Mechanisms of singlet energy transfer in doped anthracene crystals at 5 K, studied by nanosecond spectrofluorimetry

Abstract

Energy transfer at 5 K in crystalline anthracene doped with 2-methylanthracene (2MeA) has been studied by a combination of fluorescence spectroscopy and time-resolved spectrofluorimetry. The dopant induces two distinct traps, A and B, which can be assigned to specific orientations of the guest. Energy transfer to the A traps (which are 20 times more abundant) can be described in terms of first-order kinetics; the rate constant is very large, ≈ 10^–7 cm³ s^–1, corresponding to an exciton diffusion constant of 0.1 cm² s^–1. In unstrained crystals, with more than 100 p.p.m. of 2MeA, energy transfer from A to B sites occurs. This process is relatively slow, with a time-dependent rate constant, and it proceeds principally by long-range resonant exchange. Straining the crystals markedly increases energy transfer direct from host to B traps; this is attributed to the pinning of line dislocations at B sites, in which the guest molecule is more crowded, so creating an extended trapping region.