Intermolecular energy transfer in binary systems of dye polymers

We present results and physical interpretations for the energy transfer mec hanisms in two-component dye polymer systems. The data consist of fluoresce nce emission spectra and decays. Two dyes were embedded in an epoxypolymer base, and only they participated in the energy transfer. Following pulsed l aser excitation of the donor dye, energy transfer took place to the accept dye. The possible transfer paths considered here were nonradiative and radi ative transfer. The latter involves two steps, emission and absorption of a photon, and therefore is relatively slow, while nonradiative transfer is a fast single step resulting from direct Coulomb interactions. A predominant ly nonradiative transfer is desirable for applications, for instance in wav elength shifters in high energy particle detection. We studied the concentr ation effects of the dyes on the energy transfer and obtained the relative quantum efficiencies of various wavelength shifters from the fluorescence e mission spectra. For low acceptor concentrations, radiative transfer was fo und to dominate, while nonradiative transfer became dominant at increasing dye concentrations. The fluorescence decays were analyzed with a sum-of-exp onentials method and with Forster kinetics. The sum of exponential model yi elded mean decay times of the dye polymers useful for a general classificat ion. The decay times decreased as desired with increasing acceptor concentr ation. The samples, in which nonradiative energy transfer dominated, were a nalyzed with Forster kinetics. As a result, the natural decay times of the donor and acceptor dyes and the critical radii for nonradiative energy tran sfer were obtained from a global best fit. (C) 2000 American Institute of P hysics. [S0021-8979(00)07720-3].