Tuning hybridized local and charge transfer emission in pyrene derivatives by incorporating phenanthroimidazole to realize high exciton harvest in blue organic light-emitting diodes

Pyrene derivatives capable of harvesting triplet excitons and displaying blue electroluminescence are limited. A new series of pyrene-based blue-emitting hybridized local and charge transfer materials is designed by asym-metrically substituting at the 1,3-positions of pyrene with phenanthroimidazole and N-phenylcarbazole. Elec-tronic excitations due to pyrene localized and the charge transfer from phenanthroimidazole to pyrene segments dominate the absorption of the dyes. Incorporation of carbazole enhanced the charge transfer characteristics in the compounds and lead to the hybridization of local and charge transfer states. Theoretical studies revealed the possibility of exciton harvesting via the high lying triplet states, which have matching energy with the first excited singlet state. Lippert-Mataga plots for the solvatochromic data of the dyes exhibited bilinear relationship attributable to the formation of local state in the non-polar solvents and charge transfer state in the polar sol-vents. All the derivatives served as emitting layers in doped as well as non-doped multilayered organic light-emitting devices and exhibited deep-blue electroluminescence. The doped devices displayed much higher EL performance on comparing with that of the non-doped devices due to the proper alignment of energy levels leading to efficient exciton harvesting. The device containing carbazole derivative as dopant with tris(4-carbazoyl-9-ylphenyl)amine host displayed the highest external quantum efficiency of 8.4% with 50% exciton harvest.