Versatile and Resilient Hydrogen-Bonded Host Frameworks

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Versatile and Resilient Hydrogen-Bonded Host Frameworks Takuji Adachi and Michael D. Ward* Department of Chemistry and Molecular Design Institute, New York University, 100 Washington Square East, New York, New York 10003-6688, United States CONSPECTUS: Low-density molecular host frameworks, whether equipped with persistent molecular-scale pores or virtual pores that are sustainable only when occupied by guest molecules, have emerged as a promising class of materials owing to the ability to tailor the size, geometry, and chemical character of their free space through the versatility of organic synthesis. As such, molecular frameworks are promising candidates for storage, separations of commodity and fine chemicals, heterogeneous catalysis, and optical and electronic materials. Frameworks assembled through hydrogen bonds, though generally not stable toward collapse in the absence of guests, promise significant chemical and structural diversity, with pores that can be tailored for a wide range of guest molecules. The utility of these frameworks, however, depends on the resilience of n-dimensional hydrogen-bonded motifs that serve as reliable building blocks so that the molecular constituents can be manipulated without disruption of the anticipated global solid-state architecture. Though many hydrogen-bonded frameworks have been reported, few exist that are amenable to systematic modification, thus limiting the design of functional materials. This Account reviews discoveries in our laboratory during the past decade related to a series of host frameworks based on guanidinium cations and interchangeable organosulfonate anions, in which the 3-fold symmetry and hydrogen-bonding complementarity of these ions prompt the formation of a two-dimensional (2-D) quasi-hexagonal hydrogen-bonding network that has proven to be remarkably resilient toward the introduction of a wide range of organic pendant groups attached to the sulfonate. Since an earlier report in this journal that focused primarily on organodisulfonate host frameworks with lamellar architectures, this unusually persistent network has afforded an unparalleled range of framework architectures and hundreds of new crystalline materials with predictable solid-state architectures. These range from the surprising discovery of inclusion compounds in organomonosulfonates (GMS), as well as organodisulfonates (GDS), structural isomerism reminiscent of microstructures observed in soft matter, a retrosynthetic approach to the synthesis of polar crystals, separation of molecular isomers, storage of unstable molecules, formation of a zeolite-like hydrogen-bonded framework, and postsynthetic pathways to inclusion compounds through reversible guest swapping in flexible GS frameworks. Collectively, the examples described in this Account illustrate the potential for hydrogen-bonded frameworks in the design of molecular materials, the prediction of solid-state architecture from simple empirical parameters, and the importance of design principles based on robust high dimensional networks. These and other emerging hydrogen-bonded frameworks promise new advanced materials that capitalize fully on the ability of materials chemists to manipulate solid-state structure through molecular design.

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INTRODUCTION The formation of molecular crystals has been described as “supramolecular chemistry par excellence,”1 owing to the extraordinary fidelity that is characteristic of a growing crystal: a typical 1 mm3 organic crystal is the product of approximately 1018 events that occur with near perfection! The potential versatility and functions of these materials has motivated a modular approach to their synthesis, as illustrated clearly by the advances in metal−organic frameworks.2 Moreover, during the past five decades organic solid-state chemistry has produced a wide range of molecular crystals, with an equally wide range of solid-state properties, capitalizing on the versatility of organic synthesis. Solid-state properties are cooperative, however. As such, they are inextricably linked to solid-state structure, the control of which often is thwarted by the delicate, noncovalent forces that govern molecular organization in the solid state, to © XXXX American Chemical Society

the extent that even the slightest modification of a constituent can lead to unpredictable changes in crystal architecture. Although recent advances are encouraging, crystal structure prediction through computational methods, including space group, lattice parameters, and atomic positions, remains elusive.3 Consequently, organic solid-state chemists often rely on empirical strategies using structure-directing interactions such as hydrogen bonds4 to override the cumulative effect of weaker and less directional crystal packing forces. In these cases, crystal structure sometimes can be anticipated from the molecular symmetry of the building blocks, as exemplified by the two-dimensional (2D) “chicken wire” network formed by Received: July 12, 2016

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DOI: 10.1021/acs.accounts.6b00360 Acc. Chem. Res. XXXX, XXX, XXX−XXX

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Accounts of Chemical Research

