First Observation of the Key Intermediate in the “Light-Switch” Mechanism of [Ru(phen)₂dppz]²⁺

Abstract

[Ru(phen)2dppz]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)2dppz]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)2dppz]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.