the triphenylamine donor boronate 11 forms compound 46. The acceptor moiety and required linker for binding to the anode material is obtained by Knoevenagel condensation of the formyl group with cyanoacetic acid, leading to compound 47.

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Scheme 12 Synthesis of the unsymmetrical solar cell dye 47 by a two-step Vilsmeier–Haack formylation–electrophilic bromination sequence concluded by cross-coupling and Knoevenagel condensation.

The solar cell system with dye 47 (R1 = R2 = R3 = n-hex) reaches an open circuit voltage of V OC = 826 mV, with a short-circuit current J SC of 16.5 mA/cm2. The filling factor accounts to 0.69, which gives an IPCE η L of 9.4%.[27a]

A symmetrical DSSC dye based on tetrathienopyrrole was published in 2019 by Chen et al., comprising an indenonylidene-acceptor fragment 53 ([Scheme 13]).[29]

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Scheme 13 Synthetic pathway to DSSC dye 54 from dithienopyrrole 48 via dialdehyde 52 and concluding Knoevenagel condensation.

The synthetic approach to the formylated intermediate 52 was considerably different from the aforementioned process. Here, a dibrominated dithienopyrrole 48 was first isomerized, then formylated to give dialdehyde 49. This was then employed in a nucleophilic aromatic substitution using ethyl 2-mercaptoacetate (50), which in situ cyclized to form the pentacene 51. After that, correcting the oxidation state of the substituent is necessary to obtain the dialdehyde 52. Condensation with the acceptor component 53 then leads to DSSC dye 54. The compound was not explicitly examined in DSSC measurements, however, with another dye as a coadditive.


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3.2 Bis[1]benzothieno-Fused Pyrrole

Analogously to indolo-fused thiophenes shown in Section 2.2, also pyrroles have been fused with two heteroindeno systems, i.e. benzothiophenes. The resulting bis[1]benzothienopyrroles have found various applications in organic electronics. However, synthetic approaches to these two compound classes are orthogonal. While the favored route for most bis(benzothieno)pyrroles is the formation of the central pyrrole ring in the last step of the synthesis ([Scheme 14]), diindolothiophenes are rather established from an existing thiophene moiety ([Scheme 7]). Rasmussen's group reported on the synthesis and properties of a series of bis(benzothieno)pyrroles 58 with varying N-substituents from aliphatic to aromatic and also allylic moieties ([Scheme 14]).[30] The route resembles the synthesis of bis(benzothieno)thiophenes,[31] and also starts from benzo[b]thiophene (55), which is brominated using NBS towards 56 and then oxidatively dimerized to give 57a, but from that instead of lithiation and nucleophilic S-cyclization rather Buchwald–Hartwig amination proceeds to form the central pyrrole moiety in the products 58.

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Scheme 14 Synthesis of bis[1]benzothienopyrroles 58 by concluding cyclizing twofold Buchwald–Hartwig amination of benzothiophene dimer 57a.

Rasmussen examined these products and a Korean research group proceeded with the functionalized derivative 59a [32] and synthesized donor–acceptor dyes for DSSC application ([Scheme 15]).[33] Compound 59a was obtained by electrophilic bromination of aliphatically substituted precursor 58e, with a subsequent Miyaura borylation to give the diboronic acid 59a. The acceptor moiety was introduced by Suzuki coupling with acceptor-substituted thienylbromides 60.

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Scheme 15 Modular approach to bis[1]benzothienopyrrole-based donor–acceptor systems 62 by electrophilic bromination, Miyaura borylation, and Suzuki coupling followed by acidic deprotection.

The intermediate diboronic acid 59 also acts as a copolymer building block for low-bandgap conjugated polymers as reported by Yoon et al. in 2015.[32] Using the diboronic acid 59b, which bears long chiral aliphatic chains to increase solubility, multiple polymeric structures 64 and 66 were readily feasible. Polymerization proceeded via Suzuki coupling using the electron-poor dibromo arene linker units 63 and 65 ([Scheme 16]).

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Scheme 16 Synthesis of polymeric donor–acceptor dyes 64 and 66 for use in solar cell application.

