A Simple Molecular Design of Red TADF Emitters via Donor Unit Engineering: A Theoretical Approach

This study advances the design of red thermally activated delayed fluorescence (TADF) emitters, crucial for the development of organic light-emitting diode (OLED) technologies. We present a straightforward and effective donor-engineering strategy by replacing the conventional triphenylamine (TPA) donor with an oxygen-bridged benzo[5,6][1,4]oxazino[2,3,4-kl]phenoxazine (BOP) donor. This substitution yields four newly designed emitters—BOPAP, BOPAQ, DCPA-BOP, and BOPAZ—that exhibit red-shifted and enhanced near-infrared (NIR) emission characteristics. Vertical excitation energies were calculated using both ground-state (S0) and excited-state (S1) geometries for the reported and newly designed molecules. In particular, vertical excitation energies derived from S0 geometries were compared with experimentally observed photoluminescence (PL) λmax values for near-infrared (NIR) TADF materials incorporating pyrazine-based acceptors. These calculations, performed using TD-DFT at both the B3LYP/6-31G* and HSE06/6-31G* levels of theory, showed good qualitative agreement with the experimental emission data. This validates the use of S0-based vertical excitation energies as reliable predictors of emission trends in reported TADF emitters. Based on these findings, among the newly designed compounds, BOPAZ demonstrates the most promising performance, featuring a small singlet–triplet energy gap (ΔEST = 0.045 eV), a high predicted reverse intersystem crossing (RISC) rate of 3.47 × 105 s–1, and a vertical excitation wavelength of 814 nm—indicative of strong NIR emission potential. In contrast, the reference compound TPAAZ shows a lower RISC rate (2.46 × 104 s–1; experimental: 6.83 × 101 s–1) and a predicted vertical excitation of 686 nm, compared to its experimental PL λmax of 742 nm. These findings underscore the importance of rational donor modification in optimizing charge-transfer interactions and emission efficiency. Overall, our results provide valuable insights into the development of next-generation red and NIR TADF materials for advanced OLED applications.