Thienothiophenes, Dithienothiophenes, and Thienoacenes

Review pubs.acs.org/CR

Thienothiophenes, Dithienothiophenes, and Thienoacenes: Syntheses, Oligomers, Polymers, and Properties Mehmet Emin Cinar† and Turan Ozturk*,†,‡ †

Department of Chemistry, Istanbul Technical University, Maslak, 34469 Istanbul, Turkey Chemistry Group, Organic Chemistry Laboratory, TUBITAK UME, P.O. Box 54, 41470 Gebze-Kocaeli, Turkey

‡

1. INTRODUCTION Heterocyclic compounds comprising various biochemical materials are an indispensable member of our life such as nucleic acids. Moreover, many naturally occurring pigments, vitamins, and antibiotics contain heterocyclic compounds. In modern life, synthetic heterocycles as drugs, pesticides, dyes, and plastics play an important role. The Encyclopaedia Britannica unequivocally delineates a heterocyclic compound, also known as heterocycle, as “Any of a class of organic compounds whose molecules contain one or more rings of atoms with at least one atom (the heteroatom) being an element other than carbon, most frequently oxygen, nitrogen, or sulfur.”1 While the structure of doubly unsaturated five-membered homocyclic compound cyclopentadiene does not show aromatic behavior, pyrrole, furan, and thiophene possessing nitrogen, oxygen, and sulfur atoms, respectively, in place of CH2 have aromatic character on account of at least one lone electron pair occupying a p-orbital perpendicular to the ring plane. As a consequence, this lone pair contributes to conjugated diene resulting in Hückel’s aromaticity. Among these five-membered heterocycles, thiophene has the highest resonance energy (29 kcal mol−1), while pyrrole possesses a resonance energy of 24 kcal mol−1 and furan demonstrates the least aromatic behavior with a resonance energy of 19 kcal mol−1.2 Thiophene has emerged from the combination of the words theion and phaino (sulfur and shining in Greek, respectively). The discovery of thiophene was realized in 1883 by Victor Meyer, who isolated it from blue dye consisting of 1H-indole2,3-dione (isatin) and sulfuric acid in crude benzene.3 This stable liquid compound, which is more reactive than benzene toward electrophiles due to its high π-electron density, has a fivemembered flat structure possessing one sulfur as a heteroatom and described with the molecular formula of C4H4S.3 Conjugated organic materials have been attracting interest due to their role in organic thin film transistors (TFT)4,5 and solar cells.6 They are predicted to have significant advantages over their silicon analogues in terms of processing time and cost because their deposition from solution provides a fast and large-area fabrication.7 The charge transfer mobility and the current on/off ratio are the primary properties that describe the performance of the constracted device. Low conductivity in the off-state, together with a charge carrier mobility of higher than 1 × 10−3 cm2 V−1 s−1, is required as a property of a good organic semiconductor. Besides, stability against oxidation is

CONTENTS 1. Introduction 2. Thienothiophenes 2.1. Thieno[3,2-b]thiophene (1) 2.1.1. Synthesis and Properties 2.1.2. Synthesis and Applications of Oligomers and Polymers 2.2. Thieno[3,4-b]thiophene (2) 2.2.1. Synthesis and Properties 2.2.2. Synthesis and Applications of Oligomers and Polymers 2.3. Thieno[2,3-b]thiophene (3) 2.3.1. Synthesis and Properties 2.3.2. Synthesis and Applications of Oligomers and Polymers 2.4. Thieno[3,4-c]thiophene (4) 2.4.1. Synthesis and Properties 3. Dithienothiophenes 3.1. Dithieno[3,2-b;2′,3′-d]thiophene (491) 3.1.1. Synthesis and Properties 3.1.2. Synthesis and Applications of Oligomers and Polymers 3.2. Dithieno[3,4-b;3′,4′-d]thiophene (492) 3.3. Dithieno[2,3-b;3′,2′-d]thiophene (493) 3.4. Dithieno[2,3-b;2′,3′-d]thiophene (494) 3.5. Dithieno[3,4-b;3′,2′-d]thiophene (495) 3.6. Dithieno[3,4-b;2′,3′-d]thiophene (496) 4. Thienoacenes 4.1. S-Anellated α-Oligothiophenes 4.2. S-Anellated β-Oligothiophenes 5. Conclusion Author Information Corresponding Author Notes Biographies Acknowledgments References © XXXX American Chemical Society

A B B B G AJ AJ AM AT AT AV AW AX AX AY AY BL BX BZ CC CD CD CE CE CP CS CT CT CT CT CU CU

Received: May 21, 2014

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DOI: 10.1021/cr500271a Chem. Rev. XXXX, XXX, XXX−XXX

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one of the inevitable criteria as device performance deteriorates upon oxidation.8 Extension of the π system increases the conjugated length of the molecule, bringing about decreased band gap and often an increased charge carrier mobility, to which most of the research has been devoted to tune the band gap of organic semiconducting polymers.9,10 One of the expedient methods for fine-tuning band gaps involves the utilization of fused thiophene subunits,11 which incites a greater conjugation in the ground state.12 Contrary to thiophenes, fused thiophenes have more rigid structures possessing an extended π-conjugation, which could be employed for adjusting the band gap of organic materials and increasing intermolecular interactions in the solid state.9 The simplest fused thiophenes have two annulated thiophene units named as thienothiophenes (TTs). They are electron-rich structures, which enable them to construct conjugated and low band gap organic semiconductors.9,11 Fusing another thiophene ring gives rise to fused three thiophene units identified as dithienothiophenes (DTTs), which have also been widely used to build promising organic materials due to their flat and rigid delocalized systems.13 Further addition of thiophene units provides thienoacenes, possessing more than three fused thiophene rings, which have also been investigated vide infra.14−17 In this Review, the syntheses and properties of oligomers and polymers based on fused thiophenes, that is, thienothiophenes, dithienothiophenes, and thienoacenes made up exclusively of two or more thiophenes, as well as their applications will be addressed.

Scheme 1. Thienothiophenes 1−4 and Their Resonance Structures

Scheme 2. Synthesis of TT (1) and Tetrabromothieno[3,2-b]thiophene (9) from 3-Bromothiophene (5)22

2. THIENOTHIOPHENES Thienothiophenes (TTs), the simplest fused thiophenes, contain two annulated thiophene rings, with four isomers, thieno[3,2-b]thiophene (1), thieno[3,4-b]thiophene (2), thieno[2,3-b]thiophene (3), and thieno[3,4-c]thiophene (4) (Scheme 1).9,11,18−20 Syntheses and polymerizations of all of the TTs, excluding unstable TT (4), have been investigated comprehensively extensively. These three TTs (1−3), having stable and electron-rich structures, are useful building blocks for the construction of organic semiconductors possessing different conjugation lengths.21 While 1 and 2 provide conjugated systems with a low band gap, 3, known as a cross-conjugated system, cuts the conjugation and hence increases the band gap. Although 2 demonstrates almost the same conjugation length as thiophene, in a conjugated system, its quinoid form at the oxidized state is stabilized by the second fused aromatic thiophene, which provides an extra stability to the system.

Another synthesis of 1, also starting from 3-bromothiophene 5, began by successive additions of n-BuLi, elemental sulfur, and methyl 2-bromoacetate leading to the formation of methyl 2-(thien-3-ylthio)acetate (10), which was followed by regioselective Vilsmeier−Haack formylation using POCl3 and DMF to obtain methyl 2-(2-formylthien-3-ylthio)acetate (11). Aldol cyclocondensation of 11 furnished the ring-closed product methyl thieno[3,2-b]thiophene-2-carboxylate (12). Hydrolysis of 12 with LiOH followed by removal of the carboxylic group using Cu and quinoline gave TT (1) (Scheme 3).132 Henssler et al.24 developed a new two-step synthesis of 1 with an overall yield of 67% (Scheme 3). Reaction of 3-bromothiophene (5) with t-BuLi and then 1,2-bis(2,2-diethoxyethyl)disulfide (13) resulted in the acetal 14. Thermal ring closure of 14 provided TT (1) in the presence of moderately acidic Amberlyst 15. The synthesis of 3-bromothieno[3,2-b]thiophene (16) was achieved from 3,4-dibromothiophene (15) using a similar strategy.25 Schroth et al.26 synthesized the TT (1), starting with halogen/ lithium exchange of 3-bromothiophene (5) with n-BuLi,

2.1. Thieno[3,2-b]thiophene (1)

2.1.1. Synthesis and Properties. A convenient four-step synthesis of TT (1) was reported by Fuller et al., which had an overall yeld of 51%.22 They treated 3-bromothiophene (5) with lithium diisopropylamide (LDA) and then N-formylpiperidine to obtain 3-bromothiophene-2-carbaldehyde (6) in 80% yield, which was followed by a reaction with ethyl 2-sulfanylacetate to obtain ethyl thieno[3,2-b]thiophene-2-carboxylate (7) in 81% yield. Saponification of the ester 7 with LiOH provided thieno[3,2-b]thiophene-2-carboxylic acid (8) in excellent yield (90%). Finally, the carboxylic acid group was removed using Cu powder in the presence of quinoline at elevated temperature, yielding TT (1) in 88% yield (Scheme 2).23,44,132,225,233 Compound 8 reacts with bromine affording 2,3,5,6-tetrabromothieno[3,2-b]thiophene (9) in 70% yield. B


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of 12%. Treatment of 22 with benzylthiol afforded 3-(benzylthio)2-(2-(benzylthio)vinyl)thiophene (23) as a mixture of E/Z configurations, in a ratio of 34/66 and in a total yield of 87%. The E/Z-mixture of 3-acetylthio-2-(2-acetylthiovinyl)thiophenes E/Z-24 was obtained in 40% yield by successive addition of LDMAN, prepared in situ from N,N-dimethyl-l-naphthylamine and lithium, then acetyl chloride. Treatment of E/Z-24 with KOH and then with I2 gave thieno[3,2-c][1,2]dithiin (25), which was converted into the TT (1) in daylight within 120 min dissolved in DMSO. The same strategy was applied for the synthesis of benzannulated TT (28), starting from 3-bromobenzothiophene (26) and through the formation of 25, the rearrangement of which under UV light gave 28 (Scheme 4). A modified approach was applied to synthesize 3-hydroxythieno[3,2-b]thiophene (30) from 3-bromothiophene (5). Formation of 3-lithiothiophene was followed by addition of elemental sulfur and then α-chloroacetic acid to obtain the carboxylic acid 29, which under hot acidic conditions formed 3-hydroxythieno[3,2-b]thiophene (30) in 14% yield. On the other hand, cyclization of 4-((5-methylthiophen-3-yl)thio)-3-oxobutanal (31) in the presence of polyphosphoric acid (PPA) in chlorobenzene furnished the corresponding TT (32) (Scheme 5).19

Scheme 3. Synthesis of TT (1) and 3-Bromothieno[3,2-b]thiophene (16)24 Starting from 3-Bromothiophene (5) and 3,4-Dibromothiophene (15),25 Respectively132

Scheme 5. Synthesis of Hydroxyl (30) and Aldehyde (32) Possessing TTs by Using H2SO4 and PPA, Respectively19 followed by quenching with dibenzyl disulfide to form 3-benzylthiothiophene (17) in 78% yield (Scheme 4). Compound 17 was used to obtain 3-benzylthio-2-ethynylthiophene (22) following two different paths. The first involved the successive 2-bromination using NBS, then Sonogashira coupling with 2-methylbut-3-yn-2-ol and removal of acetone by treatment with a catalytic amount of NaOH to furnish 22 in an overall yield of 43%. In the second route, Vilsmeier formylation at the α-carbon ortho to the SCH2C6H5 group was followed by Corey−Fuchs ethynylation via 3-(benzylthio)-2-(2,2dibromovinyl)thiophene (21) providing 22 in an overall yield Scheme 4. Synthesis of TT (1) and Benzannulated TT (28) from Dithiins 25 and 27, Respectively26

C


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The synthesis of TT (35), having a 3-mercaptoprop-1-yl group, next to the sulfur atom, was obtained from the reaction of 3-bromothiophene-2-carbaldehyde (6) with lithium-2trimethylsilyl-1,3-dithian-2-ide (33), which was followed by treatment of the product 34 with tributylstannane (Scheme 6).27

Scheme 7. Synthesis of 2,3,5,6-Tetraphenylthieno[3,2b]thiophene (46)33,35

Scheme 6. Synthesis of TTs 35, 37, 39, 42, and 44 Having Functional Groups27−32

(Scheme 7).35 Tetra-aryl-substituted TTs were also synthesized by using the double Suzuki−Miyaura coupling reaction thermally.36 Ozturk and co-workers37 synthesized TTs 49a−f, possessing para-substituted phenyl groups at C-3, via a ring closure reaction of monoketones 48a−f, using P4S10, in moderate to good yields (Scheme 8). Optical studies revealed that the TT Scheme 8. Synthesis of TTs 49a−f Having Aryl Units at C337

Table 1. Physical Properties of 49a−d,f37 compd 49a 49b 49c 49d 49f

Eoxb [V] a,c

1.68,1.39 1.60,1.36a,c 1.99,1.09a,c 1.85,1.12a,c 0.74/1.33,0.97a,c

EgCV [eV] a,d

2.46 1.86a,d 1.98a,d 2.35a,d 2.27a,d

Egopt [eV]

λabs(DCM) [nm]

3.75,e2.05a,e 3.70,e1.99a,e 2.92,e2.16a,e 3.68,e2.19a,e 3.54,e2.01a,e

300f 302f 365f 302f 302f

a

Polymer. bVersus Ag wire. cOxidation maxima. dFrom the difference between the onsets of the oxidation and the reduction processes. e From the onsets of the absorption peaks. fAbsorption maxima in dichloromethane.

Heating a mixture of 5-formylthiophen-2-ylacrylic acid (36) and thionyl chloride in pyridine provided 3-chloro-5-dichloromethylthieno[3,2-b]thiophene-2-carbonyl chloride (37) in 52% yield.28,29 The reaction of ethyl 4-chloro-5-formylthiophene-3carboxylates (38) with ethyl thioglycolate produced diethyl thieno[3,2-b]thiophene-2,6-dicarboxylate (39) in a higher yield of 76%.30 Kuszman et al.31 applied Pummerer rearrangement on disulfoxide (40) in acetic anhydride to form 2-acetyloxythieno[3,2-b]thiophene (41) and 2,5-diacetyloxythieno[3,2-b]thiophene (42) in 5.6% and 27% yields, respectively (Scheme 6). Capozzi et al. used two equivalents of AlCl3 to synthesize 3-chloro-2-methylthieno[3,2-b]thiophene (44) from N-vinylthiophthalimide 43.32 In another method, a regioselective photodimerization of 2,3-diphenylcyclopropenethione (45) produced 2,3,5,6tetraphenylthieno[3,2-b]thiophene (46) in 30% yield.33,34 The same compound was also obtained from the reaction of 45 with 4,5-diphenyl-1,2-dithiol-3-thione (47) thermally. Derivatives of 46 were prepared in the same manner by either replacing the phenyl substituents with various groups or fusing 1,2-dithiol-3-thione to the benzene or cyclohexene rings

(49c) with a nitrophenyl group has the most red-shifted absorbance at 365 nm, affording the lowest optical band gap of 2.92 eV (Table 1). The absorption maxima of the other TTs were between 300 and 302 nm (Figure 1). Electrochemical investigations indicated that TT (49f) with a dimethylaminophenyl group possesses the lowest oxidation potential maximum of 1.33 V, whereas those of the rest were between 1.60 and 199 V (vs Ag wire). The electrochromic devices of 49a,b,f showed a good optical memory in oxidized and reduced states.38 Kong et al.39 started from a commercially available compound, 2,5-dimethylhex-3-yne-2,5-diol (50), the reaction of which with elemental sulfur led to the formation of 3,6-dimethylthieno[3,2-b]thiophene (51) in 26% yield (Scheme 9).40 Hergué et al.41 reported the synthesis of the 3,6-dialkoxy analogue of 51 (57) starting from dimethyl acetylenedicarboxylate (52) and methyl thioglycolate (Scheme 10). Their initial D


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Scheme 10. Synthesis of 3,6-Dialkoxythieno[3,2-b]thiophenes (57)41

Figure 1. (A) UV measurements of 49a−d and 49f in DCM. (B) Absorbance and (C) emission spectra of the corresponding polymers P49a−c and P49f. Reprinted (adapted) with permission from ref 37. Copyright 2012 American Chemical Society.

Scheme 11. Preparation of Mono- and Dialkyl-Substituted Thieneo[3,2-b]thiophenes, 60 and 6442

Scheme 9. Synthesis of 3,6-Dimethylthieno[3,2-b]thiophene (51) from 5040

Michael reaction was followed by an intramolecular cyclization to provide dimethyl 3-hydroxythiophene-2,5-dicarboxylate (53) in 85% yield. Treatment of 53 with methyhl iodide and then consecutive addition of LDA, sulfur, and methyl bromoacetate furnished 55 in 77% yield. Alkylation of 55 in the presence of K2CO3 and decarboxylation led to the formation of 3,6-dialkoxythieno[3,2-b]thiophenes (57). Zhang et al.42 investigated the effect of alkyl substitution at the 3-position of thieno[3,2-b]thiophene using mono- and dialkyl-substituted TTs. For the synthesis of mono alkylsubstituted TT 60, 3-bromothiophene (5) was subjected to Friedel−Crafts acylation with decanoyl chloride to form 5- and 2-acyl-substituted 3-bromothiophenes, 58a and 58b, respectively. The major product 58b was reacted with ethyl thioglycolate along with a catalytic amount of NaOH, giving 3-nonylthieno[3,2-b]thiophene-2-carboxylate (59) in 70% yield. Treatment of 59 with NaOH followed by heating in refluxing quinoline with copper powder afforded mono alkyl-substituted TT 60 in 77% yield in two steps. Similar methodology was used by He43,44 to obtain 3-n-hexyl- and 3-n-decyl-substituted TTs. Synthesis of dialkyl-substituted TT 64 began from 3,4-dibromothiophene (15). The key intermediate 3-bromo-6-nonylthieno[3,2-b]thiophene (63) was obtained in four steps in an overall yield of 58%, applying a comparable strategy. Sonogashira cross-coupling of 63 with

1-nonyne in the presence of Pd(0) was followed by hydrogenation to yield dialkylated TT 64 in 77% yield (Scheme 11).42 Skabara and co-workers45 reported the synthesis of two novel TT-based conjugated molecules 67 and 69 in 72% and 61% yields, respectively, from Stille coupling reactions of dibromoTT (65) and stannylthiophenes (66 and 68) (Scheme 12). Electrochemical and optical investigations provided the repeating band gaps as 2.21 and 3.01 eV for 67, and 2.45 and 3.14 eV for 69. While noncovalent S···O interactions in 67 resulted in a planar conformation and hence high degree of conjugation E


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Scheme 12. Synthesis of TT-Based Conjugated Molecules 67 and 69, and Structures of 70 and 7145−47

Scheme 14. Benzannulated TT (BTT, 28) from Benzothiophene (74)49

trimethylsilyl acetylene afforded 1-(3-bromobenzo[b]thiophen2-yl)-2-trimethylsilylacetylene (76) in 80% yield. Tandem ring closure and deprotection reactions using Na2S in N-methyl-2pyrrolidone (NMP) resulted in BTT (28) in 52% yield. Fouad et al.50 reported the synthesis of 28 using a strategy similar to that applied for the synthesis of 1 (Scheme 15). Scheme 15. BTT (28) from 3-Chloro[1]benzothiophene-2carbaldehyde (77)50

length, the twisted conformation was observed for the 3,4-ethylenedithiothiophene (EDTT) analogue (69). The hole mobilities for 67 and 69 were reported to be 4.0 and 1.5 × 10−2 cm2 V−1 s−1, respectively. A similar phenomenon was observed by Frère,46 who investigated the oligomer (70) possessing 3,6-dimethoxythieno[3,2-b]thiophene and 3,4-ethylenedioxythiophene (EDOT) units. Here, also the S···O intramolecular interactions along with rigid TT unit led to a planar conformation. Moreover, Skabara and co-workers47 unveiled the S···N intramolecular interaction in 2,6-bis(3,6-dihexylthieno[3,2-b]thiophen-2-yl)benzo[1,2-d:4,5-d′]bis(thiazole) (BDHTT-BBT, 71), which provided the molecule with a rigid structure. Electrochemical measurements revealed the HOMO and LUMO energy levels as −5.7 and −2.9 eV, respectively. The hole mobility of vacuum deposited films of BDHTT-BBT was observed for bottom-contact, top-gate devices as 3.0 × 10−3 cm2 V−1 s−1 with an on/off ratio of 105. Benzannulated thieno[3,2-b]thiophene 73 was synthesized through the reaction of 2-(benzo[b]-thiophen-2-yl)-3,3dimethylbutan-2-ol (72) with thionyl chloride (Scheme 13).48

3-Chloro[1]benzothiophene-2-carbaldehyde (77) was treated with methyl thioglycolate in the presence of sodium methoxide followed by successive hydrolysis and decarboxylation using copper bronze in quinolone to provide BTT (28). Thienothiophenes can also be synthesized by heating aliphatic or aromatic hydrocarbons in the presence of sulfur at elevated temperatures up to 320 °C and under high pressure, a method that was first applied by Friedmann in 1916.51 Later, flash vacuum pyrolysis (FVP) of a phosphorus ylide of 1,2-bis(2-(methylthio)phenyl)ethane-1,2-dione (79) at 850 °C gave benzo[b]thieno-[3,2-b]benzo[b]thiophene (80) in 36% yield by tandem radical cyclization (Scheme 16).52 Scheme 16. Preparation of Benzo[b]thieno[3,2-b]benzo[b]thiophene (80) through Flash Vacuum Pyrolysis of 79 and via Intramolecular Ring Formation Reaction of Bis-sulfide Diradical 8252,53

Scheme 13. Synthesis of Benzannulated TT 7348

Huang et al.49 developed a strategy for the synthesis of benzannulated TT (benzo[b]thieno[2,3-d]thiophene) (BTT, 28), in three steps and in an overall 36% yield starting from commercially available benzothiophene (74) (Scheme 14). Dibromination of 74 followed by Sonogashira coupling with F


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The synthesis of benzo[b]thieno[3,2-b]benzo[b]thiophene (80) was also realized in 49% yield through a double intramolecular ring formation reaction of bis-sulfide diradical (82), prepared in situ from the reaction of 2,2′-dibromodiphenylacetylene (81) with t-BuLi, then sulfur (Scheme 16).53 Another method, involving a double intramolecular cyclization of diethyl (E)-stilbene-4,4′-dicarboxylate (83), substituted by ethyl carbonodithioates at 2- and 2′-positions, in the presence of bromine produced diethyl benzo[b]thieno[3,2-b]benzo[b]thiophene-2,7-dicarboxylate (84) in 42% yield (Scheme 17). For the synthesis of the precursor 83, the

Scheme 18. Bis- and Mono-pyrrole Fused Products 86 and 88 from 85 and 87, Respectively55

Scheme 17. Synthesis of Diethyl Benzo[b]thieno[3,2-b]benzo[b]thiophene-2,7-dicarboxylate (84)54

Scheme 19. Synthesis of Dinaphtho[2,3-b:2′,3′-f ]thieno[3,2-b]thiophene (92, DNTT)56

[1]benzothieno[3,2-b]anthra[2,3-b]thiophene, and naphtho[3,2-b]thieno[3,2-b]anthra[2,3-b]thiophene, using two consecutive thiophene formation reactions from o-ethynyl-thioanisole and an arylsulfenyl chloride. The highly π-extended bis[1]benzothieno[2,3-d:2′,3′-d′]naphtho[2,3-b:6,7-b′]dithiophene (BBTNDT, 96) came from 2,6-bis(methylthio)-3,7bis[(trimethylsilyl)ethynyl]naphthalene (93) in four steps. Reaction of 93 with PhSCl, followed by treatment with TBAF, provided 94 in 64% yield, iodination or bromination of which produced 95a and 95b in 63% and 81% yields, respectively. Palladium-mediated aryl−aryl coupling gave 96 in 81% and 47% yields from 95a and 95b, respectively (Scheme 20). 2.1.2. Synthesis and Applications of Oligomers and Polymers. According to the computational studies by Zhang et al.,58 TT (1) was a p-type semiconductor, considering its high electron injection barrier. Replacement of the functional groups and the hydrogen atoms with electronwithdrawing groups changed its p-type property to n-type or to ambipolar. Investigation of the effect of halogen groups on the injection barrier relative to the work function of Au electrodes, reorganization energy, and transfer mobility indicated that halogen units do not improve the injection barrier. They rather increased the reorganization energy and, hence, diminished transfer mobility. On the basis of their theoretical calculations at the B3LYP/6-31G(d) level, HOMO and LUMO energy levels were obtained as −5.85 and −0.72 eV, respectively. The band gap energy driven from the difference between the energy levels of HOMO and LUMO was reported as 5.13 eV, along with the hole and electron reorganization energies (λ+ and λ−)

nitro groups of 2,2′-dinitro-(E)-stilbene-4,4′-dicarboxylate (81) were reduced with iron, providing the corresponding diamine 82, which was subjected to diazotization, followed by treatment with potassium ethyl carbonodithioate to afford 83.54 Facchetti et al.55 performed ring closure reactions of 4-(methylthio)-5-((E)-2-(4-(methylthio)-1-(pentan-2-yl)-1Hindol-5-yl)vinyl)-1-(pentan-2-yl)-1H-indole (85) in the presence of iodine and 5-(benzo[b]thien-2-yl)-4-(methylsulfinyl)-1(pentan-2-yl)-1H-indole (87) in the presence of trifluoromethanesulfonic acid to furnish the corresponding fused bis- and monopyrrole fused products 86 and 88 in 40% yields, respectively (Scheme 18). The calculated HOMO and LUMO energy levels of 86 (alkyl group: Me) were found to be −4.86 and −0.42 eV. Yamamoto et al.56 obtained dinaphtho[2,3-b:2′,3′-f ]thieno[3,2-b]thiophene (92, DNTT) via a three-step synthesis. Reaction of 2-naphthaldehyde (89) with lithium N,N,N′trimethylethylenediamide and excess n-BuLi, followed by addition of dimethyl disulfide, gave the corresponding methylthiosubstituted 2-naphthaldehyde (90) in 58% yield. McMurry coupling of 90 with TiCl4/Zn resulted in the formation of the alkene 91 in 80% yield, the ring closure reaction of which in the presence of excess iodine provided DNTT (92) in 85% yield as thermally stable yellow crystals (Scheme 19). Mori et al.57 developed a new synthetic methodology to obtain [1]benzothieno[3,2-b][1]benzothiophene (BTBT) and unsymmetric, [1]benzothieno[3,2-b]naphtho[2,3-b]thiophene, G


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Scheme 20. Preparation of Bis[1]benzothieno[2,3-d:2′,3′d′]naphtho[2,3-b:6,7-b′]dithiophene (BBTNDT, 96)57

of 0.41 and 0.68 eV, and hole and electron-transfer mobilities (μ+ and μ−) of 0.13 and 0.006 cm2 V−1 s−1, respectively. 2.1.2.1. Oligomers. Zhang et al.59 studied TT oligomers, constructed from various combinations of four thiophene units, 2,5-di(thien-2-yl)thieno[3,2-b]thiophene (97), 2-(5-(thien-2yl)thien-2-yl)thieno[3,2-b]thiophene (98), 2-(thieno[3,2-b]thien-2-yl)thieno[3,2-b]thiophene (99), and 2-(thien-2-yl)dithieno[3,2-b:2′,3′-d]thiophene (100), along with fully fused pentathienoacene (101). Absorption maxima of the oligomers 97−99 blue-shifted in the solid state from 371, 370, 350 nm (in DCM) to 330, 332, 308 nm (thin films), respectively, due to the herringbone packing, which renders H-aggregation (Chart 1, Figure 2, Table 2). However, oligomer 100 having a

Figure 2. Packing motifs of 99−101, which have herringbone, herringbone and slip π−π, and π-stacked arrangements, respectively. Reprinted (adapted) with permission from ref 59. Copyright 2006 American Chemical Society.

Table 2. Absorption Wavelengths of 97−102 in Solution and Solid State59−61

Chart 1. Oligomers 97−101, Constructed from Various Combinations of Four Thiophene Units, and 2,2′:5′,2″Terthiophene (102)59−61

a

compound

λabs(DCM) [nm]

λabs(Solid) [nm]

97 98 99 100 101 102

371 370 350 352 357 354b

330 332 308 330−400a 344 364

H- and J-aggregations. bIn chloroform.

chloroform, which was well correlated to the wavelengths of 99−101 and shifted to higher wavelength (364 nm) in the solid state.60,61 To clarify the solid-state interactions and electronic transitions, TD-DFT calculations using the B3LYP method were performed, applying the 6-31G* basis set, which indicated electronic transitions at 351, 352, and 342 nm for oligomers 99, 100, and 101, respectively. The predicted λmax values were in good agreement with the absorption maxima obtained in solution. Takamiya et al.62 investigated thermal, optical, and electrochemical properties of 2,5-bis(5-hexylthien-2-yl)thieno[3,2b]thiophene (105) and 2,5-bis(5-dodecylthien-2-yl)thieno[3,2-b]thiophene (107), which were synthesized from 2,5-dibromo-TT (103), prepared from TT (1), using 2 equiv of NBS, under Suzuki and Stille couplings, respectively. Thermal analyses revealed melting points and degradation temperatures to be 190 and 340 °C for 105 and 170 and 390 °C for 107, respectively. Optical studies indicated an

dithieno[3,2-b;2′,3′-d]thiophene (DTT) unit has both herringbone and slipped π−π arrangements, and, therefore, both effects of H- and J-aggregations appeared. Interaction of both adopted arrangements displayed a broad absorption band over a range of 330−400 nm for thin film, as H-aggregation caused a blue shift and J-aggregation was responsible for the bathochromic shift. Its solution UV−vis spectrum had an absorption peak appearing at 352 nm. In contrast to the oligomers 97−100, pentathienoacene 101 had only π-stacked packing and demonstrated a λmax of 344 nm, which was a blue-shift from the absorption maxima obtained in DCM, by 13 nm. 2,2′:5′,2″Terthiophene (α-terthienly, 102), having the same conjugation length as 99−101, had an absorption maxium of 354 nm in H


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absorption maximum of 383 nm with the optical band gap of 2.8 eV for 105 and 380 nm with band gap of 2.7 eV for 107. An OFET device constructed from 105 exhibited a hole mobility of 1.6 × 10−2 cm2 V−1 s−1 with an on/off ratio of 105 in air, whereas that of 107 had values of 4.9 × 10−3 cm2 V−1 s−1 and 105 in air, respectively. The long alkyl chain helped the hole transport against the diffusion of oxidants (Scheme 21).

Scheme 22. Synthesis of Oligomers 111−113 Starting from 103, and Structure of 11464,65

Scheme 21. Synthesis of 2,5-Bis(5-hexylthien-2-yl)-TT (105) and 2,5-Bis(5-dodecylthien-2-yl)-TT (107) from 2,5-Dibromo-TT (103)62

Mazur et al.63 replaced the hexyl groups in 105 with octyl units, which demonstrated ordered lamellar mesophases and provided the hole mobility of 0.07 cm2 V−1 s−1. Ahmed et al.64 thermally and optically investigated three derivatives of thieno[3,2-b]thiophene 111−113, possessing phenyl groups at both ends, which indicated that these thermally stable compounds are promising p-type organic semiconductors. The reaction of 2,5-dibromothieno[3,2-b]thiophene (103) with tributyl(thien-2-yl)stannane (108) under a Stille cross-coupling protocol resulted in the formation of 2,5-di(thien-2-yl)thieno[3,2-b]thiophene (97) as a yellow solid in 63% yield. The sequential treatment of 97 with t-BuLi and Bu3SnCl gave 2,5-bis(5-tributylstannylthien-2-yl)thieno[3,2-b]thiophene (109), which was subjected to Stille coupling with bromobenzene to obtain 2,5-bis(5-phenylthien-2-yl)thieno[3,2-b]thiophene (111) as a bright yellow solid in 40% yield. Dibromination of 97 was achieved using NBS, which provided 2,5-bis(5-bromothien-2-yl)thieno[3,2-b]thiophene (110) as a yellow solid in 83% yield. Suzuki coupling reactions of 110 with 4-dodecylphenylboronic and 4-(trifluoromethyl)phenylboronic acids afforded 2,5-bis(5-(4-dodecylphenyl)thien-2-yl)thieno[3,2-b]thiophene (112) as yellow and 2,5-bis(5-(4-(trifluoromethyl)phenyl)thien-2-yl)thieno[3,2-b]thiophene (113) as orange color crystals in 69% and 74% yields, respectively (Scheme 22). Optical studies revealed the absorption maxima and optical band gaps of 111−113 at 415, 401, and 408 nm, and 2.50, 2.65, and 2.80 eV, respectively. Compound 114, analogous of 111 with alkyl chain, was also studied by Ahmed et al.,65 providing the absorption maximum of 389 nm and the optical band gap of 2.71 eV. Their top-contact OFET devices fabricated using 114 demonstrated a high mobility of 0.031 cm2 V−1 s−1 with an on/off ratio of 104. Zhang et al.42 studied the planarity and conformations of backbones of polymers by investigating head-to-tail (HT) organized dimers 116 and 117, which were synthesized through Stille cross-coupling and oxidative coupling reactions, respectively. On the basis of their experimental results, substitution at 3-positions of TTs leads to steric hindrance and, hence, a blue shift in UV−vis absorption spectra (Chart 2, Figure 3). San Miguel and Matzger66,67 reported that the absorption

Chart 2. Oligomers Studied To Investigate the Planarity of Corresponding Polymers42,54,67

I


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Scheme 23. Synthesis of Dimer 123 and Polymer 124 from Dimethoxy-Substituted TT 12268

Figure 3. (Left) UV−vis absorption spectra of 99, 116, and 117 in chloroform.42 Reprinted (adapted) with permission from ref 42. Copyright 2004 American Chemical Society. (Right) X-ray crystal structure of 115 demonstrating two views.67 Reprinted (adapted) with permission from ref 67. Copyright 2007 American Chemical Society.

maximum of 97 at 373 nm shifts to 353 nm after introducing two alkyl groups at positions 3 and 6 of the TT (115). The larger is the steric hindrance, the larger is the deviation from coplanarity, which was supported by DFT calculations. Moreover, the energy change of the TT (120), having one thiophene at a 2-position and two alkyl groups (Me) at 3- and 6-positions with varying dihedral angle from 180°−90° by 10° was calculated, using the DFT method, which indicated a minimum energy torsion angle of 140°. The X-ray crystal structure of 115 showed a torsion angle of 161° (Figure 3). DFT calculations and X-ray analyses demonstrated a deviation from planarity and, hence, a blue shift. While the dimer 99 had a λmax of 353 nm, the other dimers, such as disubstituted 116, 118 and tetrasubstituted 117 were reported to have λmax values of 337, 336, and 296 nm, respectively. The use of an ethynyl linkage (119) diminished the steric hindrance and hence resulted in a red shift, as compared to 117. The absorption maximum of 119 was 387 nm. The same phenomenon was noticed for the corresponding polymers. The absorption maxima of the polymers of 117, 118, 116, 115, and 119 were 359, 446, 447, 453, and 463 nm, respectively, which were in good agreement with the observed wavelengths of the dimers (Chart 2). Roncali and co-workers68 synthesized 122, substituted with two methoxy groups at positions 3 and 6, and its dimer 123. Removal of two bromines from the 2-positions of tetrabromoTT (9), prepared from TT with bromine, gave 3,6-dibromo-TT (121), which was reacted with NaOMe in the presence of KI and CuO to obtain 122 in an overall yield of 55%. For the synthesis of the dimer 123, initially, the lithiated monomer was reacted with Bu3SnCl to prepare its stannic derivative, which was coupled with itself in the presence of CuCl2 and palladium. Crystal structure analysis of the dimer 123 indicated noncovalent sulfur−oxygen interactions ca. 2.8 Å, which provided a more rigid structure. Electrochemical polymerization of the monomer 122 gave 124, the band gap of which was 1.7 eV (Scheme 23). The 3,3-linked dimer 126 resulted from a reaction of 3-bromothieno[3,2-b]thiophene (16) and 3-trimethylstannylthieno[3,2-b]thiophene (125).69 Dimer 126 and trimer 127, which was synthesized by following the same strategy, had reduced C−H···π interactions in their crystalline state and formed a π-stack (Figure 4). On the basis of the single-crystal structure analysis, the molecules had π-stack of edge-to-face. An alternative method for the synthesis of 126 had the reductive coupling of two keto forms of 30, prepared from treatment of 29 with thionyl chloride followed by ring closure using AlCl3 (Scheme 24).70

Figure 4. Crystal structure diagram of β-linked dimer of TT (126).69 Reprinted (adapted) with permission from ref 69. Copyright 2007 American Chemical Society.

