Probing the Diphosphine Ligand’s Impact within Heteroleptic, Visible-Light-Absorbing Cu(I) Photosensitizers for Solar Fuels Production

In an effort to further develop earth-abundant photosensitizers for use in solar fuels production, a series of heteroleptic, visible-light-absorbing Cu­(I) photosensitizers (PSs) of the design [Cu­(PP)­(NN)]­PF₆ were prepared (PP = bidentate diphosphine, Xantphos = 4,5-bis­(diphenylphosphino)-9,9-dimethylxanthene, DPEphos = bis­[(2-diphenylphosphino)­phenyl]­ether, Nixantphos = 4,6-bis­(diphenylphosphino)-10H-phenoxazine; NN = bidentate diimine, biq = 2,2′-biquinoline). The Xantphos ligand was modified to impart flexibility (PP = DPEphos) and redox activity (PP = Nixantphos) to the diphosphine backbone to probe their influence on the light-absorbing properties and excited state dynamics using steady-state and time-resolved (nanosecond) spectroscopic techniques. Following visible light excitation to populate redox-active Cu­(dπ) → biq­(π*) metal-to-ligand charge transfer excited states (MLCT ES), intermolecular electron transfer via reductive quenching by the N,N-dimethylaniline (DMA) electron donor produces the one-electron reduced [Cu­(PP)­(biq^–)] species. The strong reducing potential of [Cu­(PP)­(biq^–)] PS in DMF solvent (E_1/2(PS^0/–) = −1.57, −1.62, and −1.58 V vs Fc⁺/Fc for PP = Xantphos, DPEphos, and Nixantphos, respectively) permits thermodynamically favorable ground state (GS) electron transfer to reduce the Rh­(III)-based lowest unoccupied molecular orbitals (E_p^c(Rh^III/II/I) = −1.46 V) within the cis-[Rh^III(Me₂bpy)₂­Cl₂]⁺ (Me₂bpy = 4,4′-dimethyl-2,2′-bipyridine) water reduction catalyst (CAT). In combination with nanosecond transient absorption (nsTA) and GS electronic absorption measurements, the initial photophysical and photochemical processes are proposed that contribute to formation of the [Rh^I­(Me₂bpy)₂]⁺ intermediate for H₂O reduction to H₂. Subsequently, DMF solutions of concentration-matched Cu­(I) PSs (360 μM) were photolyzed at λ^irr = 447.5 nm in the presence of Rh­(III) CAT (36 μM), DMA electron donor (1.55 M), and H₂O substrate (1.09 M), whereby Cu­(I) PSs with PP = Xantphos or Nixantphos produced comparable amounts of H₂ (48 ± 3 vs 33 ± 1 μmol) after 3 h. Conversely, the PP = DPEphos ligand produced significantly lower levels of H₂ (2.6 ± 0.6 μmol after 3 h), indicating that the structurally flexible DPEphos ligand hinders photosensitizer stability and results in more rapid decomposition to produce the photocatalytically inactive homoleptic [Cu­(biq)₂]⁺ complex.