ULTRAFAST EXCITED-STATE PLANARIZATION OF THE HEXAMETHYLSEXITHIOPHENE OLIGOMER STUDIED BY FEMTOSECOND TIME-RESOLVED PHOTOLUMINESCENCE

The ultrafast excited-state dynamics of hexamethylsexithiophene in sol ution are investigated by femtosecond time-resolved photoluminescence, The spontaneous recombination lifetime of the singlet excited state i s 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 e xcitation and its time dynamics is characterized. The effect is attrib uted to an increase of the effective conjugation length of the oligome r in the excited state caused by planarization of the backbone. The ki netics of the process are controlled by excess energy redistribution v ia vibrational and torsional coupling. A spectral drift of the emissio n peak persists in the 10-10(2) ps time domain. The possible assignmen ts of this slower phenomenon are discussed. (C) 1998 Elsevier Science B.V. All rights reserved.