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

Abstract

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 (λ^abs_max and λ^fl_max) of BBT-PhtBu4, BBT-PhCN4, and BBT-PhtBu2PhCN2 appear in a longerwavelength region, 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) curves of 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. Density functional theory (DFT) calculations as well as the experimental results revealed that increasing the number of electron-donating and electron-withdrawing phenyl groups on the thiophene ring results in the increase of the HOMO energy level and the lowering of the LUMO energy level, respectively, that leads to a decrease in the HOMO–LUMO band gap, that is, a bathochromic shift of the photoabsorption band. Thus, this work provides not only facile synthetic methods for 1,1′,3,3′-tetraaryl-4,4′-BBT derivatives with the same or different aryl substituents on the thiophene rings but also useful methods to precisely adjust their optical and electrochemical properties.