Electronic Structure and Emission Process of Excited Charge Transfer States in Solids

Excited charge-transfer complexes, or exciplexes, have attracted significant attention due to their potential applications to improving the performance of organic light-emitting diodes (OLEDs) and organic photovoltaic cells (OPVs). In solid states, exciplexes exhibit extraordinary characteristics, including broad emission spectra, multiexponential photoluminescence (PL) decay curves, and spectral red shifts as time delays in transient PL. Here, we present experimental and theoretical evidence that all of the emission characteristics of solid-state exciplexes originate from differences in their dimer configurations, which have different charge transfer rates, emission energies, singlet–triplet energy gaps, kinetic rate constants, and emitting dipole orientations. This conclusion is based on experimental observations, quantum chemical calculations, and molecular dynamics simulations. These results enabled us to develop a model of the electronic structure of an exciplex in a solid-state medium. This comprehensive model accommodates all of the characteristics of the exciplex and can be used to further our understanding of OLEDs and OPVs.