Direct Participation of Solvent Molecules in the Formation of Supramolecular Polymers

Pathway-dependent supramolecular polymerization by planarity breaking

In controlled supramolecular polymerization, planar π-conjugated scaffolds are commonly used to predictably regulate stacking interactions, with various assembly pathways arising from competing interactions involving side groups. However, the extent to which the nature of the chromophore itself (planar vs. non-planar) affects pathway complexity requires clarification. To address this question, we herein designed a new BOPHY dye 2, where two oppositely oriented BF2 groups induce a disruption of planarity, and compared its supramolecular polymerization in non-polar media with that of a previously reported planar BODIPY 1 bearing identical substituents. The slightly non-planar structure of the BOPHY dye 2, as evident in previously reported X-ray structures, together with the additional out-of-plane BF2 group, allow for more diverse stacking possibilities leading to two fiber-like assemblies (kinetic 2A and thermodynamic 2B), in contrast to the single assembly previously observed for BODIPY 1. The impact of the less rigid, preorganized BOPHY core compared to the planar BODIPY counterpart is also reflected in the stronger tendency of the former to form anisotropic assemblies as a result of more favorable hydrogen bonding arrays. The structural versatility of the BOPHY core ultimately enables two stable packing arrangements: a kinetically controlled antiparallel face-to-face stacking (2A), and a thermodynamically controlled parallel slipped packing (2B) stabilized by (BF2) F⋯H (meso) interactions. Our findings underscore the significance of planarity breaking and out-of-plane substituents on chromophores as design elements in controlled supramolecular polymerization.

Molecular and supramolecular adaptation by coupled stimuli

Adaptation transcends scale in both natural and artificial systems, but delineating the causative factors of this phenomenon requires urgent clarification. Herein, we unravel the molecular requirements for adaptation and establish a link to rationalize adaptive behavior on a self-assembled level. These concepts are established by analyzing a model compound exhibiting both light- and pH-responsive units, which enable the combined or independent application of different stimuli. On a molecular level, adaptation arises from coupled stimuli, as the final outcome of the system depends on their sequence of application. However, in a self-assembled state, a single stimulus suffices to induce adaptation as a result of collective molecular behavior and the reversibility of non-covalent interactions. Our findings go beyond state-of-the-art (multi)stimuli-responsive systems and allow us to draw up design guidelines for adaptive behavior both at the molecular and supramolecular levels, which are fundamental criteria for the realization of intelligent matter.

Solvent Geometry Regulated J- and H-Type Aggregates of Photoswitchable Organogelator: Phase-Selective Thixotropic Gelation and Oil Spill Recovery

We demonstrate supramolecular polymerization and formation of 1D nanofiber of azobenzene based organogelator (AZO-4) in cyclic hydrocarbon solvents (toluene and methylcyclohexane). The AZO-4 exhibits J- and H-type aggregates in toluene: MCH (9 : 1) and MCH: toluene (9 : 1) respectively. The type of aggregate was governed by the geometry of the solvents used in the self-assembly process. The J-type aggregates with high thermal stability in toluene is due to the enhanced interaction of AZO-4 π- surface with the toluene π-surface, whereas H-aggregate with moderate thermal stability in MCH was due to the interruption of the cyclic hydrocarbon in van der Waals interactions of peripheral chains of AZO-4 molecule. The light induced reversible photoisomerization is observed for both J- and H-aggregates. The macroscopic property revealed spontaneous and strong gelation in toluene preferably due to the strong interactions of the AZO-4 nanofibers with the toluene solvent molecules compared to the MCH. The rheological measurements revealed thixotropic nature of the gels by step-strain experiments at room temperature. The thermodynamic parameter (ΔHm) of gel-to-sol transition was determined for all the gels to get more insight into the gelation property. Furthermore, the phase selective gelation property was extended to the oil spill recovery application using diesel/water and petrol/water mixture.

Supramolecular gels - a panorama of low-molecular-weight gelators from ancient origins to next-generation technologies

Supramolecular gels, self-assembled from low-molecular-weight gelators (LMWGs), have a long history and a bright future. This review provides an overview of these materials, from their use in lubrication and personal care in the ancient world, through to next-generation technologies. In academic terms, colloid scientists in the 19th and early 20th centuries first understood such gels as being physically assembled as a result of weak interactions, combining a solid-like network having a degree of crystalline order with a highly mobile liquid-like phase. During the 20th century, industrial scientists began using these materials in new applications in the polymer, oil and food industries. The advent of supramolecular chemistry in the late 20th century, with its focus on non-covalent interactions and controlled self-assembly, saw the horizons for these materials shifted significantly beyond their historic rheological applications, expanding their potential. The ability to tune the LMWG chemical structure, manipulate hierarchical assembly, develop multi-component systems, and introduce new types of responsive and interactive behaviour, has been transformative. Furthermore, the dynamics of these materials are increasingly understood, creating metastable gels and transiently-fueled systems. New approaches to shaping and patterning gels are providing a unique opportunity for more sophisticated uses. These supramolecular advances are increasingly underpinning and informing next-generation applications - from drug delivery and regenerative medicine to environmental remediation and sustainable energy. In summary, this article presents a panorama over the field of supramolecular gels, emphasising how both academic and industrial scientists are building on the past, and engaging new fundamental insights and innovative concepts to open up exciting horizons for their future use.

