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) 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 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.