Laser‐Induced Crystallization and Crystal Growth

Recent streams of laser studies on crystallization and crystal growth are summarized and reviewed. Femtosecond multiphoton excitation of solutions leads to their ablation at the focal point, inducing local bubble formation, shockwave propagation, and convection flow. This phenomenon, called “laser micro tsunami” makes it possible to trigger crystallization of molecules and proteins from their supersaturated solutions. Femtosecond laser ablation of a urea crystal in solution triggers the additional growth of a single daughter crystal. Intense continuous wave (CW) near infrared laser irradiation at the air/solution interface of heavy‐water amino acid solutions results in trapping of the clusters and evolves to crystallization. A single crystal is always prepared in a spatially and temporally controlled manner, and the crystal polymorph of glycine depends on laser power, polarization, and solution concentration. Upon irradiation at the glass/solution interface, a millimeter‐sized droplet is formed, and a single crystal is formed by shifting the irradiation position to the surface. Directional and selective crystal growth is also possible with laser trapping. Finally, characteristics of laser‐induced crystallization and crystal growth are summarized.     

Visible‐Light‐Induced Photodimerization of a Photoactive Yellow Protein (PYP) Chromophore Model in a Single Crystal

The chromophore of the photoactive yellow protein (PYP), the photoreceptor in the photomotility of the bacterium Halorhodospira halophila, is a deprotonated para‐coumaric thioester linked to the side residue of a cysteine residue. The photophysics of the PYP chromophore is conveniently modeled with para‐hydroxycinnamic thiophenyl esters. Herein, we report the first direct evidence, obtained with X‐ray diffraction, of photodimerization of a para‐hydroxycinnamic thiophenyl ester in single crystalline state. This result represents the first direct observation of [2+2] dimerization of a model PYP chromophore, and demonstrates that even very weak light in the visible region is capable of inducing parallel radical reactions in PYP from the excited state of the chromophore, in addition to the main reaction pathway (trans–cis isomerization). This PYP model system adds an interesting example to the known solid‐state photodimerizations, because unlike the anhydrous crystal (which is not capable of sustaining the stress and disintegrates in the course of photodimerization), a single water molecule “dilutes” the structure to the extent sufficient for single‐crystal‐to‐single‐crystal reaction.     

Crystallographic report: Crystal structure of tetra(4‐methyl‐5‐imidazole‐carboxyaldehyde)zinc(II) diperchlorate

The central zinc(II) atom in the title complex is tetrahedrally coordinated by four nitrogen atoms derived from 4‐methyl‐5‐imidazolecarboxyaldehyde ligands with Zn? N in the range 2.007(3) to 2.026(4) Å. Copyright © 2003 John Wiley & Sons, Ltd.     

Single Crystal‐to‐Single Crystal Site‐Selective Postsynthetic Metal Exchange in a Zn–MOF Based on Semi‐Rigid Tricarboxylic Acid and Access to Bimetallic MOFs

The metal ions in a neutral Zn–MOF constructed from tritopic triacid H₃L with inherent concave features, rigid core, and peripheral flexibility are found to exist in two distinct SBUs, that is, 0D and 1D. This has allowed site‐selective postsynthetic metal exchange (PSME) to be investigated and reactivities of the metal ions in two different environments in coordination polymers to be contrasted for the first time. Site‐selective transmetalation of Zn ions in the discrete environment is shown to occur in a single crystal‐to‐single crystal (SCSC) fashion, with metal ions such as Fe³⁺, Ru³⁺, Cu²⁺, Co²⁺, etc., whereas those that are part of 1D SBU sustain structural integrity, leading to novel bimetallic MOFs, which are inaccessible by conventional approaches. To the best of our knowledge, site‐selective postsynthetic exchange of an intraframework metal ion in a MOF that contains metal ions in discrete as well as polymeric SBUs is heretofore unprecedented.     

