Investigation on redox mechanism of 1,4-naphthoquinone by in situ FT-IR spectroelectrochemistry

• 1,4-naphthoquinone undergoes normal two-step one-electron process in DMSO.
• Anion radicals of 1,4-naphthoquinone undergo dimerization in acetonitrile.
• The dimerization also occurs when ethanol is added to DMSO and acetonitrile.
• Hydrogen-bonding forms between intermediates (final reduced products) and ethanol.

The electrochemical character of 1,4-naphthoquinone (Q) in aprotic media and proton donor mixed solvent was explored by cyclic voltammetry (CV), in situ FT-IR spectroelectrochemistry, cyclic voltabsorptometry (CVA) and derivative cyclic voltabsorptometry (DCVA) techniques. In dimethyl sulfoxide (DMSO), Q is reduced in normal two-step one-electron transfer, forming the anion radical (Q−) in the first step and the dianion (Q2 −) in the second step accordingly. However, the electrochemical behavior of Q in acetonitrile (CH3CN), acetonitrile–ethanol mixed solvent and dimethyl sulfoxide–ethanol mixed solvent is different from that in DMSO. It is found that in these solvents, Q is reduced to Q− firstly and meanwhile the neutral–anion radical dimer (Q2−) comes into being. Then Q− is reduced to Q2 − and Q2− is reduced to Q22 −. At more negative potential, Q22 − is further reduced to Q23 −. In proton donor mixed solvent, hydrogen-bonding forms between Q2 − (Q−, Q2−, Q22 −, Q23 −) and C2H5OH.