Origin and features of the electrochemiluminescence of luminol – Experimental and theoretical investigations

The electrochemiluminescence accompanying the oxidation of luminol on platinum or graphite anodes was investigated under potentiostatic and potentiodynamic conditions in order to find out how it varies in time and with electrode potential, pH of the medium, and substrate concentration. The single-stage process observed in the range between 0 and 1 V can be attributed to the oxidation of the anion occurring as a result of H+ abstraction by OH− from one of the NH sites of luminol. The radical species thus formed undergo either disproportionation or, more probably, electrochemical oxidation (preceded by the abstraction of H+ by OH− from the remaining NH sites) to yield 5-aminophthalazine-1,4-dione. Electrochemical oxidation of the addition products of OH− to one of the carbonyl carbon atoms of this molecule, accompanied by N2 elimination, leads to radical intermediates which, after the addition of successive OH− ions, are converted to the radical anion of 3-aminophthalic acid. Electrochemical oxidation of the radical anion thereby formed – accompanied by the abstraction of H+ by OH− from one of carboxylic groups – generates electronically excited mono-anions of 3-aminophthalic acid capable of emitting light. The proposed reaction scheme is confirmed by thermodynamic data obtained at the PM3, PM3(COSMO), DFT and DFT(PCM) levels of theory. The prospective analytical applications of the direct electrochemical oxidation of luminol are mentioned.