Oriented Two‐Dimensional Porous Organic Cage Crystals

Abstract The formation of two‐dimensional (2D) oriented porous organic cage crystals (consisting of imine‐based tetrahedral molecules) on various substrates (such as silicon wafers and glass) by solution‐processing is reported. Insight into the crystallinity, preferred orientation, and cage crystal growth was obtained by experimental and computational techniques. For the first time, structural defects in porous molecular materials were observed directly and the defect concentration could be correlated with crystal growth rate. These oriented crystals suggest potential for future applications, such as solution‐processable molecular crystalline 2D membranes for molecular separations.

Porous molecular materials are attracting much interest because they can be rationally designed to achieve functions such as selectivity,processability,and stability. [1] Forexample, we have developed as eries of porous organic cages (POCs) that can be used as synthetically prefabricated molecular pores for the construction of porous materials. [2] Thesynthetic versatility of POCs enables awide range of functionality and tailored properties.T he porosity of crystalline cage solids arises from both intrinsic pores within the molecules themselves and extrinsic pores between the cages.T he packing of discrete cage molecules is dictated by weak van der Waals forces that give scope for dynamic motion, flexibility,a nd response to stimuli. [3] Also,u nlike covalent organic frameworks (COFs), POCs are crystallized without any bondforming reactions;h ence,w hile single-crystalline COFs are very rare, [4] it is relatively easy to grow high-quality singlecrystal POCs.P OCs have been explored in various applications such as sensing, [5] gas storage, [2] molecular separations (for example,x ylene isomers, [6] noble gases, [7] and chiral molecules [8] ), and proton conductivity. [9] As discrete,s oluble molecules,P OCs can be processed in organic solvents in aw ay that cannot be achieved with insoluble porous frameworks.F or example,m odular mix and match assembly strategies have been used to form binary and ternary cocrystals, [10] and cage crystals can be incorporated into polymers to form composite membranes. [11] Thef abrication of functional materials into thin films, membranes,a nd oriented crystals on substrates is of importance for applications in sensors,catalysts,electronic devices, and electrodes for fuel cells. [12] Recently,amorphous cage thin films and membranes were fabricated on various substrates by spin coating. [13] Uniform and pinhole-free cage membranes were obtained and demonstrated molecular-sieving properties.H owever,i tr emains as ignificant challenge to control crystallinity,o rientation, and surface nanostructures of cage thin films;f or example,t hese amorphous spin-coated films showed dramatic ageing effects over time.A ss uch, the preparation of crystalline POC films is ahigh-value target for applications such as gas separation. Va rious studies have been carried out on the assembly of well-organized 2D molecular systems,s uch as growth and alignment of organic semiconductor thin films. [14] Likewise,porous frameworks such as zeolites and metal-organic frameworks (MOFs) have been fabricated into thin films and membranes. [15] However,t here are fewer studies of crystalline oriented MOFs or zeolite films. [16] Most of the films are polycrystalline,a nd wellcontrolled growth and orientation is challenging.Porous thinfilm materials with high crystallinity and preferred orientation should present distinct adsorption, separation kinetics, and performance characteristics compared with bulk powders or amorphous films.
Orientation is not the only factor that affects guest diffusion in porous solids:d efect engineering in porous frameworks has emerged as an active research field because defects can play av ital role in determining material performance such as sorption capacity,c atalytic activity,s tability, and mechanical strength. [17] However,our ability to characterize,u nderstand, and control defects in porous solids is limited. [18] Thep resence of defects in MOFs may explain oft-noted discrepancies between properties derived from ideal crystal structures and experimental measurements. POCs are interesting systems for investigating defects because,u nlike MOFs and COFs,t he synthesis and the crystallization steps can be separated. Thef ormation of defects such as point vacancies is thermodynamically unfavorable, [19] and it would be expected to be rare in molecular crystals.However,there is indirect evidence for the existence of defects in POCs; [20] for example,w hen cage molecules were crystallized both slowly and rapidly,the rapidly crystallized sample exhibited substantially higher surface areas,d espite both samples showing similar powder diffraction patterns. [21] Rapid crystallization would be expected to give crystals with more defects resulting in more extrinsic porosity and higher gas uptakes. [21a] It is challenging to characterize structural defects for bulk polycrystalline powders,a nd until now, defects have not been observed directly in POCs.
