Highly efficient red thermally activated delayed fluorescence emitters by manipulating the molecular horizontal orientation

Developing highly efficient red thermally activated delayed fluorescence (TADF) emitters remains a formidable challenge due to strict molecular design principles. In this contribution, two red emitters PXZ-AQPy and PXZ-AQPhPy with high structural rigidity and linear configurations are designed and synthesized based on phenoxazine (PXZ) as a donor and acenaphtho[1,2-b]quinoxaline (AQ) as an acceptor. Both molecules with horizontal orientation are endowed with TADF and aggregation-induced emission (AIE) properties. By elongating the acceptor segment via phenyl decoration, PXZ-AQPhPy exhibits higher horizontal ratios of the emitting dipole orientation (Theta|| = 85%), while the emission characteristics are almost unaltered. Consequently, the PXZ-AQPhPy based device achieves a high external quantum efficiency of 19.6% with an emission peak at 610 nm. This work provides a simple and effective design strategy to selectively promote the horizontal orientation of red TADF emitters.