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

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.

S3 analyzer (Micromeritics, USA). Before the test, samples were degassed at 90 °C for 24 h. The pore-size distributions were calculated based on the Nonlocal Density Functional Theory (NLDFT). 1 H NMR and 13 C NMR spectra were collected on a 400 MHz spectrophotometer (Bruker AVANCE III HD, Switzerland) at 298.15 K.
Isothermal titration calorimetry was carried out using the calorimeter (Malvern PeaQ-ITC, UK). The titration of water to water was performed before test.
Transmission electron microscopy (TEM) images were recorded using a field 120kV transmission electron microscope (Hitachi HT7800, Japan). High resolution transmission electron microscopy (HR TEM) images were obtained using a field emission transmission electron microscope (FEI Tecnai G2 F20 S-Twin, USA) operating at 200 kV accelerating voltage. The elemental mappings of C, N, O and Cu atoms were collected using the X-Max 80T detector (Oxford Instruments, UK) under the ADF STEM mode. The HADDF-STEM images were obtained using an atomic resolution analytical microscope (JEOL JEM-ARM200F, Japan) equipped with the spherical aberration correction system. The electron paramagnetic resonance (EPR) spectra were recorded using Bruker EMXplus-6/1 (Germany).
The deionized water used in measurement was bubbled with N2 for 30 min to remove oxygen. The electrochemical measurements were recorded on the CHI760E electrochemical workstation (Chenhua, China) with a standard threeelectrode system with the photocatalyst-coated ITO as the working electrode, Pt wire as the counter electrode and the Ag/AgCl electrode as a reference electrode.
The electrolytes were bubbled with Ar for 1h before the measurement. The lifetimes were examined with a 450-nm diode laser using Time-Correlated Single Photon Counting (TCSPC) technique and calculated by fitting with first-order S4 exponential curve.

Synthesis of Tp-based COFs
The synthesis of Tp-based COFs was carried out using the reported solvothermal method. [1] 0.08 mmol 1,3,5-triformylphloroglucinol (Tp) and 0.12 mmol diamine monomers were charged into a Pyrex tube (10 cm×1cm) and dispersed in 1 mL solvent (dioxane for TpPa-COF and a mixture of DCB and n-BuOH (v/v, 9/1) for TpBD-COF and TpTP-COF). Then 0.02 mL pyrrolidine was added and the mixture was dispersed by sonification. After three freeze-pump-thaw cycles, the tube was sealed off and kept in the oven at 120℃ for 3 days. Afterward, the precipitate was filtered, washed with THF (3×10 mL), extracted by Soxhlet with THF for 24 h, and dried under vacuum at 40℃ for 24 h. Finally, the product was obtained with a 75-85% yield.

Synthesis of TpPa amorphous polymer
33.6 mg Tp and 25.9 mg Pa were dissolved in 30 mL THF by sonication for 10 min.
The mixture was stirred at room temperature for 12 h. The product was collected by filtration and washed by Soxhlet extraction with THF for 24 h. The resulting polymer was dried under vacuum at 40℃ for 48 h to afford the red powder with a 85% yield.

Synthesis of TpPa-Cu(I)-COF and TpPa(∆)-Cu(I)-COF
L-cysteine (0.1 mol/L) was mixed with the aqueous dispersions of TpPa-Cu(II)-COF or TpPa(∆)-Cu(II)-COF (10 mg, 100 mL) and the solution was degassed in the dark. After stirring for 1h, the product was collected by filtration, washed with water several times, and dried under vacuum overnight. The product was preserved in a dark brown bottle shielded from light and protected in N2 atmosphere. If no S5 otherwise specified, TpPa-Cu(I) and TpPa(∆)-Cu(I) were prepared in the same way.

X-ray absorption fine structure (XAFS) analyses
Cu K-edge analysis was performed with Si(111) crystal monochromators at the BL11B beamlines at the Shanghai Synchrotron Radiation Facility (SSRF) (Shanghai, China). Before the analysis at the beamline, samples were pressed into thin sheets with 1 cm in diameter and sealed using Kapton tape film. The XAFS spectra were recorded at room temperature, using a 4-channel Silicon Drift Detector (SDD) Bruker 5040. Cu K-edge extended X-ray absorption fine structure (EXAFS) spectra were recorded in the transmission mode. The spectra were analyzed by the software codes Athena and Artemis. [2] The Morlet wavelet transformation was performed by codes hamaFortran. [3] In the fitting process, coordination number (N), amplitude attenuation factor (S0 2 ), interatomic distance (R), Debye-Waller factor (σ 2 ) and edge energy shift (ΔE0) were all taken into consideration. The effective scattering amplitude, effective scattering phase shift and mean free path were calculated by FEFF6. The amplitude attenuation factor (S0 2 ) was determined to be 0.875 by fitting the spectra of Cu foil with a constrained coordination number 12. The fluctuation of S0 2 was neglectable as the data were collected simultaneously. The R factor indicated the goodness of the fit.

Photoelectrochemical/electrochemical measurements
The photocurrent response profiles, electrochemical impedance spectra (EIS), Mott-Schottky plots and cyclic voltammogram were recorded on the CHI 760E electrochemical workstation with a standard three-electrode system, using the Ar- (EAg/AgCl=0.1967 V vs. NHE at 25 ℃)

AQE calculation
The apparent quantum efficiency (AQE) was measured under the irradiation of a Xe lamp equipped with different bandpass filters (λ0 ± 20 nm), followed by the calculation using the following equation: Where, M is the amount of H2 molecules (mol), NA is the Avogadro constant (6.022 ×10 23 mol -1 ), ℏ is the Planck constant (6.626 × 10 -34 J s), c is the light speed (3 ×10 8 m s -1 ), S is the irradiation area (cm 2 ), P is the irradiation intensity (W cm -2 ), t is the S8 photoreaction time (s), λ is the wavelength of the monochromatic light (m).

Calculation of Models
The DFT calculation of model structures was performed by Gaussian 16 software package. [4] The models derived from the reported crystal structure were fully optimized at the PBE0-D3BJ/ma-def2-SVP level. [5,6] The implicit solvation model The electrostatic potential and electron-hole analysis were processed using Multiwfn 3.8 [7] and visualized by VMD 1.9.3 package. [8] The photo-induced charge transfer was quantified based on the interfragment charge transfer method.
The hydrogen binding energy (HBE) [9] was calculated from the follow equation: Where *H presents for the structure absorbed H, * is the pristine model, and the 1/2 Gibbs energy of H2 was applied to approximate the energy of the H atom and single electron.
In the calculation of the electronic circular dichroism, C, N, O atoms were directly cut from the crystal structure without further relaxation, while H atoms were optimized at the level of PBE0-D3BJ/ def2-SVP. Then the vertical excitation calculations were performed at TD-PBE0-D3BJ/ def2-SVP level for at least 30 states.

Calculation in periodic boundary conditions
The calculations based on the first principle were performed using the CP2K package [10] and visualized by VESTA. [11] In the geometry optimization, the mixed Derived from the reported crystal structure of TpPa-COF, [12] the corresponding structures of parallel stacking and antiparallel stacking were built and relaxed. The optimization of H-adsorbed structures was carried out, while the carbon backbone of the parent TpPa-COF was constrained.
The Gibbs free energies (at T K) of different structures were calculated as: where ∆Gcor (T) = ∆EZPE -T∆S + ∆U(T) was calculated at 298.15K by processing the frequency analysis results with Shermo 2.3 code [13] .