EFFECTS OF SUBSTITUTION OF THE MEDIAN THIOPHENE RING ON THE ELECTRODEPOSITION AND STRUCTURE OF POLY(TERTHIENYLS)

Alpha-terthienyls (3Ts) substituted at the beta-position of the median thiophene ring by methyl (MTT), octyl (OTT), and dioxaheptyl (DHTT) groups have been sythesized and their electrooxidation and electropolymerization have been investigated with reference to unsubstituted 3T (TT). The electrooxidation process appears strongly dependent on the initial substrate concentration, and the emergence of a concentration-dependent additional oxidation wave between those corresponding to the formation of the cation radical and dication states suggests the occurrence of an aggregation process. Although substitution does not significantly affect the oxidation potential of the 3T system, the nature of the attached substituent strongly affects the electropolymerization process and the structure of the resulting material. The analysis of the electrochemical and optical properties of the various poly(terthienyls) (P3Ts) by cyclic voltammetry and UV-visible absorption spectroscopy shows that the electropolymerization of all 3Ts is accompanied with the trapping of significant amounts of starting material in the film. Although TT leads to a poorly conjugated polymer, the grafting of long alkyl or oxyalkyl chains on the 3T system produces a several hundred millivolts decrease of the oxidation potential of the polymer and a 70-80-nm bathochromic shift of its absorption maximum, changes that are indicative of a considerable extension of conjugation. These results are discussed with regard to the specific reactivity of 3T system and to effects of substitution on the solubility of their electrooxidation products.