The polymers with molecular weights of up to 74800 (64) and 50300 (66) were examined in bulk-heterojunction-type solar cells, where they display significantly enhanced properties compared to their triacene analogues. For instance, polymer 64 exhibits an open circuit voltage V OC of 0.66 V with a short-circuit current J SC of 15.8 mA/cm2 amounting to an efficiency η L of 6.8%. The dithienopyrrole as a direct triacene-based analogue only features an open-circuit voltage V OC of 0.38 V, with J SC = 14.9 mA/cm2, and an overall IPCE η L of only 2.7%. As often seen, extension of the π-conjugation by benzo-anellation vastly improves the systems' performance.[32]

By 3,3′-ligation of the two benzothiophene moieties, the arch-typed syn,syn-bis(benzothieno)pyrroles are formed. Their syntheses were reported by Mu et al. in 2018 ([Scheme 17]). Starting from benzo[b]thiophene (55) after electrophilic bromination towards 56 using NBS a Nickel catalyzed reductive homo-coupling to form 3,3'-bis(benzothienyl) 67 was utilized. The second electrophilic bromination furnished the starting material for the concluding cyclizing Buchwald–Hartwig amination to give 68. Interestingly, transitioning from linear (compound 59, see [Scheme 14]) to arch-shaped syn,syn-derivatives 68, especially, the N-aryl-substituted molecules exhibit a highly ordered crystal packing in perfectly parallel alignments. In particular, for OFET applications, this morphology is a key prerequisite. Unsurprisingly, arch-shaped structures featuring a very favorable crystallization behavior with a hole mobility μ h of 0.037 cm2/Vs can greatly exceed far less organized crystallizing linear structures with μ h of only 1.3∙10−6 cm2/Vs.[34]

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Scheme 17 Synthesis of arch-shaped bis[1]benzothienopyrroles 68.

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4 Fused 1,4-Thiazines

As S,N-heterocycles, only 1,4-thiazines ([Figure 5]) are considered in this review article since 1,2-thiazines do not generate linearly fused systems. Also, higher homologous systems, such as thiazepines or thiazocines, are not discussed here. Therefore, in this context dinaphtho-fused and bis[1]benzothieno-fused 1,4-thiazines are considered.

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Figure 5 Doubly fused 1,4-thiazines.

4.1 Dinaphtho-Fused 1,4-Thiazines

Some scattered examples of naphtho-fused thiazines are reported in the literature. Syntheses date back to the 1910s ([Scheme 18])[35]; however, recent publications only appeared in the patent literature from 2016 ([Scheme 19]).[36] Apparently, this compound class experiences a comeback. First syntheses by Knoevenagel in 1914 started from dinaphthylamines 69 and utilized Bernthsen thionation to form fused thiazines 70 or 71 in dependence of the substrate, utilizing significantly high temperatures. Such harsh conditions are a challenge for the synthesis of substituted derivatives.

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Scheme 18 Syntheses of dinaphthothiazines 70 and 71 by Bernthsen thionation.
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Scheme 19 Synthesis of dinaphthothiazine 75 by Cadogan-type cyclization.

Contemporary syntheses utilize less forceful reactions conditions, such as Cadogan-type cyclization leading to dibenzo[a,h]pheno-thiazine 75 ([Scheme 19]). Starting from 1-chloro-2-nitronaphthalene (72) and naphthalene-2-thiole (73), intermediate 74 is formed by nucleophilic aromatic substitution.[37] Cadogan cyclization with triethyl phosphite gives dinaphthothiazine 75 in good yield.

Using Buchwald–Hartwig amination, several arylated dibenzo[a,h]phenothiazines 76 can be generated that are employed as an emissive layer in OLED devices with good device performance.[36] Selected examples of reported compounds 76 showing a wide range of products are depicted in [Figure 6].

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Figure 6 Synthetic examples of dibenzo[a,h]phenothiazines 28.

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4.2 Bis[1]benzothieno-Fused 1,4-Thiazines

As an evolutionary heir of phenothiazines and dithienothiazines, di(benzothieno)-fused thiazines were first reported by Schneeweis et al. in 2018.[11c] These electron-rich fused systems are synthesized starting from benzo[b]thiophene (55) via bromination to give dibromobenzo[b]thiophene 77 and thioetherification to form 78, which is finally employed in a twofold Buchwald–Hartwig coupling to form the ring-closed syn,syn-bis(benzothieno)thiazine (BBTT) 79 ([Scheme 20]). Interestingly, depending on the conditions, not only the symmetrical syn,syn-BBTT 79 is formed, but also the unsymmetrical syn,anti-BBTT 80 is formed. In a similar fashion, the symmetrical anti,anti-BBTT 83 can be generated. Here, the thioether formation takes place on C3 of the bromobenzothiophene 56 with subsequent bromination at C2 to form substrate 82. Finally, twofold cyclizing Buchwald–Hartwig coupling furnishes anti,anti-BBTT 83.[11c]

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Scheme 20 Synthetic pathways to the three isomeric bis[1]benzothieno[1,4]thiazines 79, 80, and 83 utilizing cyclizing Buchwald–Hartwig coupling as a key step.

