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

The electrochemiluminescence (ECL) of tris(2,2'-bipyridyl)ruthenium(II) (Ru(bpy)3(2+)) 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)3(2+) 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)3(2+) 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)3(3+) 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)3(3+) is 2.29 eV, which is larger than the lowest excited singlet state energy of Ru(bpy)3(2+) (*Ru(bpy)3(2+); 2.03 eV, 610 nm). It is determined that the generated intermediate A(•-) is crucial in the Ru(bpy)3(2+) ECL reaction.