A Holstein-Peierls Approach to Excimer Spectra: The Evolution from Vibronically Structured to Unstructured Emission

The presence of excimers, as revealed by broad, structureless, and red-shifted emission, is quite common in a great many organic solution-and solid-phase systems. A fundamental theoretical account of excimer creation and relaxation has generated enormous interest over the years. Here, a model based on a Holstein-Peierls Hamiltonian is presented to account for absorption and photoluminescence in molecular dimers. The model extends the Frenkel-charge transfer (CT)-Holstein model in order to account for both local and nonlocal coupling to a slow intermolecular vibrational mode. The model shows that excimer emission can be induced either through (1) local intermolecular coupling to the dimer CT states or through (2) the nonlocal Frenkel-CT coupling terms. Finally, by incorporating both the local and nonlocal intermolecular coupling, the measured absorption and excimer emission spectra for a set of two different bis(perylene diimide) covalently linked dimers are successfully reproduced. The evolution from structured to unstructured excimer emission likely occurs via an increase in the nonlocal coupling, but it can also be induced by the stabilization of the diabatic CT state.