Ultra-fast single-crystal polymerization of large-sized covalent organic frameworks

In principle, polymerization tends to produce amorphous or poorly crystalline materials. Efficiently producing high-quality single crystals by polymerization in solvent remains as an unsolved issue in chemistry, especially for covalent organic frameworks (COFs) with highly complex structures. To produce μm-sized single crystals, the growth time is prolonged to >15 days, far away from the requirements in practical applications. Here, we find supercritical CO2 (sc-CO2) accelerates single-crystal polymerization by 10,000,000 folds, and produces two-dimensional (2D) COF single crystals with size up to 0.2 mm within 2~5 min. Although it is the fastest single-crystal polymerization, the growth in sc-CO2 leads to not only the largest crystal size of 2D COFs, but also higher quality with improved photoconductivity performance. This work overcomes traditional concept on low efficiency of single-crystal polymerization, and holds great promise for future applications owing to its efficiency, industrial compatibility, environmental friendliness and universality for different crystalline structures and linkage bonds.

Crystalline structure.Transmission electron microscope (TEM) images and selected area electron diffraction (SAED) patterns (Fig. 2a-j, Supplementary Figs. 8, 9, 10) reveal high yield and highly crystalline features of the rod-like sc-COFs.The lattice fringes are clearly observed along the length direction with an inter-lattice separation of 2.0 nm (sc-COF TB-BA ), 2.4 nm (sc-COF 5 ), 1.8 or 2.3 nm (sc-COF TP-Py ), which correspond to COF TB-BA (100), COF 5 (100), COF TP-Py (020) or COF TP-Py (110) planes, respectively, indicating the 2D COF layers are stacked along the length direction via [001].Cross-section TEM images (Fig. 2b) show clear fringes of sc-COF TB- BA with corresponding six-fold-symmetric fast Fourier transform (FFT) patterns, thus the micro-pores of 2D sc-COFs are oriented parallel to the rods.Although some rods are curved owing to the flexibility, all locations of the rod are well crystalline, and the microporous channels with open ends are continuous over the entire rod region without domain boundaries or interfaces (Fig. 2g-j, Supplementary Figs.11, 12).The same set of SAED patterns are collected from different locations, indicating the single-crystalline nature of the 2D sc-COF rods (Supplementary Fig. 13).The sc-COF TP-Py grown for 2 min also has the same feature with highquality single-crystalline structure (Supplementary Fig. 14).In contrast, all os-COFs are disorderedly polycrystalline with nm-scale domains (Fig. 2k, l, Supplementary Fig. 15).Cross-polarized OM images (Fig. 2m, n, Supplementary Fig. 16) confirm single crystallinity of the sc-COFs, as uniform polarized light extinction is observed over the entire length of the rods (Supplementary Video 1), different from os-COFs (Supplementary Fig. 17, Supplementary Video 2).Some sc-COF TP-Py crystals are larger than 0.1 mm, up to 0.20 mm, which are the largest 2D COF single crystals reported till now (Fig. 1b).

