Femtosecond vibrational and torsional energy redistribution in photoexcited oligothiophenes

Transient stimulated emission has been measured by applying the pump-and-probe technique to a series of substituted oligothiophenes in solution. The observed dynamics are assigned to the intraband population relaxation associated with the internal energy redistribution process. This leads ultimately to a molecular conformation more quinoid in character and almost planar. Two characteristic time constants are obtained: the first, about 500–800 fs, is assigned to vibrational relaxation while the second, about 4 ps, is assigned to torsional relaxation, which involves inter-ring torsional activity. We estimate average dissipation times of ca. 170 and ca. 600 fs for vibrational and torsional quanta, respectively.