Effects of Carbon Atom Parity and Alkyl Side Chain Length on the Crystallization and Morphology of Biscarbamates, A Set of Model Compounds for Polyurethanes

The role of alkyl chain length in the melt and solution crystallization of paliperidone aliphatic prodrugs

Fatty acid-derivative prodrugs have been utilized extensively to improve the physicochemical, biopharmaceutical and pharmacokinetic properties of active pharmaceutical ingredients. However, to our knowledge, the crystallization behavior of prodrugs modified with different fatty acids has not been explored. In the present work, a series of paliperidone aliphatic prodrugs with alkyl chain lengths ranging from C4 to C16 was investigated with respect to crystal structure, crystal morphology and crystallization kinetics. The paliperidone derivatives exhibited isostructural crystal packing, despite the different alkyl chain lengths, and crystallized with the dominant (100) face in both melt and solution. The rate of crystallization for paliperidone derivatives in the melt increases with alkyl chain length owing to greater molecular mobility. In contrast, the longer chains prolong the nucleation induction time and reduce the crystal growth kinetics in solution. The results show a correlation between difficulty of nucleation in solution and the interfacial energy. This work provides insight into the crystallization behavior of paliperidone aliphatic prodrugs and reveals that the role of alkyl chain length in the crystallization behavior has a strong dependence on the crystallization method.

Influence of odd-even effect and intermolecular interactions in 2D molecular layers of bisamide organogelators

Organogelators have a wide range of use in everyday life including drug delivery and controlled release, surface coating and paper industry. In this study, a series of model bisamides have been analyzed as potential organogelators. These molecules are connected by odd and even numbered methylene units (n) in length ranging from 2 to 9. By constructing layers of those molecules along the growth direction we provide an insight into the self-assembly process. A complete systematic analysis of the computational results with B3LYP/6-311+G** suggests that the self-assembly of these potential organogelators is influenced by the odd–even effect, the relative direction of amide carbonyl groups, the bridging spacer chain length and the presence of a chiral alpha carbon. The aforementioned factors alter the strength of the intermolecular hydrogen bonds as well as the van der Waals interactions, which in turn may affect the self-assembly process of gelation and result in the formation of aggregates with different shapes. It is found that molecules with short central chains have an energetic preference for antiparallel arrangement over their parallel analogues as a result of stronger hydrogen bonding interactions. As the central chain elongates, the free energy difference between antiparallel and parallel structures decreases suggesting a compromise between hydrogen bonding and van der Waals interactions. The complete structural analysis suggests ribbon-like structures for achiral even-antiparallel and woven-like structures for odd-parallel systems, respectively. Upon creation of asymmetry on the alpha carbon, a twisted ribbon-like and a coiled coil-like structure are observed for even and odd systems, respectively. Our computational results are in accordance with the experimental results and provide an insight into the self-assembly of layers of bisamides.

Our computational results rationalize the factors that affect the molecular self-assembly of bisamides and its role on gelation.

Effect of Reinforcement at Length Scale for Polyurethane Cellular Scaffolds by Supramolecular Assemblies

This study is aimed to represent the role of carbonaceous nanofillers to reinforce the commercially available polyurethane porous structure. The effect of dimensionality of fillers to anchor the construction of stable three-dimensional (3D) cellular architectures has been highlighted. The cellular frameworks of commercially available thermoplastic polyurethane (TPU) have been fabricated through the thermoreversible supramolecular self-assembly route. It was established that the minimum shrinkage of TPU lattice structures occurred when the solid-state network is strengthened by the topologically engineered 3D hierarchical nanofillers, where the amount of reinforcement was found to play a critical role. It has been established by series of structure-property correlations that reinforcing the cellular structure to endure the capillary stress is equally effective as supercritical drying for producing low-density porous morphologies. The removal of liquid phase from gel is as important as the presence of 3D fillers in the matrix for reinforcing the cellular structures when replacing the solvent phase with air to generate a two-phase solid-gas engineered morphology. The insight into the polyurethane network structure revealed that the dimensionality, amount, and distribution of fillers in the matrix are critical for reinforcing the cellular scaffolds in solid gel without any cross-linking.

Two side chains, three supramolecules: exploration of fluorenone derivatives towards crystal engineering

Structural diversity obtained through two-dimensional molecular self-assembly induced by the chain length effect has gained immense attention, not only because of its significance in crystal engineering but also for its potential application in nanoscience and nanotechnology. Three kinds of fluorenone derivative, named F-C₇C₇, F-C₁₄C₇, and F-C₁₄C₁₄, were synthesized and used for systematic exploration of their crystalline difference. At first, scanning electron microscopy and X-ray powder diffraction were performed to investigate their differences in morphology and three-dimensional crystal structure. Then scanning tunneling microscopy experiments were conducted to compare the self-assembled monolayers. Moreover, different solvents were used to repeatedly investigate the occurrence of structural diversity. F-C₇C₇ could not self-assemble into a stable monolayer on the graphite surface under ambient conditions due to its weak molecule-substrate interaction. F-C₁₄C₇ was observed to self-assemble into twist, plier-like, octamer-curve, and random structures in 1-octanoic acid, 1-phenyloctane, n-tetradecane, and dichloromethane, respectively. However, when the same solvents were used and at similar concentrations, the F-C₁₄C₁₄ molecules were arranged into interval, mixed, linear, and plier-like configurations. These self-assembled nanopatterns formed under the driving forces of dipole-dipole interactions, hydrogen bonds, and chain-chain, molecule-substrate, and molecule-solvent van der Waals interactions. Furthermore, from the viewpoint of thermal analysis, differential scanning calorimetry, as well as polarized optical microscopy, was performed to further elucidate the difference between these three compounds in the solid and liquid crystal states. The present system is believed to provide understanding of how the chain length effect induces different crystalline properties, and to open up the possibility of fabricating diverse self-assembled networks for crystal engineering.

