To develop novel long-wavelength red TADF materials, two compounds, diTPA-DPPDC and diTPA-DPPDQ, are synthesized using dipyrido[3,2-a:2',3'-c]phenazine (DPPZ) as the base receptor, modified with cyanide and cyanobenzene groups. Both materials exhibit characteristic delayed fluorescence properties, including small single-triplet energy differences (ΔE_ST) and relatively high oscillator strengths (0.0682 and 0.0794, respectively) compared to conventional donor-acceptor TADF molecules. This balance between small ΔE_ST and high photoluminescence quantum yield (PLQY) is achieved using triphenylamine (TPA) as a sterically hindered donor to suppress aggregation-induced fluorescence quenching (ACQ). Cyanide modification in diTPA-DPPDC enhances molecular conjugation, resulting in deep red emission at 686 nm, though its lower oscillator strength (0.0682) limits its PLQY. Conversely, the cyanobenzene-modified diTPA-DPPDQ emits red light at 605 nm, with a higher oscillator strength (0.0794), achieving a PLQY of 62.8%, indicative of superior luminescence performance.

In this paper, we synthesize the diTPA-DPPDC and diTPA-DPPDQ materials via Buchwald?Hartwig and Suzuki coupling reactions. Their photophysical, delayed fluorescence, and thermal properties are investigated. Comparative analysis of their luminescent properties is conducted to evaluate their performance.

The structures of diTPA-DPPDC and diTPA-DPPDQ are confirmed by ¹H NMR, ¹³C NMR, and high-resolution mass spectrometry (HRMS). Density functional theory (DFT) calculations reveal twisted molecular structures due to TPA incorporation, which effectively suppresses ACQ and minimizes ΔE_ST (Fig. 3). diTPA-DPPDC exhibits deep red emission at 686 nm, while diTPA-DPPDQ emits red light at 605 nm due to the breaking of conjugation by the benzene ring [Fig. 4(a)]. Both compounds exhibit typical delayed fluorescence behavior (Fig. 5). Fluorescence peaks of both materials red-shift with increasing solvent polarity, demonstrating intramolecular charge transfer (ICT) characteristics [Figs. 4(c) and 4(d)]. When doped into CBP thin films at 15% (mass fraction), diTPA-DPPDC shows a PLQY of 38.2%, while diTPA-DPPDQ achieves 62.8%, consistent with its higher oscillator strength. Cyclic voltammetry reveals HOMO and LUMO energy levels of -5.19/5.25 eV and -3.13/-3.00 eV for diTPA-DPPDC and diTPA-DPPDQ, respectively, aligning with theoretical predictions (Fig. 6). Both materials exhibit high thermal stability, making them suitable for OLED device fabrication through vacuum evaporation.

Based on the DPPZ receptor, two novel receptor systems with enhanced electron-withdrawing abilities are developed by incorporating two different functional groups: cyano and cyanobenzene. Using TPA as a donor, two TADF materials, diTPA-DPPDC and diTPA-DPPDQ, are designed and synthesized. Key findings are as follows: 1) Both molecules feature twisted molecular structures due to the introduction of sterically hindered donor TPA, which effectively suppresses the inherent ACQ often observed in red-light-emitting molecules. Their HOMO and LUMO orbitals are primarily localized on their respective donor and acceptor units, ensuring a small ΔE_ST of 0.07 eV for diTPA-DPPDC and 0.11 eV for diTPA-DPPDQ; 2) A slight orbital overlap at the donor-acceptor junction in both molecules contributes to increased PLQY by achieving a balance between small ΔE_ST and large oscillator strength; 3) The cyanide group in diTPA-DPPDC not only enhances the receptor’s electron-withdrawing capability but also extends the molecular conjugation, leading to a significant red-shift in the fluorescence emission peak. It emits deep red light at 686 nm, though its molecular PLQY remains moderate at 38.2%. In contrast, the cyanobenzene group in diTPA-DPPDQ disrupts the conjugation to a lesser extent, resulting in red-light emission at 605 nm, but the increased oscillator strength improves its PLQY to 62.8%, demonstrating superior luminescence performance. Both materials exhibit characteristic delayed fluorescence behavior. We diversify the range of receptor designs in the relatively underexplored field of red-light TADF materials, achieving significant redshifts in emission wavelength. These findings provide valuable insights for the development of long-wavelength red-light TADF materials.