Influence of Solvent on the Spectroscopic Properties of Cyano Complexes of Ruthenium(II)

Abstract

UV−visible spectra, emission spectra, and RuIII/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 (Eabs) or emission (Eem) band energies with solvent increase linearly with the number of cyano ligands and correlate well with the Gutmann "acceptor number" of the solvent. Intraligand π → π* band energies also correlate with acceptor number, but with only ∼30% of the shifts for the MLCT bands. The solvent dependence arises through mixing of the π → π* 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, E0, and χ'0,gs, where χ'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, χi,L, is treated classically. Both E0 and χ'0,gs correlate well with acceptor number with ΔE0/ΔAN = 44 ± 2 cm-1/AN unit and Δχ0,gs/ΔAN = 21 ± 3 cm-1/AN unit if it is assumed that χi,L is solvent independent. From electrochemical measurements and the difference in E1/2 values for metal oxidation and bpy reduction, ΔΔG°es/ΔAN ≃ 70 ± 7 cm-1/AN unit with ΔG°es 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 χ0,gs. Δχ0,gs/ΔAN can also be estimated from the relation ΔΔG°es/ΔAN = ΔE0/ΔAN + Δχ'0,gs/ΔAN, which gives Δχ0,gs/ΔAN = 26 ± 7 cm-1/AN unit. The solvent reorganizational energy of the excited state above the ground state is χ0,es. Its variation with acceptor number can be estimated from the relation ΔEabs/ΔAN − ΔG°es/ΔAN = Δχ0,es (=13 ± 8 cm-1/AN) or from ΔEabs/ΔAN − ΔEem/ΔAN − Δχ0,gs/ΔAN (=14 ± 8 cm-1/AN), if χi,L is solvent independent. These results suggest that χ0,gs is more sensitive to solvent than χ0,es by as much as a factor of 2. ΔEem/ΔAN = 45 ± 3 cm-1/AN unit ≃ ΔE0/ΔAN = 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 ν(CN) with acceptor number in cis-[Ru(bpy)2(py)(CN)]+ and in E1/2(RuIII/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 pKa measurements, and that they are additive in the number of cyanide ligands. These and H-bonding interactions in related ammine complexes perturb the internal electronic structure of the solute with important consequences. One is that χ0,gs ≠ χ0,es although they are commonly assumed to be equal.