Photo-CIDNP Reveals Different Protonation Sites Depending on the Primary Step of the Photoinduced Electron-/Proton-Transfer Process with Ru(II) Polyazaaromatic Complexes.

The excited-state quenching of [Ru(TAP)2(HAT)]2+ (TAP = 1,4,5,8-tetraazaphenanthrene, HAT= 1,4,5,8,9,12-hexaazatriphenylene) by hydroquinone (H2Q), N-acetyl-tyrosine (N-Ac-Tyr) or guanosine-5'-monophosphate (GMP) was investigated at various pH values. The quenching occurs via electron/proton transfer, as evidenced by transient absorption spectroscopy and confirmed by 1H photochemically induced dynamic nuclear polarization (photo-CIDNP). Reductive quenching also occurs in strongly acidic solution despite a much shorter lifetime of the protonated excited-state complex. Photo-CIDNP revealed a different mechanism at low pH, involving protonation before electron transfer and yielding a distinct protonated monoreduced complex. The experimental photo-CIDNP patterns are consistent with density functional theory calculations. This work highlights the power of 1H photo-CIDNP for characterizing, at the atomic level, transient species involved in electron-transfer processes.

pH Dependence of Photoinduced Electron Transfer with [Ru(TAP)3]2.

The quenching of the excited state of [Ru(TAP)3]2+ (TAP = 1,4,5,8-tetraazaphenanthrene) by guanosine-5'-monophosphate (GMP), N-acetyltyrosine (N-Ac-Tyr), and hydroquinone (H2Q) has been studied in aqueous solution over a wide range of pH values including, for the first time, strongly acidic media. This quenching by electron transfer was examined by steady-state 1H photochemically induced dynamic nuclear polarization (photo-CIDNP) as well as by more conventional techniques, among which are pulsed laser-induced transient absorption and emission experiments. A deeper knowledge of the photochemical behavior of [Ru(TAP)3]2+ has been gained thanks to the combined use of these two approaches, photo-CIDNP and electronic spectroscopies, highlighting their complementarity. In contrast to what was believed, it is found that the protonated excited state of [Ru(TAP)3]2+ may give rise to an electron transfer with N-Ac-Tyr and H2Q. Such a photoinduced electron transfer does not occur with protonated GMP, however. 1H photo-CIDNP experiments are expected to be particularly promising for characterization of the reductive quenching of excited-state ruthenium(II) polypyridyl complexes comprising several nonequivalent protonation sites.

Full assignment of the 1H NMR spectrum of heteroleptic polyazaaromatic Ru(II) complexes based on long-range interligand NOEs.

Chemical equivalence in symmetrical polyazaaromatic ligands is not necessarily retained in the corresponding heteroleptic ruthenium(II) complexes, and the assignment of the (1)H NMR spectrum of these octahedral metallic compounds is therefore not straightforward. This work shows that full assignment can be achieved on the basis of long-range interligand NOE measurements.

Density functional theory interpretation of the 1H photo-chemically induced dynamic nuclear polarization enhancements characterizing photoreduced polyazaaromatic Ru(II) coordination complexes.

The unprotonated and protonated monoreduced forms of the polyazaaromatic Ru(II) coordination complexes [Ru(tap)(3)](2+) and [Ru(tap)(2)(phen)](2+) (tap = 1,4,5,8-tetraazaphenanthrene ; phen = 1,10-phenanthroline), that is, [Ru(tap)(3)](*+), [Ru(tap)(2)(phen)](*+), [Ru(tap)(2)(tap-H)](*2+), and [Ru(tap)(tap-H)(phen)](*2+), were studied by Density Functional Theory (DFT). The electron spin density of these radical cations, the isotropic Fermi-contact, and the anisotropic dipolar contributions to the hyperfine coupling constants of the H nuclei were calculated in vacuo and using a continuum model for water solvation. For [Ru(tap)(2)(phen)](*+), as well as for its protonated form, the DFT results show that the unpaired electron is not localized on the phen ligand. For both [Ru(tap)(3)](*+) and [Ru(tap)(2)(phen)](*+), they reveal high electron spin density in the vicinity of tap H-2 and tap H-7 (the H atoms in the ortho position of the tap non-chelating N atoms). These results are in full agreement with recent steady-state (1)H photo-Chemically Induced Dynamic Nuclear Polarization (photo-CIDNP) measurements. The DFT calculations performed for the protonated species also predict major (1)H photo-CIDNP enhancements at these positions. Interestingly, they indicate significantly different polarization for tap H-9,10, suggesting that the occurrence of a photoinduced electron transfer with protonation of the reduced species might be detected by high-precision photo-CIDNP experiments.

Exploring photoreactions between polyazaaromatic Ru(II) complexes and biomolecules by chemically induced dynamic nuclear polarization measurements.

Steady-state (1)H photo-chemically induced dynamic nuclear polarization (CIDNP) experiments were conducted at 14.1 T on deoxygenated (buffered pH 7) aqueous solutions of [Ru(phen)(3)](2+), [Ru(tap)(2)(phen)](2+), and [Ru(tap)(3)](2+) (tap = 1,4,5,8-tetraazaphenanthrene; phen = 1,10-phenanthroline) in the presence of guanosine-5'-monophosphate or N-acetyltyrosine. For the first time, CIDNP arising from photo-oxidation by polyazaaromatic Ru(II) complexes is reported. In agreement with the occurrence of a photo-electron-transfer process, photo-CIDNP effects are observed with [Ru(tap)(3)](2+) and [Ru(tap)(2)(phen)](2+) but not with [Ru(phen)(3)](2+). With [Ru(tap)(2)(phen)](2+), no significant photo-CIDNP is observed for the (1)H nuclei of the phen ligand, consistent with the fact that the metal-to-ligand charge-transfer triplet excited states responsible for the photo-oxidation involve a tap ligand. Successive experiments with [Ru(tap)(3)](2+) highlight the accumulation of long-lived radical species in solution that cause (1)H NMR signal broadening and photo-CIDNP extinction. The (1)H photo-CIDNP observed for the biomolecules is rather weak, less than about 30% of the equilibrium magnetization. However, up to 60% polarization enhancement is observed for H-2 and H-7 of the tap ligands, which indicates high unpaired electron density in the vicinity of these atoms in the transient radical pair. This is consistent with the structure of known photoadducts formed, for instance, between the metallic compounds and the guanine base of mono- and polynucleotides. Indeed, in these adducts the covalent bond involves carbon C-2 or C-7 of a tap ligand. The occurrence of photo-CIDNP with polyazaaromatic Ru(II) complexes opens new perspectives for the study of this type of compound.