Fluorescence Quenching and Energy Transfer in Monomolecular Films Containing Chlorophyll

Abstract

Quenching of the fluorescence of chlorophyll a in monomolecular films by copper pheophytin a, a non‐fluorescent chlorophyll derivative, has been measured as a function of quencher concentration. For some measurements the monolayers consisted entirely of chlorophyll plus an admixture of quencher (0.001–0.2 mole fraction). For other measurements, the pigments were dissolved in monolayers of the inert diluent oleyl alcohol. The fluorescence quenching data were analyzed in terms of the inductive resonance transfer mechanism of Förster for the chlorophyll—quencher interaction. For the dilute layers, where a single‐transfer calculation is satisfactory, the range of the chlorophyll—quencher interaction in the Förster theory is 40 Å. This result is in fortuitously good agreement with values of 38–41 Å derived from optical absorption and emission spectra also measured in dilute monomolecular layers. For the undiluted monolayers, where interactions among chlorophyll molecules are much more important, the single‐transfer model is not applicable. The fluorescence quenching data have been analyzed in terms of diffusion of localized excitations. Values of approximately 13 Å for the range of the chlorophyll—quencher interaction and 20–23 Å for the chlorophyll—chlorophyll interaction were derived from this analysis. These results are to be compared with values of 11–12 Å and 18 Å, for the two interaction ranges, respectively, calculated from the optical properties. It is pointed out that a collective excitation representation would be more appropriate for describing the undiluted monolayers.