Chemiluminescence of 3-methylindole based on electrogeneration of superoxide ion in acetonitrile solutions

Chemiluminescence of 3-methylindole (3-MIH) based on electrogeneration of superoxide ion (O2−) in air-saturated acetonitrile solutions containing no base was investigated by the use of spectroelectrochemical techniques for the first time. The emission spectrum was observed around −0.2 V versus Ag ∣ AgCl ∣ NaCl(sat) after the electrogeneration and the subsequent reoxidation of O2− (formal potential, E0′O2/O2−=−0.86 V) on the electrode surface. This spectrum revealed maximum intensity around 475 nm. On the cyclic voltammogram obtained together with the chemiluminescence at the 475-nm curve, an oxidation peak apart from the reoxidation peak of O2− was observed at about −0.2 V. This oxidation process around −0.2 V had a close connection with the chemiluminescence, and was considered to be due to the electrooxidation of 3-MI− (i.e. a conjugate base of 3-MIH) to 3-MI (i.e. a one-electron oxidation product, a radical species of 3-MI−). The electrogenerated O2− could function as a proton acceptor to 3-MIH. This reaction generates 3-MI− and hydroperoxyl radical (HO2). In the chemiluminescence observed around −0.2 V, the radical–radical coupling reaction between 3-MI and HO2 was considered to result in the formation of a 1,2-dioxetane-like intermediate to lead to the emission of light. The chemiluminescent reaction mechanism of 3-MIH was discussed in detail in comparison with that of the MCLA (a Cypridina luciferin analogue)–O2− system.