[Ru(phen)(2)dppz](2+) (phen = 1,10-phenanthroline, dppz = dipyridophenazine) and closely related complexes have previously been observed to have an undetectably small quantum yield of photoluminescence in water but a moderate emission yield when bound to DNA. This so-called ''light-switch'' effect is a critical factor in the utility of these complexes as spectroscopic probes for DNA. Here we describe a detailed investigation of the photophysics of [Ru(phen)(2)dppz](2+) in aqueous solution, and in mixtures of acetonitrile and water, by time-resolved absorption and emission spectroscopies. The emission of the complex in water has been measured for the first time. A prompt initial emission, derived from a metal-to-ligand charge-transfer (MLCT) excited state typical for polypyridyl-ruthenium complexes, is observed along with a delayed emission attributed to a novel MLCT species. The small quantum yield of photoluminescence for [Ru(phen)(2)dppz](2+) in water, and in water/acetonitrile. depends upon efficient formation of a novel MLCT species, followed by its rapid radiationless decay. The MLCT interconversion is assigned to an intramolecular charge-transfer process that is induced by the polarity and proton donating ability of the solvent.