Solvent-dependent self-assembly of N-annulated perylene diimides. From dimers to supramolecular polymers

The synthesis and self-assembling features of the N-annulated perylene diimide (NPBI) 1 in different solvents are reported. Compound 1 possesses two chiral linkers, derived from (S)-(+)-alaninol, that connect the central aromatic NPBI segment and the peripheral trialkoxybenzamide units. The Ala-based linker has been demonstrated to strongly favor the formation of intramolecularly H-bonded seven-membered pseudocycles. NPBI 1 shows a strong tendency to self-assemble even in a good solvent like CHCl3 and the formation of chiral dimers is detected in this good solvent. Both experimental techniques and theoretical calculations reveal that the intramolecular H-bonded pseudocycles are very robust and the formation of chiral dimers is driven by the π-stacking of two units of the NPBI core. Unexpectedly, an efficient transfer of the asymmetry of the point chirality at the linker to the aromatic moiety is observed in the molecularly dissolved state. Changing the solvent to more apolar methylcyclohexane modifies the self-assembly process and the formation of chiral supramolecular polymers is detected. The supramolecular polymerization of 1 is demonstrated to follow an isodesmic mechanism unlike previous referable systems. In the formation of the supramolecular polymers of 1, the combination of experimental and computational data indicates that the H-bonded pseudocycles are also present in the aggregated state and the rope-like, columnar aggregates formed by the self-assembly of 1 rely on the π-stacking of the NPBI backbones.

Simulating Assembly Landscapes for Comprehensive Understanding of Supramolecular Polymer-Solvent Systems

Complexity in supramolecular polymer systems arises from interactions between different components, including solvent molecules. By varying their concentration or temperature in such multicomponent systems, complex phenomena can occur such as thermally bisignate and dilution-induced assembly of supramolecular polymers. Herein, we demonstrate that both these phenomena emerge from the same underlying interaction mechanism between the components. As a model system, amide-decorated supramolecular polymers of porphyrins were investigated in combination with aliphatic alcohols as hydrogen-bond scavengers, and thermodynamic mass-balance models were applied to map the three-dimensional assembly landscapes. These studies unveiled that the interaction between hydrogen-bond scavengers and monomers is temperature-dependent and becomes dominant at high monomer concentrations. With these insights, we could exploit competitive monomer-alcohol interactions to prompt the dilution-induced assembly of various common monomers as well as bisignate assembly events. Moreover, kinetic insights were obtained by navigating through the assembly landscape. Similar to phase diagrams of covalent polymers, these assembly landscapes provide a comprehensive picture of supramolecular polymerizations, which helps to precisely regulate the system properties. The generality of this approach using assembly landscapes makes it relevant for any supramolecular system, and this enhanced control will open the door to build complex and functional supramolecular polymer systems.

Low-Grade Thermal Energy Harvesting and Self-Powered Sensing Based on Thermogalvanic Hydrogels

Thermoelectric cells (TEC) directly convert heat into electricity via the Seebeck effect. Known as one TEC, thermogalvanic hydrogels are promising for harvesting low-grade thermal energy for sustainable energy production. In recent years, research on thermogalvanic hydrogels has increased dramatically due to their capacity to continuously convert heat into electricity with or without consuming the material. Until recently, the commercial viability of thermogalvanic hydrogels was limited by their low power output and the difficulty of packaging. In this review, we summarize the advances in electrode materials, redox pairs, polymer network integration approaches, and applications of thermogalvanic hydrogels. Then, we highlight the key challenges, that is, low-cost preparation, high thermoelectric power, long-time stable operation of thermogalvanic hydrogels, and broader applications in heat harvesting and thermoelectric sensing.

