Screw dislocation-induced pyramidal crystallization of dendron-like macromolecules featuring asymmetric geometry

We report herein that dendron-shaped macromolecules ABn crystallize into well-ordered pyramid-like structures from mixed solvents, instead of spherical motifs with curved structures, as found in the bulk. The design of the asymmetric molecular architecture and the choice of mixed solvents are applied as strategies to manipulate the crystallization process. In mixed solvents, the solvent selection for the Janus macromolecule and the existence of dominant crystalline clusters contribute to the formation of flat nanosheets. Whereas during solvent evaporation, the bulkiness of the asymmetric macromolecules easily creates defects within 2D nanosheets which lead to their spiral growth through screw dislocation. The size of the nanosheets and the growth into 2D nanosheets or 3D pyramidal structures can be regulated by the solvent ratio and solvent compositions. Moreover, macromolecules of higher asymmetry generate polycrystals of lower orderliness, probably due to higher localized stress.


Synthetic procedures
The ABn type molecules were constructed via esterification reactions, copper catalyzed azide−alkyne cycloaddition (CuAAC) "click" chemistry, and thiol-ene reaction following the similar routes reported previously. 3,4 As an example, VPOSS-BPOSS4 was synthesized via "click" chemistry using 1 equiv of VPOSS-2Yne and 2 equiv of 2BPOSS-N3 as shown in Scheme 1. Afterwards, VPOSS in the VPOSS-BPOSS4 sample can be further modified to be hydrophilic through thiol-ene reaction.
The detailed synthesis and characterization information were provided in ACS Central Science 2017, 3, 860-867. Similarly, VPOSS-BPOSS6 was synthesized via "clicking" 1 equiv of VPOSS-3N3 and 3 equiv of 2BPOSS-Yne as shown in Scheme 2. VPOSS in the VPOSS-BPOSS6 sample can be further modified to be hydrophilic via thiol-ene reaction. The detailed synthesis and characterization information were provided as following.

Synthesis of compound BPOSS-COOH
In an uncapped 20ml vial equipped with a stirring bar, allylIsobutyl POSS (BPOSS-vinyl, 1.0 g, 1.2 mmol), thioglycolic acid (290 mg, 3.2 mmol) and photoinitiator Irgacure 2959 (3 mg) were added, followed by the addition of 10ml THF to completely dissolved the solids. The mixture was applied to 365nm UV light for 15 min. Then THF was evaporated under vacuum. The residue was purified by chromatography on silica gel using CH2Cl2:EA=20:1 as the eluent to acquire the product (Yield: 95%). 1

Synthesis of compound 2BPOSS-Yne
In a 100 mL round-bottom flask with a magnetic stirring bar, Yne-2OH (30.2 mg, 0.18 mmol), BPOSS-COOH (500 mg, 0.53mmol) and DMAP (4.2 mg, 0.03 mmol) were fully dissolved in 20 mL freshly dried THF. The mixture was capped by a rubber septum, cooled to 0 o C and stirred at that temperature for 10 min, and then DIPC (67 mg, 0.53 mmol) was added dropwise via syringe. The mixture was warm up to room temperature and stirred for another 24 h. After that, the white precipitate was filtered off, and the filtrate was evaporated under vacuum. The residue was purified by flash column chromatography on the silica gel with CH2Cl2/EA (v/v = 80/1) as eluent to afford final product (Yield: 80%). 1

Synthesis of compound VPOSS-BPOSS6
VPOSS-3N3 (30 mg, 0.03 mmol), 2BPOSS-Yne (266 mg, 0.13 mmol), CuBr (5mg, 0.04 mmol) were totally dissolved in 20 ml dry THF in a Schlenk flask and degassed for three times. Then PMDETA (15mg, 0.09 mmol) was added. The system was degassed again and stirred for 12 h. After the reaction was completed, the solution was added into silica gel column. THF was applied to elute off the crude product. After solvent removal, silica gel column chromatography using CH2Cl2 as the eluent was applied to purify the residue and acquire the product (Yield: 92%). 1

