A theoretical investigation of heavy atom and oxidation effects in MR-TADF emitters for OLEDs: a combined DFT, double hybrid DFT, CCSD, and QM/MM approaches

The emerging multi-resonance thermally activated delayed fluorescence (MR-TADF) emitters with organoboron and nitrogen cores highlight their significance in OLEDs. However, their efficiency is challenged by slower rate constants in the reverse intersystem crossing (<i>k</i>_RISC) process compared to conventional TADF emitters. The study entails an in-depth analysis focused on gaining a better understanding of the photophysical properties of MR-TADF emitters. Using DFT and TD-DFT analyses, 48 MR-TADF molecules are studied, incorporating heavy atoms such as sulfur and selenium, and their subsequent oxidation, and peripheral donors such as carbazole (Cz), <i>tert</i>-butyl-carbazole (<i>t</i>Cz), diphenylacridine (DPAC), and dimethylacridine (DMAC) into organo boron and nitrogen-embedded systems. Moreover, the QM/MM approach was utilized to examine the excited state properties in the crystal phase. A comprehensive assessment of this molecular framework reveals that integrating heavy atoms and donors into MR-TADF molecules results in significant enhancements in Δ<i>E</i>_ST, larger SOC, and higher-order radiative (10⁸ s^-1) rates, leading to faster <i>k</i>_ISC (∼10⁸ s^-1) and <i>k</i>_RISC (∼10⁶ s^-1) rates. Based on key criteria, eight potential molecules were selected and their excited-state properties were precisely analyzed using double-hybrid density functionals including B2PLYP and PBE0-2, along with highly correlated wave function STEOM-DLPNO-CCSD.