Water detection by "turn on" fluorescence of the quinone-containing complexes [Ru(phen)2(1,10-phenanthroline-5,6-dione)2+] and [Ru(phenanthroline-5,6-dione)3]2+.

Addition of water to the quinone functions in [Ru(phen)2(pdn)](2+) (1) and [Ru(pdn)3](2+) (2) (where phen = 1,10-phenanthroline and pdn = 1,10-phenanthroline-5,6-dione) turns on fluorescence at 605 nm, as formation of the geminal diol eliminates the predominant quinone-based non-radiative decay pathway and gives rise to a long-lived (3)MLCT state similar in nature to that seen in [Ru(phen)3](2+). Using NMR, the equilibrium constant for the hydration reaction of 1 in acetonitrile was determined to be 0.0253. From this data and experimental fitting of the luminescent titration data, the equilibrium constant for 2 of 1.62 × 10(-5) and emission yields for hydrated 1 and 2 were determined. Interestingly, all three quinone functions must be hydrated in 2 for luminescence, which is why the equilibrium constants vary so much. The 'turn on' luminescence allows for a very sensitive detection of water in aprotic solvents such as acetonitrile.