Sample preparation and characterizations:
Sample preparation: As a typical sample preparation, ABn powder was dissolved in THF or 1,4-dioxane at the concentration of 5 mg/mL as stock solution. It was filtered and diluted to desired concentrations before usage. A selective filtered solvent (acetonitrile or DI H2O) was titrated into the solution at the rate of 30 μL/h to trigger the crystallization/assembly. The initial volume of solution varied from 180 to 420 µL, so as to keep the solutions at the same final concentration and volume (600 uL). The solute concentrations mentioned in the text were the final concentration after solvent mixing.
Characterizations: Solution state Nuclear Magnetic Resonance (NMR) Spectroscopy: 1 H and 13 C NMR spectra were obtained in CDCl3 (Sigma-Aldrich, 99.8% D) solvents utilizing Varian Mercury 300 MHz NMR or 500 MHz NMR spectrometer. 1 H NMR spectra were referenced to the residual proton impurities in CDCl3 at δ 7.27 ppm and 13 C NMR spectra were referenced to 13 CDCl3 at δ 77.00 ppm.

Dynamic Light Scattering (DLS):
Experiments were performed on a commercial Brookhaven laser scattering spectrometer that was equipped with a 532 nm laser. An intensity-intensity BI-9000AT correlator was used to get the correlation function based on the particle movement. The diffusion coefficient D could be obtained by Γ=Dq 2 , which Γ was calculated by CONTIN method. The hydrodynamic radius (Rh) can be calculated according to the Stokes-Einstein equation: Rh=kbT/6πηD in which kb is the Boltzmann constant and η is the viscosity of the solution.

Transmission Electron Microscopy (TEM):
The TEM images were taken by a JEOL JEM-1230 electron microscope operated at 120 kV. The samples were prepared by dropping 8 μL of solution onto a carbon-coated copper grid, and the sample was allowed to dry at room temperature overnight. Freeze-dried sample was prepared by quenching the sample in liquid nitrogen for 30 mins and then pump in vacuum under the liquid nitrogen environment for 2 days. 5

Atomic Force Microscopy (AFM):
The AFM experiments were conducted on a Dimension Icon Atomic Force Microscopy (Bruker). Samples were prepared by drop-casting 10 μL onto the acetone rinsed Si wafer substrates. After solvent evaporation under ambient conditions overnight, images were scanned in the tapping mode and analyzed by NanoScope Analysis software. Grazing-Incidence Small-angle X-ray Scattering (GISAXS). GISAXS experiments were performed at the 12-ID-B beamline of the Advanced Photon Source (APS) at Argonne National Laboratory. The operating conditions were chosen as a wavelength of 1.33 Å and a sample-to-detector distance of 3.63 m.

Attenuated Total Reflectance Fourier Transform Infrared Spectroscopy (ATR-FTIR).
A THERMO NICOLET Is-50R FTIR SPECTROMETER equipped with an ATR setup was used to investigate the IR spectrum. 50 μL of the samples were dropped onto the ATR window and tested immediately for 10 times to obtain an average IR spectrum. X-ray Diffraction (XRD). The crystal structure of the assembled nanosheets was characterized by a Rigaku Ultima IV X-ray diffractometer with Cu Kα radiation (λ= 0.1542 nm). The scanning 2θ angle ranged between 3° and 28° with a step interval of 0.01°.      In the grazing incidence scattering geometry, αf and 2θf are the exit angles of X-ray beam along the out-of-plane scattering normal to the sample surface and along the in-plane scattering parallel to the sample surface, respectively. The scattering vector qz is defined by qz = (4π/λ) sin αf, where λ is the wavelength. Two peaks were observed from the GISAXS pattern, with the primary peak (q*) at qz=0.15 Å -1 and a higher order peak at qz=0.30 Å -1 . The ratio of q values (q/q*) are 1, 2, which is consistent with that of the Lamella structure.