Accelerating the spin–flip process of multi-resonance emitters via an advanced nitrogen group heavy atom strategy

Abstract

Multi-resonance thermally activated delayed fluorescence (MR-TADF) emitters, characterized by their high photoluminescence quantum yields and narrowband emissions, have attracted increasing attention in organic light-emitting diodes (OLEDs). However, MR-TADF OLEDs often suffer from significant efficiency roll-offs at high brightness due to a relatively slow reverse intersystem crossing (RISC) rate, on the order of 10²–10⁴ s⁻¹, which poses a significant challenge for their practical applications. In this study, we designed two series of MR-TADF molecules by introducing heavy atoms from the nitrogen (N) group (i.e., phosphorus (P) and arsenic (As)) in place of the N atom in the classical MR skeletons. Highly correlated wave function-based calculations show that the introduction of P and As significantly enhances the spin–orbit coupling (SOC) matrix element while preserving narrowband emission compared to its N-containing analogue, resulting in a sharp increase in the RISC rate (k_RISC). Notably, DABNA-As achieves an impressive RISC rate of 2.24 × 10⁷ s⁻¹, which is four orders of magnitude higher than that of its N-containing analogue DABNA-1 (1.14 × 10³ s⁻¹). This study offers a valuable strategy for the development of MR-TADF emitters with an effective RISC process by introducing P/As atoms.