Molecular organic fluorophores are currently used in organic light-emitting diodes, though non-emissive triplet excitons generated in devices incorporating conventional fluorophores limit the efficiency. This limit can be overcome in materials that have intramolecular CT excitonic states and associated small singlet-triplet energy; triplets can be converted to emissive singlet excitons resulting in efficient delayed fluorescence. However, the mechanistic details of the spin interconversion have not yet been fully resolved. We report transient ESR studies that allow direct probing of the spin conversion in a series of delayed fluorescence fluorophores with varying energy gaps between LE and CT triplets. The observation of distinct triplet signals, unusual in transient ESR, suggests that multiple triplets mediate the photophysics for efficient light emission in delayed fluorescence emitters. We reveal that as the energy separation between LE and CT triplets decreases, spin interconversion changes from a direct, singlet-triplet mechanism to an indirect mechanism involving intermediate states.
Supplermentary Info for Electron spin resonance resolves intermediate triplet states in delayed fluorescence
Additional info and figures: Synthesis Density functional theory calculations Electron spin resonance Additional compounds: trESR, photophysics and DFT Photophysics Electroluminescence performance