The Structure of 2‐[(1S)‐(+)‐10‐camphorsulfonamino]‐6‐aminopyridine in the Solid State and in Solution; Unprecedented Cocrystallization of Four Stereoisomers

The solid and solution structures of a new optically active aminopyridine compound, 2‐[(1S)‐(+)‐10‐camphorsulfonamino]‐6‐aminopyridine [(S)‐csaap], 1 , are reported. Crystal data: space group P2₁, a = 8.9729 (5), b = 10.9447 (6), c = 36.693 (2) Å, β = 96.435 (1)°, V = 3580.8 (3) ų, Z = 8, R₁ = 0.0673 and wR₂ = 0.1600 with I > 2σ(I). This chiral compound shows an unprecedented cocrystallization of four stereoisomers, which are characterized by X‐ray crystallography and NMR spectroscopy.     

Crystal Structures and Emitting Properties of Trifluoromethylaminoquinoline Derivatives: Thermal Single‐Crystal‐to‐Single‐Crystal Transformation of Polymorphic Crystals That Emit Different Colors

2,4‐Trifluoromethylquinoline (TFMAQ) derivatives that have amine ( 1 ), methylamine ( 2 ), phenylamine ( 3 ), and dimethylamine ( 4 ) substituents at the 7‐position of the quinoline ring were prepared and crystallized. Six crystals including the crystal polymorphs of 2 (crystal GB and YG) and 3 (crystal B and G) were obtained and characterized by X‐ray crystallography. In solution, TFMAQ derivatives emitted relatively strong fluorescence (${\lambda {{{\rm f}\hfill \atop {\rm max}\hfill}}}$ =418–469 nm and Φ_f(s)=0.23–0.60) depending on the solvent polarity. From Lippert–Mataga plots, Δμ values in the range of 7.8–14 D were obtained. In the crystalline state, TFMAQ derivatives emitted at longer wavelengths (${\lambda {{{\rm f}\hfill \atop {\rm max}\hfill}}}$ =464–530 nm) with lower intensity (Φ_f(c)=0.01–0.28) than those in n‐hexane solution. The polymorphous crystals of 2 and 3 emitted different colors: 2 , ${\lambda {{{\rm f}\hfill \atop {\rm max}\hfill}}}$ =470 and 530 nm with Φ_f(c)=0.04 and approximately 0.01 for crystal GB and YG, respectively; and 3 , ${\lambda {{{\rm f}\hfill \atop {\rm max}\hfill}}}$ =464 and 506 nm with Φ_f(c)=0.28 and approximately 0.28 for crystal B and G, respectively. In both crystal polymorphs of 2 and 3 , crystals GB and G showed emission color changes by heating/melting/cooling cycles that were representative. By following the color changes in heating at the temperature below the melting point with X‐ray diffraction measurements and X‐ray crystallography, the single‐crystal‐to‐single‐crystal transformations from crystal GB to YG for 2 and from crystal B to G for 3 were revealed.     

An Optimised Small‐Molecule Stabiliser of the 14‐3‐3–PMA2 Protein–Protein Interaction

Modulation of protein–protein interactions (PPIs) is a highly demanding, but also a very promising approach in chemical biology and targeted drug discovery. In contrast to inhibiting PPIs with small, chemically tractable molecules, stabilisation of these interactions can only be achieved with complex natural products, like rapamycin, FK506, taxol, forskolin, brefeldin and fusicoccin. Fusicoccin stabilises the activatory complex of the plant H⁺‐ATPase PMA2 and 14‐3‐3 proteins. Recently, we have shown that the stabilising effect of fusicoccin could be mimicked by a trisubstituted pyrrolinone (pyrrolidone1, 1 ). Here, we report the synthesis, functional activity and crystal structure of derivatives of 1 that stabilise the 14‐3‐3–PMA2 complex. With a limited compound collection three modifications that are important for activity enhancement could be determined: 1) conversion of the pyrrolinone scaffold into a pyrazole, 2) introduction of a tetrazole moiety to the phenyl ring that contacts PMA2, and 3) addition of a bromine to the phenyl ring that exclusively contacts the 14‐3‐3 protein. The crystal structure of a pyrazole derivative of 1 in complex with 14‐3‐3 and PMA2 revealed that the more rigid core of this molecule positions the stabiliser deeper into the rim of the interface, enlarging especially the contact surface to PMA2. Combination of the aforementioned features gave rise to a molecule ( 37 ) that displays a threefold increase in stabilising the 14‐3‐3–PMA2 complex over 1 . Compound 37 and the other active derivatives show no effect on two other important 14‐3‐3 protein–protein interactions, that is, with CRaf and p53. This is the first study that describes the successful optimisation of a PPI stabiliser identified by screening.     

