Structure Calculation of an Elastic Hydrogel from Sonication of Rigid Small Molecule Components

Abstract

The formation of hydrogels by the noncovalent assembly of organic molecules into nanoscale, cross-linked fibrous aggregates is highly topical and has been recently reviewed. [1] Hydrogels have wide-ranging potential applications in tissue engineering, [2,3] as vehicles for controlled drug delivery, [4,5] in the templated synthesis of nanoparticles and inorganic nanostructures, [6,7] in template polymerization, [8] and in pollutant capture and removal.[9] Small molecule hydrogelators frequently aggregate by hydrogen bonding, p-stacking, metal coordination, [8,10,11] and hydrophobic interactions to give often quite complex morphologies in a process that is closely related to crystallization. [12] In some cases it is thought that the solid-state structure of the gel fibers is the same as the structure of the crystalline gelator. [13,14] However, more commonly, either a different structure is adopted or the structure is unknown because the poorly crystalline nature of the gel or dried gel (xerogel) does not give powder X-ray diffraction (PXRD) patterns that are amenable to structure solution and refinement by Rietveld methods. The most common classes of gelators include nucleobases, saccharides, peptides, ureas, [15,16] and steroid derivatives.[12] Nucleobases [17,18] and related planar components, such as melamine [19,20] in particular, are interesting because of the multiple hydrogen-bonding interactions between complementary nucleobase pairs, suggesting that robust supramolecular gels might arise. In general however, nucleobases and other related heterocycles must be derivatized with long alkyl chains to give appropriate solubility properties and allow extensive hydrophobic interactions before they can act as gelators. Underivatized, rigid, planar molecules with such good hydrogen bonding functionality are commonly very insoluble as a result of the formation of very stable p-stacking and infinite hydrogen bonded chains in the solid state. Recent reports have shown, however, that sonication can induce gel formation. [21,22] Sonication is more commonly used to increase the dissolution rate of insoluble compounds. We reasoned that there may be a link between sonication-induced gelation and the partial solubilization of insoluble compounds, particularly for those compounds with strong hydrogen bonding functionality. Sonication-induced dissolution of small quantities of strongly hydrogen-bonding species may result in their rapid aggregation under nonequilibrium conditions and the deposition of a fibrous material without time for full crystallization to take place. Herein we present hydrogelation by a mixed system comprising two entirely rigid, insoluble, mutually complementary, planar multifunctional hydrogenbond donor/acceptors, namely melamine and uric acid (denoted M and U; Scheme 1), and we propose the likely structure of the gel from the results of crystal structure prediction calculations. [23][24][25] The melamine-uric acid (M-U) pair is related to the wellknown melamine-cyanuric acid rosette assemblies; [26,27]...

Proton‐Conducting Supramolecular Metallogels from the Lowest Molecular Weight Assembler Ligand: A Quote for Simplicity

“…As expected, PXRD of the bulk gels differed from that of the xerogels due to substantial scattering of the X-ray beam by the large amount of solvent trapped in the interstices of the gel network ( Figure 3). [7] However, PXRD of the xerogels matched to some extent the pattern of the Cu II oxalate hemihydrate, although this was unable to induce gelation on its own.…”

“…[4,5] In this sense, the gelling agents in metallogels are often discrete metal coordination complexes of low-molecular-weight (LMW) ligands or well-defined coordination polymers, [3] which enable the formation of extended coordi-nation networks that can immobilize large volumes of solvent by different mechanisms. Recent studies clearly stimulate the necessity of developing more powerful X-ray diffraction techniques and theoretical methods to gain closer insights into the gelation mechanisms of multivariable systems, [7] including metallogels. Recent studies clearly stimulate the necessity of developing more powerful X-ray diffraction techniques and theoretical methods to gain closer insights into the gelation mechanisms of multivariable systems, [7] including metallogels.…”

Pyrene‐Based Fluorescent Ambidextrous Gelators: Scaffolds for Mechanically Robust SWNT–Gel Nanocomposites

“…In addition, the intrinsic fluorescence of pyrene can be exploited to develop light‐harvesting systems and sensors 12. 13 The linker amino acids have a strong propensity to form intermolecular hydrogen bonding, which is a crucial factor for self‐aggregation 8. We have used three different amino acids, L ‐alanine, L ‐phenylalanine and L ‐tryptophan, which show a gradual change in character from aliphatic to aromatic.…”

“…Over the last few years, chemists have continuously indulged in the synthesis of new functional molecules with the objective of task‐specific applications 9. To this end, fluorescent supramolecular gels have gained much interest due to their growing importance in organic light‐emitting diodes, photovoltaic cells, light‐harvesting systems and sensors 10–13. Depending on their solvent immobilisation nature, LMWGs are primarily categorised into two classes: low‐molecular‐weight hydrogelators (LMHGs) and low‐molecular‐weight organogelators (LMOGs) 6a.…”

Crystalline Supramolecular Nanofibers Based on Dehydrobenzoannulene Derivatives

“…Finally, information on the molecular arrangements were obtained by crystal structural predictions (CSPs) 20a. 44 The CSPs on DBAs 1 and 3 were performed under the following conditions. The three conformers of DBAs 1 and 3 ( hor‐hor , hor‐ver , and ver‐ver conformers; see Scheme S4 and Figure S1 in the Supporting Information) were optimized by performing DFT calculations at the B3LYP/6‐31G++(d,p) level of theory.…”