Quantum chemistry has been used to investigate the key factors that determine the (reverse) intersystem crossing processes in boron-based organic light-emitting diode emitters. The ortho-linked through-space charge-transfer compounds exhibit smaller singlet–triplet splittings than their para-linked through-bond charge-transfer regioisomers. Spin-vibronic coupling to a locally excited triplet state is essential for efficient upconversion of the triplet population.

The photophysical properties of a series of thermally activated delayed fluorescence emitters, comprising a nitrogen-based donor, a phenylene bridge and a boron-based acceptor, are investigated using a combination of density functional theory and multi-reference configuration interaction methods. In addition to singlet and triplet charge-transfer (CT) states, an acceptor-localized low-lying triplet state is found in all compounds. The size of the singlet–triplet gap and the energetic order of the CT and locally excited (LE) states can be modulated by regioisomerism (ortho- or para-linkage) and the chemical modification of the subunits. Spin-vibronic interactions, introduced through a Herzberg–Teller-type approach, are found to accelerate the intersystem crossing process considerably provided that the CT and LE states are close in energy.