The presence of excimers, as revealed by broad, structure-less and red-shifted emission, is quite common in a great many organic solution- and solid-phase systems. While excimers are relatively easy to identify experimentally, theoretical interpretations often rely on original solution-phase arguments. For instance, solid-phase excimers can exhibit an increase in "monomer"-like vibronic structure with temperature, even in the absence of any obvious dissociative pathway. We have modified the dimer Frenkel-charge transfer (CT)-Holstein Hamiltonian in order to account for both local and non-local coupling to a slow intermolecular vibrational mode, with which we successfully simulate both the absorption and excimer emission spectra for a set of two different bis(perylene diimide) covalently-linked dimers, all within a single model framework. We demonstrate that excimer emission can be induced through either 1) local intermolecular coupling to only the dimer CT states or 2) through the non-local Frenkel-CT coupling terms. By comparing to experiment, we demonstrate that an evolution from structured to unstructured excimer emission likely occurs via an increase in the non-local coupling, but can also be induced by the stabilization of the CT state, both of which stabilizes the excimer state.

Structured to unstructured excimer emission in bis(perylene-diimide) dimers: Insight into the role of charge transfer states and intermolecular vibrations through a Frenkel-Holstein-style framework

ACS Spring 2022

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