Influence of solvent on the spectroscopic properties of cyano complexes of ruthenium(II)

UV-visible spectra, emission spectra, and RU(III/II) reduction potentials have been measured for cis[Ru(bpy)(2)(py)(CN)](+) (bpy is 2,2'-bipyridine; py is pyridine), cis-Ru(bpy)(2)(CN)(2), [Ru(tpy)(CN)(3)](-) (tpy is 2,2': 6',2''-terpyridine), [Ru(bpy)(CN)(4)](2-), and [Ru(MQ(+))(CN)(5)](2-) (MQ(+) is N-methyl-4,4'-bipyridinium cation) in twelve solvents. The shifts in the metal-to-ligand charge transfer (MLCT) absorption (E(abs)) or emission (E(em)) band energies with solvent increase linearly with the number of cyano ligands and correlate well with the Gutmann ''acceptor number'' of the solvent. Intraligand pi --> pi* band energies also correlate with acceptor number, but with only similar to 30% of the shifts for the MLCT bands, The solvent dependence arises through mixing of the pi --> pi* transitions with lower energy MLCT transitions. MLCT absorption and emission spectra are convolutions of overlapping vibronic components, and a Franck-Condon analysis of emission spectral profiles for cis-Ru(bpy)(2)(CN)(2)* has been used to evaluate the energy gap, E(0), and chi'(0.gs), where chi'(0.gs) is the sum of the solvent reorganizational energy for the ground state below the excited state and the inner-sphere reorganizational energy of the low-frequency modes, chi(i,L), is treated classically. Both E(0) and chi'(0.gs) correlate well with acceptor number with Delta E(0)/Delta AN = 44 +/- 2 cm(-1)/AN unit and Delta chi(0.gs)/AN 21 +/- 3 cm(-1)/AN unit if it is assumed that chi(i,L) is solvent independent. From electrochemical measurements and the difference in E(1/2) values for metal oxidation and bpy reduction, Delta Delta G(es)(0)/Delta AN similar or equal to 70 +/- 7 cm(-1)/AN unit with Delta G(es)(0) the free energy of the excited state above the ground state, These correlations show that the energy gap is far more sensitive to solvent than chi(0,gs). Delta chi(0,gs)/Delta AN can also be estimated from the relation Delta Delta G(es)(0)/Delta AN = Delta E(0)/AN + Delta chi'(0,gs)/Delta AN, which gives Delta chi(0,gs)/Delta AN = 26 +/- 7 cm(-1)/AN unit. The solvent reorganizational energy of the excited state above the ground state is chi(0,es). Its variation with acceptor number can be estimated from the relation Delta E(abs)/Delta AN - Delta G(es)(0)/Delta AN = Delta chi(0,es) (=13 +/- 8 cm(-1)/AN) or from Delta E(abs)/Delta AN - Delta E(em)/Delta AN - Delta chi(0,gs)/Delta AN (=14 +/- 8 cm(-1)/AN), if chi(i,L) is solvent independent. These results suggest that chi(0,gs) is more sensitive to solvent than chi(0,es) by as much as a factor of 2. Delta E(em)/Delta AN = 45 +/- 3 cm(-1)/AN unit similar or equal to Delta E(0)/Delta AN = 44 +/- 2 cm(-1)/AN unit, showing that the emission maximum gives accurate information about the solvent dependence of the energy gap. A model is invoked to explain the acceptor number dependence. It is based on electron pair donation from the lone pair on cyanide to individual solvent molecules through donor-acceptor interactions. The model is consistent with variations in v(CN) with acceptor number in cis-[Ru(bpy)(2)(py)(CN)](+) and in E(1/2)(Ru-III/II) for the series of complexes. In this model it is assumed that donor-acceptor interactions are more important in the ground state than in the excited state, consistent with pK(a) measurements, and that they are additive in the number of cyanide ligands. These and I-I-bonding interactions in related ammine complexes perturb the internal electronic structure of the solute with important consequences. One is that chi(0,gs) not equal chi(0,es) although they are commonly assumed to be equal.