EPR and UV–vis spectroelectrochemical studies of diketopyrrolopyrroles disubstituted with alkylated thiophenes

Abstract EPR and UV–vis spectroelectrochemical behavior of a series of ambipolar donor–acceptor–donor (DAD) compounds, consisting of a diketopyrrolopyrrole (DPP) central unit symmetrically disubstituted with mono-, bi- and terthiophene, is reported. All derivatives studied undergo a reversible reduction to radical anions at E ca. −1.55 V to ca. −1.65 V vs Fc/Fc+. Electrochemically generated radical anions are sufficiently stable to be detected at RT. Two-step oxidation of these compounds involves the formation of radical cations in the first step followed by their transformation into spinless dications in the second one. The formed radical cations are detectable at RT, however, contrary to the case of radical anions they yield unresolved EPR spectra. The potential of their formation depends on the length of D segment decreasing from ca. 0.50 V vs Fc/Fc+ for monothiophene disubstituted DPP to ca. 0.25 V in the case of the terthiophene derivative. Combining differential pulse voltammetry with EPR and UV–vis spectroelectrochemistry allows for unequivocal identification of all new, electrochemically induced UV–vis bands to particular forms of the studied molecules (radical anions, radical cations or dications). Both electrochemical reduction and electrochemical oxidation processes involve profound and distinctly different spectral changes in the visible range of the spectrum. Since they are reversible, the studied compounds can be considered as promising multicolor electrochromophores.