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Abstract: The emission spectral band shapes of several polypyridine-ligand (PP) bridged bis-ruthenium(II) complexes imply that Ru(II)/Ru(III) electronic coupling is weaker in their lowest energy metal to ligand charge transfer (MLCT) excited states than in their corresponding mixed valence ground states. In general, the amplitudes of the vibronic contributions to emission band shapes decrease markedly with the excited state-ground state energy differences, and it is expected that complexes with degenerate, or mixed valence excited states will have very weak vibronic side bands if configurational mixing of the degenerate MLCT excited states is substantial. However, the bimetallic PP-bridged ruthenium complexes emit at significantly lower energy than their monometallic analogs, but the vibronic contributions to their 77K emission spectra are very similar to those of their monometallic complexes analogs. This indicates that the mixed valence excited states of the bimetallic complexes are electronically localized. [Copyright &y& Elsevier]

Abstract: The low energy sidebands of an emission spectrum contain information about the difference between the ground and the excited state molecular structures. The structural information that can be extracted from the sideband intensities and structure decreases as the component bandwidths increase, but there is significant structural information in the sidebands of the charge transfer emission spectra of even the relatively broad (Δν 1/2 ∼1000cm−1) charge transfer (CT) emission spectra of transition metal complexes in frozen solutions. A Gaussian band shape model for the contributions of vibronic components is described and applied to the analysis of transition metal CT emission band shapes in frozen solution. The uncertainties of this approach are examined with respect to the emission spectra calculated from reported resonance-Raman (rR) parameters; such calculated spectra reproduce the frozen solution emission spectra of [Ru(bpy)3]2+ and [Ru(NH3)4bpy]2+ very well. Patterns of excited state distortions can be expressed in terms of variations in the vibrational reorganizational energies, λ k (proportional to the squared displacements), of the normal modes that correlate with the differences in excited and ground state geometries. The excited state distortions usually correspond to displacements in a large number of ground state molecular vibrational modes (more than 11 for the Ru–bpy complexes), and the bandwidths characteristic of CT spectra in frozen solutions preclude their resolution in frozen solution spectra. Thus, the convolution of the overlapping spectral contributions of these individual distortion modes results in a vibronic sideband that is broad and sometimes weakly structured, and the variations of the sideband amplitude provides information about variations in molecular structure for a series of closely related complexes. The emission sidebands can readily be converted into a reorganizational energy profile (emrep) in which the variations in amplitude are more readily interpreted in terms of molecular distortions and in which the contributions from the distortions in the highest frequency vibrational modes are more evident. This approach has been used to analyze the patterns of variations of the vibronic sideband structure of the frozen solution CT emission spectra of [Ru(L)4bpy]2+, [Ru(bpy)2PP]2+, [{(bpy)2Ru}2PP]4+ (bpy, 2,2′-bipyridine; PP, a tetraazapolypyridyl bridging ligand) and cyanide-bridged Cr(III)/Ru(II) complexes. The observed emission energies of the bpy complexes span a range of about 8000cm−1 and the vibronic sideband amplitudes tend to decrease appreciably (over about a two-fold range for the [Ru(L)4bpy]2+ complexes) and systematically with decreasing emission energy in each class of these complexes. This attenuation of the vibronic sideband intensities is ascribed to the increases in ground state–excited state configurational mixing with decreasing energy differences between the states and to the large electronic matrix elements. The variations in emreps also suggest that there is a great deal of excited state–excited state configurational mixing (probably ligand field/metal to ligand CT) in most, but not all, [Ru(L)4bpy]2+ complexes and relatively little such configurational mixing between the different MLCT valence isomer excited states of the [{(bpy)2Ru}2PP]4+ complexes. The comparison of the emreps of NH/ND isotopomers of the [Ru(L)4bpy]2+ and of the cyanide-bridged Cr(III)/Ru(II) complexes has permitted the resolution of the very weak contributions of distortions of the C–H (in the former) and N–H (in the latter) stretching vibrational modes, and it appears that decay pathways that involve only nuclear tunneling in the C–H modes cannot account for the decay behavior of the [Ru(L)4bpy]2+ complexes while such pathways in the N–H modes may account for that of the Cr(III)/Ru(II) complexes. [Copyright &y& Elsevier]

