Precious-Metal-Free CO2 Photoreduction Boosted by Dynamic Coordinative Interaction between Pyridine-Tethered Cu(I) Sensitizers and a Co(II) Catalyst

Abstract

Improving the photocatalytic efficiency of a fully noble-metal-free system for CO₂ reduction remains a fundamental challenge, which can be accomplished by facilitating electron delivery as a consequence of exploiting intermolecular interactions. Herein, we have designed two Cu(I) photosensitizers with different pyridyl pendants at the phenanthroline moiety to enable dynamic coordinative interactions between the sensitizers and a cobalt macrocyclic catalyst. Compared to the parent Cu(I) photosensitizer, one of the pyridine-tethered derivatives boosts the apparent quantum yield up to 76 ± 6% at 425 nm for selective (near 99%) CO₂-to-CO conversion. This value is nearly twice that of the parent system with no pyridyl pendants (40 ± 5%) and substantially surpasses the record (57%) of the noble-metal-free systems reported so far. This system also realizes a maximum turnover number of 11 800 ± 1400. In contrast, another Cu(I) photosensitizer, in which the pyridine substituents are directly linked to the phenanthroline moiety, is inactive. The above behavior and photocatalytic mechanism are systematically elucidated by transient fluorescence, transient absorption, transient X-ray absorption spectroscopies, and quantum chemical calculations. This work highlights the advantage of constructing coordinative interactions to fine-tune the electron transfer processes within noble-metal-free systems for CO₂ photoreduction.

Figure 1

Cu(I) PSs. (a) Chemical structures, (b) UV–vis absorption, (c) emission, and (d) excited-state decay traces of the Cu(I) PSs. Deaerated 50 μM CH₃CN solutions were used for the above spectra. Trace color: CuBCP, blue; CuPBCP, red; CuPPBCP, gold.

Figure 2

(a) Tr-XAS spectra (laser on/off) corresponding to the excited states of 0.8 mM CuBCP (blue), CuPBCP (red), and CuPPBCP (gold) via ³MLCT transitions at a delay of 100 ps between laser and X-ray pulses. Experimental Fourier transforms of k²-weighted Cu EXAFS of the laser-off (solid lines) and reconstructed (dotted lines) excited states of (b) CuBCP assuming 35% excited state, (c) CuPBCP assuming 27% excited state, and (d) CuPPBCP assuming 9.5% excited state. Insets in (b–d) show the back Fourier transformed experimental (solid lines) and fitted (dashed lines) k²[χ(k)] of the laser off and reconstructed spectra for k values of 2–9 Å^–1.

Figure 3

Photoreduction of CO₂. Time profiles of photocatalytic CO (star) and H₂ (circle) formation using 0.5 mM PS and 50 μM CoTCPc (red) under 425 nm of irradiation (20 mW cm^–2). Trace colors: CuBCP, blue; CuPBCP, red; CuPPBCP, gold; IrBPY, green. The error bars represent the standard deviations of three independent measurements.

Figure 4

Nanosecond TAS. Nanosecond TAS results of (a–d) CuBCP, (e–h) CuPBCP, and (i–l) CuPPBCP. TAS evolution of Cu(I) PSs (a,e,i) without or (b,f,j) with excess BIH. (c,g,k) Kinetic traces of Cu(I) PSs with BIH and CoTCPc. (d,h,i) Plot of (τ₁/τ – 1) versus [CoTCPc] with linear fitting and error bars from the deviations of decay traces. The data were collected by following the spectra at 360 nm in Ar-saturated CH₃CN upon excitation at 425 nm.

Figure 5

Tr-XAS for ET kinetics. (a) Experimental differential spectrum (laser on-laser off) corresponding to the Co(I) transient signal at a time delay of ∼12.3 μs between laser and X-ray pulses (red) in the Cu/Co multimolecular assemblies. (b) Pump–probe time delay scans recorded at 7714 eV reflecting the formation of the Co(I) photoinduced species in the multimolecular Co/Cu assemblies consisting of 0.75 mM Cu complex, 1.5 mM CoTCPc with 5 mM BIH in 47% CH₃CN/47% DMF/6% H₂O, from left to right for CuBCP (blue), CuPBCP (red), and CuPPBCP (gold). Kinetic fits for the Co(I) formation time scales (τ₂) are shown.

Figure 6

Proposed photocatalytic mechanism. Suggested photocatalytic mechanism for the reduction of CO₂ using the CuPPBCP/CoTCPc system was a representative example.