Energy Decomposition Analysis of Excited States Based on Time-Dependent Density Functional Theory Calculations...

Controlling the alignment of low-lying excited states is central to molecular design efforts aimed at improving the performance in targeted optoelectronic applications. Typically, the energies of these states are governed by the gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), with exchange repulsion destabilizing the lowest singlet state (S₁) relative to the triplet (T₁). However, through rational molecular design, the S₁-T₁ energy gap can be minimized─or even inverted. Such excited-state scenarios can enable mechanisms like thermally activated delayed fluorescence (TADF), which enhance the light-emission efficiency. In this work, we apply energy component analysis to time-dependent DFT (TDDFT) calculated excited states to better understand the role of the dielectric environment on S₁ and T₁ state energies. The analysis is performed using the SRSH-PCM approach, which provides a consistent treatment of environmental effects by combining a dielectric-screened range-separated hybrid (SRSH) functional with a polarizable continuum model (PCM). We demonstrate the utility of this approach by analyzing the excited-state properties of three representative molecular systems relevant to optoelectronics. Our results offer valuable insight into energy trends governing the alignment and coupling of excited states, which are key to optimizing efficiency in relevant applications.