S0 → Sn and S1 → Sn absorption spectra of thio-distyrylbenzenes

The molecular structures and the S0 → Sn and S1 → Sn absorption spectra of 1,4-distyrylbenzene and four thio-analogues, where the side phenyl rings are replaced by 2′-thienyl or 3′-thienyl groups and the central benzene ring is replaced by a 2,5-disubstituted thiophene ring, have been investigated by a combined theoretical and experimental approach. The rotational isomerism of these flexible molecules has been analyzed by ab initio quantum chemistry methodologies. The S0 → Sn one-photon absorption spectra and the S1 → Sn transient spectra have been calculated using the CS INDO S-CI and SDT-CI procedures. The calculations on conformational isomers indicate that the stable molecular structures are those where the quasi-single bonds of the vinylene groups have almost planar s-trans configuration. In the 1,4-compounds, there may be an equilibrium between two molecular forms of C2h and C2v symmetry in the model of planar conformations. As concerns the UV-vis absorption spectra, the importance of the cis band, as probe to investigate the molecular structure of the 2,5-compounds, has been pointed out, and the presence of an equilibrium between two rotamers in the compound bearing side 3′-thienyl groups has been confirmed. The S1 → Sn absorption spectra are interpretable only with configuration interaction calculations including the multiple excited configurations. In this respect, the role played by the H2 → L2 double excitation has been emphasized.