Excimer Energies

A multistate energy decomposition analysis (MS -EDA) method is introduced for excimers using density functional theory. Although EDA has been widely applied to intermolecular interactions in the ground state, few methods are currently available for excited-state complexes. Here, the total energy of an excimer state is separated into exciton excitation energy Delta EEx(|psi X center dot psi Y⟩*), resulting from the state interaction between locally excited monomer states |psi X*center dot psi Y⟩ and |psi X center dot psi Y*⟩ , a superexchange stabilization energy Delta ESE, originating from the mutual charge transfer between two monomers |psi X+center dot psi Y⟩ and |psi X-center dot psi Y +⟩ , and an orbital-and-configuration delocalization term Delta EOCD due to the expansion of configuration space and block-localized orbitals to the fully delocalized dimer system. Although there is no net charge transfer in symmetric excimer cases, the resonance of charge-transfer states is critical to stabilizing the excimer. The monomer localized excited and charge-transfer states are variationally optimized, forming a minimal active space for nonorthogonal state interaction (NOSI) calculations in multistate density functional theory to yield the intermediate states for energy analysis. The present MS-EDA method focuses on properties unique to excited states, providing insights into exciton coupling, superexchange and delocalization energies. MS-EDA is illustrated on the acetone and pentacene excimer systems; three configurations of the latter case are examined, including the optimized excimer, a stacked configuration of two pentacene molecules and the fishbone orientation. It is found that excited-state energy splitting is strongly dependent on the relative energies of the monomer excited states and the phase-matching of the monomer wave functions.