Influence of molecular configuration and functional substituents on excited state energy levels in two naphthyl-based phosphine oxide hosts

Abstract

Two phosphine oxide hosts, 1,8-bis(diphenylphosphoryl)naphthalene (NAPO) and 2,2′-bis(diphenylphosphoryl)-1,1′-binaphthyl (BiNAPO), were designed and synthesized to investigate the influence of molecular configuration and the density of functional substituents on the excited energy levels. With the much bigger conjugated area, BiNAPO has the T1 of 2.46eV, which is only 0.17eV less than that of naphthalene. This should be owing to the orthogonal configuration of BiNAPO. Meanwhile, NAPO with the higher density of functional substituents has the much lower T1 of 2.26eV. It is showed that shortening the conjugated length by tuning molecular configuration and reducing the density of functional substituents can effectively remain T1 of the chromophore. Simultaneously, Gaussian simulation showed that the larger conjugated area and the rational distribution of functional substituents endow BiNAPO with more balanced carrier injecting/transporting ability, which was further proved by nominal singly-carrier-only devices. The low-voltage driving and efficient green-emitting electrophosphorescent devices based on BiNAPO were demonstrated.