Number of Hydrogen Bonds per Unit Solvent Accessible Surface Area: A Descriptor of Functional States of Proteins

Proteins function close to native and near-native conformations. These states are evolutionarily selected to ensure the effect of mutations is minimized. The structural organization of a protein is hierarchical and modular, which reduces the dimensionality of the configurational space of the native states. Thus, finding appropriate descriptors that define the native state among all possible states of a protein is a problem of immense interest. The present study explores the correlation between solvent accessible surface areas (SASAs) and different intraprotein as well as protein-water hydrogen bonds of 55 single-chain globular proteins from four different structural classes (all α, all β, α+β, and α/β), 16 multichain proteins, and 4 macromolecular complexes. A systematic analysis of the solvent accessible surface area and intraprotein and protein-water hydrogen bonds suggests a linear relationship between SASAs and hydrogen bonds. The number of protein-water hydrogen bonds per unit SASA ranges from 3 to 4 for all the different structural protein classes. In contrast, the number of intramolecular hydrogen bonds per unit SASA, including the mainchain-mainchain, mainchain-sidechain, and sidechain-sidechain, varies between 0.75 to 2. The solvation free energy of a protein linearly decreases with SASA. Our study also shows that the solvation free energy/SASA varies from -75 to -105 kJ mol^-1 nm^-2 across all the native states studied here. The number conservancy of intraprotein hydrogen bonds per unit SASA possibly imparts structural stability to the native structure. On the other hand, 3-4 protein-water hydrogen bonds per unit SASA are possibly required to maintain a balance between the solubility and functionality of the native states. This study provides a basis for synthetic biologists to design new folds with improved functionality.

Thermoreversible Polymorph Transitions in Supramolecular Polymers of Hydrogen-Bonded Squaramides

Hydrogen-bonded squaramide (SQ) supramolecular polymers exhibit uncommon thermoreversible polymorph transitions between particle- and fiber-like nanostructures. SQs 1-3, with different steric bulk, self-assemble in solution into particles (AggI) upon cooling to 298 K, and SQs 1 and 2, with only one dendronic group, show a reversible transformation into fibers (AggII) by further decreasing the temperature to 288 K. Nano-DSC and UV/Vis studies on SQ 1 reveal a concentration-dependent transition temperature and ΔH for the AggI-to-AggII conversion, while the kinetic studies on SQ 2 indicate the on-pathway nature of the polymorph transition. Spectroscopic and theoretical studies reveal that these transitions are triggered by the molecular reorganization of the SQ units changing from slipped to head-to-tail hydrogen bonding patterns. This work unveils the thermodynamic and kinetic aspects of reversible polymorph transitions that are of interest to develop stimuli-responsive systems.

Luminescence and Length Control in Nonchelated d 8 ‐Metallosupramolecular Polymers through Metal‐Metal Interactions

Supramolecular polymers (SPs) of d⁸ transition metal complexes have received considerable attention by virtue of their rich photophysical properties arising from metal‐metal interactions. However, thus far, the molecular design is restricted to complexes with chelating ligands due to their advantageous preorganization and strong ligand fields. Herein, we demonstrate unique pathway‐controllable metal‐metal‐interactions and remarkable ³MMLCT luminescence in SPs of a non‐chelated Pt^II complex. Under kinetic control, self‐complementary bisamide H‐bonding motifs induce a rapid self‐assembly into non‐emissive H‐type aggregates (1A). However, under thermodynamic conditions, a more efficient ligand coplanarization leads to superiorly stabilized SP 1B with extended Pt⋅⋅⋅Pt interactions and remarkably long ³MMLCT luminescence (τ^77 K=0.26 ms). The metal‐metal interactions could be subsequently exploited to control the length of the emissive SPs using the seeded‐growth approach.

Effect of solvent isomers on the gelation properties of tri-aryl amine organogels and their hybrid thermoreversible gels with poly[vinyl chloride]

The gelation properties of tri-aryl amine (TATA) in two isomers of dichlorobenzene, namely ortho-dichlorobenzene (o-DCB) and meta-dichlorobenzene (m-DCB), have been studied by calorimetry for mapping out the temperature-concentration phase diagram. It is shown that both systems behave differently, yet both form molecular compounds. The occurrence of these compounds is further demonstrated by neutron diffraction in o-DCB. Depending on the isomer used, X-ray investigations highlight a difference in the position of the diffraction peaks. The morphology observed by scanning electron microscopy reveals fibrillar systems in both cases, yet the fibrils' cross-sections are far larger in m-DCB with respect to o-DCB. The body of these results is discussed in light of current opinions on the role of the solvent. Ternary gels obtained with PVC have also been investigated by means of the same experimental approach. Calorimetry investigations and X-ray diffraction data show no significant difference for the behavior of the TATA moiety. The morphology studied by scanning electron microscopy allows one to distinguish clearly the TATA fibrils and the PVC network. This again confirms the possibility of preparing functional materials with an organogelator and a polymer through the making of hybrid gels.