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 re orientation time measurements (by optical Kerr effect) of push-pull di phenyl-polyenes (4-dimethylamino 4'-cyano stilbene: DCS, ,N-dimethylam ino)-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 stat e decay time of the new species is measured for the first time at room temperature as well as the spectral characteristics of the fluorescen ce. The intensity of the new fluorescence band has been found to be pr oportional to the square of the excitation intensity. Furthermore, the reorientation time at high concentration has been determined to be ne arly twice as long as that at low concentration, indicating the possib ility of the formation of complexes in the ground state with nearly tw ice the molecular volume compared with the monomer. This bimolecular a nd 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 a lready called 'bicimer' in preliminary studies and formed by the assoc iation of two electronically excited molecules.