Simple carbazole based deep-blue emitters: The effect of spacer, linkage and end-capping cyano group on the photophysical and electroluminescent properties

Carbazole-based deep-blue emitters containing a cyano substituent and differing in the linkage and spacer moieties are synthesized by either Suzuki or Sonogashira coupling reactions. The acetylene derivatives show a red-shift in absorption over the directly linked oligomers. However, all exhibit similar emission profiles, due to the pronounced structural reorganization in the excited state of the directly linked oligomers. Linear compounds containing a 2,7-carbazole show solvent insensitive absorption and emission profiles while the 3,6-analogs exhibit positive solvatochromism in the emission indicating the presence of a donor-acceptor interaction and charge transfer excited state. While retaining similar HOMO energies that of oligomers, the acetylene derivatives display variation in LUMO energies attributable to its electron-accepting nature. The high thermal decomposition temperature of acetylene derivatives over directly connected congeners attests the rigidity of acetylene unit. When compared to the parent compounds, the cyano-derivatized compounds show better luminance in the OLED attributable to the beneficial role of cyano group. All the dyes exhibited deep blue electroluminescence with external quantum efficiencies as high as 4.4%.