Scheme 24. Synthesis of Dimer 126 Linked via 3,3′-Positions and Structure of Trimer 12769,70

Noh et al.71 used TT (1) for the synthesis of co-oligomers to establish the semiconductors 128 and 129, possessing fluorene and biphenyl groups, respectively, for OFET applications (Chart 3). While the crystal structure of 129 is almost planar, a slight J


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Chart 3. Co-oligomers 128 and 129 Used for OFET Applications71,74−76

Scheme 26. Syntheses of Rod-Type Oligomers 136 and 138−140, Using Sonogashira Cross-Coupling and Horner− Wadsworth−Emmons78,79,395

deviation of fluorene units from planarity was observed in 128. The planar and rigid structure of 129 provided good charge transport mobility (0.09 cm2 V−1 s−1) in thin film OFET devices as compared to the TT 128, having a mobility of 0.06 cm2 V−1 s−1.74,75 Noh et al.76 investigated organic phototransistors of 129, whose sensitivity and ratio of photocurrent to dark current were recorded as 82 A/W and 2.0 × 105 at 380 nm, respectively. The photocurrent amplification was attributed to photovoltaic (turn-on) and photocurrent (turn-off) effects. Nakayama et al.77 synthesized the dimer 131 and the tetramer TT 134, starting from 51 (Scheme 25). Bromination Scheme 25. Synthesis of Dimer 131 and Tetramer 134 from 5177

dialdehyde 137. Its double Horner−Wadsworth−Emmons reaction with diethyl [4-(t-butylsulfanyl)benzyl]-phosphonate furnished 138 (Scheme 26). On the basis of optical measurements, the band gaps of 136 and 138 are 3.0 and 2.7 eV, respectively, smaller than the corresponding phenylene analogue by 0.3 eV. These results were supported by calculations (TD-B2PLYP/ ccpVDZ//B3LYP/ccpVDZ level) of the energy needed for electron excitation to the first singlet state. Liu et al.395 and Ito et al.79 synthesized the derivatives 139c−f, which had TT unit end-capped with styrene having H and alkyl groups at the para positions, in place of t-butylsulfanyl, and their analogues containing bithieno[3,2-b]thiophenes 140c−f. The optical studies revealed broad absorption bands for all of the oligomers 139 and 140, ranging from 350 to 500 nm (Table 3). In the solid state, a blue-shifted absorption peak was obtained, due to the herringbone packing, which resulted in H-aggregation and hence significant intermolecular interactions.395 Replacement of TT with bi-TT resulted in a red shift of 39 nm (λ139c: 427 nm) for R = H and 37 nm (λ139d−f: 435 nm) for R = alkyl. The optical band gaps were estimated to be 2.93 (2.53 eV395) and 2.90 eV for 139c and 139d−f, and 2.67 and 2.63 eV for 140c and 140d−f, respectively, by computational studies at the B3LYP/6-31+G(d,p)//B3LYP/6-31G(d) levels. Electrochemical investigations disclosed HOMO energy levels only for of 139c−f, due to the low solubility of 140c−f. The HOMO levels of 139c and 139d−f were found to be −5.46 (5.43 by Liu et al.395) and −5.39 eV, respectively. Although Liu et al.395 presented a high hole mobility of 3.0 × 10−2 cm2 V−1 s−1 for

of 51 with NBS was followed by coupling reaction using Ni as a catalyst in the presence of Zn and PPh3 to yield the dimer 131. The tetramer 134 was obtained following a similar approach in 84% yield. Bromination of 131 with NBS and then coupling the obtained product 132 with its Grignard reagent 133 in the presence of NiCl2(dppp) furnished tetramer 134. Oxidation potentials of the dimer 131 and the tetramer 134 were measured to be 0.65 and 0.44 V (E1/2 (ferrocene/ferrocenium); 0.2 V vs Ag/Ag+) in CH3CN and CH2Cl2, respectively. Seidler et al.78 synthesized the rod-type oligomers oligo(p-phenylene-ethynylene) (136) and oligo(p-phenylene-vinylidene) (138). Sulfur units were attached to both ends of the molecules to provide “alligator clips”, which were used for the measurement of their single-molecule conductance by means of a mechanically controllable break junction (MCBJ) technique. Syntheses of the compounds 136 and 138 were realized by applying Sonogashira cross-coupling of the bisethynyl compound 135 with S-(4iodophenyl)ethanethioate giving 136 in 32% yield. For the synthesis of 138, 1 was successively reacted with n-BuLi/N,Ndimethylformamide (DMF) and then t-BuLi/DMF resulting in K


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levels of 141−143, obtained from their CV measurements, were −5.52, −5.61, and −5.58 eV, whereas the LUMO levels were calculated to be −4.32, −4.31, and −4.33 eV, respectively. These results were in good agreement with the DFT results. The CV measurements gave band gaps of 1.20, 1.30, and 1.25 eV for the compounds 141−143, respectively. Thin film and solution UV−vis absorption spectra of 141−143 demonstrated that the compounds 141 and 143 have hypochromic shifts of 36 (λmax(solid) = 674 nm) and 100 nm (λmax(solid) = 605 nm), respectively, in the solid state, whereas compound 143 had a broader absorption band without any change of peak maximum (λmax(solution,solid) = 720 nm). This indicated different molecular packings of compounds 141−143 in their solid states. Thermal annealing studies on the organizations of the molecules in thin films revealed that compounds 141 and 142 had H-aggregation in the solid state, whereas a thin film of compound 143 demonstrated J-type aggregation, which was observed by the red-shifted absorption, as compared to the deposited thin film. OFET measurements indicated a change in the charge transport performance with the location of alkyl groups. While compound 142, having alkyl chains at 3,3′-positions, demonstrated the highest electron mobility of 0.22 cm2 V−1 s−1, the electron mobilities of the compounds 141 and 143 were lower than that of compound 142. This phenomenon unveiled the significant effects of orientations of alkyl chains on OFET devices. Liu et al.81 synthesized thieno[3,2-b]thiophenes substituted by both four phenyl (144) and four 2-thienyl groups (145), along with two 2-thienyl units (146). Suzuki coupling of tetrabromo-TT (9) with phenyl- or thiophene-2-boronic acids resulted in the production of 144 as colorless and 145 as yellow crystal blocks in 56% and 50% yields, respectively. From the reaction of thiophene-2-boronic acid, disubstituted compound (146) was also obtained as yellow crystalline prisms in low yield (15%). Tributyl(5-hexylthien-2-yl)stannane (147) was treated with tetrabromo-TT (9) to obtain thieno[3,2-b]thiophene (148), having tetra 5-hexylthien-2-yl groups, in 75% yield, as a yellow solid (Scheme 27). Because of the four hexyl subunits, the solubility of 148 was the best, as compared to the other compounds 144−146. According to their differential scanning calorimetry (DSC) and thermogravimetric

Table 3. Optical Properties of 136, 138, 139c−f, and 140c−f along with Hole Mobilities of 139c−f and 140c−f78,79,395 compound

λabs [nm]

136 138 139c 139d 139e 139f 140c 140d 140e 140f

375 409 427 435 435 435 466 472 472 472

a

HOMO [eV]

−5.46 (−5.43)a −5.39 −5.39 −5.39

EgOpt [eV]

mobility (×10−2) [cm2 V−1 s−1]

3.0 2.7 2.93 (2.53)a 2.90 2.90 2.90 2.67 2.63 2.63 2.63

0.21 (3.0)a 2.6 1.7 1.9 1.8 3.5 0.37 0.29

Recorded by Liu et al.395

139c, Ito et al.79 recorded the highest hole mobilities for 139d and 140d, containing butyl groups, which were 2.6 × 10−2 cm2 V−1 s−1 with an on/off ratio of 4 × 104 and 3.5 × 10−2 cm2 V−1 s−1 with on/off value of 2 × 104, respectively. Wu et al.80 explored the photophysical and electrochemical properties of dicyanomethylenes, possessing 2,5-di(thien2-yl)thieno[3,2-b]thieno-quinoids 141−143, substituted with alkyl chains (2-decyltetradecyls) at 2,2′-, 3,3′-, and 6,6′positions (Chart 4). The chains conveyed good solubility Chart 4. Oligomers 141−14380

Scheme 27. Synthesis of Tetrasubstituted TTs 144, 145, and 148 and Disubstituted TT 14681

in tetrahydrofuran and chlorinated organic solvents. While the solubility of 141 was the highest, 142 had the lowest, due to the strong influence of side alkyl chain orientations on intermolecular interactions in the solid state. They also provided good molecular packing in the solid state, due to the short π−π stacking distance between the intermolecular layers. According to DFT calculations (B3LYP/6-31G(d) level) on the HOMO and LUMO levels of the molecules, 141−143 were predicted to be higher in energy than those of the unsubstituted parent molecules (HOMO = −5.85 eV and LUMO = −4.39 eV). While the HOMO levels of the compounds 141−143 were −5.61, −5.73, and −5.68 eV, the LUMO energy levels were −4.16, −4.19, and −4.26 eV, respectively. The HOMO energy L


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analysis (TGA) studies, all of the compounds were thermally stable and did not have any glass transitions on account of their dense molecular packings. On the basis of X-ray crystallographic analysis, the phenyl groups in 144 are arranged in a twisted form with dihedral angles of 35.8° and 59.4° between TT and the phenyl rings. In contrast to the other fused thiophenes, which had intermolecular S···S interactions, tetraphenyl-substituted TT (144) formed a C−H···π stack with an interplanar distance of 3.85 Å. In the case of 145, the thienyl groups were oriented to the TT core unit with dihedral angles of 22.0° and 77.8°. However, compound 146 had an almost planar structure with a small dihedral angle of 7.6°. UV−vis spectra of 144−146 and 148 displayed absorption maxima of 339, 361, 367, and 374 nm, respectively. Moreover, shoulder peaks were observed at 275 nm for 145 and 290 nm for 148, emerging from the 3-linkages between the oligothiophenes. The band gaps, obtained from the longest wavelength absorption edges, of 145, 146, and 148 were 2.92, 2.98, and 2.82 eV, which were smaller than that of 144 (3.23 eV). According to cyclic voltammetry investigations, the increase of HOMO levels was directly proportional to the number of thiophene units. While the HOMO values of compounds 144−146 and 148 were −5.61, −5.37, −5.55, and −5.28 eV, LUMOs were reported to be −2.38, −2.45, −2.57, and −2.46 eV, respectively. Electropolymerizations of 144 and 148 failed on ITO, which was explained as being due to the blocking groups at 2-positions. However, the other monomers, 145 and 146, could be polymerized under the same conditions. Kim et al.82 connected two thieno[3,2-b]thiophenes, possessing hexyl groups, with a π-conjugated spacer, naphthalene and anthracene. The reaction of 2-hexylthieno[3,2-b]thiophene (149) with 2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane afforded 2-(2′-hexyl-5′-thieno[3,2-b]thienyl)-4,4,5,5-tetramethyl1,3,2-doxaborolane (150) in 41% yield; Suzuki coupling of this with 2,6-dibromonaphthalene or 2,6-dibromoanthracene gave thermally stable 2,6-bis(5′-hexyl-thieno[3,2-b]thiophen-2′-yl)naphthalene (151) and 2,6-bis(5′-hexyl-thieno[3,2-b]thiophen2′-yl)anthracene (152) in 82% and 70% yields, respectively (Scheme 28). DSC studies showed the crystalline properties of each oligomer. Optical investigations in dilute chloroform showed that 151 has two maxima appearing at 296 and 380 nm, whereas oligomer 152 demonstrated three absorption maxima at 344, 412, and 438 nm. Electrochemical studies showed smaller oxidation and reduction potentials of 0.85 and 1.56 V, respectively, for the oligomer 152 than that of the oligomer 151 of 1.00 and 1.65 V (vs Ag/AgNO3), respectively. The estimated HOMO, LUMO energy levels and therefrom calculated band gap energies of the oligomers 151 and 152 were −5.40, −2.75, 2.65 and −5.25, −2.84, 2.41 eV, respectively. The oligomers 151 and 152 formed highly ordered polycrystalline vacuum-evaporated films, and their hole transport motilities were reported to be 0.084 cm2 V−1 s−1 with an on/off ratio of 3.2 × 106 at 100 °C and 0.14 cm2 V−1 s−1 with an on/off ratio of 6.3 × 106 at 120 °C, respectively. Choi and co-workers83 assembled four alkylated thieno[3,2-b]thiophenes, located around a benzene through ethylene linkages, for OFET applications. The reaction of 5-hexylthieno[3,2-b]thiophene-2-carbaldehyde (153) with octamethyl(benzene1,2,4,5-tetrayltetrakis(methylene))-tetrakis(phosphonate) (154) afforded 2-(2,4,5-tris((E)-2-(2-hexylthieno[3,2-b]thiophen-5-yl)vinyl)styryl)-5-hexylthieno[3,2-b]thiophene (155) (Scheme 29). As the compound has a good solubility in various organic

Scheme 28. Preparation of Oligomers 151 and 152 through Suzuki Coupling82

solvents, it was studied for solution processed device applications. According to DSC analysis, it had phase transition and endothermic cold crystallization temperatures at 172 and 90 °C, respectively. The absorption maximum of the oligomer in solution was 395 nm, and the PL emission appeared at 498 nm (Figure 5). The optical band gaps in solution and at annealed solid state (film) were 2.54 and 2.40 eV, respectively. The HOMO and LUMO energy levels driven from the electrochemical studies were −5.17 and −2.77 eV, respectively. However, charge carrier mobility was measured as low as 2.5 × 10−4 cm2 V−1 s−1 with an on/off ratio of ca. 103 on account of the solid film possessing arbitrary oriented molecules. Liang et al.84 explored the optical and electronic properties of 1,2,4,5-tetrakis(phenylethynyl)benzene derivatives computationally (Scheme 29). According to the theoretical studies at the CAM-B3LYP/6-31G(d) and TDCAM-B3LYP/6-31G(d,p) levels, compound 158, possessing 2-vinyl-thieno[3,2-b]thiophene branches had higher lying HOMO and lower lying LUMO levels, and consequently a smaller band gap energy. The calculated HOMO−LUMO gaps were 4.67, 5.05, and 4.55 eV for the molecules 156−158, respectively. Moreover, larger absorption wavelength and oscillator strength values were obtained. However, replacement of thieno[3,2-b]thiophene with thiophene (156) provided the molecule with the broadest absorption region, suggesting that 156 was a suitable material for solar cell applications if performance was based on the broadest absorption region alone. Metri et al.85 introduced triphenylamine in place of benzene as a center of star shape oligomers 159−161, in a combinatorial manner, which involved Suzuki coupling of dioxaborolane-TPA derivative and 2-bromo-3-nonylthieno[3,2-b]thiophene and Stille coupling of stannyl thieno[3,2-b]thiophene (Scheme 30). Optical investigations revealed λmax values of 354, 386, and 396 nm resulting in optical band gaps of 2.99, 2.90, and 2.65 eV for 159−161, respectively, which showed that the oligomer M


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Scheme 30. Synthesis of Star Shape Oligomers 159−16185

Scheme 29. Synthesis of Star Shape Oligomer 155 from the Aldehyde 153, and Computationally Studied Star Shape Oligomers 156−15883,84

Durso et al.86 synthesized thieno(bis)imide-based n-type semiconductors 162−164, which had electron mobilities of up to 0.3 cm2 V−1 s−1, by reacting brominated thieno(bis)imide (N−Br) and corresponding distannyl derivatives through Stille cross-coupling reactions (Chart 5). DSC measurements demonstrated the liquid crystalline properties of 162 and 163. According to UV−vis studies, the compound possessing a Chart 5. n-Type Semiconductors 162−16486

Figure 5. UV−vis and PL (excited at 400 nm) spectra of 155. Solid black line, solution; dashed brown line, pristine film; and dash-dotdashed line, annealed film. PL spectra were recorded in solution states.83 Reprinted (adapted) with permission from ref 83. Copyright 2007 American Chemical Society.

161 had the lowest optical band gap, due to the extended π-conjugation system through thiophene unit between TPA and TT groups. N


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Chart 6. Dye-Sensitized Solar Cells 165a−d and 16687,88

bithiazole 164 had an absorption maximum blue-shifted by about 15 and 20 nm, as compared to 163 and 162, respectively. Measurements also revealed the blue-shifted maximum emission wavelengths for 164 at 489 and 514 nm. 162 and 163 had emission wavelengths red-shifted by 67 and 32 nm with respect to 164, respectively. The estimated optical gap energy of 164 was reported to be 2.57 eV, and compounds 162 and 163 had smaller optical gap energies of 2.39 and 2.49 eV, respectively. On the basis of cyclic voltammetry and UV−vis studies, while 164 has the lowest HOMO energy level of −5.81 eV, 162 and 163 have HOMO energy levels of −5.65 and −5.69 eV, respectively, which were supported by computational predictions at the B3LYP/6-31G(d) level. DFT calculations indicated that, unlike 162 and 163, both HOMO and LUMO orbitals of 164 are uniformly delocalized over the whole molecule, which results in better intermolecular overlapping, and hence improved charge transport. OFET measurements showed the highest electron charge transport of 3.0 × 10−1 cm2 V−1 s−1 for 163, while 164 had a charge mobility of 1.6 × 10−1 cm2 V−1 s−1, and 162 had 1.3 × 10−5 cm2 V−1 s−1. Li et al.87 investigated indoline-based dye-sensitized solar cells 165a−d, containing 3-propyl-thieno[3,2-b]thiophene, which hindered the interactions between molecules and diminished the π-aggregation. The red shifts of 3−11 nm were observed from the comparison of the absorption spectra of TiO2 sensitized films of 165a−d with 165a−d together with chenodeoxycholic acid (CDCA) (Table 4). TTs were also

Chart 7. Donor−Acceptor (D−A) Systems 167−170, Which Show Nonlinear Optical (NLO) Properties90

Table 4. Absorption Maxima of 165a−d87

a

compound

λmaxa [nm]

λmaxb [nm]

λmaxc [nm]

165a 165b 165c 165d

492 495 465 500

435 416 416 445

444 423 427 448

In DCM. bTogether with CDCA. cSensitized TiO2 films.

used as a π-spacer in asymmetric cyanine dyes (for example, 166), possessing donor (D) and acceptor (A) units for photovoltaic applications, particularly in dye-sensitized solar cells (DSSC) (Chart 6).88 Fuse et al.89 performed a rapid synthesis of a 112-membered D−π−A dye library, applying a one-pot procedure, which did not show any obvious correlations between ε and the η. However, the HOMO and LUMO energy levels affected η. The dyes with a more positive HOMO energy level and with a LUMO energy level 105. Liu et al.110 investigated the influence of the gate dielectric on the contact resistance (Rc) in a device constructed from 2,7didecyl[1]benzothieno[3,2-b][1]benzo[b]thiophene (C8-BTBT, 199). It displayed a drastic change at Rc from 10 to 66 kΩ cm with gate dielectric interfaces due to the trapped charges, which decelerated the carrier mobility. Tsukagoshi and co-workers111 used 199 (C8-BTBT) to optimize the fabrication process. They performed in situ purification via spin-coating of 199/PMMA (poly(methyl methacrylate)) blend whereby they removed the effect of impurities on the electrical properties of 199, which was expected to reduce the cost during fabrication. The naphthalene fused derivatives (203a−g) along with anthracene embedded analogue (204) were also investigated resulting in the charge mobilities varying from 0.6 to 7.9 cm2 V−1 s−1. While 7.9 cm2 V−1 s−1 was obtained from 203d (R1 = C10H21), the anthracene analogue produced a charge mobility of 3.0 cm2 V−1 s−1.112,113 The HOMO of oligothiophene has the nodal planes located on the sulfur atoms, that is, no coefficients of HOMOs on the sulfur atoms. However, as the holes have to migrate through the HOMO of each molecule in the p-channels of OFETs, replacement of a sulfur atom by another chalcogen does not result in a significant difference in the hole mobilities. Moreover, large sulfur atoms encumber the intermolecular interactions of HOMOs. Therefore, large HOMO coefficients of sulfur atoms were considered to make the molecules promising p-channel organic semiconductors for OFET application. Shinamura et al.114 performed computational studies at the B3LYP/ 6-31G(d) level to resolve the HOMOs of various sulfurcontaining molecules (Chart 12). According to their DFT

obtained using vapor deposition, had the mobility of 2.0 cm2 V−1 s−1, which is closer to the highest mobility of 3.0 cm2 V−1 s−1 achieved by the best pentacene OFET. Moreover, it was more stable than pentacene in air, which makes this material more attractive for thin-film OFET applications. Huang et al.49 synthesized derivatives of BTT, having phenyl (206) and benzo[b]thiophene (207) substituents along with its dimer (208), applying Stille coupling in the key step (Scheme 34). α-Proton abstraction from 28 using Scheme 34. Synthesis of Derivatives of BTT 206−20849 and Their DTT Containing Analogues 209−211115

n-BuLi was followed by treatment with tributylchlorostannane to provide 205. Coupling with aryl bromides in the presence of Pd(0) furnished the corresponding products 206−208 in 68%, 52%, and 51% yields, respectively. Their DSC analyses demonstrated sharp endothermic peaks above 160 °C. The highest melting point (334 °C) and weight loss were recorded for compound 208. The absorption maxima for 206−208 were 330, 366, and 388 nm, which were considerably blue-shifted with respect to the absorption maxima of their DTT derivatives 209−211 (Scheme 34),115 which were 358, 378, and 411 nm in o-dichlorobenzene, respectively. As 208 had the smallest HOMO−LUMO gap of 2.91 eV, the largest one was observed with 206 (3.32 eV). Compound 207 had a HOMO−LUMO gap of 3.04 eV. According to differential pulse voltammetry (DPV) investigations of the BTT derivatives in dichlorobenzene at 50 °C, oxidation peaks for 206 and 207 were +1.61 and +1.48 V, respectively, and in the case of more-conjugated 208, the oxidation peak was lowered to +1.27 V. The same trend was observed with compounds 209−211, the oxidation potential peaks of which varied from +1.33 to +1.14 V (vs a nonaqueous Ag reference electrode). The HOMO−LUMO gaps, calculated from DPV, were in good agreement with the optical band gaps

Chart 12. HOMOs of Various Sulfur-Containing Molecules114

calculations, α-quaterthiophene and pentathienoacene had nodal planes on the sulfur atoms, whereas benzo[1,2-b:4,5-b′]thiophene and [1]benzothieno[3,2-b][1]benzothiophene had HOMO coefficients on sulfur atoms. They concluded that benzo[1,2b:4,5-b′]dichalcogenophene is a good candidate for p-channel organic semiconductors. A homogeneous thin film of [1]benzothieno[3,2-b][1]benzothiophene on Si/SiO2 substrates, T


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Yamamoto et al.56 investigated dinaphtho[2,3-b:2′,3′-f ]thieno[3,2-b]thiophene (92, DNTT), the optical band gap of which was found to be 3.0 eV, and its HOMO energy level, obtained using cyclic voltammetry was −5.44 eV (Scheme 19). These findings were supported by MO calculations. According to the X-ray crystallographic studies, 92 has an almost planar structure with a deviation of 0.058 Å. OFET investigations, performed using “top-contact”-type devices on Si/SiO 2 substrates, indicated p-channel FET properties with a mobility of higher than 0.3 cm2 V−1 s−1 and an on/off ratio of higher than 106. Even higher mobility of >2.0 cm2 V−1 s−1 and on/off ratio of 107 were observed after the surfaces were treated with octyltrichlorosilane (OTS) at 60 °C. Northrup et al.121 unveiled the effect of alkyl substituents, located at 2- and 9-positions, on the electronic structure of dinaphtho[2,3-b:2′,3′-f ]thieno[3,2b]thiophene (92), using DFT calculations with a combination of the local atomic potential (LAP) approach. Their results indicate that alkyl chains decreased intermolecular distances, in good agreement with the experimental results and explained as being due to van der Waals interactions between the alkyl chains. Yagi et al.122 measured the ultraviolet photoelectron spectra (UPS) of 92 providing the low-lying HOMO, which rendered high stability, supported by the DFT calculations. The ionization potential (IP) of 92 was obtained from UPS measurement, which, unlike CV, determined the HOMO level directly without any approximations, to be 5.44 eV, significantly higher than the IP of 4.85 eV, of amorphous pentacene. Sakai et al.123 observed pressure-dependent conductivity with the single crystals of 92, and the mobility changed drastically with the applied pressure. Yogev et al.124 obtained the gap density of states (DOS) and the Fermi level (FL) position in devices constructed from pentacene and 92, using Kelvin probe force microscopy, which indicated a smaller gap DOS for 92 as compared to pentacene grown on the same surface, and hence drastically reduced FL pinning. Kang et al. 125 investigated X-ray crystallography and OFET properties of isomeric 2,9-diphenyldinaphtho[2,3-b:2′,3′-f ]thieno[3,2-b]thiophene (DNTT) (217) and 3,10-diphenyl-DNTT (218) (Chart 14). Like the parent DNTT, herringbone packing was

of 3.56, 3.39, and 3.16 eV for 206−208, respectively. On the basis of optical and electrochemical investigations, they were found to be very stable, as they had lower HOMO energy levels and larger band gaps with respect to pentacene. Single-crystal X-ray crystal analysis of the compound 208 revealed that the phenyl rings are almost coplanar to the thienothiophene units. The C−C single bond between the two TTs is 1.45 Å, and the dihedral angle between two flat TTs is 0°. The distance between the two stacking planes is 3.53 Å, and the angle between two planes, forming herringbone packing, is 54.2°. The shortest intermolecular S−S distance was recorded to be 3.97 Å. Thin film transistor (TFT) properties of their devices were investigated, which were found to be TFT active and formed p-channel semiconductors. A device fabricated from the most conjugated compound, 208, showed the highest device performance among them with hole mobilities of 0.08−0.22 cm2 V−1 s−1 and on/off ratios of 106−107. Kugler et al.116 conducted surface modification using organic semiconductors (28, 80, 212a, and 213), having partially fluorinated fullerenes (C 60 F 36 and C 60 F 48 ) for OLED applications (Chart 13). Luca et al.117 improved the interaction Chart 13. Some Organic Semiconductors That Were Modified for OLED Applications116,117

between TT-donor (212b) and perylene acceptor (214) in a blend by performing a selective solvent vapor annealing, which resulted in the enhancement of the interfacial charge transfer in BHJ. Müllen and co-workers118 recorded the high mobility of 1.7 cm2 V−1 s−1 with an on/off ratio 107 for a solutionprocessed OFET fabricated from 212b. Soeda et al.119 synthesized insoluble 92 from its precursor 215 in ionic liquid, which enabled two-dimensional crystal growth of 215, making fabrication of an insoluble organic semiconductor simpler (Scheme 35). The average mobility was 1.55 cm2 V−1 s−1. The effect of improper arrangement of semiconductor on dynamic TFT performance was also investigated using 92.120

Chart 14. Some TT Derivatives for OFET Applications56,122−128

Scheme 35. Synthesis of 92 in Ionic Liquid119

obtained for both isomers in single crystal and thin film X-ray analyses, regardless of the positions of phenyl substituents. On the basis of OFET measurements, slightly higher mobilities of 3.4 and 3.5 cm2 V−1 s−1 were observed for 217 and 218, U


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than that for 221 by 0.1 eV. The results for 222 and 221 were 1.8 and 1.9 eV, respectively.133 Copolymers constructed through the reaction of 1 with thiophene and bithiophene were investigated to explain the improvement of the charge mobility and other semiconducting properties.134,135 Gustafson et al.336 used the polymer 222 together with PEDOT to prepare an alloy photoelectrocatalyst for water oxidation, which provided enhanced water oxidation reaction. Polymerization of 3-nonylthieno[3,2-b]thiophene (60) was performed using three different methods: (i) oxidative coupling in the presence of FeCl3 to provide the polymer 223 with a low solubility in THF and large polydispersity indice (Mn = 1895, Mw = 5514 g mol−1, PDI = 2.91, obtained from gel permeation chromatography (GPC)), (ii) dibromination with NBS, followed by a Kumada coupling to furnish the regiorandom polymer 225, which also had low solubility in THF (Mn = 2184, Mw = 2672 g mol−1, GPC), and (iii) preparation of the corresponding stannane derivative and its Stille coupling to produce regioregular red dark polymer 228, which was sparingly soluble in THF and had a molecular weight of Mn = 2714, Mw = 3119 g mol−1 (Scheme 36). The red dark

respectively, as compared to the unsubstituted DNTT (92). Incorporation of two phenyl subunits enhanced the thermal stabilities. While the OFET performance of 92 decreased drastically, even at 100 °C, 218 showed thermal stability up to 200 °C with a slight loss of performance. 2,9-Diphenyl-DNTT (217) demonstrated a good thermal stability up to 250 °C, which makes it a good candidate for high temperature processes. An optimized device, fabricated from 2,9-didecyl-DNTT (216), had a charge mobility of 4.3 cm2 V−1 s−1 with an on/off ratio of 108, and it was operated with a low voltage of 2−3 V,126 which enabled a megahertz operation at low voltage.127 The charge mobility of the top-contact OFET devices was found to be 6.0 cm2 V−1 s−1, which were constructed by forming electrodes upon solution-crystallization of 216.128 Mori et al.57 explored the photovoltaic properties of bis[1]benzothieno[2,3-d:2′,3′d′]naphtho[2,3-b:6,7-b′]dithiophene (BBTNDT, 96), which displayed a very high mobility of >5 cm2 V−1 s−1 in an OFET device (Scheme 20). Jin and Zhang129 studied the substituent effects on the charge transport and optical properties of 2-(2-hydroxyphenyl)-5-phenyl1,3,4-oxadiazole (219), computationally, possessing subunits at the 5-position (Chart 15). Benzo[d]thieno[3,2-b]thiophene

Scheme 36. Syntheses of Regiorandom (223 and 225) and Regioregular (228) Polymers, and Polymer 22942

Chart 15. Substituent Effects on the Charge Transport and Optical Properties of 219129

substituent (220) made 219 a promising luminescent material for organic light-emitting diodes (OLEDs) and electron transport materials. Both absorption and fluorescence maxima of 220 had bathochromic shifts with respect to parent 219. Moreover, due to its phosphorescence emission, the efficiency of electroluminescence was improved to the required level for OLED flat panel display. According to the reorganization energies obtained, it is considered to be a good candidate for electron transport materials. 2.1.2.2. Polymers. In contrast to TT (2), the electropolymerization of which was performed in 2001 by Lee et al.,130 electrochemical and chemical polymerizations of thieno[3,2b]thiophene (1) were carried out in the 1980s, producing poly(thieneno[3,2-b]thiophene) (PTT, 222), a highly insoluble polymer.131,132 The electrical conductivity of 222 was 3 × 10−6 S cm−1 (Chart 16). Later, Taliani et al. reported the

poly(3-nonylthieno[3,2-b]thiophene), possessing alkyl chains on each TT unit, had, nevertheless, limited solubility in THF due to the increased stiffness of the molecule through TT units. Polymerization of 64 was conducted applying oxidative coupling in the presence of FeCl3 in CHCl3. Contrary to polymers composed of mono alkyl substituted TTs, polymer 229 containing dialkyl-substituted monomers was yellow in color and soluble in THF, CHCl3, and toluene. Polymer 229 had a large molecular weight of Mn = 54 372, Mw = 70 210 g mol−1. Purification was easily carried out by precipitation from a mixture of dichloromethane and MeOH. Cyclic voltammetry showed irreversible oxidation and reduction processes for the synthesized polymers. Moreover, all of the polymers had low oxidation potentials, which make them useful p-type materials. According to electrochemical investigations, polymer 229 has

Chart 16. Polythiophene (221) and Poly(thieneno[3,2-b]thiophene) (222)131,132

conductivity of 222, as 8 × 10−4 S cm−1. It demonstrated an optical band gap energy (Eg = 2.0 eV) similar to that of poly(thiophene) (PT, 221), contradicting the computational results, conducted using the valence effective Hamiltonian technique, which predicted a band gap energy for 222 smaller V


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The copolymers 234a−234c, possessing TT and bithiophene repeating units with high molecular weights of 28 500/51 300, 29 600/54 000, and 33 000/59 600 g mol−1 (Mn,Mw), were less sterically hindered and soluble (Chart 18). They are possible

the largest band gap among the other three polymers 223, 225, and 228, reflecting the steric effect of the alkyl units.42 Koeckelberghs and co-workers136 prepared chiral 3,6dialkoxy-substituted poly(thieno[3,2-b]thiophene)s (PTTs) (230), 3,6-dialkylthio-substituted PTTs (231), and alternating copolymers (232) of 3,6-dialkoxy-TTs and 3,6-dialkyl-TTs via Stille-coupling reaction, which had molecular weights of 27 700, 5700, and 18 700 g mol−1 (GPC) of Mn with PDI values of 2.2, 1.5, and 2.5, respectively (Chart 17). They explored the

Chart 18. Copolymers 234a−c and 235a−c, and Tetrafluorotetracyanoquinodimethane138−148

Chart 17. Chiral 3,6-Dialkoxy-Substituted-PTTs (230), 3,6-Dialkylthio-Substituted PTTs (231), and Alternating Copolymers of 3,6-Dialkoxy-TT with 3,6-Dialkyl-TT (232)136,137 and cyclopenta[2,1-b:3,4-b′]dithiophene (233)

candidates for fabrication of “quasi-single-crystal” transistors, due to their liquid crystalline properties and high charge carrier mobilities of 0.30, 0.30, and 0.63 cm2 V−1 s−1 in the absence of air (N2), respectively.138 Han and his co-workers139 investigated the nanofibril morphology of poly(2,5-bis(3-dodecylthien-2-yl)thieno[3,2-b]thiophene) (234b) indicating that the fibril morphology of the polymer was best obtained by vapor atmosphere of the solvent, which had a solubility parameter (10.0 (cal cm−3)1/2 for CS2) similar to that of polymer 234b (10.5 (cal cm−3)1/2). Optical studies found an absorption peak at 497 nm. The same group140 succeeded in forming nanofibrils from 234a, using additives (for 234b, 1-dodecanethiol was found to be the best additive), which decreased the aggregation size in solution, thereby forming single chains assumed to be nanofibrils. 1-Dodecanethiol, occupying the space generated by alkyl side-chains, enlarged the interlayer distance of crystalline planes and, hence, changed the crystalline structure. Himmelberger et al.141 examined the effect of the film thickness of one of the best polymer semiconductors, poly(2,5-bis(3-tetradecylthien-2-yl)thieno[3,2-b]thiophene) (234c), that is, thermal processing on the microstructure and performance in OFETs, which indicated a significant influence on the crystalline texture and film crystallinity. Experimental results revealed that crystal formation was hindered when the film thickness was not enough; hence, a diminishing of the charge mobility was observed. Moreover, annealing resulted in a narrower angular distribution of crystallites; as a result, enhancement of the charge mobility was observed. For example, keeping the thickness of the film constant (20 nm) and annealing the film provided an improvement of the charge mobility from 3.8 (unannealed) to 6.6 cm2 V−1 s−1. Li et al.142 performed computational studies on an important electrooptical polymer, poly(2,5-bis(3-tetradecylthien-2yl)thieno[3,2b]thiophene) (234c), to explain the crystal structure and unveil the impact of the side-chains on the properties of the electronic structures. Results indicated a small effect emerging from the patterns of side-chains on HOMO−LUMO energy levels and a significant effect caused by the π−π stacking distance

(chir)optical properties of polymers A−C in both solution and film. The strong S···O intramolecular interactions resulted in a rigid rod-like conformation. Increase of the steric hindrance between the adjacent monomer goups and decrease of the strength of the intramolecular interactions, as observed in dialkylthio case and the alternating copolymers, led to the formation of random coils in solution. Large Cotton effects were recorded in chloroform as well as in films. The CD spectra of some of the semicrystalline-polymer-films were a combination of “real” CD and pseudo” CD, and contributions emerged from macroscopic order. Optical investigations of the conjugated polymer 233 possessing 3,6-dialkoxy-TT and cyclopenta[2,1b:3,4-b′]dithiophene units (Mn = 908 000 and 598 000 g mol−1 for 233 with R1 and R2, respectively) demonstrated rigid and highly conjugated strand forms of the polymers in solution (Chart 17).137 The CD spectrum showed a superposition of chiral exciton coupling and another contribution evolving from a chiral, helical conformation of the polymer backbone. The formation of (chiral) aggregation of the copolymer 233 emerged from closely approached two polymer chains due to the creation of additional space for the second aromatic moiety. The monomers 49a−f, possessing one aryl group at β-positions of TT, were polymerized electrochemically and chemically with thiophene through Suzuki coupling reaction (Scheme 8).37 Polymers, obtained electrochemically, demonstrated oxidation potential maxima between 1.09 and 1.39 V (vs Ag wire) except for 49f, which had the lowest oxidation potential maximum of 0.97 V. The electrochemical band gaps were between 1.86 and 2.46 eV, which were comparable to the optical band gaps estimated between 1.99 and 2.19 eV. Optical and electrochemical band gaps of the corresponding copolymers were calculated to be between 2.0 and 2.5 eV. While the copolymer with a methoxyphenyl group had the highest quantum yield of 0.64, the one possessing a nitrophenyl group demonstrated the lowest quantum yield of 0.003. W


