Facile modification of multi-resonance acceptors and donors in intramolecular TSCT-TADF emitters for OLEDs: a computational study

Abstract

Recently, through-space charge transfer (TSCT) has attracted considerable interest in the development of thermally activated delayed fluorescence (TADF) emitters with high radiative (k_r) and reverse intersystem crossing (k_RISC) rates, along with narrow emission. However, achieving these properties simultaneously remains challenging. To address this, a series of eighteen TSCT-TADF molecules were designed by extending the reported carbazole (Cz) donor with additional diphenylamine (DPA) and dimethylacridine (DMAC) donors, combined with multi-resonance organoboron acceptors (MRAs) containing oxygen (OB) and heavy atoms such as sulfur (OBS) and selenium (OBSe). The molecular frameworks are based on phenyl and pyridine core scaffolds, connected via a tert-butyl carbazole (tCz) linker, enabling a systematic investigation of the effects of donors and heavy-atom substitution on the photophysical and electronic properties using density functional theory (DFT) and time-dependent density functional theory (TD-DFT). The findings reveal that phenyl diphenylamine (PDPA) containing molecules with MRA exhibit low ΔE_ST, moderate SOC, and enhanced radiative (k_r) and effective RISC rates. The desired properties are achieved in the PDPA with the OB molecule showing reduced structural relaxation and achieving a narrow FWHM compared to the experimental molecule, while maintaining high PLQY. Furthermore, the incorporation of OBS with PDPA leads to broader emission than OBSe, attributed to increased structural relaxation and higher reorganization energy. These findings highlight that donor strength, along with the MRA acceptor, plays a key role in determining the efficiency of TSCT-TADF molecules and offers a rational design strategy for next-generation emitters, enabling high-performance OLEDs with improved colour purity and emission efficiency.