Scheme 1. (A) Quasi-hexagonal GS Sheet, Illustrating the Major and Minor Hydrogen-Bonded Ribbons and (B) “Shifted Ribbon” Motif Observed Occasionally in GS Compounds

trimesic acid5 and the three-dimensional (3D) diamond-like network formed from adamantane-1,3,5,7-tetracarboxylic acid.6 In 2001, an Account from our laboratory reported lowdensity molecular frameworks based on a robust 2D network assembled through charge-assisted (guanidinium)N−H···O(sulfonate) hydrogen bonds,7 wherein guanidinium organomonosulfonates (GMS) formed guest-free phases while guanidinium organodisulfonates (GDS) readily formed frameworks with guests included. Since that Account, more than 500 crystalline compounds based on the guanidinium sulfonate (GS) network with interchangeable sulfonates, exhibiting various lamellar, cylindrical, and cubic architectures, have been discovered, a benchmark in crystal engineering and one that continues to surprise. In this Account, we describe examples that illustrate the versatility and unique character of these frameworks, including the discovery of GMS inclusion compounds, new architectures in GMS and GDS frameworks, structural isomerism reminiscent of soft matter, strategies for the synthesis of polar crystals, selective encapsulation for separation and chemical storage, a zeolite-like hydrogen bonded framework assembled from molecular tiles, and postsynthetic modification through guest swapping in flexible frameworks.

attributed to the strength of the charge-assisted hydrogen bonds and the unique ability of the GS sheet to pucker like an accordian, which provides a pathway to close packing with retention of the hydrogen-bond connectivity.

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ARCHITECTURAL DIVERSITY IN GS FRAMEWORKS The organic groups appended to sulfonates can be viewed as posts (for monosulfonates) or pillars (for disulfonates and some polysulfonates) for constructing a GS framework in the third dimension from the 2D GS sheets. The GS sheet can be described as consisting of one “major” (M) and two “minor” (m) ribbons, and the “up-down” projections of the organosulfonate groups from the opposing sides of a GS sheet can be depicted as filled or open circles (Figure 1). The number of possible “up-down” arrangements of organosulfonate groups is actually indefinite. For example, guest-free guanidinium organomonosulfonates (GMS) can assemble through interdigitation of organosulfonate posts that project from the same side of each GS sheet to form a bilayer architecture (PT-I), while equal numbers of rows of organic groups projecting from opposite sides form a simple continuously layered (s-CL) architecture (PT-II). The architecture formed depends on the cross-sectional area of the organic group, with the s-CL more accommodating of groups with larger footprints (Figure 2A,B). Guanidinium organodisulfonates (GDS) assemble via organodisulfonate “pillars” that connect opposing GS sheets, thus enforcing inclusion cavities in the gallery regions between adjacent GS sheets (Figure 2C,D). Numerous low-density frameworks have been realized, discrete bilayer, simple brick, double brick, zigzag brick, V-brick, that can be attributed to templating by differently sized and shaped guests during framework assembly.8 Despite the facile formation of the guest-free GMS compounds and the absence of predestined inclusion cavities like those in related GDS host frameworks, GMS inclusion compounds form readily.9,10 Using a combinatorial library of 24 GMS hosts and 26 guest molecules, a total of 304 inclusion compounds out of a possible 624 host−guest combinations were realized, demonstrating a remarkable capacity of the GMS hosts to form inclusion compounds while revealing the role of guest templating. Many GMS−guest combinations generated a lamellar (simple continuous layered inclusion compounds: sCLIC) architecture with the PT-II projection topology, identical to the guest-free s-CL and the GDS s-CLIC compounds but with guests confined between the organic

