Vibronic Coupling in ESR Spectra of Triplet States in Molecules: Temperature Effects

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Abstract
The vibronic coupling between triplet exciton and nuclear motions is studied in two limiting cases: (1) exciton localized on a part of a molecule (trimers, dimers) and (2) exciton ``delocalized'' over a molecule (such as coronene). In both cases, the effect of temperature upon ESR spectra is treated. This temperature dependence results from the trapping of the exciton, either in one component of the trimer (localized states), or in a deformed state of the molecule (delocalized state). In the latter case, the nuclear vibrations are also important in mixing different electronic states. The theoretical results obtained are compared to experimental data on the lifetime of an exciton on one component of a trimer, Case (1), and on the temperature dependence of D (zero‐field splitting) in molecules such as coronene, Case (2).