Promoting Photocatalytic Direct C–H Difluoromethylation of Heterocycles using Synergistic Dual-Active-Centered Covalent Organic Frameworks

Difluoromethylation reactions are increasingly important for the creation of fluorine-containing heterocycles, which are core groups in a diverse range of biologically and pharmacologically active ingredients. Ideally, this typically challenging reaction could be performed photocatalytically under mild conditions. To achieve this separation of redox processes would be required for the efficient generation of difluoromethyl radicals and the reduction of oxygen. A covalent organic framework photocatalytic material was, therefore, designed with dual reactive centers. Here, anthracene was used as a reduction site and benzothiadiazole was used as an oxidation site, distributed in a tristyryl triazine framework. Efficient charge separation was ensured by the superior electron-donating and -accepting abilities of the dual centers, creating long-lived photogenerated electron–hole pairs. Photocatalytic difluoromethylation of 16 compounds with high yields and remarkable functional group tolerance was demonstrated; compounds included bioactive molecules such as xanthine and uracil. The structure–function relationship of the dual-active-center photocatalyst was investigated through electron spin resonance, femtosecond transient absorption spectroscopy, and density functional theory calculations.


Characterization
Fourier transform infrared (FT-IR) spectra were collected on a VARIAN 1000 FT-IR spectrometer in the region of 400-4000 cm -1 by potassium bromide pressed-disk technique.The morphologies and microstructures were probed utilizing Scanning electron microscope (SEM, Nova NanoSEM 230) and transmission electron microscope (TEM, FEI TECNAI G2 F20).The cumulative apparent surface areas for N2 were calculated on a Micro ASAP 2020 using a Brunauer-Emmett-Teller (BET) model range from 0.01 to 0.1 bar for all samples.The microporous volumes were calculated using the t-plot method, while the total porous volumes were obtained from the N2 isotherm at P/P0=0.99.Pore size distributions were derived from the N2 adsorption isotherms using DFT methods.UV/Vis/NIR absorption spectra of the polymers were recorded on a UV-2600 spectrometer (Shimadzu, Japan) as powders in the solid state.Electrochemical measurements of these materials were carried out on a Metrohm Autolab PGSTAT204 at room temperature in a threeelectrode cell, with the glassy carbon auxiliary electrode as working electrode, the platinum wire as counter electrode.And the Hg/Hg2Cl2 electrode was used as reference electrode.The polymer samples were coated onto the glassy carbon electrode.The solid-state 13 C NMR spectra were performed on a Bruker AVANCE III 400 MHz NMR spectrometer. 1 H and 13 C NMR spectra were obtained in deuterated solvents on Bruker AM-400 MHz using tetramethyl silane (TMS) as an internal standard.Electrostatic potential map was carried out by Standard ab initio molecular orbital theory and density functional theory calculations with the Gaussian 09 software package and Gauss View visualization program.The electron spin resonance (ESR) signals of the radicals that were spin-trapped by DMPO or TEMPO were recorded on the JES FA200 spectrometer (JEOL, Japan).Specimens for the ESR measurement were prepared by mixing the photocatalyst with 40mM DMPO or TEMPO solution in a beaker and illuminating it with visible light (λ > 400 nm).The ultrafast TA spectra of the samples were obtained using a laser system with amplification at a 1 KHz repetition rate and a Helios spectrometer with an ultrafast system.The laser pump pulse signal in the range of 425-900 nm was focused onto the sapphire disk to generate an ultrafast continuous probe beam.The probe light, which contained the sample and reference beam, was collected by the lens of the visiblelight responsive fiber silicon or infrared-light responsive InGaAs diode array.The TA decay kinetic trace was measured by recording the average ΔA over the corresponding detection wavelength range for each delay.Thin films were prepared for all photocatalysts.To be specific, 30.0 mg of catalyst were dispersed in 1 mL of ethanol solution by sonication.The above suspension was then sprayed on 15×15×3 mm quartz slides and dried in air for 12 hours.In situ 1 H NMR: To a 10 mL Schlenk tube equipped with a magnetic stir bar, added quinoxalin-2(1H)-ones (0.2 mmol), NaSO2CF2H (0.4 mmol) and COFs (8 mg) in DMSO-d6 (2.0 mL).Then the mixture was stirred and irradiated by the
General procedure of the photocatalytic reactions using COFs. 3 To a 10 mL Schlenk tube equipped with a magnetic stir bar, added quinoxalin-2(1H)-ones (0.2 mmol), NaSO2CF2H (0.4 mmol) and COFs (8 mg) in DMSO (2.0 mL).Then the mixture was stirred and irradiated by the 3W blue LEDs at room temperature for 24 h.The residue was added water (10 mL) and extracted with ethyl acetate (5 mL x 3).The combined organic phase was dried over Na2SO4.The resulting crude residue was purified via column chromatography on silica gel to afford the desired products.

Photocatalytic recycling experiments.
The general procedure was followed for setting up the reactions.After the completion of a reaction cycle after 24h, the reaction mixture was centrifuged at 10 000 rpm for 1 min.Then, the supernatant was removed and fresh CH2Cl2 (2mL) was added for washing.The centrifugation was repeated and residual CH2Cl2 was removed.The nanoparticles were then dried for the next reaction.

NMR spectra of products
The 1 H-NMR Spectrum of 1a.
The 1 H-NMR Spectrum of 1b.
The 1 H-NMR Spectrum of 1c.
The 13 C-NMR Spectrum of 1c.
The 13 C-NMR Spectrum of 1d.
The 19 F-NMR Spectrum of 1d.
The 1 H-NMR Spectrum of 1e.
The 13 C-NMR Spectrum of 1e.
The 13 C-NMR Spectrum of 1f.
The 19 F-NMR Spectrum of 1f.
The 1 H-NMR Spectrum of 1g.
The 13 C-NMR Spectrum of 1g.
The 19 F-NMR Spectrum of 1i.
The 1 H-NMR Spectrum of 1j.
The 19 F-NMR Spectrum of 1j.
The 1 H-NMR Spectrum of 1k.
The 19 F-NMR Spectrum of 1k.
The 1 H-NMR Spectrum of 1l.
The 13 C-NMR Spectrum of 1l.
The 19 F-NMR Spectrum of 1l.
The 1 H-NMR Spectrum of 1m.
The 13 C-NMR Spectrum of 1m.
The 19 F-NMR Spectrum of 1n.
The 1 H-NMR Spectrum of 1o.
The 13 C-NMR Spectrum of 1o.
The 19 F-NMR Spectrum of 1o.

Figure. S8
Figure.S8 Mott−Schottky plot and flat band potentials of the COFs.

Figure. S11 1 H
Figure.S11 1 H NMR spectrum of detection of hydrogen peroxide.
Figure.S15.Time-resolved PL spectra of the COFs.

Figure .
Figure.S20 X-ray photoelectron spectroscopy (XPS) analysis was conducted on V-COF-AN-BTto investigate its surface chemical composition.

Table S1
Some reported materials on photocatalytic direct C-H difluoromethylation