Structural Design and Applications of Stereoregular Fused Thiophenes and Their Oligomers and Polymers

Abstract

Stereoregular fused thiophenes (SFTs: especially thieno[3,2-b]thiophene (TT) and dithieo[3,2-b:2′,3′-d]thiophene (DTT)), as stable conjugated structures deriving from thiophene ring enlargement, possess outstanding properties and special configuration, such as the superior carrier transfer efficiency and a high degree backbone of planarity. In comparison to stand-alone SFTs structures, oligomers and polymers containing different heteroaromatic units have been much widely researched and used in many fields. In decade, several important reviews have summarized the broad field of fused thiophenes including SFTs, and their synthesis and optoelectronic applications. Here, we critically present the structure–performance relationships and application of oligomers and polymers containing SFTs (exhibiting thiophene ring number from 2 to 7) units. First, the basic structures and properties of SFTs are briefly stated. Then, oligomers classified by extra conjugated heterocyclic attachments are carefully discussed, focusing on the structure–performance relationships for their optoelectronic applications including organic photovoltaic cells and organic field-effect transistors. Moreover, such relationships in polymers have been applied in much wider fields such as organic light-emitting diodes, electrochromic devices, thermoelectric devices, and supercapacitors are discussed. Finally, a summary and prospect are given. Through this review, instruction for molecular design and novel ideas for the future development of SFTs-contained are provided.     

Band gap tuning of narrow‐polydispersity two‐dimensional conductive polymers with electroactive side‐chains

A series of two‐dimensional donor–acceptor–donor (D1–A(D2)) type of conducting polymers (CPs) all with electroactive bulky side chain structure has been designed, synthesized, and investigated by introducing the donor–acceptor (D1–A) thiophene–quinoxaline moiety in the main chain alongside and additional donor and hole transporting units in the side chain. All the UV‐vis spectra of the 2D polymers, PTPQT, PFPQT, and PCPQT, each with triphenylamine, fluorene, and carbazole units as the D2 side chain, possess strong intramolecular charge transfer absorption, thus resulting in better light harvesting. Their optical and electronic properties were thoroughly explored experimentally and computationally. The effect of molecular weight of the narrow polydispersity polymers on their optoelectronic property was studied in detail. In summary, the 2‐D CPs show potential for use as an active material in optoelectronic devices. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 1217–1227     

Effect of thiophene spacers in benzodithiophene‐based polymers for organic electronics

Poly{2,6‐bis(thiophene‐2‐yl)‐4,8‐bis(5‐dodecylthiophen‐2‐yl)benzo[1,2‐b :4,5‐b' ]dithiophene} [poly(Th‐bDTBDT‐Th)] was successfully synthesized through Stille coupling polymerization. The addition of the thiophene spacer groups between the benzodithiophene units resulted in improved performance in optoelectronic devices. This was attributed to the reduced lamellae stacking distance in thin film with prominent π–π stacking peak indicating close assembly of poly(Th‐bDTBDT‐Th). Spacing between the benzodithiophene units in poly(Th‐bDTBDT‐Th) helped the close packing of dodecyl chains and generated improved π stacking interaction. For poly(Th‐bDTBDT‐Th), the measured average field effect mobility was 2.32 × 10^?3 cm² V^?1 s^?1 and average hole mobility in vertical direction was 2.92 × 10^?5 cm² V^?1 s^?1. Charge transport in both directions was improved by one order of magnitude with the presence of the thiophene spacer. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55 , 3942–3948     

Hybrid Conjugated Organic Oligomers Consisting of Oligodiacetylene and Thiophene Units: Synthesis and Optical Properties

