Insight into the Reductive Quenching of a Heteroleptic Cu(I) Photosensitizer for Photocatalytic H2 Production

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© 2019 American Chemical Society. Heteroleptic Cu(I) photosensitizers undergo photoinduced electron transfer through either an oxidative or reductive quenching mechanism. We have previously shown that visible light excitation of [CuI(Xantphos)(biq)]PF6 (Xantphos = 4,5-bis(diphenylphosphino)-9,9-dimethylxanthene; biq = 2,2′-biquinoline) and N,N-dimethylaniline (DMA) electron donor generates a reduced photosensitizer and oxidized donor via excited-state reductive quenching. Here we expand the series of DMA-based donors using (1) methyl substituents (-CH3) located around the aryl ring and (2) electron donating (e.g., -OCH3, -CH3) or withdrawing (-Br, -C(O)OEt) substituents at the para position to establish the thermodynamic threshold for reductive quenching of [CuI(Xantphos)(biq)]+. Using a combination of electrochemical and spectroscopic methods, we have determined the necessary oxidation potentials for electron donors (labeled E(ED+/0)) to participate in excited-state reductive quenching to afford [CuI(Xantphos)(biq-)]0. Subsequent ground-state electron transfers to cis-[RhIII(Me2bpy)2Cl2]PF6 catalyst (Me2bpy = 4,4′-dimethyl-2,2′-bipyridine) result in catalyst activation (Rh(I) formation) and H2 evolution. Importantly, stability of the one-electron oxidized donor (ED•+) impacts the competing forward and back electron transfer reactions, where 4-methoxy-N,N-dimethylaniline (MeO-DMA) is the strongest reducing agent used yet the chemical stability of MeO-DMA•+ hinders electron accumulation on the Rh-polypyridyl catalyst. Our findings show the combination of thermodynamics and chemical stability of electron transfer products is an important consideration for not only the Cu(I) photosensitizer and Rh catalyst but also the electron source.