Stereochemical Control of Polymorph Transitions in Nanoscale Reactors

Crystallization of glycine in the cylindrical nanopores of anodic aluminum oxide (AAO) revealed the formation of metastable β-glycine in pores having diameters less than 200 nm. Two-dimensional X-ray microdiffraction indicated that the [010] axis of the embedded β-glycine nanocrystals coincided with the pore direction, identical to behavior observed previously in the cylindrical nanopores of polymer monoliths. Whereas the β-glycine nanocrystals were stable indefinitely in ambient air and persisted upon heating, they transformed to the α polymorph upon standing at room temperature and 90% relative humidity (RH). The α-glycine nanocrystals were oriented with the [010] axis nearly perpendicular to the pore direction, reflecting a nearly 90° rotation of the glycine molecules during the transition. When the β-glycine nanocrystals were formed in the AAO cylinders in the presence of small amounts of racemic hydrophobic amino acid auxiliaries, which are known to bind selectively to the (010) and (01̅0) faces on the fast-growing end of β-glycine enantiomorphs, the β → α phase transition at 90% RH was suppressed. In contrast, β-glycine nanocrystals grown in the presence of an enantiopure amino acid auxiliary, which binds to the fast-growing end of only one of the enantiomorphs, thus suppressing its formation and leaving the other enantiomorph unperturbed, transformed into the α polymorph under the same conditions. This observation confirms that binding of an amino acid to the {010} faces is stereoselective and that access of water to these faces is essential for the transition to the α polymorph

Isolation and Stabilization of a Pheromone in Crystalline Molecular Capsules

The active monomer form of the male-produced pheromone of the Mediterranean fruit fly can be isolated selectively from its equilibrating trimer species by encapsulation within a calixarene pocket built into a hydrogen-bonded framework from guanidinium 4-sulfocalix[4]­arene. Encapsulation of the Δ¹-pyrroline guest significantly perturbs the assembly of the quasihexagonal two-dimensional guanidinium-sulfonate network of the guest-free framework, to the extent that guanidinium ions are excluded from some sites to accommodate the steric requirements of the guest. Nonetheless, single crystal X-ray diffraction reveals the preservation of a layered structrure in which the calixarene capsules stack in an antiparallel configuration. These observations illustrate that the binding of the pheromone monomer by the calixarene is sufficiently strong to overcome the loss of guanidinium-sulfonate hydrogen bonds, which is corroborated by the strong binding constants measured in solution. The solid-state encapsulation stabilizes the otherwise volatile unstable monomer form, suggesting an effective strategy for the storage, application, and controlled release of an important agricultural adjuvant

Guest Exchange through Single Crystal–Single Crystal Transformations in a Flexible Hydrogen-Bonded Framework

A molecular framework based on guanidinium cations and 1,2,4,5-tetra­(4-sulfonatophenyl)­benzene (TSPB), an aromatic tetrasulfonate with nominal 2-fold and mirror symmetry, exhibits three crystallographically unique one-dimensional channels as a consequence of molecular symmetry and complementary hydrogen bonding between the guanidinium (G) ions and the sulfonate (S) groups of TSPB. Unlike previous GS frameworks, this new topology is sufficiently flexible to permit reversible release and adsorption of guest molecules in large single crystals through a cyclic shrinkage and expansion of the channels with retention of single crystallinity, as verified by single crystal X-ray diffraction. Moreover, the G₄TSPB framework permits guest exchange between various guest molecules through SCSCTs as well as exchange discrimination based on the size and character of the three different channels. The exchange of guest molecules during single crystal–single crystal transformations (SCSCT), a rare occurrence for hydrogen-bonded frameworks, is rather fast, with diffusivities of approximately 10^–6 cm² s^–1. 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. Surprisingly, rapid guest exchange also is observed in a smaller channel having a cross-section that accommodates only one guest molecule, which can only be explained by guest-assisted single-file unidirectional diffusion. Several single crystals of inclusion compounds can be realized only through guest exchange in the intact framework, suggesting an approach to the synthesis of single crystalline inclusion compounds that otherwise cannot be attained through direct crystallization methods

