Electrochemiluminescence of Tris(2,2′-bipyridyl)ruthenium(II) with Ascorbic Acid and Dehydroascorbic Acid in Aqueous and Non-aqueous Solutions

Abstract

The electrochemiluminescence (ECL) of tris(2,2′-bipyridyl)ruthenium(II) (Ru(bpy)₃²⁺) is studied in non-aqueous media using dehydroascorbic acid (DHA) as coreactant to validate the evidence for the mechanism of the ascorbic acid (H₂A)/Ru(bpy)₃²⁺ ECL system in an aqueous media. DHA is electrochemically reduced around –1.2 V vs. Ag/Ag⁺ in pure acetonitrile to generate the ascorbyl radical anion (A^•−), which is confirmed by in-situ UV-visible absorption measurements using a thin-layer spectroelectrochemical cell. The ECL of the DHA/Ru(bpy)₃²⁺ system in non-aqueous media is not observed in the potential range from 0 to +1.4 V in anodic potential sweep mode; however, distinct ECL is detected using double potential step electrolysis from –1.2 to +1.4 V vs. Ag/Ag⁺. The ECL may be generated by a homogeneous charge-transfer process between A^•− produced during the first pulse potential step (–1.2 V) and Ru(bpy)₃³⁺ generated during the second pulse potential step (+1.4 V). The calculated standard enthalpy (–ΔH°) for the charge-transfer reaction between A^•− and Ru(bpy)₃³⁺ is 2.29 eV, which is larger than the lowest excited singlet state energy of Ru(bpy)₃²⁺ (*Ru(bpy)₃²⁺; 2.03 eV, 610 nm). It is determined that the generated intermediate A^•− is crucial in the Ru(bpy)₃²⁺ ECL reaction.