Photoacid Behavior versus Proton-Coupled Electron Transfer in Phenol–Ru(bpy)₃²⁺ Dyads

Abstract

Two dyads composed of a Ru­(bpy)₃²⁺ (bpy = 2,2′-bipyridine) photosensitizer and a covalently attached phenol were synthesized and investigated. In the shorter dyad (Ru–PhOH) the ruthenium complex and the phenol are attached directly to each other whereas in the longer dyad there is a <i>p</i>-xylene (xy) spacer in between (Ru–xy–PhOH). Electrochemical investigations indicate that intramolecular electron transfer (ET) from phenol to the photoexcited metal complex is endergonic by more than 0.3 eV in both dyads, explaining the absence of any ³MLCT (metal-to-ligand charge transfer) excited-state quenching by the phenols in pure CH₃CN and CH₂Cl₂. When pyridine is added to a CH₂Cl₂ solution, significant excited-state quenching can be observed for both dyads, but the bimolecular quenching rate constants differ by 2 orders of magnitude between Ru–PhOH and Ru–xy–PhOH. Transient absorption spectroscopy shows that in the presence of pyridine both dyads react to photoproducts containing Ru­(II) and phenolate. The activation energies associated with the photoreactions in the two dyads differ by 1 order of magnitude, and this might suggest that the formation of identical photoproducts proceeds through fundamentally different reaction pathways in Ru–PhOH and Ru–xy–PhOH. For Ru–PhOH direct proton release from the photoexcited dyad is a plausible reaction pathway. For Ru–xy–PhOH a sequence of a photoinduced proton-coupled electron transfer (PCET) followed by an intramolecular (thermal) electron transfer in the reverse direction is a plausible reaction pathway; this two-step process involves a reaction intermediate containing Ru­(I) and phenoxyl radical that reacts very rapidly to Ru­(II) and phenolate. Thermal back-reactions to restore the initial starting materials occur on a 30–50 μs time scale in both dyads; i.e., due to proton release the photoproducts are very long-lived. These back-reactions exhibit inverse H/D kinetic isotope effects of 0.7 ± 0.1 (Ru–PhOH) and 0.6 ± 0.1 (Ru–xy–PhOH) at room temperature