Novel and highly soluble hybrid conjugated organic oligomers consisting of oligodiacetylene and thiophene units have been synthesized in high purity through iterative and divergent approaches based on a sequence of Sonogashira reactions. The series of thiophene‐containing oligodiacetylenes (ThODAs) and homocoupled ThODAs (HThODAs) show—both in solution and in the solid state—a strong optical absorption, which is progressively red shifted with increasing chain length. The linear correlation of the absorption maximum (λ^A_max) with the inverse of conjugation length (CL=number of double and triple bonds) shows that the effective conjugation length of this system is extended up to at least CL=20. Furthermore, absorption measurements of dropcast thin films display not only a bathochromic shift of the absorption maxima but also a higher wavelength absorption, which is attributed to increased π–π interactions. The wavelength of the maximum fluorescence emission (λ^E_max) also increases with CL, and emission is maximal for oligomers with CL=7–12 (fluorescence quantum yield Φ_F=～0.2). Both longer and shorter oligomers display marginal emission. The calculated Stokes shifts of these planar materials are relatively large (0.4 eV) for all oligomers, and likely due to excitation to the S₂ state, thus suggesting that the presence of enyne moieties dominates the ordering of the lowest excited states. The fluorescence lifetimes (τ_F) are short (τ_F,_max=?1 ns) and closely follow the tendency obtained for the fluorescence quantum yield. The anisotropy lifetimes show a near‐linear increase with CL, in line with highly rigid oligomers.     

Comprehensive Analysis of Fragment Orbital Interactions to Build Highly π‐Conjugated Thienylene‐Substituted Phenylene Oligomers

π‐Conjugated thienylene? phenylene oligomers with fluorinated and dialkoxylated phenylene fragments have been designed and prepared to understand the interactions in fragment orbitals, the influence of the substituents (F, OMe) on the HOMO–LUMO gap, and the role of intramolecular non‐covalent cumulative interactions in the construction of π‐conjugated nanostructures. Their strong conjugation was also evidenced in the gas phase by UV photoelectron spectroscopy and theoretical calculations. These results can be explained by the crucial role of the relative energetic positions of the π orbitals of the dimethoxyphenylene, which was used to model the dialkoxyphenylene entity, in determining the π/π^* orbital levels of the fluorinated phenylene entity. Dialkoxyphenylenes raise the HOMO orbitals, whereas fluorinated phenylenes lower the LUMO orbitals in the oligomers. In addition, the presence of S???F and H???F interactions in the fluorinated phenylene? thienylene compounds add to the S???O interactions in the mixed targets and contribute to the full conjugation in the oligomer, inducing weak inter‐ring angles between the involved aromatic cycles. These results, which showed extended conjugation of the π system, were corroborated by a narrow HOMO–LUMO gap (according to DFT calculations) and by a relatively strong maximum wavelength (as obtained by TD‐DFT calculations and experimental UV/Vis measurements). The crystallographic data of two mixed thienylene? (fluorinated and dialkoxylated phenylene) five‐ring oligomers agree with the above results and show the formation of quasi‐planar conformations with non‐covalent S???O, H???F, and S???F interactions. These studies in the solid and gas phases show the relevance of associating dialkoxyphenylene and fluorinated phenylene fragments with thiophene to lead to oligomers with improved electronic delocalization for electronic or optoelectronic devices.     

Synthesis of π‐conjugated polymer containing both thiophene and phosphole sulfide in the main chain by simultaneous post‐element transformation of organotitanium polymer

The synthesis and unique optoelectronic features of a π‐conjugated polymer containing both thiophene and 1‐phenylphosphole sulfide units (multiple heteroles) in the main chain by the post‐element transformation of a regioregular organometallic polymer possessing titanacyclopentadiene‐2,5‐diyl unit are described. The π‐conjugated polymer containing multiple heteroles was obtained in 73% yield by the simultaneous reaction of the organotitanium polymer with sulfur monochloride and dichlorophenylphosphine (0.6 equiv each), whose number‐average molecular weight (M_n) and the molecular‐weight distribution (M_w/M_n) were estimated to be 11,000 and 3.4, respectively, by the size exclusion chromatography (SEC). The π‐conjugated polymer thus obtained was found to have the high HOMO and the low LUMO energy levels due to the electron‐rich thiophene and electron‐deficient phosphole sulfide units, respectively, as supported by its cyclic voltammetry (CV) analysis. Compared to a mixture of a polymer containing sole thiophene‐unit and that containing sole phosphole sulfide units, the π‐conjugated polymer‐containing multiple heteroles proved to exhibit interesting optical properties. For example, a specific emission peak was observed at 608 nm in the photoluminescence spectrum, which was not observed in the case of the thiophene‐containing polymer, the phosphole‐containing polymer, and their mixture. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019 , 57, 2519–2525     