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Chart 19. Copolymers 236 and 236-T39

on the band gap energy. Moreover, it was also suggested that by changing the side-chains, the band gap could be adjusted. Hu et al.143 studied the relationship of the properties of a single chain folding with chemical structure and stiffness of the polymer backbone in the polymer 234c, which demonstrated an almost rigid polymer chain. There was an absorption maximum of 474 nm and an emission peak at 568 nm with a small Stokes shift of 94 nm, which, with respect to that of the polymer, possessing only 3-hexylthiophene unit (125 nm), emerged from backbone rigidity of the polymer, resulting in less folding. Sirringhaus and co-workers144 explained the polaronic charge carriers in a device, fabricated from the polymer 234c using electrooptical charge modulation spectroscopy (CMS), which indicated a charge-induced absorption similar to that of neutral bulk polymer, resulting in a similar degree of polymer alignment at the interface. Bruner et al.145 suggested the enhancement of PCE with the intercalation of fullerene between the side-chains of the polymer 234c. Poelking et al.146 investigated the dynamic disorder of 234c in solid state by changing the temperature computationally, which concluded that the polymer had crystalline lamellar arrangements. The alkyl side-chains had a large influence on the backbone paracrystallinity and raised the energetic disorder. They observed that the decrease of the side-chain dynamic resulted in backbone paracrystallinity, hence, exchanged the charge carrier mobility. On the basis of the B3LYP/6-311G(d,p) level calculations, reorganization energy decreased with increasing repeating units (Table 8). Because of the planar geometry of the polymer in the

peak on cooling, associated with its recrystallization. As the optical studies indicated, the absorption maxima of 382 and 394 nm for 236 in chlorobenzene and the solid state, respectively, 236-T showed three absorption maxima, appearing at 499, 556, and 601 nm in solution and two red-shifted λmax of 552 and 598 nm for the spin-coated thin film, indicating the formation of well-ordered molecular packing in the solid state. The optical band gaps estimated for thin films of the polymers 236 and 236-T were 2.74 and 1.94 eV, respectively, showing the contribution of the additional thiophene, next to the core unit, which extends the π-conjugation length and decreases the steric hindrance emerging from the interactions of long alkyl chains substituted on one thiophene group. Cyclic voltammetry measurements, carried out on thin films, provided ionization potentials of 5.5 and 5.0 eV for 236 and 236-T, respectively. Although 236-T, possessing higher IP energy, was more stable toward oxidation, it caused poor hole injection, and hence poor TFT properties. XRD studies indicated that 236-T is more ordered than 236, due to the additional thiophene unit. This was supported by relaxed potential energy surface (PES) calculations as a function of dihedral angles, performed at the B3LYP/3-21G(d) level. DFT calculations suggested that the extent of core space and the intramolecular repulsions between alkyl units and neighboring groups affected the structural ordering significantly, and consequently the properties of TFT. An atomic force microscopy (AFM) image of the polymer 236 showed a rough granular crystalline surface, whereas that of the polymer 236-T displayed larger grains, indicating a more ordered molecular packing. It provided 236-T with a relatively high charge carrier mobility of 0.03 cm2 V−1 s−1 and an on/off ratio of 106. However, the charge mobility of polymer 236 was only 8.1 × 10−7 cm2 V−1 s−1 with an on/off ratio of 101. Light-emitting field-effect transistors (LEFETs) combine the advantages of light-emitting diodes and field-effect transistors (FETs). Hsu et al.149 used hole-transporting polymer poly(3,6dialkylthieno[3,2-b]thiophene-co-bithiophene) (237-a) to enhance the charge mobility, that is, current density, which is currently a drawback for LEFETs (Chart 20). An increase of current density improves the brightness. Polymer films, having long-term order, enhanced the hole transport on the gate dielectric and the hole injection. The highest transistor mobility of around 1 cm2 V−1 s−1 was obtained with a polymer concentration of 0.5% (wt), resulting in terrace-like morphology. However, annealing above 200 °C caused a decrease of mobility on account of the influence of the ribbon boundaries on the charge transport. Heeney et al.23 reported a mobility of 0.02 cm2 V−1 s−1 for 237-b, possessing alkyl groups on

Table 8. Calculated Reorganization Energies for GeometryOptimized (opt) and Constrained (con) 234c (n = 1−6)146 λopt λcon

monomer

dimer

trimer

tetramer

pentamer

hexamer

0.516 0.304

0.378 0.231

0.306 0.176

0.239 0.131

0.131 0.098

0.098 0.084

solid state, optimization in plane constrained backbone resulted in even smaller reorganization energies. Moreover, all of the dynamic order parameters of the backbone, including the unit cell expansion, were temperature dependent. Cochran et al.147 investigated the packing motif of 234c with the acceptor tetrafluorotetracyanoquinodimethane (F4TCNQ) in thin and bulk films by using solid-state NMR, synchrotron X-ray scattering, and optical spectroscopy, which indicated that they aligned in a cofacial arrangement whereby charge transfer was almost unity (Chart 18). Heeney and co-workers148 demonstrated the effect of the side chains along the conjugation length of the polymer and charge carrier mobilities of OFETs. Among the investigated polymers 235a−c, 235c provided the highest hole mobility of 4.6 cm2 V−1 s−1 due to the very small backbone torsion that emerged from long alkyl chains situated away from each other. Kong et al.39 synthesized solution-processable regioregular semiconductors, poly(2,5-bis(3-dodecylthien-2-yl)-3,6dimethylthieno[3,2-b]thiophene) (236) and poly(2,5-bis(3′dodecyl-2,2′-bithien-5-yl)-3,6-dimethylthieno[3,2-b]thiophene) (236-T), with molecular weights of 12 100 (Mw/Mn = 1.84, GPC) and 13 300 g mol−1 (Mw/Mn = 1.89, GPC), respectively, to explain the effects of the methyl groups of TT and additional thiophene rings on the improvement of the intermolecular interactions (Chart 19). Both polymers 236 and 236-T were found to be thermally stable up to 400 °C and possessed two transition temperatures 85, 131 and 72, 221 °C, respectively. Unlike the polymer 236, 236-T demonstrated an exothermic X


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substituted thienothiophene units made the fine-tuning of the energy levels feasible. Rutherford et al.132 investigated the thermal and electrical properties of poly[2,5ethynylene-(thieno[3,2-b]thiophenediyl)ethynylene] (241), constructed from TT and a diethynyl spacer (Scheme 38). Chemical polymerization of 135 in the

Chart 20. Hole Transporting Polymers 237a−c23,149,150

Scheme 38. Synthesis of the Polymer 241, Possessing Diethynyl Spacer, and Its Polymerization at 150 °C, and Structures of Polymers 222 and 222-Me132

thiophene units and molecular weights of Mn = 8750 and Mw = 19 200 g mol−1 (GPC). Durrant and his co-workers150 investigated the charge photogeneration yields of a blend of a polymer 237-c (Mn = 32 000 and Mw = 100 000 g mol−1, GPC), comprising one less thiophene unit with respect to polymer 237-a, with PC61BM, using transient absorption spectroscopy (Chart 20). The singlet exciton and ionization potential energies of the polymer 237-c were shown to be 2.1 and 5.0 eV, respectively. The energy of dissociated charges was observed as 0.80 eV. The concentration of PC61BM in the blend affected the charge separation yield significantly. A substantial improvement in the charge separation was found on increasing the concentration of PC61BM in the blend from 5% to 50% (wt). He et al.151 synthesized poly(3,6-dihexyl-thieno[3,2-b]thiophene vinylene) (238), using Stille-coupling method (Scheme 37). 238 had an average molecular weight of

presence of CuCl and O2 furnished poly[2,5-ethynylene(thieno[3,2-b]thiophenediyl)-ethynylene] (241) as an orange powder. Unfortunately, it was totally insoluble in common organic solvents. Thermal investigations indicated that 241 had an exothermic signal at 141 °C in DSC, which was attributed to a cross-linking reaction. Corresponding cross-linked thermally stable polymer was then obtained by heating the polymer 241 to 150 °C (Scheme 38). The molecular weight of 241 could not be determined due to its insolubility. They also prepared poly(2,5-thieno[3,2-b]-thiopheneylene) (222) and poly[2,5(3-methylthieno[3,2-b]thiopheneylene)] (222-Me), using both electropolymerization and chemical oxidation reaction with FeCl3. The electrical conductivities of 241 and its polymer were 5.1 × 10−13 and 4.8 × 10−13 S cm−1, respectively; polymers 222 and 222-Me had 2.4 × 10−6 and 9.4 × 10−8 S cm−1.132 Some aromatic units, such as fluorene, benzo[1,2,5]thiadiazole, thiazole, benzoxadiazole, benzodithiophene, etc., were also utilized to form copolymers with TTs. Lim et al.72 synthesized and investigated the thermal, electrochemical, optical, and OLED-device properties of poly(9,9′-dioctylfluorene-alt-thieno[3,2-b]thiophene) (243), which was synthesized applying the Suzuki cross-coupling protocol to 2,5-dibromothieno[3,2-b]thiophene (103) and 2,7-bis(4,4,5,5-tetramethyl-1,3,2dioxaborolan-2-yl)-9,9′-dioctylfluorene (242) (Scheme 39). GPC measurements revealed the slightly lower molecular weight (Mn/Mw) of 238 as 13 600/24 500 g mol−1. DSC demonstrated a phase transition behavior between the crystalline and liquidcrystalline states, after heating above 300 °C and cooling to room temperature gradually. The glass transition (Tg) was detected at 112 °C. According to the optical studies, while 243 in chloroform had two absorption maxima of 452 and 470 nm, its PL emissions consisted of one maximum with a shoulder and a tail appearing at 478, 510, and 554 nm, respectively. In the solid state, absorption maxima at 448 and 471 nm and PL emissions at 495, 511, and 548 nm were recorded. The estimated optical band gap was 2.48 eV (500 nm). Electrochemical investigations showed the HOMO, LUMO, and the electrochemical band gap to be −5.38, −240, and 2.98 eV, respectively. An OLED device, fabricated as ITO/PEDOT/243/LiF/Al, showed a green light emission with a low turn-on voltage of 3.3 V. The experimental results showed that a better electroluminescence was observed with the polymer 243 as compared to fluorene- and

Scheme 37. Synthesis of Poly(3,6-dihexyl-thieno[3,2-b]thiophene vinylene) (238) and the Structures of Alkylsulfanyl (239) and Alkylsulfonyl (240) Side-Chain Possessing Polymers151,152

32 000 g mol−1 with a PDI of 1.13 (GPC). The solutionprocessed organic field-effect transistor (OFET) fabricated with bottom-gate top-contact geometry provided the highest hole mobility of 0.032 cm2 V−1 s−1 after thermal annealing at 180 °C. Optical studies demonstrated an absorption maximum of 538 nm in chloroform, which was red-shifted up to 542 nm in film. The optical band gap was calculated from the onset of absorption peak as 1.77 eV, which was lower than the electrochemical band gap of 2.07 eV calculated from electrochemically recorded HOMO (−5.04 eV) and LUMO (−2.97 eV) energy levels. Schneider et al.152 used the alkylsulfanyl (239) and alkylsulfonyl (240) side chains to tune the electronic properties of organic semiconducting polymers, possessing thieno[3,2-b]thiophene unit and vinyl spacer (Scheme 37). Replacement of sulfanyl group by sulfonyl lowered the HOMO and LUMO energy levels. The use of the combination of sulfanyl and sulfonyl Y


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Scheme 39. Preparation of 243, 245, and 24672,73,153

thiophene-based copolymers. Lim et al.72,73 also indicated the importance of two octyl groups attached to the fluorene group, providing a higher crystallinity and a more ordered morphology, which enhanced the mobilities up to 1.1 × 10−3 cm2 V−1 s−1. The charge carrier mobility of the polymers containing TT repeating units was higher than that of the polymers having thiophenes, due to the rigid structure of TT. Kong et al.153 synthesized new amorphous semiconducting materials possessing fluorene-based thiophene copolymers, poly(2-(5-(9,9-dibutyl-9H-fluoren-2-yl)3-hexylthiophen-2-yl)-5-(3-hexylthiophen-2-yl)thieno[3,2-b]thiophene) (245) and poly(2-(5-(9,9-dibutyl-9H-fluoren-2-yl)-4hexylthiophen-2-yl)-5-(4-hexylthiophen-2-yl)thieno[3,2-b]thiophene) (246), by insertion of two alkylthiophenes using Suzuki coupling protocol (Scheme 39). The number-average molecular weights (Mn) obtained from GPC measurements were 18 300 (Mw/Mn = 1.8) and 15 800 (Mw/Mn = 1.7), respectively. The UV−vis investigations revealed absorption maxima of 245 and 246 as 436 and 427 nm in solution and 441 and 431 nm in the film state, respectively. While X-ray crystallographic analysis demonstrated randomly oriented polymer chains, atomic force microscopy (AFM) images indicated the formation of film morphology. Field-effect transistor mobilities of 245 and 246 films were recorded as 5.4 × 10−4 and 1.6 × 10−4 cm2 V−1 s−1 under ambient conditions, respectively. Kawabata et al.154 performed the asymmetric electrochemical polymerization of TT (1), 2,2′-bithieno[3,2-b]thiophene (bis-TT, 99), and 4,7-bis-thieno[3,2-b]thiophen-2-yl-benzo[1,2,5]thiadiazole (TT-Btdaz)155 in cholesteric liquid crystals (CLC) (Scheme 40). For electropolymerization, the place of the highest spin density of the radical cation was predicted by DFT calculations revealing the potential reactive sites of monomers to be the 2-positions of fused thiophenes, which resulted in the formation of polymers linked through 2-carbons. While optical investigations of poly(TT) (222) and poly(bisTT) demonstrated similar absorption spectra, the intensity of the circular dichroism (CD) was smaller for 222 than for poly(bis-TT). UV−vis absorption studies of poly(TT-Btdaz) (247-TT) showed a red-shifted charge-transfer-type band (580 nm) by 20 nm with respect to its furan (247-F) and thiophene (247-T) analogues, which showed the stronger donor effect of TT (1) as compared to thiophene (Figure 7).

Scheme 40. Electropolymerization of TT-Btdaz Furnishing 247-TT, and Structures of 247-F and 247-T154,155

Figure 7. UV−vis spectra of the polymers 247-F, 247-T, and 247-TT in the neutral state.154 Reprinted (adapted) with permission from ref 154. Copyright 2013 American Chemical Society.

Zhou and co-workers156 investigated the effect of thiophene spacers in the backbone connecting TT, possessing two dioctylthiophene units in the 3,6-positions, with benzothiadiazole (BT), on the photovoltaic properties of two-dimensional polymers (Scheme 41). While optical measurements resulted in the optical band gaps of 1.60 and 1.58 eV, electrochemical studies gave the band gaps as 2.00 and 1.97 eV for 248 and 249, respectively. The polymer with two thiophene spacers (249) demonstrated a broader absorption and higher PCE of 3.34% with a short circuit current (Jsc) value of 12.6 mA cm−2 in BHJ polymer solar cells, whereas 248 provided only 1.5% of PCE with Jsc value of 4.18 mA cm−2 under the same conditions. Palai et al.157 reported optical band gaps of the polymers 250a−c, containing unsubstituted TT, between 1.53 and 1.54 eV, and their number-averaged molecular weights Mn as 57.0, 59.5, and 71.0 kDa, respectively. Z


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Scheme 41. D−A Copolymers 248, 249, and 250a−c Possessing One and Two Thiophene Spacers156,157

Scheme 42. Synthesis of Copolymers 253 and 255, Possessing TT Units, and the Structures of Benzothiadiazole Copolymers 256 and 257, Having Thiophene and TT, Respectively158,159

Lee et al.159 explored the effects of thiophene and TT on the backbone structure of polymers possessing benzothiadiazole, which affected the PCE and the charge mobility of the polymers (Figure 8). While thiophene possessing polymer

158

Li et al. synthesized the copolymers poly(4,8d i d o d e c y l o x y b e n z o [ 1 , 2 - b : 4 , 5 -b ′] d i t h io p h en e- a l t -3 pentylthieno[3,2-b]thiophene) (253) and poly(4,8didodecyloxybenzo[1,2-b:4,5-b′]dithiophene-alt-5,6-bis(octyloxy)-4,7-bis(6-pentylthieno[3,2-b]-thiophen-2-yl)benzo[c][1,2,5]thiadiazole (255) through Stille coupling reactions of benzo[1,2-b:4,5-b′]dithiophene (251) with electron-donating thieno[3,2-b]thiophene unit (252) and benzothiadiazole derivative (254), respectively (Scheme 42). While 253 was very soluble in common organic solvents, 255 was slightly soluble, due to its stiff backbone. Their molecular weights (Mn/Mw) were obtained from GPC measurements as 270 000/ 288 000 (PDI = 1.1) and 170 000/204 000 g mol−1 (PDI = 1.2), respectively. The long alkyl and alkoxy chains provided higher solubility and hence might be responsible for the high degree of polymerization. They were thermally stable up to 293 °C. The UV−vis absorption maxima of 253 and 255 in chloroform were at 500 nm, attributed to the π−π* transition between benzodithiophene and TT units, and 543 nm emerging from the intramolecular charge transfer (ICT). The UV−vis absorption maxima were red-shifted in the solid state by 20 and 48 nm, resulting in optical band gaps of 2.07 and 1.79 eV for 253 and 255, respectively. According to the electrochemical investigations, HOMO/LUMO energy levels of the polymers 253 and 255 were −5.14/−3.03 and −5.12/−3.26, respectively, which provided electrochemical band gaps of 2.11 and 1.85 eV, matching well with the optical band gaps. A device, fabricated from the polymer 255 and PC71BM, had a PCE of 3.54% with Jsc of 9.15 mA cm−2, whereas that of 253 with PC61BM demonstrated a PCE of only 1.12% with Jsc of 4.02 mA cm−2. A maximum EQE of 50% was obtained from the device made from 255 with PC61BM.

Figure 8. Schematic backbone curvature of polymers 256 and 257.159 Reprinted (adapted) with permission from ref 159. Copyright 2014 American Chemical Society.

(256) demonstrated curved geometry due to the bond angle of thiophene linkage, that is, 160°, TT containing polymer (257) exhibited linear structure, which is indicative of a higher degree of interchain ordering with edge-on orientation. As a result, polymer 257 afforded the charge mobility of 0.26 cm2 V−1 s−1; on the other hand, the thiophene containing polymer provided only 0.02 cm2 V−1 s−1 of charge mobility. However, the curved polymer 257 had enhanced photovoltaic properties, which rendered a PCE of 5.56%. Wang et al.160,161 synthesized a series of D−π−A copolymers, consisting of a bisalkoxysubstituted benzodithiophene (BDT) donor and bis(octyloxy)-substituted benzodiathiazole (BT) acceptor units, connected through furan (258), thiophene (259), and thieno[3,2-b]thiophene (260) (Chart 21). The AA


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Chart 21. D−π−A Copolymers 258−260160,161

level, providing band gaps of 2.48, 2.35, and 2.28 eV. Photovoltaic measurements revealed that while a solar cell, made of 260 as a donor and PC71BM as an acceptor, exhibited the highest PCE of 4.93%, solar cells fabricated from furan and thiophene π bridges demonstrated 2.81% and 3.72% of PCE. A morphological analysis of 260 indicated a smooth surface of the active layer with an average surface roughness (Ra) of 0.42 nm. For this blend, the highest external quantum efficiency (EQE) of 60% was at 500 nm. However, replacement of TT with furan and thiophene diminished the EQE to 40% at 472 nm and 52% at 465 nm, respectively. The experimental and computational results suggest that TT is a promising candidate to improve the properties of photovoltaic materials. Lee et al.159 investigated D−A-type semicrystalline copolymers possessing thiophene (T) and thienothiophenes (TT) along with benzodiathiazole (BT) acceptor. They showed almost planar structures emerged from noncovalent S···O and C−H···N interactions in the polymer backbone. However, TT containing polymer provided a higher charge transport of 0.26 cm2 V−1 s−1 with respect to that of thiophene analogue (0.02 cm2 V−1 s−1), which was attributed to the higher degree of interchain ordering with edge-on orientation in TT possessing polymer. The use of donor−acceptor (DA) systems in copolymers has attracted interest in enhancing the efficiency of photovoltaic devices. TT (1), having a rigid structure with donor property, has been used in combination with various acceptors to decrease the band gap of conducting polymers. Bithiazole (BTz) is a coplanar acceptor unit having two electron-withdrawing CN groups. Inclusion of BTz in a polymer containing TT units (261) results in a lowering of the HOMO energy level and provides a band gap of 1.89 eV (Chart 22). The film produced

molecular weights (Mn/Mw) of 258−260 were measured (GPC) to be 6.2/13.2 (PDI = 2.1), 10.4/23.2 (PDI = 2.2), and 32.8/65.0 kDa (PDI = 2.0), respectively. Replacement of furan or thiophene with TT as a π bridge resulted in a change in the shape of the molecular chains from z-shaped to almost linear and, consequently, an increase of overlapping of π-orbitals. Hence, bathochromic shifts from 534 to 568 and 631 nm were realized, respectively (Figure 9). Moreover, the use of TT

Chart 22. Donor−Acceptor Systems 261 and 262162,163

from the copolymer of TT and BTz (Mw = 6.73 K, Mn = 4.2 K, and PDI = 1.6) possessed proper crystallinity, due to their similar structures.162,163 A hole mobility was recorded to be 6.45 × 10−3 cm2 V−1 s−1. The PCE of the polymer solar cell (PSC) of this polymer and indene-C60 bisadduct (ICBA) (w/w, 1:1) was 5.4% with a higher Voc of 1.03 V. Moreover, on the basis of photovoltaic measurements, Jsc and FF were obtained as 8.6 mA cm−2 and 0.61, respectively. Another acceptor unit, benzoxadiazole, decreased both the HOMO and the LUMO energy levels, resulting in air-stable polymers with high values of Voc for the blended form with fullerene.164 Benzoxadiazole is symmetric, planar, and thermally stable, and has a low-lying HOMO. Thus, it formed low-band gap polymers with thiophene derivatives (262) (Chart 22). A high value of Voc and a PCE

Figure 9. (Top) Top and side views of B3LYP/6-311++G(3df, 3pd) level optimized geometries of the three copolymers’ (258−260) backbone units with a chain length n = 1 (gray, C; white, H; red, O; blue, N; and yellow, S). (Bottom) Absorption spectra of 258−260 (a) in chloroform and (b) solid film on a quartz plate.160 Reprinted (adapted) with permission from ref 160. Copyright 2012 American Chemical Society.

caused a high-lying HOMO energy level (−5.21 eV), as compared to furan and thiophene, the HOMOs of which are −5.44 and −5.35 eV, respectively. A TT located between D and A diminished the band gap from 1.96 for 258 to 1.82 for 259 to 1.78 eV for 260. The same trend was predicted by computational studies at the B3LYP/6-311++G(3df,3pd) AB


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were shown to be 0.85 V and 5.3%, respectively, for a device fabricated using benzoxadiazole, TT, thiophene, and PC61BM. Mishra et al.165 prepared poly(3-alkylthiophene)s, having TT and electron-deficient thiazolo[5,4-d]thiazole rings, through Stille coupling (Scheme 43). The synthesis was started with

Chart 23. Copolymers 271a and 271b Holding Diketopyrrolopyrrole (DPP) Unit166−168

Scheme 43. Synthesis of Copolymers 270a−c through Stille Coupling Approach165 271a demonstrated an absorption maximum appearing at 810 nm in chloroform, which was red-shifted by 15 nm in thin film. The optical band gap derived from the onset absorption of the thin film is 1.23 eV, which was 0.62 eV smaller than the electrochemically obtained band gap, on account of the exciton binding energy of conjugated polymers. Xu et al.167 studied the hole and electron transport in a copolymer 271a, using charge modulation spectroscopy (CMS), photoinduced absorption, and chemical doping spectroscopy, which displayed twodimensional delocalization in hole and electron polarons, due to its significant intermolecular couplings, the electron and hole mobility of which were 0.1 and 1 cm2 V−1 s−1, respectively (Chart 23). On the basis of the analyses of all three possible configurations, neutral, cofacial dimer, and staggered dimer, by performing the TD-CAM-B3LYP/6-31+G(d) level of calculations, staggered dimer stacking of the copolymers caused a subgap charge-induced optical transition, due to the asymmetrical donor and acceptor moieties. Janssen and co-workers168 studied the copolymer 271b, having a high molecular weight of 447 kg mol−1, which was helpful to achieve a phase separation and morphology of active. 271b had a solid-state absorption up to 920 nm with a small band gap of 1.35 eV. Electrochemical measurements revealed HOMO and LUMO energy levels of −5.10 and −3.68 eV, providing an electrochemical band gap of 1.42 eV, which was somewhat higher than the optical band gap. The FET measurements with a bottom-contact top-gate configuration pointed out the ambipolar property of the polymer. The electron and hole mobilities were shown to be 1.5 and 0.8 cm2 V−1 s−1, respectively. The device, fabricated from the polymer and PC71BM, had a PCE of 6.9% and a fill factor of 0.70 for a thin film, possessing a thickness of 220 nm, which make it a good candidate for solar cell applications. While the short circuit current (Jsc) was 14.8 mA cm−2, the open circuit voltage (Voc) was obtained to be 0.66 V.15 Bronstein et al.169 reported TT-based diketopyrrolopyrrole (DPP) copolymers (272 and 277), having 2-octyl-1-dodecyl alkyl groups and molecular weights (Mn/Mw) of 16/78 and 14/75 kDa with PDI of 4.9 and 5.4, respectively, which had PCEs of 3.0 %and 5.4%, respectively (Chart 24, Table 9). A top-gate bottom-contact OFET device of 277 had a hole mobility of 1.95 cm2 V−1 s−1 (Figure 10). Although the observed performance of the device was good, the processability was limited. Meager et al.170 increased the number of carbons in the alkyl chains to improve the processability by increasing the molecular weight and solubility of the copolymer. Four copolymers of TT 273−276 were synthesized possessing an acceptor unit of DPP, substituted with a large alkyl group, 2-decyl-1-tetradecyl, to enhance the solubility (Chart 24). The average molecular weights (Mn/Mw) of 273−276 were determined (GPC) to be 148/385 (PDI = 2.6), 100/280 (PDI = 2.8), 50/78 (PDI = 1.6), and 23/42 kDa (PDI = 2.6), respectively. While Pd-catalyzed Stille coupling using microwave irradiation was applied for the preparation of 273 and 274, conventional Suzuki coupling was used to access 275 and 276.

NBS bromination of 3-alkylthiophene (263), which was followed by formylation in the presence of n-BuLi and DMF, leading to aldehyde 265. Reaction of 265 with dithiooxamide (266) gave thiazolo[5,4-d]thiazole oligomers (267) in 25−32% yields. NBS dibromination and then polymerization with distannyl compound 269 under the Stille coupling protocol furnished the air-stable polymers 270a−c, which were thermally stable up to 336−344 °C, soluble in common organic solvents, and had low band gaps. The number-averaged molecular weights of 248a−c were determined (GPC) to be 15 900 (PDI = 3.1), 15 000 (PDI = 3.1), and 16 500 g mol−1 (PDI = 2.9), respectively. Optical studies revealed their absorption maxima in the range of 564 and 568 nm with shoulders at around 614−616 nm. The optical band gaps, estimated from the edge of UV−vis peaks, were between 1.82 and 1.85 eV. Electrochemical studies demonstrated the ionization potentials (HOMO) to be 5.62−5.65 eV and the electron affinities (LUMO) as 3.77−3.83 eV. They had strong red fluorescent emission in the range of 649−679 nm. Li et al.166 explored the electrochemical and optical properties of poly(N,N′-dialkyl-1,4-diketopyrrolo[3,4-c]pyrrole-alt-2,5-dithienylthieno[3,2-b]thiophene (271a) comprehensively (Chart 23). The number-average molecular weight (Mn) of the senthesized copolymer 271a was obtained from GPC analysis as 89 700 g mol−1 (PDI = 2.36). Electrochemical investigations using cyclic voltammetry revealed the onset oxidation and reduction potentials as 0.82 and −1.03 V (vs Ag/ AgCl), respectively, which resulted in the electrochemical band gap of 1.85 eV. The HOMO and LUMO energy levels were provided to be 5.25 and 3.40 eV, respectively. The copolymer AC


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Chart 24. Diketopyrrolopyrrole Copolymers 272−283169−174

Table 9. Physical Properties of 272−280169−173 compound

λabs [nm]

272 273 274 275 276 277 278 279 280

779,a787b 857,a784b 771,a783b 796,a774b 755,a755b 764,a790b 812,a810b 804,a803b 768,a772b

a

Eg

Opt

[eV]

1.30 1.40 1.30 1.30 1.50 1.40 1.40 1.40 1.43b

PCE [%] 3.0 4.1 3.8 1.1 4.0 5.4 7.3 6.9 6.2

2

−1

mobility [cm V

and TT in the polymers 276 and 273, respectively, was to obtain a planar structure, which was required to improve the charge transport, benzo[c][1,2,5]thiadiazole (BT) and phenyl units were applied due to their noteworthy transistor ambipolarity in polymer 275 and encouraging solar cell efficiencies in polymer 276. 272 had absorption maxima of 779 nm (in solution) and 787 nm (in solid state). Polymer 274, possessing TT instead of thiophene, showed a broader and redshifted absorption in both solution and solid state. The reason for the red shift in the solid state could be due to aggregation of conjugated TT units. A red-shifted absorption maximum was also observed for polymer 275, 796 and 774 in solution and solid state, respectively. Polymer 276 did not show any change of absorption maximum of 755 nm, obtained in solution and in the solid state. The p-phenylene unit resulted in a higher band

−1

s ]

0.037 0.450 0.090 0.003 0.060 1.95 0.052 0.066 0.116

In solution. bIn solid state.

The polymers had high molecular weights with narrow polydispersities (PDI). While the reason for the use of thiophene AD


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not exhibit any red shift. The HOMO (−5.1 eV), estimated from photoelectron spectroscopy in air (PESA), and LUMO (−3.7 eV) energies, derived from PESA and UV−vis studies, indicated an almost complete lack of influence of alkyl chain on frontier orbitals. The band gap energy was calculated to be 1.4 eV for 277−279. A solar cell device, fabricated from 277−279 and PC71BM, showed PCE of 5.9%, 7.3%, and 6.9%, and hole mobilities of 1.4 × 10−2, 5.2 × 10−2, and 6.6 × 10−2 cm2 V−1 s−1, respectively. Zhang et al.173 modified 280 by replacing 2-thienyl groups with dodecyloxy units to obtain the low band gap donor 281 to study the influence of ZnO layer on spectral response of the device (Chart 24). An improvement of the photocurrent by means of the ZnO spacer was realized. PCE of 3.31% with Jsc of 8.8 mA cm−2 was recorded for the device with the ZnO layer, but in the absence of ZnO, the PCE was 3.12% with Jsc of 9.4 mA cm−2. Ryu et al.174 replaced TT with thiophene in 281 to obtain 282, which provided also a PCE of 3.31% (Chart 24). On the other hand, the copolymer 283, possessing dithienobenzodithiophene unit, exhibited a PCE of 4.75%, due to the improved delocalization and intermolecular π−π interactions. Tsoi et al.175 monitored in situ the molecular vibrations of the copolymer 277 and PC70BM in organic photovoltaic blends, using Raman spectroscopy, noting the changes in Raman spectra, which were attributed to crystallization of the copolymer and PC70BM. Transition temperatures to crystalline phases were 150 °C for copolymer 277 and 170 °C for PC70BM. Shahid et al.176 synthesized a D−A copolymer, poly(selenophen-2,5-diyl-2,5-di(2-octyldodecyl)-1,4-diketopyrrolo[3,4-c]pyrole-3,6-diyl-selenophen-2,5-diyl-thieno[3,2-b]thiophen-2,5-diyl) (285), with a lower molecular weight (Mn/Mw) of 100/250 kDa, as a dark green thermally stable solid in 80% yield, by using Stille coupling of distannyl-TT with diselenophene-substituted DPP in the presence of Pd2(dba)3 under microwave conditions (Scheme 44). The UV−vis

Figure 10. (Above) The top-gate bottom-contact OFET device structure.169 Reprinted (adapted) with permission from ref 169. Copyright 2011 American Chemical Society. (Below) AFM images of blends of 273−276.170 Reprinted (adapted) with permission from ref 170. Copyright 2013 American Chemical Society.

gap (1.5 eV) due to the formation of a twisted conformation and, consequently, loss of conjugation. While polymer 272 had a band gap of 1.3 eV, polymers 273 and 274 displayed band gaps of 1.4 and 1.3 eV, respectively. Solar cells (PSC), made of polymer:fullerene bulk heterojunction (BHJ), showed efficiencies of 4.1%, 3.8%, 1.1%, and 4.0%, and the mobilities of the TFT devices were 0.45, 0.09, 0.003, and 0.06 cm2 V−1 s−1, for 273−276, respectively. On the basis of AFM measurements, while 272, showing the highest BHJ efficiencies, had a coarse morphology, 274 and 276 possessed less coarse morphology (Figure 10). However, an AFM image of 275 demonstrated the finest morphology, which resulted in a lower Jsc (2.9 mA cm−2) with a fill factor (FF) of 0.55 and Voc of 0.68 V. Zhang et al.171 incorporated a benzodithiophene, substituted with two thien-2-yl units, into the polymer 272, to obtain the thermally stable polymer 280, which had an extension of conjugation in two dimensions. Optical studies revealed absorption maxima of 768 and 772 nm in solution and in the solid state with the optical band gap of 1.43 eV (Table 9). Electrochemical explorations disclosed the HOMO and, with the help of optical band gap, the LUMO energy levels as −5.05 and −3.62 eV, respectively, providing the electrochemical band gap of 1.43 eV. The device constructed from this polymer and PC71BM had a Jsc of 16.25 mA cm2, PCE of 6.18%, and a hole mobility of 1.16 × 10−1 cm2 V−1 s−1. Meager et al.172 studied the effect of alkyl chains, located on the DPP units, on the efficiency of solar cells. Optical studies showed absorption maxima of 764, 812, and 804 nm in chlorobenzene for 277−279, respectively. The average molecular weights (Mn/Mw) were recorded as 24/89 (PDI = 3.7), 45/83 (PDI = 3.7), and 80/154 (PDI = 3.7), kDa, respectively. While a red-shifted λmax was observed for 277 with 26 nm, solid-state UV−vis measurements of 278 and 279 did

Scheme 44. Preparation of D−A Copolymer 285 from TT and Diselenophene-Substituted DPP Monomers176

absorption maximum in chloroform was a broad band at 823 nm, and a red-shift of 41 nm at λmax was observed in the solid state. The optical band gap, estimated for the film, was 1.21 eV, which was smaller than Egap of thiophene analogue copolymer 271a (1.23 eV, 825 nm). The ionization potential of the thin film, measured using photoelectron spectroscopy, was 5.1 eV, and the LUMO energy level was reported to be −3.9 eV. Introduction of selenophene into a copolymer decreased the LUMO level and hence diminished the optical AE


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Scheme 45. Synthesis of Copolymers 291−294 from TT (1) and Isoindigo Monomers177

Egap. Moreover, incorporation of selenophene made the polymer an excellent ambipolar material. Additionally, introduction of larger and more polarizable units (here that is Se) into the polymer backbone provided an enhancement of electron transport. Hole and electron mobilities of the top-contact device, fabricated from 250 °C annealed copolymer, were 0.79 and 0.1 cm2 V−1 s−1. Xu et al.177 synthesized the copolymer 291 from TT (1) and isoindigo monomers for bulk-heterojunction polymer solar cell (BHJ PSCs) applications (Scheme 45). They also linked these monomers via thiophene bridges and bithiophene units to produce polymers 292−294 having modified band gaps and HOMO energy levels. The LUMO energy levels were tuned by isoindigo units. Polymers 291−294 were found to be thermally stable for solar cell applications, and the degradation onset temperatures were 389, 374, 325, and 359 °C with 5% weight loss. The molecular weights (Mn/Mw) of 291, 293, and 294 were determined using GPC analysis as 28.5/65.5 (PDI = 2.3), 23.5/44.6 (PDI = 1.9), and 21.8/45.7 kDa (PDI = 2.1). The molecular weight of 292 could not be recorded due to its solubility problem in THF. Unlike the other polymers, 292 was not soluble in common organic solvents such as THF, CHCl3, and toluene, but moderately soluble in warm chlorobenzene and dichlorobenzene. Optical studies indicated a weak aggregation of 291 in the solid state, resulting in almost the same λmax values in chloroform and as a thin film, 610 and 609 nm,

respectively. The optical band gap of 291, estimated from the band edge (765 nm), was 1.62 eV. Polymers 293 and 294 had two absorption maxima, observed in the ranges of 350−500 and 500−800 nm. The absorption maxima obtained in solution at 630 and 625 nm for 293 and 294 were red-shifted to 644 and 647 nm in the solid state, providing optical band gaps of 1.49 and 1.52 eV, respectively. The electrochemical band gaps were 1.91, 1.55, and 1.62 eV for 291, 293, and 294, respectively. The decrease in the band gaps of 293 and 294, as compared to 291, emerged from the insertion of thiophene units between TT and isoindigo units, which increased the HOMO energy levels of 293 and 294 to −5.34 and −5.43 eV, respectively, from −5.71 eV of 291 (Scheme 45). BHJ PSCs were fabricated from the polymers 291 (in chlorobenzene), 292 (in dichlorobenzene), and 294 (in chlorobenzene) with PC71BM demonstrated PCEs of 1.25%, 4.69%, and 3.24%, respectively. According to an analysis of the morphologies, all of the blend films were smooth and uniform, with surface rms values of 2.1, 2.3, and 4.8 nm for 291, 293, and 294, respectively. Becuse the highest PCE was obtained for polymer 293, its hole mobility was found to be 1.9 × 10−4 cm2 V−1 s−1. Bérubé et al.215 performed DFT calculations in combination with Scharber’s model to explain the properties of the promising polymers for photovoltaic cells. On the basis of the predicted PCEs, the copolymer 295, possessing a low optical AF


