Medium dependence of the spectroscopic and photophysical properties of Ru(bpy)2(HAT)2+. The effect of solvent, pH and binding to polyelectrolytes

In order to assess its usefulness as a probe for medium properties, the absorption, emission and excited state decay behaviour of Ru(bpy)2(HAT)2+ (bpy,2,2′-bipyridyl; HAT, 1,4,5,8,9,12-hexaazatriphenylene) were studied in various solvents, low temperature ethanol—methanol glasses, perfluorinated ionomer membranes (Nafion), in the presence of DNA or synthetic polynucleotides and in a solution of sodium polystyrenesulphonate (PSSS). The emission intensity and lifetime are quenched in acid solution and in protic solvents, increasing in the order H2O < ethanol (EtOH) < dimethylsulphoxide) DMSO) < CH3CN. The pKa values for the first two protonations of the HAT ligand in Ru(bpy)2(HAT)2+ (pKa1 = − 2.5, pKa2 = −4.3) have been determined. The spectroscopic and emission lifetime properties of the complex in Nafion membranes depend markedly on the swelling solvent and membrane hydration. The acidity of the H+-form membrane at different degrees of hydration was estimated by comparison with the absorption behaviour in acid solution. In the presence of DNA in aqueous solutions, the absorption of the complex is markedly altered, resembling that in low temperature glasses, and a 400% increase in the emission intensity and lifetime is observed. Spectroscopic changes are less marked with single-stranded than with double-stranded polynucleotides and very sensitive to the presence of salt cations. They increase in the order poly(dG).poly(dC) < poly[d(GC)] < poly(dA).poly(dT) < calf thymus DNA (CT-DNA) < poly[d(AT)] (dG, deoxyguanylic acid; dC, deoxycytidylic acid; dA, deoxyadenylic acid; dT, deoxythymidylic acid). Results are consistent with the partial intercalation of the complex into DNA, with a preference for AT stretches.