Effect of substituted nitrogen atom location on the photophysical and charge transporting properties of carboline derivatives

In this work, three carboline derivatives, 9,9′-m-phenylenedi-α-carboline (NmCP1), 9,9′-m-phenylenedi-β-carboline (NmCP2), and 9,9′-m-phenylenedi-γ-carboline (NmCP3) were designed and synthesized to examine the effect of nitrogen atom position on the photophysical and charge transport properties of carboline derivatives. NmCP1 with α-carboline showed high hole and electron current density in the single carrier device owing to formation of π-π stacking by weak hydrogen bond between α-carbaoline and a phenyl linker from single crystal analysis. Whereas, NmCP2 and NmCP3 were advantageous to increase the triplet energy due to the distortion of the carboline unit from the central phenyl unit. Among the three compounds, the NmCP1 achieved high external quantum efficiency of 18.6% in the deep blue phosphorescent organic light-emitting diodes. Therefore, the control of the nitrogen position could manage the carrier transport properties, photophysical properties and device performances of the carboline derivatives.