Fine-Tuning the Physicochemical and Electroluminescence Properties of Multiply-Substituted Bipolar Carbazoles by Functional Group Juggling

Abstract

The nature of chromophores often plays a decisive role in imparting properties to functional materials. In this work, we demonstrate that the position of the chromophoric unit on the carbazole core is crucial to tune the photophysical and electroluminescence properties. A series of bipolar compounds featuring carbazole donor and cyano acceptor groups are used as model compounds to evaluate the proposed hypothesis. It is revealed that the conjugation connectivity and chromophore density significantly affects the functional properties of the dyes. A compound featuring carbazole on the C2 and C7 positions of a carbazole core displayed a more bathochromic absorption band than its structural isomer with carbazole on C1 and C8 positions. A major change in emission properties is achieved on changing the position of the carbazole and cyano units on carbazole nucleus. The emission spectra of the dyes are sensitive to solvent polarity and displayed positive solvatochromism attributable to the photoinduced charge transfer from the peripheral carbazole donor to the cyano acceptor. They are demonstrated as dopant emitters in solution-processed multilayer organic light emitting devices and exhibited deep-blue or sky-blue emission with CIE coordinates varying in the range: 0.14≤x≤0.20, 0.07≤y≤0.16. A compound containing a carbazole donor on C3 and C6 and a cyano acceptor on C2 and C7 positions of the carbazole core exhibited the best performance in an OLED with a maximum external quantum efficiency of 3.4 %.