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

• Published in: Journal of physical chemistry (1952) 1996-02, Vol.100 (8), p.2915-2925
• Main Authors: Timpson, Cliff J, Bignozzi, Carlo A, Sullivan, B. Patrick, Kober, Edward M, Meyer, Thomas J
• Format: Article
• Language: English
• ISSN: 0022-3654; 1541-5740
• DOI: 10.1021/jp953179m

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 (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 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, E 0, 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 E 0 and χ‘0,gs correlate well with acceptor number with ΔE 0/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 E 1/2 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 = ΔE 0/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 ΔE abs/ΔAN − ΔG°es/ΔAN = Δχ0,es (=13 ± 8 cm-1/AN) or from ΔE abs/ΔAN − ΔE em/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. ΔE em/ΔAN = 45 ± 3 cm-1/AN unit ≃ ΔE 0/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 depen