Covalent Triazine Framework Nanoparticles via Size‐Controllable Confinement Synthesis for Enhanced Visible‐Light Photoredox Catalysis

Abstract For metal‐free, organic conjugated polymer‐based photocatalysts, synthesis of defined nanostructures is still highly challenging. Here, we report the formation of covalent triazine framework (CTF) nanoparticles via a size‐controllable confined polymerization strategy. The uniform CTF nanoparticles exhibited significantly enhanced activity in the photocatalytic formation of dibenzofurans compared to the irregular bulk material. The optoelectronic properties of the nanometer‐sized CTFs could be easily tuned by copolymerizing small amounts of benzothiadiazole into the conjugated molecular network. This optimization of electronic properties led to a further increase in observed photocatalytic efficiency, resulting in total an 18‐fold enhancement compared to the bulk material. Full recyclability of the heterogeneous photocatalysts as well as catalytic activity in dehalogenation, hydroxylation and benzoimidazole formation reactions demonstrated the utility of the designed materials.


Preparation of CTF nanoparticles
Scheme S1 Synthetic route for covalent triazine framework nanoparticles in confinement.

S4
Step 1: Tetraethoxylsilane (TEOS) (0.52 g, 2.5 mmol) and 2,5-dicyano-3-hexylthiophene (0.4 g, 1.83 mmol) were first mixed to form a homogeneous oil phase. Then an aqueous solution of cetyltrimethylammonium chloride (6.2 mg in 8 mL) was poured into the oil mixture under vigorous stirring. The obtained microemulsion was further sonicated for 3 min using an ultrasonic tip (Branson 450 W, at 70% amplitude). The resulting miniemulsion was stirred at 1000 rpm for another 24 h at room temperature. During this process, the silica capsule was formed by slow hydrolysis of TEOS at the interface of water and the oil phase.
Step 2: The freeze-dried particles were directly polymerized under trifluoromethane sulfonic acid (TfOH) at 100°C in a degassed sealed desiccator for 24 hours. The formed yellow solid was washed with water and diluted aqueous ammonia to remove the TfOH. Further purification was conducted by continuous washing the sample with DCM in a Soxhlet extractor.
Step 3: The resulting yellow powder was stirred with 4M NH4HF2, followed by careful washing with water and ethanol. After drying the material at 80°C under vacuum overnight, CTF NPs were obtained as a yellow to orange powder.
Nanoparticles with larger sizes were synthesized by increasing the volume ratio between the dispersed phase and water phase of the emulsion to two and four times.
In a control experiment, chloroform (260µL, 3.25 mmol) was used instead of DCHT as oil phase. The above procedure was followed. After freeze drying and evaporation of all solvents, TEM pictures (see Figure S3) were taken.

Preparation of bulk CTF
A vial of 2,5-Dicyano-3-hexylthiophene (100 mg) was placed into a conical flask, in which there was another vial with trifluoromethanesulfonic acid (0,3 mL). The conical flask was degassed with nitrogen and sealed followed by heating to 100°C in a sand bath for 24h. After cooling down to room temperature, the product was immersed in water and washed well with ammonia solution (10%) and Milli Q water. The resultant material was further purified with Soxhlet extraction with DCM/MeOH (1:1). After drying under vacuum at 80°C over night, the polymer (48 mg, 48%) was obtained.

Photocatalytic dibenzofurane synthesis
General procedure: To a 20 mL vial with a magnetic stir bar was added nanoparticle photocatalyst (4 mg), alkene (1.30 eq.), phenol (1.0 eq.), (NH4)2S2O8 and nitromethane. Then the vial was capped, degassed for 5 min with nitrogen and placed under the irradiation of a blue LEDs lamp (λ=460 nm, 0.061 W/cm 2 ). The reaction mixture was stirred at room temperature. The conversion and yield was determined by GC-MS with trimethylsilican as internal standard. Afterwards, the mixture was transferred to a separatory funnel containing DCM and H2O (v/v, 1/1). The organic S5 layers were separated and extracted thrice with DCM. The combined organic layers were washed with brine, dried over anhydrous Na2SO4 and concentrated by rotary evaporation. The crude product was purified via column chromatography on silica using EtOAc/Hexane as elute to afford the pure compound.

Photocatalytic recycling experiments
The general procedure was followed for setting up the reactions. After the completion of a reaction cycle after 24h, the conversion was analysed by GCMS and the reaction mixture was centrifuged at 10 000 rpm for 1 min. Then, the supernatant was removed and fresh CH3NO2 (2 mL) was added for washing. The centrifugation was repeated and residual CH3NO2 was removed. The nanoparticles were then dried in a nitrogen stream for 30 min and weighed. Eventual weight loss was accounted for by adjusting the amounts of trans-anethole, mequinol, (NH4)2S2O8 and solvent. After degassing, the next reaction cycle was started by LED irradiation.

