A KINETIC-STUDY OF MONOMER AND EXCIMER FLUORESCENCE OF PYRENE LECITHIN IN LANGMUIR-BLODGETT-FILMS

The fluorescence of Langmuir-Blodgett films of 1-palmitoyl-2-pyrenedecanoylphosphatidylcholine in matrices composed of dipalmitoyl-, 1-palmitoyl-2-oleyl- and dioleylphosphatidylcholine was studied quantitatively at different concentrations and temperatures. Stationary quantum yields, time-resolved life-times and spectral data were used to determine the kinetics of pyrene fluorescence. Possible schemes for monomer and excimer decays are presented and rate parameters for the individual steps are evaluated. The main paths for the monomer decay involve two kinds of non-interacting singlet species. The first is quenched by interaction with another pyrene moiety or an aggregate in the ground state and does not form excimers. The other pyrene moiety is aggregated in such a way that only a small rotational motion is required for it to attain a dimeric configuration which might be a precursor of an excimer. The formation of pyrene excimer is mainly determined by a reorganization, diffusional rotation in or out of the plane of the pyrene moieties in the lattice. Evidence for energy migration in the lattice in the excimer formation process is presented. The excimer is kinetically distinguishable from the monitored monomeric species. Relative intensities of excimer emission are considerably reduced when the temperature decreases, indicating diffusional excimer formation at higher temperatures. Activation energies for excimer formation are evaluated at different temperatures and concentrations. Evidence for the reorganization of the molecular lattice by light and temperature is presented.