2, University of Oxford, Oxford, , United Kingdom
Electrically injected charge carriers in organic light-emitting devices (OLEDs) undergo recombination events to form singlet and triplet states in a 1:3 ratio, representing a fundamental hurdle for achieving high quantum efficiency in devices. Dopants based on thermally activated delayed fluorescence (TADF) traditionally rely on a small singlet-triplet exchange energy to activate luminescence pathways for weakly emissive triplet states. We use transient electron spin resonance (TrESR) and density functional theory (DFT) to show that when the photoexcitation is at the lowest singlet excitation energy, the spin-orbit coupling (SOC) mechanism drives spin conversion for the benchmark 4CzIPN and 2CzPN TADF molecules, even in the absence of heavy metals. Furthermore, the polarization of the observed triplet ESR signals support the proposal for second-order, vibronically-mediated SOC. Departure from a ‘static’ picture for the excited states involved in TADF removes the problem of balancing the promotion of fluorescence and the rate of reverse intersystem crossing; these are contradictory molecular properties under the adiabatic approximation. Critically, torsional freedom between the electron acceptor and donor components open additional ISC pathways between S1 and T1, and should be considered as a possible design rule for new TADF molecules.