Control of Dual Conformations: Developing Thermally Activated Delayed Fluorescence Emitters for Highly Efficient Single-Emitter White Organic Light-Emitting Diodes.

In this work, we propose a novel concept to develop two fluorophores 2-(10 H-phenothiazin-10-yl)thianthrene 5,5,10,10-tetraoxide (PTZ-TTR) and 2-(4-(10 H-phenothiazin-10-yl)phenyl)thianthrene 5,5,10,10-tetraoxide (PTZ-Ph-TTR) showing dual conformations for highly efficient single-emitter white organic light-emitting diodes (WOLEDs). Both molecules exist in two stable conformations. Their nearly orthogonal forms own lower energy levels and show thermally activated delayed fluorescence (TADF) characteristics, whereas their nearly planar conformers possess higher energy levels and show only prompt fluorescence. These dual conformers were exploited for fabricating WOLEDs with complementary emission colors contributed by the two conformations. Moreover, the originally wasted triplet energy on the nearly planar conformation can be transferred to the nearly orthogonal one and then harvested via the TADF channel, realizing full exciton utilization. A PTZ-TTR-based single-emitter device exhibits standard white emission with a CIE coordinate of (0.33, 0.33) and a high color rendering index value of 92. On the other hand, the PTZ-Ph-TTR-based single-emitter device realizes an emission approaching warm white light and a high maximum external quantum efficiency of 16.34%. These results demonstrate an alternative approach for designing high-performance WOLEDs based on single TADF emitters.