Crystallographic report: Crystal structure of tris(2,6‐di‐tert‐butyl‐4‐methylphenolato‐O) (tetrahydrofuran‐O) erbium toluene solvate

A low‐coordinate aryloxo erbium complex, [(ArO)₃Er(THF)](MePh), has been synthesized by the reaction of anhydrous ErCl₃ with three equivalents of NaOAr in tetrahydrofuran. The central erbium atom is coordinated by three oxygen atoms of the aryloxo ligands and one oxygen atom of the tetrahydrofuran molecule, resulting in a distorted tetrahedron. Copyright © 2004 John Wiley & Sons, Ltd.     

Controlling Nucleation and Crystal Growth of a Distinct Polyoxovanadate Cluster: An In Situ Energy Dispersive X‐ray Diffraction Study under Solvothermal Conditions

The formation of the antimonato polyoxovanadates [V₁₄Sb₈(C₆H₁₅N₃)₄O₄₂(H₂O)] ? 4H₂O ( 1 ), (C₆H₁₇N₃)₂[V₁₅Sb₆(C₆H₁₅N₃)₂O₄₂(H₂O)] ? 2.5H₂O ( 2 ), {C₆H₁₅N₃}₄[V₁₆Sb₄O₄₂] 2H₂O ( 3 ) (C₆H₁₅N₃=1‐(2‐aminoethyl)piperazine, AEP) has been studied under solvothermal conditions by using in situ energy dispersive X‐ray diffraction (EDRXD). The syntheses were performed with an identical ratio for Sb₂O₃ and NH₄VO₃. If the reactions slurries are not stirred during the solvothermal reaction and by applying 70–75 % amine concentration, the products contain all three compounds, whereas 3 is observed at 80 %. Under stirring conditions, variation of the concentration of AEP led to crystallization of the three different compounds at distinct concentrations, that is, 1 is formed at 75 %, 1 and 2 between 75 and 80 % and 3 at 80 %. At an amine concentration of 77.5 %, first reflections of 2 occurred and at later stages, compound 1 started to crystallize. The sample with the lowest number of V^IV species was formed at the lowest amine concentration, whereas crystallization of 3 required the highest concentration. The formation of the compounds occurred without crystalline intermediates and/or precursors. With increasing reaction temperature, the incubation time was significantly reduced.     

Supramolecular Synthons in Designing Low Molecular Mass Gelling Agents: L‐Amino Acid Methyl Ester Cinnamate Salts and their Anti‐Solvent‐Induced Instant Gelation

Easy access to a class of chiral gelators has been achieved by exploiting primary ammonium monocarboxylate ( PAM ), a supramolecular synthon. A combinatorial library comprising of 16 salts, derived from 5 l ‐amino acid methyl esters and 4 cinnamic acid derivatives, has been prepared and scanned for gelation. Remarkably, 14 out of 16 salts prepared (87.5 % of the salts) show moderate to good gelation abilities with various solvents, including commercial fuels, such as petrol. Anti‐solvent induced instant gelation at room temperature has been achieved in all the gelator salts, indicating that the gelation process is indeed an aborted crystallization phenomenon. Rheology, optical and scanning electron microscopy, small angle neutron scattering, and X‐ray powder diffraction have been used to characterize the gels. A structure‐property correlation has been attempted, based on these data, in addition to the single‐crystal structures of 5 gelator salts. Analysis of the FT‐IR and ¹H NMR spectroscopy data reveals that some of these salts can be used as supramolecular containers for the slow release of certain pest sex pheromones. The present study clearly demonstrates the merit of crystal engineering and the supramolecular synthon approach in designing new materials with multiple properties.     