Alkyl bicarbamates supramolecular organogelators with effective selective gelation and high oil recovery from oil/water mixtures

A series of alkyl bicarbamates supramolecular organogelators were synthesized with different structures and lengths of alkyl chains. The driving forces for the self-assembly of small molecules, including the intermolecular H bonding, π-π stacking and van der Waals interactions, played an important role in the formation of different 3D network structures, i.e., fibers, ribbons, sheets, and prisms. And a probable formation process of the gel networks was proposed. Furthermore, the phase-selective gelling performances were investigated for oil removal from aqueous solution. Interestingly, the gelling properties were found to be affected by the length and structure of alkyl chains, while some gelators with intermediate alkyl chain lengths could effectively gel all the tested oils from water surface within 15 min, such as Russian crude oil, diesel, gasoline, soybean oil, peanut oil, olive oil, cyclohexane, hexane and ethyl acetate. Advantageously, fast gelation, high rate of oil removal (>95%) and excellent oil retention rate (close to 100%) were realized in the recovery of oil spills from water surface. This kind of supramolecular gelators demonstrates good potential applications in the delivery or removal of organic pollution from oil/water mixtures.

Inhibiting the Self-Sorting Behavior in the Blends of a Homologous Set of Polyurethane Model Compounds

Self-sorting is the phenomenon in which there is high fidelity recognition and preference only for self and avoidance of nonself (narcissistic self-sorting). It has been observed in a number of biological systems and chiral synthetic molecules. We found that blends of biscarbamates, which are model compounds for polyurethanes, self-sort during crystallization [ J. Phys. Chem. B 2008 , 112 , 4223 - 4232 ], although these are not chiral molecules. Even if the two components in the blend differ only by a couple of CH2 groups in the side chain length, no intercomponent hydrogen bond forms, and the molecules self-sort. They do not show any cocrystallization despite being part of a homologous series. We believe that it is the first reported example such behavior among synthetic nonchiral molecules. This is similar to the behavior of blends of hydrogen-bonding polymers including polyurethanes. We show that the difference in the growth rates of the individual species is responsible for the self-sorting behavior in these nonchiral synthetic compounds. While self-sorting might be advantageous for separation of blends, it poses a challenge for modifying properties such as the melting temperatures, spherulite size, etc., for various applications. We will discuss methods that were attempted to bridge the self and nonself that would lead to a more homogeneous system. We evaluated the miscibility using differential scanning calorimetry (DSC), since the occurrence of a single or multiple endotherms would indicate molecular level miscibility. This is similar to the behavior of glass transition temperatures in the case of polymer blends. Optical microscopy (OM) and X-ray diffraction (XRD) were also used. It is concluded that irrespective of the protocol followed for preparing the mixtures, mutual plasticization occurred in most cases (i.e., mixing of domains of the two species) and not molecular mixing.

Structural transition control between dipole–dipole and hydrogen bonds induced chirality and achirality

This study presents efficient strategies on manipulation of hydrogen bonds and dipole–dipole induced chiral and achiral self-assembly nanostructures.

Odd–even effect on the thermal properties of Schiff base functionalized dicyanate esters and thermo-mechanical properties of their blends with epoxy resins

An odd–even effect was observed in the case of dicyanate esters described in this work.

Multi odd–even effects on cell parameters, melting points, and optical properties of chiral crystal solids based on S-naproxen

Chiral solids based on S-naproxen alternatively crystallize in P2₁ and P2₁2₁2₁, respectively, which show the odd–even effects on cell parameters, melting points, and luminescence.

Succinate esters: odd-even effects in melting points

Dialkyl succinates show a pattern of alternating behavior in their melting points, as the number of C atoms in the alkane side chain increases, unlike in the dialkyl oxalates [Joseph et al. (2011). Acta Cryst. B67, 525-534]. Dialkyl succinates with odd numbers of C atoms in the alkyl side chain show higher melting points than the immediately adjacent analogues with even numbers. The crystal structures and their molecular packing have been analyzed for a series of dialkyl succinates with 1-4 C atoms in the alkyl side chain. The energy difference (ΔE) between the optimized and observed molecular conformations, density, Kitaigorodskii packing index (KPI) and C-H...O interactions are considered to rationalize this behavior. In contrast to the dialkyl oxalates where a larger number of moderately strong C-H...O interactions were characteristic of oxalates with elevated melting points, here the molecular packing and the density play a major role in raising the melting point. On moving from oxalate to succinate esters the introduction of the C2 spacer adds two activated H atoms to the asymmetric unit, resulting in the formation of stronger C-H...O hydrogen bonds in all succinates. As a result the crystallinity of long-chain alkyl substituted esters improves enormously in the presence of hydrogen bonds from activated donors.