A Theoretical Study of Cohesion, Structural Deformation, Inclusion, and Dynamics in Porous Hydrogen-Bonded Molecular Networks

Anti-friction gold-based stretchable electronics enabled by interfacial diffusion-induced cohesion

Stretchable electronics that prevalently adopt chemically inert metals as sensing layers and interconnect wires have enabled high-fidelity signal acquisition for on-skin applications. However, the weak interfacial interaction between inert metals and elastomers limit the tolerance of the device to external friction interferences. Here, we report an interfacial diffusion-induced cohesion strategy that utilizes hydrophilic polyurethane to wet gold (Au) grains and render them wrapped by strong hydrogen bonding, resulting in a high interfacial binding strength of 1017.6 N/m. By further constructing a nanoscale rough configuration of the polyurethane (RPU), the binding strength of Au-RPU device increases to 1243.4 N/m, which is 100 and 4 times higher than that of conventional polydimethylsiloxane and styrene-ethylene-butylene-styrene-based devices, respectively. The stretchable Au-RPU device can remain good electrical conductivity after 1022 frictions at 130 kPa pressure, and reliably record high-fidelity electrophysiological signals. Furthermore, an anti-friction pressure sensor array is constructed based on Au-RPU interconnect wires, demonstrating a superior mechanical durability for concentrated large pressure acquisition. This chemical modification-free approach of interfacial strengthening for chemically inert metal-based stretchable electronics is promising for three-dimensional integration and on-chip interconnection.

Redox potential tuning in bio-relevant heterocycles via (anti)aromaticity modulated H-bonding (AMHB)

Hydrogen bonds are arguably the most important non-covalent interactions in chemistry and biology, and their strength and directionality have been elegantly exploited in the rational design of complex structures. We recently noted that the variable responses of cyclic π-systems upon H-bond formation reciprocally lead to modulations of the H-bonds’ strengths, a phenomenon that we dubbed (anti)aromaticity-modulated hydrogen bonding (AMHB) [J. Am. Chem. Soc. 2016, 138, 3427–3432]. Species that switch from aromatic to antiaromatic or vice versa upon changing π-electron counts should be oppositely stabilized by the AMHB effects, so their redox potentials should be significantly “tuned” by H-bond formation. Herein, using quantum chemical simulations, we explore the effects of these H-bond induced π-electron polarizations on the redox potentials of (anti)aromatic heterocycles. The systems chosen for this study have embedded amide groups and amidine moieties capable of forming two-point H-bonds in their cyclic π-systems. Thus, as the 4-electron and 6-electron π-systems in redox-capable monocycles (e.g., quinones) can be differentially stabilized, their redox potentials can be modulated by H-bond formation by as much as 6 kcal/mol (258 mV for one electron transfer). In fused rings, the connectivity patterns are as important as the π-electron counts. Extending these ideas to flavin, a biologically relevant case, we find that H-bonding patterns like those found in its crystals can vary its redox potential by up to 1.3 kcal/mol.

Modular Construction of Porous Hydrogen-Bonded Molecular Materials from Melams

Ordered materials with predictable structures and properties can be made by a modular approach, using molecules designed to interact with neighbors and hold them in predetermined positions. Incorporating 4,6-diamino-1,3,5-triazin-2-yl (DAT) groups in modules is an effective way to direct assembly because each DAT group can form multiple N-H⋅⋅⋅N hydrogen bonds according to established patterns. We have found that modules with high densities of N(DAT)₂ groups can be made by base-induced double triazinylations of readily available amines. The resulting modules can form structures held together by remarkably large numbers of hydrogen bonds per molecule. Even simple modules with only 1-3 N(DAT)₂ groups and fewer than 70 non-hydrogen atoms can crystallize to form highly open networks in which each molecule engages in over 20 N-H⋅⋅⋅N hydrogen bonds, and more than 70 % of the volume is available for accommodating guests. In favorable cases, guests can be removed to create rigorously porous crystalline solids analogous to zeolites and metal-organic frameworks.

Switching hydrogen bonds to readily interconvert two room-temperature long-term stable crystalline polymorphs in chiral molecular perovskites

Two room-temperature polymorphic forms could be long-term stable yet easily interconvertible by switching the inter-cationic H-bonds in chiral molecular perovskites.

Hydrogen-bonded organic frameworks: design, structures and potential applications

This paper highlights the current key progress on HOF-based materials, including their design, structural characteristics, and applications.

