Singlet-Exciton Energy Transfer in Tetracene-doped Anthracene Crystals as Studied by Time-Resolved Spectroscopy

Abstract

The time-resolved sensitized fluorescence of tetracene-doped anthracene crystals following a two-photon picosecond-pulse excitation was investigated experimentally as a function of dopant concentration and temperature. Even at the optimum time resolution achieved in these experiments (about 5 ps) the time dependence of the host fluorescence was always purely exponential, that of the guest fluorescence a superposition of two exponentials. Thus no time dependence of the energy-transfer rate could be identified. The temperature dependence of the energy-transfer rate is explained tentatively in terms of a “hopping model” (hopping time th ≈ 5.8 · 10−14 s) and thermal reactivation from shallow exciton traps (trap depth 43 cm−1) in the vicinity of the guest molecules. In pure anthracene crystals the intrinsic exciton lifetime (observed at 20 K) is (5.0 ± 0.5) ns. At higher temperatures it increases to about 15.5 ns at 300 K due to reabsorption processes. Below 20 K it decreases to about 1.5 ns at 1.6 K because of exciton trapping at shallow X-traps (trap depth 23 cm−1). When using conventional one-photon excitation exciton-exciton annihilation was observed with an annihilation constant γss ≈ 1.0 × 10−8 cm3 s−1. © 1982, Walter de Gruyter. All rights reserved.