Spatially optimized quaternary phosphine oxide host materials for high-efficiency blue phosphorescence and thermally activated delayed fluorescence organic light-emitting diodes

Two quaternary ambipolar phosphine oxide host materials, 9-(4-(9-(4'-(2-(diphenylphosphoryl) phenoxy)biphenyl-4-yl)-9H-fluoren-9-yl)phenyl)-9H-carbazole (9CzFDPESPO) and 9-(4-(9-(3'-(diphenylphosphoryl)-4'(2-(diphenylphosphoryl)phenoxy)biphenyl-4-yl)-9H-fluoren-9-yl)phenyl)-9H-carbazole (9CzFDPEPO), were designed and prepared with mixed indirect and multi-insulating linkages to investigate the spatial effect on the electroluminescence performance of high-energy-gap hosts in blue phosphorescence and thermally activated delayed fluorescence light-emitting diodes (PHOLEDs and TADF OLEDs), in comparison to a ternary analogue 9-(4-(9-(4-(diphenylphosphoryl) phenyl)-9H-fluoren-9-yl) phenyl)-9H-carbazole (9CzFSPO). The donor-acceptor (D-A) distance in 9CzFDPESPO is elongated through the involvement of diphenylene as a p-extender, while the second phosphine oxide acceptor on diphenylene of 9CzFDPEPO is utilized to make its D-A distance comparable to that of 9CzFSPO. The single-molecular optoelectronic properties of these three hosts were uniform, owing to the effectively suppressed intramolecular electronic coupling by indirect and multi-insulating linkages. Their similar high triplet energy of 3.0 eV and the suitable energy levels of the highest occupied and the lowest unoccupied molecular orbitals around -6.1 and -2.5 eV, respectively, provide effective energy transfer and carrier injection. In contrast, 9CzFDPESPO shows the highest hole mobility owing to its longest D-A distance, directly indicating the spatial effect. As expected, with the most effectively suppressed spatial effect, 9CzFDPESPO endowed its blue phosphorescence and TADF devices with a state-of-the-art performance, e.g. external quantum efficiencies of 22.5% and 16.7%, respectively, which were the best results for quaternary host materials reported so far.