Band-gap tuning of carbazole-containing donor–acceptor type conjugated polymers by acceptor moieties and π-spacer groups

Carbazole-containing donor–acceptor type conjugated polymers were prepared by Sonogashira cross-coupling reaction of 3,6-diethynyl-9-hexadecylcarbazole and arylene dibromides, and the optical and electrochemical properties were extensively investigated. Copolymerization with 4,7-dibromo-2,1,3-benzothiadiazole furnished the orange-colored polymer with a charge-transfer band at 440 nm in CH2Cl2, indicating efficient intramolecular donor–acceptor interactions. On the other hand, the color of the copolymer with 2,5-dibromopyridine was ocher and the longest wavelength absorption band was 381 nm in CH2Cl2, suggesting almost negligible donor–acceptor interactions. This difference was also reflected by the emission spectra, solvatochromic behaviors, and electrochemical redox potentials. Comparison of the ethynylene spacer between the donor and acceptor moieties to other π-spacers revealed that the ethynylene spacer serves as a more efficient π-spacer than vinylene and direct connection (without spacer) for strong donor–acceptor coupling in this polymer system.