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.