Vinylene‐Linked Covalent Organic Frameworks by Base‐Catalyzed Aldol Condensation

Abstract Two 2D covalent organic frameworks (COFs) linked by vinylene (−CH=CH−) groups (V‐COF‐1 and V‐COF‐2) are synthesized by exploiting the electron deficient nature of the aromatic s‐triazine unit of C3‐symmetric 2,4,6‐trimethyl‐s‐triazine (TMT). The acidic terminal methyl hydrogens of TMT can easily be abstracted by a base, resulting in a stabilized carbanion, which further undergoes aldol condensation with multitopic aryl aldehydes to be reticulated into extended crystalline frameworks (V‐COFs). Both V‐COF‐1 (with terepthalaldehyde (TA)) and V‐COF‐2 (with 1,3,5‐tris(p‐formylphenyl)benzene (TFPB)) are polycrystalline and exhibit permanent porosity and BET surface areas of 1341 m2 g−1 and 627 m2 g−1, respectively. Owing to the close proximity (3.52 Å) of the pre‐organized vinylene linkages within adjacent 2D layers stacked in eclipsed fashion, [2+2] photo‐cycloadditon in V‐COF‐1 formed covalent crosslinks between the COF layers.

4 NMR measurements: 1 H NMR for the samples dissolved in suitable solvents were carried on Bruker Avance II 200. 13 C Solidstate NMR (cross polarization magic-angle spinning (CP/MAS)) spectra were carried out on a Bruker Avance 400 MHz spectrometer operating at 100.6 MHz.

Solid-state diffuse reflectance Ultraviolet-visible spectroscopy (UV-DRS) analysis:
Solid-state diffuse reflectance Ultraviolet-visible spectroscopy (UV-vis) spectra of the as pristine COF powders and starting monomers have been collected on Varian Cary 300 UV-Vis Spectrophotometer.

Physisorption measurements:
N2 and Ar sorption measurements were performed on a volumetric sorption instrument (AutosorbiQMP).
Prior to the gas sorption studies of COFs, the samples were dried under a dynamic vacuum (<10 -3 Torr) at room temperature (RT) followed by heating 150 °C for 24 h. Using the Ar adsorption isotherms, the surface areas were calculated over a pressure range 0.010.9 = p/p0 using Brunauer-Emmett-Teller (BET), and pore size distributions were calculated using the quenched solid density functional theory (QSDFT) method on the Ar adsorption branch.

Fluorescence Emission Spectroscopy:
Fluorescence emission was measured with a Fluoromax-2 spectro-fluorometer (Horiba Jobin Yvon, Bensheim, Germany). Slits were set to 4 nm for excitation and 2 nm for emission, while the integration time was 0.5 s and the increment 1 nm. The sample was excited at 365 nm, and emission spectra were recorded in a suitable range centred around the emission maximum between 370 and 700 nm.
Synthesis of 2,4,6 trimethyl s-triazine: The synthesis of 2,4,6 trimethyl s-triazine was adapted from a reported procedure. [2] To prepare the free base, 17.28 g (0.14 moles) of ethyl acitimidate hydrochloride was added quickly at 20-25 °C to a vigorously stirred mixture of 40 mL of methylene dichloride and 31.8g (0.23 moles) of potassium carbonate saturated solution in water. After 10 min., the organic phase was collected and the aqueous phase was extracted 2-3 times with 50 mL dichloromethane. All the extracts were combined and dried over anhydrous sodium sulphate. The solution was filtered and then stripped of solvents on a rotary evaporator to a concentrated solution. To this solution 1.8 mL of glacial acetic acid was added slowly (20-30 min) and the temperature was held at 25 o C for 1 hour. The mixture was then allowed to stand for 1 day. The reaction mixture was then stripped of solvents and the residue was diluted by adding dichloromethane. The solution was filtered from crystalized acetamidine acetate and residual acetic acid was neutralized with a concentrated aqueous potassium carbonate and dried with sodium sulphate. The solution was then stripped of most of the solvent and then kept at room temperature for the evaporation of the rest to finally get a solid. After solvent evaporation colourless crystals of 2,4,6 trimethyl s-triazine were obtained after purification by sublimation. Sublimated crystals were also submitted to determine the single crystal x-ray structure. After four days, the autoclave was cooled down and the formed precipitate was collected by filtration and thoroughly washed with water, methanol and acetone. V-COF-1 was obtained as a (fluffy) yellow powder after a further drying step under vacuum at 100 °C .

Section S4. Structure Modeling and Atomic Coordinates of V-COFs:
Structure models were generated using the Material Studio Modelling 5.0 package. [4] Geometry optimization was performed using the Forcite module and UFF forcefield. [5] Full profile pattern fitting (Pawley) was performed against the experimental powder pattern using the Reflex module.

Section S10. [2+2]-photocycloaddition:
Conditions for [2+2] cycloaddition on V-COF-1 powder: 50mg of V-COF-1 powder was kept inside a jacketed glass vessel inside a glove box under argon atmosphere. The glass vessel was then sealed with a rubber septum and para film and taken out from the glove box and connected to a water circulation setup maintaining a temperature of 10 °C. Light (λ~320-500 nm) was irradiated from the bottom to ensure a close distance to the sample. The sample was occasionally shaken to provide a homogeneous light exposure to all parts of the samples.