Substituent effect on the optoelectronic properties of alternating fluorene-thiophene copolymers

A novel series of soluble alternating conjugated copolymers, comprised of 9,9-dihexylfluorene and thiophene or substituted thiophene moieties (P1-P5), were synthesized via the palladium-catalyzed Suzuki coupling reaction. Substitutents on the thiophene include the electron-donating groups of hexyl and hexyloxymethyl group and the electron-withdrawing groups of hexyl carboxylate and cyano. The steric effects of the bulkier substituents outweigh the electronic effects of the substituents on the observed absorption and photoluminescence properties of the copolymers. Therefore, only the cyano substituted copolymer (P4) exhibits a red shift of the electronic spectra with 1.6 times enhancement in the fluorescence quantum yield as compared with the unsubstituted copolymer (P5). The substituents slightly reduce the values of T-g and T-d of P5, but all of the reported copolymers have a T-d larger than 300 degrees C.