Deep blue-emissive bifunctional (hole-transporting + emissive) materials with CIEy 0.06 based on a U-shaped phenanthrene scaffold for application in organic light-emitting diodes

Introduction Development of deep blue-emissive materials (CIEy o 0.10) with improved physical as well as electroluminescence properties is of tremendous contemporary interest in organic light-emitting diodes (OLEDs). It is due to the fact that deep blue-emissive materials are more energy efficient than sky blue-emissive materials, and can be utilized as hostmaterials to produce any color by a cascade of energy transfer. Although highly stable and efficient green-, redand sky blue-emissive materials have been developed, those that show deep blue emission and exhibit desirable efficiency, stability and color purity are underdeveloped. Consequently, one observes significant focus directed toward developing deep blue-emissive materials based on fluorescence, phosphorescence and thermally activated delayed fluorescence. In recent years, several new materials that emit deep blue light have been reported, often with benzimidazole as the emissive fragment, cf. Fig. 1. Incidentally, multilayer device fabrication is often a requirement to sequester deep blue emission from these materials. Only a few materials that emit deep blue light when employed in simple double layer devices have been reported. Insofar as deep blue emission in double layer devices is concerned, a careful analysis shows that an additional layer of PEDOT:PSS/MoO3 is invariably present in the fabricated devices. We showed a few years ago that N,N-diphenylaminobiphenyl-functionalized bimesitylene leads to deep blue emission with CIE coordinates of (0.15, 0.10) from a true double layer device. However, realization of deep blue-emission with CIEy ca. 0.06 from a true double layer device constructed from a bifunctional (hole-transporting + emissive) small molecular organic material is heretofore unknown to the best of our knowledge. To engineer deep blue emission in a double layer device, it is necessary that the material is a bifunctional one. Not surprisingly, such materials are of much commercial demand, as they minimize the production cost by precluding casting of a separate layer. In continuation of our studies on bifunctional materials based on rigid bimesitylene and Troger’s base (TB) as core scaffolds, we reasoned that deep blue-emissive bifunctional materials could be developed based on a U-shaped phenanthrene scaffold functionalized at 3 and 6 positions with suitable secondary aromatic amines, Scheme S1 (ESI†). Our choice of phenanthrene as the scaffold rested on the following considerations: first, the Fig. 1 Structures of some representative deep blue emitters and the results of the device fabrication of these compounds.