On the Improvement of Blue Emission for All sp2-Hybridized Bistriphenylenyls: Incorporating Phenylenyl Moieties To Enhance Film Amorphism

The film amorphism of 2,2'-bistriphenylenyl (BTP), a blue-light emitter with all sp(2)-hybridized polyaromatic, is enhanced by incorporating phenyl groups between the two triphenylene (TP) planes. This series of compounds are BTP, 1,4-di(triphenylen-2-yl)benzene (T1), and 4,4'-di(triphenylen-2-yl)biphenyl (T2), where the latter preserve the characteristics of all sp(2) hydrocarbons and improve the optimized electroluminescence (EL) performance. Specifically, the most efficient T2-based device can reach a maximum external quantum efficiency of 5.2% and a very high luminescence of 47776 cd/m(2) in the deep-blue region with Commission Internationale d'Enclairage (CIE) coordinates of (0.15, 0.12). The efficiency approaches the upper theoretical limit of 5-5.5%, estimated for a singlet emitter with a refractive index of 1.7-1.5. For comparison, the optimized BTP-based devices confer the corresponding values of 4.2%, 21 204 cd/m(2), and (0.14, 0.11). Such an improvement is ascribed to the effect of introducing the phen ylene moiety on the film amorphism, examined by single-crystal X-ray analysis, atomic force microscopy, photoluminescent spectroscopy, and HOMO/LUMO energy levels for the materials involved in EL devices.