Effects of Electronic Mixing in Ruthenium(II) Complexes with Two Equivalent Acceptor Ligands. Spectroscopic, Electrochemical, and Computational Studies

Abstract

The lowest energy metal to ligand charge transfer (MLCT) absorption bands found in ambient solutions of [Ru(NH(3))(4)(Y-py)(2)](2+) and [Ru(L)(2)(bpy)(2)](+) complexes (Y-py a pyridine ligand and (L)(n) a substituted acetonylacetonate, halide, am(m)ine, etc.) consist of two partly resolved absorption envelopes, MLCT(lo) and MLCT(hi). The lower energy absorption envelope, MLCT(lo), in these spectra has the larger amplitude for the bis-(Y-py) complexes, but the smaller amplitude for the bis-bpy the complexes. Time-dependent density functional theory (TD-DFT) approaches have been used to model 14 bis-bpy, three bis-(Y-py), and three mono-bpy complexes. The modeling indicates that the lowest unoccupied molecular orbital (LUMO) of each bis-(Y-py) complex corresponds to the antisymmetric combination of individual Y-py acceptor orbitals and that the transition involving the highest occupied molecular orbital (HOMO) and LUMO (HOMO-->LUMO) is the dominant contribution to MLCT(lo) in this class of complexes. The LUMO of each bis-bpy complex that contains a C(2) symmetry axis also corresponds largely to the antisymmetric combination of individual ligand acceptor orbitals, while the LUMOs are more complex when there is no C(2) axis; furthermore, the energy difference between the HOMO-->LUMO and HOMO-->LUMO+1 transitions is too small (<1000 cm(-1)) to resolve in the spectra of the bis-bpy complexes in ambient solutions. Relatively weak MLCT(lo) absorption contributions are found for all of the [Ru(L)(2)(bpy)(2)](m+) complexes examined, but they are experimentally best defined in the spectra of the (L)(2) = X-acac complexes. TD-DFT modeling of the HOMO-->LUMO transition of [Ru(L)(4)bpy](m+) complexes indicates that it is too weak to be detected and occurs at significantly lower energy (about 3000-5000 cm(-1)) than the observed MLCT absorptions. Since the chemical properties of MLCT excited states are generally correlated with the HOMO and/or LUMO properties of the complexes, such very weak HOMO-->LUMO transitions can complicate the use of spectroscopic information in their assessment. As an example, it is observed that the correlation lines between the absorption energy maxima and the differences in ground state oxidation and reduction potentials (DeltaE(1/2)) have much smaller slopes for the bis-bpy than the mono-bpy complexes. However, the observed MLCT(lo) and the calculated HOMO-->LUMO transitions of bis-bpy complexes correlate very similarly with DeltaE(1/2) and this indicates that it is the low energy and small amplitude component of the lowest energy MLCT absorption band that is most appropriately correlated with excited state chemistry, not the absorption maximum as is often assumed.

Metal-to-Ligand Charge-Transfer Emissions of Ruthenium(II) Pentaammine Complexes with Monodentate Aromatic Acceptor Ligands and Distortion Patterns of their Lowest Energy Triplet Excited States

“…The difference between the 300 K absorption and 77 K emission maxima of pentaammine− and tetraammine−Ru complexes (9.9 ± 0.5 and 7.6 ± 0.8 cm −1 /10 3 , respectively) is significantly larger than those found for the Ru−bpy chromophores (about 5−6 cm −1 / 10 3 ) 36 and about 30% larger for the pentaammines than for the other Ru−MDA complexes.…”

“…Literature syntheses were used for the following compounds: [Ru(NH 3 ) 5 (L)] 2+ complexes with L = py, ac-py, ph-py, 4,4′-bipyridine (4,4′-bpy), and pyrazine (pz) and 17,33−35 cis-/trans-[Ru(NH 3 ) 4 (L) 2 ](PF 6 ) 2 with L = pz, py, ac-py, and ph-py ( Figure 1). 36 Table 1. prepared by dissolving the corresponding [Ru(NH 3 ) 5 (MDA)](PF 6 ) 2 complex in D 2 O and then precipitating it with the addition of saturated NaPF 6 /D 2 O solution. This procedure was repeated several times as described previously.…”

“…The slope of the correlation in Figure 3 is about 0.8 ± 0.2 rather than 1.0, but this is probably a consequence of the scatter of data, the relatively limited energy range of the observations, and the DFT-based inference that the orbital occupations of the Franck−Condon excited states reached by absorption do not always correlate with those of the lowest energy 3 MLCT excited state. 19,36 The [Ru(NH 3 ) 5 (MDA)] 2+ complexes emit at lower energies, but 20 We have now examined the relaxation properties in Ru−MDA complexes that span an energy range of about 0.5 eV. The emission yields of the Ru− MDA chromophores tend to be much smaller than those of the simple monobpy complexes over this energy range, 19 and a more extensive report and discussion of the implications of this will be presented elsewhere.…”

Energy Dependence of the Ruthenium(II)-Bipyridine Metal-to-Ligand-Charge-Transfer Excited State Radiative Lifetimes: Effects of ππ*(bipyridine) Mixing

“…[13][14][15][16][17][18] However, our recent work combining spectroscopic studies with density functional theory (DFT) modeling has found several excited state properties of ruthenium-(D)/polypyridyl ligand-(A) complex excited states that differ from expectation based on simple limiting models. [19][20][21][22][23][24] Thus, when the donor and 3 acceptor are covalently linked, and since the energy differences between excited states are often not large in heavy metal complexes, theoretical models based on such weak coupling limits can be misleading. 25 In the present paper we examine the 77 K radiative properties (spectra, quantum yields and lifetimes) of simple Ru-bpy complexes in order to gain more critical insight into the effects of electronic mixing on metal to ligand charge transfer (MLCT) excited state properties.…”

Effect of pH in the photoluminescence of a ruthenium complex featuring a derivative of the ligand pyrazine[2,3-f][1,10]-phenanthroline

“…The maximum MLCT wavelength in polypyridinic ruthenium compounds can be roughly estimated by the energy related to the oxidation of the metal and the first reduction of the ligands [21]. According to the electrochemical data the maximum MLCT wavelength for this complex should be observed at ca.…”