Herein, we report the oriented assembly of POC crystals on surfaces such as silicon wafers and glass substrates.C age crystals were grown as 2D hexagonal layers,a ligned in parallel with the substrate.T his new morphology was fabricated by the simple technique of dip coating. Local point defects were directly observed, for the first time,using atomicforce microscopy (AFM). A" perfect" aligned cage crystal was obtained using as low crystallization process,w hile molecular vacancies were formed by rapid removal of the solvents.T he concentration of defects was also found to be related to the crystal growth rate. CC3 was synthesized as described previously. [2] This cage molecule has tetrahedral symmetry with an internal void and four open triangular windows (Figure 1A,B). Thec rystal structure,C C3a,s hows aw indow-towindow packing motif,leading to an interconnected 3-dimensional (3D) pore channel, as shown in Figure 1D-F. [22] The Brunauer-Emmett-Teller (BET) surface area of CC3a is 409 m 2 g À1 when it forms ahighly crystalline solid. [21a] We developed as imple and efficient method to create oriented CC3 structures on substrates by dip coating. As illustrated in Figure 1C and the Supporting Information, Figure S1, the substrate was immersed into asolution of CC3 in chloroform or dichloromethane for an appropriate period of time to grow oriented seed crystals.Bypulling the substrate upward at ac onstant speed, oriented cage crystals were formed on the substrate upon solvent evaporation. Thec age molecules preferentially nucleate and adhere to the surface of the substrate via van der Waals interactions,a nd are subsequently assembled into aligned crystalline layers or films ( Figure 1G,H).
Scanning electron microscopy (SEM) images revealed that cage molecules formed hexagonal shaped crystals on the surface of as ilicon wafer ( Figure 1I,J), in contrast to the octahedral morphology of bulk CC3 crystals (Figure 2, insets; Supporting Information, Figure S2). Thed iameter of the hexagonal shaped crystals was 3-5 mmw ith an average thickness of about 200 nm, and these microcrystals were formed discontinuously on the substrate (that is,the substrate was not fully covered;S upporting Information, Figure S3). These hexagonal cage crystals could also be fabricated on other substrates such as glass and carbon TEM grids (Supporting Information, Figures S4,S5).
Thec rystallinity and preferred orientation of these aligned CC3 crystals were further characterized by powder X-ray diffraction (PXRD). TheP XRD patterns of oriented CC3 crystals fabricated on the surface of silicon wafer show that they possess cubic F4 1 32 symmetry with a = 25.4 ,i n good agreement with bulk CC3 crystals (Figure 2; Supporting Information, Figure S7). Only three diffraction peaks are observed for the surface deposited CC3 crystals,i ndicating the oriented nature of the materials.T he peaks can be indexed as (111), (222), and (333). Therefore,C C3 cage molecules were grown in the (111) direction on the silicon wafer surface.T he cage packing along (111) orientation is illustrated in the Supporting Information, Figure S8. AFM was also used to characterize these oriented crystals.F igure 3A shows the AFM image of an entire hexagonal shape cage crystal grown on as ilicon wafer substrate.F igure 3C,D show the individual terraces on top of the cage crystals.T he height of these terraces is measured as 1.41 AE 0.18 nm, which agrees well with the size of cage molecule as measured from the single crystal structure.Atopographic study of oriented CC3 crystals ( Figure 3E)shows the cage packing structure on the crystal surface.T he height profile showed that the cage molecules have an intermolecular spacing of 1.41 AE 0.18 nm ( Figure 3F). Both PXRD analysis and AFM images suggest that the cage molecules are assembled by al ayer-by-layer growth mechanism with preferential (111) orientation.
