Mind the gaps: what the STGABS27 set can teach about second-order excited state methods, solvent models, and charge transfer

Charge-transfer (CT) states are ubiquitous in modern organic electronics, yet their accurate theoretical description poses a challenge for common excited state methods. The recently introduced STGABS27 benchmark set provides highly accurate experimentally measured adiabatic energy gaps (Δ<i>E</i>_ST) between the lowest singlet and triplet excited states of thermally activated delayed fluorescence (TADF) emitters. While first studies revealed a remarkable performance of orbital-optimized state-specific ΔDFT and mixed results with TD-DFT and DFT/MRCI, this work explores the performance of correlated wave-function methods, namely second-order algebraic diagrammatic construction (ADC(2)) and second-order approximate coupled-cluster singles and doubles (CC2) in their canonical and spin-scaled variants. Owing to the polar nature of the states, a particular emphasis is placed on the dielectric solvent models. The results show that only a few models, namely the iterative state-specific COSMO solvation model in combination with spin-component-scaled or scaled opposite-spin (SCS/SOS) ADC(2) or CC2, are competitive with ΔDFT/PCM and achieve sub-kcal mol^-1 agreement with experimental singlet-triplet gaps, which is confirmed by cross-checks on emission energies. However, this performance comes with a hefty cost, as both models are roughly 100 times slower than similarly accurate ΔDFT/PCM-based models.