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THE GUANIDINIUM SULFONATE HYDROGEN-BONDED NETWORK The 3-fold symmetry and hydrogen bond complementarity of the guanidinium cation (G = C(NH2)3+) and the sulfonate moieties of organodisulfonate anions (DS; S = -O3S-R-SO3-) typically affords a two-dimensional quasi-hexagonal hydrogenbonded GS sheet that can be described as 1-D GS “ribbons” fused along the ribbon edges by lateral (G)N−H···O(S) Hbonds (Scheme 1, left), which serve as flexible “hinges” that permit puckering of the GS sheet. The interribbon puckering angle (θIR), determined from the centroid of two sulfur atoms on a selected GS ribbon and the nearest sulfur atoms on the two adjacent ribbons, dictates the repeat distance normal to the ribbon direction (b1). The observed values of b1 range from 13.0 Å (twice the width of a GS ribbon) for a perfectly flat sheet to as little as 7.0 Å for a highly puckered sheet, while retaining the quasi-hexagonal motif. Occasionally, the GS sheet adopts a “shifted-ribbon” motif (Scheme 1, right) in which adjacent connected ribbons are shifted from the quasihexagonal arrangement, by as much as a1/2. The GS network is remarkably resilient to a wide range of organosulfonates and guests (in the case of inclusion compounds), which can be B


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Figure 1. Projection topologies (PT) observed for GS sheets and their corresponding architectures. Filled and open circles depict organic groups projecting from the sulfonate nodes above and below the sheet, respectively. The G ions sit on the undecorated nodes of the hexagonal tiling. The major ribbon (M) and minor ribbons m(1) and m(2) are depicted on PT-1. The parallelograms depict the translational repeat unit of each sheet. The loops sketched on the cylindrical projection topologies (CPT-I and CPT-II) denote hydrogen-bonded fusion of the edges of the GS ribbons at the top and bottom of each diagram, which results in formation of cylinders. s-CL = simple continuous layered; s-CLIC = simple continuous layered inclusion compound; dCL = double continuous layered; d-CLIC = double continuous layered inclusion compound; zz-CL = zigzag continuous layered; zz-CLIC = zigzag continuous layered inclusion compound (two versions); TIC = tubular inclusion compound. Adapted with permission from ref 10. Copyright 2007 American Chemical Society.

groups protruding from opposing GS sheets (Figure 2E), akin to “molecular jaws.” The ubiquity of the GMS inclusion compounds can be attributed to fewer constraints on packing compared with the GDS compounds due to the absence of a covalent connection between the GS sheets, thereby removing the requirement of registry between adjacent sheets that otherwise may frustrate optimal packing. A double continuously layered inclusion compound (d-CLIC) with projection topology PT-III and “double-wide” channels formed when guests were too large for the s-CLIC architecture, as well as two distinct architectures with “zigzag” channels (zz-CLICs) with PT-IV and PT-V configurations directed by guest shape as well as size. Although the assembly of so many constituents seems counterintuitive, GMS inclusion compounds are likely favored by the gain in entropy associated with the loss of structured solvent (usually water or methanol) around the nonpolar guest molecules during crystallization.

Figure 2. (A,B) Guest-free GMS bilayer and s-CL architecture (PT-I topology). (C) GDS bilayer inclusion compound (PT-I topology). (D) GDS s-CLIC architecture (PT-II topology). (E) GMS s-CLIC architecture (PT-II topology). 4CBS = 4-chlorobenzenesulfonate; 4IBS = 4-iodobenzenesulfonate; NDS = naphthalenedisulfonate. The metrics denote distance along the puckering direction, distance between GS layers, and puckering angle.

Surprisingly, GMS inclusion compounds are not constrained to layered architectures. Certain host−guest combinations afforded crystalline hexagonal rods, dubbed tubular inclusion compounds (TICs), in which the GS sheet curled into a cylinder consisting of six GS ribbons while retaining the quasihexagonal motif (Figure 3). The organic groups attached to the C


