Delayed Fluorescence and Triplet Excition Kinetics in Tetracene Doped Anthracene Crystals

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Abstract
The triplet exciton kinectics in the host-guest system anthracene-tetracene is studied. Intensity and time dependence of the delayed fluorescence emission of both host and guest are investigated for guest concentrations between 10−8 and 10−5 mol/mol. The host triplet state is excited using a xenon are lamp or a krypton ion laser for steady state and a specially shaped xenon flash or a pulsed ruby laser for the transient measurements. The guest concentrations are measured down to 5 × 10−9 mol/mol with an accurcay of 20% by observing the prompt guest fluorescence excited directly with a suitable line of an argon ion laser. For relative guest concentrations smaller than 2 × 10−7 mol/mol the measurements can be described satisfactorily by assuming that the guest molecules act as traps with a lifetime of 0.8ms, a trapping rate for the host excitons of γ∗N with γ∗ = 1.8 × 10.11 cm3 s−1 (N = density of guest molecules), and a detrapping rate of about 100s−1. The analysis involves host-host as well as host-guest annihilations. The observed magnetic field dependence of the delayed fluorescence (directional resonances) agrees with the kinetic measurements. At higher guest concentrations additional traps govern the triplet kinetics.