Single crystal of a one-dimensional metallo-covalent organic framework

Although polymers have been studied for well over a century, there are few examples of covalently linked polymer crystals synthesised directly from solution. One-dimensional (1D) covalent polymers that are packed into a framework structure can be viewed as a 1D covalent organic framework (COF), but making a single crystal of this has been elusive. Herein, by combining labile metal coordination and dynamic covalent chemistry, we discover a strategy to synthesise single-crystal metallo-COFs under solvothermal conditions. The single-crystal structure is rigorously solved using single-crystal electron diffraction technique. The non-centrosymmetric metallo-COF allows second harmonic generation. Due to the presence of syntactic pendant amine groups along the polymer chains, the metallopolymer crystal can be further cross-linked into a crystalline woven network.


Instrumentation and characterization
Liquid 1 H and 13 C NMR spectra were recorded on a Bruker AVANCE I 300 MHz or a Bruker AVANCE I 500 MHz NMR spectrometer and chemical shifts (δ-scale) were reported in parts per million (ppm). Solid-state NMR experiments were performed on a Bruker Avance III HD 600 MHz wide-bore NMR spectrometer. The 13 C cross-polarization magic-angle spinning (CP/MAS) NMR spectra were recorded with a 4 mm double-resonance MAS probe. High-resolution mass spectrometry (HRMS) data were obtained on a Finnigan/MAT 95XL-T spectrometer. Fouriertransform infrared spectroscopy (FTIR) was carried out on a Bruker VERTEX 80v spectrometer in transmission mode under vacuum. Photoluminescence (PL) spectra were obtained with a laser confocal microscope (NT-MDT, NTEGR Spectra) with 532 nm (continuous wave (cw)) excitation lasers under ambient conditions. H, C and N contents were determined via elemental analysis performed on an Elementar vario MICRO cube. Thermogravimetric analysis (TGA) was carried out on a Discovery TGA within the temperature range of 30 °C to 800 °C with a heating rate of 10 °C/min. Powder X-ray diffraction (PXRD) data were collected on a Bruker D8 Focus Powder X-ray Diffractometer using Cu Kα radiation (40 kV, 40 mA) at room temperature. Synchrotron powder X-ray diffraction (SPXD) data were collected on the sample in a 0.5 mm capillary on the BL14B1 beamline (λ = 0.69005 Å) at the Shanghai Synchrotron Radiation Facility in Shanghai, China. The collected data were ranging from 2.0 to 20.0°, with 0.004° data binning.
Nitrogen sorption analysis was performed on Quantachrome Instruments Autosorb-iQ (Boynton Beach, Florida USA) with extra-high pure gases.
Scanning electron microscopy (SEM) images were obtained with a JEOL JSM-6701F microscope. A JEM-2100Plus (LaB6 filament) operated at 200 kV was used for high resolution transmission electron microscopy (HRTEM) imaging. Single-crystal electron diffraction (SCED) data were collected using a JEOL JEM2100 TEM (LaB6 filament) operating at 200 kV. The reciprocal space reconstruction was carried out using the RED software, and the reflection intensity extraction was conducted by the program XDS 2,3 .
For the second harmonic generation (SHG) characterization, a Yb:KGW femtosecond PHAROS laser system was used as the pump of a collinear optical parametric amplifier ORPHEUS with a LYRA wavelength extension option (Light Conversion Ltd, pulse duration of 150 fs, tunable excitation wavelength of 310 nm -2700 nm, and repetition rate of 100 Hz -100 kHz). The laser beam was focused on the COF samples with a 100x (NA = 0.9) air objective from Nikon (1 μm 2 spot size). The nonlinear emission was collected in a back-scattering configuration via the same objective and detected by a spectrograph (PI Acton SP2300 by Princeton Instruments) for spectral measurements. The power of the collected SHG emission was measured using a calibrated silicon photodetector (Newport), while the excitation power was measured using a germanium photodetector (Thorlabs). For polarization dependent SHG measurements, a half-wave plate was used to vary the orientation of the linearly polarized laser pulses.
Atomic force microscopy (AFM) nano-indentation experiments were performed using the Dimension Icon instrument operating under the indentation mode, equipped with a Bruker Tap525A probe. Its spring constant and contact sensitivity have been calibrated as 156.760 N/m and 83.01 nm/volt, respectively.