Precise control of the dip-coating method (see the Supporting Information) allowed us to adjust the nanostructure of the crystals to produce either near-perfect crystals with very few defects or crystals with ah igh number of vacancy defects.AFM showed that the local morphology was affected by the growth conditions.H igh-resolution AFM deflection imaging of as lowly crystalized sample showed af lat, hexagonal shape and defect-free crystal surfaces ( Figure 3A;S upporting Information, Figure S9). We also prepared quickly grown oriented CC3 crystals.A FM images revealed ah exagonal crystal with at riangular nucleation point in the center ( Figure 3B;S upporting Information, Figure S10) surrounded by six segments relating to the hexagonal packing of cages in the crystal structure.Molecular vacancies were observed on the surface of the crystal, as shown in Figure 3B and Figure 4. Thed efects on the crystal surface are localized within three of the six segments (Figure 3B,a nd Figure 4), producing an alternating pattern of high and low defect concentration. More AFM images of other quickly grown CC3 crystals also show asimilar pattern and alarge number of vacancy defects (Supporting Information, Figures S11,S12). This pattern is related to the growth of   the segments and the crystallographic directions (Supporting Information, Figure S13). Thesegments of the crystal formed at the apex of the central triangular defect have 2% surface vacancies while the segments formed at the edges of the triangle have 10-12 %s urface vacancies.I nitial formation of atriangular {111} face by growth of the crystal parallel to the surface,followed by propagation from the vertices (parallel to h100i)and edges (along h110i)would account for the observed crystal shape.D ifferences in vacancies that is," missing cages", and void defect concentrations between the sectors can be related to edge versus point growth, with the probability of imperfections higher for growth from the edge,o wing to the larger area of the growth front and potential differences in the both intermolecular interactions presented by the cages in this direction. Thesize of the defects ranges from individual molecular vacancies up to 27.5 nm multiple vacancyp ores,w hich indicates that multiple cage molecules are absent during the rapid crystal growth. Furthermore,F igures 3B and S10 show that the defects are not just present at the surface.The formation of anew layer of cage molecules growing on top of the crystal surface with defects beneath it suggests that additional pore volume owing to defects is retained in the subsurface crystal structure.This is the first direct evidence for the existence of vacancy defects, which have been invoked previously to explain the properties of porous molecular materials. [20,21] This explains our observation, for example,t hat rapidly crystallized bulk CC3 has significantly higher surface area than slowly crystallized CC3.
Theability to control and to quantify vacancies as adirect function of crystallization rate demonstrates av iable "defect engineering" strategy for POCs via controlled solution processing.
We also tried to grow oriented cage films on glass substrates.M icroscopy showed large hexagonal crystals grown continuously on the glass surface (Supporting Information, Figure S14), suggesting potential for forming conformal porous crystalline coatings.T he key to successful growth of these uniformly oriented large crystals was appropriate solvent evaporation conditions.T he resultant bulk oriented crystals exhibited multiple cage layers (Figure 1K;S upporting Information, Figure S14). PXRD shows three main peaks at 2q = 6.28 8 (111), 12.48 8 (222), and 18.68 8 (333) (Supporting Information, Figure S15), indicating that oriented growth on glass occurs parallel to the (111) crystal planes,a sf or the silicon surface.A fter the oriented crystals were ground to fine powders,the PXRD was fully consistent with the known CC3 crystal structure [2] (Supporting Information, Figure S16). Hence,t he oriented CC3 crystals pack window-to-window,b ut grow in ap referentially oriented manner.I na ddition, am ultiple dip-coating process was carried out to promote secondary growth of oriented cage crystals.A fter more than 100 cycles of dip coating,t he substrate was densely covered by discrete hexagonal crystals with as urface coverage of up to 85 %, although the orientation was lost on the uppermost layers (Supporting Information, Figure S17).
Simulations were used to generate representative structural models of the interactions between CC3 and the silicon surface.T here are two possible geometries for the growth of oriented cage crystals on silicon, with either the cage window or cage arene face attached to the surface.T he atomistic model for each of these cases ( Figure 5A-D) was geometryoptimized at the PBE-D3 level of theory,u sing the CP2K package. [23] Surface binding energies derived from these models showed that the cage arene interacts with the silicon wafer more strongly by 16.1 kJ mol À1 .The structural model of oriented cage crystals ( Figure 5E)w as constructed from as tarting model based on the reported crystal structure of CC3a,w ith 2D layers of cage molecules grown in the (111) direction.
In summary,t his study demonstrates the controlled surface growth of aligned cage crystals for the first time. Cage molecules can be grown in ap referentially oriented manner on several substrates.Astructural model was generated to represent the cage packing motifs on as ilicon substrate.T he dip-coating approach is as imple and efficient way to fabricate porous molecular materials into thin films with control over defect concentration. This is the first time that defects have been observed directly in crystalline POCs, and the defect concentration can be correlated with the crystallization rate.T hese results suggest new opportunities for these molecular cage materials;f or example,l arge coherent crystalline POCs thin films might be useful for molecular sieving, allowing the excellent potential that has been demonstrated for bulk POCs [7,24] to be transferred into more practicable and scalable membrane technologies.