Simultaneous delayed fluorescence and phosphorescence in organic luminescent material employing multiple excited...

Triplet dynamics play a key role in room temperature phosphorescence (RTP) and thermally activated delayed fluorescence (TADF). In this work, we report a model emitter with three emission components: prompt fluorescence (PF) in nanoseconds, delayed fluorescence in microseconds, and RTP in milliseconds, with the emission spectrum ranging from ultraviolet to deep blue. We experimentally and theoretically verify that a second triplet excited state, T₂, below the singlet state S₁ is involved in facilitating simultaneous PF, TADF, and RTP in the model emitter. The reverse intersystem crossing (rISC) from T₂ to S₁ contributes to the TADF, while the radiative transition from T₁ to the ground state is the origin of the long-lived RTP. By transferring the energy of multiple excited states to a series of conventional fluorescence emitters, a multi-color emissive system covering the entire visible wavelength range has been realized, with the photoluminescence decay ranging from 10^-9 s to 10^-1 s. By slightly tuning the energy difference between these excited states in the model molecule, a highly efficient organic luminescent material with only PF and RTP emission has been obtained with an RTP quantum yield above 30%. This work provides insights into the key role of higher-lying triplet states in the development of efficient TADF and RTP materials.