Hexakis(2,4,6‐triisopropylphenyl)cyclotristannoxane – a Molecular Diorganotin Oxide with Kinetically Inert Sn–O Bonds

The single‐crystal X‐ray structure analysis of hexakis(2,4,6‐triisopropylphenyl)cyclotristannoxane, cyclo‐[(2,4,6‐i‐Pr₃‐C₆H₂)₂SnO]₃ ( 1 ), is reported and reveals this compound to contain an almost planar six‐membered ring. Redistribution reactions of 1 with cyclo‐(t‐Bu₂SnO)₃ and t‐Bu₂SiCl₂, respectively, failed and indicate an unusual kinetic inertness of the Sn–O bonds in 1 as compared to related molecular diorganotin oxides containing less bulkier substituents. The redistribution reaction of cyclo‐(t‐Bu₂SnO)₃ with cyclo‐(t‐Bu₂SnS)₂ leads to an equilibrium involving the trimeric diorganotin oxysulphides cyclo‐t‐Bu₂Sn(OSnt‐Bu₂)₂S ( 2 a ) and cyclo‐t‐Bu₂Sn(SSnt‐Bu₂)₂O ( 2 b ).     

Synthesis and Antibacterial Activities of 2‐(1‐Aryl‐5‐Methyl‐1,2,3‐triazol‐4‐yl)‐1,3,4‐Oxadiazole Derivatives

Eighteen novel 2‐(1‐aryl‐5‐methyl‐1,2,3‐triazol‐4‐yl)‐1,3,4‐oxadiazole derivatives and two acylhydrazone intermediate compounds were synthesized by various pathways starting from 1‐aryl‐5‐methyl‐1,2,3‐triazol‐4‐formhydrazide ( 1 ). All products were identified by spectroscopic analysis, and 2‐(1‐aryl‐5‐methyl‐1,2,3‐triazol‐4‐yl)‐5‐benzalthio‐1,3,4‐oxadiazole was further validated by X‐ray crystallography. Results from primary antibacterial activity tests indicated that most of the compounds were effective against E. coli, P. aeruginosa, B. subtilis and S. aureus.     

Structures of X 34‐nylons in chain‐folded lamellae and gel‐spun fibers

Structural studies and morphological features of a new family of linear, aliphatic even–even, X 34‐nylons, with X = 2, 4, 6, 8, 10, and 12, are investigated with X‐ray diffraction and electron microscopy. Solution‐grown crystals were obtained by isothermal crystallization from N,N‐dimethylformamide solutions. The thickness of lamellar‐like crystals was orders of magnitude less than the chain lengths of the polymer samples used, implying that the chains fold to form chain‐folded lamellae. The results bear a close resemblance, with the noticeable exception of 2 34‐nylon, to those reported for nylon 6 6 and other even–even nylon chain‐folded lamellar crystals. The basic structure of the straight‐stem lamellar core is similar to that of the classic nylon 6 6 triclinic α structure, and the chains tilt ≈42° relative to the lamellar normal. In the case of 2 34‐nylon, the structure resembles the 2 Y nylon series, and the chain tilt angle reduces to 36.6°. These combined results suggest that, even with a relatively low frequency of amide units along the backbone of these molecules, hydrogen bonding is still the dominant element in controlling the behavior, structure, and properties of these polymers. In addition, gels were prepared in concentrated sulfuric acid, and gel‐spun fibers were studied using X‐ray diffraction. The data are interpreted in terms of a modified nylon triclinic α structure that bears a resemblance to the structure of even–even nylons at elevated temperatures. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 2685–2692, 2002