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Chart 25. Copolymer 295215

band gap and large electron affinity, with a PCE of 8.6% was a good candidate for photovoltaic applications (Chart 25). Wu et al.178 synthesized the copolymers 296−298, having thieno[3,4-c]pyrrole-4,6-dione (TPD) and 2,5-di(thiophen-2yl)thieno[3,2-b]thiophene units, substituted with two dodecyl groups at different positions, 2,5-bis(4-dodecylthiophen-2yl)thieno[3,2-b]thiophene for 296, 2,5-bis(3-dodecylthiophen2-yl)thieno[3,2-b]thiophene for 297, and 3,6-didodecyl-2,5di(thiophen-2-yl)thieno[3,2-b]thiophene for 298, using Stille coupling (Chart 26). While 297 was soluble in common organic Chart 26. Copolymers 296−298, Having Thieno[3,4-c]pyrrole-4,6-dione (TPD) and 2,5-Di(thiophen-2-yl)thieno[3,2-b]thiophene Units178

Figure 11. (a) UV−vis absorption spectra of 296−298 in hot tetrachloroethane and on a quartz substrate, (b) CVs of 296−298 films on a Pt-electrode in CH3CN (electrolyte, Bu4NPF6; scan rate, 50 mV s−1; and internal standard, ferrocene), and (c) optimized structures of 296−298 (the alkyl chain clusters are depicted by blue ovals).178 Reprinted (adapted) with permission from ref 178. Copyright 2013 American Chemical Society.

solvents, the solubilities of 296 and 298 were poor, and they are only soluble in hot chlorinated solvents. The average molecular weights (Mn) were determined (GPC) to be 154 (PDI = 4.99), 10.5 (PDI = 1.2), and 16 kDa (PDI = 2.06), respectively. Optical studies, performed in 1,1,2,2-tetrachloroethane at 80 °C, showed absorption maxima of 525, 479, and 492 nm for the polymers 296−298, respectively. While the absorption maxima of 296 and 298 were red-shifted by 50 and 83 nm, respectively, displaying good intermolecular interactions in thin films, 297 had a slightly red-shifted λmax in the solid state, emerging from the near absence of strong intermolecular interactions (Figure 11). On the basis of the electrochemical investigations, the HOMO energy levels were −5.07, −5.06, and −5.05 eV for the polymers 296−298, respectively. The highest hole mobility of 1.29 cm2 V−1 s−1 with an on/off ratio of 106−7 was obtained at

260 °C for a device fabricated from the polymer 298, due to the change in the polymer chain packing (Figure 11), whereas 296 and 297 showed hole mobilities of 0.15 cm2 V−1 s−1 at 230 °C (on/off: 104−5) and 1.1 × 10−2 cm2 V−1 s−1 at 230 °C (on/off: 103). 2D-GIXD indicated π−π interactions and formation of lamellar packing in all of the polymers 296−298 with interlayer distances of 3.63, 3.67, and 3.74 Å. Zhong et al.179 combined two thieno[3,2-b]thiophene groups via a dialkyl germanium bridge, resulting in the formation of dithienogermolodithiophene 302, which was subsequently Stille coupled with 5-octyl-4H-thieno[3,4-c]pyrrole-4,6(5H)dione (304) to obtain the corresponding copolymer 305 as a dark solid (Scheme 46). The molecular weight (GPC) of 305 was reported to be Mn/Mw = 12 000/17 000 Da (PDI = 1.4). In the key step, involving the formation of the dimer 300, 5-bromothieno[3,2-b]thiophen-2-yl)-(trimethyl)silane (299) was reacted with LDA to produce the rearranged intermediate 5-lithio-6-bromothieno[3,2-b]thiophen-2-yl)-(trimethyl)silane, via a halogen-dance, in situ oxidative dimerization of which with CuCl2 provided the dimer 300 in 70% yield. Lithiation of 300 with n-BuLi at −90 °C, followed by addition of dibromobis(2-ethylhexyl) germane (301), provided the corresponding fused product 302 in 60% yield. Thermal analysis of the polymer 305 demonstrated a good stability up to 340 °C. However, DSC analysis did not show any transitions up to 300 °C. Optical studies revealed an absorption maximum with a shoulder, attributed to the aggregation of the polymer, at 595 and 643 nm, respectively. In the solid state, a clear red shift of λmax by 68 nm was observed (Figure 12). The optical band gap of the polymer was estimated as 1.75 eV, which is in well agreement with the electrochemical band gap of 1.8 eV. Electrochemical studies indicated the HOMO and LUMO energy levels to be −5.68 and −3.88 eV, respectively. The charge mobility of 0.11 cm2 V−1 s−1 was obtained from bottom-contact and bottom-gate transistor devices assembled using the polymer 305. A BHJ solar cell constructed from a blend of copolymer and PC71BM had an efficiency of up to 7.2%. Liu et al.180 used benzothiadiazole electron-withdrawing units, benzo[c][1,2,5,]thiadiazole (306a), 5-fluorobenzo[c][1,2,5]thiadiazole (306b), AG


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a band gap energy of 1.22 eV. On the other hand, electrochemical investigations showed a lower HOMO energy level for 308a, as compared to its DPP analogues, −5.49 and −5.25 eV, respectively, which demonstrated the stronger electron-withdrawing effect of PzDP over DPP. The LUMO energy level was −4.17 eV. An OFET device, fabricated from this polymer and dodecyltrichlorosilane, had an ambipolar property. The hole and electron mobilities were 0.053 and 0.021 cm2 V−1 s−1, respectively, with an on/off ratio of 105−106. Rumer et al.182 prepared a dihydropyrroloindoledione, which was responsible for the extension of the conjugation, and TT containing copolymer (308b) with vinyl spacers, applying microwave Stille coupling polymerization in chlorobenzene (Scheme 47). The use of the vinyl linkage diminished the steric effect, emerging from the side-chains, and hence reduced the interlayer distance, which resulted in closer packing. Optical studies revealed a λmax value of 669 nm in dilute chlorobenzene and a maximum of broad absorption at 672 nm in the solid state with an optical band gap of 1.54 eV, which makes this polymer a good candidate for organic solar cells. Electrochemical, photovoltaic, and other investigations of 308b were hampered by poor solubility. Yuan et al.183 investigated the copolymer 309, synthesized through Stille coupling of naphthalenediimide and the TT monomers with a molecular weight (Mn/Mw) of 62.5/206.3 kDa (PDI = 3.3) (Chart 27), the optical studies of which indicated that the thin film of 309 had an absorption maximum of 368 nm and an optical band gap of 1.57 eV. Intramolecular charge transfer appeared at 560 nm. HOMO and LUMO energies were −5.45 and −3.88 eV, respectively. The electron mobility of 7.0 × 10−3 cm2 V−1 s−1 with an on/off ratio of 104 was recorded. Donaghey et al. 184 performed the copolymerization of diazaindacenodithiophene (NIDT) with thiophene (T) and thienothiophene (TT), using microwave-assisted Stille cross-coupling of the distannylated NIDT (285) with 2,5-dibromothiophene (311) and 2,5-dibromo-TT (103) to obtain highly planar and electron-rich polymers with moderate average molecular weights (Mn/Mw) of 33/56 and 13/28 kDa, respectively (Scheme 48). Compound 312 had two absorption bands in solution at 556 and 587 nm, indicating aggregate formation, whereas in the solid state, the peak at 587 nm was red-shifted to 602 nm and became more intense. The band gap of 312, obtained from solid-state UV−vis measurement, was 1.9 eV. However, 313 had only one absorption maximum of 551 nm in solution, and it had two absorption maxima in the solid state at 551 and 592 nm, the calculated band gap energy of which was 2.0 eV. The reason for the higher band gap energy, with respect to that of 313, is the low-lying HOMO emerging from the reduced delocalization along the polymer backbone due to the higher aromatic stabilization of TT. While ambient photoelectron spectroscopy (PESA) measurements gave the HOMO energy of 313 (−4.9 eV), lower than 312 by 0.1 eV, the same LUMO energies for both polymers (−2.9 eV) were obtained. To explain the electronic structure of the polymer, DFT calculations at B3LYP/6-31G(d) level were performed on a trimer, which indicated that the HOMO orbitals were fully delocalized over the backbone, enhancing the charge transport. Like the HOMO, the LUMO orbitals were fully delocalized over the polymer backbone, indicating the quinoidal character of the trimer. Both trimers had a twisted geometry with a torsion angle of >10°. DSC analyses did not show any melting and crystallization transitions for 312 and 313. OFET measurements, using bottom-gate top-contact

Scheme 46. Preparation of Copolymer 305 by Using Dialkyl Germanium Bridging and TPD, and Benzothiadiazole Electron-Withdrawing Units 306a−c179,180

Figure 12. Absorption spectra of 305 in chlorobenzene and as a thin film.179 Reprinted (adapted) with permission from ref 179. Copyright 2013 American Chemical Society.

and 5,6-difluorobenzo[c][1,2,5]thiadiazole (306c), which provided PCEs of 7.0%, 8.2%, and 8.4%, respectively (Scheme 46). Facchetti and co-workers181 synthesized a copolymer 308a, possessing an extended iso-DPP unit (dipyrrolo[2,3-b:2′,3′-e]pyrazine-2,6(1H,5H)-dione, PzDP, 307) as a strong acceptor group and TT (1), applying Stille coupling polymerization (Scheme 47). The average molecular weights in number and weight (Mn/Mw) were retrieved from GPC measurements to be 28 561/73 094 (PDI = 2.56). The absorption maxima of the copolymer 308a were 788 and 806 nm in solution and in the solid state, respectively, providing an optical band gap of 1.32 eV, and the copolymer, containing DPP and TT, exhibited AH


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Scheme 47. Synthesis of Copolymer 308a Possessing an Extended Iso-DPP Unit, and Copolymer 308b181,182

was achieved using a mixture of sulfuric acid/acetic acid to obtain the compound 318 in 83% yield. Dibromination of 318 was conducted with NBS in 92% yield, followed by Suzuki coupling with 2,3-diphenyl-5,8-bis(4,4,5,5-tetramethyl-1,3,2dioxaborolan-2-yl)quinoxaline (320) to give the polymer 321. The polymer was soluble in common organic solvents and thermally stable up to 404 °C. The number-averaged molecular weight (Mn) was determined using GPC as 21 500 g mol−1 (PDI = 2.2). However, it did not have any melting point and glass transition temperature. UV−vis studies indicated an absorption maximum of 629 nm in chloroform, which was red-shifted by 11 nm in the solid state. Optically and electrochemically estimated band gaps were 1.70 and 1.66 eV, respectively, which were supported by DFT level calculations at the B3LYP/6-31G(d) level. Moreover, it had a low-lying HOMO energy level (−5.39 eV). FET measurements revealed a hole mobility of 1.5 × 10−4 cm2 V−1 s−1. A BHJ solar cell device was constructed from 321, for which PC71BM was employed. It had a PCE of 5.1%. Xia et al.186 synthesized a copolymer 323, analogues of 321, using TT instead of 320 and indacenodithiophene monomer unit lacking two fused thiophenes (Scheme 49). It was soluble in common organic solvents and had the averaged molecular weight in number (Mn) of 16 500 g mol−1 (PDI = 1.47). The optical studies displayed only a 5 nm redshifted absorption maximum in the solid state with respect to λmax of 510 nm in THF. The optical band gap of the polymer 323 was estimated to be 2.12 eV, which matched well with the electrochemical band gap of 2.18 eV. HOMO and LUMO energy levels, obtained from electrochemical investigations, were −5.29 and −3.11 eV, respectively. A photovoltaic device, constructed from polymer 323 and poly[(9,9-bis(3′-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)] (PFN) interlayer, exhibited a PCE of 4.19% with a Jsc of 7.68 mA cm−2 and a Voc of 0.88 V. Biniek et al.187 performed the polymerization of tetradodecylthieno[3′,2′:6,7][1]-benzothieno[3,2-b]thieno[3,2-g][1]benzothiophene (DTBTBT, 324) with benzothiadiazole (Bz, 325), thiophene (326), and 176, to obtain the corresponding copolymers 327−329 with average molecular weights (Mn/Mw) of 19/132, 29/77, and 9/17 kDa (Scheme 50). They were thermally stable up to 430 °C. The absorption maxima of 403/ 613, 497, and 494 nm in solution were almost the same as those in the solid state. The second absorption maximum of 327 was

Chart 27. Copolymer 309 Having Naphthalenediimide and TT Monomers183

Scheme 48. Copolymers 312 and 313 Containing Diazaindacenodithiophene (NIDT)184

(BG-TC) devices, provided hole mobilities for 312 and 313 of 5 × 10−4 and 2 × 10−4 cm2 V−1 s−1, respectively. Xu et al.185 synthesized a ladder-type conjugated polymer 321, possessing thieno[3,2-b]thiophene-phenylene-thieno[3,2-b]thiophene structure as a planar π-conjugated building block and diphenylquinoxaline as an electron-deficient monomer (Scheme 49). The reaction of TT (1) with 2,5dibromoterephthalate (314) under Negishi cross-coupling conditions provided diethyl 2,5-di(thieno[3,2-b]thiophen-2-yl)benzene-1,4-dioate (315) in 78% yield, which was followed by treatment with excess Grignard reagent, generated from 1-bromo-4-(2-butyloctyloxy)benzene (316), to form the corresponding diol 317 in 63% yield. Intramolecular ring-closure AI


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Scheme 49. Preparation of Copolymers 321 and 323185,186

of 0.92 V, when PC71BM was employed as an acceptor, was measured for 327, 0.70 and 0.66 V were obtained for 328 and 329, respectively. The PCE of 3.7% with Jsc of 6.78 mA cm−2 was achieved for 327, and the PCE values of 3.3% and 2.3% with Jsc of 8.10 and 6.41 mA cm−2 were realized for 328 and 329, respectively. While the hole mobility of 328 was 0.1 cm2 V−1 s−1, those of 327 and 329 were shown to be 0.01 cm2 V−1 s−1. Peeters et al.188 synthesized the copolymers 330a and 330b, possessing TT and meta-arylne substituted with a oligo(phenylenevinylene) side-chain, which had distal (in polymer 330a) and proximal (in polymer 330b) chiral units (Chart 28). The molecular weights were obtained as 10.6 and 13.4 kg mol−1 with a PDI of 1.3, respectively. They studied the optical properties of the polymers to explain the effects of the chiral side-chains, which indicated the prompted chiral effects in the molecules when the chiral unit was located at the β-position proximal to the polymer backbone. These results were supported by CD and DSC experiments. Although the polymer 330a (n = 0), possessing a chiral unit at the β-position, did not show any melting point in DSC analysis nor any aggregation, a helical formation in poor solvent was observed by CD analysis. Moreover, the long side-chains hindered the formation of a helical conformation and provided lateral stacks.

Scheme 50. Synthesis of Copolymers 327−329 from DTBTBT (324) with Benzothiadiazole, T, and TT, Respectively187

attributed to an internal charge transfer (ICT) transition. Optical band gaps, extracted from the edge of UV−vis bands, were 1.8 eV for 327 and 2.2 eV for 328 and 329. The HOMO level of 327 was −5.2 eV, which was the lowest among the others (−4.9 eV for 328 and 329), and the LUMO levels were −3.4 eV for 327 and −2.7 eV for 328 and 329. While a VOC

2.2. Thieno[3,4-b]thiophene (2)

2.2.1. Synthesis and Properties. Thieno[3,4-b]thiophene (2) was investigated comphrehensively comprising both experimental and computational studies.130,189 Its UV−vis AJ


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Huo et al.192 synthesized 339, containing ester and cyano groups at 2- and 3-positions, from commercially available 2-ethylhexyl-4,6-dihydrothieno[3,4-b]thiophene-2-carboxylate (336). Bromination of 336 with NBS, and then treatment with CuCN, provided 2-ethylhexyl-3-cyano-4,6-dihydrothieno[3,4-b]thiophene-2-carboxylate (337). Reaction of 337 with m-chloroperbenzoic acid (m-CPBA), followed by refluxing in acetic anhydride, yielded 2-ethylhexyl-3-cyanothieno[3,4-b]thiophene-2-carboxylate (339) as a yellow solid in 23% over two steps (Scheme 52).

Chart 28. Copolymers 330a and 330b, Possessing TT and meta-Arylne Substituted with an Oligo(phenylenevinylene) Side-Chain188

Scheme 52. Synthesis of 339 via Dehydration of 338 in Refluxing Acetic Anhydride192

absorption spectrum indicated a red shift with respect to the other stable TTs 1 and 3. Computational studies, applying the method of partial retention of diatomic differential overlap (PRDDO), predicted a band gap of the corresponding polymer as 1.54 eV, whereas the band gap of its quinoid structure was reported to be 1.63 eV.189 The formation of a quinoid structure results in the formation of aromatic thiophene unit, which stabilizes quinoid resonance structure and decreases the HOMO−LUMO band gap (Chart 29).190 The TT (2) had a

Scheme 53. Synthesis of 2 and 342 from 3,4Dibromothiophene (15)193,194

Chart 29. Quinoid Resonance of Thieno[3,4-b]thiophene Polymer190

low oxidation potential peak of 1.05 V (vs Ag/Ag+) to generate the corresponding radical cation for polymerization.130 Wynberg et al.191 published the first synthesis of air-sensitive thieno[3,4-b]thiophene in 1967. They followed the method developed by Cava and Pollack, which was applied for the synthesis of thiophenes from dihydrothiophene sulfoxides. The sulfoxide (331) was converted into 332 in 95% yield in acetic anhydride, which was followed by hydrolysis to form thieno[3,4-b]thiophene-2-carboxylic acid (333). Reaction with diazomethane gave methyl thieno[3,4-b]thiophene-2-carboxylate (334) in quantitative yield. Removal of the carboxylic acid group was achieved using copper and quinoline (Scheme 51).

Patra et al.193 synthesized 2 from 3,4-dibromothiophene (15) in two steps in an overall yield of 39% (Scheme 53). Their synthesis started with a Sonogashira cross-coupling with trimethylsilylacetylene, which provided 3-bromo-4(trimethylsilyl)ethynylthiophene (340) in moderate yield. Treatment of 340 with n-BuLi, followed by addition of elemental sulfur, gave the corresponding thiolate anion, thermal intramolecular cyclization of which afforded TT (2) as a colorless oil in 63% yield. The UV−vis spectrum of 2 consisted of three absorption maxima at 234, 266, and 293 nm. The results of TD-DFT calculations at the TD-B3LYP/6-311+G(2df,p)//B3LYP/6-31G(d) level were in good agreement with the UV−vis results. Similar methodology was used to make 2-phenylthieno[3,4-b]thiophene (342) by Neef et al.194 3,4-Dibromothiophene (15) was reacted with phenylacetylene leading to formation of 3-bromo-4-(2-phenylethynyl)thiophene (341) in 58% yield. Compound 341 was reacted with n-BuLi and elemental sulfur successively, and then ring closure was performed with KOH to furnish the TT 342 in 32% yield (Scheme 53). Cyclic voltammetry studies indicated that 342

Scheme 51. Synthesis of TT (2), Starting from Sulfoxide (331)191

AK


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had an oxidation potential peak of 0.9 V (vs Ag+ reference electrode), the electronic and optical band gaps of which were calculated to be 0.85 and 0.82 eV. The band gap was found to be lower than that of the parent TT, due to enhancement of its resonance through the phenyl substituent. Park et al.195 introduced an efficient methodology for the synthesis of 2-(hept-1-en-1-yl)thieno[3,4-b]thiophene (347), trans-2-styrylthieno[3,4-b]thiophene (348), and (Z)-2-phenyl3-(thieno[3,4-b]thien-2-yl)acrylonitrile (349) (Scheme 54). Scheme 54. Preparation of 347−349

polyphosphoric acid (PPA) to obtain the ring-closed product 2-decylthieno[3,4-b]thiophene-6-carboxylic acid (353) in 44% yield. Removal of the carboxylic acid group was achieved using barium-promoted copper chromate in quinoline affording the monomer 354d in 69% yield (Scheme 55). Scheme 55. Synthesis of 2-Decylthieno[3,4-b]thiophene (354d)197

195

Dey et al.198 followed another methodology to synthesize the TT (2) and its alkyl-substituted derivatives (354b−e). Addition of a second carboxylic acid group to thiophen-2-carboxylic acid was performed using n-BuLi and CO2 in 56% yield. Alkylation of 355a was conducted with n-BuLi and an alkyl bromide to afford the alkylated products 355b−e in 71−79% yields. The carboxylic acid groups were converted into primary alcohols, then into bromides using PBr3. Reaction of 357a−e with Na2S furnished the precursors 358a−e, which were aromatized with DDQ resulting in 2 and 354b−e in 53−62% yields (Scheme 56).

3,4-Dibromothiophene (15) was first lithiated using n-BuLi followed by addition of N,N-dimethylformamide (DMF) to obtain 4-bromothiophene-3-carbaldehyde (343) in 88% yield. This was then reacted with ethyl mercaptoacetate in the presence of CuO nanoparticles and K2CO3 to afford ethyl thieno[3,4-b]thiophene-2-carboxylate (344) in 85% yield as a brown solid, the ester group of which was reduced with lithium aluminum hydride to obtain thieno[3,4-b]thiophene-2-ylmethanol (345) in 90% yield. This, in turn, was converted into thieno[3,4-b]thiophene-2-carbaldehyde (346) using pyridinium chlorochromate (PCC) oxidation. Wittig or Knoevenagel reactions of 346 furnished the monomers 347 as a pale yellow oil, 348 as a light green solid, and 349 as a pale yellow solid, in 43%, 45%, and 50% yields, respectively. Electrochemical studies revealed that while 348 had the lowest oxidation potential peak at 1.33 V (Eonset: 0.96 V), those of 347 and 349 are at 1.42 (Eonset: 1.10 V) and 1.47 V (Eonset: 1.20 V) (vs Ag wire as a reference electrode), respectively. On the basis of electrochemical studies, introduction of an electron donating and resonance improving group, such as phenyl, increased the energy level of the HOMO, consequently decreasing the oxidation potential, whereas an electron-withdrawing unit increased the oxidation potential of the monomers. Alkyl-substituted TTs were used as monomers for the preparation of soluble conducting polymers. In 1997, Pomerantz et al.196,197 synthesized the monomer 2-decylthieno[3,4-b]thiophene (354d), starting from thiophene-2-carboxylic acid (350). It was reacted with 2 equiv of n-BuLi and 2,2′bis(1,1-dimethoxydodecan-2-yl)disulfide (351), successively, to obtain 3-(1,1-dimethoxydodecan-2-ylthio)thiophene-2-carboxylic acid (352) in 47% yield, which was followed by treatment with

Scheme 56. Preparation of TT (2) and Alkyl-Substituted TTs (354b−e), Starting from 350198

Fukazawa et al.199 synthesized diaryl-substituted bi(thieno[2,3-c]thiophene)s (361a) applying an intramolecular double 5-exo-dig cyclization of bis[(arylcarbonyl)thienyl]acetylenes (359a) (Scheme 57). Because fused rings extend π-conjugation and affect the alignment of the molecules in their solid states, it enhanced the formation of π-stacks and provided good charge carrier mobility. Fusing thiophene rings resulted in symmetrical and unsymmetrical bi(thieno[2,3-c]thiophene)s. AL


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Scheme 57. Synthesis of Diaryl-Substituted Bi(thieno[2,3-c] thiophene) (361) (Top) and Structures of Symmetrical/ Unsymmetrical Bi(thieno[2,3-c]thiophenes) (Bottom)a199

a

Arrow shows the direction of dipole moment given in debye.

The latter had a dipole moment of 1.67 D. The reaction of bis(3-phenylcarbonyl-2-thienyl)acetylene (359a) with Lawesson’s reagent provided thiocarbonyl compound (360a), thermal 5-exo-dig cyclization of which gave the diaryl-substituted bi(thieno[2,3-c]thiophene) (361a) in the yield of 82%. Various diaryl-substituted bi(thieno[2,3-c]thiophene)s were obtained in the same way, possessing 4-(trifluoromethyl)phenyl (361b), 3,5-bis(trifluoromethyl)phenyl (361c), 4-methoxyphenyl (361d), 4-(N,N-dimethylamino)phenyl (361e), and 5-tertbutyldimethylsilylthien-2-yl (361f) groups, in 65−84% yields. Cyclic voltammetry indicated two oxidation potentials (E1/2ox) appearing at +0.23 and 0.86 V (vs ferrocene/ferroceneium couple). The difference between the first oxidation potential of 361a (+0.30 V) and that of 5,5′-diphenyl-2,2′-bithiophene (+0.67 V) revealed an improvement of the electron-donating ability by fusing the thiophene unit. Optical studies in solution and solid state showed an interesting behavior of unsymmetrical analogue of 361b (Figure 13). While in DCM, it had a green color and absorption and fluorescence emission wavelengths of 473 and 520 nm, respectively, in the solid state, it had absorption at 610 nm and red luminescence emission at 667 nm. X-ray crystallographic analysis indicated that the aryl units of 361b were coplanar with bi(TT) and the molecules had a slipped stacked arrangement causing significant interlayer interactions, which was supported by B3LYP/6-31G(d) level caluclations (Figure 13). The unsymmetrical analogue of 361b, however, had two polymorphs, the thermal stabilities of which were also different. Despite the discrepancy between the solid state of 361b and 361c, their UV−vis spectra were almost the same. X-ray analysis showed that the solid-state arrangement of one of the polymorphs of 361c was the same as that of 361b. Yet, the other had π-stacking with a small shift in the direction of the short axis. The slipped stacked form of 361c

Figure 13. (Above left) Electronic spectra of 361b. Black solid line, absorption spectrum in DCM; black broken line, emission spectrum in DCM; red solid line, excitation spectrum in the crystal; red broken line, emission spectrum in the crystal. (a) X-ray crystal structure of 361b (50% probability for thermal ellipsoids), (b) overlap of two adjacent molecules, and (c) packing structure shown as a ball-and-stick model.199 Reprinted (adapted) with permission from ref 199. Copyright 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

had an anisotropic property and a high charge mobility of 4.0 cm2 V−1 s−1 along the longer axis, whereas the mobility for an isotropic character was 1.3 cm2 V−1 s−1. 2.2.2. Synthesis and Applications of Oligomers and Polymers. Lee et al.130 were the first to perform the electrochemical polymerization of unsubstituted TT (2), applying a constant potential of 1.2 V on an indium tin oxide (ITO)-coated glass plate. The polymer had UV−vis absorption maximum at 804 nm (1.54 eV) and its band edge was observed at 1459 nm, resulting in a band gap as low as 0.85 eV. Poly(thieno[3,4-b]thiophene) was concluded to be a good candidate for electrochromic devices21 due to its stability and optical properties. It is sky-blue in reduced form and transparent when oxidized.200 A stable and water-processable sulfonated poly(thieno[3,4-b]thiophene) was obtained through reaction with fuming sulfuric acid, which possessed a low band gap energy, ranging from 1.05 to 1.18 eV. Tuning of the optical properties of the polymer with changing of the sulfonation level was possible; the higher is the sulfonation level, the larger is the blue-shift of the π−π* transitions and the higher is the oxidation potential, on account of the decreased electron density on the conjugated backbone. The oxidation potential peaks of 56% and 65% sulfonated polymers were −0.2 and +0.2 V (vs Ag/Ag+), respectively.201 AM


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Patra et al.193 brominated 2 with 2 equiv of NBS to obtain 4,6-dibromothieno[3,4-b]thiophene (362) as a white crystalline solid in 93% yield, which was further used for solid-state polymerization (SSP) (Scheme 58). The selectivity of bromination

Scheme 59. Chemical Polymerization of 2-Decylthieno[3,4-b]thiophene (354d), Providing Polymer 364196,197

Scheme 58. Solid-State Polymerization (SSP) of 362 and Bond Lengths of 362193

polymerization of 108. Computational studies by Hong et al.189 revealed that both aromatic and quinoid repeating units had almost the same energy. However, when alkyl groups were introduced at C2, polymer having aromatic groups had lower energy (0.3 kcal/mol), as compared to the polymer possessing the quinoid form. The optical absorption maximum and the band gap energy for the film were 925 nm and 0.92 eV, respectively, the band edge of which reached to 1350 nm.197 In chloroform a blue-shifted absorption maximum by 186 nm with a band gap of 0.98 eV was obtained. The films doped by FeC13, iodine, or NOBF4 had conductivities of 3.1 × 10−3, 1.0 × 10−6, and 4.2 × 10−6 S cm−l, respectively. TGA analysis indicated decomposition of the polymer 364 at about 145 °C bringing about 44% of weight loss attributed to cleavage of the C9H19 alkyl unit and consequently formation of an arylmethyl radical. In 2005, Lee et al.201,202 reported the polymerization of 2 as an aqueous dispersion using a polyelectrolyte, such as poly(styrenesulfonate) (PSS), for the formation of poly(thieno[3,4-b]thiophene)-poly(styrenesulfonic acid) (365), which is a water dispersible conducting polymer with a band gap of less than 1.0 eV (Ag/Ag+) (Scheme 60). Persulfate ion and hydrogen

was proven by NMR and X-ray crystallographic studies. The 1 H NMR spectrum of 362 has two doublets at δ 7.35 and 6.78 ppm with a coupling constant of J = 5.6 Hz, for the ortho-related protons. X-ray analysis demonstrated that 362 is planar and had two different packings, π−π stacking and a herringbone. The distance between two planes in π−π stacking packing was 3.49 Å. The bond lengths of peripheral thiophene ring were different from those of the main thiophene ring, which matched well with the presence of two exocyclic double bonds (Figure 14). DSC analysis showed an endothermic peak

Scheme 60. Polymerization of 2 as an Aqueous Dispersion, Furnishing Polymer 365201,202

Figure 14. (Top) Crystal structure of 362 (presents two independent molecules in the unit cell). (Bottom) Crystal packing diagram of 362 showing short Br···Br contacts (red, Br; yellow, S; gray, C).193 Reprinted (adapted) with permission from ref 193. Copyright 2011 American Chemical Society.

peroxide were used to prepare the dispersions. While 365 was highly transmissive and had a green color upon oxidation, it was blue in its neutral state. A thin film of the polymer was prepared using ferric sulfate, and its fabrication was performed from drop casting on a glass. The measured conductivity of the thin film was 10−2 S cm−1. The UV−vis spectrum of the polymer, obtained using ferric sulfate hydrate, indicated bathochromic shifts of λmax from 100−188 to 237−350 nm for neutral and oxidized polymers, respectively.202 Sotzing and co-workers203 synthesized three dimers of thieno[3,4-b]thiophene, 2,2′-bis(thieno[3,4-b]thiophene) (368), 4,4′-bis(thieno[3,4-b]thiophene) (371), and 6,6′-bis(thieno[3,4-b]thiophene) (372), using Ullmann coupling (Scheme 61). The dimer 368 was synthesized through lithiation of TT (2) with 2 equiv of n-BuLi and then addition of chlorotriisopropylsilane (TIPSCl) to give 4,6-bis(triisopropylsilyl)thieno[3,4-b]thiophene (366) in 54% yield, which was followed by treatment with n-BuLi and CuCl2 to obtain the coupled product 2-(4,6-bis(triisopropylsilyl)thieno[3,4-b]thiophen-2-yl)-4,6-bis(triisopropylsilyl)thieno[3,4-b]thiophene

associated with melting followed by exothermic polymerizations. For SSP, which provided mild polymerization conditions, 362 was heated to a temperature below the melting point (50 °C) resulting in the bromine doped polymer 363, insoluble in common organic solvents (Scheme 58). The conductivity of 363 was 6 S cm−1, which was higher than that of the polymer (only 2 S cm−1 in the oxidized state) obtained from chemical polymerization with FeCl3.201 Periodic boundary condition (PBC) calculations at the DFT level predicted a planar structure having a band gap of 0.96 eV for 363. In 1997, Pomerantz et al.196,197 polymerized 2-decylthieno[3,4-b]thiophene (354d), using 1.2 equiv of FeCl3 in chloroform to obtain the polymer 364 in moderate yield (66%), which was characterized by its blue-green color and solubility in common organic solvents (Scheme 59). The average molecular weight (Mn) was 30 000 g mol−1 with a degree of AN


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the EDOT/TT ratio of the copolymers, produced in TBAP and TBAPF6, demonstrated ca. 1:4 and 1:2 compositions, respectively. As the copolymer obtained using TBAP electrolyte possessed TT in higher ratio, optical measurements indicated a 100 nm red-shift in UV−vis absorption band (750 nm), as compared to the one prepared in TBAPF6. Conductivities of the polymers 368, 371, and 372 were found to be ca. 2 × 10−5 S cm−1. When they were doped with iodine, 368 had a conductivity of 0.007 S cm−1, and 371 and 372 had 0.2 S cm−1. Zhang et al.205 synthesized two-dimensional π-expanded quinoidal terthiophenes 375 and 376 from the combination of thieno[3,4-b]thiophene as a donor group and 5-alkyl-4Hthieno[3,4-c]pyrrole-4,6(5H)-dione as an acceptor unit with a flat configuration emerging from the weak intramolecular S−O/S interactions. Oligomer 375 demonstrated relatively higher electron mobility (3.0 cm2 V−1 s−1) with respect to 376 (0.44 cm2 V−1 s−1) due to the increased crystallinity and orientational order in 375 (Chart 30).

Scheme 61. Synthesis of the Dimers 368, 371, and 372, and Preparation of the Copolymer 374203,204

Chart 30. Two-Dimensional π-Expanded Quinoidal Terthiophenes 375 and 376205

(367) in 38% yield as an orange solid. Desilylation was conducted using tetra-n-butylammonium fluoride (TBAF), which furnished the monomer 2,2′-bis(thieno[3,4-b]thiophene) (368) in 87% yield as a yellow powder. Comparable methodology was applied for the syntheses of 371 and 372. Of these three monomers, 368 had the highest oxidation peak of 0.73; values for 371 and 372 were 0.49 and 0.53 V (vs nonaqueous Ag/Ag+), respectively (Figure 15). Their corresponding

Son et al.206 fabricated devices from semiconducting polymers 377a−c, consisting of TT (2) and benzodithiophene units along with F atoms on TT and benzodithiophene unit (Chart 31). The molecular weights (GPC) were determined as 97.5 (PDI = 1.38), 26.7 (PDI = 2.38), and 78.4 kg mol−1 (PDI = 2.61), respectively. Their studies revealed the substantial effect of modification of backbone with fluorine atoms on the physical properties of polymers. Fluorination rendered the phase separation of the polymer and PC71BM film, resulting in a drastic change in the PCE of solar cells. They recorded the efficiency of BHJ solar cell, a blend film of 377a and PC71BM, as high as 7.4%. The values of Voc, Jsc, and FF for 377a were reported to be 0.74 V, 14.5 mA cm−2, and 0.69, respectively (Table 10, Figure 16). The blend film, made from the copolymer 378, containing planar π-conjugated tetrathienoanthracene, TT (2), and PC61BM, demonstrated a PCE of 5.62%. Voc, Jsc, and FF values were 0.66 V, 15 mA cm−2, and 0.58, respectively.207 Soon et al.208 examined the transient optical and singlet oxygen generation, which indicated that the film prepared from 377a was relatively unstable due to the generation of singlet oxygen from high energy triplet excitons, formed via a nongeminate charge recombination. The transient optical studies revealed a broad photoinduced absorption maximum at 1130 nm. This absorption decayed monoexponentially with a lifetime of 1.2 μs, even shorter with 0.7 μs in the presence of oxygen, suggesting the triplet states of the polymer. Kumar et al.209 explored the surface morphology of a blend of 377a with PC71BM to explain the effect of the morphology on the PCE by analyzing an active layer spray-coating using AFM, which indicated that the optimized spray-coated film demonstrated a PCE of 5.96%. Guo et al.210 investigated the properties of the device fabricated from poly(thienothiophene-benzodithiophene) (377d) and PC61BM,

Figure 15. First CVs obtained for the electrochemical polymerizations of 2, 368, 371, and 372 in lithium trifluoromethanesulfonate in acetonitrile/nitrobenzene (1:1 w/w) (monomer concentration: 3 mM; scan ratem 50 mV s−1).203 Reprinted (adapted) with permission from ref 203. Copyright 2006 American Chemical Society.

electropolymers displayed lower optical band gaps of about 0.9 eV (1377 nm) comparable to the band gap of the copolymer 374 (0.80 eV), synthesized electrochemically from TT (2) and 3,4-ethylenedioxythiophene (EDOT) (373).204 Calculation of AO


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Chart 31. Copolymers 377−380206−214

Figure 16. (Left) I−V curves of solar cells of prepared from 377a−c with mixed solvent of 97% 1,2-dichlorobenzene (DCB)/3% 1,8-diiodoctane (DIO).206 Reprinted (adapted) with permission from ref 206. Copyright 2011 American Chemical Society. (Right) Sketch of the arrangements of 377d. The interstack (horizontal) and π−π stacking (vertical) distances are depicted by da and db, respectively.210 Reprinted (adapted) with permission from ref 210. Copyright 2010 American Chemical Society.

