Dielectric control of reverse intersystem crossing in thermally activated delayed fluorescence emitters

Thermally-activated delayed fluorescence (TADF) enables organic semiconductors with charge transfer (CT)-type excitons to convert dark triplet states into bright singlets via reverse intersystem crossing (rISC). However, thus far the contribution from the dielectric environment has received insufficient attention. Here, we study the role of the dielectric environment in a range of TADF materials with varying changes in dipole moment upon optical excitation. In dipolar emitters, we observe how environmental reorganisation after excitation triggers the full CT exciton formation, minimising the singlet-triplet energy gap, with the emergence of two (reactant-inactive) modes acting as a vibrational fingerprint of the CT product. In contrast, the dielectric environment plays a smaller role in less dipolar materials. The analysis of energy-time trajectories and their free-energy functions reveal that the dielectric environment significantly reduces the activation energy for rISC in dipolar TADF emitters, increasing the rISC rate by three orders of magnitude versus the isolated molecule.