Chain-Length Dependent Stationary and Time-Resolved Spectra of α-Oligothiophenes

By analyzing stationary absorption and fluorescence spectra of several α-oligothiophenes in dichloromethane at room temperature, the S 1 ← S 0 zero−zero transition energies and Huang−Rhys parameters are determined. Reorientational behaviors of the α-oligothiophenes in dichloromethane are also investigated by femtosecond time-resolved fluorescence spectroscopy. The reorientational relaxation observed for each α-oligothiophene in its first excited state is interpreted in terms of revolution of a hydrodynamic prolate ellipsoid of which the emission transition dipole is aligned along the long molecular axis. Our results show that a hydrodynamic slip model could successfully explain the reorientation times of α-terthiophene, α-quaterthiophene, and α-quinquethiophene in dichloromethane at 288 K, by modeling these molecules as prolate ellipsoids with dimensions consistent with their van der Waals volumes.