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spaced guests. One-half of the guests are contained within the cylinders and the other half between the cylinders, resulting in the overall stoichiometry GMS·2/3(guest). The large number of GMS inclusion compounds permitted the sorting of the architectural isomers in a phase diagram based on simple molecular parameters, specifically the sulfonate volume/guest volume ratio (Vsulf/Vguest) and the guest eccentricity (εguest), both measured readily from molecular models (Figure 4). Inspection of the phase diagram reveals that the TIC and d-CLIC architectures reside primarily in separate sectors, with the TIC architecture templated by guests that are small and disk-shaped and the d-CLIC architecture, with its wide channels, templated by larger guests. The ability to sort architectures by relatively simple molecular parameters enables a more informed design of new compounds, and it is a significant step in crystal engineering. The isomerism displayed by the various CLICs and TICs is somewhat reminiscent of lamellar and hexagonal cylinder phases observed in soft matter surfactant assemblies and block copolymers. The correspondence to soft matter is especially apparent for guanidinium phenyl alkanesulfonates, biphenylalkanesulfonate, and alkanesulfonates, which are crystalline with lamellar architectures at room temperature but form smectic liquid crystal phases upon heating and lyotropic lamellar liquid crystals in certain organic solvents.11,12 Like soft matter microstructures, the phase diagram in Figure 4 is described by relatively simple parameters, and the GMS inclusion compounds are equipped with a welldefined, elastic interface, the GS sheet, although the length scale defining curvature and periodicity in the GMS compounds is smaller. The tubular inclusion compounds prompted the question as to whether the 3-fold packing of the cylinders, driven by dispersive interactions between the organosulfonate groups could be replicated with rigid trisulfonates having 3-fold symmetry. Indeed, the guanidinium salts of 1,3,5-benzenetrisulfonate (BTS), tri(4-sulfophenyl)methane (TSPM), and 1,3,5tri(4-sulfophenyl)benzene (TSPHB) crystallized in a cylindrical architecture (G3BTS and G3TSPHB in space group P63/m; G3TSPM in P63), but with neighboring GS cylinders connected through covalent nodes provided by the trisulfonates rather than dispersive interactions (Figure 5).13 This topological equivalence demonstrates that symmetry can be preserved from

Figure 3. (A) Schematic of the GMS TIC architectures. (B) Spacefilled rendering of a single cylinder in G4BBS·2/3(o-xylene) (4BBS = 4-bromobenzenesulfonate). (C) Seven adjoining cylinders. (D) GS tube, illustrating the commensurism between guest stacks and the intersulfonate distance along the major ribbon. Hydrogen atoms and all but the α-C atoms of 4BBS are omitted for clarity. Adapted with permission from reference 10. Copyright 2007 American Chemical Society.

sulfonate nodes project outward from the surface of each cylinder (Figure 1, CPT-II), interdigitating to form hexagonal arrays of cylinders with a pore diameter of approximately 9 Å. Each cylinder contains a commensurate stack of uniformly

Figure 4. Structural phase diagram for 206 GMS inclusion compounds. Each data point represents a unique host−guest combination. Reproduced with permission from ref 10. Copyright 2007 American Chemical Society. D


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the guest molecules, once again demonstrating the templating role of guest molecules. Interestingly, the rigid tetrasulfonates afforded architectures with differently sized and shaped cylinders in the same crystal.

Figure 5. (left) Schematic representation of the tubular architecture generated by rigid trisulfonates (right) and space-filled model of G3(1,3,5-trisulfonatobenzene). Adapted with permission from reference 13. Copyright 2015 American Chemical Society.

a well-defined blueprint even though significantly different intermolecular forces direct assembly. This concept was limited to rigid connectors, however, as trisulfonates with flexible arms tend to form lamellar architectures with “molecular baskets” because the sulfonate groups on each molecule can bend and insert into the same sheet.13

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EXPANDING VALENCY The series of guanidinium organotetrasulfonates, G4T4SMB, G4TSPB, and G4TSP, revealed a trend from lamellar to cylindrical with increasing rigidity (Figure 6).13 Whereas the flexible arms allow lamellar structures due to metric compatibility of the sulfonate groups and their positions in the GS sheet, rigid G4TSP formed cylindrical architectures with a topology enforced by the disposition of the sulfonate groups. The moderate conformational freedom in G4TSPB, however, allowed both lamellar and cylindrical structures, depending on

Figure 7. (top) Reversible single crystal−single crystal transformation accompanying guest exchange in the flexible framework between G4TSPB·(dioxane)5 (left) and G4TSPB·(tetrahydrofuran)5 (right). The tetrahydrofuran molecules are depicted as green to distinguish them from the dioxane molecules in G4TSPB·(dioxane)5. (bottom) Schematic illustration of reversible single crystal−single crystal transformations based on G4TSPB framework. Adapted with permission from ref 14. Copyright 2014 American Chemical Society.