379d and PC61BM had the highest efficiency of 6%, whereas the lowest efficiency of 2.26% was recorded for the solar cell containing 379f. Son et al.214 synthesized dithieno[2,3-d:2′,3′d′]benzo[1,2-b:4,5-b′]dithiophene-based conjugated polymers 380a−c and investigated the photovoltaic properties of 380b and 380c, because 380a had low solubility in common organic solvents. The weight-averaged molecular weights (GPC) of 380b and 380c were reported to be 108.6 (PDI = 2.84) and 105.5 kDa (PDI = 2.32), respectively. While TGA analyses of the polymers 380b,c exhibited thermal stability up to 250 °C, DSC did not show any glass transition. Both polymers had broad absorption bands from 450 to 800 nm, resulting in optical band gaps of 1.67−1.68 eV. Electrochemical investigations revealed the HOMO energy levels to be −5.24 and −5.30 eV for 380b,c, respectively. On the basis of space-chargelimited current (SCLC) measurements, the charge mobility of 1.69 × 10−4 cm2 V−1 s−1 observed for polymer 380b was higher than the mobility of 380c, 0.71 × 10−4 cm2 V−1 s−1, possibly due to large bis(2-ethylhexyl)methyl groups, having a nonconductive property. Devices fabricated from 380b,c donors with PC71BM provided PCEs of 7.6% and 4.9%, respectively. Their photovoltaic measurements such as Voc, Jsc, FF, and PCE are given in Table 10. Huo et al.192 performed copolymerization of TT with benzo[1,2-b:4,5-b′]difuran (382) to obtain low band gap polymers, containing various electron-withdrawing moieties such as fluorine, ester, carbonyl, sulfonyl, and cyano units, on the TT unit, which helped to tune the molecular energy levels of the polymers (Scheme 62). Dibromination of 339 with NBS

which had a PCE of higher than 5%, which is much higher than that of the copolymer (0.87%) having bithiophene instead of benzodithiohene.211 Grazing incidence X-ray scattering (GIXS) analysis indicated that the lamellar packing formed between the conjugated backbone planes and side-chains was parallel to the substrate (Figure 16). This arrangement resulted in enhancement of the charge mobility up to 4.5 × 10−4 cm2 V−1 s−1. Liang et al.212,213 demonstrated the synthesis and photovoltaic investigations of a series of semiconducting polymers possessing alternating thieno[3,4-b]thiophene and benzodithiophene units, the molecular weights (Mw) of which were recorded as 19.3− 25.0 kg mol−1 with PDI of 1.25−1.50. While electrochemical measurements indicated the band gaps varying from 1.70 to 1.88 eV for 379a−f, the optical band gaps were estimated between 1.58 and 1.63 eV. The alkoxy side chains and alkyl introduction of electron-withdrawing fluorine dimininshed the HOMO energies of the polymers. The solar cell fabricated using

Table 10. Optical and Photovoltaic Results Obtained from Polymers 377a−c, 378, 379a−f, and 380b,c206−214 377a 377b 377c 378 379a 379b 379c 379d 379e 379f 380b 380c

HOMO [eV]

LUMO [eV]

EgCV [eV]

EgOpt [eV]

VOC [V]

JSC [mA cm−2]

FF

PCE [%]

−5.15 −5.41 −5.48 −5.04 −4.90 −4.94 −5.04 −5.12 −5.01 −5.01 −5.24 −5.30

−3.31 −3.60 −3.59 −3.28 −3.20 −3.22 −3.29 −3.31 −3.24 −3.13

1.93 1.81 1.89 1.76 1.70 1.72 1.75 1.81 1.77 1.88

1.68 1.75 1.73 1.69 1.58 1.59 1.60 1.63 1.62 1.61 1.68 1.68

0.74 0.68 0.68 0.66 0.56 0.66 0.72 0.74 0.66 0.62 0.89 0.88

14.5 11.0 9.10 15.0 15.1 12.8 13.9 13.0 10.7 7.74 13.0 10.7

0.69 0.43 0.39 0.58 0.63 0.66 0.58 0.61 0.58 0.47 65.3 52.1

7.40 3.30 2.70 5.62 5.60 5.10 5.85 6.10 4.10 2.26 7.60 4.90

AP


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Scheme 62. Synthesis of Copolymers 383a−e by Stille Coupling192

Chart 32. Promising Copolymers 384−388 for Photovoltaic Cell Applications135,215−217

provided 381 in 66% as an orange-yellow solid. Copolymerization of 381 with 2,6-bis(trimethyltin)-4,8-bis(octyloxy)benzo[1,2-b:4,5-b′]difuran (382) was performed through Stille coupling, which afforded 38% of polymer 383a. The other polymers 383b−e were produced in the same manner. All of the polymers were soluble in chloroform, chlorobenzene, and dichlorobenzene. The number-averaged molecular weights (Mn) obtained from GPC analyses were reported to be 30.0 (PDI = 1.3), 40.7 (PDI = 1.7), 61.0 (PDI = 1.7), 45.3 (PDI = 2.4), and 37.5 (PDI = 1.3), respectively (Table 11).

384a−387, which had PCEs of less than 8.0% (Chart 32). However, Chen et al.217 tuned the Voc of the device by changing the functional group of the semiconductor and obtained a higher Voc of 0.76 V for the solar cell fabricated from 384b. They realized a PCE of 6.77% from this solar cell with high short-circuit current density. Park et al.195 investigated the electrochemical properties of 2-(1-heptenyl)thieno[3,4-b]thiophene (347), trans-2styrylthieno[3,4-b]thiophene (348), and (Z)-2-phenyl-3(thieno[3,4-b]thiophene-2-yl)acrylonitrile (349), and their conjugated polymers 389a−c, respectively (Scheme 63).

Table 11. Optical and Electrochemical Properties of the Copolymers 383a−e192 polymer 383

λmax,solution (nm)

λmax,film (nm)

Eg (eV)

HOMO (eV)

LUMO (eV)

EgCV (eV)

a b c d e

603 681 659 661 628

623 692 671 700 644

1.60 1.53 1.53 1.52 1.61

−5.44 −5.03 −5.07 −5.11 −5.25

−3.70 −3.63 −3.61 −3.60 −3.61

1.74 1.40 1.46 1.51 1.64

Scheme 63. Electropolymerizations of Monomers 347−349195

Polymer 383b, substituted by only a carbonyl group, had an absorption maximum at the longest wavelength of 681 nm, and polymer 383a, containing ester and cyano units, showed an absorption maximum at shorter wavelength, both in solution and in the solid state (603 and 623 nm, respectively). However, in the case of spin-coated films, the absorption maximum of the polymer 383d, having carbonyl and fluoro substituents, appeared at the longest wavelength of 700 nm. While their optical band gap energies varied from 1.52 eV (383d) to 1.61 eV (383e), the band gap energies obtained from CV measurements were between 1.40 eV (383b) and 1.74 (383a) (Table 10). Thus, the copolymers with the stronger electron-withdrawing groups had lower HOMO and LUMO levels. The highest PCE of 5.23% with Voc of 0.63 V and Jsc of 13.88 mA cm−2 was obtained for the device fabricated from 383d and PC71BM. Bérubé et al.215 used Kohn−Sham energy levels of density functional theory in combination with Scharber’s model to shed light on the experimental findings135,216 of the promising polymers for possible photovoltaic cell applications. On the basis of predicted PCEs, the copolymer 388 possessing thienopyrroledione (TPD) with power conversion energy of 8.9% was found to be the best candidate among the polymers

Electrochemical studies revealed that the monomer 348 had the lowest oxidation potential peak at 1.33 V (Eonset: 0.96 V), whereas those of the monomers 347 and 349 were 1.42 (Eonset: 1.10 V) and 1.47 V (Eonset: 1.20 V) (vs Ag/Ag+), respectively. Introduction of electron-donating groups improved the resonance effect and increased the energy level of the HOMO, consequently decreasing the oxidation potential of the monomers, whereas an electron-withdrawing unit increased the oxidation potential of the monomers. The same trend was observed in the case of the corresponding polymers. While polymer 389b held the lowest onset potential of 0.23 V, those of 389a and 389c had potentials of 0.32 and 0.38 V (vs Ag/ Ag+), respectively. The lowest band gaps of 1.0 eV (1148 nm), 0.89 eV (1304 nm), and 0.78 eV (1390 nm) were found for polymers 389a−c, respectively (Scheme 63). Gong 218 and co-workers synthesized low band gap copolymers possessing fluorinated-thienothiophenes to obtain AQ


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Scheme 64. Synthesis of Copolymer 392218

Scheme 65. Preparation of Copolymer 396 by Using Stille Coupling Approach, and the Structure of Fluorine Containing Copolymer 397220,221

inverted polymer solar cells and to explain the influence of electronegative fluorine atoms on the electronic properties and morphology of thin films (Scheme 64). They concluded that the copolymer holding 20−40% fluorine units had the highest hole mobility, matching well with a dispersed interconnected morphology of the polymer. While a solar cell of the polymer having 20% fluorinated units showed an efficiency of 5.85%, increasing these units unfortunately diminished the PCE. This was explained by AFM images indicating that the phase morphology of polymer was roughened, which affected the performance of the device inversely (Figure 17). The optical

afforded 393 in 96% yield. Dibromination of 393 with NBS provided the corresponding product 394 in 81% yield, Stille cross-coupling of which with 2,6-bis(trimethylstannyl)-4,8bis(2,3-didecylthiophene-5-yl)-benzo[1,2-b:4,5-b′]dithiophene (395) resulted in the copolymer 396 in 80% yield. The numberaveraged molecular weight was determined by using GPC to be 46.0 kDa (PDI = 1.6). DSC measurement revealed no remarkable transitions, and the property of low-crystallinity for the copolymer was unambiguously supported by X-ray crystallographic analysis. Optical studies showed λonset of 1050 nm resulting in a low band gap of 1.2 eV, possibly due to the presence of TT units possessing electron-withdrawing perfluorinated alkylcarbonyl groups. CV investigations indicated a relatively low HOMO energy level (−5.6 eV). Calculated from the HOMO level and the optical band gap, the LUMO energy level was estimated to be −4.36 eV. The polymer 396 had a sufficiently low LUMO energy level to be used in organic photovoltaic cell (OPV) applications. On the other hand, the bulk heterojunction (BHJ) of 396 with CdSe colloidal nanocrystals demonstrated a high performance photodetector as a result of high charge carrier mobility of 396, recorded to be 0.02 cm2 V−1 s−1, and charge trapping ability of the CdSe colloidal nanocrystals. Liu et al.221 investigated the influence of fluorine content in the polymer 397 on the performance of polymer cells. A PCE of 8.75% was recorded with the highest fluorinated polymer (Scheme 65). Kim et al.222 synthesized the copolymer poly{4,8-bis((2ethylhexyl)-thieno[3,2-b]thiophenyl)-benzo[1,2-b:4,5-b′]dithiophene-alt-2-ethylhexyl-4,6-dibromo-3-fluorothieno[3,4b]thiophene-2-carboxylate} (399), applying Stille coupling of

Figure 17. AFM phase images of the 392:PC71BM with varying content of fluorinated unit: (a) 0%, (b) 20%, (c) 40%, (d) 60%, and (e) 80%.218 Reprinted (adapted) with permission from ref 218. Copyright 2013 American Chemical Society.

band gap of the polymers was ca. 1.6 eV, indicating the lack of the effect of fluorine percentage in the copolymer on tuning the band gap. Park et al.219 prepared low-temperature-annealed ZnO films as electron-transporting layers for invertedstructured polymer solar cells, which provided a 20% higher power-conversion efficiency. The optimized low-temperatureannealed ZnO films with blends of copolymer 392, possessing 20% fluorine coupled TT and PC71BM, produced a PCE of 6.42% (Scheme 64). Yun et al.220 synthesized a low band gap copolymer 396 from dialkyl thienylated benzodithiophene (395) and perfluororalkylcarbonyl-substituted thieno[3,4-b]thiophene (394) monomers (Scheme 65). Thus, reaction of thieno[3,4-b]thiophene-2carbaldehyde (346) with heptadecafluorooctyl iodide in the presence of methyllithium followed by oxidation with MnO2 AR


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Scheme 66. Synthesis of Copolymer 399222

Scheme 67. Syntheses of the Copolymers 401-H and 401-F223

Low HOMO levels of −5.44 and −5.51 eV with band gaps of 1.65 and 1.64 eV were recorded for the polymers 401-H and 401-F, respectively. The λmax values were 684 nm for 401-H and 680 nm for 401-F. PCEs of 4.49% and 5.16% and charge mobilites of 3.5 × 10−4 and 5.7 × 10−4 cm2 V−1 s−1 were achieved for 401-H and 401-F, respectively. Kleinhenz et al.190 examined six copolymers synthesized through the Stille couplings of benzo[1,2-b:4,5-b′]dithiophene (BnDT, 403), dithieno[3,2-f:2′,3′-h]quinoxaline (QDT, 404), and naphtho[2,1-b:3,4-b′]dithiophene (NDT, 405) with distannyl TT (402-H) and fluorinated distannyl TT (402-F) under microwave and toluene reflux conditions in 23−68% yields (Scheme 68). The number-average molecular weights (Mn) of the resultant polymers varied from 12.1 to 42.7 kg mol−1 with PDI of 1.47−2.56. The thienothiophene unit was reported to be significantly effective on the low-band gap property of the

bis(2-ethylhexylthieno[3,2-b]thiophenylbenzo[1,2-b:4,5-b′]dithiophene (398) with 2-ethylhexyl 3-fluorothieno[3,4-b]thiophene-2-carboxylate (390-F) in 48% yield (Scheme 66). The number-averaged molecular-weight (Mn) was recorded as 25 000 g mol−1 (PDI = 2.46). The optical band gap of 399 was obtained to be 1.55 eV, and HOMO and LUMO energy levels were reported to be −5.31 and −3.73 eV, respectively. The device, fabricated from thin film of the copolymer, demonstrated a mobility of 0.021 cm2 V−1 s−1. PCE of the photovoltaic cells constructed from inverted 399:PC71BM and P3HT:ICBA-based cells was measured to be 8.66%, which is superior to the conventional device (Figure 18).

Scheme 68. Synthesis of Copolymers 406-H(F)−408-H(F) Using Stille Coupling Reaction190

Figure 18. External quantum efficiencies (EQEs) of the conventional and inverted devices, which were fabricated from 399.222 Reprinted (adapted) with permission from ref 222. Copyright 2014 American Chemical Society.

Koti et al.223 synthesized and investigated the optical, electrochemical, and photovoltaic properties of the polymers 401-H and 401-F, constructed from the Stille cross-coupling polymerization of the donor group 3,8-bis(2-butyloctyloxy)naphtho[3,2-b:7,6-b′]dithiophene (NDT) (400), possessing two Me3Sn with dibrominated thienothiophene 390-H and fluorinated TT 390-F (Scheme 67). The molecular weights (Mn/Mw) of 401-H and 401-F were recorded from GPC analyses as 21 408/ 68 877 (PDI = 3.2) and 61 018/136 054 (PDI = 2.23). AS


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Table 12. Optical and Electrochemical Properties of Copolymers 406-H(F)−408-H(F)190

a

polymer

λmaxa [nm]

Egopta [eV]

λmaxb [nm]

Egoptb [eV]

HOMO [eV]

LUMO [eV]

EgCV [eV]

mobility [×10−6, cm2/V·s]

406-H 406-F 407-H 407-F 408-H 408-F

655 633 619 655 633 599

1.51 1.57 1.43 1.43 1.39 1.48

650 638 635 650 661 638

1.43 1.44 1.39 1.41 1.38 1.43

−5.12 −5.31 −4.99 −5.16 −4.70 −5.00

−2.95 −2.97 −2.94 −3.15 −2.94 −3.11

2.16 2.34 2.05 2.01 1.76 1.89

0.64 7.22 5.15 2.66 5.12 6.24

In chlorobenzene. bFilm.

polymers. Morover, HOMO energy level was adjusted by tuning the electronic properties of the monomer units by introducing fluorine substituent on TT. While the highest HOMO was recorded as −4.70 eV for 408-H, the lowest one was obtained with 406-F (Table 12). Optical properties of the polymers were investigated both in solution and in film, which gave band gaps varying from 1.39 to 1.57 eV and 1.38 to 1.44 eV, respectively (Figure 19). Electrochemical studies provided slightly higher Eg

Scheme 69. Synthesis of 3-Bromothieno[2,3-b]thiophene (411)224

Scheme 70. Synthesis of 413 and 415 from 412 and 414, Respectively224,225

Figure 19. UV−vis spectra of polymers 406-H(F)−408-H(F) in chlorobenzene (solid line) and in solid film (dash line). Reprinted (adapted) with permission from ref 190. Copyright 2011 American Chemical Society.

values (1.76−2.34 eV) with respect to the optical ones. The hole mobilities of the polymers measured by space charge limited current (SCLC) were reported as (0.64−7.22) × 10−6 cm2 V−1 s−1. 2.3. Thieno[2,3-b]thiophene (3)

Scheme 71. Ring Closure of 419 with I2 Furnishing 420227

2.3.1. Synthesis and Properties. Because thieno[2,3-b]thiophene (3) is a cross-conjugated system, it decreases the conjugation length in a polymer so as to decrease the HOMO energy level. Synthesis of 3 and its derivatives was achieved by applying several protocols. Formylation of 15 with n-BuLi and DMF was followed by consecutive addition of n-BuLi, elemental sulfur, and then methyl bromoacetate to furnish 409. Synthesis of the ring closure product, 4-bromothieno[2,3-b]thiophene-2-carboxylic acid (410), was achieved through the reaction of 409 with sodium alkoxide. 1,5-Diazabicyclo[4.3.0]non-5-ene (DBN) was also reported to be used to convert 409 (without Br) into 410.226 Decarboxylation of 410 resulted in 3-bromothieno[2,3-b]thiophene (411) (Scheme 69).224 Another method involved the reaction of 2,4,5-tribromothiophene-3-carbaldehyde (412) with ethyl thioglycolate in the presence of potassium iodide and CuO, which afforded ethyl 4,5-dibromothieno[2,3-b]thiophene-2-carboxylate (413) in 58% yield (Scheme 70).225 Ring closure was also achieved in the presence of 1,5-diazabicyclo[4.3.0]non-5-ene (DBN).226 Reaction of thien-2-ylthioacetoacetates (414) in PPA gave the TT 415.224 Reaction of thiophene-3-carbaldehyde (416) with 2-thioxothiazolidin-4-one (417) provided 2-mercapto-3-(3-thienyl)acrylic

acid (419) through intermediate 418. Follow-up reaction of 419 with 2 equiv of iodine furnished thieno[2,3-b]thiophene-2carboxylic acid (420) in 70% yield (Scheme 71).227 Another strategy involves a condensation reaction of thiophene3-carbaldehyde (416) with malonic acid (421) in the Doebner modification of the Knoevenagel reaction (Scheme 72). The reaction was conducted in a mixture of pyridine and piperidine to afford (E)-3-(3-thienyl)acrylic acid (422), which was followed by a reaction with thionyl chloride to obtain AT


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2,5-Disubstituted thieno[2,3-b]thiophenes (436) were obtained from the reaction of 1,3-diketones (434) with alkyl halides (435), possessing an electron-withdrawing unit, and carbon disulfide in the presence of anhydrous KF as a promoter (Scheme 76). The reaction was also conducted by using phasetransfer catalysis.235,236

Scheme 72. Synthesis of 423 under Doebner Condensation Protocol228,229

Scheme 76. Synthesis of 2,5-Disubstituted Thieno[2,3-b]thiophenes (436)235,236

3-chlorothieno[2,3-b]thiophene-2-carboxylic acid chloride (423) in 51% yield.228,229 3,3-Sigmatropic rearrangement of allyl 2-thienyl sulfide (424) afforded 2-methylthieno[2,3-b]thiophene (426) and 3-chloro-3,4-dihydro-2H-thieno[2,3-b]thiopyran (427) in a total yield of 60% (Scheme 73).230 Scheme 73. 3,3-Sigmatropic Rearrangement of Allyl 2-Thienyl Sulfide (424) Providing 426230

A similar approach was used to synthesize oxy- and aminosubstituted TTs (439) in 61−87% yield, from 3-keto esters, malononitrile, ethyl cyanoacetate, and 3-ketonitrile (437) with alkylating agents such as ethyl bromoacetate and bromoacetonitrile (438) (Scheme 77).237

TT (3) can be synthesized from trimethylsilylpenta-1,3-diyne (428) under mild condition. The alkyne and CS2 are added to a solution of n-butyllithium and KOtBu, followed by introduction of a solution of tBuOH in HMPA to furnish 3 in 47% yield (Scheme 74).231

Scheme 77. Synthesis of Oxy- and Amino-Substituted TTs (439)237

Scheme 74. Synthesis of 3, Starting from Trimethylsilylpenta-1,3-diyne (428)231

Treatment of CS2 with malononitrile (437) in the presence of NaOMe results in the formation of dicyanodithioacetate (440),238 which on reaction with γ-bromocrotonic acid (441) gives diamino-substituted thieno[2,3-b]thiophenes (442) (Scheme 78).239 Other TT derivatives were produced by modifying this protocol.240−242

Another methodology involved the synthesis of 2-(2,2dimethoxyethylthio)thiophene (431) from the reaction of 2-mercaptothiophene (429) with 2-bromoacetaldehyde dimethyl acetal (430), the ring closure of which with PPA furnished TT (3) in 45% yield (Scheme 75).232,233 The same starting material Scheme 75. Synthesis of 3 and 433 from 429 via Acetal (431) and Ketone (432) Formation, Respectively232−234

Scheme 78. Synthesis of Diamino-Substituted TTs (442)238,239

429 was used to obtain 3-alkyl-TT (433) by Heeney et al.,234 applying a different methodology. In the first step, alkylation of 429 with an α-chloroketone gave 432. In the next step, ring closure was achieved in the presence of an acid catalyst (Amberlyst 15) to form the alkylated TT (433) in 56% yield (Scheme 75).

The successive reactions of malonic acid dimethyl and diethyl ester (443) with sodium amide, carbon disulfide, and methyl chloroacetate (435) afford 3,4-dihydroxythieno[2,3-b]thiophenedicarboxylate (412) as a brown solid in 47% and AU


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45% yields, respectively. Treatment of 445 with 1,2-dibromoethane results in the production of 3,4-ethylenedioxythieno[2,3b]thiophenedicarboxylate (446) as a white solid in 63% and 70% yields, the structure (R = Et) of which was unequivocally proven by X-ray single-crystal analysis. Saponification followed by decarboxylation gave 3,4-ethylenedioxythieno[2,3-b]thiophene (447) as a white solid in 75% yield (Scheme 79).243

Scheme 81. Synthesis of Heterohelicenes 454 and 455, Starting from 3245

Scheme 79. 3,4-Ethylenedioxythieno[2,3-b]thiophene (447) from Malonic Acid Esters (443)243

into heterohelicenes 454 and 455, respectively, via a double photocyclization (Scheme 81).245 2,3,6,7-Tetrathiabenzo[l,3-cd:4,6-c′d′]dipentalene (458) was prepared via dimerization of TT (3).246 Another method for the synthesis of 458 involved the coupling of 3,4-dibromo-TT (456) with 3,4-bis(trimethylstannyl)-TT (457) in the presence of a palladium catalyst giving 458 in 13% yield, which is isoelectronic with perylene, and its functionalization was realized using alkyating reagents to provide the tetra-alkylsubstituted 459. Alkylation of 458 was achieved via lithiation with LDA, followed by addition of an alkyl iodide or a dialkyl disulfide (Scheme 82).247 Synthesis of 3,4-di(p-tolyl)thieno[2,3-b]thiophene (450) was performed using 1,3-di(p-tolyl)propan-1,3-dione (448) as a starting material. Treatment of 448 with KF, CS2, and ethyl bromoacetate afforded ketene dithioacetal (449) in 66% yield followed by a Dieckmann-type cyclization in the presence of NaOEt. Hydrolysis and decarboxylation complete the synthesis of 450 (Scheme 80).244

Scheme 82. Preparation of Dipentalene 458 from 456 and Its Alkylation in the Presence of LDA247

Scheme 80. Dieckmann Cyclization for the Synthesis of 450244

The chemical polymerizations of the TT (433) were conducted reacting dibromo-TT 460, prepared by treatment of 433 with NBS, with MeMgBr in the presence of Ni(dppp)2 (Scheme 83).234 2.3.2. Synthesis and Applications of Oligomers and Polymers. Contrary to the other isomers, insertion of TT (3), which is known as a cross-conjugated unit, into conjugated oligomers or polymers restrains the effective conjugation length and consequently raises its ionization potential, that is, provides the low-lying HOMO energy level and enhances the stability of materials.248 Lithiation of thieno[2,3-b]thiophene (3) with n-BuLi, followed by addition of N,N-dimethylformamide, resulted in the formation of the dialdehyde 451 in 49% yield. The reaction of 451 with 2 equiv of Wittig reagents (452 and 453) furnished the corresponding vinylene compounds, which were converted

Scheme 83. Polymerization of a Dibromo TT (460) in the Presence of MeMgBr and Ni(dppp)2234

McCulloch and co-workers248 synthesized copolymers 464a and 464b having bithiophene and thieno[2,3-b]thiophene (3) monomers, applying Stille coupling polymerization of 2,5-bis(trimethylstannyl)thieno[2,3-b]thiophene (462) with AV


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absorption maximum of 468 at 332 nm. On the basis of DFT calculations at the B3LYP/6-31G(d,p) level, the HOMO and LUMO energy levels were predicted to be −4.73 and −0.87 eV, respectively; therefrom the estimated band gap energy was 3.86 eV. A first irreversible oxidation peak at 0.81 V (vs Ag/ AgCl) was shifted to the anodic region by 150 mV as compared to trimeric EDOT. The S···O intramolecular interactions in 468 provided a molecule with a more planar structure and a lower HOMO energy level with respect to PEDOT. The experimental results indicated that oxidation of the polymer 469, obtained electrochemically, was difficult as compared to PEDOT, due to the interruption of the conjugation through cross-conjugated units.243 Campbell and co-workers250 synthesized the liquid-crystal copolymer 472 possessing 9,9-dioctylfluorene and 3,4-dimethylsubstituted TT (3) units in 97% yield by Suzuki cross-coupling reaction of 2,5-dibromo-3,4-dimetylthieno[2,3-b]thiophene (470) with 9,9-dioctylfluorene-2,7-diboronic acid bis(1,3propanediol) ester (471) (Scheme 86). The weight-averaged

Scheme 84. Preparation of TT-Terthiophene Copolymers 464a,b248,249

5,5-dibromo-4,4-dialkyl-2,2-bithiophenes (463) (Scheme 84). The number-averaged molecular weights (Mn) were reported to be 29 000 (PDI = 2.0) for 464a and 26 000 (PDI = 1.9) for 464b. The absorption maxima of 464a were observed at 421 and 470 nm in solution and in the solid state, respectively, which were lower than those obtained for poly(3-hexylthiophene) (P3HT). The red shift indicated aggregation in the solid state. While the ionization potential of a thin film of 464a was 5.3 eV, P3HT had an ionization potential of 4.8 eV. DSC analyses of both polymers demonstrated two endothermic signals at room temperature and 180 °C attributed to the side-chain and backbone melting points, respectively. Wide-angle X-ray scattering studies of 464a revealed lamellar arrangements with a π−π stacking distance of 3.7 Å. High charge carrier mobilities of 0.15 and 0.12 cm2 V−1 s−1 were measured for fabricated thin film transistors of the copolymers 464a and 464b under ambient air and light conditions, respectively, along with an on/off ratio of around 105. The high charge carrier mobilities of 0.30 cm2 V−1 s−1 of 214a and 214b, possessing TT-1 unit, as compared to 464a and 464b were recorded under inert (nitrogen) atmosphere. Shkunov et al.249 prepared solution-processable ambipolar blends containing 464a and PCBM, the FET measurements of which demonstrated n- and p-type conduction properties. They introduced silanes, containing alkyl chains, to provide a blend with balanced charge transport. The HOMO, to which the hole is injected, was −5.3 eV, and the LUMO of PCBM, into which the electron is injected from the gold surface, was −3.7 eV. On the basis of the FET measurements, using octyltrichlorosilane (OTS), the highest electron and hole mobilities were 9 × 10−3 and 4 × 10−3 cm2 V−1 s−1, respectively. Dibromination of 465 with NBS provided 466 as a pale yellow solid in 80% yield. Stille coupling of 466 with 467 furnished 2,5-bis(3,4-ethylenedioxythiophene)-3,4diethylenedioxythieno[2,3-b]thiophene (468) as a pale yellow solid in 18% yield (Scheme 85).243 Optical studies revealed an

Scheme 86. Synthesis of 472 Containing TT (3) and Fluorine Units250

molecular weight (Mw) was recorded as 90 kg mol−1 (PDI = 2.6). Because of the presence of TT (3), the conjugation was hampered, resulting in deep-blue photoluminescent (417 nm) and electroluminescent (410 nm) emissions. 2.4. Thieno[3,4-c]thiophene (4)

Contrary to the other TTs, thieno[3,4-c]thiophene (4) has not been investigated comprehensively as it is unstable; its existence could only be proved by trapping experiments.251−254 It has a 10-π-electron system and a sulfur atom with an oxidation state of +4, which is called a “nonclassical” thienothiophene. The analogues 473 and 474,255,256 and the first isolable thieno[3,4-c]thiophene (475), holding four phenyl substituents, were reported by Cava et al. (Chart 33).257,258 Clark259 performed Chart 33. Analogues of TT (4) and the First Isolable Thieno[3,4-c]thiophene (475)255−258

Scheme 85. Preparation of Oligomer 468 and Its Electropolymerization243 the self-consistent field Pariser−Parr−Pople calculation to shed light on the π-electronic structure of TT (4). It had either charge separated resonance structure or double bond possessing sulfur atom. On the basis of the calculations, the molecule had a triplet state lying 0.11 eV lower than the lowest singlet state. The computations on the relative heat of formation of 4, as compared to the other TTs (1−3), unveiled that it was unstable with energy of 46 kcal mol−1 with respect to the most stable TT (1). While TT (3) was less stable by only 1.2 kcal mol−1, TT (2) had a relative energy of 7.2 kcal mol−1. AW


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Tetraphenylthieno[3,4-c]thiophene (475) was obtained as shiny reddish-purple needles possessing a melting point of 245−248 °C. Its solution in benzene does not show an ESR signal, indicating its singlet ground state. The UV−vis absorption spectrum of its intense permanganate-like colored solution in 1,2-dichloromethane had an absorption maximum at 553 nm. On the basis of fragmentations (base peak: 444 m/z [M]+•), recorded from its mass spectrum, tetraphenylthieno[3,4-c]thiophene (475) is a stable molecule. Its 1H NMR spectrum contained only one single sharp peak at 7.12 ppm, for which orientations of the phenyl groups with respect to the core unit were explained by X-ray studies. While the phenyl units at C1 and C4 were found to rotate out of plane by 39.6°, the rotations of the phenyl groups at C3 and C6 were measured as 58.4°. Moreover, shorter C−S, 1.706 Å, and longer Cα−Cβ, 1.407 Å, and Cβ−Cβ, 1.452 Å, bond lengths were observed with respect to corresponding values for thiophene −1.714, 1.370, and 1.423 Å, respectively. 2.4.1. Synthesis and Properties. Three-step synthesis starting from tetrabenzoylethane (476) afforded tetraphenylthieno[3,4-c]thiophene (475) in an overall yield of 38%. The double ring closure in the presence of P4S10260 resulted in the formation of 4,6-dihydro-1,3,4,6-tetraphenylthieno[3,4-c]thiophene (477). Reaction of 477 with NaIO4 furnished the corresponding sulfoxide (478) in 96% yield. Dehydration with acetic anhydride provided the isolable “nonclassical” thienothiophene 475 in 87% yield (Scheme 87).257,258

Scheme 88. Synthesis of Tetra-(2-thienyl)-Substituted Thieno[3,4-c]thiophene (484) from 47918

Scheme 89. Synthesis of Thieno[3,4-c]thiophene 486, Holding Four Electron-Withdrawing Units, Starting from Thienyl-dinitrile (485)262,263

Scheme 87. Synthesis of Tetraphenyl-Substituted Thieno[3,4-c]thiophene (475), Starting from 476257,258 (489), substituted by alkylthio and arylthio groups, with PPh3, resulting in 1,4-bis(t-butylthio)-3,6-diarylthieno[3,4-c]thiophenes (490) in moderate yields (Scheme 90, Figure 20). Scheme 90. Dimerization Product 488 from 487264,265 and Synthesis of 490 Starting from 489266,267 Like tetraphenylthieno[3,4-c]thiophene (475), tetra-2thienylthieno[3,4-c]thiophene (484) was synthesized by Ishii et al.18 from the sodium salt of di-2-thienoylmethane (479) and bromodi-2-thenoylmethane (480) in an overall yield of 27%. In the key step, Lawesson’s reagent (LR)261 was used to obtain fused thiophene 482 (Scheme 88). Tetra-2-thienylthieno[3,4-c]thiophene (484) is a dark purple crystalline substance with a melting point of 187−188 °C and is stable for a long time in air at room temperature. 1H and 13C NMR spectra demonstrated that four 2-thienyl units are equivalent and two different carbon signals appear at 116.49 and 141.63 ppm, corresponding to the thieno[3,4-c]thiophene core unit. The UV−vis spectrum showed an intense absorption at 576 nm with a 20 nm bathochromic shift with respect to 475. The redox potential of 484 was measured in dichloromethane, displaying a reversible wave, with a value of +0.61 V (vs Ag/Ag+). Thieno[3,4-c]thiophene 486, substituted by four electronwithdrawing units, two cyano and two carboxylic acid ester groups, was prepared from the reaction of thienyl-dinitrile (485) with thionyl chloride in the presence of triethylamine (Scheme 89).262,263 Treatment of 2,3-di(alkylthio)cyclopropenethiones (487) with triphenyl- or tributylphosphine resulted in the dimerization product 1,3,4,6-tetra(alkylthio)thieno[3,4-c]thiophenes (488).264,265 Later, Matsumura et al.266,267 reacted cyclopropenethione

Summaries on the properties of some important thienothiophenes and their oligomers and polymers are given in Chart 34 and Table 13.

3. DITHIENOTHIOPHENES Three annulated thiophene rings, that is, the annulation of bithiophenes by bridging the α,α′, α,β′, and β,β′ positions with a sulfur atom, known as DTTs (dithienothiophenes), have been used widely to build functional materials, for example, p-type AX


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(493), dithieno[2,3-b;2′,3′-d]thiophene (494), dithieno[3,4b;3′,2′-d]thiophene (495), and dithieno[3,4-b;2′,3′-d]thiophene (496) (Chart 35).13 3.1. Dithieno[3,2-b;2′,3′-d]thiophene (491)

3.1.1. Synthesis and Properties. The first synthesis and spectroscopic assignments of 491 were reported by Jong and Janssen in 1971.294 Treatment of 3-bromothiophene (5) with n-BuLi and (PhSO2)2S resulted in the formation of 3,3′-dithienyl sulfide (497) in 68% yield, which was followed by a second treatment with n-BuLi and then CuCl2 to obtain 491 in 52% yield (Scheme 91). Chen et al.272 performed the synthesis of 491 starting from 3-bromothiophene (5) using a one-pot approach. Initially, 5 was lithiated with n-BuLi followed by addition of sulfur and then TsCl. The mixture was reacted with 3-lithiothiophene, dilithiated with n-BuLi, and finally ring closure with CuCl2 furnished 491 in >30% yield. Huang et al.273 studied the electronic, optical, and conducting properties of 491 computationally at the DFT level (B3LYP/6-311G(d,p)). Internal reorganization energies for hole and electron (λ+ and λ−) were calculated to be 358 and 333 meV, respectively. The electron affinity of 491 was predicted to be −0.35 eV, whereas vertical ionization potential (IPv) was found to be 7.56 eV, which aligned well with the experimentally obtained IPv (7.78 in the gas phase).274 Theoretical studies provided the HOMO and band gap as −5.84 and 4.53 eV, respectively.273 Matzger and co-workers69 synthesized 2,6-bis(trimethylsilyl)substituted DTT (500a), using 4-bromo-2-trimethylsilylthiophene (498a) as a starting material. Reaction of S(SnBu3)2 with 2 equiv of 498a provided the coupled product 499a in a very

Figure 20. ORTEP drawing of 490 (R = Ph), viewed perpendicular and parallel to the thieno[3,4-c]thiophene ring.267 Reprinted (adapted) with permission from ref 267. Copyright 1998 American Chemical Society.

semiconductors for OFETs, due to their planar, sulfur-rich, rigid, conjugated, and highly thermal- and photostable structures.8,11,13,14,20,21,268 Moreover, DTT was also utilized as a photosensitizer together with co-initiator onium salts (diphenyliodonium hexafluorophosphate, triphenylsulfonium hexafluorophosphate, N-ethoxy-2-methylpyridinium hexafluorophosphate, etc.) in long wavelength free radical photopolymerization of (meth)acrylic monomers, in which intermolecular electron transfer took place from photoexcited DTT to the onium salt.269−271 Six isomers have appeared in the literature: dithieno[3,2-b;2′,3′-d]thiophene (491), dithieno[3,4-b;3′,4′-d]thiophene (492), dithieno[2,3-b;3′,2′-d]thiophene

Chart 34. Thienothiophene Possessing Oligomers and Polymers, of Which Fabricated Devices Provided Excellent Hole (μ+)57,104,112,140 and Electron (μ−)86,167,168,176,205 Mobilities, and Power Conversion Efficiencies (PCE)98,221,222

AY


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Table 13. Summary of the Properties of Some Thienothiophenes

AZ


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Table 13. continued

BA


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Table 13. continued

BB


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Table 13. continued

BC


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Table 13. continued

BD


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Table 13. continued

BE


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Table 13. continued

BF


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Table 13. continued

BG


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Table 13. continued

a

In solution. bSolid. cIn CHCl3. eElectron mobility. PPolymer of the corresponding compound.

