Preparation and spectroelectrochemical behaviour of a new alternate copolymer of 3,3′-di-n-octyl-2,2′-bithiophene and fluoren-9-one

Abstract

The synthesis and spectroelectrochemical behaviour of a new solution processible conjugated polymer, namely poly[{5,5′-(3,3′-di-n-octyl-2,2′-bithiophene)}-alt-(2,7-fluoren-9-one)] (abbreviated as PDOBTF), are described. PDOBTF can be considered as the first member of a new family of conjugated copolymers—poly(oligothiophene-alt-fluoren-9-one)s—whose properties can be tuned by changing the length of the oligothiophene segments and their regiochemistry. PDOBTF can be obtained by oxidative polymerisation of 2,7-bis(4-octylthien-2-yl)-fluoren-9-one or by condensation polymerisation of 2,7-bis(5-bromo-4-octylthien-2-yl)-fluoren-9-one using a modification of Yamamoto coupling. Both preparation methods lead to a mixture of polymeric and oligomeric species and require post-polymerisation fractionating if high molecular fractions are to be obtained. Oxidative polymerisation gives product of a higher molecular weight (M_n = 41.0 kDa, M_w/M_n = 1.81 for the highest molecular weight fraction) as compared to the one prepared by Yamamoto condensation polymerisation (M_n = 13.3 kDa, M_w/M_n = 1.45 for the highest molecular weight fraction).

Electrochemical oxidation of PDOBTF in an nonaqueous electrolyte (0.1 M Bu₄NBF₄/acetonitrile) gives rise to an anodic peak at E = 835 mV, which can be ascribed to the p-type doping of the copolymer. The extension of the potential to E = 1500 mV results in the oxidative degradation of the copolymer and induces total loss of its electroactivity. UV-Vis-NIR and Raman spectroelectrochemical data are consistent with the oxidative doping. The latter technique enables the monitoring of the doping-induced changes in both structural sub-units of the copolymer: the bithiophene sub-unit and the fluoren-9-one one.