Picosecond time-resolved dual fluorescence, transient absorption and reorientation time measurements of push-pull diphenyl-polyenes: Evidence for 'loose' complex and 'bicimer' species

Picosecond streak camera measurements, time-resolved absorption and reorientation time measurements (by optical Kerr effect) of push-pull diphenyl-polyenes (4-dimethylamino 4'-cyano stilbene: DCS, 1-(p-N,N-dimethylamino)-4-(p-cyanophenyl)-1,3-butadiene: DCB) in solution have revealed, in polar solvents at high concentration and high excitation intensity, the presence of a new emitting species characterized by an anomalous red-shifted fluorescence band. The formation rate and the excited state decay time of the new species is measured for the first time at room temperature as well as the spectral characteristics of the fluorescence. The intensity of the new fluorescence band has been found to be proportional to the square of the excitation intensity. Furthermore, the reorientation time at high concentration has been determined to be nearly twice as long as that at low concentration, indicating the possibility of the formation of complexes in the ground state with nearly twice the molecular volume compared with the monomer. This bimolecular and biphotonic process, demonstrated by the experimental results, makes it possible to reveal a 'loose' complex formation in the ground state, leading upon high excitation intensity to the new emitting species already called 'bicimer' in preliminary studies and formed by the association of two electronically excited molecules.