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 (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 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 E 1/2(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 pK a measurements, and that they are additive in the number of cyanide ligands. These and H-bonding interactions in related ammine c...

An improved synthesis, crystal structures, and metallochromism of salts of [Ru(tolyl-terpy)(CN)3]−

“…The complexes show very similar UV/Vis spectroscopic behaviour, with a manifold of closely-spaced 1 MLCT absorptions peaking in the 500 -600 nm region in non hydrogen-bonding solvents, which blue shift in protic solvents up to a limit of 427 nm in water. This is similar in principle to the solvatochromic behaviour of [Ru(bipy)(CN) 4 ] 2- [4] but the solvatochromic shift in the tpy-based complexes is smaller because of the presence of only three cyanide groups rather than four.…”

“…1a), with the major deviation from ideality arising from the limited bite angle (Fig. 1b) Table 1 [4,5]. The complexes show very similar UV/Vis spectroscopic behaviour, with a manifold of closely-spaced 1 MLCT absorptions peaking in the 500 -600 nm region in non hydrogen-bonding solvents, which blue shift in protic solvents up to a limit of 427 nm in water.…”

“…The ligand ttpy [9] and the metal complex Ru(dmso) 4 Cl 2 [10] were prepared according to published procedures. The following instruments were used for spectroscopic characterisation and analysis: UV/Vis spectra, a Cary-50 spectrophotometer; luminescence spectra, a Jobin-Yvon Fluoromax 4 fluorimeter; Electrospray mass spectra, a Micromass LCT instrument; NMR spectra, a Bruker Avance-III 400 instrument.…”

“…We [1,2] and others [1,3,4] have been investigating the chemistry of luminescent cyanometallates of the [Ru(bipy)(CN) 4 ] 2-type. These are very interesting molecular components to use in supramolecular assemblies for a variety of reasons, including (i) the ability of the externally-directed cyanide groups to form supramolecular assemblies by coordination to other metal cations, or by hydrogen-bonding or halogen-bonding; (ii) the remarkable sensitivity of the spectroscopic and photophysical properties of the metal complex to its environment (solvatochromism and 'metallochromism') which allow applications such as switching and sensing; and (iii) the availability of the CN groups to act as spectroscopic handles in time-resolved IR spectroscopy which allows the excited state dynamic behaviour of complexes containing these fragments to be measured on the picosecond timescale [1].…”

Synthesis, Characterization, Spectroscopic, and Electrochemiluminescence Properties of a Solvatochromic Azacrown‐Containing Cyanoruthenate(II): Potential Applications in Separation and Indirect Photometric Detection of Cations and Amino Acids in HPLC

“…[16,17] The newly synthesized complex gave satisfactory elemental analyses and was characterized by UV/Vis specA C H T U N G T R E N N U N G tros-A C H T U N G T R E N N U N G copy, negative ESI-MS, IR and 1 H NMR spectroscopy. The IR spectrum of the newly synthesized complex shows three nA C H T U N G T R E N N U N G (C N) stretches in the region of 2030-2090 cm…”