Nonsymmetrical connection of two identical building blocks: constructing donor-acceptor molecules as deep blue emitting materials for efficient organic emitting diodes

We propose a strategy to construct deep blue emission molecules based on the concept of nonsymmetrical connection of two identical-conjugated groups. It was demonstrated that the nonsymmetrical connection strategy indeed resulted in the separation of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) and the formation of a donor-acceptor (D-A) structure. For D-A molecules constructed by two identical groups, the degree of charge transfer is weaker and deep blue emission is easily achieved. Two D-A molecules (PIpPI and PImPI) were synthesized by employing diphenyl-phenanthroimidazole (PI) as a building block. The nonsymmetric connection of PI groups endows these molecules with a D-A feature that can result in a bipolar transport property. The nondoped organic light-emitting diodes with PIpPI and PImPI as emitter exhibit deep-blue emission and maximum external quantum efficiencies of 8.84% and 6.83%, respectively.