Thieno[3,4‐c]pyrrole‐4,6‐dione‐3,4‐difluorothiophene Polymer Acceptors for Efficient All‐Polymer Bulk Heterojunction Solar Cells

Branched‐alkyl‐substituted poly(thieno[3,4‐c]pyrrole‐4,6‐dione‐alt‐3,4‐difluorothiophene) (PTPD[2F]T) can be used as a polymer acceptor in bulk heterojunction (BHJ) solar cells with a low‐band‐gap polymer donor (PCE10) commonly used with fullerenes. The “all‐polymer” BHJ devices made with PTPD[2F]T achieve efficiencies of up to 4.4 %. While, to date, most efficient polymer acceptors are based on perylenediimide or naphthalenediimide motifs, our study of PTPD[2F]T polymers shows that linear, all‐thiophene systems with adequately substituted main chains can also be conducive to efficient BHJ solar cells with polymer donors.     

Side chain engineering of naphthalene diimide–bithiophene‐based polymer acceptors in all‐polymer solar cells

Two novel polymeric acceptors based on naphthalene diimide (NDI) and 2.2′‐bithiophene, named as P(NDI2THD‐T2) and P(NDI2TOD‐T2), were designed and synthesized for all polymer solar cells application. The structural and electronic properties of the two acceptors were modulated through side‐chain engineering of the NDI units. The optoelectronic properties of the polymers and the morphologies of the blend films composed of the polymer acceptors and a donor polymer PTB7‐Th were systemically investigated. With thiophene groups introduced into the side chains of the NDI units, both polymers showed wider absorption from 350 nm to 900 nm, compared with the reference polymer acceptor of N2200. No redshift of absorption spectra from solutions to films indicated reduced aggregation of the polymers due to the steric hindrance effect of thiophene rings in the side chains. The photovoltaic performance were characterized for devices in a configuration of ITO/PEDOT:PSS/PTB7‐Th:acceptors/2,9‐bis(3‐(dimethylamino)propyl)anthra[2,1,9‐def:6,5,10‐def]diisoquinoline‐1,3,8,10(2H,9H)‐tetraone (PDIN)/Al. With the addition of diphenyl ether as an additive, the power conversion efficiencies (PCEs) of 2.73% and 4.75% for P(NDI2THD‐T2) and P(NDI2TOD‐T2) based devices were achieved, respectively. The latter showed improved J_sc, Fill Factor (FF), and PCE compared with N2200 based devices. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55, 3679–3689     

Towards Molecular Design Rationalization in Branched Multi‐Thiophene Semiconductors: The 2‐Thienyl‐Persubstituted α‐Oligothiophenes

The introduction of branching in multi‐thiophene semiconductors, although granting the required solubility for processing, results in an increased molecular fluxionality and a higher level of distortion, thus hampering π conjugation. Accordingly, branched oligothiophenes require rationalization of their structure–reactivity relationships for target‐oriented design and optimization of the synthetic effort. Our current research on spiderlike oligothiophenes affords deep insight into the subject, and introduces new, easily accessible molecules with attractive functional properties. In particular, a regular series, T′X _Y , of five new multi‐thiophene systems, T′5₃ , T′8₄ , T′11₅ , T′14₆ , and T′17₇ , constituted by five, eight, 11, 14, and 17 thiophene units, respectively, their longest α‐conjugated chain consisting of tri‐, tetra‐, penta‐, hexa‐, and heptathiophene moieties, respectively, has been synthesized and fully characterized from the structural, spectroscopic, and electrochemical point of view. The electronic properties of the monomers and their electropolymerization ability are discussed and rationalized as a function of their molecular structure, particularly in comparison with the series of 5‐(2,2′‐dithiophene)yl‐persubstituted α‐oligothiophenes ( TX _Y ) previously reported by us. These oligothiophenes are easily accessible materials, with promising properties for applications as active layers in multifunctional organic devices including solar cells.     

