Cyclopenta[c]thiophene-Based D-A Conjugated Copolymers: Effect of Heteroatoms (S, Se, and N) of Benzazole Acceptors on the Properties of Polymers

Three new donor-acceptor (D-A) type copolymers P1, P2, and P3 have been synthesized by Stille condensation between the distannyl derivative of thiophene-capped cyclopenta[c]thiophene (CPT) with 4,7-dibromo[2,1,3]benzothiadiazole, 4,7-dibromo[2,1,3]benzoselenadiazole, and 4,7-dibromo[2,1,3]benzotriazole, respectively. These new CPT-based D-A copolymers showed an interesting trend of visible color (red, green, and blue) in solution as the acceptor was varied keeping the donor constant. The optical band gaps of the polymers, which were estimated by measuring the absorption onset in the UV-vis spectra of the film, were found to be 1.57, 1.44, and 1.86 eV for P1, P2, and P3, respectively. DFT calculations correlated the strength of the acceptors with the interesting trend in the colors of these (D)(nonvariant)-(A)(variant) copolymers. Compared with the solution, the film state absorption of P2 and P3 was significantly red-shifted compared to that of P1, indicating the presence of strong interchain interactions due to efficient self-pi-stacking in the solid state.