Management of Triplet Energy and Charge-Transport Properties of Hosts by CN Position Engineering

Novel hosts strategically designed to have high triplet energy and bipolar carrier-transport features were derived by substituting carbazole and CN-modified carbazole at the ortho position of the ortho-connected biphenyl core with a CN functional group. The position of the CN unit in the biphenyl core was controlled, and the effect of the CN position on the photophysical properties and device performances of the blue phosphorescent organic light-emitting diodes (PHOLEDs) was studied. The CN substitution position changed the dihedral angle between the aromatic units of the host materials, which managed the singlet energy, triplet energy, and carrier-transport properties of the hosts. Four hosts were synthesized to study the effect of the CN substitution position, and the hosts with the CN unit at a sterically hindered position increased the triplet energy, whereas the hosts with the CN unit at other positions with less steric hindrance improved the charge-transport properties. The management of the CN position could increase the triplet energy of the hosts over 3.0 eV, and the best blue PHOLED showed over 20% external quantum efficiency.