Abstract: The emission spectra of several classes of transition metal complexes are examined for relationships between strong donor/acceptor (D/A) electronic coupling (expressed by the matrix element, HRP) and the Franck-Condon (FC) parameters in electron-transfer systems. The effects of large values of HRP on the FC parameters are expressed in terms of the configurational mixing of the diabatic electron-transfer states are interpreted in terms of the fraction of electron density that is delocalized (), and they are evaluated with respect to the limit that . Ion pair charge transfer spectra and outer-sphere electron-transfer systems are used to define this limit. One effect of values of is the dramatic attenuation of electron-transfer reorganizational energies, and this effect is examined with respect to the variations in the vibronic contributions of the bpy ligand to the 77K emission spectra of am(m)ine-polypyridine ruthenium(II) complexes. These contributions of high frequency vibrational modes are manifested in the carefully calibrated near infrared emission spectra as changes in the relative intensities of the vibronic sidebands or in the band shape on the low energy side of the spectrum. These vibronic contributions are most clearly exhibited in empirical reorganizational energy profiles (emreps) based on Λx = hνvib(ΔIobsd/Imax(f)), where ΔIobsd is the intensity difference between the observed emission spectrum and the fundamental component. The emreps provide a useful approach for examining the experimental spectral data for contributions of high frequency vibrational modes to the excited state distortion. The respective emreps clearly display the attenuation of the bpy-centered vibronic contributions with the fraction of charge delocalized. The implications for the electron-transfer coordinate in these strongly coupled D/A complexes are considered. Strong electronic coupling between the donor and the acceptor (or between the initial and the product of electron-transfer states) to the ligand that links them can dramatically change the electron-transfer properties of the D/A complex. Work of the Taube group illustrates the variations in HRP that are induced by the bridging ligand. Other effects of strong coupling with the bridging ligand are: (1) reductions of the energy of the D/A electron-transfer absorption band that are roughly proportional to HRP; (2) bridging ligand distortions that alter HRP and also result in contributions of the bridging ligand vibrational modes to the electron-transfer reorganizational energy. The first of these effects is a characteristic of superexchange coupling and the substituted-dipyridyl ligand-bridged Ru(NH3)52+/Ru(NH3)53+ complexes (studied by Sutton and Taube) are considered as models. Cyanide-bridged metal complexes are models for the second effect. The entanglement of the nuclear coordinates of the bridging cyanide with HRP (i.e., vibronic coupling, with ; where QCN is the CN bond length and b is a constant) results in anti-kinematic shifts of the ground state CN stretching frequencies of M(CN)Ru(NH3)5 complexes and in a failure of superexchange coupling in a series of [{Ru(NH3)5}2M(MCL)(CN)2]6+ complexes (MCL = a tetraazamacrocyclic ligand). The electron-transfer emissions of these complexes, displayed as emreps indicate that the CN stretch contributes more than 100cm-1 to the overall electron-transfer reorganizational energy in CN-bridged complexes, and implicate a dominant NH stretching mode-mediated nuclear tunneling pathway for back electron-transfer in these complexes. Excited state electron-transfer relaxation by means of NH mediated nuclear tunneling pathways appear to dominate for {CrII(CN)RuIII} → {CrIII(CN)RuII} at 77K, but a more efficient relaxation pathway appears to be important for the am(m)ine-polypyridyl RuII complexes. [Copyright &y& Elsevier]

The lowest energy metal to ligand charge transfer (MLCT) absorption bands found in ambient solutions of [Ru(NH3)4(Y-py)2+ and [Ru(L)2(bpy)2]+ complexes (Y-py a pyridine ligand and (L)n a substituted acetonyl-acetonate, halide, am(m)ine, etc.) consist of two partly resolved absorption envelopes, MLCTlo and MLCThi. The lower energy absorption envelope, MLCTlo, 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 antisymmettic 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 MLCTlo in this class of complexes. The LUMO of each bis-bpy complex that contains a C2 symmetry axis also corresponds largely to the antisymmetric combination of individual ligand acceptor orbitals, while the LUMOs are more complex when there is no C2 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 bts-bpy complexes in ambient solutions. Relatively weak MLCTlo 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)4bpy]m+ complexes indicates that it is too weak lobe 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 (ΔE1/2) have much smaller slopes for the bis-bpy than the mono-bpy complexes. However, the observed MLCTlo and the calculated HOMO→LUMO transitions of bis-bpy complexes correlate very similarly with ΔE1/2 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 slate chemistry, not the absorption maximum as is often assumed. [ABSTRACT FROM AUTHOR]