cis peak as probe to investigate the molecular structure. Application to the rotational isomerism of 2,5-diphenylethenyl(hetero)arenes

A combined theoretical and experimental study on rotational isomerism, excited electronic states structure and cis peak effect is presented for flexible conjugated molecules containing two styryl groups coupled by an unsaturated pentatomic ring (furan, thiophene, pyrrole, cyclopentadiene). The several conformers of the ground electronic state have been investigated by means of Hartree-Fock ab initio methods and density functional theory. The electronic spectra have been calculated with the CS INDO CI approach in the singly-excited scheme and in that involving multiple excitations. By means of the theoretical findings and of the analysis of the cis peak of UV-vis absorption spectra, it is shown that the stable rotamer of these molecules is the trans-trans conformer. The trans-cis conformer may have also a relative presence at room temperature, mainly in 2,5-(PhE)₂P. The cis-cis form is the most destabilized. The importance of the doubly excited electronic configurations in the theoretical treatments of the excited electronic states, in particular of the one responsible for the cis peak, the 2¹A₁ state, is shown. Some useful considerations on the S₁ → S_n transient spectra are reported.