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

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 [Cu^I(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 [Cu^I(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 [Cu^I(Xantphos)­(biq^–)]⁰. Subsequent ground-state electron transfers to cis-[Rh^III(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.