Synthesis, characterization, and systematic structure-property investigation of a series of carbazole-thiophene derivatives

A series of carbazole-thiophene oligomers linked at the 3,6-positions of the carbazole fragment of 4,4'-bis(carbazol-9-yl)biphenyl (CBP) and 4,4'-bis(carbazol-9-yl)-2,2'-dimethylbiphenyl (CDBP) with systematically elongated molecular lengths were synthesized via the Suzuki-Miyaura and Ullmann coupling reactions. Their electronic properties were studied by UV-Vis, cyclic voltammetry, and theoretical calculations. The coupling of CBP and CDBP with thiophene and bi- and terthiophene residues stabilized the HOMO and LUMO energy levels. The absorption and emission spectra exhibited a gradual red shift. The compounds with oligothiophene units had greatly decreased band gaps compared with CBP and CDBP. Therefore, these units may be introduced into the backbone of pi-conjugated small molecules to develop new materials with low band gaps that may have potential applications in optoelectronics.