Theoretical design and validation of [1,2,4]triazolo[1,5-a]pyridine-based TADF emitters through donor and linkage variations†

Achieving an optimal balance between donor and acceptor strength, coupled with strategic variations in molecular linkage, is pivotal for designing efficient thermally activated delayed fluorescence (TADF) materials. In this study, we incorporate the electron-accepting [1,2,4]triazolo[1,5-a]pyridine (TP) unit to explore high-performance TADF emitters. Through a systematic investigation, we elucidate the relationship between molecular structure and photophysical properties by modulating donor strength and the linkage between donor and acceptor units via a phenyl spacer. Eighteen molecules featuring diverse donor substituents (Cz, DPA, DMAc, PTZ, PXZ, and NPP) and linkage configurations (ortho-, meta-, para-) were designed, and their photophysical properties were thoroughly analysed using density functional theory (DFT) and time-dependent DFT (TD-DFT) methods. Key parameters such as the singlet–triplet energy gap (ΔE_ST), spin–orbit coupling (SOC), charge transfer (CT) indices, root-mean-square deviation (RMSD), reorganization energies, and excited-state rate constants were evaluated to characterize their emission properties. Our results reveal that molecules with weak donors exhibit large ΔE_ST values due to significant overlap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). In contrast, molecules with moderate to strong donors demonstrate smaller ΔE_ST values, attributed to enhanced HOMO–LUMO separation and improved CT character. Notably, molecules with meta-linkages and moderate to strong donor groups exhibit ΔE_ST values below 0.1 eV, moderate SOC, and higher rates of intersystem crossing (k_ISC), reverse intersystem crossing (k_rISC), and radiative decay (k_r), making them ideal candidates for TADF emission. This study provides a comprehensive framework for the rational design of highly efficient TP-based TADF materials for OLED applications and serves as a valuable guide for experimentalists in developing next-generation TADF emitters.