The cylindrical architecture of G4TSPB exhibits three crystallographically unique one-dimensional channels (Figure 6). The framework is sufficiently flexible to permit reversible release and adsorption of various guest molecules with retention of single crystallinity (Figure 7).14 The release and adsorption of dioxane between G4TSPB·(dioxane)4 and G4TSPB·(dioxane)5 revealed a reversible single crystal−single crystal transformation accompanied by cyclic “breathing” of the framework. Reversible guest exchange between G4TSPB·(dioxane)5 and tetrahydofuran, toluene, aniline, and nitrobenzene occurred with retention of single crystallinity, signaling a postsynthetic approach to inclusion compounds that could not be synthesized directly. Moroever, the different channels discriminate between exchanging guests (Figure 7). Whereas all five dioxane guests in G4TSPB·(dioxane)5 could be replaced by five tetrahydrofuran guests, only partial exchange could be achieved with toluene to generate G4TSPB·(toluene)3(dioxane). Partial exchange also was observed in the conversion of G4TSPB·(tetrahydrofuran)5 to G4TSPB·(toluene)3(tetrahydrofuran)0.5. This surprising

Figure 6. Tendency to form cylindrical architectures increasing with tetrasulfonate rigidity. E


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Figure 8. (A, B) Complementary [G3NO3]2+ (yellow) and HSPB6− (green) tiles, and their corresponding edge lengths. (C) An unfolded quasitruncated octahedron (q-TO) based on the complementary [G3NO3]2+ (yellow) and HSPB6− (green) tiles. (D) The quasi-truncated octahedron. The squares in panels C and D correspond to the openings that define the channels between adjacent q-TOs in the solid state. (E, F) The quasitruncated octahedron viewed perpendicular to a HSPB6− tile and a (G3NO3)2+ tile. The purple sphere represents the maximum isotropic volume accommodated by the interior cavity, measuring approximately 12 Å in diameter. (Photo) Plastic model of the zeolite-like hydrogen bonded framework. Adapted from ref 15. Reprinted with permission from AAAS.

Figure 9. (top) Protocol for an inclusion-based separation, wherein guest inclusion results in rapid crystallization of an inclusion compound. (bottom) Selectivity profiles of pairwise competition for inclusion of xylene isomers by (left) G2NDS and (right) G4CBS, where X and Y are the mole fractions for a particular isomer in solution and in the inclusion compound retrieved after crystallization, respectively. The offset of the triangle from the center is a measure of the selectivity for competition experiments performed with an equimolar ratio of the three isomers. The curves represent fits to the data, from which selectivity coefficients (K) can be calculated. The 45° line corresponds to K = 1 (no selectivity). Adapted with permission from ref 16. Copyright 2001 American Chemical Society.

invoking some kind of cooperative peristaltic motion wherein entry of a new guest on one side of the channel forces all the molecules in the channel to move and expel an original guest molecule from the other end. The other possible explanation would invoke defects, but single crystallinity is preserved during these exchange processes. Rounding out the de novo framework design is a zeolite-like hydrogen bonded framework formed with the hexa(sulfonatophenyl)benzene hexaanion (HSPB6−) by recognizing that the assembly of two complementary hexagonal tiles of

discrimination can be attributed to the size constraints of the different channels, but the mechanism of exchange is puzzling. The diffusivities of molecules in the channels during exchange, estimated from 1H NMR assays, were estimated to be ∼10−6 cm2 s−1, which is surprisingly large. Rapid diffusion in the two channels having cross sections sufficient to accommodate two guest molecules can be explained by two-way or ring diffusion, most likely vacancy assisted. Guest exchange in the smaller channel (in the center of each schematic in Figure 7) requires single-file diffusion, which is difficult to explain without F


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Accounts of Chemical Research comparable size could fold into a quasi-truncated octahedron (q-TO), similar to the Archimedean truncated octahedron. As such, four HSPB6− tiles and four supramolecular [G3NO3]2+ tiles, hexagonal, complementary, and metrically matched along the edges, formed a convex polyhedron through 72 hydrogen bonds, with a [46.64.64] tiling and 4̅3m symmetry, assembling further into a body-centered cubic framework with features resembling those of sodalite and zeolite A (Figure 8).15 Single crystal X-ray diffraction revealed I43̅ m space group symmetry and a = 26.7 Å, with q-TOs interconnected by 4 Å channels formed by sodium bridges between sulfonate oxygen atoms. The q-TO can accommodate a sphere with a diameter of 12 Å, corresponding to a sphere volume of 905 Å3. The total free volume in the interior of each approaches 2200 Å3. The q-TO and its framework exhibit a remarkable ability to encapsulate and tolerate an assortment of molecular species with wideranging shapes, sizes, substituents, and charges, ranging from transition metal complexes to “ship-in-a-bottle” metal-iodide nanoclusters. The pervasiveness of the q-TO framework for such a wide range of guests signifies an inherent thermodynamic stability of the framework alone, rather than assembly directed by templating as observed for most other GS compounds.