Improved EL efficiency of fluorene‐thieno[3,2‐b]thiophene‐based conjugated copolymers with hole‐transporting or electron‐transporting units in the main chain

New electroluminescent polymers (poly(9,9′‐dioctylfluorene‐co‐thieno[3,2‐b]thiophene‐co‐benzo[2,3,5]thiadiazole) ( P1) and poly(9,9′‐dioctylfluorene‐co‐thieno[3,2‐b]thiophene‐co‐benzo[2,3,5]thiadiazole‐co‐[4‐(2‐ethylhexyloxyl)phenyl]diphenylamine ( P2) ) possess hole‐transporting or electron‐transporting units or both in the main chains. Electron‐deficient benzothiadiazole and electron‐rich triphenylamine moieties were incorporated into the polymer backbone to improve the electron‐transporting and hole‐transporting characteristics, respectively. P1 and P2 show greater solubility than poly(9,9′‐dioctylfluorene‐co‐thieno[3,2‐b]thiophene ( PFTT ), without sacrificing their good thermal stability. Moreover, owing to the incorporation of the electron‐deficient benzothiadiazole unit, P1 and P2 exhibit remarkably lower LUMO levels than PFTT , and thus, it should facilitate the electron injection into the polymer layer from the cathode electrode. Consequently, because of the balance of charge mobility, LED devices based on P1 and P2 exhibit greater brightness and efficiency (up to 3000 cd/m² and 1.35 cd/A) than devices that use the pristine PFTT . © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 243–253, 2006     

High‐Performance All‐Polymer Solar Cells: Synthesis of Polymer Acceptor by a Random Ternary Copolymerization Strategy

Demonstrated in this work is a simple random ternary copolymerization strategy to synthesize a series of polymer acceptors, PTPBT‐ET_x, by polymerizing a small‐molecule acceptor unit modified from Y6 with a thiophene connecting unit and a controlled amount of an 3‐ethylesterthiophene (ET) unit. Compared to PTPBT of only Y6‐like units and thiophene units, PTPBT‐ET_x (where x represents the molar ratio of the ET unit) with an incorporated ET unit in the ternary copolymers show up‐shifted LUMO energy levels, increased electron mobilities, and improved blend morphologies in the blend film with the polymer donor PBDB‐T. And the all‐polymer solar cell (all‐PSC) based on PBDB‐T:PTPBT‐ET_0.3 achieved a high power conversion efficiency over 12.5 %. In addition, the PTPBT‐ET_0.3‐based all‐PSC also exhibits long‐term photostability over 300 hours.     

From Nongelating to Gelating: Synthesis and Structural Self‐Assembling Property Relationships of a Homologous Series of Oligo(amide–triazole)s

A homologous series of oligo(amide–triazole)s (OAT) [ OAT‐CO₂H‐2 n and OAT‐COPrg‐(2 n +1) ] with an increasing number of primary amide (CONH) and triazole hydrogen‐bonding functionalities was prepared by an iterative synthetic procedure. It was found that their self‐assembly and thermoreversible gelation strength had a strong correlation to the number of hydrogen‐bonding moieties in the oligomers. There also existed a threshold value of the number of CONH units, above which all the oligomers became organogelators. Hence, oligomers with ≤4 CONH units are devoid of intermolecular hydrogen bonding and also non‐organogelating, whereas those that contain >4 CONH units show intermolecular association and organogelating properties. For the organogelators, the T_gel value increases monotonically with increasing number of CONH units. On the basis of FTIR measurements, both the CONH and triazole C? H groups were involved in the hydrogen‐bonding process. A mixed xerogel that consisted of a 1:1 weight ratio of two oligomers of different lengths ( OAT‐CO₂H‐6 and OAT‐CO₂H‐12 ) was found to show microphase segregation according to differential scanning calorimetry, thus indicating that oligomers that bear a different number of hydrogen‐bonding units exhibited self‐sorting to maximize the extent of intermolecular hydrogen bonding in the xerogel state.     

Thiophene,Selenophene, and Tellurophene‐based Three‐Dimensional Organic Frameworks

3D frameworks are important because of their potential to combine the advantageous properties of porous materials with those associated with polymers. A series of novel 3D aromatic frameworks are presented that incorporate the heterocycles thiophene, selenophene, and tellurophene. The specific surface area and pore width of frameworks depends on the element that is used to build the framework. Optoelectronic properties are element‐dependent, with heavy atoms red‐shifting the optical properties and decreasing the energy gap of the solid. The metalloid nature of tellurophene allows the properties of this material to be tuned based on its oxidation state, even as an insoluble solid. The incorporation of the optoelectronic active thiophene, selenophene, and tellurophene units and the effect that they have on properties was studied. A supercapcitor device was fabricated using these frameworks, showing that these 3D frameworks are promising for optoelectronic uses.     

