Phenanthroimidazole-based bipolar carbazoles featuring cyano substituents to realize efficient deep-blue electroluminescence with an external quantum efficiency of nearly 6%

Abstract

Developing deep-blue bipolar materials with high quantum yields remains a crucial issue in organic light-emitting diodes. The excited-state characteristics of dyes play a pivotal role in controlling the photoluminescence (PL) and electroluminescence (EL) outcomes. Here, a series of carbazole–phenanthroimidazole dyads with and without phenyl spacer units have been designed and synthesized. The functional properties are tunable by substitution of cyano units at various positions on the carbazole. The cyano-featured dyes exhibit a bathochromic shift in their emission spectra that is attributable to the contribution of a charge-transfer component in the excited state, which is further confirmed by solvatochromism studies. Accordingly, the dyes showed larger Stokes shifts due to a significant reorganization of the molecular structure in the excited state. The enhanced CT component is expected to benefit the exciton utilization efficiency of singlet energy states, thereby offering better EL performances. All dyes showed excellent thermal stability upon cyanation of carbazole–phenanthroimidazole derivatives. Electrochemical studies revealed stabilization of the lowest unoccupied molecular orbitals (LUMOs) upon the addition of cyano units to the central carbazole and realized maximum stabilization for the dyes containing cyano units at the C2 and C7 positions. The dyes were utilized as dopant emitters in solution-processed OLED devices and exhibited pure blue emission with a CIE_x,y coordinate of (0.16, 0.06) and an external quantum efficiency of 5.7%.