Synthesis, optical and electrochemical properties of 1,1',3,3'-tetraaryl-4,4'-bibenzo[c]thiophene derivatives with same or different aryl substituents on the thiophene rings

We designed and synthesized 1,1',3,3'-tetraaryl-4,4'-bibenzo[c]thiophene derivatives, 1,1',3,3'-PhtBu-4,4'-BBT (BBT-PhtBu4), 1,1',3,3'-PhCN-4,4'-BBT (BBT-PhCN4), and 1,1'-PhtBu-3,3'-PhCN-4,4'-BBT (BBT-PhtBu2PhCN2) which have four electron-donating tert-butylphenyl groups, four electron-withdrawing cyanophenyl groups, and two tert-butylphenyl groups and two cyanophenyl groups, respectively, on each thiophene ring by the Stille coupling reaction using 1,1'-diaryl-3,3'-distannyl-4,4′-BBT or 1,1',3,3'-tetrastannyl-4,4′-BBT. It was found that the photoabsorption and fluorescence maximum wavelengths (λmaxabs and λmaxfl) of BBT-PhtBu4, BBT-PhCN4, and BBT-PhtBu2PhCN2 appear in a longer wavelength, respectively, in comparison with those of 1,1′-diaryl-4,4′-BBT derivatives BBT-PhtBu2 with a tert-butylphenyl group and BBT-PhCN2 with a cyanophenyl group on each thiophene ring. Moreover, the cyclic voltammetry (CV) for the 4,4′-BBT derivatives indicated that the introduction of the electron-donating tert-butylphenyl group and/or the electron-withdrawing cyanophenyl group into the benzo[c]thiophene skeleton leads to the lowering of the oxidation potential. In addition, an electron-donating phenyl substituent is more effective than an electron-withdrawing phenyl substituent in the lowering of the oxidation potential. The density functional theory (DFT) calculations as well as the experimental results revealed that increasing the number of electron-donating and electron-withdrawing phenyl group on the thiophene ring results in the rising of HOMO energy level and the lowering of LUMO energy level, respectively, that leads to the decrease in the HOMO–LUMO band gap, that is, the bathochromic shift of photoabsorption band. Thus, this work provides not only facile synthetic methods for 1,1',3,3'-tetraaryl-4,4'-BBT derivatives with same or different aryl substituents on the thiophene rings but also useful methods to precisely adjust their optical and electrochemical properties.

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