Dielectric Effects on Charge-Transfer and Local Excited States in Organic Persistent Room-Temperature Phosphorescence

If thermally activated delayed fluorescence (TADF) in organic molecules operates on the premise of a minimal change between the singlet and triplet excited states, persistent room-temperature phosphorescence (PRTP) seems to rely on distinct changes among the excited states. Understanding the nature of such changes at the density functional theory level requires that the accuracy be preserved in the geometries of both the ground and excited states. We first show that adding dielectric effects is necessary not only for identifying the relevant charge-transfer excited states but also for calculating the geometries, nearly free of the size consistency error inherent in the optimally tuned range-separated hybrid functional. We use PTZ-BzPN (phenothiazine bonded to benzophenone), a conformationally bistable molecule known to alternate between PRTP and TADF, to investigate different roles played by individual molecular parameters, namely, the reorganization energy, singlet-triplet energy gap, and spin-orbit coupling. We find that PRTP is active when both the reorganization energy and singlet-triplet gap are finite, which is realized by the spin-adaptive geometry of a carbonyl linkage that switches on orbital orthogonality in the singlet and off in the triplet. With increasing dielectric constant, the forward intersystem crossing rate remains constant. In TADF, on the other hand, the forward and reverse intersystem crossing rates decrease to converge with each other.