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SELECTIVE ENCAPSULATION BY DESIGN Traditional separation methods for molecular isomers often can be unfeasible. Unlike covalent host frameworks such as zeolites or metal−organic frameworks, which rely on selective sorption and diffusion through pre-existing pores, the tailored inclusion cavities in the GS frameworks are created during crystal assembly. Trapped guests can be retrieved subsequently by dissolution of the crystallized inclusion compound and extraction under mild conditions, and the host material can be recycled. This concept was demonstrated by our laboratory for the GS frameworks,16 most notably the separation of 2,6dimethylnaphthalene from its nine other isomers in approximately 95% purity after crystallization of its inclusion compound with the guanidinium biphenylsulfonate framework. The guanidinium naphthalene-2,6-disulfonate (G2NDS) framework exhibited a selectivity for the isomers of xylene in the order p-xylene ≫ o-xylene > m-xylene, considerably greater than that typically exhibited by industry-standard zeolitic materials. In contrast, the GMS host frameworks with 4methylbenzenesulfonate (4MBS), 4-chlorobenzenesulfonate (4CBS), and 4-bromobenzenesulfonate (4BBS) preferentially included m-xylene (Figure 9, bottom right for G4CBS). Though these examples are limited to rather simple isomers, they suggest protocols for separation of high value-added compounds, for example, enantioselective separations using chiral organosulfonates. Selective encapsulation of a different kind was demonstrated for a lamellar GS architecture built from a tetrasulfonated calixarene, in which the footprint of the four sulfonate groups on the upper rim of the calixarene match up with the sulfonate sites on a GS sheet, creating an “endo-inclusion” cavity, illustrated schematically in Figure 10.17 These capsules can incorporate 3,4-dihydro-2H-pyrrole (Δ1-pyrroline),18 the active pheromone of the Mediterranean fruit fly (Medfly), one of the most destructive fruit pests. The Δ1-pyrroline monomer coexists with its corresponding trimer in both solution and neat liquid form (by 1H NMR), but the monomer is the active form. Whereas the pure liquid pheromone gradually polymerizes after four months storage in neat liquid form, the monomer

Figure 10. (top) Schematic representation of the calixarene footprint on the GS sheet and the appended “capsules”. (bottom) Selective encapsulation of the monomer during crystallization of the endoinclusion compound G1.5SC⊃Δ1-pyrroline. Adapted with permission from ref 18. Copyright 2013 American Chemical Society.

can be stored indefinitely without decomposition when included in the endo-inclusion cavities of the GS calixarene framework and released slowly by heating above ambient, suggesting an agricultural adjuvant to trap female Medflies.

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SEPARATING STRUCTURE FROM FUNCTION The GS frameworks present an opportunity for synthesizing functional materials, wherein the function can be introduced through guest inclusion organized in a manner directed by a reliable host framework scaffold, obviating the need to use molecular components that direct structure and provide function simultaneously, a daunting prospect that severely limits crystal engineering. In this way, the GS frameworks resemble a skyscraper: floors, ceilings, and walls articulate the structure but the function is added separately, from lighting to computers to office workers. This can be exemplified by the synthesis of a polar framework that produced a series of inclusion compounds with predictable space group symmetry and lattice parameters, while guiding the assembly of various acentric guest molecules into polar arrays.19 Puckering of the GS sheet in a simple brick GDS framework forces straight pillars in adjacent layers to tilt in opposite directions. A retrosynthetic analysis suggested that the same puckering could be achieved with banana-shaped pillars, such that the pillars would point in the same direction G


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Figure 11. A retrosynthetic approach to a 3D polar host frameworks (and, consequently, polar inclusion compounds): Linear organodisulfonate pillars of a puckered simple brick GS framework (PT-II) can be cleaved virtually, rotated 180° with respect to each other, and reassembled to produce banana-shaped pillars and a polar framework. This framework can be made directly with “bent” pillars, such as meta-substituted arene disulfonates. From ref 19. Reprinted with permission from AAAS.

