Femtosecond spectral relaxation of -conjugated hexamethylsexithiophene in solution

The effects of ring rotation on both thermalization and decay kinetics of the excited state in methyl-substituted sexithiophene are investigated in solution by femtosecond pump and probe experiments. Electron-ring torsion coupling is proposed to play a fundamental role in leading to a slow spectral relaxation of the emitting state and driving the internal conversion to the ground state. Within our model, planarization of the excited state takes place in 4 ps in chloroform solution. The recovery of the excited-state population has been measured up to 500 ps in a broad range of probe wavelength from 490 nm up to 1 m. © 1995 The American Physical Society.