Excitation intensity-dependent fluorescence behaviour of some luminescent polymers

The fluorescence lifetime, fluorescence signal output and fluorescence spectral distribution of three highly diluted luminescent polymers (one para-phenylenevinylene and two para-phenylene-ethynylene polymers) in tetrahydrofuran are studied as a function of picosecond laser pulse excitation intensity. The results are compared with coumarin 2 in methanol and a distyrylbenzene molecule in tetrahydrofuran. The fluorescence lifetime and the spectral distribution are practically independent of excitation intensity, indicating the absence of excitation-dependent cooperative effects and the absence of emission from preferred exciton states. The fluorescence signal saturation of the polymer solutions at high excitation intensities indicates the photoexcitation of polymer segments (excitons or polarons) extending on average over two para-phenylene-ethynylene or four para-phenylenevinylene monomeric units.