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 (E1/2(PS0/–) = −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 (Epc(RhIII/II/I) = −1.46 V) within the cis-[RhIII(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 [RhI(Me2bpy)2]+ intermediate for H2O reduction to H2. Subsequently, DMF solutions of concentration-matched Cu(I) PSs (360 μM) were photolyzed at λirr = 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.