Porous Hydrogen-Bonded Organic Frameworks

Ordered porous solid-state architectures constructed via non-covalent supramolecular self-assembly have attracted increasing interest due to their unique advantages and potential applications. Porous metal-coordination organic frameworks (MOFs) are generated by the assembly of metal coordination centers and organic linkers. Compared to MOFs, porous hydrogen-bonded organic frameworks (HOFs) are readily purified and recovered via simple recrystallization. However, due to lacking of sufficiently ability to orientate self-aggregation of building motifs in predictable manners, rational design and preparation of porous HOFs are still challenging. Herein, we summarize recent developments about porous HOFs and attempt to gain deeper insights into the design strategies of basic building motifs.

Crankshafts: using simple, flat C2h-symmetric molecules to direct the assembly of chiral 2D nanopatterns

Linear D2h-symmetric bisisophthalic acids 1 and 2 and related substances have well-defined flattened structures, high affinities for graphite, and strong abilities to engage in specific intermolecular interactions. Their adsorption produces characteristic nanopatterns that reveal how 2D molecular organization can be controlled by reliable interadsorbate interactions such as hydrogen bonds when properly oriented by molecular geometry. In addition, the behavior of these compounds shows how large-scale organization can be obstructed by programming molecules to associate strongly according to competing motifs that have similar stability and can coexist smoothly without creating significant defects. Analogous new bisisophthalic acids 3a and 4a have similar associative properties, and their unique C2h-symmetric crankshaft geometry gives them the added ability to probe the poorly understood effect of chirality on molecular organization. Their adsorption shows how nanopatterns composed predictably of a single enantiomer can be obtained by depositing molecules that can respect established rules of association only by accepting neighbors of the same configuration. In addition, an analysis of the adsorption of crankshaft compounds 3a and 4a and their derivatives by STM reveals directly on the molecular level how kinetics and thermodynamics compete to control the crystallization of chiral compounds. In such ways, detailed studies of the adsorption of properly designed compounds on surfaces are proving to be a powerful way to discover and test rules that broadly govern molecular organization in both 2D and 3D.

Crystalline molecular machines: function, phase order, dimensionality, and composition

The design of molecular machines is stimulated by the possibility of developing new materials with complex physicochemical and mechanical properties that are responsive to external stimuli. Condensed-phase matter with anisotropic molecular order and controlled dynamics, also defined as amphidynamic crystals, offers a promising platform for the design of bulk materials capable of performing such functions. Recent studies have shown that it is possible to engineer molecular crystals and extended solids with Brownian rotation about specific axes that can be interfaced with external fields, which may ultimately be used to design novel optoelectronic materials. Structure/function relationships of amphidynamic materials have been characterized, establishing the blueprints to further engineer sophisticated function. However, the synthesis of amphidynamic molecular machines composed of multiple "parts" is essential to realize increasingly complex behavior. Recent progress in amphidynamic multicomponent systems suggests that sophisticated functions similar to those of simple biomolecular machines may eventually be within reach.

Crystal engineering rescues a solution organic synthesis in a cocrystallization that confirms the configuration of a molecular ladder

Treatment of an achiral molecular ladder of C(2h) symmetry composed of five edge-sharing cyclobutane rings, or a [5]-ladderane, with acid results in cis- to trans-isomerization of end pyridyl groups. Solution NMR spectroscopy and quantum chemical calculations support the isomerization to generate two diastereomers. The NMR data, however, could not lead to unambiguous configurational assignments of the two isomers. Single-crystal X-ray diffraction was employed to determine each configuration. One isomer readily crystallized as a pure form and X-ray diffraction revealed the molecule as being achiral based on C(i) symmetry. The second isomer resisted crystallization under a variety of conditions. Consequently, a strategy based on a cocrystallization was developed to generate single crystals of the second isomer. Cocrystallization of the isomer with a carboxylic acid readily afforded single crystals that confirmed a chiral ladderane based on C(2) symmetry. The chiral ladderane and acid self-assembled to generate a five-component hydrogen-bonded complex that packs to form large solvent-filled homochiral channels of nanometer-scale dimensions. Whereas cocrystallizations are frequently applied to structure determinations of proteins, our study represents the first application of a cocrystallization to confirm the relative configuration of a small-molecule diastereomer generated in a solution-phase organic synthesis.

The weak help the strong: low-molar-mass organogelators harden bitumen

Low-molar-mass organogelators (LMOG) can turn liquids into thermoreversible gels because they self-assemble into a fibrous network. In contrast, using the same kind of low-molar-mass additives to harden materials, which are already solidlike on their own, has been hardly exploited. We show here that simple dicarboxylic acids are very efficient low-molar-mass organogelators (LMOG) for bitumen. Indeed, they increase the range of temperature where bitumen is a solid. Moreover, the hardness and elastic modulus of bitumen at room temperature are also improved. This concept of improving the mechanical properties of a solid with an LMOG can probably be applied to other materials.