Unexpectedly, compounds 83 are essentially planarized. Usually, fused 1,4-thiazines are neither planar in solution nor in the solid state, but exhibit angled bent, butterfly-like structures. This stereoelectronic feature is caused by the formal antiaromaticity of the planar 8π-electron system. Therefore, puckering from coplanarity into a nonaromatic, boat-conformation-like butterfly structure circumvents the antiaromatic energetic bias. For anti,anti-BBTTs 83, however, the steric bias of the N-substituent obviously overrides the antiaromatic bias, assisted by delocalization of the thiazine π-electrons into the benzo extensions. Furthermore, planarized anti,anti-BBTTs 83 exhibit some additional significant electronic deviations from their butterfly counterparts. While syn,syn-BBTTs 79 are nonluminescent in solution and in the solid state, only weak emission can be observed. However, a significant increase of emission is detected for partially planarized anti,anti-derivatives 83. Their longest wavelength absorption bands are bathochromically shifted against 79 and 80 by about 8500 cm−1.

By introducing substituents on the benzo[b]thiophene structure at the outset of the synthesis of BBTT, substituted derivatives are feasible, which are not accessible by functionalization of the parent system. For example, bromination neither proceeded with bromine nor with other electrophilic bromination agents due to oxidation to stable radical cations instead. For the brominated structure 88, the synthetic route starts with bromination of benzo[b]thiophene (55) to form the tribrominated derivative 84 ([Scheme 21]). The bromo substituent at the benzo-ring then persists throughout the following dehalogenation to give dibromo derivative 85 and even the thioetherification to furnish thioether 86, although both steps include bromo-lithium exchange reactions. After bromination to give compound 87, the substrate for thiazine formation, twofold Buchwald–Hartwig coupling furnishes dibrominated anti,anti-BBTTs 88 as final products with an excellent yield.[38]

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Scheme 21 Synthesis of 3,9-dibromo substituted anti,anti-bis[1]benzothieno[1,4]thiazines 88.

From anti,anti-BBTT 88, arylation occurs smoothly via the bromo–lithium-exchange–borylation–Suzuki (BLEBS) sequence ([Scheme 22]). This concluding Suzuki coupling with aryl halides 89 furnishes a variety of aryl-substituted derivatives 90 in overall good yields. All reported compounds show strong solution luminescence with fluorescence quantum yields Φ F of 31–47%.[38]

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Scheme 22 Syntheses of (hetero)arylated anti,anti-bis[1]benzothieno[1,4]thiazines 90 by BLEBS sequence.

Furthermore, dibromo BBTT 88 can be coupled with three (hetero)arylamines 15 by Buchwald–Hartwig amination to form 91 in overall excellent yields ([Scheme 23]). These compounds are also highly fluorescent (Φ F = 17–30%); however, more importantly, their HOMO levels are as high as E HOMO = −4.52 eV as determined from the electrochemical data.[38]

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Scheme 23 Synthesis of electron-rich diaminated anti,anti-bis[1]benzothieno[1,4]thiazines 91 by Buchwald–Hartwig amination of dibromide 88 and diarylamines 15.

Taking into account the structural similarity of this class to those reported in Sections 2.1, 2.2, 3.1, or 3.2, anti,anti-BBTT 83 and syn,syn-BBTT 79 are well suited for comparable electronic applications ([Scheme 24]).

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Scheme 24 Structural similarities and relations between 1,4-thiazine-based heteropentacenes and dipyrrolothiophene and dithienopyrrole congeners.

