Non-Aqueous Poly(dimethylsiloxane) Organogel Sponges for Controlled Solvent Release: Synthesis, Characterization, and Application in the Cleaning of Artworks

Polydimethylsiloxane (PDMS) organogel sponges were prepared and studied in order to understand the role of pore size in an elastomeric network on the ability to uptake and release organic solvents. PDMS organogel sponges have been produced according to sugar leaching techniques by adding two sugar templates of different forms and grain sizes (a sugar cube template and a powdered sugar template), in order to obtain materials differing in porosity, pore size distribution, and solvent absorption and liquid retention capability. These materials were compared to PDMS organogel slabs that do not contain pores. The sponges were characterized by Fourier-transform infrared spectroscopy with attenuated total reflectance (FTIR-ATR) and compared with PDMS slabs that do not contain pores. Scanning electron microscopy (SEM) provided information about their morphology. X-ray micro-tomography (XMT) allowed us to ascertain how the form of the sugar templating agent influences the porosity of the systems: when templated with sugar cubes, the porosity was 77% and the mean size of the pores was ca. 300 μm; when templated with powdered sugar, the porosity decreased to ca. 10% and the mean pore size was reduced to ca. 75 μm. These materials, porous organic polymers (POPs), can absorb many solvents in different proportions as a function of their polarity. Absorption capacity, as measured by swelling with eight solvents covering a wide range of polarities, was investigated. Rheology data established that solvent absorption did not have an appreciable impact on the gel-like properties of the sponges, suggesting their potential for applications in cultural heritage conservation. Application tests were conducted on the surfaces of two different lab mock-ups that simulate real painted works of art. They demonstrated further that PDMS sponges are a potential innovative support for controlled and selective cleaning of works of art surfaces.

Halogen- and hydrogen-bonded self-assembled fibrillar networks of substituted 1,3:2,4-dibenzylidene-D-sorbitols (DBS)

Substituting the sole primary hydroxyl group of the low molecular weight organogelator (LMOG), 1,3:2,4-dibenzylidene-D-sorbitol (DBS), with a halogen atom (Cl, Br, or I; i.e., 6-Cl-DBS, 6-Br-DBS, or 6-I-DBS) drastically alters the supramolecular self-assembled fibrillar network (SAFiN) that forms when the molecules aggregate. The SAFiN varies depending on the solvent properties, impacting the role of non-covalent hydrogen- and halogen-bonding interactions along and between fibers. The halogenated DBS derivatives have more coherent crystalline fibers than DBS, with larger length-to-width aspect ratios. High-resolution synchrotron powder X-ray diffraction of each wet-state gel in toluene and DFT optimization obtained complete structures for the three halogenated DBS derivatives in their SAFiNs. The presence of a halogen atom reduces the reliance on hydrogen bonding by enabling new halogen bonding interactions that impact the self-assembly behavior, especially in solvents of higher polarity. For 6-I-DBS and 6-Br-DBS, the primary forces driving molecular self-assembly are C-H⋯π and intermolecular halogen-to-halogen interactions, and there is one unique molecule in each unit cell. However, the Cl atoms of 6-Cl-DBS are not close, and its SAFiN structures rely more on hydrogen bonding. As a result, the enhanced hydrogen bonding, electronic differences among the halogens, and spatial factors allow its unit cell to include two independent molecules of 6-Cl-DBS.

1D Supramolecular Assemblies That Crystallize and Form Gels in Response to the Shape-Complementarity of Alcohols

N-9-Fluorenylmethyloxycarbonyl (Fmoc)- and C-tertiary butyl (t-Bu)-protected glutamate (L-2), bearing a phenanthroline moiety at the side residue, forms 1D supramolecular assemblies via H-bonding as well as undergoing π-stacking interactions to afford crystals or gels that depend on the shape-complementarity of coexisting alcohols, as demonstrated by structural analyses on these assemblies by means of single-crystal X-ray diffractometry and supplemented with small- and wide-angle X-ray scattering data. Moreover, the rheological measurements on the gels help to define a model for when gels and crystals are expected and found. These observations and conclusions highlight an important, but not very appreciated, aspect of solute-solvent interactions within supramolecular assemblies that can allow the constituent-aggregating molecules in some systems to exhibit high selectivity toward the structures of their solvents. The consequences of this selectivity, as demonstrated here by single-crystal and powder X-ray diffraction data, can lead to self-assembled structures which alter completely the bulk phase properties and morphology of the materials. In that regard, rheological measurements have helped to develop a model to explain when gels and phase-separated mixtures of crystals and solvents are expected.

Molecular and Aggregate Structural, Thermal, Mechanical and Photophysical Properties of Long-Chain Amide Gelators Containing an α-Diketo Group in the Presence or Absence of a Tertiary Amine Group

Three structurally related gelators, each containing octadecyl chains, an α-diketo group at the 9,10 positions, and each with a different N-amide group-isobutyl (DIBA), isopentyl (DIPA) or N-(2-(dimethylamino)ethyl) (DMEA)-have been synthesized. Their neat structures as well as the thermal mechanical, and photophysical properties in their gel states with various liquids have been investigated. The gelator networks of DIBA and DIPA in octane, hexylbenzene and silicone oil consist of bundles of fibers. These gels are partially thixotropic and mechanically, thermally (to above their melting or silicone oil gelation temperatures), and photophysically stable. They are mechanically and thermally stronger than the gels formed with DMEA, the gelator with a tertiary amine group. The lone pair of electrons of the tertiary amine group leads to an intra-molecular or inter-molecular charge-transfer interaction, depending on whether the sample is a solution, sol, or gel. Neat, solid DMEA does not undergo the charge-transfer process because its amino and diketo groups are separated spatially by a large distance in the crystalline state and cannot diffuse into proximity. However, the solution of DIPA upon the addition of triethylamine becomes unstable over time at room temperature in the dark or (more rapidly) when irradiated, which initiates the aforementioned charge-transfer processes. The eventual reaction of the gelators in the presence of a tertiary amine group is ascribed to electron transfer from the lone-pair on nitrogen to an α-diketo group, followed by proton transfer to an oxygen atom on the anion radical of the α-diketo group from a methyl or methylene group attached to the nitrogen atom of the cation radical. Finally, the formation of an α-diketyl radical leads to irreversible electronic and structural changes that are observed over time.

