Asymmetric Diphosphane Dioxides With A-?-A-?'-D Scaffolds for High-Purity Deep-Blue Luminescence

Abstract

Phosphane oxides represent a cornerstone unit in modern optoelectronic materials due to their strong electron-withdrawing character, high thermal stability, and ability to tune excited-state properties. Herein, we report a streamlined design strategy toward donor-pi-acceptor-pi'-acceptor (Gamma-type) systems incorporating dual phosphane oxide units as both intermediate and terminal acceptors. A series of diphosphane dioxides (1-3) was synthesized and characterized, revealing pronounced deep-blue fluorescence (lambda em = 385-418 nm in toluene, lambda em = 415-457 nm in solid) that stems from charge-transfer and locally excited states with high photoluminescence yields up to 95%. Structure-property relationships demonstrate that subtle variations in pi-conjugation (pi = phenyl, biphenyl) and donor strength govern optical bandgap, oscillator strength, excited-state dipole moments, and intersystem crossing pathways. Notably, 2 and 3 exhibit favorable thermal stability and high triplet energies, enabling their application as emitters in deep-blue OLEDs. Devices based on 2 exhibit a maximum external quantum efficiency (EQE) of 4.86%, delivering high blue color purity with CIE coordinates of (0.15, 0.07), while maintaining low turn-on voltages and minimal efficiency roll-off. Transient electroluminescence studies indicate a significant contribution from triplet-triplet annihilation, enhancing exciton utilization beyond the conventional singlet limit.