Optimizing triazine-based thermally activated delayed fluorescence molecules for enhanced organic light-emitting diode applications

Abstract

In pursuit of efficient organic light-emitting materials, we employed a rational design strategy to develop eleven cyanotriazine-based donor–π–acceptor emitters for thermally activated delayed fluorescence (TADF) applications. A comprehensive computational investigation using DFT and TD-DFT methods was carried out to evaluate their geometric and optoelectronic properties. The designed molecules are based on 4,4′-(6-phenyl-1,3,5-triazine-2,4-diyl)dibenzonitrile, where the dibenzonitrile unit acts as an acceptor and various donor units are introduced via a phenyl π-linker. Steric interactions between the donor units and π-linker induce twisted donor–acceptor geometries, which are favorable for TADF. Initial screening based on vertical excitation energies identified eight promising candidates with singlet–triplet energy gaps (ΔE_ST) below 0.3 eV. These systems were further analyzed in terms of spin–orbit coupling matrix elements (SOCMEs) and reorganization energies, which govern reverse intersystem crossing (RISC). Among the studied molecules, TZCN–PXZ, TZCN–PTZ, and TZCN–ICbz exhibit particularly favorable combinations of small ΔE_ST and enhanced SOCME values, indicating efficient TADF behavior. While nearly orthogonal geometries promote strong HOMO–LUMO separation and minimal ΔE_ST, moderately twisted conformations (≈70°) provide an optimal balance between ΔE_ST reduction and spin–orbit coupling enhancement via LE-CT orbital mixing. These findings establish clear structure–property relationships and provide useful design guidelines for developing high-performance TADF emitters.