Harmonizing Triplet Level and Ambipolar Characteristics of Wide-Gap Phosphine Oxide Hosts toward Highly Efficient and Low Driving Voltage Blue and Green PHOLEDs: An Effective Strategy Based on Spiro-Systems

A series of phosphine oxide (PO) hosts based on diphenylphosphine oxide and spiro[fluorene-9,9'-xanthene] (SFX) moieties, SFX2PO, SFX27PO, SFX2'PO, and SFX2'7'PO, were designed and synthesized. On the basis of the different electrical properties of xanthene and fluorene in SFX, the influence of substitution position on the chemical and optophysical properties of the ambipolar-core based systems were investigated in detail. The effective strategy of introducing electron-withdrawing PO moieties in electron-deficient moieties in the molecules accompanied with suitable linkages insulating the electron-rich and -deficient moieties was convincingly demonstrated, which can endow the hosts with much better carrier injecting and transporting ability and high enough T-1 for blue and green phosphors. As the results, the operating voltages of the devices based on SFX2PO and SFX27PO were much lower than those of the devices based on SFX2'PO and SFX2'7'PO. Simultaneously, the efficiencies of the SFX2PO based devices were about twice of those of the devices based on SFX2'PO and SFX2'7'PO. We suppose that it is not necessary to achieve too high T1 and improved electron injection through PO moieties at the cost of sacrificing the hole injecting ability of the chromophores. An ideal strategy is preserving high enough T1 and improving electron injection by utilizing PO moieties without reducing hole injection and transportation in the hosts.