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.