Redox Behaviour and Redox Potentials of Dyes in Aqueous Buffers and Protic Ionic Liquids

Abstract

Organic dyes hold promise as inexpensive electrochemically‐active building blocks for new renewable energy technologies such as redox‐flow batteries and dye‐sensitised solar cells, especially if they display high oxidation and/or low reduction potentials in cheap, non‐flammable solvents such as water or protic ionic liquids. Systematic computational and experimental characterisation of a representative selection of acidic and basic dyes in buffered aqueous solutions and propylammonium formate confirm that quinoid‐type mechanisms impart electrochemical reversibility for the majority of systems investigated, including quinones, fused tricyclic heteroaromatics, indigo carmine and some aromatic nitrogenous species. Conversely, systems that generate long‐lived radical intermediates ‐‐ arylmethanes, hydroquinones at high pH, azocyclic systems ‐‐ tend to display irreversible electrochemistry, likely undergoing ring‐opening, dimerisation and/or disproportionation reactions.