Engineering crystal symmetry and polar order in molecular host frameworks.

A crystal design strategy is described that produces a series of solid-state molecular host frameworks with prescribed lattice metrics and polar crystallographic symmetries. This represents a significant advance in crystal engineering, which is typically limited to manipulation of only gross structural features. The host frameworks, constructed by connecting flexible hydrogen-bonded sheets with banana-shaped pillars, sustain one-dimensional channels that are occupied by guest molecules during crystallization. The polar host frameworks enforce the alignment of these guests into polar arrays, with properly chosen guests affording inclusion compounds that exhibit second harmonic generation because of this alignment. This protocol exemplifies a principal goal of modern organic solid-state chemistry: the precise control of crystal symmetry and structure for the attainment of a specific bulk property.

The generality of architectural isomerism in designer inclusion frameworks.

We describe herein new structural isomers of a lamellar host system based on organodisulfonate "pillars" that connect opposing hydrogen-bonded sheets, consisting of topologically complementary guanidinium (G) ions and sulfonate (S) groups, to generate inclusion cavities between the sheets. These new isomers-zigzag brick, double brick, V-brick, and crisscross bilayer-expand significantly on our earlier report of architectural isomerism displayed by the discrete bilayer and simple brick forms. We demonstrate here that the discrete bilayer-simple brick isomerism, which was limited to several host-guest combinations based on the G(2)(4,4'-biphenyldisulfonate) host and one pair of compounds based on the G(2)(2,6-naphthalenedisulfonate), can be generalized to other organodisulfonate pillars. Furthermore, in many cases the selectivity toward the different framework isomers reflects a rather systematic templating role of the guest molecules and host-guest recognition during assembly of the lattice. We also describe a convenient approach to identifying and classifying the innumerable possible host architectures based upon the pillar projection topologies for the GS sheets and the intersheet connectivities. The discovery of these new architectures reveals a structural versatility for this class of materials that exceeds initial expectations and observations. Each topology produces different connectivities between the sheets in the third dimension that endows each framework isomer with uniquely shaped and sized inclusion cavities, enabling this host system to conform readily to different guests. The unlimited number of architectures available, combined with the inherent conformational softness and structural tunability of these host lattices, suggests a near universality for the GS system with respect to guest inclusion.

Metric engineering of soft molecular host frameworks.

The self-assembly and solid-state structures of host-guest inclusion compounds with lamellar architectures based on a common building block, a resilient hydrogen-bonded sheet consisting of guanidinium ions and sulfonate moieties of organodisulfonate "pillars", are described. The pillars connect adjacent sheets to generate galleries with molecular-scale cavities occupied by guest molecules. The size, shape, and physicochemical character of the inclusion cavities can be systematically adjusted by interchanging framework components while maintaining the lamellar architecture, enabling prediction and control of crystal lattice metrics with a precision that is unusual for "crystal engineering". The reliability of the lamellar architecture is a direct consequence of conformational flexibility exhibited by these hosts that, unlike rigid systems, enables them to achieve optimal packing with guest molecules. The adaptability of these hosts is further reflected by an architectural isomerism that is driven by guest templating during assembly of the inclusion compounds. Host frameworks constructed with various pillars display metric interdependences among specific structural features that reveal a common mechanism by which these soft frameworks adapt to different guests. This unique feature facilitates structure prediction and provides guidance for the design of inclusion compounds based on these hosts.