Multistate Energy Decomposition Analysis of Molecular Excited States

A multistate energy decomposition analysis (MS-EDA) methodis describedto dissect the energy components in molecular complexes in excitedstates. In MS-EDA, the total binding energy of an excimer or an exciplexis partitioned into a ground-state term, called local interactionenergy, and excited-state contributions that include exciton excitationenergy, superexchange stabilization, and orbital and configuration-statedelocalization. An important feature of MS-EDA is that key intermediatestates associated with different energy terms can be variationallyoptimized, providing quantitative insights into widely used physicalconcepts such as exciton delocalization and superexchange charge-transfereffects in excited states. By introducing structure-weighted adiabaticexcitation energy as the minimum photoexcitation energy needed toproduce an excited-state complex, the binding energy of an exciplexand excimer can be defined. On the basis of the nature of intermolecularforces through MS-EDA analysis, it was found that molecular complexesin the excited states can be classified into three main categories,including (1) encounter excited-state complex, (2) charge-transferexciplex, and (3) intimate excimer or exciplex. The illustrative examplesin this Perspective highlight the interplay of local excitation polarization,exciton resonance, and superexchange effects in molecular excitedstates. It is hoped that MS-EDA can be a useful tool for understandingphotochemical and photobiological processes.