Three nitrosyl-
dye conjugates, namely, [(Me 2bpb)Ru(NO)(Resf)] ( 1-Resf), [(Me 2bQb)Ru(NO)(Resf)] ( 2-Resf), and [((OMe) 2bQb)Ru(NO)(Resf)] ( 3-Resf) have been synthesized via direct replacement of the
chloride ligand of the parent {Ru-NO} (6) nitrosyls of the type [(R 2byb)Ru(NO)(L)] with the anionic tricyclic
dye resorufin (Resf). The structures of 1-Resf- 3-Resf have been determined by X-ray crystallography. The
dye is coordinated to the
ruthenium centers of these conjugates via the phenolato-O atom and is trans to NO. Systematic red shift of the d pi(Ru) --> pi*(NO) transition of the parent nitrosyls [(R 2byb)Ru(NO)(L)] due to changes in R and y in the equatorial tetradentate
ligand R 2byb (2-) results in its eventual merge with the intense absorption band of the
dye around 500 nm in 3-Resf. Unlike the UV-sensitive parent [(R 2byb)Ru(NO)(L)] nitrosyls, these
dye-sensitized nitrosyls rapidly release NO when exposed to visible light (lambda >/= 465 nm). Comparison of the photochemical parameters reveals that direct coordination of the light-harvesting chromophore to the
ruthenium center in the present nitrosyls results in a significantly greater extent of sensitization to visible light compared to nitrosyls with appended chromophore (linked via alkyl chains). 1-Resf has been employed as a "trackable" NO donor to promote NO-induced apoptosis in MDA-MB-231 human
breast cancer cells under the control of light. The results of this work demonstrate that (a) the d pi(Ru) --> pi*(NO) transition (photoband) of {Ru-NO} (6) nitrosyls can be tuned into visible range via careful alteration of the
ligand frame(s) and (b) such nitrosyls can be significantly sensitized to visible light by directly ligating a light-harvesting chromophore to the
ruthenium center. The potential of these photosensitive nitrosyl-
dye conjugates as (i)
biological tools to study the effects of NO in cellular environments and (ii) "trackable" NO donors in
photodynamic therapy of
malignancies (such as
skin cancer) has been discussed.