Growth rate and crystal quality.
To study the growth rate, 2D COFs are grown for different times.SEM images (Supplementary Figs. 7, 18) show rod-like crystals with sizes up to several tens (sc-COF TP-Py ) of micrometers are obtained within 2-5 min, with similar morphologies and lengths to sc-COFs grown for 2 h and 12 h (Supplementary Fig. 19), indicating that most single-crystal polymerization finishes within 5 min owing to the consuming monomer.The os-COFs have distinct morphologies (Supplementary Fig. 20), which are disorderedly aggregated with small crystallites, even if the growth lasts for 3 days.The crystal growth rate of sc-COFs reaches 40 μm min −1 in sc-CO 2 , up to 7 orders of magnitude faster compared with that of os-COFs (Fig. 1b).
The crystal quality is measured by PXRD.High-intensity diffraction peaks of sc-COF TP-Py appear after 2-5 min (Fig. 3a), which are similar to PXRD patterns of sc-COF TP-Py grown for 8-12 h and os-COF TP-Py grown for 3 days, indicating high-quality COFs are obtained in sc-CO 2 within minutes.Weak diffraction peaks of the solvothermal samples appear only after 2 h, indicating the crystallization in organic solvent is much slower.The full width at half maximum (FWHM) of the diffraction peak (Fig. 3b) is related to the crystal quality 29 .In sc-CO 2 , FWHM of the sc-COF TP-Py (110) peak reaches 0.39°after 5 min, decreases slightly to 0.35°after 2 h, and maintains around 0.34°until 8 h.However, in organic solvent, FWHM of the os-COF TP-Py (110) peak is only 0.62°after 4 h.A time up to 1-3 days is required to obtain high-quality COFs with FWHM around 0.35°.Moreover, we use a mixture solvent of sc-CO 2 and n-BuOH (Fig. 3c).Higher sc-CO 2 content leads to a larger growth rate and smaller FWHM of COF TP-Py (110) peak, indicating that the solvent medium plays a pivotal role in not only ultra-fast crystallization but also the formation of high-quality crystalline structures.
Single-crystal polymerization mechanism.Ultra-fast singlecrystal polymerization is related to the special properties of sc-CO 2 .In organic solvent, the COF crystals are obtained by initial polymerization and nucleation followed by a subsequent crystalline growth or transformation process 3,5,6,[11][12][13] .Similarly, the growth of 2D COFs in sc-CO 2 involves extension of the lattice in the lateral [100] direction by covalent bond formation as well as in the vertical [001] direction by non-covalent stacking (Fig. 4) 12,13 .The assembly of monomers into the ordered framework by polymerization normally encounters many mistakes, such as overlays, defect sites, ill-defined agglomerations, interlayer cross-links, etc 4 .To obtain an ordered framework, a time-consuming reversible selfcorrection is required along with the polymerization for error-checking and proof-reading 4,[6][7][8][9][10][11][12][13] .Considering the large surface tension (20-50 dyn cm −1 ) and viscosity (0.3-4 cp, normally) of organic solvent, the transport of precursors (including monomers and oligomers) and byproducts to or from the reaction sites is restricted by the slow diffusion rate in the solvent as well as in the micro-pores or interstices of COFs.As a result, the precursor concentration is low while the byproduct concentration is high near the reaction sites, which drastically slows down the reversible reaction, decelerates the framework formation, and brings a fundamental limitation in ultra-fast polymerization of high-quality single crystals.Supercritical fluid technologies have been generally utilized in pharmaceutical, food, chemical manufacturing, and industries since 1960s 30 , which realize a green, scalable and efficient chemical or biochemical synthesis and processing in a special medium distinct from normal gases and liquids [31][32][33] .Considering the inertness, sc-CO 2 functions as an inert solvent in the reactions 34 , and the reaction thermodynamics is similar to that in liquid mediums.The difference is that the supercritical fluid [001] [001] [001]  combines gas-like viscosity and surface tension with liquid-like density and solvating properties 35 .The viscosity of sc-CO 2 is about 0.02 cp (80 °C, 8 MPa), while the surface tension is near zero 36 .The diffusion coefficient is 1-2 orders of magnitude higher than that in organic solvent 37 .As a result, sc-CO 2 allows increased penetrating ability and much faster diffusion of precursors and by-products in the medium and in the micropores or interstices of COFs 38 .It is expected that the precursors can be sufficiently supplied to the reaction sites while the byproducts be fast removed from there.Thus, sc-CO 2 greatly influences the reaction kinetics and accelerates the polymerization along with the error-checking and proof-reading process, resulting in ultra-fast nucleation and extension of lateral covalent