Tuning the Acceptor Core in Cyano PyridineBased Framework to Realize Efficient TADF Emitters.

For next-generation organic light-emitting diode (OLED) displays and lighting, achieving stable, efficient deep-blue and sky-blue emission remains a challenge. Herein, we report a cyanopyridine-based modular scaffold, an asymmetric dual core (2tCz2PyCN) that suppresses exciton loss to achieve sky-blue emission, and an asymmetric single core analog (2tCzPyCN). The single core 2tCzPyCN exhibits deep blue emission (λmax = 437 nm) with a CIE of (0.15, 0.14), asymmetric 2tCz2PyCN emitter exhibits sky-blue emission (λmax = 478 nm) with a CIE of (0.19, 0.38). The reduced conjugation in the single core 2tCzPyCN widens the energy gap, resulting in deep-blue color (CIEy = 0.14), it also maximizes faster reverse intersystem crossing (RISC) through spin orbit coupling (SOC). The extended bipyridine core of 2tCz2PyCN lowers the excited state reorganization energy (0.82 vs. 0.94 eV), suppressing non-radiative decay and raising the PLQY from 41.6% to 80.7%. The 2tCzPyCN-based device achieves an EQEmax of 8.80% in the deep blue region, whereas the asymmetric 2tCz2PyCN shows EQEmax of 16.63% in the sky-blue region. This work demonstrates that engineering the excited state via an asymmetric acceptor is a practical pathway for color tuning and developing efficient TADF emitters.