Chart 35. Six Isomers of Dithienothiophenes 491−49613

Scheme 92. Synthesis of 2,6-Disubstituted DTTs (500a−c) Starting from 498a−c69,275

Scheme 91. Synthesis of DTT (491) Starting from 3-Bromothiophene (5) via Two Different Routes272,294

Scheme 93. Synthesis of 491 Starting from Tetrabromothiophene (501)276

good yield (94%), which was followed by treatment with n-BuLi and then CuCl2 to form 2,6-bis(trimethylsilyl)-DTT (500a) in an excellent yield (96%) (Scheme 92). Syntheses of 2,6-disubstituted DTTs (500b,c) were also achieved by means of C−S cross-coupling followed by oxidative dehydro C−H coupling.275 Frey et al.276 reported the synthesis of dithieno[3,2-b;2′,3′-d]thiophene (491) starting from tetrabromothiophene (501) in an overall yield of 47% (Scheme 93). Treatment of 501 with 2.1 equiv of n-BuLi was followed by addition of 1-formylpiperidine to form 3,4-dibromothiophene-2,6-dicarbaldehyde (502). Next, 502 was reacted with ethylmercaptoacetate providing 2,6-DTT-dicarboxylate (503), which was converted to 491 after hydrolyzing with LiOH and then removing CO2. He402−404 synthesized 3,5-didecyldithieno[3,2-b:2′,3′-d]thiophene (509) in five steps starting from 501 using the

same strategy as applied by Frey et al.276 Reaction of 501 with n-BuLi, followed by treatment with undecanal, furnished 1,1′-(3,4-dibromothiophene-2,5-diyl)bis(undecan-1-ol) (505) in 72% yield. Oxidation of 505 with chromic acid led to the formation of 1,1′-(3,4-dibromothiophene-2,5-diyl)bis(undecan1-one) (506) in 90% yield, and then addition of ethyl 2-mercaptoacetate produced dicarboxylate-substituted DTT (507) in 59% yield. Hydrolysis of 507 gave 3,5-didecyldithieno[3,2-b:2′,3′-d]thiophene-2,6-carboxylic acid (508) in 98% yield. In the final step, 508 was heated in the presence of copper powder in quinoline to furnish 509 in 47% yield (Scheme 94). BH


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Scheme 94. Synthesis of DTT (509) from 501 by Using 2-Mercaptoacetate in the Key Step402−404

Scheme 96. Synthesis of DTTs (514a−e) from the Reactions of 1,8-Diketones (513a−e) in the Presence of P4S10278−285

of 1.0 s and a satisfactory stability after 500 repeated cycles.281 Electropolymerization of DTT 514b, possessing a strong electron-donating MeO, and 514c with a mild electronwithdrawing Br group indicated the deposition of the polymers on the electrode surface as a function of the rise of oxidation current at 1.33 mV and the rise of reduction peaks at 1.06 mV. Spectroelectrochemical investigations indicated the oxidation and reduction potentials to be 0.96 and 0.79 V for 514b, and 1.1 and 0−92 V for 514c, respectively, which resulted in operating the device between yellow and blue colors for 514b and between green and blue colors for 514c. The electrochemical band gaps were measured and shown to be 2.20 and 2.28 eV, respectively. After repeated cycles, they showed good stability, making them promising candidates for electrochromic devices. The DTT 514b was used along with a silver salt for the preparation of a silver−epoxy nanocomposite. Electron transfer reaction was successful between photoexcited 514b and the silver salt under visible light irradiation.282 Ozturk and coworkes283,284 also synthesized a DTT 514e, possessing 2-thienyl, and conducted its electro-copolymerization with ethylenedioxythiophene (EDOT) on a glassy carbon electrode (GCE) and Pt electrodes. The experimental results revealed that, unlike the homopolymer of DTT, which resulted in a lower frequency capacitance (CLF) value of 0.89 mF cm−2, the copolymer demonstrated a higher CLF value of 1.11 mF cm−2. A B3LYP/6-311+G(d,p) calculation on the mechanism of the formation of DTT in the presence of P4S10, indicated that, first, attack of the carbonyl oxygen on the P atom of P4S10 took place resulting in an electrophilic carbon, of which, in turn, intramolecular attack from an α-position of the thiophene furnished an intermediate and then the DTT.285 Schroth et al.26 followed a different methodology, starting from the thiophene (515, R = H). Oxidative coupling of 515 in the presence of n-BuLi and CuCl2 was followed by dibromination to obtain 3-bromo-2-(3-bromothien-2-yl)thiophene (517), treatment of which with n-BuLi and then addition of the elemental sulfur afforded 1,2-dithiin 519. This was converted into DTT (491) by treatment with copper bronze at 180−200 °C (Scheme 97). Following the same methodology, tetramethyl- and tetraphenyl-substituted DTTs 520 and 521 were synthesized. The DTT derivative 520 was synthesized, by a different methodology, in four steps in an overall yield of 17%. Monoiodination of 2,3-dimethylthiophene (515) was followed by dimerization in the presence of a nickel catalyst leading to the formation of bithiophene 516, which was subjected to dibromination and then dilithiation. The dilithiated intermediate was reacted with (PhSO2)2S to obtain the tetramethyl-DTT 520 (Scheme 98).287 Another alternative methodology for the synthesis of 2,6disubstituted DTTs was reported by Paradies,275 who utilized a two-step synthesis involving oxidative dehydrocoupling of 498b,c under Moris conditions followed by C−S cross coupling

Huang et al.273 investigated the electronic, optical, and conducting properties of 509 computationally at the DFT level (B3LYP/6-311G(d,p)). Internal reorganization energies for hole and electron (λ+ and λ−) were calculated to be 775 and 338 meV, respectively. The electron affinity of 509 was predicted to be −0.38 eV, and the vertical ionization potential (IPv) was 7.46 eV. Theoretical studies provided the HOMO and HOMO−LUMO gap values of −5.65 and 4.53 eV, somewhat larger than the experimentally obtained band gap of 3.80 eV.273 Hong et al.277 followed the same methodology to produce tetracarboxylic acid (512) from tetrabromothiophene (501) in three steps and in an overall yield of 20%. Double lithiation of 501, followed by introduction of oxoacetate groups, resulted in 510. Addition of ethyl 2-mercaptoacetate provided the corresponding tetracarboxylate (511), and hydrolysis of 511 furnished 512 (Scheme 95). Scheme 95. Preparation of Tetracarboxylic Acid-Substituted DTT (512)277

Ozturk and co-workers13,278−280 reacted 1,8-diketones (513a−e) with P4S10260 to synthesize DDTs, possessing aryl units (Ph, 4-MeOC6H4, 4-BrC6H4, and 4-O2NC6H4) in 53−95% yields (Scheme 96). Electropolymerization of 514a using a potentiodynamic method in DCM/acetonitrile gave a homopolymer with reduction and oxidation potentials 0.75 and 0.88 V (vs Ag/Ag+), respectively, which correspond to operating an electrochromic device between yellow and blue colors. Spectroelectrochemical investigations revealed the band gap of the thin film as 2.18 eV. Two absorption maxima were obtained, attributed to π−π* transitions. A device fabricated from its polymer and PEDOT afforded a good switching time BI


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Scheme 97. Use of Dithiin in the Synthesis of DTTs 491, 520, and 52126,286

Scheme 100. Double Ring Closure with SOCl2 Afforded DTT (525)288,289

Scheme 101. Synthesis of Benzannulated DTTs 528a−c from Diacetylenes 526a−c and Structures of 528d Possessing Two DTT Units290,408

Scheme 98. Synthesis of DTT 520 Starting from 515287

form the disulfides 527a−c in moderate to good yields (47%, 49%, and 80% for R = CF3, MeO, and H, respectively). Heating 527a−c, in the presence of copper nanopowder, furnished the benzannulated DTTs 528a−c in 58−84% yields. X-ray crystallographic analysis indicated an almost coplanar structure for 528a (R = H). Optical studies showed an absorption band between 340−420 nm. Huang et al.273 studied the electronic, optical, and conducting properties of 528a computationally at the DFT level (B3LYP/6-311G**). Internal reorganization energies for hole and electron (λ+ and λ−) were calculated to be 560 and 283 meV, respectively. The electron affinity of 528a was predicted to be 0.36 eV, and the vertical ionization potential (IPv) was 7.49 eV. The HOMO is at −5.77, in agreement with the experimental value (−5.94 eV), and the HOMO−LUMO gap is 4.04 eV. Xiong et al.290 studied the optical and electrochemical properties of π-extended nine-ringfused linear thienoacenes 528d (R1 = OC8H17, C6H13, C11H23) (Figure 21), possessing two DTT units (Scheme 101), with which, contrary to 528a−c and other thienoacenes, high device reproducibility and nearly no dependence on substrate temperatures were recorded. The highest performance was measured for the device, constructed from 528d (R1 = C11H23), with a mobility of 1.0 cm2 V−1 s−1 and an on/off ratio of 107. The maximum mobilities obtained from 528d (R1 = OC8H17 and C6H13) were 0.011 and 0.5 cm2 V−1 s−1, respectively.

Scheme 99. Synthesis of 2,6-Disubstituted DTTs (500b,c) Starting from 498b,c275

in the presence of palladium catalysts to form 500b,c in 71% and 36% yields, respectively (Scheme 99).275 In another method, reaction of 2,5-dibromothiophene (311) with ethyl acrylate gave the coupled product 524 in 83% yield, the reaction of which with thionyl chloride provided 525, through double ring closure, in 40% yield (Scheme 100).288,289 Okamoto et al.408 prepared 528a−c in a two-step synthesis, starting from diacetylenes 526a−c end-capped with o-bromophenyl units (Scheme 101). First, 526a−c were lithiated using 4 equiv of t-BuLi, followed by successive reactions with elemental sulfur and NaOH and K3[Fe(CN)6] to BJ


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Scheme 103. Oxidation of DTTs 491 and 529 with H2O2, and Structures of 531 and 532294,295,297,313

Figure 21. (Top) Absorption spectra of 528d: (a) R1, OC8H17 (green line, in DCM; blue line, on film); (b) R1, C6H13 (red line, in DCM; black line, on film); and (c) R1, C11H23 (violet line, in DCM; pink line, on film). (Bottom) Cyclic voltammograms of 528d: (a) R1, OC8H17 in hot ClPh; (b) R1, C6H13 on film; and (c) R1, C11H23 in hot ClPh (supporting electrolyte, Bu4NPF6; counter electrode, Pt wire; scan rate, 50 mV s−1; reference electrode, SCE for solution and Ag/AgNO3 for thin film).290 Reprinted (adapted) with permission from ref 290. Copyright 2014 American Chemical Society.

at the 2- and 6-positions, enhanced the PL quantum yield up to 48% in the solid state, which was explained as being due to arrangement in an antiparallel fashion rather than formation of cofacial dimmer (Scheme 103, Figure 22).297 Easy

Benzannulated (528a) can also be prepared in a three-step synthesis, from benzo[b]thiophene (74).291 Dibromination using Br2 followed by treatment with LDA and then copper chloride led to (3,3′-dibromo-[2,2′]-bis[benzo[b]thiophenyl]) (529). Reaction of 529 with n-BuLi and then bis(phenylsulfonyl)sulfide in diethyl ether provided the benzannulated DTT (528a) (Scheme 102). A large HOMO−LUMO gap Scheme 102. Benzannulated DTT (528a) from Coupled Intermediate 529291 Figure 22. Crystal structure of 531 (ac plane is depicted with blue parallelogram).297 Reprinted (adapted) with permission from ref 297. Copyright 2003 American Chemical Society.

modification of DTT-S,S-dioxide, making it possible to tune the fluorescence by using NCS or N-succinimidyl ester functional groups, was utilized for the applications in OLED devices and in biological systems.298,299 Instead of two methyl groups, as in 530b, an analogue substituted by two decyl units was synthesized by He402 in 36% yield, using m-CPBA. Barbarella et al.313 introduced dimesitylborane units to the 2- and 6-positions of 3,5-dimethyl-DTT-S,S-dioxide by reacting it with dimesitylboron fluoride to give 532 in 16% yield (Scheme 103). TD-DFT calculations at the B3LYP/SVP level on dimethyl-substituted DTT-S,S-dioxide (530b) and its bis(dimesitylboron) analogue 532 showed that while 530b has an excitation from HOMO to LUMO with an energy of 3.52 eV and an oscillator strength of 0.20, the HOMO− LUMO excitation of compound 532 was 2.82 eV with an oscillator strength of 0.72, which indicated that boron substitution resulted in a red-shifted excitation energy.314 Computational studies by Zhang et al.58 indicated DTT as a p-type semiconductor, considering its high electron injection barrier, to which introduction of electron-withdrawing groups changed its semiconduction property from p-type to n-type and to ambipolar. Investigation of the effect of halogen groups on the injection barrier, relative to the work function of Au electrodes, reorganization energy, and transfer mobility, revealed that the halogen units did not improve the injection barrier,

of 3.46 eV and a very low-lying HOMO of −5.60 eV provided 528a with a high oxidation stability in device applications. A high charge mobility of 0.51 cm2 V−1 s−1 with an on/off ratio larger than 106 for a vacuum-deposited device was recorded. Kugler et al.116 used dialkyl-substituted DB-DTT along with partially fluorinated fullerenes (C60F36 and C60F48) for OLED applications. The synthesis of thiophene-S,S-dioxide, through oxidation of thiophene with H2O2 or m-CPBA, provides thiophenes with increased electron affinity and fluorescence quantum yield. Unlike an aromatic thiophene, thiophene-S,S-dioxide has diene properties.292,293 Attachment of two oxygen atoms to a DTT and its methylated derivative with H2O2 leads to DTT-S,Sdioxides in 57% (530a)294 and 36% (530b)295 yields, respectively (Scheme 103). These had absorption bands of 350 and 364 nm in dichloromethane and of 357−360 nm in the solid state, respectively. Higher photoluminescence (PL) quantum yields of 75% and 77% were obtained for 530a and 530b, respectively, in dichloromethane, which were considerably decreased to 12% and 16% in the solid state, due to the formation of the corresponding excimers. Their X-ray single-crystal analyses suggested that the excimers are developed through a cofacial sandwich-type arrangement with interplanar distances of ca. 3.7 Å.296 However, 531, possessing 3-methylthiophen units BK


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whereas they resulted in an increase of the reorganization energy and, hence, diminished transfer mobility. HOMO and LUMO energy levels of −5.60 and −1.09 eV, respectively, were obtained from calculations at the B3LYP/6-31G(d) level. The band gap energy derived from the difference between the energy levels of HOMO and LUMO was 4.52 eV. They also calculated the hole and electron reorganization energies (λ+ and λ−) of 0.35 and 0.32 eV, lower than those of TT (1) (λ+ = 0.41 eV and λ− = 0.68 eV) and hole and electron-transfer mobilities (μ+ and μ−) of 0.34 and 0.37 cm2 V−1 s−1, respectively, higher than those of TT (1) (μ+ = 0.13 and μ− = 0.006 cm2 V−1 s−1). 3.1.2. Synthesis and Applications of Oligomers and Polymers. Li et al.300 synthesized the α-linked DTT dimer (533), the crystal structure of which showed it to have a completely planar structure, a well-ordered face-to-face π−π stacking and strong intermolecular S···S interactions (Figure 23). Abstraction of α-proton with BuLi and then

Figure 24. Crystal structure packing of β-linked dimer of DTT (538).69 Reprinted (adapted) with permission from ref 69. Copyright 2007 American Chemical Society.

replaced with Me3Sn to obtain 2-(trimethylsilyl)-5-(trimethylstannanyl)-DTT (536) in 79% yield. Stille coupling of 536 with 535 in the presence of Pd(0) in toluene afforded the dimer 537, and desilylation with TBAF afforded the β,β-dimer-DTT (538) in 98% yield (Scheme 105). Scheme 105. Dimerization of 500a through β-Positions69

Figure 23. (Left) Single-crystal packing view of 533 perpendicular to the bc plane. (Right) The absorption and emission spectra of 533. UV−vis was recorded in dilute CHCl3. PL excitation wavelengths for solution and film are 390 and 340 nm, respectively.300 Reprinted (adapted) with permission from ref 300. Copyright 1998 American Chemical Society.

Scheme 104. Dimerization of 491 through α-Positions Furnishing 533300 For OFET applications, Sun et al. 301 synthesized α-substituted DTT derivatives 541a−c, through Pd-catalyzed coupling of 2,6-dibromo-DTT (539) with thien-2-yl- (540a), phenyl- (540b, R = H), and biphenylboronic acids (540c) (Scheme 106). UV−vis spectra of all of the products 541a−c displayed a small blue shift in their solid-state spectra with respect to measurements in THF. However, significant differences were detected in the emission spectra due to a possible aggregation or excimer formation. Large optical band gaps for 541a−c were estimated, indicating their stability against oxidation. All three compounds formed highly crystalline films, and 541b (R = H) showed the highest charge carrier mobility of 0.42 cm2 V−1 s−1 and an on/off ratio of 5 × 106. Liquied crystal properties of 541a possessing different lengths of alkyl chains varying from C7H15 to C18H37 at both end points of 541a were also investigated, which indicated a strong influence of the length of the alkyl chain on the mesomorphic behavior.302 Huang et al.273 studied the electronic, optical, and conducting properties of 541b (R = H) computationally at the DFT level (B3LYP/6-311G(d,p)). Internal reorganization energies for hole and electron (λ+ and λ−) were calculated to be 319 and 435 meV, respectively. The electron affinity of 541b (R = H) was predicted to be 0.61 eV, which is below the threshold value of 2.8 or 3.0 eV, and vertical ionization potential (IPv) was obtained as 6.72 eV, matching well with

oxidation with Fe(acac)3 gave 533 in 57% yield (Scheme 104). DSC measurement gave a high melting point of 316 °C for 533, as compared to that (67 °C) of DTT (491). The HOMO− LUMO gap, estimated from the absorption band edge of 440 nm in solution, was found to be 2.8 eV, whereas Egap, calculated from the emission peak appearing at 547 nm in the solid state, is 2.3 eV. Good charge carrier mobilities of 0.02−0.05 cm2 V−1 s−1 were obtained for 533 in OFET devices. The β,β′-linked dimer (538) was synthesized by Matzger et al.69 The two linked DTT units were demonstrated to be in the same plane and had face-to-face stacking with an intermolecular distance of 3.57 Å (Figure 24). A blue shift on the absorption maximum was observed as compared to the α-linked dimer (533), due to cross-conjugation between the fused rings. Reaction of 500a with NBS in DMF resulted in 2-bromo-6-(trimethylsilyl)-DTT (534) in 55% yield, the bromine atom of which was then rearranged to the β-position upon its reaction with lithium diisopropylamide (LDA) in THF, yielding compound 535. The bromine atom of 535 was BL


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(540e). Chen and co-workers306,307 studied the adsorption of DTT molecules 541e and 541h, which had one and two pentafluorophenyl substituents, respectively, on a Au(111) electrode. Like 541b (R = H), 541h was arranged in a lamella structure (Figure 25), whereas 541e had a different arrangement in which dipoles were aligned in opposite directions to their neighbors, according to experimental results from in situ scanning tunneling microscopy (STM) measurements in perchloric acid. Mieno et al.308 synthesized the organic semiconducting oligomers 541f,g having comparatively large band gaps and low-lying HOMO energy levels (2.6/−5.12 and 2.5/−5.06 eV, respectively), which rendered good environmental stability. X-ray crystallographic analysis indicated strong intermolecular S···S interactions between two 541f molecules, which provided high charge carrier mobility. The OFET device constructed using single crystals of 541f resulted in a good electrical performance with a mobility of 0.54 cm2 V−1 s−1. On the other hand, the device fabricated using 541g realized only 0.05 cm2 V−1 s−1 of charge mobility. Chen and co-workers272 prepared 2,6-diperfluorobenzoyl functionalized DTT 542 using n-BuLi and AlCl3, considering that the DTT, having perfluorobenzoyl unit, can demonstrate a good n-type property (Scheme 107). While treatment of 491 with n-BuLi and then addition of perfluorobenzoyl chloride provided 542 in 36% yield, AlCl3 gave 542 with lower yield (20−30%), along with the monofunctionalized DTT 543 (30% yield), which was further reacted with benzoyl chloride in the presence of AlCl3 to furnish asymmetric DTT 544 in 57% yield. They synthesized benzoyl- and 2-thienoyl-substituted DTTs 545 and 546 applying the same strategy in 63% and 41% yields, respectively. Optical investigations revealed the redshifted λmax appearing at around 400 nm in o-dichlorobenzene with respect to the absorption maximum of 454 recorded around ∼300 nm. The optical gaps of 542, 544−546, obtained from the onset of the absorption peaks, were reported to be 2.86, 2.88, 2.93, and 2.82 eV, respectively, while differential pulse voltammograms provided the electrochemical band gaps as 2.97, 3.07, 3.08, and 2.99 eV in o-dichlorobenzene. The semiconductor properties of 542, 544−546 were investigated in top-contact OFETs, fabricated by vapor-deposition

Scheme 106. Synthesis of 2,6-Diaryl-DTTs (541a−g) from Suzuki Coupling Reactions and the Structure of 541h301−306,308

the expermintal IPv with a deviation of less than 0.07 eV. Theoretical studies provided the HOMO value of −5.43 eV and the band gap of 3.59 eV;273 the experimentally obtained band gap was 3.1 eV.301 Zhu et al.303 demonstrated the effect of alkyl units, which were substituted at the para positions of the phenyl groups, located at 2- and 6-positions of DTT, on the charge mobility, using various alkyl groups (R = CH3, C4H9, C6H13, and C8H17). These had almost the same band gaps of 2.5 eV (HOMO, −5.5 eV; LUMO, −3.0 eV). However, 541b (R = Me) afforded the highest charge mobility of 0.54 cm2 V−1 s−1 and an on/off ratio of 2.2 × 106 as compared to the other alkyl-substituted DTTs. The octyl-substituted 541b (R = C8H17) single-crystal microribbon had a mobility of 1.1 cm2 V−1 s−1. Fluorene-substituted DTT (541d), synthesized by Iosip et al.,304 displayed reversible oxidation waves and suitable HOMO energy levels (Scheme 106). Takimiya et al.305 prepared pentafluorophenyl-substituted DTT (541e) from the reaction of 539 with pentafluorophenylboronic acid

Figure 25. (Left) Crystal structure of 541b (R = H). (A) Front view of the molecular stacking, (B) side view of herringbone packing, and (C) top view of the molecular stacking with the shortest S···S distance of 3.42 Å. (Right) Crystal structure of 541h. (A) Top view of molecular stacking, (B) front view of molecular stacking, (C) side view of molecular stacking with the S···S stacking distances of 3.76 Å, and (D) intermolecular F···H and S···S distances with the shortest S···S distance of 3.53 Å.307 Reprinted (adapted) with permission from ref 307. Copyright 2014 The Royal Society of Chemistry. BM


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para positions of phenyl units of 514a was synthesized310 and electropolymerized onto a glassy carbon electrode in sodium dodecyl sulfate solution together with multiwalled carbon nanotubes. Electrochemical impedance spectroscopy demonstrated a good capacitive property for 548c composite. The specific capacitance of the composite was calculated to be 20.17 F g−1. Electrochemical investigations indicated a longterm stability for the capacitor with a decrease of the specific capacitance by only around 13% after 500 repeat cycles. Improvement in the long-term redox stability of DTTs for electrochromic and electroconductive applications was attempted by introducing bulky aryl substituents at the 2- and 6-positions of DTT, which had a large cage structure. Stability over 17 × 103 and 20 × 103 times in repeated oxidation was demonstrated for compounds 549 and 550, respectively (Chart 36).311

Scheme 107. Synthesis of 2,6-Difunctionalized DTTs 542, 544−546 from 491272

Chart 36. Compounds 549 and 550 Having a Long-Term Redox Stability Achieved by Bulky Aryl Substituents311

method, which indicated the n-type charge carrier property of 542 and 544 with a mobility of ∼0.03 cm2 V−1 s−1. The others (545 and 546) were classified as p-type semiconductors with mobilities of 0.01 and 2.0 × 10−4 cm2 V−1 s−1, respectively. Ozturk and co-workers309 attached two 2-thienyl units to the 2- and 6-positions of DTT (514a), through bromination to obtain 2,6-dibromo-3,5-diphenyldithieno[3,2-b;2′,3′-d]thiophene (547a) and then Suzuki coupling with a thiopheneboronic ester to furnish 3,5-diphenyl-2,6-dithiophene2-yl-dithieno[3,2-b;2′,3′-d]thiophene 548 (Scheme 108).

Cicoira et al.312 synthesized a DTT derivative (553), possessing two bithiophenyl units at 2- and 6-positions in 52% yield using microwave-assisted Suzuki coupling of 3,5-dimethyl-2,6-diiodo-DTT (551) with bithiophenylboronic acid ester (552) (Scheme 109). They fabricated organic

Scheme 108. Incorporation of Thienyl Groups to DTTs 548a,c309,310

Scheme 109. Preparation of DTT 553 Having Bithiophenyl Units at 2- and 6-Positions312

light-emitting transistors (OLETs) by both vacuum sublimation and drop-casting methods. FET measurements indicated that the method of deposition of the active layer affected the FET properties considerably. While vacuum-sublimed film displayed a charge mobility of ∼2 × 10−2 cm2 V−1 s−1 and an on/off ratio of ∼106, solution-processing resulted in a mobility of ∼1 × 10−2 cm2 V−1 s−1 and an on/off ratio of ∼10. A possible reason for the diminished mobility of the device was explained to be inefficient control of the drop-cast process. Barbarella et al.313 replaced bithiophenyl groups with dimesitylborane units by reacting 3,5-dimethyl-DTT (529)

Electrochemical investigations revealed the oxidation and reduction potentials of 0.90 and 0.75 V (vs Ag wire reference electrode), respectively, and its corresponding polymer was obtained by the potentiodynamic method, the electronic band gap for which was 1.94 eV. This was lower than that of the corresponding polymer of 514a, due to the extended π-conjugation of polymer 548a through its thiophenes. A device constructed from this polymer and PEDOT was operated in a potential range of 0.0−2.0 V, providing red and blue colors. 548c with four 2-thienyl groups located at α-positions and BN


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Scheme 110. Addition of Boron Acceptor to DTT 529313,314

Interplanar distance was recorded as 2.23 Å, emerging from the intermolecular S···S interactions. While their optical studies revealed the optical band gaps of 2.40 and 2.58 eV for 558a and 558b, respectively, electrochemical investigations showed the HOMO energy levels of −5.39 and −5.22 eV, respectively, indicating higher stabilities under ambient conditions. Compound 558a demonstrated a good OFET property with a charge mobility of 0.17 cm2 V−1 s−1 and an on/off ratio of ∼105 at 70 °C. The carrier mobility, measured for the device fabricated with 558b, was reported to be 0.013 cm2 V−1 s−1 with an on/off ratio of ∼104 at the same temperature. Navarette315 and Kim316 used a DTT unit as a conjugated spacer to form D−π−D and D−π−A systems, 559a−d, for nonlinear optical materials (Chart 37). The optical invesChart 37. D−π−D and D−π−A Systems315−321,323

with dimesitylborane fluoride, resulting in bis(dimesitylboryl)DTT (554) and dimesitylboryl-DTT (555) as a byproduct in 43% and 15% yields, respectively (Scheme 110). Compound 554 displayed broad blue-green and weak red emissions. Its OLED exhibited both broad blue-green emission arising from the isolated molecules and narrow red emission (at 680 nm). Superposition of both emissions provided white light up to 3800 cd m−2 with the external quantum efficiency (EQE) of 0.35%. TD-DFT calculations, performed on 529 and 554 to unveil the electronic excitations, indicated that while the electronic excitation of 529 was obtained from HOMO to LUMO with an energy of 4.29 eV and oscillator strength of 0.37, the HOMO−LUMO excitation for compound 554 had an energy of 2.98 eV with an oscillator strength of 0.98, indicating that boron substitution resulted in a red-shifted excitation energy.314 Liu et al.395 synthesized two DTTs end-capped with styrene, possessing H (558a) and alkyl units (558b) at the para position, through Stille coupling reactions of 557a and 557b. They were prepared by initial treatment of aryl acetylene with HSnBu3, and then Stille coupling of the resultant 557a and 557b with 2,6-dibromo-DTT to obtain 558a and 558b in 65% and 70% yields, respectively (Scheme 111). While their TGA Scheme 111. Preparation of DTTs End-Capped with Styrene (558a) and Alkyl-Substituted Styrene (558b) through Stille Coupling395

tigations in DCM revealed four absorption maxima for both compounds 559a,b at 391, 419, 439, and 464 nm, and 393, 429, 450, and 474 nm, respectively (Figure 26). The experimentally obtained band gap of 559a was 2.82 eV and that of 559b was 2.65 eV, which were supported by computational studies. TD-DFT at the B3LYP/3-21G(d) level demonstrated excitation energies of 2.59 eV with an oscillator strength of 2.49 and 2.39 eV with an oscillator strength of 2.45 for 559a and 559b, respectively. These spacers were also utilized for

demonstrated decomposition temperatures of 343 and 359 °C for 558a and 558b, DSC indicated endothermic peaks at 288 and 221 °C, respectively. Single-crystal structure of only 558a was achieved, which showed the trans−trans configuration with a twisted geometry having a dihedral angle of 130 °C. BO


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Chart 38. Organic Dyes 562a−c324

Figure 26. (Left) Normalized absorption (dashed lines) and emission (solid lines) spectra of 559a,b in DCM.315 Reprinted (adapted) with permission from ref 315. Copyright 2004 American Chemical Society. (Right) UV/vis and PL spectra of 560 in DCM.318 Reprinted (adapted) with permission from ref 318. Copyright 2011 American Chemical Society.

(386 and 374 nm), emerged from weak π−π* transitions. Their PL emissions were 557 (562a), 624 (562b), and 610 (562c) nm. The optical band gaps were estimated to be 2.45, 2.19, and 2.15 eV for 562a−c, respectively. Electrochemical studies demonstrated HOMO/LUMO energy levels as −5.70/ −3.25, −5.26/−3.07, and −5.32/−3.17 eV, for 562a−c, respectively. Compound 562b has mesomorphic properties due to the high proportion of flexible side-chains to the stiff backbone. DSSC of 562b provided the PCE of 3.72% with Voc of 0.58 V and Jsc of 9.98 mA cm−2. The PCEs of 562a,c were 2.69% and 3.82%. Youn et al.325 synthesized various benzo[d,d′]thieno[3,2b;4,5-b′]dithiophene (BTDT) derivatives via a one-pot reaction method mentioned in an early section115 in 35−85% yields and examined the molecular packings of them by considering both bulk 3D crystals and thin films (Chart 39). All of the

two-photon absorption materials.317 The asymmetric molecules 559c and 559d, having D−π−A systems, had small red-shifted (∼5 nm) absorption maxima with respect to symmetric 559a and 559b possessing a D−π−D system, due to the charge transfer in the excited of 559c and 559d.316 A DTT derivative 560, having D−π−A system, was synthesized for a dyesensitized solar cell (DSC). HOMO and LUMO energy levels were estimated to be −5.15 and −2.88 eV, respectively. While absorption maxima of 301, 423, and 489 nm were observed in optical studies, PL emission was obtained at 613 nm (Figure 26). 560 provided a PCE of 7.3% along with open-circuit voltage (Voc) of 697 mV, short-circuit photocurrent density (Jsc) of 14.4 mA cm−2, and fill factor (FF) of 0.73 (Chart 37).318 A uniform, ultrathin inverse opal (IO) electrode, sensitized with 560, furnished PCE of 5.78% with open-circuit voltage (Voc) of 67 mV, short-circuit photocurrent density (Jsc) of 14.3 mA cm−2, and fill factor (FF) of 0.60.319 Later, 560 was prepared via a continuous flow synthesis, using a commercial benchtop flow reactor.320 Akhtaruzzaman et al.321 synthesized three organic dyes, 561a−c, possessing donor−acceptor systems, linked with DTT to explain the effect of electron donors on photocurrent and photovoltage for dye-sensitized solar cell (DSC) applications. Absorption maxima were 428, 481, and 485 for 561a−c in ethanol, respectively. The highest efficiency of 6.38% for the photovoltage of 561c indicated that indoline is a better donor than carbazole (Chart 37). Cheng et al.322 investigated the similar compounds, lacking of olefinic spacer, and reported that DTT-S,S-dioxide shows a better performance with respect to the corresponding DTT. Qin et al.323 synthesized an organic sensitizer (C203) 561-d, which had an emission peak at 718 nm and an excitation transition energy of 2.04 eV. Its device, fabricated using an acetonitrile-based electrolyte, demonstrated, under an irradiance of AM 1.5G full sunlight, the short-circuit photocurrent density (Jsc) of 14.33 mA cm−2, open-circuit photovoltage (Voc) of 734 mV, fill factor (FF) of 0.76, and yielded an overall conversion efficiency (η) of 8.0%. The other device of 561-d, fabricated with a solvent-free ionic liquid electrolyte, provided Jsc of 14.06 mA cm−2, open-circuit photovoltage (Voc) of 676 mV, fill factor (FF) of 0.74, and yielded an overall high conversion efficiency (η) of 7.0%. Shellaiah et al.324 synthesized the organic dyes 562a−c, possessing 3,4,5-tris(dodecyloxy)phenyl as electron-donor and cyanoacrylic acid as electron-acceptor units, which were linked with a DTT π-spacer (Chart 38). The absorption maxima were 443, 386/476, and 374/473 nm in dilute THF for 562a−c, respectively. The longer wavelengths were attributed to intramolecular charge transfer (ICT) from the donor (3,4,5-tris(dodecyloxy)phenyl) to the acceptor (cyanoacrylic acid), whereas the shorter wavelengths, observed for 562b,c

Chart 39. BTDT Derivatives 563a−e325

compounds had high melting points above 269 °C, and were thermally stable up to 366 °C, except 563a and 563d, which had a weight loss at 287 and 247 °C, respectively. While optical studies, conducted in o-dichlorobenzene, revealed a range of absorption maxima between 351 and 411 nm, electrochemical investigations, performed in the same solvent at 60 °C, provided the oxidation potential peaks varying from 1.14 to 1.61 V, and the reduction potentials recorded between −1.75 and −1.98 V (vs a nonaqueous Ag reference electrode). Optical and electrochemical band gaps of 563a−e were reported to be 3.21/3.37, 3.07/3.28, 3.08/3.27, 3.22/3.36, and 2.74/2.89 eV, respectively (Table 14). The results, obtained from single-crystal Table 14. Physical Properties of 563a−e325 compound

Eoxa,b [V]

Ereda,c [V]

HOMO [eV]

LUMO [eV]

EgCV [eV]

EgOpt [eV]

563a 563b 563c 563d 563e

1.39 1.30 1.31 1.61 1.14

−1.98 −1.98 −1.96 −1.75 −1.75

−5.59 −5.50 −5.51 −5.81 −5.34

−2.22 −2.22 −2.24 −2.45 −2.45

3.37a 3.28a 3.27a 3.36a 2.89a

3.21a 3.07a 3.08a 3.22a 2.74a

a

A nonaqueous Ag reference electrode was used. obenzene at 60 °C. cIn o-dichlorobenzene. BP

b

In o-dichlor-


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Chart 40. π-Extended Seven- (568) and Nine-Ring-Fused Linear Copounds 569a−c328,329

diffraction and grazing incidence wide-angle X-ray scattering (GIWAXS) measurements, showed two strained lattices with respect to single-crystal motifs for phenylbenzo[d,d′]thieno[3,2-b;4,5-b′]dithiophene (563a), 2-biphenylbenzo[d,d′]thieno-[3,2-b;4,5-b′]dithiophene (563b), 2-naphthalenyl benzo[d,d′]thieno[3,2-b;4,5-b′]dithiophene (563c), and 2-pentafluorophenylbenzo[d,d′]thieno[3,2-b;4,5-b′]dithiophene (563d). Compound 563a, possessing more strained lattice as a major part relative to the bulk-like lattice, might contribute to the high carrier mobility. On the other hand, poor crystallinity caused a decreased OFET efficiency. Cheng et al.326 blended 563a with p-octylphenyl, possessing 563, in a ratio of 0.33/1.00, which demonstrated a relatively high hole mobility of 0.65 cm2 V−1 s−1 with an on/off ratio of 104. Skabara and co-workers327 performed the synthesis and investigated electrochemical and optical properties of three oligomers 565−567, possessing diindenodithienothiophene (Scheme 112). The absorption maxima of the oligomers

Scheme 113. Synthesis of Linear Fused DTT-Diimides 571a−d277

Scheme 112. Preparation of Oligomers 565−567327

provided 570 in 75% yield. Treatment of 570 with amines furnished 571a−d in 46−59% yields (Scheme 113). Optical studies, conducted in DCM, indicated red-shifted absorption maxima of 405−410 nm with respect to DTT (512), which had a λmax of around 300 nm. The optical band gaps were estimated from the onset of the absorption maxima and were found to be between 2.80 and 2.84 eV. Electrochemically estimated LUMO energy levels were −3.52, −3.56, −3.66, and −3.62 eV for 571a−d, respectively. DFT calculations at the B3LYP/ 6-31G(d) level, to explain the effects of diimide extension on frontier molecular orbitals, demonstrated reduction of the HOMO and LUMO energy levels by fusing the diimide unit onto the DTT core. X-ray crystallographic analysis showed a layered herringbone arrangement with a herringbone angle of ∼55.7°, which possessed S···π and C−O···π interactions. Marco et al.90 synthesized donor−acceptor (D−A) systems 572 and 573, possessing DTT units as π-spacer, which had nonlinear optical (NLO) properties (Chart 41). TGA analyses

565−567 were reported to be 284, 273, and 419 nm, respectively, and while their optical HOMO−LUMO gaps were 3.4, 3.4, and 2.6 eV, those obtained from the electrochemical studies were recorded to be 3.3, 3.5, and 2.8 eV, respectively. Cyclic voltammetry indicated the enhancement of the stability for the oligomer with alkyl groups (566). Corresponding S,S-dioxide (567) demonstrated strong photoluminescence with a quantum yield of 0.72 in solution, but only 0.14 in the solid state, possibly due to aggregation-induced quenching. The field-effect transistor fabricated using oligomer 565 provided a hole mobility of around 10−4 cm2 V−1 s−1. The OFET properties of π-extended seven-ring-fused linear compound 568,328 having two DTT units fused to benzene, and nine-ring-fused linear compounds 569a−c329 were investigated (Chart 40). While the hole mobility of 2.6 × 10−4 cm2 V−1 s−1 was recorded for 568, the devices constructed with 569a and 569b provided mobilities of 0.011 and 0.5 cm2 V−1 s−1, respectively. The highest value was obtained for 569c to be 1.05 cm2 V−1 s−1, due to the disorientation of the grains detected from GIXRD analysis, which had a significant role in the OFET performance. Hong et al.277 prepared linear fused DTT-diimides from tetracarboxylic acid-substituted DTT (512) in a two-step synthesis. Dehydration of 512 in refluxing acetic anhydride

Chart 41. Donor−Acceptor (D−A) Systems 572 and 57390

showed them to be thermally stable compounds. From electrochemical investigations in CH2Cl2, the oxidation/reduction potentials of 572 and 573 were +0.55/−0.84 and +0.54/ −0.63 V (vs Ag/AgCl), respectively, a trend supported by computational studies. Calculations at the B3P86/6-31G(d) BQ


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Scheme 114. Star Oligomer 577 from 49183

level predicted HOMO and LUMO energy levels for 572 as −5.61 and −3.45 eV, and for 573 as −5.75 and −3.87 eV. Absorption λmax values were at longer wavelengths, indicating intramolecular charge transfer (ICT). The observed absorption maxima in DCM for 572 and 573 were 659 and 708 nm, respectively; thus compound 573 is a stronger acceptor than 572. Muraoka et al.330 investigated the electrochemical properties of DTT 574 substituted by two ferrocene units at the 2 and 6 positions. Double redox waves with ΔE1/2 of 81 mV were observed at +0.25 and +0.33 V depicting the electronic communication of two metal centers in the mixed-valence state. Shah et al.331 used DTT together with acetylene units as a π-spacer between two ferrocene units to have long-range intramolecular interactions, indicating the ΔE1/2 value of 575 to be 52 mV, despite the large distance of about 14 Å between the two redox centers (Fe) (Chart 42). Optical studies showed λmax Chart 42. DTT as a π-Spacer between Two Ferrocene Units330,331

Horner−Emmons reaction, afforded 577 (Scheme 114). The oligomer has good solubility in common organic solvents. DSC analysis demonstrated a broad melting point, starting at 240 °C, indicating a thermal decomposition. While optical investigations revealed a λmax of 426 nm for the oligomer 577 in chloroform, the emission maxima for its thin film were at 523 and 547 nm, resulting in an optical band gap of 2.20 eV (Figure 28).