Formal ring contraction by sulfur extrusion directly correlates syn,syn-bis[1]benzothienothiazines 75 ([Scheme 24], right path) to bis[1]benzothienopyrroles 59, which has found application as solar cell dyes ([Scheme 15]). Further thieno exchange leads to dithienothienopyrroles 41, again featuring wide applicability in DSSC or perovskite-based solar cell devices. Likewise, formally anti,anti-bis[1]benzothieno[1,4]thiazines 83 ([Scheme 24], left path) can by formal N-extrusion be traced back to bis[1]benzothienothiophenes as all-S heteropentacenes (not shown). However, N-analogous diindolothiophenes 25 through S/N-exchange and their ring-contracted congeners dithienopyrrolothiophenes 6 are S,N-heteropentacenes and find application in OLED devices ([Scheme 5]) or solar cells ([Scheme 4]).


#
#

5 Conclusions and Outlook

The class of S,N-heteropentacenes with five and six ring anellations encompasses interesting electron-rich pentacyclic systems with a plethora of applications in the field of molecule-based electronics. They are easily accessible using classical and modern state-of-the-art methodologies ([Scheme 25]). While C–N bonds are formed by either Buchwald–Hartwig amination or Cadogan-type reactions, C–S bonds are either introduced as preformed (fused) thiophenes or via nucleophilic attack of a (hetero)aryllithium at a sulfur electrophile, like SCl2 or bis(phenylsulfonyl)sulfide. Prudent choice of substrates and reaction conditions then opens the full variety of pentacene structures, ranging from fused thieno systems to an increasing number of pyrrole units. Often, thiophenes are favored as substrates, due to their low tendency to undergo side reactions, and their stability makes them well-to-handle materials. Another factor advocating for thiophenes as building blocks also lies within their well addressable reactivity. Halogens, as precursors for cross-coupling reactions but also for bromo–lithium-exchange methodologies, help in building up fused systems as substrates for final cyclizing Buchwald–Hartwig couplings to form pyrrole units. In addition, α-lithiation of thiophenes opens avenues to beneficial side-chain modification, e.g. in OPV dye compounds. More challenging with this respect are pyrroles, which easily tend to oxidize, especially when halogenated. Finally, also fused 1,4-thiazines, formed by (5 + 1)-Buchwald–Hartwig cyclizations from bromo-benzothiophenes, provide access to a quite novel class of particularly electron-rich S,N-heteropentacenes, some with considerable antiaromatic character with concomitant high stability.

Zoom Image
Scheme 25 General retrosynthetic scheme of S,N-heteropentacenes.

In summary, as a consequence of their concise syntheses, tunability of the molecular electronic properties as well as solid-state morphology, ranging from amorphous to highly ordered crystallinity, S,N-heteropentacenes are promising electronic molecular building blocks for functional organic materials in the highly vibrant, rapidly developing field of molecular electronics and photonics. Novel structures based on these scaffolds are literally waiting to be unraveled, disclosed, and conceived in the near future.


#
#

No conflict of interest has been declared by the author(s).

Acknowledgment

The authors cordially thank the Fonds der Chemischen Industrie for financial support.

Dedicated to Prof. Dr. Peter Bäuerle on the occasion of his 65th birthday.