Isolation and Stabilization of a Pheromone in Crystalline Molecular Capsules

The active monomer form of the male-produced pheromone of the Mediterranean fruit fly can be isolated selectively from its equilibrating trimer species by encapsulation within a calixarene pocket built into a hydrogen-bonded framework from guanidinium 4-sulfocalix[4]­arene. Encapsulation of the Δ¹-pyrroline guest significantly perturbs the assembly of the quasihexagonal two-dimensional guanidinium-sulfonate network of the guest-free framework, to the extent that guanidinium ions are excluded from some sites to accommodate the steric requirements of the guest. Nonetheless, single crystal X-ray diffraction reveals the preservation of a layered structrure in which the calixarene capsules stack in an antiparallel configuration. These observations illustrate that the binding of the pheromone monomer by the calixarene is sufficiently strong to overcome the loss of guanidinium-sulfonate hydrogen bonds, which is corroborated by the strong binding constants measured in solution. The solid-state encapsulation stabilizes the otherwise volatile unstable monomer form, suggesting an effective strategy for the storage, application, and controlled release of an important agricultural adjuvant

Non-Topotactic Phase Transformations in Single Crystals of β‑Glycine

The metastable β polymorph of glycine exhibits a single-crystal-to-single-crystal transformation (SCSCT) to either the α or γ phase with retention of the crystal habit of the parent β phase. X-ray diffraction and optical microscopy reveal that of 51 single crystals of the β parent phase, 24 form single domain crystals of α or γ with a single orientation matrix, confirming an SCSCT. The remaining 27 β parent crystals transform to crystals with a few domains of either α and/or γ, each domain arising from a single nucleation event. In three cases the β → α and β → γ transformations occurred within the same glycine single crystal. In all cases, regardless of the number of domains, the transformation occurred with retention of the original habit of the parent phase. Both β → α and β → γ transformations proceed in a non-topotactic manner, as evident from the random orientations of the daughter phases in the crystallographic reference frame of the parent β phase. The transformation rates, as measured by advancement of the growth front with polarized optical microscopy, vary from crystal to crystal as well as for different regions within the same crystal. Moreover, the transformation rate increases substantially with relative humidity, and at a relative humidity of 90% α-glycine was observed as the only product

Non-Topotactic Phase Transformations in Single Crystals of β‑Glycine

The metastable β polymorph of glycine exhibits a single-crystal-to-single-crystal transformation (SCSCT) to either the α or γ phase with retention of the crystal habit of the parent β phase. X-ray diffraction and optical microscopy reveal that of 51 single crystals of the β parent phase, 24 form single domain crystals of α or γ with a single orientation matrix, confirming an SCSCT. The remaining 27 β parent crystals transform to crystals with a few domains of either α and/or γ, each domain arising from a single nucleation event. In three cases the β → α and β → γ transformations occurred within the same glycine single crystal. In all cases, regardless of the number of domains, the transformation occurred with retention of the original habit of the parent phase. Both β → α and β → γ transformations proceed in a non-topotactic manner, as evident from the random orientations of the daughter phases in the crystallographic reference frame of the parent β phase. The transformation rates, as measured by advancement of the growth front with polarized optical microscopy, vary from crystal to crystal as well as for different regions within the same crystal. Moreover, the transformation rate increases substantially with relative humidity, and at a relative humidity of 90% α-glycine was observed as the only product

Non-Topotactic Phase Transformations in Single Crystals of β‑Glycine

The metastable β polymorph of glycine exhibits a single-crystal-to-single-crystal transformation (SCSCT) to either the α or γ phase with retention of the crystal habit of the parent β phase. X-ray diffraction and optical microscopy reveal that of 51 single crystals of the β parent phase, 24 form single domain crystals of α or γ with a single orientation matrix, confirming an SCSCT. The remaining 27 β parent crystals transform to crystals with a few domains of either α and/or γ, each domain arising from a single nucleation event. In three cases the β → α and β → γ transformations occurred within the same glycine single crystal. In all cases, regardless of the number of domains, the transformation occurred with retention of the original habit of the parent phase. Both β → α and β → γ transformations proceed in a non-topotactic manner, as evident from the random orientations of the daughter phases in the crystallographic reference frame of the parent β phase. The transformation rates, as measured by advancement of the growth front with polarized optical microscopy, vary from crystal to crystal as well as for different regions within the same crystal. Moreover, the transformation rate increases substantially with relative humidity, and at a relative humidity of 90% α-glycine was observed as the only product

Relationship between Tribology and Optics in Thin Films of Mechanically Oriented Nanocrystals

