The effect of enantioselective chiral covalent organic frameworks and cysteine sacrificial donors on photocatalytic hydrogen evolution

Abstract

Covalent organic frameworks (COFs) have constituted an emerging class of organic photocatalysts showing enormous potential for visible photocatalytic H₂ evolution from water. However, suffering from sluggish reaction kinetics, COFs often cooperate with precious metal co-catalysts for essential proton-reducing capability. Here, we synthesize a chiral β-ketoenamine-linked COF coordinated with 10.51 wt% of atomically dispersed Cu(II) as an electron transfer mediator. The enantioselective combination of the chiral COF-Cu(II) skeleton with L-/D-cysteine sacrificial donors remarkably strengthens the hole extraction kinetics, and in turn, the photoinduced electrons accumulate and rapidly transfer via the coordinated Cu ions. Also, the parallelly stacking sequence of chiral COFs provides the energetically favorable arrangement for the H-adsorbed sites. Thus, without precious metal, the visible photocatalytic H₂ evolution rate reaches as high as 14.72 mmol h^-1 g^-1 for the enantiomeric mixtures. This study opens up a strategy for optimizing the reaction kinetics and promises the exciting potential of chiral COFs for photocatalysis.

Fig. 1. Characterizations of the Cu(II)-coordinated TpPa-COF.

a Structure of the TpPa-Cu(II)-COF complex. b Cu 2p XPS spectra of TpPa-Cu(II)-COF(10.76 wt%), TpPa-Cu(II) model compound and Cu(OAc)₂•H₂O. c N 1 s XPS spectra of TpPa-COF, TpPa-Cu(II)-COF(10.76 wt%) and TpPa-Cu(II) model compound. d Structural tautomerization of keto to enol for coordination of Cu²⁺ with N-salicylideneaniline moiety. e PXRD patterns of TpPa-COF before and after coordination with Cu(OAc)₂. f Nitrogen adsorption (solid circle) and desorption (open circle) isotherm profiles and pore-size distribution (inset) of the TpPa-Cu(II)-COF(10.76 wt%). g The r-space distributions and h the wavelet analysis calculated from the k²-weighted Cu K-edge EXAFS spectra of TpPa-Cu(II)-COF(10.76 wt%) and Cu foil (without phase correction). i HADDF-STEM image of the atomically dispersed Cu in the TpPa-Cu(II)-COF(10.76 wt%). Source data are provided as a Source Data file.

Fig. 2. Energy-band structures, photocatalytic H2 evolution, XPS spectra, proposed mechanism, and excited-state simulation.

a Energy-band structures of TpPa-COF and TpPa-Cu(II)-COFs containing 4.99, 8.47, 10.76, and 12.77 wt% of Cu²⁺ ions, respectively. b Time courses for photocatalytic H₂ evolution under visible irradiation for TpPa-COF, TpPa-Cu(II)-COF(10.76 wt%), and a mixture of TpPa-COF and Cu(OAc)₂. c Cu 2p XPS and Cu LMM auger (inset) spectra of the recycled TpPa-Cu(II)-COF after 1-h photocatalysis. d Illustration of photocatalytic mechanism including proton reduction and Cu-mediated cysteine oxidation. e Top: theoretical prediction of photogenerated electron pathway. Bottom: real-space distribution of photo-induced electrons (green regions) and holes (blue regions) on the calculated TpPa-Cu(I) model. f Calculation of the transferred electron numbers in the cutout models with different linkers. g Turnover frequency of photocatalytic H₂ evolution vs. the linker lengths of the different Tp-based COFs. Source data are provided as a Source Data file.

Fig. 3. Chiral dichroism spectra, photocatalytic H2 evolution and polarization curves.

a Circular dichroism spectra of the achiral TpPa-COF, TpPa(Δ)-COF, TpPa(Λ)-COF, and double-layered TpPa(Δ) model. b Proposed structural response to the circular dichroism signals in the chiral COF. c Circular dichroism spectra of the achiral TpPa-Cu(II)-COF, TpPa(Δ)-Cu(II)-COF and TpPa(Λ)-Cu(II)-COF. d Time-dependent photocatalytic H₂ evolution and e Sacrificial oxidation turnover frequency of the various combinations of TpPa(Δ)-Cu(II)-COF, TpPa(Λ)-Cu(II)-COF and TpPa-Cu(II)-COF with L-/D-cysteine. The stars on the histograms represent the two groups of enantiomeric mixtures. f Polarization curves and Tafel curves (inset) of the achiral TpPa-COF and chiral TpPa(Δ)-COF. Source data are provided as a Source Data file.

Fig. 4. Binding affinity between chiral COF and cysteine.

a Calculation of relative Gibbs free energy for the binding of TpPa(Δ)-Cu(I)-COF model with L-/D-cysteine, respectively. b ITC thermogram resulting from titration of a TpPa(Δ)-Cu(II)-COF dispersion (2 mmol/L) with a L-/D-cysteine aqueous solution (20 mmol/L) (up) and fitting with two sets of sites model (bottom). Source data are provided as a Source Data file.

Fig. 5. Proposed kinetic mechanism of H2 production on the TpPa-Cu-COF.

a Electrostatic potential (ESP) of the TpPa-Cu(I) model. b Calculated binding free energy of H atom on the TpPa model. c Schematic Gibbs free energy diagrams for the H₂ evolution pathway on the two COF models. d Top-view and side-view of the parallelly stacked chiral TpPa-COF and antiparallelly stacked achiral TpPa-COF lattice models. Source data are provided as a Source Data file.