Effects of Electronic Mixing in Ruthenium(II) Complexes with Two Equivalent Acceptor Ligands. Spectroscopic, Electrochemical, and Computational Studies
Allard, Marco M
Lee, Mandy M
Endicott, John F
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The lowest energy metal to ligand charge transfer (MLCT) absorption bands found in ambient solutions of [Ru(NH3)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, 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 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 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 bis-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 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 (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 state chemistry, not the absorption maximum as is often assumed.