NO-releasing agents are potential therapeutic candidates for a range of disease states. Owing to the stability of ruthenium nitrosyl complexes, they may serve as NO-releasing agents in a controlled manner. Photochemical techniques provide the opportunity to limit NO release to selected sites within the body. In this context, we have studied the photo-reactivity of an aqueous solution containing [Ru(NH3)5(pyrazine)]2+ (I) and [RuCl(macrocycle)NO]+ (II) (macrocycle = cyclam or [15]aneN4). We have found that NO can be produced by light irradiation between 400 - 550 nm. The amount of NO depends on the ratio I/II with the NO quantum yield range from 2 x 10-4 to 7 x 10-2 mol.Einstein-1. It is also dependent on the O2 concentration and speculate that this is the result of superoxide formation due a secondary photochemical reaction of [Ru(NH3)5(pyrazine)]2+. The optical spectroscopy and transient absorption lifetimes of [Ru(NH3)5(pz)]2+ are affected by the presence of the nitrosyl complex. Under argon the lifetime of I is 115 ps but is 132 ps in the presence of II. This effect was interpreted as evidence for ground state association between the two ruthenium complexes, and the photochemical properties interpreted in terms of electron transfer from the excited state of [Ru(NH3)5(pz)]2+ to trans-[RuCl([15]aneN4)NO]2+. The pharmacological assays of a physiological solution containing I and II in solution as well as in sol-gel and silicone matrices show vasodilation action, and further studies are directed toward therapeutic applications based on NO photo-release agents.

Acknowledgments: FAPESP, CAPES, CNPq, CNPq/millennium and US National Science Foundation (CHE-0352650).