Weakly Conjugated Phosphine Oxide Hosts for Efficient Blue Thermally Activated Delayed Fluorescence Organic...

The development of host materials with high first-triplet state (T₁) energy and high charge mobility is a key to achieve efficient true-blue organic light-emitting diodes (OLEDs), employing phosphorescence and thermally activated delayed fluorescence (TADF). An ether-bridged double triphenylphosphine oxide (TPPO) compound, bis(2-(diphenylphosphino)phenyl)ether oxide (DPEPO), was reported to have a very high T₁ energy of 3.3 eV but suffers from poor charge mobility. Here, five bridge-controlled multi-TPPO derivatives were studied through a combination of experiments and theory. We demonstrate that the push-pull electron capability of the bridge group governs the T₁ energy and electron mobility of these materials. Replacing the ether bridge by a bis(trifluoromethyl)methylene group can reduce the energy barrier for intramolecular electron exchange and consequently enhance the electron mobility by two orders of magnitude without lowering the T₁ energy. A blue TADF OLED employing this bis(trifluoromethyl)methylene-bridged compound achieves the same high external quantum efficiency but a much higher current density compared to the control device employing DPEPO. In contrast, a bridge group with strong electron-withdrawing capability, such as phosphine oxygen or sulfone, lowers the T₁ energy of the compound by enhancing the electronic coupling between TPPO subunits and inhibits intermolecular electron transfer by trapping the electron charge around the bridge.