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 ΔE_Ex (|Ψ_X•Ψ_Y >^*), resulting from the state interaction between locally excited monomer states |Ψ_X^*•Ψ_Y> and |Ψ_X•Ψ_Y^*>, a super-exchange resonance energy ΔE_SE, originating from the mutual charge transfer between two monomers |Ψ_X^+•Ψ_Y^-> and |Ψ_X^-•Ψ_Y^+>, and an orbital-and-configuration delocalization term ΔE_OCD 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, super-exchange 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.