Radiationless transitions in p-terphenyl crystals doped with anthracene, tetracene and pentacene

Abstract

The fluorescence spectra and lifetimes of anthracene, tetracene and pentacene molecules in a host p-terphenyl single crystalline lattice have been measured over the temperature range 10–300 K. At low temperatures the molecules occupy different sites in the host lattice: five for anthracene (O₁, O₂, O₃, O₄, O₅), six for tetracene (O₁, O₂, A₁, A₂, A₃, A₄) and four for pentacene (O₁, O₂, O₃, O₄). The high-temperature fluorescence spectra are dominated by particular sites (O₄/O₅ for anthracene, A₂/A₄ for tetracene and O₁/O₂ for pentacene) and the temperature dependence of the fluorescence lifetime of these sites is recorded. Neither O₄/O₅ for anthracene nor A₂/A₄ for tetracene exhibit a marked temperature dependence whereas the lifetime of O₁/O₂ for pentacene decreases with increasing temperature with an activation energy of 60–90 cm^–1. This temperature dependence is associated with an increased probability for intersystem crossing from O₁/O₂ to a triplet level (T₂) of the pentacene molecule which is located at 16 960 ± 15 cm^–1. For anthracene and tetracene T₂ is inaccessible from all sites over the entire temperature range and for anthracene the short lifetime reflects the facile transfer of excitation from S₁ to T₁.