  • References

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    • 17b Li G, Li D, Ma R, Liu T, Luo Z, Cui G, Tong L, Zhang M, Wang Z, Liu F, Xu L, Yan H, Tang B. J. Mater. Chem. A 2020; 8: 5927
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  • 20 Leitner T, Vogt A, Popović D, Mena-Osteritz E, Walzer K, Pfeiffer M, Bäuerle P. Mater. Chem. Front. 2018; 2: 959
  • 21 Wang Z, Liang M, Tan Y, Ouyang L, Sun Z, Xue S. J. Mater. Chem. A 2015; 3: 4865
  • 22 Brier E, Wetzel C, Bauer M, Mena-Osteritz E, Wunderlin M, Bäuerle P. Chem. Mater. 2019; 31: 7007
  • 23 Schnürch M, Spina M, Khan AF, Mihovilovic MD, Stanetty P. Chem. Soc. Rev. 2007; 36: 1046
  • 24 Ahn HC, Cho YJ, Kim BO, Kim SM, Kwon HJ. Rohm and Haas Electronic Materials Korea Ltd., WO2011/132865A1, 2011
  • 25 Zhiyang L, Xueyan R, Qifeng X. Beijing Dingcai Technology Co., Ltd., CN110950887A 2020;
  • 26 Wan J.-H, Fang W.-F, Li Z.-F, Xiao X.-Q, Xu Z, Deng Y, Zhang L.-H, Jiang J.-X, Qiu H.-Y, Wu L.-B, Lai G.-Q. Chem. Asian J. 2010; 5: 2290
    • 27a Chen D, Chen J, Wang Z. Huayin Institute of Technology, CN106433187A, 2017
    • 27b Jing C, Tao G, Lihai M, Wang J, Zhihui W, Suhao Y. CN109265470A 2019;
  • 28 Ding S, Yang B, Wang Y, Wang Z, Luo Y, Cai P, Chen J, Yan B, Gao Y. ; Huayin Institute of Technology, CN111138454A, 2017
  • 29 Chen X, Liu H, Xia L, Hayat T, Alsaedi A, Tan Z. ChemComm 2019; 55: 7057
  • 30 Wolfe RM. W, Culver EW, Rasmussen SC. Molecules 2018; 23: 2279
    • 31a Mori T, Oyama T, Takeda K, Yasuda T. ; Kyocera Corp., JP2017210449A, 2017
    • 31b Alessandrini L, Braga D, Jaafari A, Miozzo L, Mora S, Silvestri L, Tavazzi S, Yassar A. J. Phys. Chem. A 2011; 115: 225
    • 31c Chen L.-H, Chen M.-C, Liang Y.-C, Yan J.-Y, Zhang X. Industrial Technology Research Institute, US2012/0012819A1, 2012
  • 32 Jung IH, Kim J.-H, Nam SY, Lee C, Hwang D.-H, Yoon SC. Macromolecules 2015; 48: 5213
  • 33 Wu H, Huang Z, Hua T, Liao C, Meier H, Tang H, Wang L, Cao D. Dyes Pigm. 2019; 165: 103
  • 34 Mu W, Sun S, Zhang J, Jiao M, Wang W, Liu Y, Sun X, Jiang L, Chen B, Qi T. Org. Electron. 2018; 61: 78
  • 35 Knoevenagel E. J. Prakt. Chem. 1914; 89: 1
  • 36 Lee J. ; Samsung Display Co., Ltd., US2016/0308144A1, 2016
  • 37 Hodgson HH, Leigh E. J. Chem. Soc. 1939; 1094
  • 38 Berens HR. V, Mohammad K, Reiss GJ, Müller TJ. J. J. Org. Chem. 2021, Manuscript submitted


Publication History

Received: 29 January 2021

Accepted: 25 February 2021

Article published online:
01 April 2021

© 2021. The Author(s). This is an open access article published by Thieme under the terms of the Creative Commons Attribution-NonDerivative-NonCommercial License, permitting copying and reproduction so long as the original work is given appropriate credit. Contents may not be used for commercial purposes, or adapted, remixed, transformed or built upon. (https://creativecommons.org/licenses/by-nc-nd/4.0/)