Figure 12. (top left) Schematic representation of GDS lamellar BL∥, simple brick, and BL architectures. The guest alignment in the bilayer structures depends on cavity size. (top right) Libraries of eight GDS hosts and nine linear π-conjugated guests, characterized by (bottom left) metric variables lS−S, the distance between sulfur atoms in the host pillar, and lg, the length of the long axis of the guest. (bottom right) Structural phase diagram for GDS inclusion compounds sorted according to distinct sectors defined by the value of lg/lS−S. Blue circles (●) denote BL⊥ compounds, red diamonds (◆,◇) denote simple brick compounds, and green triangles (▲,△) denote BL∥ compounds. Adapted with permission from ref 20. Copyright 2010 American Chemical Society.

(Figure 11), generating a polar framework. Pillars having C2v symmetry would be expected to afford a framework with orthorhombic Imm2 space group symmetry. This was reduced to practice using meta-substituted arenedisulfonates, which generated polar frameworks that enforced polar alignment of nitroarene guests, producing single crystals with SHG activity that scaled with the guest hyperpolarizablities. These results

demonstrated the potential for separating structure from function in the design of crystalline materials. Polyconjugated molecules have substantial potential in electronics, ranging from light-emitting diodes to nonlinear optical devices to thin-film transistors. The optical and electronic properties of these compounds are governed by optical absorption, emission, charge generation, and carrier transport, which typically are cooperative effects that depend on H


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Accounts of Chemical Research the arrangement of molecular constituents in the solid state. GDS frameworks were found to direct the organization of various oligothiophenes20 in bilayer and brick frameworks. Large guests typically promote the formation of the more open brick frameworks, but oligothiophene guests can be included in the bilayer architecture as well, in either a “parallel” or “perpendicular” orientation, depending on relative heights of the pillar and guest (Figure 12). Various guest configurations, edge-to-edge, face-to-edge, and end-to-end, were realized by the systematic transition from the BL∥ to simple brick to BL⊥ architecture, achieved with increasing values of the guest length relative to the pillar. By examining a total of 59 unique inclusion compounds, a structural “phase diagram” was constructed based on the parameters lg and lS−S (Figure 12), once again illustrating structure prediction of a kind based on simple parameters. Controlled guest arrangement enabled tuning of the optical properties of the guests due to confinement in the host matrix, manifested as bathochromic shifts in the absorption and emission spectra of the guests compared with methanol solutions.20 Similarly, a variety of coumarin dyes were included in GDS frameworks, bilayer, simple brick, zigzag brick, and a “chevron” brick, with larger coumarin dyes templating the more open frameworks.21 This permitted control over the formation of J- and H-aggregates, as well as their corresponding emission behavior, which was distinct from that of the monomers. The ability to regulate intermolecular association of laser dyes in high concentrations in a robust host framework may lead to opportunities for tunable solid-state lasing materials. The effect of dye aggregation in GDS frameworks also has been explored through the use of “tetris-shaped” pillars,22 such as the stilbene disulfonates PV3DS2− and DSBDS2−, wherein the only possible orientation of the pillar is with its long axis parallel to the GS sheet (Figure 13). A discrete bilayer architecture would require a Z-conformer in which the sulfonate−sulfonate distance within each pillar was commensurate with the sulfonate nodes in a single GS sheet, an unlikely possibility. Conversely, an E-conformer could assemble the framework in a continuously layered architecture because this distance criterion is relaxed, requiring only planar PV3 and DSBDS moieties and registry of the opposing sheets in a manner that avoids steric interference between neighboring pillars. Molecular models based on the lengths of the PV3DS2− and DSBDS2− pillars and the possible projection topologies of the sulfonate nodes revealed that these pillars would pack in a continuously layered architecture with their long axes diagonal to the GS major ribbon axis, along a vector defined by two sulfonate nodes in adjacent ribbons on a GS sheet. Dense packing of the DSB and PV3 residues between the GS sheets precluded the incorporation of guest molecules, allowing strong intermolecular electronic couplings between the stilbene-like fragments. G2DSBDS adopted a face-to-face brickwork packing motif (Figure 13) and G2PV3DS adopted a face-to-face herringbone motif. Strong intermolecular electronic couplings between the pillars were confirmed by the spectral shifts in the absorption and emission spectra. Collectively, these observations promise manipulation of aggregate structure for the optimization of optoelectronic properties, even for GS compounds in which the function is delivered by the host framework alone.