frameworks.As an evidence, we grow 2D COFs in the nanometer-scale interstices of highly ordered 3D monolith made of 1 μm-sized polystyrene spheres (PSs).After removing PSs, a free-standing 3D-ordered structure of sc-COFs is obtained (Supplementary Figs.21, 22).As a comparison, in 1,4-dioxane and mesitylene mixture, no COFs can grow in the interstices of the 3D monolith (Supplementary Fig. 23).This result indicates high diffusion and penetrating ability in sc-CO 2 .To obtain an eclipsed non-covalent stacking structure of 2D COFs, a new layer forms via oligomer stacking and subsequent lateral template polymerization on top of an old layer 13 .The solubility of supercritical fluid is intermediate between that of a gas and a liquid 33 .The moderate solubility of sc-CO 2 allows nucleation and efficient stacking of oligomers vertically on old layers (Supplementary Fig. 24), resulting in the growth of the crystal along [001] direction.
To prove the mechanism, control experiments tune solvent property by adding n-BuOH in sc-CO 2 .With higher n-BuOH content, the mixture solvent has lower diffusion ability and larger solubility.As a result, the products (1 h) have reduced crystal length (Supplementary Figs.25-27) and increasing FWHM of COF TP-Py (110) peak (Fig. 3c), when n-BuOH content increases from 0.25% (vol.) to 2% (vol.).Finally, the products become amorphous without any diffraction peaks when the content is >2% (vol.).This result indicates that the solvent medium plays a pivotal role in not only ultra-fast growth but also the formation of high-quality crystalline structures.Therefore, fast revisable reaction as well as efficient epitaxial nucleation and growth of the new layers in sc-CO 2 realize minutes-growth of COFs into large-sized rod-like single crystals.
Polarized photoluminescence.We measure the photoluminescence (PL) emission from sc-COF TP-Py and os-COF TP-Py (Fig. 5a).Angle-dependent emission only occurs in the case of sc-COF TP-Py (5 min).When the polarization angle (θ) changes from 0°to 180°(the long axis direction is set to 0°), the PL emission (I PL ) from the crystal (Fig. 5b) gradually becomes stronger (0°-90°), and then becomes weaker (90°-180°).The dichroic ratio (DR) (defined as I PL maximum/I PL minimum) is as high as 43.5, and the polarization ratio (ρ) 39 , defined as (DR − 1)/(DR + 1), is 0.96 (Fig. 5c).In the case of os-COF TP-Py (3 days) or COFs produced by other methods, the ρ is only ~0 (Fig. 5b).It is the first report of polarized PL emission from COFs.The ρ is among the highest values of organic or polymeric crystalline materials 40 , which verifies not only single-crystalline nature but also ultrahigh crystalline quality of sc-COF TP-Py (see Supplementary Note 1) 41 .
FP-TRMC photoconductivity.In our measurement, the spatial size of the oscillating motion of charge carriers is estimated within several nanometers at a maximum 42 .Thus, the grain and/ or domain boundary will not affect the ϕΣμ obtained by FP-TRMC.The FP-TRMC result is not related to the grain size but the actual quality (or crystallinity) of the as-grown COFs 43 .
We measure the samples by flash-photolysis time-resolved microwave conductivity (FP-TRMC) under N 2 atmosphere.In the measurement, charge carriers are generated upon photo excitation and the local motion of the carriers can be probed via dielectric loss of the low-power microwave probes.The observable region of charge carriers can be extended to a very short time region, which allows us to obtain the intrinsic chargecarrier mobility within several nanometers after laser pulse irradiation under a rapidly oscillating electric field 44,45 .This technique is devoid of grain boundaries, impurities or crystal size, which reflects long-range (>10 nm) carrier transport along the ordered structure in crystalline domains 46 .Thus, the FP-TRMC photoconductivity (ϕΣμ), where ϕ and Σμ represent the photocarrier generation yield and the sum of the photogenerated charge carrier mobilities, respectively 46 , cannot be affected by the length of the crystals, but gives an estimate of the actual crystallinity in nanometer-scale level (see Supplementary Note 2).After 5 min growth in sc-CO 2 , the photoconductivity transients of sc-COF TP-Py show a rapid rise within the time constant of the present set of apparatus in current, with a maximum ϕΣμ value of 5.8 × 10 −6 cm 2 V −1 s −1 at an excitation photon density of 1.8 × 10 16 photons cm −2 .ϕΣμ reaches ~6.8 × 10 −6 cm 2 V −1 s −1 after growth in sc-CO 2 for 20 min to 12 h (Fig. 6).As a comparison, ϕΣμ of os-COF TP-Py cannot reach the same value, even after growth for 3 days.Therefore, ultra-fast polymerization in sc-CO 2 not only obtains large-sized single crystals, but also results in comparable or higher crystal quality compared with os-COFs grown for days.