Synthesis,photophysics, theoretical modeling,and electroluminescence of novel 2,7‐carbazole‐based conjugated polymers with sterically hindered structures

2,7‐dibromo‐N‐hexylcarbazole is successfully synthesized in three steps with an overall 37% yield. Novel 2,7‐carbazole‐based sterically hindered conjugated polymers are further synthesized. In the backbone structure of polymer P1 , alkylated bithiophene moiety is β‐substituted with dodecyl chains on both thiophene rings, adopting the tail‐to‐tail configuration. While for polymers P2 and P3 , partially planarized thieno[3,2‐b]thiophene moiety ( P2 ) and β‐pentyl substituted thieno[3,2‐b]thiophene ( P3 ) are incorporated. All polymers demonstrate efficient blue‐to‐green light emission, good thermal stability (T_d ≥ 379 °C), and high glass transition temperatures (T_g = 118 °C). The optical and electronic properties of the resulted polymers are tuned by the incorporated alkyl chains. For instance, the incorporation of β‐pentyl group in thieno[3,2‐b]thiophene moiety endows P3 with blue‐shifted photophysical spectra, reduced fluorescence quantum yield and larger band gap in comparison with P2 . The steric effect of incorporated alkyl chains is further illustrated by geometry optimization of three model oligomers (analogues to the repetition units of P1–P3 ) using density functional theory. Sterically hindered polymers P1 and P2 exhibit high charge transport ability and moderate electroluminescent properties in primarily tested single‐layer light‐emitting diodes (configuration: ITO/PEDOT:PSS/Polymer/Ca/Ag). © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 7725–7738, 2008     

Comparison of ground and excited state polarizabilities of thiophene,cyclopentadiene, and fulvene oligomers and their cyano substituted derivatives—Ab initio study

Ab Initio study of the ground and excited state polarizabilities of thiophene, fulvene, and cyclopentadiene based conducting oligomers and their cyano derivatives have been performed using the restricted Hartree–Fock (RHF) and the configuration interaction singles (CIS) approaches, respectively, with 3‐21G* basis set. For comparison purposes, for some small oligomers (monomers and dimers), higher basis sets (6‐31G*, 6‐31+G*, aug‐cc‐pVTZ) were also employed in the computations of polarizabilities. The trends in polarizability as a function of oligomer length were investigated. For all systems, the RHF polarizability increases as n^1.2–1.3 as n gets larger and the CIS polarizability increases as n^1.4–1.6 for n less than seven or eight rings and then increases approximately linearly with n for larger n. For the thiophene based systems the dependence of the polarizability on bond length alternation (BLA) along the backbone of the oligomers was also investigated using the RHF, density functional (DFT), and CIS theories (with 3‐21G* basis set). For thiophene dimer, we also performed RHF/aug‐cc‐pVTZ calculations of polarizabilities versus BLA. We found that the polarizability is largest when BLA is near zero (for both ground and excited states), which correlates with the lowest excitation energy. Comparison with experimental results has been made where possible. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 1983–1995, 2007     

New selenophene‐based low‐band gap conjugated polymers for organic photovoltaics

Low‐band gap selenophene‐based polymers were synthesized. Their optoelectronic and photovoltaic properties and space‐charge limited currents were compared with those of the related thiophene‐based polymers. The band gaps of the Se‐based derivatives were approximately 0.05–0.12 eV lower than those of their thiophene counterparts. Organic photovoltaic (OPV) devices based on the blends of these polymers and 1‐(3‐methoxycarbonyl)propyl‐1‐phenyl‐[6,6]‐C₇₁ (PC₇₁BM) were fabricated, and the maximum power conversion efficiency of the OPV device based on PSPSBT and PC₇₁BM was 3.1%—with a short‐circuit current density (J_sc) of 9.3 mA cm^?2, an open‐circuit voltage (V_oc) of 0.79 V, and a fill factor of 0.42—under AM 1.5 G illumination (100 mW cm^?2). © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013 , 51, 4550–4557     