Georg Thieme Verlag KG
Rüdigerstraße 14, 70469 Stuttgart, Germany

  • References

    • 1a Zaib S, Khan I. Bioorg. Chem. 2020; 105: 104425
    • 1b Shiryaev VA, Klimochkin YN. Chem. Heterocycl. Compd. 2020; 56: 626
    • 1c Sapra R, Patel D, Meshram D. J. Med. Chem. 2020; 3: 71
    • 2a Ding L, Jonforsen M, Roman LS, Andersson MR, Inganäs O. Synth. Met. 2000; 110: 133
    • 2b Granström M, Petritsch K, Arias AC, Lux A, Andersson MR, Friend RH. Nature 1998; 395: 257
    • 2c Ramos AM, Rispens MT, van Duren JK. J, Hummelen JC, Janssen RA. J. J. Am. Chem. Soc. 2001; 123: 6714
    • 2d Liu J, Ren J, Zhang S, Hou J. Polym. Chem. 2020; 11: 5019
    • 2e Ke X, Meng L, Wan X, Sun Y, Guo Z, Wu S, Zhang H, Li C, Chen Y. Mater. Chem. Front. 2020 4. 3594
    • 2f Kozma E, Catellani M. Dyes Pigm. 2013; 98: 160
    • 2g Würfel P, Würfel U. Physics of Solar Cells: From Basic Principles to Advanced Concepts. John Wiley & Sons; New York: 2016
    • 3a Katz HE, Lovinger AJ, Johnson J, Kloc C, Siegrist T, Li W, Lin YY, Dodabalapur A. Nature 2000; 404: 478
    • 3b Würthner F. Angew. Chem. Int. Ed. 2001; 40: 1037
    • 3c Shinji A, Jun-ichi N, Eiichi F, Hirokazu T, Youji I, Shizuo T, Yoshiro Y. Chem. Lett. 2004; 33: 1170
    • 3d An TK, Jang SH, Kim S.-O, Jang J, Hwang J, Cha H, Noh YR, Yoon SB, Yoon YJ, Kim LH, Chung DS, Kwon S.-K, Kim Y.-H, Lee S.-G, Park CE. Chem. Eur. J. 2013; 19: 14052
    • 3e Lu C, Chen W.-C. Chem. Asian J. 2013; 8: 2813
    • 3f Garnier F. Chem. Phys. 1998; 227: 253
    • 4a Pinner DJ, Friend RH, Tessler N. Appl. Phys. Lett. 2000; 76: 1137
    • 4b Inganäs O, Berggren M, Andersson MR, Gustafsson G, Hjertberg T, Wennerström O, Dyreklev P, Granström M. Synth. Met. 1995; 71: 2121
    • 4c Chen D, Su S.-J, Cao Y. J. Mater. Chem. C 2014; 2: 9565
    • 4d Senthil Kumar N, Arul Clement J, Mohanakrishnan AK. Tetrahedron 2009; 65: 822
    • 5a Beaujuge PM, Fréchet JM. J. J. Am. Chem. Soc. 2011; 133: 20009
    • 5b Mas-Torrent M, Rovira C. Chem. Rev. 2011; 111: 4833
    • 5c Durban MM, Kazarinoff PD, Luscombe CK. Macromolecules 2010; 43: 6348
    • 6a Wang Y, Sun L, Wang C, Yang F, Ren X, Zhang X, Dong H, Hu W. Chem. Soc. Rev. 2019; 48: 1492
    • 6b Chen D, Zhu D, Lin G, Du M, Shi D, Peng Q, Jiang L, Liu Z, Zhang G, Zhang D. RSC Adv. 2020; 10: 12378
    • 7a Bellido MN. Chem. Phys. Lett. 1985; 122: 562
    • 7b van Duijnen PT, Swart M. J. Phys. Chem. A 1998; 102: 2399
    • 7c Bernasconi CF, Kittredge KW. J. Org. Chem. 1998; 63: 1944
  • 8 Gronowitz S, Hörnfeldt A.-B. Thiophenes. Elsevier; Amsterdam: 2004
  • 9 Cordell FR, Boggs JE. J. Mol. Struct. 1981; 85: 163
    • 10a Miu L, Yan S, Yao H, Chen Q, Zhang J, Wang Z, Cai P, Hu T, Ding S, Chen J, Liang M, Yang S. Dyes Pigm. 2019; 168: 1
    • 10b Meyer T, Ogermann D, Pankrath A, Kleinermanns K, Müller TJ. J. J. Org. Chem. 2012; 77: 3704
    • 10c Al-Busaidi IJ, Haque A, Al Rasbi NK, Khan MS. Synth. Met. 2019; 257: 116189
    • 11a May L, Müller TJ. J. Molecules 2020; 25: 2180
    • 11b Dostert C, Wanstrath C, Frank W, Müller TJ. J. Chem. Commun. 2012; 48: 7271
    • 11c Schneeweis AP. W, Hauer ST, Reiss GJ, Müller TJ. J. Chem. Eur. J. 2019; 25: 3582
