Electrogenerated chemiluminescence in mechanistic investigations of electroorganic reactions: Part III. Reduction of some disulfides at the dropping mercury electrode

In the simultaneous cathodic reduction of diphenyl disulfide (Ph?S?S?Ph) or dibenzoyl disulfide (Ph?CO?S?S?CO?Ph) and a series of fluorescent aromatic hydrocarbons A at the dropping mercury electrode in dimethylformamide the emission of A is observed. This electrogenerated chemiluminescence (ECL) originates from the electron transfer between A^? and the radicals Ph?S or Ph?COS, which are formed in a one-electron reductive cleavage of the disulfide bond by A^?. As an intermediate the anion radical Ph?S?S?Ph^? or Ph?CO?S?S?CO?Ph^? is assumed. In the case of Ph?S?S?Ph the ECL intensity is enhanced by proton donors (water or benzoic acid), which increase the cleavage rate of Ph? S?S?Ph^? in an electrophilic attack by the proton. The relatively negative threshold reduction potential of A (?1.4 to ?1.6 V) for the ECL in comparison with the half-wave potential (?0.85 V) supports a mercury-assisted heterogeneous reduction mechanism of Ph?S?S?Ph. The intensity-potential curves and the intensity—time curves at the mercury drop were measured for different concentrations of Ph?S?S?Ph and A and for different mercury pressures. No luminescence could be detected with o,o′-dinitrodiphenyl disulfide and diethyl disulfide.