Luminescence and excitation energy transfer in monolayer assemblies incorporating organic dye molecules. II

Abstract

Excitation energy transfer in the monolayer assembly is studied in which the same combination of organic dye molecules as in our preceding paper [J. Appl. Phys. 62, ▪▪▪ (1987)] is employed, but donor molecules are organized into the J aggregate, which consists of large arrays of densely packed chromophores and is characterized by a strong and narrow absorption band (J band) considerably red shifted from the corresponding monomer absorption band and a sharp resonance fluorescence. The acceptor molecules can be dissolved in the J aggregate, resulting in quenching of the resonance fluorescence and appearance of sensitized fluorescence of the acceptor. Quenching of resonance fluorescence by the acceptor obeys the Stern–Volmer relation with a temperature-dependent Stern–Volmer constant. Lowering the temperature from T=293 to 77 K causes the decrease of both the Stern–Volmer constant and the intensity ratio of the sensitized fluorescence to the resonance fluorescence. Comparison of the Stern–Volmer constants and luminescence lifetimes of the donor in the isolated and aggregated states reveals that energy transfer from the J aggregate occurs far more efficiently than from the monomer. The observed temperature dependence is analyzed under an assumption that the rate constant of radiative transition is proportional to 1/T. Such an assumption is rationalized by the model that the coherent exciton extending over a certain domain moves over about 3000 donor molecules, occasionally reaching the vicinity of an acceptor.