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

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journal contribution
posted on 07.10.2017, 00:00 by Ludovic Troian-Gautier, Epiphanie Mugeniwabagara, Luca Fusaro, Emilie Cauët, Andrée Kirsch-De Mesmaeker, Michel Luhmer
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.