Many crystalline dyes, when rubbed unidirectionally with cotton on glass slides, can be organized as thin films of highly aligned nanocrystals. Commonly, the linear birefringence and linear dichroism of these films resemble the optical properties of single crystals, indicating precisely oriented particles. Of 186 colored compounds, 122 showed sharp extinction and 50 were distinctly linearly dichroic. Of the latter 50 compounds, 88% were more optically dense when linearly polarized light was aligned with the rubbing axis. The mechanical properties of crystals that underlie the nonstatistical correlation between tribological processes and the direction of electron oscillations in absorption bands are discussed. The features that give rise to the orientation of dye crystallites naturally extend to colorless molecular crystals

Strong Intermolecular Electronic Coupling of Chromophores Confined in Hydrogen-Bonded Frameworks

Guanidinium organodisulfonate (GDS) hydrogen-bonded frameworks constructed from “tetris-shaped” ortho-substituted disulfonated stilbene derivatives display crystal architectures in which the stilbenes serve as pillars that connect opposing guanidinium sulfonate (GS) sheets in a continuously layered architecture while guiding the organization of the stilbene residues into packing motifs that produce unique optical properties. The constraints imposed by ortho-substitution result in a heretofore unreported topology of the pillars projecting from the two-dimensional GS sheet, while the dense packing of stilbene constituents, confined between the GS sheets, results in strong intermolecular electronic coupling. Stilbene 420 (2,2″-([1,1′-biphenyl]-4,4′-diyldi-2,1-ethenediyl)­bis-benzenesulfonate) pillars pack in a face-to-face brickwork motif, producing a large bathochromic shift (∼100 nm) of the absorbance and emission spectra relative to stilbene 420 in methanol. The distyrylbenzenedisufonate (2,2′-((1<i>E</i>,1′<i>E</i>)-1,4-phenylenebis­(ethene-2,1-diyl))­dibenzenesulfonate) pillars, which pack in a face-to-face herringbone motif between the GS sheets, afford both hypsochromic and bathochromic shifts in their absorption spectrum, indicative of an unusually large Davydov splitting. The observation of both bathochromic and hypsochromic shifts can be attributed to the herringbone arrangement, in which both transitions are allowed due to the nonzero vector sum of the transition dipoles in both states. The large magnitude of the Davydov splitting reflects the strong intermolecular coupling between the chromophores, enforced by confinement in the GS framework. The newly discovered GS architectures evoke a new design rule that permits prediction of GS topologies in the case of longer tetris-shaped pillars

Strong Intermolecular Electronic Coupling of Chromophores Confined in Hydrogen-Bonded Frameworks

Guanidinium organodisulfonate (GDS) hydrogen-bonded frameworks constructed from “tetris-shaped” ortho-substituted disulfonated stilbene derivatives display crystal architectures in which the stilbenes serve as pillars that connect opposing guanidinium sulfonate (GS) sheets in a continuously layered architecture while guiding the organization of the stilbene residues into packing motifs that produce unique optical properties. The constraints imposed by ortho-substitution result in a heretofore unreported topology of the pillars projecting from the two-dimensional GS sheet, while the dense packing of stilbene constituents, confined between the GS sheets, results in strong intermolecular electronic coupling. Stilbene 420 (2,2″-([1,1′-biphenyl]-4,4′-diyldi-2,1-ethenediyl)­bis-benzenesulfonate) pillars pack in a face-to-face brickwork motif, producing a large bathochromic shift (∼100 nm) of the absorbance and emission spectra relative to stilbene 420 in methanol. The distyrylbenzenedisufonate (2,2′-((1<i>E</i>,1′<i>E</i>)-1,4-phenylenebis­(ethene-2,1-diyl))­dibenzenesulfonate) pillars, which pack in a face-to-face herringbone motif between the GS sheets, afford both hypsochromic and bathochromic shifts in their absorption spectrum, indicative of an unusually large Davydov splitting. The observation of both bathochromic and hypsochromic shifts can be attributed to the herringbone arrangement, in which both transitions are allowed due to the nonzero vector sum of the transition dipoles in both states. The large magnitude of the Davydov splitting reflects the strong intermolecular coupling between the chromophores, enforced by confinement in the GS framework. The newly discovered GS architectures evoke a new design rule that permits prediction of GS topologies in the case of longer tetris-shaped pillars