Photosensitization of Ruthenium Nitrosyls to Red Light with an Isoelectronic Series of Heavy-Atom Chromophores: Experimental and Density Functional Theory Studies on the Effects of O-, S- and Se-Substituted Coordinated Dyes

Three ruthenium nitrosyl-dye conjugates, namely, [((OMe)2bQb)Ru(NO)(Resf)] (RuNO-Resf), [((OMe)2bQb)Ru(NO)(Thnl)] (RuNO-Thnl), and [((OMe)2bQb)Ru(NO)(Seln)] (RuNO-Seln) have been synthesized using the tetradentate N4 dicarboxamido ligand H2(OMe)2bQb. Each nitrosyl of this series is conjugated to a phenoxazine-type heterotricyclic chromophore which has been systematically varied in a central position to test the effects of “heavy-atom” substitution (O = Resorufin; S = Thionol; Se = Selenophore) in photosensitization. The structure of the chloride-bound precursor {Ru-NO}6 nitrosyl [((OMe)2bQb)Ru(NO)(Cl)] (RuNO-Cl) and three nitrosyl-dye conjugates, namely, RuNO-Resf, RuNO-Thnl and RuNO-Seln, have been determined by X-ray crystallography. All three nitrosyl-dye conjugates exhibit sharp 1H NMR spectra (S = 0 ground state) and νNO stretches in the IR spectrum in the region 1825−1855 cm−1, typical of {Ru-NO}6 nitrosyls. The presence of a heavy atom in the bound dye gives rise to a systematic red-shift in the electronic absorption spectrum, shifting from RuNO-Resf (λmax = 500 nm) to RuNO-Thnl (λmax = 530 nm) to RuNO-Seln (λmax = 535 nm). Results of careful measurements with monochromatic light sources indicate that heavy-atom substitution in the coordinated dye makes the resulting nitrosyl-dye conjugates more susceptible to light of longer wavelength (lower energy). Density functional theory (DFT) calculations on RuNO-Cl, RuNO-Resf, RuNO-Thnl, and RuNO-Seln have been performed to gain insight into the electronic structure of the {dye-Ru-NO} frame and the nature of transition(s) that sensitizes these conjugates to lights of longer wavelengths and promote NO photolability. Results of this study provide an explanation for the sensitization observed in our strategy of direct attachment of dye molecules to {Ru-NO}6 nitrosyls. This strategy could lead to eventual isolation of designed metal nitrosyls that are sensitive to red- or near-infrared light and hence potential photodynamic therapy (PDT) agents for treatment of malignancies with high doses of NO.