Spectroscopic signatures for planar equilibrium geometries in methyl-substituted oligothiophenes

In recent studies it was demonstrated that temperature-dependent optical spectroscopy is a valuable tool for revealing the differences in the geometries of flexible molecules like oligothiophenes (OTs) in the ground (S0) and first excited (S1) electronic states, by examining the symmetry relations between the absorption and emission spectra: while at low temperature the spectra show mirror symmetry, pointing to planar geometries in S0 and S1, the symmetry relation breaks down at ambient temperature due to thermal population of torsional modes. In the present joint spectroscopic and theoretical study, we demonstrate that this behavior is also observed for di- and tetramethyl-substituted OTs, suggesting an essentially planar equilibrium geometry not only in S1 but also in S0, despite the increasing sterical hindrance which is imposed by the substituents. This rather surprising result is rationalized by the softness of the carbon-sulfur bond, which is able to adapt to the geometrical constraints.