Theoretical Study on MR-TADF Materials Based on CzBN

Multi-resonance thermally activated delayed fluorescence (MR-TADF) materials have garnered significant research interest owing to their remarkably narrow emission spectra with full width at half maximum (FWHM) below $40~\text{nm}$, demonstrating substantial advantages over conventional donor-acceptor (D--A) type TADF materials in spectral purity. However, conventional N--B--N resonant framework materials are fundamentally constrained by their intrinsically low reverse intersystem crossing rates ($k_{\text{RISC}} < 10^{3}~\text{s}^{-1}$), presenting a persistent challenge for achieving high-efficiency TADF. This study proposes a triple collaborative design strategy based on CzBN to break through this limitation: (1) Enhance the separation of HOMO and LUMO by $\pi$-conjugation expansion and reduce $\Delta E_{\text{ST}}$; (2) Introduce O/S heteroatoms to control the excited state charge transfer (CT) characteristics and further reduce $\Delta E_{\text{ST}}$; (3) Enhance the spin-orbit coupling (SOC) effect through the synergy of extended $\pi$-system and heteroatoms. Based on this, five new MR-TADF molecules were designed and studied. Among them, the $k_{\text{RISC}}$ of CzBN\_S reached $3.48 \times 10^{6}~\text{s}^{-1}$, two orders of magnitude higher than CzBN, while maintaining $\Delta E_{\text{ST}} < 0.1~\text{eV}$ and FWHM at $40~\text{nm}$.