Synthesis and Electronic, Photophysical, and Electrochemical Properties of a Series of Thienylcarbazoles

A series of thienylcarbazoles were synthesized by Suzuki–Miyaura and Ullmann coupling reactions. In these compounds, the 2-thienyl or 2,2′-bithiophen-5-yl group is connected at the N-, 1,8-, 3,6-, 2,7-, 2,7,N-, or 1,8,N-positions of the carbazole ring. The effects of structural variations on their electronic, photophysical, and electrochemical properties were explored by UV–vis and fluorescence spectroscopies, cyclic voltammetry (CV), and DFT calculations in evaluation of their potential as material components. The thienyl substituents at the 2,7-positions in 4, 5, and 10 are responsible for a high degree of π-conjugation and strong emission with fluorescence quantum yields up to 0.61. The CV on a series of thienylcarbazoles revealed a good electron-donating ability of 3,6-substituted carbazoles 3 and 9. The number of thiophene units was found to affect the extent of π-conjugation, the resulting HOMO–LUMO gaps, and fluorescence efficiency. The crystal structures of 5 and 9 were also disclosed.