Evidence of dexter energy transfer in NO photolability of dye-sensitized ruthenium nitrosyls

Direct attachment of light-harvesting dye molecules to the ruthenium center of designed {Ru–NO}6 nitrosyls has been shown to exhibit enhanced light-induced NO photorelease upon exposure to visible light. Theoretical studies have indicated that orbital overlap between the MOs of the dye and the nitrosyl units are key for such sensitization. In order to check whether altering the electronic conjugation between these two units causes reduction in the extent of sensitization, we have synthesized a pyridinyloxy-fluorescein dye PyFlEt and attached it to two designed ruthenium nitrosyls through the pyridine-N donor of the pyridinyloxy-end. The quantum yield values of NO photodissociation at 500 nm (ϕNO) and fluorescence quantum yield values (ϕFl) of these two nitrosyl–dye conjugates namely, [(Me)2bpb)Ru(NO)(PyFlEt)]ClO4 (1-PyFlEt) and [((OMe)2IQ1)Ru(NO)(PyFlEt)]BF4 (2-PyFlEt) have been compared with those of [(Me)2bpb)Ru(NO)(FlEt)]ClO4 (1-FlEt) and [((OMe)2IQ1)Ru(NO)(FlEt)]BF4 (2-FlEt) in which the same FlEt dye is directly attached through the phenolato-O donor. This minor alteration in the linkage between the dye and the nitrosyl unit has caused significant reduction in the ϕNO values of 1-PyFlEt and 2-PyFlEt while their ϕFl values have shown moderate improvement. These results strongly suggest that the light energy absorbed by the dye unit is transferred to the Ru–NO moiety through the Dexter pathway. If the electronic overlap is disrupted, only part of the energy is used in NO photorelease and a significant portion is lost through fluorescence.