Electrochemistry and electron transport properties of copolymers of electron deficient fluorenes with thiophene

Copolymers of fluoren-9-one and (fluoren-9-ylidene)malononitrile with thiophene have been prepared by the electrochemical polymerization of 2-(2-thienyl)fluoren-9-one, 2,7-di(2-thienyl)fluoren-9-one, and (2,7-di(2-thienyl)fluoren-9-ylidene)malononitrile. 2-(2-Thienyl)fluoren-9-one gave polymer films with low conjugation lengths, indicating poorly defined coupling, while both monomers with two terminal thiophenes gave high quality copolymers. Poly-2,7-di(2-thienyl)fluoren-9-one] was the most easily p-doped material, and gave the highest p-type conductivity of ca. 0.4 mS cm⁻¹. In contrast poly-(2,7-di(2-thienyl)fluoren-9-ylidene)malononitrile] was more easily reduced by over 0.5 V (formal potential = −0.60 V versus Ag/AgCl), and exhibited the higher n-type conductivity of ca. 6 μS cm⁻¹. Comparisons between the three copolymers reveal differences in the mechanisms of p-type and n-type conduction, with the former dominated by hole transport along the polymer chains and the latter involving electron hopping between localized sites.