Visible-Light-Driven Photosystems Using Heteroleptic Cu(I) Photosensitizers and Rh(III) Catalysts To Produce H₂

The synthesis of two new heteroleptic Cu­(I) photosensitizers (PS), [Cu­(Xantphos)­(NN)]­PF₆ (NN = biq = 2,2′-biquinoline, dmebiq = 2,2′-biquinoline-4,4′-dimethyl ester; Xantphos = 4,5-bis­(diphenylphosphino)-9,9-dimethylxanthene), along with the associated structural, photophysical, and electrochemical properties, are described. The biquinoline diimine ligand extends the PS light absorbing properties into the visible with a maximum absorption at 455 and 505 nm for NN = biq and dmebiq, respectively, in CH₂Cl₂ solvent. Following photoexcitation, both Cu­(I) PS are emissive at low energy, albeit displaying stark differences in their excited state lifetimes (τ_MLCT = 410 ± 5 (biq) and 44 ± 4 ns (dmebiq)). Cyclic voltammetry indicates a Cu-based HOMO and NN-based LUMO for both complexes, whereby the methyl ester substituents stabilize the LUMO within [Cu­(Xantphos)­(dmebiq)]⁺ by ∼0.37 V compared to the unsubstituted analogue. When combined with H₂O, <i>N,N</i>-dimethylaniline (DMA) electron donor, and <i>cis</i>-[Rh­(NN)₂Cl₂]­PF₆ (NN = Me₂bpy = 4,4′-dimethyl-2,2′-bipyridine, bpy = 2,2′-bipyridine, dmebpy = 2,2′-bipyridine-4,4′-dimethyl ester) water reduction catalysts (WRC), photocatalytic H₂ evolution is only observed using the [Cu­(Xantphos)­(biq)]⁺ PS. Furthermore, the choice of <i>cis</i>-[Rh­(NN)₂Cl₂]⁺ WRC strongly affects the catalytic activity with turnover numbers (TON_Rh = mol H₂ per mol Rh catalyst) of 25 ± 3, 22 ± 1, and 43 ± 3 for NN = Me₂bpy, bpy, and dmebpy, respectively. This work illustrates how ligand modification to carefully tune the PS light absorbing, excited state, and redox-active properties, along with the WRC redox potentials, can have a profound impact on the photoinduced intermolecular electron transfer between components and the subsequent catalytic activity