Photophysical properties of Ru-cyano-poly(pyridine) complexes. Acidity and temperature tuning of luminescence properties

Abstract

The emission spectra and lifetimes of Ru(biq)2(CN)2, Ru(DMCH)2(CN)2 and Ru(i-biq)2(CN)2(biq = 2,2′-biquinoline, DMCH = 6,7-dihydro-5,8-dimethyldibenzo[3,2-b : 2′,3′-j][1,10]phenanthroline, i-biq = 2,2′-bi-iso-quinoline) have been studied in several methanol–concentrated H2SO4 solvent mixtures at 77 K and in nitrile mixtures from 84 to 310 K. For Ru(biq)2(CN)2 and Ru(DMCH)2(CN)2, where the lowest metal-to-ligand charge-transfer (MLCT) level lies at much lower energy than the lowest ligand-centred (LC) level, protonation reverses the energy ordering of the levels and leads to an LC emission. For Ru(i-biq)2(CN)2 the MLCT levels lie at higher energies and the emission is substantially LC in nature in methanol, but clearly becomes ‘purer’ LC on addition of H2SO4, as shown by the strong increase in the emission lifetime. The temperature dependence (in nitrile solution) of emission spectra and emission lifetimes shows that for Ru(biq)2(CN)2 and Ru(i-biq)2(CN)2 relaxation of the solvent cage, made possible by melting of the rigid matrix, stabilizes the MLCT level. In the biq complex this is the emitting level in a rigid matrix, whereas for the i-biq complex the stabilization of the MLCT level causes a change in the energy ordering and thus in the nature of the emitting state, which is essentially LC in a rigid matrix and MLCT in fluid solution. Detailed experimental results, including pH-dependent, temperature-dependent and time-dependent emission spectra are illustrated and discussed.