Photocatalytic dehalogenation
To a 20 mL vial with a magnetic stir bar was added nanoparticle photocatalyst CTF2BT (4 mg), αchloroacetophenone (77.3 mg, 0.50 mmol, 1.0 eq.), Hünig's base (71.1 mg, 0.55 mmol, 1.1 eq.), Hantzsch ester (253 mg, 1.0 mmol, 2.0 eq.) and acetonitrile (4 mL). Then the vial was capped, degassed by N2 bubbling and placed under the irradiation of a blue LEDs lamp (0.061 W/cm 2 ). The reaction mixture was stirred at room temperature for 8h. The GCMS conversion was found to be 99%. Afterwards, the mixture was transferred to a separatory funnel containing DCM and H2O (v/v, 1/1). The organic layers were separated and extracted thrice with DCM. The combined organic layers were washed with brine, dried over anhydrous MgSO4 and concentrated by rotary evaporation. The crude product was purified via column chromatography (Petrol ether / ethyl acetate, 6:1) on silica to afford benzaldehyde (57 mg, 95%).

Photocatalytic hydroxylation of boronic acids
To a 20 mL vial with a magnetic stir bar was added nanoparticle photocatalyst CTF2BT (4 mg), 4biphenylboronic acid (99.0 mg, 0.50 mmol, 1.0 eq.), Hünig's base (129 mg, 1.0 mmol, 2.0 eq.) and N,N-dimethylformamide (4 mL). Then the vial was capped and placed under the irradiation of a blue LEDs lamp (0.061 W/cm 2 ). The reaction mixture was stirred at room temperature for 12h. The GCMS conversion was found to be 90%. Afterwards, the mixture was transferred to a separatory funnel containing DCM and H2O (v/v, 1/1). The organic layers were separated and extracted thrice with DCM. The combined organic layers were washed with brine, dried over anhydrous MgSO4 and concentrated by rotary evaporation. The crude product was purified via column chromatography (Petrol ether / ethyl acetate, 3:1) on silica to afford 4-phenylphenol (71 mg, 83%

Photocatalytic benzoimidazole formation
To a 20 mL vial with a magnetic stir bar was added nanoparticle photocatalyst CTF2BT (4 mg), ophenylenediamine (54.1 mg, 0.50 mmol, 1.0 eq.), benzaldehyde (53.1 mg, 0.50 mmol, 1.0 eq.), Hünig's base (129 mg, 1.0 mmol, 2.0 eq.) and acetonitrile (4 mL). Then the vial was capped, degassed by N2 bubbling and placed under the irradiation of a blue LEDs lamp (0.061 W/cm 2 ). The reaction mixture was stirred at room temperature for 15h. The GCMS conversion was found to be 95%. Afterwards, the mixture was transferred to a separatory funnel containing DCM and H2O (v/v, 1/1). The organic layers were separated and extracted thrice with DCM. The combined organic layers were washed with brine, dried over anhydrous MgSO4 and concentrated by rotary evaporation. The crude product was purified via column chromatography (Petrol ether/Ethyl acetate, 1:1, 3% of acetic acid was added to the ethyl acetate used) on silica to afford 2phenylbenzimidazole (71 mg, 73%) .      The nitrogen sorption isotherms are typical for non-porous materials. It is supposed, that porosity, if present, would not be detectable due to the long alkyl chains of DCHT blocking N2 from accessing pores.  Figure S19. Apparent quantum yield (AQY) for CTF-2BT when irradiated at four different wavelengths.
For determining the apparent quantum yield, CTF-2BT was used as photocatalyst. The general procedure of the photocatalytic dibenzofurane benchmark synthesis was followed. The product formation was studied using four different LEDs (385 nm, 450 nm, 525 nm, 620 nm, ~60 mW/cm²). An area of 1.44 cm² was illuminated for 4h. The apparent quantum yields were estimated using the following equation Φ AQY = number of product molecules formed number of incident photons S30 Figure S20. Bar diagram of yields after 24h of reaction using CTF80 as photocatalyst in the benchmark reaction. The error bar represents two independent measurements. UV/Vis and IR spectra before and after (b,c) as well as TEM image after the application (d). Figure S21. Overview of the substrate scope for different 2,3-dibenzofurans catalysed using CTF-2BT NPs. Experimental detail is given in the general procedure.