    • 12a Vogt A, Henne F, Wetzel C, Mena-Osteritz E, Bäuerle P. Beilstein J. Org. Chem. 2020; 16: 2636
    • 12b Wetzel C, Mishra A, Mena-Osteritz E, Walzer K, Pfeiffer M, Bäuerle P. J. Mater. Chem. C 2016; 4: 3715
  • 13 Wetzel C, Brier E, Vogt A, Mishra A, Mena-Osteritz E, Bäuerle P. Angew. Chem. 2015; 127: 12511 . Angew. Chem. Int. Ed. 2015, 54, 12334
  • 14 Wetzel C, Vogt A, Rudnick A, Mena-Osteritz E, Köhler A, Bäuerle P. Org. Chem. Front. 2017; 4: 1629
  • 15 Schmuck C, Rupprecht D. Synthesis 2007; 2007: 3095
  • 16 Cadogan JI. G, Todd MJ. Chem. Commun. 1967; 178
    • 17a Förtsch S, Vogt A, Bäuerle P. J. Phys. Org. Chem. 2017; 30: e3743
    • 17b Li G, Li D, Ma R, Liu T, Luo Z, Cui G, Tong L, Zhang M, Wang Z, Liu F, Xu L, Yan H, Tang B. J. Mater. Chem. A 2020; 8: 5927
  • 18 Kamada T, Kuraray C, Mitsudo K, Suga S, Sugioka T, Tsuruta M. Univ. Okayama Nat. Univ. Corp.; Yoshimoto, J.; Samsung Display Co. Ltd., KR20150039459A, 2015
  • 19 Chung C.-L, Chen C.-H, Tsai C.-H, Wong K.-T. Org. Electron. 2015; 18: 8
  • 20 Leitner T, Vogt A, Popović D, Mena-Osteritz E, Walzer K, Pfeiffer M, Bäuerle P. Mater. Chem. Front. 2018; 2: 959
  • 21 Wang Z, Liang M, Tan Y, Ouyang L, Sun Z, Xue S. J. Mater. Chem. A 2015; 3: 4865
  • 22 Brier E, Wetzel C, Bauer M, Mena-Osteritz E, Wunderlin M, Bäuerle P. Chem. Mater. 2019; 31: 7007
  • 23 Schnürch M, Spina M, Khan AF, Mihovilovic MD, Stanetty P. Chem. Soc. Rev. 2007; 36: 1046
  • 24 Ahn HC, Cho YJ, Kim BO, Kim SM, Kwon HJ. Rohm and Haas Electronic Materials Korea Ltd., WO2011/132865A1, 2011
  • 25 Zhiyang L, Xueyan R, Qifeng X. Beijing Dingcai Technology Co., Ltd., CN110950887A 2020;
  • 26 Wan J.-H, Fang W.-F, Li Z.-F, Xiao X.-Q, Xu Z, Deng Y, Zhang L.-H, Jiang J.-X, Qiu H.-Y, Wu L.-B, Lai G.-Q. Chem. Asian J. 2010; 5: 2290
    • 27a Chen D, Chen J, Wang Z. Huayin Institute of Technology, CN106433187A, 2017
    • 27b Jing C, Tao G, Lihai M, Wang J, Zhihui W, Suhao Y. CN109265470A 2019;
  • 28 Ding S, Yang B, Wang Y, Wang Z, Luo Y, Cai P, Chen J, Yan B, Gao Y. ; Huayin Institute of Technology, CN111138454A, 2017
  • 29 Chen X, Liu H, Xia L, Hayat T, Alsaedi A, Tan Z. ChemComm 2019; 55: 7057
  • 30 Wolfe RM. W, Culver EW, Rasmussen SC. Molecules 2018; 23: 2279
    • 31a Mori T, Oyama T, Takeda K, Yasuda T. ; Kyocera Corp., JP2017210449A, 2017
    • 31b Alessandrini L, Braga D, Jaafari A, Miozzo L, Mora S, Silvestri L, Tavazzi S, Yassar A. J. Phys. Chem. A 2011; 115: 225
    • 31c Chen L.-H, Chen M.-C, Liang Y.-C, Yan J.-Y, Zhang X. Industrial Technology Research Institute, US2012/0012819A1, 2012
  • 32 Jung IH, Kim J.-H, Nam SY, Lee C, Hwang D.-H, Yoon SC. Macromolecules 2015; 48: 5213
  • 33 Wu H, Huang Z, Hua T, Liao C, Meier H, Tang H, Wang L, Cao D. Dyes Pigm. 2019; 165: 103
  • 34 Mu W, Sun S, Zhang J, Jiao M, Wang W, Liu Y, Sun X, Jiang L, Chen B, Qi T. Org. Electron. 2018; 61: 78
  • 35 Knoevenagel E. J. Prakt. Chem. 1914; 89: 1
  • 36 Lee J. ; Samsung Display Co., Ltd., US2016/0308144A1, 2016
  • 37 Hodgson HH, Leigh E. J. Chem. Soc. 1939; 1094
  • 38 Berens HR. V, Mohammad K, Reiss GJ, Müller TJ. J. J. Org. Chem. 2021, Manuscript submitted