Discussion
Practical application of COFs received limited success due to challenges in synthesizing high-quality products rapidly, industrycompatibly and environmentally friendly.Supercritically solvothermal polymerization reduces the time needed for growing large-sized 2D COF single crystals from months to minutes, which is hard or even impossible previously.Despite ultra-fast growth rate, it increases the crystal size up to 0.2 mm (sc-COF TP-Py ), 2 orders of magnitude larger than the largest single crystals of 2D COFs reported previously 8 , and also improves the quality and photoconductivity performance.Sc-COFs can be further purified and activated in sc-CO 2 (Supplementary Fig. 28).Although the solubility of sc-CO 2 isn't so strong as organic solvents, which results in a few of uncleaned monomers or oligomers in the nanochannels of 2D COFs, it is impressive that the surface area still reaches 1710 m 2 g −1 , comparable to that of os-COF TP-Py (1947 m 2 g −1 ).Thus, the entire procedure is environmentally friendly without using amounts of organic solvents.Considering these as well as wide usage of sc-CO 2 in industry, it solves all the above challenges with universality for different linkages of 2D COFs, which greatly shortens the gap between synthesis and practical applications.Moreover, obstacle in synthesis hampers researches and applications of 2D COF single crystals.This work solves the problem and finds that the high quality and singlecrystalline nature of sc-COFs give rise to highly polarized PL, showing that the 2D COF single crystals produced in sc-CO 2 have great potential in special application fields which previous polycrystalline samples haven't.It is worth noting that there is a general belief that slow crystallization is necessary to grow high-quality single crystals 1 .In contrary, this result indicates ultra-fast polymerization in sc-CO 2 can produce single crystals with both larger size and higher quality, compared with slow crystallization, overcoming the contradiction between growth time and product quality.This work provides a new understanding of ultra-fast single-crystal polymerization in supercritical liquid.Besides boronate ester and imine 2D COFs, it also holds great promise in the efficient and precise construction of various COFs and other covalent crystalline materials, opening up other opportunities for not only fundamental research but also practical application of these materials and their single crystals.
(2) os-COF TB-BA : A mixture of 1,3,5-triformylbenzene (TB, 9.7 mg, 0.06 mmol) and 4,4′-biphenyldiamine (BA, 16.6 mg, 0.09 mmol) was dissolved in 0.5 mL mesitylene and 0.5 mL dioxane, and transferred to a reaction tube.After sonication for 5 min at room temperature, the mixture was added 0.1 mL 6 M aqueous acetic acid.Then, the tube was degassed through three freeze-pump-thaw cycles, sealed under vacuum and heated at 120 °C for 3 days.After reaction, the precipitate was collected by filtration, washed by acetone, tetrahydrofuran and dried in vacuum oven at 100 °C27 .
Characterization.The samples were measured by PXRD (PXRD, Bruker, D8).All the COFs were recorded in the 2θ range between 2˚and 40˚.The radiation was Cu Kα (λ = 1.54 Å), and the data collection was carried out using an Aluminum holder at a scan speed of 1°min −1 and a step size of 0.02˚.Fourier transform infrared (FT-IR) spectra were collected using a thermofisher Nicolet 6700 spectrometer.
TEM images were collected using FEI Tecnai G2 F20 S-Twin (acceleration voltage: 200 kV).SEM images were collected using Zeiss Gemini SEM500.The SEM samples were prepared by evaporating a drop of alcohol with COFs on a clean Si/SiO 2 wafer.OM and cross-polarized OM images were collected using a polarized OM (Leica DM2500P).
The N 2 adsorption-desorption experiments were conducted on an Autosorb iQ 2 (Anton Paar) surface area analyser.The os-COF TP-Py sample was washed in THF by Soxhlet's extraction for 3 days, dried in vacuum for 12 h, and degassed at 120 °C for 12 h before the measurement.The sc-COF TP-Py sample was purified by liquid CO 2 for 5 times, kept in sc-CO 2 (40 °C 8 MPa) for one hour (Samdri®-PVT-3D, Tousimis) and degassed at 0.13 MPa min −1 before the measurements 49 .N 2 isotherms were recorded at 77 K by using ultra-high purity N2 (99.999% purity).The surface area was determined using Brunauer-Emmett-Teller (BET) adsorption model.
Growth and characterization of 3D-ordered sc-COFs.After being treated with aqueous hydrogen peroxide and concentrated sulfuric acid (vol./vol.= 3:1), Si/SiO 2 wafer was cleaned with isopropanol, acetone, and DI water, respectively.Then, the wafer was blown dry by a N 2 gun and suspended vertically in 10% wt.aqueous polystyrene (PS) microspheres dispersion (Thermo Corp.).The dispersion was placed in an oven at 40 °C overnight.By controlling the evaporation rate, highly ordered 3D monoliths of PS were obtained on the SiO 2 /Si wafer 50 .
To grow the sc-COF TB-BA in the highly ordered 3D monolith of PS, 16 mg 1,3,5benzenetricarboxaldehyde and 27.6 mg 4,4′-diaminobiphenyl were mixed within 0.1 mL n-BuOH and 0.1 mL acetic acid, transferred into stainless steel reactor, then a piece of PS colloid crystal was immerged into the solution.Then the reactor was heated to 80 °C and charged with 8 MPa CO 2 .After a reaction for 12 h, the reactor was cooled to room temperature and slowly depressurized at a rate of 1-2 MPa min −1 . The wafer was washed with THF and acetone, and characterized by SEM.
Angle-dependent photoluminescence measurement.The angle-dependent photoluminescence (PL) spectra were measured by a Horiba iHR550 spectrometer equipped with a silicon CCD detector array 51 .A 532 nm laser (spot size ∼1 μm, laser power of 70 μW) was used as the excitation source.The incident laser was focused by a Nikon Eclipse Ti microscope with a 50 × (NA = 0.7) objective.In the measurements, the polarization direction of the incident laser light was parallel to the long axis of COF crystals.A linear polarizer was placed in the collecting optical path to detect the angle dependence of the PL emission.The polarized PL images were collected using Olympus BX-51-P fluorescence microscope at different polarization angles (by rotating the stage).We used a filter set which provided a narrow wavelength range (450 to 490 nm) for excitation, and allowed that emitted light at wavelengths above 515 nm passed to the detector.All optical experiments were performed in ambient environment and at room temperature.
Flash photolysis time-resolved microwave conductivity test.Flash photolysis time-resolved microwave conductivity (FP-TRMC) was performed by in situ TRMC system 52 .A high degree of sensitivity in the conductivity measurement was realized in a resonant cavity.The resonant frequency and power of microwave probes were set at ~9.1 GHz and 3 mW, respectively.Charge carriers were generated upon direct excitation of solid samples with third-harmonic generation (λ = 355 nm) from a Spectra-Physics model INDI-HG Nd: YAG laser (Continuum Inc., Surelite II, 5-8 ns pulse duration, 10 Hz), and the conductivity transients were recorded by a digital oscilloscope (Tektronix, TDS 3032B).All the experiments were performed at 25 °C in nitrogen.

