Photophysics of Molecular Aggregates from Excited State Diabatization

A detailed understanding of photophysical processes in molecular aggregates requires the precise characterization of electronic excited states resulting from the interaction between chromophoric units. Theoretical descriptions of such systems are usually achieved by means of excitonic models, using effective Hamiltonians built on a basis of diabatic states that enable physical interpretations in terms of local excitations, charge transfer, or multiexcitonic configurations. In this work, we develop an alternative approach based on a diabatization scheme, which allows the decomposition of the adiabatic excited state energies of molecular aggregates into contributions issued from intermolecular couplings, without requiring any a priori definition of diabatic states. The general equations describing the deconvolution of adiabatic energies into different types of contributions are presented for various conformations of symmetrical and nonsymmetrical model dimers and compared to the energy expressions derived from excitonic models. We show that, while perturbative approximations typically employed in the construction of excitonic Hamiltonians assume weak intermolecular interactions, the presented methodology is valid within the entire range of coupling regimes. It should therefore constitute a useful tool to extract accurate ab initio diabatic state energies and interstate couplings for eventual derivation of model excitonic Hamiltonians