Efficient Synthesis of Alkyl End‐Capped Oligoheterocycles via the Use of Palladacycle Catalyst

The preparation of mixed thiophene/furan oligomers with alkyl groups at α,α′‐position by the method which we discovered recently in our lab is presented. Thus, the mixed thiophene/furan oligomers can be prepared in good yields from monoiodoarene in the presence of 5 mol % of palladacycle catalyst and 1.2 equiv of N,N‐diisopropylethylamine in DMF at 100°C for 8 h.     

Synthesis and characterization of naphtho[2,1‐b:3,4‐b′]dithiophene‐based polymers with extended π‐conjugation systems for use in bulk heterojunction polymer solar cells

Two novel polymeric semiconductor materials based on naphtho[2,1‐b:3,4‐b']dithiophene (NDT), PNDT‐TTT and PNDT‐TET , were designed and synthesized. These synthesized polymers were tested in bulk heterojunction solar cells as blends with the acceptor [6,6]‐phenyl‐C71‐butyric acid methyl ester (PC₇₁BM). PNDT‐TTT contained tri‐thiophene units, and PNDT‐TET contained bi‐thiophene units coupled by ethylenic linkages. Comparison to the properties of PNDT‐T , which contained single thiophene units, these polymers exhibit red‐shifted absorption spectra as a result of the enhanced conjugation lengths. These effects resulted in high short circuit currents (J_SC) in the organic solar cells. The PNDT‐TET ‐ and PNDT‐TTT ‐based devices exhibited considerably better photovoltaic performances, with power conversion efficiencies of 3.5 and 3.3%, respectively, compared to the PNDT‐T ‐based device (1.3%). © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013 , 51, 4742–4751     

One donor–two acceptor (D–A1)‐(D–A2) random terpolymers containing perylene diimide,naphthalene diimide,and carbazole units

A series of one donor–two acceptor (D–A₁)‐(D–A₂) random terpolymers containing a 2,7‐carbazole donor and varying compositions of perylene diimide (PDI) and naphthalene diimide (NDI) acceptors was synthesized via Suzuki coupling polymerization. The optical properties of the terpolymers are weighted sums of the constituent parent copolymers and all show strong absorption over the 400 to 700 nm range with optical bandgaps ranging from 1.77 to 1.87 eV, depending on acceptor composition. The copolymers were tested as acceptor materials in bulk heterojunction all‐polymer solar cells using poly[(4,8‐bis‐(2‐ethylhexyloxy)‐benzo[1,2‐b;4,5‐b′]dithiophene)‐2,6‐diyl‐alt‐(4‐(2‐ethylhexanoyl)‐thieno[3,4‐b]thiophene)‐2,6‐diyl] (PBDTTT‐C) as the donor material. In contrast to the optoelectronic properties, the measured device parameters are not composition dependent, and rather depend solely on the presence of the NDI unit, where the devices containing any amount of NDI perform half as well as those using the parent polymer containing only carbazole and PDI. Overall this is the first example of a one donor–two acceptor random terpolymer system containing perylene diimide (PDI) and naphthalene diimide (NDI) acceptor units, and demonstrates a facile method of tuning polymer optoelectronic properties while minimizing the need for complicated synthetic and purification steps. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 3337–3345     

A semiempirical study of heterocycle oligomers and polymers in different dielectric media

Four common five-membered heterocycles—pyrrole, phosphole, thiophene, and furan—and their oligomers with the chain length of 2, 4, 6, and 10 units have been studied quantum chemically using the semiempirical PM 3 parameterization. The oligomers of pyrrole and phosphole with the homolytically dissociated N? H bond and P? H bond, respectively, and oligomers of thiophene and furan with one electron removed per monomer unit (4n + 2 π-electron bipolaron systems) have also been studied. The electronic properties of the respective polymers were extrapolated from the oligomer data. Bulk polymer effects on the electronic structure were modeled using the self-consistent reaction field theory in the multicavity approximation (MC a SCRF ). © 1995 John Wiley & Sons, Inc.