Fig. 1
Fig. 1 Supercritically solvothermal polymerization.a Schematic of the COF TP-Py synthesis.b Crystal size and growth rate in this work (sc-COFs and os-COFs) compared with some best-reported results.The red star indicates the best result in sc-CO 2 .c, d SEM images of sc-COF TP-Py (5 min) and os-COF TP-Py (3 days).e PXRD patterns of sc-COFs (5 or 90 min), os-COFs (3 days), os-COFs (30 or 90 min), and the simulated patterns.The characteristic peaks are marked by stars.The scale bars are 10 μm.

Fig. 2
Fig. 2 Characterization of the crystalline structure.a Top and side view of COF TB-BA .b Cross-section TEM image of sc-COF TB-BA .The inset is the FFT pattern obtained from the dashed square.c-g TEM images of sc-COF TB-BA (c), sc-COF 5 (d), and sc-COF TP-Py (e-g).The insets show the corresponding SAED patterns.h-j High-resolution TEM images of a sc-COF TP-Py crystal taken from the regions marked by red circles in (g).k, l TEM and enlarged TEM images of os-COF TP-Py .m, n OM and cross-polarized OM images of a 0.20 mm long sc-COF TP-Py crystal (m) and sc-COF TP-Py crystals (n).The scale bars are 20 nm in (b-d), (h-j), 50 nm in (f, l), 200 nm in (k), 2 μm in (g), 10 μm in (e), 50 μm in (m) and (n).

Fig. 3
Fig. 3 Ultra-fast crystal growth of sc-COFs.a PXRD of the samples of os-COF TP-Py and sc-COF TP-Py grown for different times.b FWHM of sc-COF TP-Py (110) and os-COF TP-Py (110) peaks as a function of growth time.c FWHM of the COF TP-Py (110) peak and average crystal sizes of the products grown for 1 h in a mixture solvent of n-BuOH and sc-CO 2 .

Fig. 4 Fig. 5
Fig.4Single-crystal polymerization mechanism.Schematic illustration of polymerization in organic solvent or sc-CO 2 .In sc-CO 2 , the ultra-fast diffusion of monomers, oligomers, and byproducts in medium and micro-pores accelerates polymerization as well as error-checking and proof-reading, while the suitable solubility accelerates vertically epitaxial growth, resulting in minutes-growth of single crystals.

Fig. 6 FP
Fig. 6 FP-TRMC measurement.a Photoconductivity (ϕΣμ) of sc-COF TP-Py and os-COF TP-Py as a function of the growth time.b FP-TRMC transients of the sc-COF TP-Py grown for 5 min, 20 min and 12 h, respectively.