Systematic Structure–Property Investigations on a Series of Alternating Carbazole–Thiophene Oligomers

A series of alternating carbazole–thiophene oligomers, namely 2,7-linked carbazole–thiophene oligomers 1, 3, 5, 7, and 9 and 3,6-linked ones 2, 4, 6, 8, and 10, in which the molecular length was systematically elongated, were synthesized by Suzuki–Miyaura coupling reactions. The effects of the conjugation connectivity between the carbazole and thiophene moieties and the molecular length on the electronic, photophysical, and electrochemical properties of 1–10 were comprehensively investigated. In the present oligomer architectures, the connection with thiophene at the 2,7-positions of carbazole ensures π-conjugation to a high extent and high fluorescence quantum yields, while that at the 3,6-positions enhances the donor ability. The increase in the molecular length of the 2,7-linked oligomers effectively extends π-conjugation. The relationship between structural variations and photophysical properties was examined by fluorescence lifetime measurements in detail. The X-ray crystal structure of 6 was also disclosed.