Excited State Dynamics of a MR-TADF Emitter in Host Matrices

Multi-resonance thermally activated delayed fluorescence (MR-TADF) emitters are beneficial for color pure organic light-emitting diodes (OLEDs) due to their narrow emission bandwidths. Understanding the photophysics of these materials is crucial for developing devices with better performance. In this letter, we demonstrate the excited state photophysical properties of the MR-TADF emitter OQAO(mes)₃ embedded in different host matrices with different polarities, including the OLED-relevant hosts. Transient absorption and time-resolved fluorescence experiments reveal a second lower lying triplet state in high polarity matrices, consistent with solution studies. The two triplet populations may interconvert; thus, the low-lying triplets can be partly harvested to bright singlets via reverse intersystem crossing (rISC) generating delayed fluorescence. Interestingly, we find no evidence of exciplex-like interactions in host-guest systems affecting the TADF process, contrary to previous reports.