Figure 27. (Left) Simulation curves (red circles) matched with cyclic voltammograms (black line) for 575 in dichloroethane (DCE) at 25 °C (supporting electrolyte: 0.1 M [n-Bu4N]PF6; scan rate, 100 mV s−1). (Right) UV−vis spectra of 575 in a DCE at different potentials applied: (i) neutral, (ii) 630 mV, and (iii−v) 600, 630, and 660 mV, respectively (supporting electrolyte, [n-Bu4N]PF6).331

Figure 28. Absorption (UV/vis) and emission (PL) spectra of 577. The PL excition wavelength was 400 nm (solid line, solution; dashed line, pristine film; dash-dot-dashed line, annealed film; PL spectra were taken in solution states).83 Reprinted (adapted) with permission from ref 83. Copyright 2007 American Chemical Society.

values of 322, 370, 396, and 478 for neutral molecule and 265, 302, 342, 510, and 1020 for the radical cation (Figure 27). λmax values smaller than 400 nm emerged from the π−π* electronic transition, and 478 nm belonged to the FeII d−d electronic transition. While intervalence charge transfer (IVCT) appeared at 1020 nm in the case of radical cation, metal to ligand charge transfer (MLCT) was assigned to 510 nm. Choi et al.83 synthesized a star oligomer for OFET applications, possessing a phenyl unit in the center, linked to four alkylated DTTs through ethylene spacers. The reaction of DTT (491) with n-BuLi and then with 1-hexyl bromide, followed by a formylation reaction with n-BuLi and DMF, formed 5-hexyl-2-carbaldehyde-DTT (576). Subsequent treatment of 576 with [1,2,4,5-tetra-(diethoxy-phosphorylmethyl)benzyl]-phosphonic acid diethyl ester (154), applying a

Electrochemical HOMO and LUMO energy levels were −5.25 and −3.05 eV, respectively, providing an electrochemical band gap of 2.20 eV, in good agreement with the optical band gap. A higher charge carrier mobility of 2.5 × 10−2 cm2 V−1 s−1 with an on/off ratio of >103 was observed for the oligomer 577. Wang et al.332 studied the photochromic properties of unsymmetrical compounds 578−581, possessing DTT units (Chart 43). Their X-ray crystal structures indicated that each has antiparallel arrangements. Electrochemical studies demonstrated the HOMO−LUMO energy gaps for 578−581 to be 2.43, 2.46, 2.79, and 2.69 eV, respectively, whereas those for the closed compounds 578c−581c, obtained by irradiation with UV and visible light at certain wavelengths (365 nm), and compounds 578−581 under visible light at 500 nm, were 2.15, 1.99, 2.14, and 2.10 eV, respectively, showing that the closed BR


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Arbizzani et al.337 carried out the electropolymerization of DTT on transparent conductive tin oxide covered glass and studied the polymer electrochemically. While p-doped polymer had an onset potential of 0.37 V, that for n-doped polymer was −1.23 V (vs Ag). Spectroelectrochemical measurements revealed λmax of 480 nm with an optical band gap of 1.76 eV. The obtained optical band gap of 1.68 eV was in good agreement with that predicted from quantum chemical calculations (1.51 eV). Electrochemical explorations revealed a band gap of 1.69 eV.338 DTT and DTT-S,S-dioxides were reacted with thiophene to make copolymers 588 and 589 for photovoltaic applications. Thien-2-yltributylstannane (584) was coupled with dibromoDTT (539) or dibromo-DTT-S,S-dixode (585) to synthesize the monomers 586 and 587, respectively. Polymerization using FeCl3 provided the soluble copolymers 588 and 589, respectively (Scheme 115). The molecular weight of 588

Chart 43. Photochromic Compounds 578−583332,333

Scheme 115. Synthesis of Copolymers 588 and 589 from Monomers 586 and 587, Respectively, in the Presence of FeCl3339,340 compounds 578c−581c had smaller band gaps as compared to their open analogues 578−581. These compounds demonstrated good photochromisim in both solution and solid state. Optical investigations of diarylethene (582) and its dimer (583) demonstrated λmax values and the optical band gaps for 582 and 583 to be 364 and 384 nm, and 3.01 and 2.86 eV, respectively. However, electrochemical studies showed smaller band gaps of 2.61 and 2.72 eV, in the same order. Diarylethene dimer 583, possessing two sides to allow ring closure reactions, exhibited higher fluorescent efficiency due to the formation of the corresponding one-side closed compound under UV light (380 nm).333 Fujitsuka et al. 334,335 investigated the photochemical polymerization of 491 in the presence of an appropriate electron acceptor such as p-dinitrobenzene (DNB) and CCl4, using nanosecond laser flash photolysis and time-resolved fluoresence spectroscopy. They suggested that photoinduced electron transfer from the singlet excited state of 491 to an electron acceptor caused formation of a radical cation of 491 and a radical anion of DNB, which were proved by transient absorption spectra in the visible and near-IR regions (Figure 29). was determined by size exclusion chromatography as 36 000. A C60 blend of copolymer 588 demonstrated a photoinduced electron transfer; copolymer 589 had a good photoluminescence quenching in copolymer/inorganic composites.339,340 Neo et al.341 prepared the highly soluble and thermally stable conjugated polymers poly(2,6-(dithieno[3,2-b:2′,3′-d]thiophenyl)-alt-2,5-(3,4-dialkoxy)thiophenyl) (591a,b) from Stille coupling of 539 with 2,5-bis(trimethyltin)-3,4dialkoxythiophene (590a,b) (Scheme 116). The molecular weights (Mn/Mw) were reported to be 29 100/49 600 (PDI = 1.70) for 591a and 35 400/62 500 (PDI = 1.76) for 591b.

Figure 29. (Left) Transient absorption spectrum of 491 at 100 ns after laser excitation by XeCl excimer laser (308 nm). Inset: Absorption− time profile at 380 nm excited by Nd:YAG laser (266 nm). (Right) Absorption spectrum of radical cation of 491 generated by γ irradiation in freon solution at 77 K. Lower: Transition energies and oscillator strengths calculated by the ZINDO method.335 Reprinted (adapted) with permission from ref 335. Copyright 1997 American Chemical Society.

Scheme 116. Preparation of Copolymers 591a,b, Containing DTT and Dialkoxythiophene Monomers341

Gustafson et al.336 used a polymer of 491 together with PEDOT to prepare an alloy photoelectrocatalyst for water oxidation, which provided an enhanced water oxidation reaction. BS


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Chart 44. Oligomer 59591 and Polymers 596−598174,344−347

Optical studies revealed the UV−vis absorption and PL emission with PL quantum yield (ΦPL) as 518, 573 nm, and 2.7% for 591a, and 514, 574 nm, and 2.6% for 591b, respectively. The optical band gaps of 591a,b were estimated to be 1.75 and 1.79 eV, in the same order, which were well aligned with the electrochemical band gaps of 1.75 and 1.89 eV. Polymer films, which were purple in a neutral state, changed to gray and then to transmissive light blue when oxidized. The polymer having decyl grops (591a) showed high optical contrasts of 46% in the visible region and 72% in the near-infrared region. An electrochromic device fabricated from 591a demonstrated a contrast ratio of 15% at λmax = 534 nm with coloration efficiency of 449 cm2/C and 20% at 1321 nm with coloration efficiency of 471 cm2/C. Wang et al.342 prepared soluble poly(3,5-didecanyldithieno[3,2-b:2′,3′-d]thiophene-alt-9-fluorenone (594) from the reaction of bis(trimethylstannyl)-DTT (592) and dibromo-9fluorenone (593) using Stille cross-coupling polymerization in the yield of 57%. It had a high glass transition temperature (152 °C) and low molecular weight (Mn/Mw) of 9.1/ 12.8 kg mol−1 (PDI = 1.41) (Scheme 117). Polymer 594 in Scheme 117. Preparation of Copolymer 594, Possessing DTT and Fluorenone342

THF solution and as a thin film demonstrated a broad absorption band up to 620 nm with a λmax value of 356 nm in THF, and a red-shifted absorption up 700 nm in the solid state. The optical band gap obtained from the thin film was 1.68 eV. Electrochemical investigations gave HOMO and LUMO energy levels as −5.09 and −3.41 eV, respectively, and an electrochemical band gap of 1.68 eV, in good agreement with the optical band gap. A device fabricated from 594 and PCBM afforded a PCE of 0.374%. Annealing at 175 °C improved the efficiency of the device up to 0.923%. Lu et al.91 synthesized the oligomer 595, having a D−A system, end-capped with thiophene units (Chart 44). Thermal analysis indicated that the oligomer had a degradation temperature of 203 °C, and DSC revealed crystallization at 160 °C. Electrochemical investigations demonstrated the HOMO/ LUMO energy levels and the electrochemical band gap to be −5.08/−3.27 and 1.81 eV, respectively, lower than the corresponding TT analogue, indicating a more effective electrondonating property of DTT as compared to a TT unit. A thin film of the oligomer showed λmax 625 nm, providing an optical band gap of 1.61 eV, supported by B3LYP/6-31G(d,p) level calculations. OFET devices, fabricated using the oligomer, gave a hole mobility of 4.78 × 10−3 cm2 V−1 s−1 with an on/off ratio of >107. Park et al.343 investigated the effects of solvent additives 1,8-diiodooctane (DIO) and 1-chloronaphthalene (CN) on the energy levels, OFET properties, and morphology

prepared from derivative of 595, having 5-hexyl-2-thienyl substitutents at both ends. This showed that the additives did not influence the HOMO and LUMO levels of the oligomer and its blend with PC71BM. On the other hand, the efficiency of the charge transport was increased and the morphology of the film was changed, hence improving the device performance. For example, the use of CN increased the hole mobility from 8.6 × 10−5 cm2 V−1 s−1 with an on/off ratio of 1.8 × 105 (without additive) to 5.3 × 10−4 cm2 V−1 s−1 with an on/off ratio of 8.7 × 105. Lee et al.344 performed Stille cross-coupling polymerization of these acceptors and donors to obtain the low band gap D−A copolymer 596a with a molecular weight of (Mn/Mw) of 22.7/47.3 kDa (Chart 44). Optical studies showed a broad absorption band from 651 to 950 nm with a low optical band gap of 1.31 eV. DSC exhibited a high glass transition temperature of 128 °C. HOMO and LUMO energy levels were estimated to be −5.01 and −3.70 eV, respectively. A device, fabricated from 596a and PCBM, exhibited a PCE of 0.55% with a short circuit current of −1.8 mA cm2. Open circuit voltage and fill factors were 0.59 V and 0.52, respectively. Kim et al.345 synthesized the copolymer 596b, which had a highly crystalline structure with energy levels suitable for a p-type semiconductor and had a molecular weight of (Mn) of BT


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12 035 g mol−1 with a PDI of 6.98. Optical studies exhibited absorption maxima of 783 and 794 nm in solution and solid state, respectively, attributed to intramolecular charge transfer (ICT). The optical band gap of the thin film, estimated from the edge of the absorption band, was found to be 1.43 eV, matching well with the electrochemical band gap of 1.46 eV. HOMO and LUMO energy levels were measured to be −5.17 and −3.71 eV, in the same order. This high-performance conjugated copolymer (596b) demonstrated a good tendency to form a nanowire, a device from which had high photoresponsivities at 632 and 830 nm. Ryu et al.174 investigated the solar cell efficiency of 596c, which had an absorption maximum of 814 nm both in solution and in thin film. The optical band gap was estimated to be 1.43 eV. The device fabricated using 596c provided a PCE of 6.11%. Li et al.346 performed the D−A random polymerization, using DPP and DTT units, resulting in the copolymers 597a−f with the weight-averaged molecular weights (Mw) varying from 52.4 to 69.4 kDa (Chart 44). Although DSC analyses did not show any transition temperatures or melting points for the copolymers 597a−d, 597e,f had melting points of 284 and 258 °C, respectively. There were two λmax for the solutions and thin films of 597a−f, appearing between 370 and 600 nm, which were attributed to π−π* transitions from donor, and from 600 to 910 nm, emerging from D−A charge transfer transitions. Optical band gaps were estimated to be 1.37 eV. Electrochemical studies demonstrated HOMO energy levels as −5.25, −5.24, and −5.19 for 597a−c, respectively. HOMO levels for the copolymers 597d−f were between −5.15 and −5.18 eV (Figure 30). A solar cell device,

of 0.15 cm2 V−1 s−1 with an on/off ratio of 106 (Chart 44). There were two absorption maxima, at 460 and 760 nm, and an optical band gap of 1.34 eV. A PCE of a device constructed from the copolymer and PC71BM was 3.44% along with Voc of 0.53 V, Jsc of 9.91 mA cm−2, and FF of 65%. Li et al.348 investigated the random donor−acceptor semiconducting copolymer 598b, which had a high PCE of above 5% under 100 mW cm−2 AM1.5G for its BHJ solar cells fabricated using PC71BM. Bérubé et al.215 performed DFT calculations with the combination of Scharber’s model to explain the findings on promising polymers for photovoltaic cells. On the basis of the predicted PCEs, the copolymer 599, containing thieno[3,4-c]pyrrole-4,6-dione (TPD) with a PCE of 8.9%, was a promising material for photovoltaic applications (Chart 45). The thermally Chart 45. Copolymer 599 and 600, Containing Thieno[3,4-c]pyrrole-4,6-dione (TPD) as an Acceptor215,349

stable (up to 382 °C) conjugated copolymer 600 with a molecular weight (Mn/Mw) of 7.92/17.6 kg mol−1 and PDI of 2.22,349 possessing DTT as a donor and TPD as an acceptor, exhibited strong thermochromic properties in solution (PhCl), and had a peak shift up to 170 nm. At room temperature, it had two absorption maxima at 630 and 578 nm, and in hot chlorobenzene (up to 120 °C) these two peaks disappeared while a new peak at 460 nm appeared, which was attributed to rotation around the single bond between donor and acceptor and disruption of the conjugated system by forming a twisted structure at higher temperatures. A device constructed from this polymer with PC71BM had a PCE of 2.1%. Balan et al.350 investigated the optical and electrical properties of the copolymers 601−603, possessing a DTT donor group and weak, medium, and strong acceptor units, respectively (Chart 46). Their molecular weights (Mn/Mw) (GPC) were reported to be 5800/10 090 (PDI = 1.74), 6600/10 100 (PDI = 1.53), and 12 600/21 500 Da (PDI = 1.70), respectively. Optical studies demonstrated a good correlation with the strength of the acceptors. The stronger was the acceptor unit, the more effective was the D−A system, and the longer was the absorption maximum obtained. Indeed, λmax values of 472, 515, and (349) 535 nm were recorded in dilute chloroform for 601−603, respectively. Although the thin films did not show any red shift with varying copolymers of 602 and 603 due to the ordering of the polymer chains, the absorption maximum of 601 (537 nm) was blue-shifted by 21 nm as compared to 602 and 603. Emission wavelengths in solution and optical band gaps were 566, 577, and 667 nm and 1.84, 1.74, and 1.73 eV for the polymers 601−603, respectively (Figure 31). However, electrochemical band gaps (1.45, 1.57, and 1.35 eV in the same order) were somewhat smaller than the optical band gaps. While the device constructed from the copolymer 603 did not

Figure 30. (a and c) Solution UV−vis−IR spectra for 597a−f in chlorobenzene, and (b and d) their thin film spectra. (e) Cyclic voltammograms of 597a−f (internal reference: ferrocene−ferrocenium couple).346 Reprinted (adapted) with permission from ref 346. Copyright 2013 The Royal Society of Chemistry.

Table 15. Photovoltaic Parameters of the Polymers 597a−f346 polymer 597

Jsc (mA cm−2)

Voc (V)

FF (%)

PCE (%)

a b c d e f

8.71 10.93 10.17 13.75 12.76 3.24

0.480 0.536 0.536 0.548 0.584 0.593

49.33 61.00 56.21 50.33 67.27 67.60

2.06 3.58 3.06 3.79 5.02 1.30

fabricated from 597e and PC71BM, exhibited the highest PCE of 5.02% (Table 15). Seah et al.347 investigated the OFET properties of the copolymer 598a, which was thermally stable up to 400 °C. It had a large number-averaged molecular weight (Mn) of 290 kg mol−1 with a PDI of 2.6 and a hole mobility BU


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which had molecular weights (Mn/Mw) (GPC) of 38/412 (PDI = 11) and 4/14 kDa (PDI = 3.5), respectively. Absorption maxima were at 516 nm in solution and at 540 nm as a thin film of the copolymer 604, and the copolymer 605 showed blueshifted maxima at 485 nm in chloroform and 506 nm in the solid state. The optical band gaps were estimated to be 1.78 eV for 604 and 1.63 eV for 605. Electrochemical investigations revealed HOMO/LUMO energy levels of −5.64/−3.87 and −5.67/−3.82 eV for 604 and 605, respectively, meaning electrochemical band gaps of 1.77 and 1.85 eV. The results obtained from photovoltaic measurements demonstrated a PCE of 0.14% for the copolymer 604 and 0.21% for the copolymer 605. The open-circuit voltage (Voc), short circuit current density (Jsc), and fill factor (FF) of the copolymer 579 were 0.49 V, 0.83 mA cm2, and 34.4%, respectively. For the copolymer 605, values of 0.35 V, 1.57 mA cm−2, and 38.2% were obtained, in the same order. Experimental studies indicated that the speed of the switching time increased with the introduction of the DTT unit. Osken et al.352 investigated the optical, electrochemical, and photovoltaic properties of the copolymers 606−608, possessing fluorene and different ratios of DTT-S,S-dioxide (5%, 15%, 25%, and 50%) (Chart 48). According to TGA analyses, the

Chart 46. Copolymers 601−603, Possessing a DTT Donor Group and Weak, Medium, and Strong Acceptor Units, Respectively350

Chart 48. Copolymers 606−608352

Figure 31. Normalized absorption spectra of 601−603 in dilute CHCl3 solution (inset shows the photographs of the corresponding solutions under room light) (a), in the thin film state (spin-cast from chloroform solutions) (b), and normalized fluorescence spectra of 601−603 in CHCl3 solution (λex = 472 nm for 601, 515 nm for 602, and 535 nm for 603).350 Reprinted (adapted) with permission from ref 350. Copyright 2013 The Royal Society of Chemistry.

copolymers 607, having 5−25% of DTT-S,S-dioxide, and 608 were thermally stable. The weight-averaged molecular weights of 606−608 varied from 9.5 to 859 kDa. Copolymers 606 and 607 had the longest λmax of 447 and 472 nm, respectively, corresponding to the increased percentage of DTT-S,S-dioxide, and this trend was also observed for their emission maxima appearing at the longest wavelength of 558 and 592 nm, in the same order. However, 608 exhibits a reverse case; that is, with an increasing amount of DTT-S,S-dioxide, a blue shift was obtained in both solution and solid state. Optical band gaps were estimated to be between 2.47 and 2.95 eV for the copolymers 606, and those of the copolymers 607 and 608 had optical band gaps varying from 2.17 to 2.95 eV, and from 2.97 to 2.99 eV, respectively (Table 16). The OLED devices, constructed from these copolymers, displayed a wide range of colors, varying from sky blue to red. Zhao et al.353 used Sonogashira cross-coupling to make a conjugated copolymer 609, possessing perylene diimide354 and DTT units linked by ethynylene bridges, which provided a planar

have any FET properties, the charge mobility of 601 was 1.7 × 10−5 cm2 V−1 s−1 with an on/off ratio of 103, and that of 602 had 6.0 × 10−4 cm2 V−1 s−1 with an on/off ratio of 105. Cevher et al.351 investigated the electrochemical and spectroelectrochemical properties of copolymers 604 and 605 (Chart 47). Their TGA analyses indicated thermally stable copolymers, Chart 47. Copolymers 604 and 605 Holding Benzo[D][1,2,3]triazole Unit351

BV


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Table 16. Optical and Electrochemical Properties of 606− 608352 polymer 606-5 606-15 606-25 606-50 607-5 607-15 607-25 607-50 608-5 608-15 608-25 608-50

Egopt [eV]

HOMO [eV]

2.95 2.48 2.47 2.47 2.95 2.26 2.26 2.17 2.99 2.97 2.97 2.97

−5.81 −5.81 −5.74 −5.78 −5.81 −5.65 −5.66 −5.60 −5.72 −5.73 −5.82 −5.89

LUMO [eV] −2.49 −2.27 −2.30 −3.10 −2.24 −2.57 −2.61 −3.22 −2.44 −2.36 −2.34 −2.83

Table 17. Electrochemical and Optical Properties of 609−611353−356

Egcv [eV] 3.32 3.54 3.44 2.68 3.57 3.08 3.05 2.38 3.28 3.37 3.48 3.06

structure, resulting in good packing (Chart 49, Table 17). They also synthesized the copolymer (610a) without ethynylene spacer. TGA analysis of 609 demonstrated a thermally stable polymer; however, DSC did not show any transition up to 250 °C. The molecular weights (Mn/Mw) (GPC) of 609 were reported to be 15/17 kDa (PDI = 1.1). Similar molecular weight values were obtained for 610a. Optical investigations in chloroform indicated a red-shifted absorption maximum of 609 (714 nm) by 89 nm as compared to 610a (Figure 32). The optical band gaps were estimated to be 1.55 and 1.60 eV for 609 and 610a, respectively. HOMO and LUMO energy levels obtained from electrochemical measurements were −5.70 and −3.90 eV for the copolymer 609, and those of the copolymer 610a had energy levels of −5.80 and −4.00 eV, which were supported by DFT calculations at the B3LYP/6-31G(d,p) level (Figure 32). The LUMO energy level of 609 was lower than that of 610a by 0.1 eV, which increased the air stability of the copolymer 584. While a top-contact bottom-gate device fabricated from the copolymer 609 demonstrated an electron mobility of 0.06 cm2 V−1 s−1 in air, the device constructed using copolymer 610a did not work at all. Bottom-contact bottom-gate OFETs of 609 and 610a afforded electron mobilities of 0.75 and 0.038 cm2 V−1 s−1, respectively. Zhang et al.355 investigated the thermal, optical, electrochemical, and OFET properties of the

compound

HOMO [eV]

LUMO [eV]

EgCV [eV]

λabs [nm]

EgOpt [eV]

609 610a 610b 611

−5.70 −5.54 −5.31 −5.42

−3.90 −3.90 −4.01 −3.89

1.80 1.64 1.30 1.53

714 625 714 685

1.55 1.60 1.30 1.24

Figure 32. (Above) Frontier molecular orbitals (FMO) of 609 and 610a at the B3LYP/6-31G(d,p) level. (Below) UV−vis absorption spectra of 609 and 610a in chloroform and thin films.353 Reprinted (adapted) with permission from ref 353. Copyright 2013 American Chemical Society.

copolymers 610a,b and 611. TGA analyses indicated thermally stable polymers. The molecular weights (Mn/Mw) of 610b were 25/38 kDa with a PDI of 1.5, whereas those of 611 were 9.0/16 kDa with a PDI of 1.7. Optical studies demonstrated broad absorptions, with maxima of 369, 482, 736, and 369, 483, 747 nm for 610a in solution and as a thin film, respectively. The copolymer 610b showed absorption maxima of 369, 486, 714 in

Chart 49. Copolymers 609−612, Possessing Perylene Diimide353−356

BW


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thin film of 613. HOMO and LUMO energies were recorded to be −5.31 and −3.91 eV, respectively. It had the electron mobility of 4.8 × 10−3 cm2 V−1 s−1 with an on/off ratio of 108. Solvay et al.357 reported the ruthenium complexes 614a and 614b, comprising two tridentate ligands and DTT units, for dye-sensitized solar cell (DSSC) applications (Chart 51). Fang et al.358 prepared two thermally stable metallo-polymers 615a (R = H) and 615b (R = COOH), possessing aryl-imidazophenanthrolines (AIP) ligands located at the 2- and 6-positions of DTT. A broad absorption band between 270 and 650 nm with three maxima indicated π−π transitions, intramolecular charge transfer (ICT), and metal-to-ligand charge transfer (MLCT), respectively. While λmax values of 429 and 407 nm for 615a and 165b, respectively, in the solid state were attributed to ICT, absorption bands of 517 and 572 nm were assigned as due to MLCT. UV−vis measurements indicated that extending the π-conjugation and introducing COOH units helped to decrease the band gaps, which were 2.0 and 1.9 eV for 615a and 615b, respectively. Electrochemical studies afforded the HOMO energy level of −5.9 eV for metallo-polymer 615a and −5.7 eV for 615b. The metallo-polymers displayed electrochromic properties upon electrochemical oxidation. The H-bond effect on the surface-modified pyridyl-ZnO (ZnOpy) nanoparticles was studied by spectroscopic measurements. Unlike nanocomposites, 615a/ZnOpy, 615b/ZnOpy exhibited higher crystallinities with a layered-structure, based on XRD analyses. Moreover, 615b/ZnOpy nanoparticles formed a more homogeneous surface through H-bonds, as compared to that of 615a/ZnOpy, which lacked H-bonds.

solution and 372, 492, 754 in the solid state. The polymer 611 had red-shifted λmax values, as compared to 610a,b. The optical band gaps were estimated to be 1.37, 1.30, and 1.24 eV, respectively. Electrochemical investigations revealed HOMO/ LUMO energy levels of −5.54/−3.90, −5.31/−4.01, and −5.42/ −3.89 eV, providing electrochemical band gaps of 1.64, 1.30, and 1.53 eV, respectively. On the basis of OFET measurements, 610b showed ambipolar properties with a hole mobility of 4 × 10−5 with an on/off ratio of 103 and electron mobility of 4 × 10−4 with an on/off ratio of 104. Polymers 610a and 611 exhibited electron mobilities of 5 × 10−3 and 7 × 10−4 cm2 V−1 s−1, respectively, with an on/off ratio of 104. Zhang et al.356 explored the oligomer 612, possessing two DPI units connected via a DTT group. The absorption maxima were 461, 492, and 529 nm in chloroform, and red-shifted λmax values were 494 and 551 nm as a thin film. The device constructed from this copolymer afforded an electron mobility of 0.013 cm2 V−1 s−1 with an on/off ratio of 5 × 103. Yuan et al.183 investigated the copolymer 613, prepared by Stille coupling of naphthalenediimide and DTT (Chart 50). The molecular weights (Mn/Mw) were recorded to be 21.3/ 61.8 kDa with a PDI of 2.9. Optical studies revealed an absorption maximum of 391 nm and an optical band of 1.40 eV for Chart 50. Copolymer 613 Having Naphthalenediimide and DTT Monomers183

3.2. Dithieno[3,4-b;3′,4′-d]thiophene (492)

In 1971, Janssen294 reported the synthesis of 492, starting from 3,4-dibromothiophene (15) in an overall yield of 13% (Scheme 118). First, 15 was reacted with n-BuLi and the Chart 51. Ruthenium Complexes 614a and 614b, and Metallo-polymers 615a,b357,358

BX


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couple), respectively, with a low ΔE of 320 mV, indicating the ability of methylthio units stabilizing the corresponding radical cation. The experimental findings indicated that 621 forms charge-transfer and radical-cation salts.365 Aita et al.366 synthesized a planar cyclooctatetraene (COT) 627 fused to dithieno[3,4-b:3′,4′-d]thiophene from 622. Reaction of 622 with n-BuLi and DMF afforded the corresponding aldehyde, which was treated with a Wittig reagent and n-BuLi to form 623 in 26% yield in two steps overall. Subsequent formylation of 623 with n-BuLi and DMF gave 624 in 37% yield, which was subjected to addition of allylmagnesium bromide, and then protection of newly formed secondary alcohol with TMSCl gave 625 in 64% yield. Ring-closing metathesis of 625, using Grubbs’ second generation catalyst, was followed by dehydration with Dess−Martin sulfurane and potassium tert-butoxide to yield 627 in 41% yield (Scheme 119).

Scheme 118. Synthesis of 492, Starting from 15 Using Two Routes294,359,429

lithiated thiophene was treated with (PhSO2)2 to form 616, the reaction of which with n-BuLi and then its oxidative ring closure reaction in the presence of CuCl2 furnished dithieno[3,4-b;3′,4′-d]thiophene (492). Alternative methodology involving the use of Pd(PPh3)4 and hexamethylditin furnished 492 in 78% yield.13,359,429 Catellani et al.360 performed an X-ray crystallographic analysis, which demonstrated that the ring in the center of the structure of 492 in the monoclinic crystal had a different bond order with respect to the side rings, which resulted in improved aromaticity. They also studied the singlecrystal structure of a charge-transfer complex of DTT with tetracyanoquinodimethane (TCNQ), which demonstrated the DTT-TCNQ complexes to be oriented face-to-face. The DC conductivity was measured to be 2 × 10−10 Ω−1 cm−1.361 The optical band gap of 1.15 eV obtained matches well with that predicted from quantum chemical calculations (1.12 eV). Electrochemical investigations indicated an electrochemical band gap of 0.93 eV for its polymer.338 Cravino et al.362 demonstrated the existence of paramagnetic positive and negative charge carriers for a polymer with high g-factors (p-/n-doped: 2.0045/2.0054) with the help of in situ ESR spectroscopy. Substitution of two methyl units on 492 provided a mixture of dimethyl-substituted DTTs 617−619 in a ratio of 100:28:1, respectively (Chart 52).363 Electropolymerization and FeCl3

Scheme 119. Synthesis of a Planar Cyclooctatetraene 627 Fused to a DTT366

Chart 52. Dimethyl-Substituted DTTs 617−619, and Dioctyl (620) and Tetra-methylthiol (621) Possessing DTTs363−365 The planarity of the COT unit was determined by X-ray crystallographic analysis. B3LYP/6-31G(d,p) level of calculations demonstrated a C2v point group (Figure 33). Computational studies indicated the COT ring to have magnetic antiaromaticity. Electrochemical investigations revealed a band gap of 2.4 eV, which was in good agreement with that predicted by DFT calculations (2.5 eV). Optical studies showed a λmax value of 650 nm and therefrom an estimated optical band gap of 1.9 eV. Cyclic tetrathiophene 629,367 planarized by the sulfur-bridge, has an antiaromatic cycloocatatetraene (COT) core. Treatment of 622 with n-BuLi and then addition of CuCl2 resulted in the coupled product 628 in 45% yield. Desilylation with TBAF resulted in the planar cyclic tetrathiophene 629 (Scheme 120). X-ray crystallographic analysis indicated a bent angle of 3.0° for both 628 and 629 (Figure 34), that for 629 being in agreement with the one predicted from computational studies at the RB3LYP/6-31G(d,p) level (4.3°). The theoretically estimated HOMO−LUMO gap for 629 is 2.62 eV. The longest absorption maximum for 628 was at 618 nm, attributed to the planar COT core. Electrochemical investigations demonstrated an oxidation halfwave potential (E1/2ox) of 0.54 V

promoted chemical polymerizations of 617 and 1,3dioctyldithieno[3,4-b:3′,4′-d]thiophene 620 were performed by Inaoka et al.364 The polymer obtained from 620 was soluble in common organic solvents. A thin film of the polymer had electrochromic properties, and its oxidation resulted in diminishing the absorption peak at 570 nm. Remonen et al.365 studied the electrochemical properties of 621, possessing four methylthiol units at α-positions of the outer thiophenes, demonstrating a lower oxidation potential with respect to its tetramethyl analogue. Halfwave potentials (E11/2 and E21/2) were recorded in CH2Cl2 as 0.82 and 1.14 (vs ferrocene/ferrocenium BY


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The constant oxidation potential of electropolymerization was 1.04 V (vs standard calomel electrode). The polymer has strong electrochromic properties. While it was opaque in its reduced state, the oxidized state was described as semitransparent. It had an absorption maximum of 590 nm (2.1 eV) attributed to a low energy of the first π−π* electronic transition. The calculated band gap was 1.1 eV, which demonstrated efficient π-electron delocalization.369 The conductivity of the film was 1 Ω−1 cm−1.370 Formation of structure 630 is the most probable of the four possible polymers (630−633) (Chart 53).371,372 Arbizzani et al.337,373 Chart 53. Four Possible Polymers 630−633 from 492369−372

carried out the electropolymerization of DTT on transparent conductive tin oxide covered glass and studied the polymer electrochemically. While p-doped polymer had an onset potential of 0.22 V, that for n-doped polymer was −0.71 V (vs Ag). Spectroelectrochemical measurements revealed an absorption maximum of 610 nm with an optical band gap of 1.09 eV. They374,375 also reported that polydithieno[3,4-b:3′,4′-d]thiophene is a good candidate for the polymer-based redox supercapacitors, due to the possible positive and negative charge injections, that is, the property of a p- and n-dopable polymer. Reaction of 492 with H2O2 in acetic acid resulted in the production of the sulfone 634, together with sulfoxide 635 (Chart 54).294 Ohmae et al.367 applied the method of Jong and

Figure 33. (a) At the B3LYP/6-31G(d,p) level optimized and (b) X-ray (ORTEP 50% probability) structures of 627, and (c) its crystal packing (the slight bending in B emerges from the short C−H intermolecular interactions in crystal packing).366 Reprinted (adapted) with permission from ref 366. Copyright 2013 American Chemical Society.

Scheme 120. Preparation of Cyclic Tetrathiophenes 628 and 629 from 622367

Chart 54. Reaction of 492 with H2O2 Provides 634 along with 635294

Janssen,294 which involved the use of mCPBA in DCM, to improve the formation of sulfone 634 up to a 94% yield. Ohmae et al.367 synthesized cyclic tetrathiophene 638 planarized by a sulfone-bridge, possessing an antiaromatic cycloocatatetraene (COT) core. α-Proton abstraction from 634 followed by addition of TMSCl afforded 636 in 94% yield. Treatment of 636 with n-BuLi and then addition of CuCl2 resulted in the coupled product 637 in 32% yield. Desilylation with TBAF furnished 638 quantitatively (Scheme 121). X-ray crystallographic analysis indicated a bent angle of 3.2° for 637 (Figure 35). The theoretically (RB3LYP/6-31G(d,p) level) estimated band gap for 638 is 2.72 eV. The longest absorption maximum for 637 was at 575 nm attributed to the planar COT core. Electrochemical investigations demonstrated the effects of an electron-withdrawing sulfonyl group on the oxidation potential of 637 by lowering its HOMO level. Its two reduction halfwave potentials (E1/2red) were −1.43 and −1.93 V (vs Ag/AgNO3).

