Star-Shaped Asymmetrically Substituted Blue Emitting Carbazoles: Synthesis, Photophyscial, Electrochemical and Theoretical Investigations

A series of star-shaped blue-emitting 1,3,6,8-tetrasubstituted carbazoles containing different functional groups are synthesized and characterized. Electron-transporting N-phenylbenzimidazole is attached to C3 and C6 positions while the chromophore at C1 & C8-positions varied from fluorene to triphenylamine. The effect of the nature of the C1 and C8 chromophores on optical, electrochemical and thermal properties is unravelled. Absorption spectra revealed that the choice of meta- mode of connectivity between the chromophores on carbazole nucleus interrupted the interchromophoric electronic interactions in the ground state. However, the molar extinction coefficients of the dyes increased significantly on the addition of chromophores at C1 and C8 attributable to the increment in the chromophore density. Interestingly, triphenylamine and pyrene containing compounds displayed positive solvatochromism in emission attributable to intramolecular charge transfer and excimer excited states, respectively. Fluorescence and thermal properties are strongly influenced by the nature of chromophores present on C1 & C8. Electrochemical studies revealed low oxidation potential for arylamine containing derivatives and led to high-lying highest occupied molecular orbital in the series. However, the lowest unoccupied molecular orbital is stabilized by pyrene substituent due to its -acceptor character.