Optical spectra of oligothiophenes: vibronic states, torsional motions, and solvent shifts

Low temperature fluorescence and absorption spectra of α,α′-quinquethiophene (5T) are almost mirror symmetrical due to small differences in the nuclear displacements of the vibronic modes in the electronic S 0 and S 1 states. The spectra are in good accordance with Franck–Condon calculations based on ab initio Hartree–Fock (HF/6-311G ∗) and restricted configuration singles (RCIS/6-311G ∗) quantum chemical calculations. With increasing temperature the spectra are broadened and blue-shifted, and the mirror symmetry between fluorescence and absorption is lost. The spectral shifts can be quantitatively correlated to the changes in the polarizabiltity of the environment. Inhomogeneous broadening is simulated by convolution of the spectra with appropriate distribution functions. The loss of mirror symmetry, which occurs exactly at the melting point of the solvent, is well described by the excitation of torsional modes. Measurements of the oligothiophenes in different solvents allow the determination of the adiabatic transition energies in vacuo, which are compared to the quantum chemical calculations.