Spirobicyclic host material with pseudo-intramolecular charge transfer: Improving color purity of high-performance pure-blue and white thermally activated delayed fluorescence diodes

Developing high-performance pure-blue and white organic light-emitting diodes (OLED) is one of the biggest challenges for thermally activated delayed fluorescence (TADF) technology, although great efforts have been made in designing new blue TADF emitters. Herein, we demonstrate that by virtue of solvatochromic characteristics for TADF molecules, host engineering can provide a feasible alternative to realize the high color purity and the state-of-the-art efficiencies on the basis of conventional blue TADF dyes. An electron-withdrawing diphenylphosphine oxide (DPPO) group is introduced at 4 position of symmetrical spirobi[xanthene] to construct 9,9'-spirobi[xanthen]-4-yldiphenylphosphine oxide (SXSPO) with the asymmetry of configuration and optoelectronic properties. Besides forming the ambipolar property and suppressing quenching, the steric and inductive effects of DPPO are spatially confined, which prevents the formation of high-polarity charge transfer excited states and the interaction between polar P=O group and TADF dopants. Based on a conventional TADF emitter bis[4-(9,9-dimethyl-9,10-dihydroacridine)phenyl]sulfone (DMAC-DPS), SXSPO dramatically improves the color purity of its pure-blue and white devices and realizes the external quantum efficiencies (eta(EQE)) beyond 20%. The power efficiency of the white devices reaches to similar to 60 lm W-1, demonstrating the potential applications as lighting source.