Rational design of crystalline two-dimensional frameworks with highly complicated topological structures

Constructing two-dimensional (2D) polymers with complex tessellation patterns via synthetic chemistry makes a significant contribution not only to the understanding of the emergence of complex hierarchical systems in living organisms, but also to the fabrication of advanced hierarchical materials. However, to achieve such tasks is a great challenge. In this communication we report a facile and general approach to tessellate 2D covalent organic frameworks (COFs) by three or four geometric shapes/sizes, which affords 2D COFs bearing three or four different kinds of pores and increases structural complexity in tessellations of 2D polymers to a much higher level. The complex tessellation patterns of the COFs are elucidated by powder X-ray diffraction studies, theoretical simulations and high-resolution TEM.


Fourier transform infrared spectroscopy (FT-IR)
Fourier transform infrared spectroscopy (FT-IR) was carried out with a Nicolet 380 FT-IR spectrometer. The samples for IR study were prepared as KBr pellets.

Solid-state nuclear magnetic resonance (NMR) spectroscopy
The 13 C CP/MAS NMR spectra of the COFs were recorded on an Agilent DD2 600 Solid NMR System with 4 mm zirconia rotors. The spinning rate is 9 kHz and the contact time is 3 ms.

Thermal gravimetric analysis (TGA)
Thermal gravimetric analysis was conducted on a Waters TGA Q500 by heating the samples from 25 to 1000 °C under nitrogen atmosphere with a heating rate of 10 °C/min.

Scanning electron microscopy (SEM)
Scanning electron microscopy was carried out using a FEI NOVA NANOSEM 450 scanning electron microscope. The samples were dispersed over the slices of silicon wafers adhered to flat copper platform sample holders and then coated with gold using a sputter coater (ambient temperature, 85 torr pressure in an nitrogen atmosphere, puttered for 30 s from a solid gold target at a current at 30 mA) before being submitted to SEM characterization.

Transmission electron microscopy (TEM)
Transmission electron microscopy was performed on a JEOL JEM-2100F instrument with an accelerating voltage of 200 kV. The samples were dispersed over the carbon coated copper grids.

Powder X-ray diffraction
Powder X-ray diffraction measurement was carried out with an PANalytical X'Pert

S4
Powder system using monochromated Cu/Kα(λ= 0.1542 nm). The samples were spread on the square recess of XRD sample holder as thin layers.

Nitrogen adsorption-desorption isotherm measurement
The measurements were carried out using Micromeritics ASAP 2020 or Quantanchrome autosorb iq systems. Before gas adsorption measurements, the as-synthesized COFs were activated by being immersed in anhydrous 1,4-dioxane for 6 h for 3 times. The solvent was decanted and the samples were dried under dynamic vacuum at 120 C for 4 h. The resulting samples were then activated by degassing at 200 o C for 4 h and used for gas adsorption measurements from 0 to 1 atm at 77 K. The Brunauer-Emmett-Teller (BET) method was utilized to calculate their specific surface areas.

Structural simulation and powder X-ray diffraction analysis
The models of the triple-pore and tetrad-pore COFs with eclipsed stacking (AA, space group P2) and staggered stacking (AB, space group P1) were established using Accelarys Materials Studio 7.0 software. The structural models were optimized by the Forcite module, which gave the total energies at the same time. The stimulated PXRD patterns were determined by the Reflex module. Pawley refinements of the experimental PXRD profiles were conducted by TOPAS software.
After three evacuation/nitrogen fill cycles, the mixture was stirred at room temperature for 24 h. Then, 20 mL water was added into the system, and the mixture was extracted with CH 2 Cl 2 . The organic layer was washed with brine and dried over sodium sulfate. After removal of the solvents, the crude product was purified by silica S8 gel chromatography using petroleum ether/ethyl acetate (8/1) as eluent to obtain 2d as a white solid (0.18 g, 58%

Synthesis of 3b
A 250 mL three-necked round-bottomed flask was charged with compound 3a (9.20 g, 43.19 mmol), CuI (250 mg, 1.31 mmol), Pd(PPh 3 ) 2 Cl 2 (910 mg, 1.30 mmol) in THF (100 mL) and diisopropylamine (45 mL) under nitrogen atmosphere and stirred for 5 min at room temperature. Trimethylsilylacetylene (11 mL, 77.81 mmol) was added dropwise to the mixture under high nitrogen flow and the reaction continued at room temperature for 3 h. The solvent was removed under vacuum and the crude product was purified by silica gel chromatography using a mixture of petroleum ether/CH 2 Cl 2 (4/1) as eluent to afford 3b as a light yellow solid (6.37 g, 64%). 1 H NMR (500 MHz,

Synthesis of 3c
A mixture of compound 3b (2.14 g, 9.29 mmol) and K 2 CO 3 (1.28 g, 9.26 mmol) was dissolved in methanol (60 mL). After three evacuation/nitrogen fill cycles, the mixture was stirred at room temperature for 3 h. Then, 50 mL water was added into the system, and the mixture was extracted with CH 2 Cl 2 . The organic layer was washed with brine and dried over sodium sulfate. After removal of the solvents, the crude product was purified by silica gel chromatography (petroleum ether/CH 2 Cl 2 = 1/1) to give 3c as a