Effect of Cyano on the Functional Properties of Phenanthroimidazole-Substituted Carbazole Derivatives

A series of carbazole-based structural isomers appended with phenanthroimidazole and/or cyano chromophores are designed and synthesized. The structure–property relationship of bifunctionalized dyes is studied and compared with that of monofunctionalized dyes. The position of the chromophoric connectivity to the core carbazole is found to determine the physiochemical and electroluminescence properties of the isomeric dyes. The dyes with phenanthroimidazole incorporated at the C2 position of the carbazole offer a higher π-conjugation as compared to the dye with the C2 cyano-functionalized derivative. The incorporation of a phenylene spacer between the carbazole and phenanthroimidazole led to a red shift in emission, which is attributable to the reorganization of dye in the excited state. The C2 and C7 substituted bipolar dye offers more red-shifted emission because of the increased charge transfer component, which is further confirmed by its polar excited state in solvatochromism studies. The bifunctionalized dyes displayed high thermal/morphological stability with a thermal decomposition and glass transition temperature greater than 411 and 119 °C, respectively. Moreover, electrochemical measurements of the dyes reveal that the addition of a cyano substitution on the carbazole facilitates the stabilization of the LUMO. The materials are employed as dopant emitters in solution-processed multilayer organic light-emitting diode (OLED) devices and display deep-blue electroluminescence with Commission International L’Eclairage (CIE) coordinates of (0.16, 0.06) and external quantum efficiency of 5.7%, indicating the potential application for efficient blue OLEDs.