Effect of donor/acceptor on the charge transfer and exciton harvest of pyrene-phenanthroimidazole dyads

Designing blue emitters with polyaromatic hydrocarbons such as anthracene and pyrene is more challenging as their derivatives are prone for aggregation caused emission quenching. We report herein three new pyrene-based deep blue emitting materials containing phenanthroimidazole moiety and a diphenylamine donor or cyano acceptor. Though the absorption of the compounds is similar, the emission were altered by the peripheral unit, due to the modulation in the donor-acceptor interaction and the intramolecular charge transfer (CT) in the excited state. The solvatochromic emission data indicated the presence of hybridized local and CT excited state for the parent core and its diphenylamine derivative. However, the cyano derivative inherits more CT characteristics. Additionally, all the dyes were found to exhibit acidochromism due to the facile interaction of phenanthroimidazole moiety with the acid. All the dyes were used for organic light-emitting diodes (OLED) applications with tris(4-carbazoyl-9-ylphenyl)amine (TCTA) host material, which exhibited deep blue or cyan-blue color electroluminescence with high external quantum efficiency. The device consisting of a dye with parent core as a dopant in TCTA host exhibited highest external quantum efficiency (EQE) of 7.5% with current efficiency 3.3 cd/A and luminance 2734 cd/m2 attributable to the effective utilization of excitons harvested through the HLCT channel.