Precise Modulation of Donor–Acceptor Spatial Configuration and Excited-State Energy Alignment for Efficient Red-Shifted TADF Emitters

The development of efficient deep-red thermally activated delayed fluorescence (TADF) materials remains challenging for the application of OLED due to limitations imposed by the energy-gap law. For both narrowband multiple resonance TADF emitters and conventional donor-acceptor (D-A)-type TADF emitters, extending the π-conjugation or enhancing the intramolecular charge transfer (ICT) strength is a common strategy to achieve red-shifted emission. However, balancing the emission wavelength, quantum efficiency, and reverse intersystem crossing (RISC) kinetics remains a critical challenge. Herein, we address this trade-off through precise D-A distance and angle modulation to optimize the excited-state energy alignment. A series of emitters (6(7)-PXZ-FQ and 6(7)-MeODPA-FQ) were designed, wherein 6-PXZ-FQ and 6-MeODPA-FQ achieved emission peaks at 654 and 618 nm, respectively, demonstrating significant bandgap narrowing via D-A distance compression. Structural modifications concurrently enhanced ICT strength and transition dipole moments, inducing red-shifted emission while maintaining high photoluminescence quantum yields. Furthermore, spatial tuning optimized the excited-state energy landscape, accelerating the RISC rates. Consequently, red OLEDs based on 6-MeODPA-FQ attained a maximum external quantum efficiency (EQE) exceeding 16%. A white OLED (WOLED) incorporating this emitter achieved an EQE of 19.7% with a color rendering index of 82, enabled by balanced exciton allocation and energy transfer modulation. This work provides critical insights for the design of high-performance red TADF materials and WOLED devices.