Conducting polymers are an interdisciplinary research area involving close collaborations between chemists, material scientists, physicists, and engineers. The field has developed during the last 30 years due to an interest in academic research as well as in possible commercial applications. Much research has been performed on conjugated polyacetylenes, polythiophenes, polypyrroles, polyphenylenes, poly(p-phenylene vinylene)s and other conducting polymers. Polythiophenes are the most studied conjugated polymers, yet, despite the thousands of papers published on polythiophene and its derivatives, very little is known about its close analogue, polyselenophene. Polyselenophenes should have some advantages over polythiophenes, however no reasonably conducting polyselenophene was reported until recently. Oligo-and polyselenophenes have a more quinoid character, lower band gap, and, importantly, they are more difficult to twist than oligo-and polythiophenes. Parent polyselenophene has been studied, while polyselenophene derivatives were practically unexplored until lately. Significant progress in polyselenophenes has been reported over the last two years, leading to the availability of promising polyselenophene materials. This feature article gives a general overview of polyselenophenes and highlights recent progress in this field, mostly from the authors' own work.

The first highly conductive polyselenophene, namely, poly(3,4-ethylenedioxyselenophene) (PEDOS), was synthesized by taking advantage of a novel method for efficiently contracting the selenophene ring. PEDOS shows a relatively low band gap (1.4 eV), very high stability in the oxidized state, and a well-defined spectroelectrochemistry.

A series of new low-band-gap thieno-or selenolo-fused polyselenophenes (P5 and P6) and selenolo-fused polythiophene (P4) (as well as previously reported thieno-fused polythiophene, P3) was prepared systematically by electropolymerization (P4-P6) and by solid-state polymerization (P3, P5 and P6). The 2,5-dibrominated monomers (3Br 2 , 5Br 2 , and 6Br 2 ) undergo solid-state polymerization under slight heating and produce insoluble P3, P5, and P6 as black conducting powders. The spectroelectrochemically measured optical band gaps of P4-P6 films are 0.96, 0.72, and 0.76 eV, respectively. DFT calculations performed on P3-P6 provide excellent estimations of the experimental band gaps of these polymers. The band gap of the polyselenophenes (P5 and P6) is 0.2 eV lower than that of the corresponding polythiophenes (P3 and P4). We introduced a new scheme for band gap control in conjugated polymers by replacing the sulfur atom with a selenium atom in the main and/or peripheral ring, which leads to significant and predictable changes in the band gap of the polymers. This is due to the lower aromaticity of a selenophene ring compared to a thiophene ring. Thus, we have achieved band gap control in very low band gap (∼0.7-1.0 eV) polymers through the use of different combinations of selenium and sulfur atoms in the main and peripheral rings.

Since the discovery of high conductivity in iodine-doped polyacetylene, many interesting conducting polymers have been developed. Of these, polythiophenes have been most studied as electronic materials, with poly(3,4-ethylenedioxythiophene) (PEDOT) and the water-soluble PEDOT-PSS being the most successful commercially used conducting polymers. The polyselenophene family together with poly(3,4-ethylenedioxyselenophene) (PEDOS) and its derivatives have been shown to have slightly different properties compared to these of polythiophene and PEDOT because of their different electron donating characters, aromaticities (selenophene vs thiophene), oxidation potentials, electronegativities, and polarizabilities (Se vs S). As a result, the polyselenophenes, especially PEDOS and its derivatives, show a lower band gap and higher-lying highest occupied molecular orbital (HOMO) levels compared with those of thiophene and the PEDOT family. In an organic materials context, the PEDOS family offers some advantages over PEDOT derivatives. This Account draws on computational studies, synthetic methods, electrochemical polymerizations, chemical polymerizations, and the materials properties of PEDOS and its derivatives to demonstrate the importance of these novel materials, which lie at the frontier of conducting polymer research. In particular, we show that (i) PEDOS derivatives have a lower band gap (about 0.2 eV) than the corresponding PEDOT derivatives. Consequently, PEDOS derivatives can absorb the solar spectrum more efficiently compared to PEDOT derivatives and the properties of optoelectronic devices based on neutral and doped PEDOS should be somewhat different from these of PEDOT. (ii) EDOS derivatives have a greater tendency to undergo electrochemical polymerization compared to EDOT derivatives and offer stable and smooth polymer films. (iii) The PEDOS backbone is more rigid than the PEDOT backbone. (iv) PEDOS derivatives are excellent electrochromic materials with high transparency, and have higher contrast ratio and coloration efficiency. (v) The PEDOS/C electrode offers better control over the formation and size of nanoparticles through Se···Pt interactions compared with the PEDOT/C electrode. In addition to this, we summarize the synthesis, electrochemical polymerization, materials properties, and computational studies of fused polyselenophene analogues, namely, poly(cyclopenta[c]selenophene), and a series of low band gap thieno- or selenolo-fused polyselenophenes and selenolo-fused polythiophene. Additionally, we discuss oxidative and solid state polymerization to obtain conducting PEDOS, and its derivatives, and made throughout comparison with S-analogue where applicable. We found that EDOS-based derivatives have a greater tendency toward solid state polymerization and working at a temperature about 20 °C lower than that required for EDOT-based compounds. Our results demonstrate the utility of EDOS unit for generating promising materials PEDOS and its derivatives for electronic devices. Consequently, EDOS structure is ...

A systematic study of the electrochemical, spectroelectrochemical and electrochromic (EC) properties of poly (alkyl-3,4-ethylenedioxyselenophene) 2, 4, 6, 8 and 12) films is reported. The alkyl substituents lead PEDOS-C n films to exhibit sharper redox peaks than the parent PEDOS film and raise an onset of their oxidation potentials by 0.20-0.62 V. As expected, the introduction of alkyl chains significantly improves the EC properties and the electrochemical stability of PEDOS-C n films and results in high contrast ratios and coloration efficiencies, low switching voltages, and fast switching times. As the length of the alkyl chain increases, the broad absorption peak of PEDOS sharpens and splits into three distinct peaks for PEDOS-C n films starting from n ) 4. We have identified the correlations between EC properties (such as coloration efficiency) and other observed properties of PEDOS-C n films (such as their peak width, the position of the maximum absorption peak, the ratio of the absorption intensity of NIR spectra peaks to visible spectra peaks) and the contrast ratio. Interestingly, EC properties show bell-shaped behavior as a function of alkyl chain length, which reaches the maximum for PEDOS-C 6 film, whereas the EC properties of films with shorter and longer alkyl chains are poorer.

Surprisingly, despite its very high mobility in a single crystal, rubrene shows very low mobility in vacuum-sublimed or solution-processed organic thin-film transistors. We synthesized several rubrene analogues with electron-withdrawing and electron-donating substituents and found that most of the substituted rubrenes are not planar in the solid state. Moreover, we conclude (based on experimental and calculated data) that even parent rubrene is not planar in solution and in thin films. This discovery explains why high mobility is reported in rubrene single crystals, but rubrene shows very low field-effect mobility in thin films. The substituted rubrenes obtained in this work have significantly better solubility than parent rubrene and some even form films and not crystals after evaporation of the solvent. Thus, substituted rubrenes are promising materials for organic light-emitting diode (OLED) applications.

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A novel family of electrochromic materials has been discovered. The electropolymerized poly(hexyl‐3,4‐ethylenedioxyselenophene) film switches color between a highly absorbing pure blue and a nearly colorless bleached state, achieves both a high contrast ratio of 88–89% and a high CE of up to 773 cm2 C−1 while showing a fast switching time and remarkable stability with the contrast ratio remaining 48% after 10000 cycles.