Figure 13. (top) GS architecture with “Tetris-shaped pillars”. The long axes of the pillars align parallel to the GS sheets, precluding guest inclusion. The length of pillars, L, determines the azimuthal orientation with respect to the sheet. (bottom) Packing of 11 DSBDS pillars (space filling) on a GS sheet (wireframe). G ions are omitted for clarity. Adapted with permission from ref 22. Copyright 2015 American Chemical Society.

block, the guanidinium-sulfonate network, that reveal unprecedented control of solid-state architecture, spanning lamellar to cylindrical to cubic, based on relatively simple parameters such as size and shape of molecular constituents, while demonstrating the value of separating architecture from function when attempting the synthesis of functional materials. The GS compounds demonstrate the utility of constraining design to one dimension, in this case by locking in two dimensions with a resilient hydrogen-bonded sheet equipped with “builders”, the organic substituents on the sulfonates, for engineering the third dimension. One definition of engineering is “the art of skillful contrivance,” and the GS compounds accumulated in our laboratory and others23,24 are consummate examples of “crystal engineering.” Proton conduction recently was described25 in GDS frameworks previously reported by our group,26,27 further expanding on the suite of properties resulting from their unique architecture. The GS concept has been emulated in reports of analogous pillared networks constructed from organosulfonates and transition metals equipped with amine ligands,28−31 as well as hydrogen bonded guanidinium borate networks in which the guanidinium ion acts as a 3-fold connecting node to generate a cubic boracite network.32 Nonetheless, universal design principles that are applicable to molecular frameworks in general remain a challenge. We hope that this Account encourages further exploration of other classes of compounds based on common motifs that illuminate the underlying principles governing architectural forms. Moreover, hydrogen-bonding frameworks with persistent porosity, including flexible frameworks, have been reported recently with capabilities for carbon dioxide capture,33 gas separations,34 and chiral separations,35 promising further advances in functional host frameworks. Although the utility of designer frameworks ultimately will rely on their

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OUTLOOK This Account encompasses hundreds of organic solid-state compounds designed from a common supramolecular building I


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competitive economic advantage, crystal engineering remains a promising frontier for the discovery of functional molecular materials.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Funding

This work was supported primarily by the National Science Foundation under Award Number DMR-1308677. T.A. thanks the JSPS Postdoctoral Fellowships for Research Abroad for the financial support. Notes

The authors declare no competing financial interest. Biographies Takuji Adachi received a B.Eng. at Osaka University and a Ph.D. degree at the University of Texas at Austin under supervision of Paul F. Barbara and David A. Vanden Bout. After a postdoctoral research position at New York University, Adachi moved to the Institut de Science et d’Ingénierie Supramoléculaires at Université de Strasbourg, where he is a postdoctoral research associate. Michael D. Ward received a B.S. in Chemistry from the William Paterson College of New Jersey and a Ph.D. in Chemistry from Princeton University. After a postdoctoral position at the University of Texas, Austin, he held research positions at Standard Oil of Ohio and Dupont Central Research, joining the faculty of the Department of Chemical Engineering and Materials Science at the University of Minnesota in 1990. Ward moved to New York University in 2006, where he established the Molecular Design Institute and is a Silver Professor in the Department of Chemistry.

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ACKNOWLEDGMENTS The authors take this opportunity to acknowledge the contributions and support of Professor Paul F. Barbara, who unfortunately passed away in October of 2010. Paul was a renowned scientist, a former editor of this journal, the Ph.D. advisor of T.A., and a longstanding colleague and friend of M.D.W. His dedication to science was unparalleled, and he is sorely missed.

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REFERENCES

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Versatile and Resilient Hydrogen-Bonded Host Frameworks

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