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Scheme 1 Conceptual structure of this review based on the central heterocyclic cores thiophene, pyrrole, and thiazine, where various symmetrical pentacyclic fused S,N-topologies are accessible.
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Figure 1 Doubly fused thiophenes.
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Scheme 2 Synthesis of a generic parent building block 6 for various dipyrrolo-fused thiophene compounds, depicted for the synthesis of 6a.
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Scheme 3 Synthesis of an acceptor-substituted dipyrrolo-fused thiophene 8 by twofold Vilsmeier–Haack formylations followed by Knoevenagel condensation reactions.
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Scheme 4 Synthesis of the unsymmetrical counterparts 13a and 13b by single Vilsmeier–Haack formylation, followed by electrophilic bromination and subsequent Suzuki cross couplings and Knoevenagel condensations.
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Scheme 5 Electronically enriched derivatives 16 synthesized by electrophilic bromination of 6a and Buchwald–Hartwig amination employing various arylamines 15.
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Figure 2 Representative bis(thienopyrrolo)thiophenes 16 for OLED applications (BP = [1,1'-biphenyl]-4-yl).
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Scheme 6 Synthesis of higher ladder-type heteroacenes by utilization of DPTP-type heteropentacene 20 as the functional building block.
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Scheme 7 Synthesis of pivotal diindolothiophene 25 from 2,5-dibromothiophene 22 and subsequent cyclization, followed by Ullmann-type arylation and SNAr furnishing an unsymmetrically bis(N-arylated) diindolothiophene 28a.
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Scheme 8 Synthesis of sickle-shaped NH-diindolothiophene 35 by Suzuki coupling, Cadogan-type cyclization, Buchwald–Hartwig amination, and CH-activation reaction.
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Figure 3 Synthetic examples of diindolothiophenes 28.
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Figure 4 Doubly fused pyrroles.
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Scheme 9 Two-step synthesis of 3-bromothieno[3,2-b]thiophene (39) starting from 3,4-dibromothiophene (36).
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Scheme 10 Synthesis of the generic building block 41a by oxidative dimerization of 39 and ring-closing Buchwald–Hartwig amination of 40.
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Scheme 11 Synthesis of indenocarbazole-substituted derivative 43 by CH-activation.
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Scheme 12 Synthesis of the unsymmetrical solar cell dye 47 by a two-step Vilsmeier–Haack formylation–electrophilic bromination sequence concluded by cross-coupling and Knoevenagel condensation.
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Scheme 13 Synthetic pathway to DSSC dye 54 from dithienopyrrole 48 via dialdehyde 52 and concluding Knoevenagel condensation.
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Scheme 14 Synthesis of bis[1]benzothienopyrroles 58 by concluding cyclizing twofold Buchwald–Hartwig amination of benzothiophene dimer 57a.
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Scheme 15 Modular approach to bis[1]benzothienopyrrole-based donor–acceptor systems 62 by electrophilic bromination, Miyaura borylation, and Suzuki coupling followed by acidic deprotection.
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Scheme 16 Synthesis of polymeric donor–acceptor dyes 64 and 66 for use in solar cell application.
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Scheme 17 Synthesis of arch-shaped bis[1]benzothienopyrroles 68.
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Figure 5 Doubly fused 1,4-thiazines.
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Scheme 18 Syntheses of dinaphthothiazines 70 and 71 by Bernthsen thionation.
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Scheme 19 Synthesis of dinaphthothiazine 75 by Cadogan-type cyclization.
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Figure 6 Synthetic examples of dibenzo[a,h]phenothiazines 28.
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Scheme 20 Synthetic pathways to the three isomeric bis[1]benzothieno[1,4]thiazines 79, 80, and 83 utilizing cyclizing Buchwald–Hartwig coupling as a key step.
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Scheme 21 Synthesis of 3,9-dibromo substituted anti,anti-bis[1]benzothieno[1,4]thiazines 88.
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Scheme 22 Syntheses of (hetero)arylated anti,anti-bis[1]benzothieno[1,4]thiazines 90 by BLEBS sequence.
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Scheme 23 Synthesis of electron-rich diaminated anti,anti-bis[1]benzothieno[1,4]thiazines 91 by Buchwald–Hartwig amination of dibromide 88 and diarylamines 15.
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Scheme 24 Structural similarities and relations between 1,4-thiazine-based heteropentacenes and dipyrrolothiophene and dithienopyrrole congeners.
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Scheme 25 General retrosynthetic scheme of S,N-heteropentacenes.