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

Abstract

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.

The shiny side of copper: bringing copper(i) light-emitting electrochemical cells closer to application

“…All bands are structured with two emission maxima, which is a typical feature of these complexes. 21,24,32,60 For all the complexes, any alkyl substitution in the bipyridine ligand leads to a blue shi of the emission with respect to 6 ] complex emission spectra in solution, the most hypsochromically shied are the ones with 2-Etphen and 4,5,6-Me 3 bpy, where the wavelengths and proles of the emission bands are very similar (Fig. 7).…”

“…[11][12][13] The colourtuning of iTMC-based LECs is well-established in the case of iridium complexes, although the use of this element makes again the sustainability questionable. We [14][15][16][17][18][19][20][21][22][23][24][25][26][27][28] and others [29][30][31][32][33][34][35][36][37][38] have investigated copper(I) complexes of the type [Cu(P^P)(N^N)] + (P^P ¼ chelating diphosphane, N^N ¼ diimine or 1,10-phenanthroline) as alternative electroluminescent materials for LECs. These complexes are of particular interest as they exhibit the phenomenon of thermally activated delayed uorescence (TADF), in which the rst excited singlet (S 1 ) and triplet (T 1 ) states are strongly coupled, allowing intersystem crossing between the two levels without the need for a heavy metal.…”

Application of Discrete First‐Row Transition‐Metal Complexes as Photosensitisers

“…This can in turn catalyse a reaction, such as the reduction of water or CO 2 . Heteroleptic Cu complexes are also commonly used here, often but not exclusively in combination with noble metals, such as Rh [47] . For example, a Cu(I) PS containing a xantphos and a diimine ligand was used alongside a Rh(III) catalyst to produce H 2 from water under irradiation with a blue light [48] .…”

“…Heteroleptic Cu complexes are also commonly used here, often but not exclusively in combination with noble metals, such as Rh. [47] For example, a Cu(I) PS containing a xantphos and a diimine ligand was used alongside a Rh(III) catalyst to produce H 2 from water under irradiation with a blue light. [48] In this case, the Cu PS can undergo intermolecular electron transfer upon excitation, thereby reducing the Rh moiety, which can then reduce protons to H 2 .…”

Copper(I) Phosphinooxazoline Complexes: Impact of the Ligand Substitution and Steric Demand on the Electrochemical and Photophysical Properties

“…Photoactive Cu I complexes are considered as a highly promising alternative to traditional systems based on noble metals such as ruthenium, iridium, rhenium or platinum . Indeed, Cu I compounds were already successfully applied as photosensitizers in the light‐driven reduction of protons to H 2 , as photoredoxcatalysts for organic transformations or in devices such as organic light‐emitting diodes (OLEDs), dye‐sensitized solar cells (DSSCs) and light‐emitting electrochemical cells (LECs) . Unfortunately, a limited stability under operating conditions still hampers their large‐scale application in molecular solar energy conversion schemes .…”