Excitation Energy Transfer among Chlorophyll a Molecules in Polystyrene: Concentration Dependence of Quantum Yield, Polarization and Lifetime of Fluorescence

Electronic excitation energy transfer was studied for chlorophyll a in a solid solution of poly­ styrene by measuring the concentration quenching of quantum yield, polarization, and lifetime of fluorescence. The concentration quenching of the experimental fluorescence quantum yield is adequately described by Kelly and Porter’s empirical formula (Proc. Roy. Soc., Lond. A 315, 149, 1970), and of polarization of fluorescence by the Jablonski theory (Acta Phys. Pol., 14, 295, 1955). With increasing concentration of chlorophyll a, the fluorescence peak at 672 nm (mainly monomer) is red-shifted, the intensity of the emission peak at '■»■'730 nm (mainly aggregate) relative to that at the shorter wavelength is increased. The R 0 values, calculated by using total concentrations, for the emission at 672 nm and 730 nm are 73 + 2 A and 45 ± 1 A, respectively. This may suggest that the chlorophyll monomers have a greater efficiency of energy transfer than the aggregates, which fluoresce at ~ 7 3 0 nm.