A Novel Deep Blue-Emitting Zn^II Complex Based on Carbazole-Modified 2-(2-Hydroxyphenyl)benzimidazole: Synthesis, Bright Electroluminescence, and Substitution Effect on Photoluminescent, Thermal, and Electrochemical Properties

Abstract

A novel deep blue-emitting ZnII complex Zn(Lc)2 (Lc− = 2-(1-(6-(9H-carbazol-9-yl)hexyl)-1H-benzo[d]imidazol-2-yl)phenolate) based on a carbazole-functionalized N^O ligand was synthesized by a modified method. Other two ZnII complexes (Zn(La)2, La− = 2-(1H-benzo[d]imidazol-2-yl)phenolate; Zn(Lb)2, Lb− = 2-(1-ethyl-1H-benzo[d]imidazol-2-yl)phenolate) were also prepared for comparison. The remarkable substitution effect on the photoluminescent and thermal properties of the complexes was studied. The investigation indicated an unexpected amplifying hypsochromic effect of the substituents on the emission of the complex in the solid state: the larger substituent corresponded to the larger blue shift of the emission of the complex (Zn(Lc)2 has the shortest emission wavelength of 422 nm as the deep blue emission among these three complexes). The stronger steric effect induced by the bulky substitutions should be one of the most important factors. Among the three ZnII complexes, the temperature of decomposition of Zn(Lc)2 is the highest at 427 °C. Cyclic voltammetry (CV) of the complexes showed that the carbazole moieties remarkably improved the hole injection ability of Zn(Lc)2 with the HOMO energy level 0.6 eV higher than those of Zn(La)2 and Zn(Lb)2. The good hole injection and transporting ability of Zn(Lc)2 was further proved by its three-layer devices, in which the electroluminescent (EL) emission mainly originated from the electron-transporting Alq3 layer. Through the four-layer devices with the hole-blocking layer, the pure blue emission of Zn(Lc)2 at 452 nm was demonstrated. Zn(Lc)2 seems favorable among the blue-emitting ZnII complexes with a brightness more than 2000 cd m−2, a high efficiency stability, and an excellent EL spectra stability.