FIRST OBSERVATION OF THE KEY INTERMEDIATE IN THE LIGHT-SWITCH MECHANISM OF [RU(PHEN)(2)DPPZ](2+)

[Ru(phen)(2)dppz](2+) (phen = 1,10-phenanthroline, dppz = dipyridophen azine) 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 ''lig ht-switch'' effect is a critical factor in the utility of these comple xes as spectroscopic probes for DNA. Here we describe a detailed inves tigation of the photophysics of [Ru(phen)(2)dppz](2+) in aqueous solut ion, and in mixtures of acetonitrile and water, by time-resolved absor ption and emission spectroscopies. The emission of the complex in wate r has been measured for the first time. A prompt initial emission, der ived from a metal-to-ligand charge-transfer (MLCT) excited state typic al for polypyridyl-ruthenium complexes, is observed along with a delay ed emission attributed to a novel MLCT species. The small quantum yiel d of photoluminescence for [Ru(phen)(2)dppz](2+) in water, and in wate r/acetonitrile. depends upon efficient formation of a novel MLCT speci es, followed by its rapid radiationless decay. The MLCT interconversio n is assigned to an intramolecular charge-transfer process that is ind uced by the polarity and proton donating ability of the solvent.