FEMTOSECOND VIBRATIONAL AND TORSIONAL ENERGY REDISTRIBUTION IN PHOTOEXCITED OLIGOTHIOPHENES

Transient stimulated emission has been measured by applying the pump-a nd-probe technique to a series of substituted oligothiophenes in solut ion. The observed dynamics are assigned to the intraband population re laxation associated with the internal energy redistribution process. T his leads ultimately to a molecular conformation more quinoid in chara cter and almost planar, Two characteristic time constants are obtained : the first, about 500-800 fs, is assigned to vibrational relaxation w hile the second, about 4 ps, is assigned to torsional relaxation, whic h involves inter-ring torsional activity. We estimate average dissipat ion times of ca. 170 and ca. 600 fs for vibrational and torsional quan ta, respectively.