Comparison of homoleptic and heteroleptic 2,2'-bipyridine and 1,10-phenanthroline ruthenium complexes as chemiluminescence and electrochemiluminescence reagents in aqueous solution.

We have conducted a comprehensive comparative study of Ru(bipy)(3)(2+), Ru(bipy)(2)(phen)(2+), Ru(bipy)(phen)(2)(2+), and Ru(phen)(3)(2+) as chemiluminescence and electrochemiluminescence (ECL) reagents, to address several previous conflicting observations and gain a greater insight into their potential for chemical analysis. Clear trends were observed in many of their spectroscopic and electrochemical properties, but the relative chemiluminescence or ECL intensity with a range of analytes/co-reactants is complicated by the contribution of numerous (sometimes opposing) factors. Significantly, the reversibility of cyclic voltammetric responses for the complexes decreased as the number of phenanthroline ligands was increased, due to the lower stability of their ruthenium(III) form in the aqueous solvent. This trend was also evident over a longer timescale when the ruthenium(III) form was spectrophotometrically monitored after chemical oxidation of the ruthenium(II) complexes. In general, the greater stability of Ru(bipy)(3)(3+) resulted in lower blank signals, although this effect was less pronounced with ECL, where the reagent is oxidised in the presence of the co-reactants. Nevertheless, this shows the need to compare signal-to-blank ratios or detection limits, rather than the more common comparisons of overall signal intensity for different ruthenium complexes. Furthermore, our results support previous observations that, compared to Ru(bipy)(3)(2+), Ru(phen)(3)(2+) provides greater ECL and chemiluminescence intensities with oxalate, which in some circumstances translates to superior detection limits, but they do not support the subsequent generalised notion that Ru(phen)(3)(2+) is a more sensitive reagent than Ru(bipy)(3)(2+) for all analytes.