Figure 34. ORTEP drawings of X-ray structures of 628 and 629 (50% probability level, hydrogen atoms are omitted for clarity).367 Reprinted (adapted) with permission from ref 367. Copyright 2010 American Chemical Society.

(vs Ag/AgNO3) and two reduction halfwave potentials (E1/2red) of −1.79 and −2.25 V for 628 (vs Ag/AgNO3). Fujitsuka et al.334,335 investigated the photochemical polymerization of 492 in the presence of a suitable electron acceptor, that is, p-dinitrobenzene (DNB) and CCl4, using nanosecond laser flash photolysis and time-resolved fluoresence spectroscopy. They suggested that photoinduced electron transfer from the singlet excited state of 492 to an electron acceptor resulted in the formation of radical cation of 492 and radical anion of DNB, evidence for which came from transient absorption spectra in the visible and near-IR regions. Bolognesi et al.368 performed the first electropolymerization of 492.


In 1958, Pandya and Tilak376 reported the synthesis of the DTT dithieno[2,3-b;3′,2′-d]thiophene (493), starting from BZ


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Scheme 121. Synthesis of Cyclic Tetrathiophenes 637 and 638367

Scheme 123. Synthesis of 493 from 642 via Different Strategies294,359,429

level was −0.602 eV. However, incorporation of two methyl groups at the α-positions of 493 resulted in increases of both HOMO and LUMO energy levels to −5.547 and −0.430 eV, respectively (Figure 36).377

Figure 35. ORTEP drawing of X-ray structure of 637 (thermal ellipsoids are drawn at the 50% probability level; hydrogen atoms are omitted for clarity).367 Reprinted (adapted) with permission from ref 367. Copyright 2010 American Chemical Society.

thiophene (515). 2,5-Dilithiation of thiophene was followed by reaction with disulfide (639) to obtain (640), which underwent ring closure reaction in the presence of PPA in refluxing chlorobenzene affording 493 (Scheme 122). The pyrolysis of

Figure 36. B3LYP/6-31G(d,p) level electronic density contours (0.03 e bohr−3) and energies for FMOs of 493 (left) and 2,5dimethyl-substituted 493 (right).377 Reprinted (adapted) with permission from ref 377. Copyright 2007 American Chemical Society.

Scheme 122. Synthesis of 493 in Two Steps, Starting from Thiophene 515 (R = H) and in One Step Starting from 64119,376

Another method involved the coupling of two 3-bromothiophene (5) in the presence of n-BuLi and CuCl2, followed by dibromination and subsequent treatment with n-BuLi and SCl2 (or PhSO2)2S to form 493 in an overall yield of 20% (Scheme 124).378,379 Scheme 124. Synthesis of 493 from 5 via 644 Possessing Two Thiophenes Connected from 3,3′-Positions378,379

2,2′-dithienyl sulfide (641) with hydrogen sulfide at 500−600 °C under nitrogen furnished dithienothiophene (493).19 Treatment of 3-bromo-2-lithiothiophene (642) with (PhSO2)2S gave the intermediate product 643, the lithiation of which with n-BuLi followed by oxidative coupling with CuCl2 produced 493 in 21% yield (Scheme 123).294 Various methods were applied to form the remaining carbon−carbon bond from 643, giving 493 in 75%, 47%, and 69% yields by employing palladium catalysis (Pd(PPh3)4) with hexamethylditin (Me3SnSnMe3), atetype copper complexes, and organozinc compounds followed by CuCl2 promoted ring closure reactions, respectively.359,429 HOMO and LUMO energy levels of 493 were predicted from computational studies performed at the B3LYP/6-31G(d,p) level. The HOMO energy level was −5.827 eV, and the LUMO

Shi et al.380 synthesized the oligomers 2,5-diphenyl-dithieno[2,3-b:3′,2′-d]thiophene (649), 2,5-dibiphenyl-dithieno[2,3b:3′,2′-d]thiophene (650), 2,5-distyryl-dithieno[2,3-b:3′,2′-d]thiophene (651), and 1,4-di[2-dithieno[2,3-b:3′,2′-d]thiophen2-yl-vinyl]benzene (652) for organic field-effect transistors (OFETs) (Scheme 125). The first two oligomers 649 and 650 were prepared from 2,5-dibromo-DTT (646), which was prepared readily from DTT (493) and NBS, then phenyl- or biphenylboronic acids under Suzuki cross-coupling conditions. Oligomers 651 and 652 were produced from Wittig reactions of aldehyde-substituted DTTs 647 and 648. TGA analysis CA


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Scheme 125. Preparation of Oligomers 649−652 Using Suzuki and Wittig Reaction Conditions380,381

indicated thermally stable oligomers 649−652 with degradation temperatures above 307 °C. DSC investigations revelaed the example with the lowest melting point to be 649, at 272 °C, while the melting points of 650 and 651 were 368 and 305 °C; 652 had the highest melting point of 378 °C (Table 18).

Scheme 126. Reaction of 2,5-Diketones (653) with P4S10 Providing DTTs (654)382

Table 18. Physical Properties of 649−652380 compound

mpa [°C]

λabs [nm]

HOMO [eV]

EgOpt [eV]

mobility [cm2 V−1 s−1]

649 650 651 652

272 368 305 378

245 314 306 356

4.80 4.80 4.81

2.99 2.48 2.92 2.25

0.52 2.2 0.16

a

Scheme 127. Benzannulated DTT (656) from 65526

mp: Melting point.

Absorption maxima of 245, 314, 306, and 356 nm, and optical band gaps of 2.99, 2.48, 2.92, and 2.25 eV, were recorded for films of the oligomers 649−652, respectively. Electrochemical investigations provided HOMO energy levels of 4.80 eV for 650 and 651, and 4.81 eV for 652. Compounds 650−652 had field-effect mobilities of 0.52, 2.2, and 0.16 cm2 V−1 s−1, respectively, but 649 did not provide any field-effect property. Ozturk and co-workers382 synthesized DTTs (654), having para-substituted phenyl groups at the 3- and 4-positions, through ring closure reactions of 2,5-diketones (653) in the presence of P4S10260 (Scheme 126). Electrochemical studies showed oxidation potentials between 1.19 and 1.70 V (vs Ag/AgCl). Although DFT calculations revealed radical spin densities at the 2- and 5-positions of the DTTs, attempted polymerization failed due to the almost complete lack of spin densities at the carbons of the corresponding dimers, which are required for electropolymerization.383 Benzannulated DTT (656) was obtained in 31% yield from 655 with ethoxycarbonylsulfenyl chloride in the presence of TiCl4 (Scheme 127). The same product was also synthesized with a slightly higher yield (47%) from the reaction of 655 with n-BuLi and elemental sulfur.26

Zhang et al.384,385 synthesized and investigated the transistor properties of semiconductors 663a−d, which were composed of two DTT units linked through an ethenyl bridge, and 664 (Scheme 128). To synthesize the precursors 662a−c, three different methodologies were applied: (i) bromination of 657 with NBS and then Stille cross-coupling of product 658 with 2-(tri-n-butylstannyl)thiophene 108 to obtain 662a, (ii) Suzuki coupling of 659 with phenylboronic acid to produce 660, which was then formylated using POCl3 and DMF for the synthesis of 662b, and (iii) lithiation of the DTT (493) with LDA and then alkylation with n-hexyl bromide to obtain 661, which was converted into an aldehyde using POCl3 and DMF for the synthesis of 662c. In the last step, a McMurry reaction was used to form double bonds from the aldehyde functionalities of 657 and 662a−c to produce the semiconductors 663a−c. These are thermally stable with degradation temperatures ranging from CB


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Scheme 128. Preparation of Semiconductors 663a−d and 664384,385

324 to 365 °C. DSC analysis indicated melting points of 330, 360, 236, and 338 °C for 663a−d, respectively. Absorption maxima and band gaps derived from the optical investigations of 663a−d in solution and as thin films are listed in Table 19 Table 19. Absorption Maxima and Optical Band Gaps of 663a−d in Solution and Solid State384 solution (THF) oligomer 663 a b c d

λmax [nm] 382, 381, 380, 379,

401 400 400 398

Eg [eV] 2.89 2.90 2.94 2.91

thin film λmax [nm] 359, 404, 311, 333, 339,

431 428 423 423

Eg [eV] 2.34 2.47 2.53 2.48

(Figure 37). HOMO energy levels of 663a−d, estimated from electrochemical studies, were 5.42, 5.41, 5.36, and 5.39 eV. The aromatic and alkyl end-groups affected film microstructures, electronic properties, and packing arrangements significantly. Compound 663b, end-capped with phenyl units, exhibited a high performance in a thin film FET, the charge mobility of which was 2.0 cm2 V−1 s−1 with an on/off ratio of up to 108. The charge mobility of 664, synthesized by a selfcoupling reaction of 659 in the presence of Cu-bronze, was 0.005 cm2 V−1 s−1 with an on/off ratio of 105.385 Miyasaka et al.386 synthesized spiro heptathiophene 668 along with 669, and a single diastereomer dispiro octathiophene 673 from treatment of oligothiophenes 667 and 671, which possess two DTT units, with trifluoroacetic acid (TFA) in chloroform. The precursors 667 and 671 were obtained from the Negishi cross-coupling of 3-bromo-5-octylthiophene (665) with 666 and 670, respectively (Scheme 129). NMR experiments and DFT calculations at the B3LYP/6-31(d,p) level unveiled a stepwise mechanism via monospiro octathiophene, for the formation of dispiro compound 673, with stereogenic centers of the same relative stereochemistry.

Figure 37. (Above) Optical absorption spectra of dimers 663a−d (a) in THF and (b) vacuum-deposited thin films. (Below) (a) View down the crystallographic a-axis of compound 663b; (b) intermolecular interactions; d1(C···H) = 2.86 Å, d2(C···H) = 2.88 Å, d3(C···H) = 2.76 Å, d4(C···S) = 3.47 Å, d5(C···S) = 3.35 Å, and d6(C···S) = 3.41 Å.384 Reprinted (adapted) with permission from ref 384. Copyright 2009 American Chemical Society.

Reaction of 493 with H2O2 and mCPBA in acetic acid gave DTT-S,S-dioxide 674 (Chart 55). 3.4. Dithieno[2,3-b;2′,3′-d]thiophene (494)

In 1971, Jong and Janssen387 reported a three-step synthesis of the DTT dithieno[2,3-b;2′,3′-d]thiophene (494), starting from 3-bromo-2-(thien-3-yl)thiophene (675). Bromination of 675 was followed by lithiation and then addition of elemental sulfur to give 677. The reaction of 677 with CuO furnished the required product 494 (Scheme 130). Reaction of 494 with mCPBA in acetic acid provided the corresponding DTT-S,S-dioxide 678 in 65% yield (Chart 56). CC


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Scheme 129. Synthesis of Spiro Heptathiophene 668 and Dispiro Octathiophene 673386

Arbizzani et al.337,390b carried out electropolymerization of DTT (495) on transparent conductive tin oxide covered glass (ITO) and studied the polymer electrochemically. While p-doped polymer had an onset potential of 0.14 V, that for n-doped polymer was −1.12 V (vs Ag). Spectroelectrochemical measurements revealed an absorption maximum of 650 nm with an optical band gap of 1.21 eV. Catellani et al.338 performed optical and computational investigations to explain the electronic structure of the polymer. The optical band gap 1.12 eV obtained was in good agreement with that predicted from quantum chemical calculations (0.82 eV). The electrochemical band gap was 1.26 eV. Cravino et al.362 showed the formation of paramagnetic positive and negative charge carriers with high g-factors (p-/n-doped: 2.0044/2.0049) using in situ ESR spectroscopy. Ehrenfreund et al.391 determined the lowest even parity (LEP) state in the polymer using Raman spectra, which is important for light-emitting and photovoltaic devices. The energy of LEP, lower than the lowest odd parity (LOP), results in a smaller light emission efficiency and higher probability of charge separation. Experimental results demonstrated the low LEP state comparable to the optical band gap of the polymer. Reaction of 495 with mCPBA in acetic acid gave the corresponding DTT-S,S-dioxide 682 in 64% yield (Chart 57).

Chart 55. Reaction of 493 with H2O2 and mCPBA Providing 674

Scheme 130. Preparation of 494 Starting from 675387

Chart 56. DTT-S,S-Dioxide 678387

The electronic structures and spectra of 678 were investigated comprehensively.387,389 3.5. Dithieno[3,4-b;3′,2′-d]thiophene (495)

Chart 57. DTT-S,S-Dioxide 682294

De Jong et al.294 synthesized the DTT dithieno[3,4-b;3′,2′d]thiophene (495) through the reaction of 3-bromo-2lithiothiophene (679) with disulfide 680 to afford dibromodithienyl sulfide (681) in 94% yield, followed by dilithiation with n-BuLi and then oxidative ring closure in the presence of CuCl2 to furnish 495 in 29% yield (Scheme 131). Computational389 and spectro- and electrochemical studies were devoted to explain the properties of 495.337,338,390


In 1971, Janssen387 synthesized the DTT dithieno[3,4-b;2′,3′d]thiophene (496) starting from 3-bromothiophene (5). Lithiation of 5 with n-BuLi followed by treatment with disulfide 680 provided 683 in 80% yield. Next, bromination with NBS led to the formation of dibromide 684, the reaction of which with 2 equiv of n-BuLi and then oxidative ring closure in the presence of CuCl2 resulted in 496 (Scheme 132). Computational389 and spectro- and electrochemical studies were conducted to investigate the properties of 496.390,392

Scheme 131. Preparation of 495 Starting from 679 in Two Steps294

CD


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precursor (Scheme 133). Chen et al.272,394 reported two one-pot syntheses, [1+1+2] and [2+1+1], of 687, involving the lithiation of 5 and 16 with n-BuLi followed by addition of sulfur and then TsCl. The mixture then was reacted with lithiated 16 and 5, and, finally, ring closure was performed with CuCl2 to furnish 687 in an overall yield of 27% and 22% yields, repscetively. The tetrathienoacene has a large polarization energy (2.36 eV). This was explained by intermolecular orbital interactions in the solid state, which were not detected in TTs and DTTs. Huang et al.273 studied the electronic, optical, and conducting properties computationally at the DFT level (B3LYP/6-311G(d,p)). Internal reorganization energies for hole and electron (λ+/λ−) were calculated to be 332/302 and 313/277 meV, and electron affinities were predicted to be 0.04 and 0.34 eV for 687 and 689, respectively. Vertical ionization potentials (IPv) were 7.20 eV for 687, which align well with the experimentally obtained IPv (7.52 eV in the gas phase),274 and 6.95 eV for 689, which matched well with the experimental IPv of 7.25 eV.274 Theoretical studies provided the HOMO and HOMO−LUMO gap at −5.64 and 4.10 eV, respectively, for 687. A HOMO energy level of −5.52 eV, close to the experimentally estimated HOMO energy level (5.33 eV),397 was predicted for 689. The HOMO−LUMO gap was reported to be 3.80 eV for 689, which was comparable to the experimentally determined band gap of 3.29 eV.397 Shi et al.396 synthesized bisindeno-thienoacene 690, which is antiaromatic with 4nπ electrons and has small singlet biradical character (Scheme 133). Optical investigations revealed the absorption maxima of 561, 610, and 729 nm providing the optical band gap of 1.27 eV. Liu et al.395 synthesized two tetrathienoacenes end-capped with styrene possessing H (693a) and alkyl unit (693b) at the para position through Stille coupling reactions of 692a and 692b, prepared through treatment of arly acetylene with HSnBu3 in the presence of AIBN, with dibromo-tetrathienoacene (Scheme 134). While TGA demonstrated decomposition temperatures of 350 and 389 °C for 693a and 693b, respectively, DSC indicated two endothermic peaks at 216 and 265 °C for 693b. Single-crystal structre of 693a showed a trans−trans configuration and almost flat geometry. Interplanar distance was reported as 2.20 Å, indicating the intermolecular S···S interactions (Figure 38). While optical studies revealed the optical band gaps of 2.37 and 2.41 eV for 693a and 693b, respectively, the HOMO energy levels, calculated from electrochemical investigations, were −5.36 and −5.14 eV, respectively, which demonstrated the higher stabilities under ambient conditions. Compound 693a displayed a better OFET property with a charge mobility of 0.03 cm2 V−1 s−1 and an on/off ratio of ∼104 at 70 °C as compared to 693b, which showed a charge mobility of 0.017 cm2 V−1 s−1 with an on/off ratio of ∼105 at the same temperature. According to OFET studies,397 pentathienoacene displayed a higher stability in air, as compared to pentacene, which was explained by a larger HOMO−LUMO energy gap. Moreover, they recorded a good hole transport mobility of 4.5 × 10−2 cm2 V−1 s−1 for pentathienoacene. Theoretical (B3LYP/ 6-31G(d,p)) and experimental (UPS) studies398 explain the charge transport mechanism of pentathienoacene crystal, considering two mechanisms, band and hopping models. The calculated transport parameters indicated a higher intrinsic hole mobility for pentathienoacene than for pentacene in the band mechanism and that of sexithiophene in the hopping mechanism. Matzger and coworkers393 synthesized pentathienoacene (689) and heptathienoacene (701) in five steps in overall yields

Scheme 132. Preparation of 496 Starting from 5387

Arbizzani et al.337,373,390b electropolymerized DTT (496) on ITO and studied the polymer electrochemically, which had a p-doped onset potential of −0.28 V (peak at 0.77 V) and an n-doped onset potential of −0.89 V (peak at −1.61 V) (vs Ag). Spectroelectrochemical measurements showed an absorption maximum of 760 nm, having a band gap of 1.12 eV. According to the optical and computational studies by Catellani et al.,338 the observed optical band gap of 1.05 eV was comparable to that predicted from quantum chemical calculations (0.79 eV). The electrochemical band gap was 0.61 eV. Cravino et al.362 demonstrated the formation of the paramagnetic positive and negative charge carriers with high g-factors (p-/n-doped: 2.0042/2.0052) by ESR spectroscopy. Reaction of 496 with mCPBA in acetic acid provided the corresponding DTT-S,S-dioxide 685 in 75% yield (Chart 58). Summaries on the properties of some important Chart 58. DTT-S,S-Dioxide 685387

dithienothiophenes and their oligomers and polymers are given in Chart 59 and Table 20.

4. THIENOACENES 4.1. S-Anellated α-Oligothiophenes

Annulation of α-oligothiophenes with a sulfur atom to link the β-positions of the adjacent thiophenes led to the formation of linear thienoacenes. They are planar and rigid π-conjugated molecules. The existence of strong intermolecular S···S interactions leads to an efficient molecular orbital overlap, which makes them useful building blocks for optoelectronic applications.393 Two known methods have been followed for the syntheses of thienoacenes, linking 3- and 3′-positions of thiophene species with a sulfur atom, and then coupling of 2- and 2′-positions, or vice versa, first combination of 2- and 2′-positions with a single bond and, and then linking 3- and 3′-positions with a sulfur atom. Following the first strategy, Kobayashi et al.25,274 synthesized tetrathienoacene (687) and pentathienoacene (689). Thus, lithiation of 3-bromothieno[3,2-b]thiophene (16) and then reaction with bis(3-thienyl)disulfide afforded 3-(3-thienyl)thieno[3,2-b]thiophene (686). The reaction of 686 with n-BuLi and CuCl2 successively furnished tetrathienoacene (687) in 60% yield. In the same way, reaction of (bis(benzenesulfonyl)sulfide), utilized as the sulfur source, with 16 followed by treatment with n-BuLi and CuCl2 gave pentathienoacene (689) in 20% yield, the relatively lower yield being due to the low solubility of the CE


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Chart 59. Dithienothiophene Containing Oligomers and Polymers, of Which Fabricated Devices Provided Excellent Hole (μ+)329,380,384,385 and Electron (μ−)353,355,356 Mobilities, and Power Conversion Efficiencies (PCE)318,321,323

gave 695b,c, which was dimerized through introduction of a sulfur bridge with bis(tri-n-butyltin)sulfide in the presence of Pd(0) to obtain 696b,c. Their oxidative ring closure using CuCl2 resulted in the formation of 697b,c (Scheme 135). UV−vis and fluorescence emission spectra indicated a bathochromic shift by about 10 nm, as compared to its unsubstituted analogue, which was reported to decrease the band gap due to alkyl units. X-ray analyses revealed that while 697b had orthorhombic crystals, 697cb had monoclinic crystals. A similar procedure was applied for the synthesis of 699, to give heptathienoacene (700).393 Optical measurements showed that heptathienoacene had red-shifted absorption and emission spectra and, hence, a smaller band gap, as compared to that of pentathienoacene, due to the extended π-conjugation. Pentathienoacene 689 had only π-stacked packing and an absorption maximum at 344 nm in the solid state, which was blue-shifted as compared to the λmax obtained in DCM by 13 nm.

of 31% and 27%, respectively (Scheme 135). They used a TIPS group to block terminal α-positions and at the same time to enhance the solubility. Replacement of one bromine atom of 2,5-dibromothieno[3,2-b]thiophene (103) with TIPS, followed by treatment with LDA, gave 3-bromo-5-(triisopropylsilyl)thieno[3,2-b]thiophene (695a) via the halogen dance mechanism. Subsequent reaction with Bu3SnSSnBu3 in the presence of Pd(PPh3)4 gave the corresponding sulfur-bridged precursor (696a), which was subjected to ring closure, applying the n-BuLi/CuCl2 protocol to yield TIPS-substituted pentathienoacene (697a). Desilylation with TFA afforded the unsubstituted thiopentacene (689).393 Zhang et al.399 disclosed the role of dynamic disorder of 689 indicating its significant role in the 1-D and 2-D charge trasport processes. The same strategy was used by Matzger and Martin,400 who synthesized 2,6-bis-alkyl-pentathienoacene 697b,c for solution-processing. Halogen dancing of 694b,c by lithiation CF


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Table 20. Summary of the Properties of Some Dithienothiophenes

CG


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Table 20. continued

CH


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Table 20. continued

CI


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Table 20. continued

CJ


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Table 20. continued

a

In solution. bSolid. cIn CHCl3. eElectron mobility. PPolymer of the corresponding compound.

Scheme 133. Preparation of Tetrathienoacene (687) and Pentathienoacene (689) from 16, [1+1+2] and [2+1+1] One-Pot Synthesis Routes for Achieving 687, and the Structure of Bisindeno-thienoacene 69025,272,274,394,396

Scheme 134. Preparation of DTTs End-Capped with Styrene (693a) and Alkyl-Substituted Styrene (693b) through Stille Coupling395

TD-DFT calculations performed using the B3LYP/6-31G(d) basis set provided an electronic transition at 342 nm.59 Fan et al.401 investigated the effect of the length of thienoacenes (DTT, pentathienoacene, and heptathienoacene) on the transmission properties of the substituent groups through density functional theory, which showed that heptathieno[3,2-b:2′,3′d]thiophene (701), possessing NH2 and NO2 substituents, was the best oligomer for device application (Scheme 135). He et al.43,44,402−404 synthesized didecanyl-substituted tetrathienoacene (706) in an overall yield of 22% in five steps, CK


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Scheme 135. Synthesis of 697a−c, 689, and 700 Starting from 694a−c, and 534, Respectively, and the Structure of 701 Having NO2 and NH2 Groups393,400,401

Figure 38. (a) Front and side views of the molecular structures of 693a with 50% probability ellipsoids. (b) Stereographic views of unit cells of 693a (hydrogen atoms have been omitted for clarity). (c) View of stacking patterns in the 693a crystals along the b axis (for clarity, only one sheet-like array of molecules (0.5 < c 106.415 4.2. S-Anellated β-Oligothiophenes

Making a sulfur bridge between 2- and 2′-carbons of 3,3′bithiophene produces isomers of DTT. The synthesis of the CP


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Chart 62. Compound 762 Possessing Two Thieno[2,3-b]thiophenes and a Dithieno[2,3-b:3′,2′-d]thiophene419

Scheme 144. Optically Active Fused Heptamers 765 and 769420,421

having an antiarrangement to TTs, demonstrated a sandwichherringbone structure with strong C···S and S···S intermolecular interactions, demonstrated by an X-ray single-crystal analysis (Figure 42). An absorption maximum was observed at 288 nm

[7]helicenes (−)-769 and (+)-769 with enantiomeric excesses (ee) of 97% and 93%, respectively (Scheme 144). An [11]helicene was also prepared in the presence of (−)-sparteine (Figure 43).422

Figure 42. (a) Top and (b) side views for 762 (30% probability level, hydrogen atoms are omitted for clarity), and (c) sandwichherringbone arrangement with C···S and S···S interactions in the crystal packing.419 Reprinted (adapted) with permission from ref 419. Copyright 2013 American Chemical Society.

with a shoulder appearing at 355 nm, which resulted in an optical band gap of 2.7 eV. TD-DFT calculations at B3LYP/6-31G(d,p) level unveiled this λmax as HOMO to LUMO+1 excitation with an oscillator strength of 0.38. The electrochemically obtained HOMO level was −5.53 eV.419 The first optically active, fused heptamer ((−)-765) was synthesized in 20−37% yields with an enantiomeric excesses (ee) of 19−47% by Rajca et al.420,421 from the condensation reaction of 4,4′-dibromo-5,5′-bis(trimethylsilyl)bi(thieno[2,3-b:3′,2′-d]thiophene-3-yl) (763) in the presence of (−)-sparteine to form a chiral dilithiated intermediate ((−)-764). Desilylation of rac-765 was followed by treatment with LDA and then a chiral menthyl-substituted chlorosiloxane [(−)-MenthSiCl (767)] to give the protected diastereomeric mixture ((−)-768 and (+)-769). Separation by column chromatography and desilylation with TFA afforded highly optically pure

Figure 43. Molecular structure and conformation for [11]helicene (top view, A; side view, B; 50% probability level, hydrogen atoms are omitted for clarity).422 Reprinted (adapted) with permission from ref 422. Copyright 2005 American Chemical Society.

Li et al.423 prepared a racemic double helicene (772), which was D2-symmetric, from the reaction of 771 with LDA and then CuCl2 in 48% yield (Scheme 145). 771 was synthesized from 2,5-ditrimethylsilyl substituted DTT (770) in three steps in an overall yield of 66%. X-ray crystallographic analysis indicated strong intermolecular π−π and S···S interactions. Optical studies revealed a highly π-electron delocalization, which demonstrated CQ


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Scheme 145. Synthesis of Double Helicene 772 from 770 and the Structure of 773423

Figure 44. Molecular structure and conformation for [7]helicenes rac-777 (atom numbering and disorder for the C7-alkyl chains are omitted for clarity; 50% probability level).424 Reprinted (adapted) with permission from ref 424. Copyright 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Osuna et al. calculated the HOMO and LUMO levels of dimethyl-substituted [7]helicene (778) and [11]helicene (779) at the B3LYP/6-31G(d,p) level (Chart 63). The Chart 63. [7]Helicene (778) and [11]Helicene (779) Were Studied Computationally377,425

two absorption maxima at 247 and 349 nm along with a shoulder peak at 354 nm, bathochromically shifted as compared to [7]helicene (773). The synthesis of phenyl-condensed carbon−sulfur [7]helicene was performed using (−)-B-chlorodiisopinocamphenylborane (775), which was applied to reduce the racemic diketone selectively to its corresponding alcohol (+)-776 and leaving (−)-774 unreacted (Scheme 146). The alcohol was oxidized to an enantiomerically pure diketone (+)-774 with pyridinium chlorochromate (PCC). Compounds (+)-777 and (−)-777 were obtained using TiCl3/Zn/DME in an intramolecular McMurry reaction with enantiomeric excesses (ee) of 50−90% (Figure 44).424

HOMO/LUMO energy levels were −5.526/−0.859 and −5.467/−1.026 eV for 778 and 779, respectively.377 The properties of the helicenes, possessing a number of thiophene rings from 4 to 20, along with thiophene, TT, and DTT have also been studied computationally in depth.425−427 Nenajdenko and co-workers428 synthesized octathio[8]circlene, C16S8 (782), in an overall yield of 80%. Tetrathiophene (780), synthesized from 3,4-dibromothiophene (15) in 70% yield,429 was treated with LDA, elemental sulfur, and then HCl(aq) to yield the corresponding thiol (781). Vacuum pyrolysis

Scheme 146. Synthesis of Optically Active 777 by Using (−)-B-Chlorodiisopinocamphenylborane (775), Which Reduces Diketone (+)-774 Selectively424

CR


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a twist angle of ∼10° (Figure 45). Nuclear independent chemical shift (NICS) analysis demonstrated an aromatic structure for 782 with a NICS value of −9.7.431 In addition to sulflower (C16S8), the electronic and structural properties of decathio[10]circulene (C20S10) were explored but only computationally at the DFT level (B3LYP/6-311G(d)).432 Summaries on the properties of some important thienoacenes and their oligomers and polymers are given in Chart 64 and Table 21.

Scheme 147. Preparation of Highly Symmetric 782428

Chart 64. Thienoacene, Having Oligomers and Polymer, Fabricated Devices That Provided Relatively Good Hole (μ+) Mobilities (cm2 V−1 s−1)395,410,414

of 781 gave the highly symmetric 782 (Scheme 147), which was completely insoluble in common organic solvents. On the basis of solid-state magic-angle spinning 13C NMR and X-ray powder diffraction analyses, 782 possesses a highly symmetric and planar structure (Figure 45). Gahungu et al.430 investigated the

5. CONCLUSION In this Review, an in-depth survey on fused oligothiophenes, that is, from the smallest thienothiophenes to thienoacenes, has been reported, which covered their syntheses, derivatizations, and applications in material science. On the basis of the content of the number of thiophenes, the survey had three sections, thienothiophenes, dithienothiophenes, and thienoacenes, which possess two, three, and more than three annulated thiophene units, respectively. Synthetic methodologies and their use in the construction of oligomers and polymers were depicted systematically. Their intriguing properties and appealing applications in the field of organic light-emitting diodes (OLED), organic field-effect transistors (OFET), (dye sensitized) solar cells, and electrochromic devices (ECD) were highlighted within the molecular level including HOMO, LUMO, band gap, electronic excitations, and intramolecular charge transfers. Moreover, the analyses (AFM, TEM, STEM, etc.) involving amorphous and single-crystal structures shed light on the surface and packing of molecules, which shape the performances of devices. The results obtained from OLED, OFET, solar cell, and ECD investigations were elaborated by addressing the properties of HOMO−LUMO levels and electronic structures of oligomers and polymers with the help of computational studies. As these compounds are rich in sulfur, the smallest of which possesses two sulfur atoms, they are particularly good electron donors. On the other hand, when one thienyl ring is oxidized to thienyl-S,S-dioxide, an increase in their electron affinity and photoluminescence is observed. Because of their increasing importance particularly in material chemistry, various methods have now been developed for their syntheses, derivatization, and inclusion in small molecules and polymers, which have been carefully included in this Review. As these compounds have a flat structure and are rich in electrons, they literally offer many possibilities to design materials possessing a wide range of conformational properties such as rigidity, stacking, strong intermolecular (π−π and S···S) interactions, self-assembly, and

Figure 45. (Above) Packing of sulflower 782 molecules in the unit cell (yellow, S; white, C).428 Reprinted (adapted) with permission from ref 428. Copyright 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. (Below) B3P86/6-31G(d,p) level optimized conformations for (a) monocation, (b) neutral, and (c) anion of C16S8.430 Reprinted (adapted) with permission from ref 430. Copyright 2009 American Chemical Society.

oxidation and reduction of 782 using DFT calculations at the B3P86/6-31G(d,p) level, which indicated the mono- and dications to be almost planar as neutral 782, and anion having CS


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Table 21. Summary of the Properties of Some Thienoacenes

a

In solution. bSolid. cIn CHCl3. PPolymer of the corresponding compound.

demonstrated to be convenient for fine-tuning of HOMO− LUMO energy levels, thereby enhancing power conversion and charge transfer efficiencies. Thienothiophenes, dithienothiophenes, and thienoacenes are important molecules in organic heterocyclic chemistry and organic material chemistry from both a synthetic point of view and for their applications in organic materials. Thus, it is clear that they will continue to attract the interest of many scientists for new fascinating avenues.

well-defined structures. Altering such properties closely affects the properties of the resultant materials such as absorption/ emission, HOMO/LUMO energy levels, band gaps, etc. These materials also offer easy fine-tuning of chemical and physical properties of the designed materials through the varying chemical structures and functionalities of the included oligothiophenes. Among the four thienothiophene isomers, thieno[3,2-b]thiophene 1 and thieno[3,4-b]thiophene 2 are the most employed molecules in material chemistry, as they provide materials with the desired properties such as lower band gap and better absorption/emission properties and HOMO/LUMO energy levels. Concerning thieno[3,2-b]thiophene, this is due to suitable extended conjugation between two fused flat thiophenes, which also provides proper quinoid structure at its oxidation state. Although thieno[3,4-b]thiophene does not have any extended conjugation with its second fused thiophene ring, at the oxidation state, while one of the rings acquires quinoid structure, the second ring remains aromatic, which provides an extra stability to the quinoid ring. Regarding dithienothiophenes, among the six isomers, dithieno[3,2-b;2′,3′-d]thiophene 491 is the most studied, and, similar to thieno[3,2-b]thiophene, it has a proper conjugation, providing a proper quinoid form at the oxidation state. On the other hand, although thienoacenes, possessing more than three annulated thiophenes, are not as appealing as the thienothiophenes and dithienothiophenes in material science, which is mainly due to their handling difficulties, that is, synthesis and solubility, they offer new challeges from a synthetic point of view. In this Review, it was also pointed out that the genial combinations of thienothiophenes and dithienothiophenes with electron-withdrawing groups provide donor−acceptor (D−A)type systems having small HOMO−LUMO gaps, which are useful to improve the performance of OLED and OFET devices. The art of using substituents on TTs and DTTs, such as alky chains, electron-withdrawing units, halogens, etc., was

AUTHOR INFORMATION Corresponding Author

*Fax: +90 (212) 285 6386. E-mail: [email protected]. Notes

The authors declare no competing financial interest. Biographies

M. Emin Cinar completed his B.Sc. (Honors) degree at the Middle East Technical University in Ankara in 2001 and later obtained M.Sc. (Honors) and Ph.D. (Magna cum lauda) degrees at the University of CT


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Siegen, Germany, under the supervision of Prof. Michael Schmittel in 2004 and 2010, respectively. He then carried out TUBITAK-BIDEB postdoctoral research with Prof. Turan Ozturk. He is currently conducting a postdoctoral research with Prof. Michael Schmittel. His research interests involve the reaction mechanism, nonstatistical dynamics, OLED, OFET, solar cells, nanorotors and -switches, and computational chemistry.

Turan Ozturk received his Ph.D. degree from the University of East Anglia, England, and then moved to the University of Kent at Canterbury, England, as a postdoctoral fellow, where he developed a new method for the synthesis of fused 1,4-dithiin and thiophene rings from 1,8-diketones using Lawesseon’s reagent and P4S10. He took up a position at TUBITAK MRC, Turkey, then Middle East Technical University, Turkey, and joined Istanbul Technical University as a full professor. He has previously been British Council Research Fellow, NATO Research Fellow and Honorary Lecturer at the University of Kent at Canterbury, and Senior Research Fellow at University of Waterloo, Canada. His research interests concentrate on the development of new organic materials having electronic and optical properties.

ACKNOWLEDGMENTS We thank Profesor John A. Joule of The University of Manchester for reading the manuscript, The Scientific and Technological Research Council of Turkey for a grant to M.E.C. (grant no. TUBITAK BIDEB 2216), and Unsped Global Logistic for financial support. REFERENCES (1) Encyclopedia Britannica, Encyclopedia Britannica Online. Official website: heterocyclic compound, http://www.britannica.com. (2) Eicher, T.; Hauptmann, S. The Chemistry of Heterocycles, 2nd ed.; Wiley-VCH Verlag: Weinheim, 2003; Chapter 5, p 52. (3) Meyer, V. Ber. Dtsch. Chem. Ges. 1883, 16, 1465. (4) (a) Katz, H. E.; Bao, Z.; Gilat, S. L. Acc. Chem. Res. 2001, 34, 359. (b) Wang, C.; Dong, H.; Hu, W.; Liu, Y.; Zhu, D. Chem. Rev. 2012, 112, 2208. (c) Dong, H.; Wang, C.; Hu, W. Chem. Commun. 2010, 46, 5211. (5) (a) Kang, B.; Lee, W. H.; Cho, K. ACS Appl. Mater. Interfaces 2013, 5, 2302. (b) Brabec, C. J.; Heeney, M.; McCulloch, I.; Nelson, J. Chem. Soc. Rev. 2011, 40, 1185. (6) (a) Cheng, Y.-J.; Yang, S.-H.; Hsu, C.-S. Chem. Rev. 2009, 109, 5868. (b) Mishra, A.; Bäuerle, P. Angew. Chem., Int. Ed. 2012, 51, 2020. (7) Arias, A. C.; MacKenzie, J. D.; McCulloch, I.; Rivnay, J.; Salleo, A. Chem. Rev. 2010, 110, 3. (8) Bundgaard, E.; Helgesen, M.; Carlé, J. E.; Krebs, F. C.; Jørgensen, M. Macromol. Chem. Phys. 2013, 214, 1546. (9) (a) Roncali, J. Chem. Rev. 1992, 92, 711. (b) Roncali, J. Chem. Rev. 1997, 97, 173. (c) Skabara, P. J. In Handbook of Thiophene-Based CU


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