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

Abstract

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 5ps) 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 t h ≈ 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 γ ss ≈ 1.0 × 10 -8 cm 3 s -1 .