Low-molecular-weight amino-acid-based derivatives: from organogels to single crystals and mesocrystals

FLORENCIA DI SALVO; FEDERICO MOVILLA; JUAN MANUEL REY; OLGA C. SÁNCHEZ M.

Hyderabad

Congreso; Congress and General Assembly of the International Union of crystallography; 2017

International Union of Crystallography

Amino acids are able to self-assemble into ordered superstructures, also called mesocrystals, when they are subjected to certain crystallization conditions such as, pH, supersaturation level and the use of additives, being the last one the most popular strategy [1, 2]. While different morphologies of crystalline arrangements are known, the greatest interest lies in the obtaining of three-dimensional (3D) self-assemblies with hierarchical architectures that can contribute to the knowledge of the controlled manufacture of nanoscale materials and novel devices. There are several examples in recent literature of inorganic compounds which provide interesting responses to this problem however, only a few organic molecules have been considered as building blocks. Thus, the design and synthesis of 3D supernaonoestrutcures with hierarchical morphologies based on a single kind of organic molecule is an interesting challenge. Besides, self-assembly strategies applied to low-molecular weight building blocks can conduct to supramolecular gels. Their properties can be modulated by variation of temperature or solvent composition, since these changes directly affect the strength of the non-covalent interactions within the gel network [3]. The obtaining of both, supramolecular gels or crystalline materials (mesocrystals or single crystals) is governed by the presence non-covalent interactions (such as hydrogen bonding, pi?pi interactions, metal?ligand coordination, van der Waals forces, hydrophobic effects), resulting in spanning network that can immobilize solvents for the first ones or, self-assemble mainly between the low molecular weight building blocks to give place to the second ones. Thus, there is a delicate balance for a low-weight molecule to behave as a gelator or crystal.In this work we present a new family of chiral L-amino-acid-based low-weight molecules which behave as excellent building blocks to the construction of supramolecular gels or crystalline structures depending on the fine tune of the functional groups and the environment conditions (temperature, solvent and/or pH). Compounds 1-3 are obtained in their zwitterionic form after a Schiff base reaction followed by reduction, between the aldehyde piperonal and L-Alanine (Ala), L-Phenylalanine (Phe) and L-Tyrosine (Tyr), respectively (see figure). Crystallization experiments using organic solvents are only possible with very dilute solutions of the amino acid derivatives due to their low solubility. As result, compounds 1 and 3 conduct mainly to polycrystalline materials and 2 gives place to crystalline superstructures with spherical morphologies attributed to the different solubility and supramolecular properties conferred by the amino acid bone. All derivatives are soluble in basic solutions after the deprotonation of the zwitterion and for these mixtures, spherical like mesocrystals are obtained after slow evaporation of the solvent. There is also a dependence of the morphology with the ionic strength; the symmetry of the sphere apparently increases for higher ionic strengh values. The crystalline materials in all cases are studied by polarized optical microscopy (POM), scanning electron microscopy (SEM), powder XRD, FTIR and NMR. Due to their structural features, compounds 1-3 are also soluble in acids. After slow diffusion of hydrogen chloride acid vapors in basic solutions of 2, single crystals suitable for X-ray diffraction studies are obtained. The crystallographic results confirm the presence of the protonated derivative (see figure). Although 1 and 3 do not give place to single crystals of suitable quality under the same conditions, experiments using other acids are in progress. In order to increase the solubility of compounds 1-3 in their zwitterionic form in organic solvents, the temperature of the solutions can be increased. Only the derivative of Phe is soluble in higher concentrations after increasing the temperature and then after cooling, it gives place to supramolecular gels showing again a direct correlation between the identity of the building block and the supramolecular behavior when compares compound 1-3. NMR, powder XRD, SEM, FTIR, UV-visible and elemental analysis confirm the identity and properties of the gel and the reversibility of the process with temperature. As expected, the properties of the organogel (Minimum Gelation Concentration, transparency, etc.) depend on the identity of solvent (alcohols C > 4, dioxane and alkanes). In conclusion, it is possible to suggest that the presence and directionality of certain functional groups and the intermolecular interactions developed, such as the existence or not of the OH and the phenyl group, are key factors in the mechanism of self-assembly into hierarchical structures, single crystals or even supramolecular organogels. Furthermore, the strong pH dependence allows the obtaining of single crystals or the assembly to crystalline superstructures. This new family of very low-molecular-mass derivatives shows a huge versatility regarding supramolecular properties. Varying the aldehyde and/or the amino acid, opens a broad perspective for the design of novel self-assembly architectures for the further development of functional soft and/or crystalline materials.[1] Jiang, H. et al. (2011). Cryst. Growth Des, 11, 3243?3249, and references therein.[2] Ejgenberg, M. & Mastai, Y. (2012). Cryst. Growth Des, 12, 4995−5001.[3] Buerkle, L.E. & Rowan, S.J. (2012). Chem. Soc. Rev. 41, 6089?6102.