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 (ΔEₛₜ) between the lowest singlet and triplet excited states of thermally activated delayed-fluorescence (TADF) emitters. While first studies reported excellent performance for orbital-optimized, state-specific ΔDFT and mixed results for TD-DFT and DFT/MRCI, this work assesses correlated wave-function approaches—specifically second-order Algebraic Diagrammatic Construction (ADC(2)) and the second-order approximate Coupled-Cluster Singles and Doubles (CC2)—in their canonical and spin-scaled variants. Because these excited states are strongly polar, particular attention is given to the dielectric treatment of the solvent. We find that only a few combinations—most notably the iterative, state-specific COSMO model paired with spin-component- or spin-opposite-scaled (SCS/SOS) ADC(2) or CC2—match the accuracy of ΔDFT/PCM, yielding sub-kcal mol⁻¹ agreement with experimental singlet–triplet gaps, a result corroborated by emission energies. This accuracy, however, is expensive: the best-performing ADC(2)/CC2-based protocols are roughly 100 times slower than comparably accurate ΔDFT/PCM calculations.