Redox processes of 2,6-dichlorophenolindophenolate in different solvents. A combined electrochemical, spectroelectrochemical, photochemical, and theoretical study

Cyclic voltammetric and EPR/UV-vis-NIR spectroelectrochemical studies were performed to examine the cathodic reduction of 2,6-dichlorophenolindophenolate (DCIP) in proton-donating aqueous and methanol solutions, as well as in aprotic dimethylsulfoxide (DMSO), and to characterize the paramagnetic species generated upon the DCIP reduction. In situ EPR and UV-vis-NIR spectroelectrochemistry confirmed the formation of the radical anion DCIP•– in DMSO and methanol. The same radical anion was found also in the reaction system consisting of KO2 mixed under argon with DCIP in DMSO or methanol, evidencing the electron transfer from superoxide radical anion to DCIP. The expected radical anion DCIP•– was not detected in the photoexcited suspensions DCIP/TiO2/DMSO under argon, which indicates fast consecutive reactions of photogenerated DCIP•– in the vicinity of TiO2 surface. The reduction of blue-color DCIP to the final colorless product DCIPH2 can be realized in multiple reaction pathways determined mainly by the proton-donating capacity of the solvent. Following the calculated total DFT energies, the oxygen on the indophenol moiety represents the first proton acceptor site for DCIP, DCIP•–, as well as for DCIP2– species.