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)]PF 6 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 (Me 2 bpy) 2 Cl 2 ] + (Me 2 bpy = 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 (Me 2 bpy) 2 ] + intermediate for H 2 O reduction to H 2 . 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 H 2 O substrate (1.09 M), whereby Cu(I) PSs with PP = Xantphos or Nixantphos produced comparable amounts of H 2 (48 ± 3 vs 33 ± 1 μmol) after 3 h. Conversely, the PP = DPEphos ligand produced significantly lower levels of H 2 (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.

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.

The cucurbit[8]uril macrocycle can secure a platinum terpyridyl complex into a particularly reactive dimer that catalyzes the photoreduction of water.

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