4,4'-(1,10-phenanthroline-2,9-diyl)dianiline (I).
To a round bottom flask were added S2 (562.3 mg, 1.0 mmol) and 4 M HCl/1,4-dioxane (10 mL). The mixture was stirred at room temperature for 2 h, and then filtered to get a dark red solid. The solid was dissolved in deionized water (50 mL), and then neutralized with KOH solution. The resulting suspension was extracted with CH2Cl2 (3 × 40 mL), the combined organic phase was washed with brine, dried over Na2SO4, and concentrated to afford the product I (335 mg, 92% yield) as a light yellow solid. 1 150.4, 145.2, 136.5, 128.3, 126.7, 126.3, 124.9, 118.3, 113.8 Synthesis of 2,9-bis(4-(dimethoxymethyl)phenyl)-1,10-phenanthroline (II) 2,9-bis(4-(dimethoxymethyl)phenyl)-1,10-phenanthroline (II). To a two-necked round bottom flask were added S1 (676 mg, 2.0 mmol), 2-(4-(dimethoxymethyl)phenyl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (1.45 g, 5.2 mmol), K2CO3 (828 mg, 6.0 mmol), Pd(PPh3)4 (231 mg, 0.2 mmol), 1,4-dioxane (32 mL), and H2O (8 mL). The reaction mixture was deaerated by slowly bubbling N2 for 30 min, and then heated to reflux at 100 °C for 8 h. After cooling to room temperature, the solvent was removed under reduced pressure. The residue was treated with H2O (60 mL), extracted with CH2Cl2 (4 × 40 mL), the combined organic phase was washed with brine, dried over Na2SO4, and concentrated. The crude product was purified by silica gel column chromatography using hexane/ethyl acetate = 3/1 as the eluent to give the product II (758 mg, 79% yield) as a white solid. 1  Synthesis of the conjugated polymer I (11.6 mg, 0.032 mmol) and II (15.4 mg, 0.032 mmol) were weighed into a 10 mL Schlenk tube. To the mixture were added 1-butanol (0.1 mL), 1,2-dichlorobenzene (0.9 mL) and 6 M aqueous acetic (0.1 mL). The Schlenk tube was frozen in a liquid nitrogen bath, evacuated to an internal pressure of 0 mbar and sealed. After warming to room temperature, the Schlenk tube was placed into an oven and heated at 120 °C for 3 days yielding a yellow-orange solid at the bottom of the tube. The Schlenk tube was opened when the mixture was still warm and the solid was transferred into a vial, separated by centrifugation, washed copiously with DMSO, THF, and EtOH. After drying at room temperature, the resulting solid was dried under vacuum at 100 °C for 12 h to obtain the conjugated polymer. Figure 1 | Experimental PXRD patterns of the conjugated polymer (black) and mCOF-Ag (red). The synthesised conjugated polymer has a semi-crystalline feature, which is attributed to the π-π stacking in the rigid and conjugated structure of phenanthroline backbones; besides, the reversibility in imine bond formation can impart self-correction during the hydrothermal conditions.  ) and N−H bending vibration (pink band) decrease, indicating the consumption of amine groups, while the peak at 1600 cm -1 (green band) due to newly formed imine bond increases gradually. The red curve corresponds to the control experiment without glyoxal, which was heated in 1,4-dioxane at 70 °C for five days. All KBr pellets were dried at 100 °C before testing to eliminate background signals due to water.

Supplementary Figure 19 | SEM image of wCOF-Ag.
Supplementary Figure 20 | 3D reciprocal lattice of wCOF-Ag reconstructed from the SCED data. As wCOF-Ag was synthesized via post-synthesis, the local bonding is not very order. It is difficult to directly observe the newly formed bonds via SCED data.