PCM-ROKS for the Description of Charge-Transfer States in Solution: Singlet–Triplet Gaps with Chemical Accuracy from Open-Shell Kohn–Sham Reaction-Field Calculations

The adiabatic energy
gap between the lowest singlet and triplet
excited states ΔEST is a central
property of thermally activated delayed fluorescence (TADF) emitters.
Since these states are dominated by a charge-transfer character, causing
strong orbital-relaxation and environmental effects, an accurate prediction
of ΔEST is very challenging, even
with modern quantum-chemical excited-state methods. Addressing this
major challenge, we present an approach that combines spin-unrestricted
(UKS) and restricted open-shell Kohn–Sham (ROKS) self-consistent
field calculations with a polarizable-continuum model and range-separated
hybrid functionals. Tests on a new representative benchmark set of
27 TADF emitters with accurately known ΔEST values termed STGABS27 reveal a robust and unprecedented
performance with a mean absolute deviation of only 0.025 eV (∼0.5
kcal/mol) and few deviations greater than 0.05 eV (∼1 kcal/mol),
even in electronically challenging cases. Requiring only two geometry
optimizations per molecule at the ROKS/UKS level in a compact double-ζ
basis, the approach is computationally efficient and can routinely
be applied to molecules with more than 100 atoms.