Balancing TADF Properties in π-Bridged Donor–Acceptor Systems by Sterical Constraints: The Best of Three Worlds

Key parameters that steer the efficiency of thermally activated delayed fluorescence (TADF) are the energetic splitting ΔEST between S1 and T1, their mutual spin–orbit coupling (SOC) and the transition dipole strength μ of the S1 emission. Small ΔEST values, resonable SOC and high values of μ are difficult to achieve simultaneously. Using high-level quantum chemical methods, we have investigated the course of these parameters as functions of the donor–acceptor torsion angle in a series of conformationally constrained triarylamine–terephthalonitrile systems. Potential energy surface crossings between triplet states of charge-transfer and local-excitation character close to 90° indicate that a three-state model of the TADF kinetics might not be appropriate. The smallest adiabatic ΔEST values are obtained for a hybrid solvent model comprising two explicit toluene molecules in addition to a polarizable continuum model of solvation. Due to the substantial geometrical displacements of the S1 and T1 potentials in the torsion angle, the adiabatic Hessian method does not provide meaningful rate constants for reverse intersystem crossing. The recently developed vertical Hessian approach remedies this problem.