Ultrafast excited-state planarization of the hexamethylsexithiophene oligomer studied by femtosecond time-resolved photoluminescence

The ultrafast excited-state dynamics of hexamethylsexithiophene in solution are investigated by femtosecond time-resolved photoluminescence. The spontaneous recombination lifetime of the singlet excited state is measured to be of the order of 400 ps. A fast spectral red-shift of the photoluminescence band is observed in the first picosecond after excitation and its time dynamics is characterized. The effect is attributed to an increase of the effective conjugation length of the oligomer in the excited state caused by planarization of the backbone. The kinetics of the process are controlled by excess energy redistribution via vibrational and torsional coupling. A spectral drift of the emission peak persists in the 10–102 ps time domain. The possible assignments of this slower phenomenon are discussed.