Alternating copolymers of thiadiazole and quaterthiophenes – Synthesis, electrochemical and spectroelectrochemical characterization

A series of copolymers consisting of alternating thiadiazole and unsubstituted or alkyl disubstituted quaterthiophene units, were synthesized by electropolymerization of 2,5-bis(2,2′-bithiophene-5-yl)-1,3,4-thiadiazole (polymer P1), 2,5-bis(4′-octyl-2,2′-bithiophene-5-yl)-1,3,4-thiadiazole (polymer P2), 2,5-bis(3′-octyl-2,2′-bithiophene-5-yl)-1,3,4-thiadiazole (polymer P3), 2,5-bis(3-decyl-2,2′-bithiophene-5-yl)-1,3,4-thiadiazole (polymer P4). For comparative reasons P3 was also obtained via Suzuki coupling of 2,5-bis(5-bromo-2-thienyl)-1,3,4-thiadiazole and neopentyl ester of 4,4′-dioctyl-2,2′-dithienyl-5,5′-diboronic acid. As evidenced by cyclic voltammetry studies the presence of electron accepting thiadiazole unit in the main polymer chain results in an increase of the reductive doping potential of the studied compounds as compared to polythiophene or poly(alkylthiophene). Electrochemically determined electron affinities values were found in the range from −3.10 eV to −3.14 eV, showing a negligible effect of the alkyl substituent on this parameter. To the contrary, the oxidation potential of the studied copolymers strongly depended on the presence and the position of the alkyl group. For P3 the electron donating properties of the substituents were particularly pronounced leading to a decrease of its oxidative doping potential by 210 mV, as compared to the case of the unsubstituted polymer (P1), and the corresponding drop of the ionization potentials from +5.75 eV to +5.54 eV. The presence of a characteristic capacitive plateau's following the reductive and oxidative dopings suggests that both redox reactions are true doping reactions and the synthesized polymers can be transformed either in n-type or p-type conductors. For P3 and P4 these findings are additionally corroborated by UV–vis-NIR spectroelectrochemical data which unequivocally show the formation of polaronic/bipolaronic bands upon reductive and oxidative dopings. The analysis of the Raman spectroelectrochemical data obtained for P3, supported by theoretical calculations of the vibrational model, leads to the conclusion that the mechanism of the electrochemical doping in this polymers is the same as in poly(alkylthiophene) homopolymers and involves the transformation of the benzoid-like structure into quinoid one. As judged from the redox properties of the synthesized copolymers, P3 seems to be the most promising candidate for application in such organic electronic devices such as p-channel field effect transistors (FETs), photodiodes (PD) or photovoltaic cells (PC), however its use in air operating n-channel and ambipolar FETs seems to be excluded due to relatively high absolute value of electron affinity.