Chemiluminescence of indole and its derivatives induced by electrogenerated superoxide ion in acetonitrile solutions

Electrochemical behavior of the dioxygen (O₂)/superoxide ion (O₂ ⁻) redox couple in acetonitrile (AN) solutions containing indole compounds and no proton acceptor, and their chemiluminescence have been examined using spectroelectrochemical techniques. In AN solutions, in the presence of indole compounds having a substituent at the N-position (e.g. 1-methylindole and 1,2-dimethylindole), these indole compounds did not work as a proton donor to electrogenerated O₂ ⁻, and did not affect the redox reaction of the O₂/O₂ ⁻ couple. On the other hand, the indole compounds bearing a hydrogen at the N-position (e.g. indole, 3-methylindole (3-MI), tryptamine (TP), 3-indolemethanol and 3-indoleacetic acid) acted as a proton donor to O₂ ⁻. In addition, chemiluminescence was observed for some of the indole compounds having a hydrogen at the N-position and an electron-releasing group at the 3-position (i.e. TP, 2,3-dimethylindole, 2-(3-indole)-ethanol and 3-MI). The intensity of chemiluminescence of 3-MI was by far the strongest among those of the above four compounds. The chemiluminescent reaction of the indole compounds seemed to have close connection with electrogeneration of O₂ ⁻, which functioned as a proton acceptor to the indole compounds, and the chemical stability of a radical species (i.e. a one-electron oxidized form of conjugate base of indole compounds) due to the substituent which is present at the 3-position. The chemiluminescence was considered to occur via the formation of 1,2-dioxetane-like intermediates. The reaction mechanisms of chemiluminescence based on the electrogenerated O₂ ⁻ were briefly discussed.