Alternating copolymers of diketopyrrolopyrrole or benzothiadiazole and alkoxy-substituted oligothiophenes: spectroscopic, electrochemical and spectroelectrochemical investigations

A series of solution processable semiconducting donor-acceptor (DA) copolymers consisting of either diketopyrrolopyrrole or benzothiadiazole A units and alkoxy- or alkyl-substituted oligothiophene D units were synthesized. For all prepared copolymers the measured XPS spectra (C1s, S2p, N1s and O1s) were in a very good agreement with the expected chemical constitution. Spectroscopic studies of the synthesized copolymers showed that their optical band gaps were governed by the presence of the alkoxy substituents whose electron donating properties led to additional gap narrowing yielding semiconductors with band gaps of below 1.3 eV in the case of the polymers with the diketopyrrolopyrrole A unit. The same trend was observed with the electrochemical band gaps, whose values were however found to be ca. 0.4 eV superior to the corresponding optical band gaps values. Vibrational model was calculated for two of the synthesized copolymers with the goal to unequivocally attribute the observed Raman modes and to support the interpretation of the spectral changes induced by the electrochemical oxidation. It was established that the electrochemical oxidative doping of the copolymer with the benzothiadiazole A unit is limited to the oligothiophene segment in which the charge of the formed polycation is localized. To the contrary, in the case of the polymer with the diketopyrrolopyrrole A segment the charge imposed on the oligothiophene segment delocalizes towards the diketopyrrolopyrrole unit. These findings are in perfect agreement with the UV-vis-NIR spectroelectrochemistry data which show strong localization of electrochemically created charge carriers in the benzothiadiazole - oligothiophene copolymer and their metallic-like delocalization in the diketopyrrolopyrrole one. The latter seems to be very interesting not only as a potential low band gap component of organic photovoltaic cells but also, in the doped state, as electronic conductor of metallic character.