Nonplanar structure accelerates reverse intersystem crossing of TADF emitters: nearly 40% EQE and relieved efficiency roll off†

Abstract

Exploring strategies to enhance reverse intersystem crossing (RISC) is of great significance to develop efficient thermally activated delayed fluorescent (TADF) molecules. In this study, we investigate the substantial impact of nonplanar structure on improving the rate of RISC (k_RISC). Three emitters based on spiroacridine donors are developed to evaluate this hypothesis. All molecules exhibit high photoluminescent quantum yields (PLQYs) of 96–98% due to their rigid donor and acceptor. Leveraging the synergistic effects of heavy element effect and nonplanar geometry, S2-TRZ exhibits an accelerated k_RISC of 24.2 × 10⁵ s⁻¹ compared to the 11.1 × 10⁵ s⁻¹ of S1-TRZ, which solely incorporates heavy atoms. Additionally, O1-TRZ possesses a further lower k_RISC of 9.42 × 10⁵ s⁻¹ because of the absence of these effects. Remarkably, owing to the high PLQYs and suitable TADF behaviors, devices based on these emitters exhibit state-of-the-art performance, including a maximum external quantum efficiency of up to 40.1% and maximum current efficiency of 124.7 cd A⁻¹. More importantly, devices utilizing S2-TRZ as an emitter achieve a relieved efficiency roll-off of only 7% under 1000 cd m⁻², in contrast to the 12% for O1-TRZ and 11% for S1-TRZ, respectively. These findings advance our fundamental understanding of TADF processes for high-performance electroluminescent devices.