Effects of cyclic and acyclic amidine side-chains on the properties of polysiloxane ionomers constructed in situ from three uncharged components

Ionomers, polysiloxanes with imidazolinium dithiocarbamate side chains, have been synthesized in situ from three uncharged components-a polysiloxane with imidazole side chains, CS₂, and hexylamine or octadecylamine. Their structural and dynamic properties are compared over a temperature range of 0-50 °C with those of the analogous ionomers in which the polysiloxanes have amidinium side chains. The results, primarily from differential scanning calorimetry, powder X-ray diffraction measurements, and rheology show that the small structural (and smaller electronic) differences between the cyclic 5-membered ring imidazolinium and acyclic amidinium groups have marked effects on the bulk properties of the ionomers. These include their shear strengths and the manner in which the microcrystalline portions of the ionomers with dithiooctadecylcarbamate anions are packed. Thus, it is possible to finely tune the natures of the ionomers from one polysiloxane by changing temperature, the chain length of the alkylamine, and the nature of the base attached to the polysiloxane chain. Why these changes occur to the various properties is discussed.

Benzil Photoperoxidations in Polymer Films and Crosslinking by the Resultant Benzoyl Peroxides in Polystyrene and Other Polymers

Benzil (BZ) can be converted almost quantitatively to benzoyl peroxide (BP) in aerated polymer films upon irradiation at >400 nm (i.e., the long-wavelength edge of the n→π* absorption band of BZ, where BP does not absorb). Here, we summarize results for the photoperoxidation of BZ structures with molecular oxygen, principally in glassy polymer matrices. Some of the polymers are doped directly with BZ or its derivatives, and others, contain covalently attached BZ pendant groups from which BP groups are derived. While the decomposition of low-molecular-weight BP doped into polymer films (such as those of polystyrene (PS)) results in a net decrease in polymer molecular weight, thermal decomposition of pendant BP groups is an efficient method for chain crosslinking. Crosslinking of PS films doped with a molecule containing two covalently linked BZ or BP groups proceeds in a similar fashion. Free radicals from the covalently attached BP allow grafting of new monomers, as well. Additionally, the use of radiation filtered through masks has been used to create patterns of polymers on solid surfaces. Crosslinking of photodegradable poly(phenyl vinyl ketone) with BP structures obtained by photoperoxidation of BZ structures for the preparation of photodegradable polymer networks is described as well. In sum, the use of BZ and BP and their derivatives offers simple and convenient routes for modifying polymer chains and, especially, for crosslinking them. Specific applications of each use and process are provided. Although applications with PS are featured here, the methodologies described are amenable to a wide variety of other polymers.

NMR spectral fingerprint patterns as diagnostics for the unambiguous configurational analysis of the classic organo-gelator 1,3:2,4-dibenzylidene-d-sorbitol (DBS) and its derivatives

On the basis of experimental data and density functional theory (DFT) chemical shift and scalar coupling predictions, simple spectral nuclear magnetic resonance (NMR) fingerprint patterns have been established for the determination of the configuration in 1,3:2,4-dibenzylidene-d-sorbitol (DBS), a classic low molecular weight gelator, and its derivatives. The results rigorously prove the orientation of the phenyl rings in DBS that had been previously assumed in the literature on the basis of thermodynamic arguments.

Thiol- and Disulfide-Based Stimulus-Responsive Soft Materials and Self-Assembling Systems

Properties and applications of synthetic thiol- and disulfide-based materials, principally polymers, are reviewed. Emphasis is placed on soft and self-assembling materials in which interconversion of the thiol and disulfide groups initiates stimulus-responses and/or self-healing for biomedical and non-biomedical applications.

Luminescent Behavior of Gels and Sols Comprised of Molecular Gelators

We present a brief review of some important conceptual and practical aspects for the design and properties of molecular luminescent gelators and their gels. Topics considered include structural and dynamic aspects of the gels, including factors important to their ability to emit radiation from electronically excited states.

Multiple-State Emissions from Neat, Single-Component Molecular Solids: Suppression of Kasha's Rule

Three rigid and structurally simple heterocyclic stilbene derivatives, (E)-3H,3'H-[1,1'-biisobenzofuranylidene]-3,3'-dione, (E)-3-(3-oxobenzo[c] thiophen-1(3H)-ylidene)isobenzofuran-1(3H)-one, and (E)-3H,3'H-[1,1'-bibenzo[c] thiophenylidene]-3,3'-dione, are found to fluoresce in their neat solid phases, from upper (S₂ ) and lowest (S₁ ) singlet excited states, even at room temperature in air. Photophysical studies, single-crystal structures, and theoretical calculations indicate that large energy gaps between S₂ and S₁ states (T₂ and T₁ states) as well as an abundance of intra and intermolecular hydrogen bonds suppress internal conversions of the upper excited states in the solids and make possible the fluorescence from S₂ excited states (phosphorescence from T₂ excited states). These results, including unprecedented fluorescence quantum yields (2.3-9.6 %) from the S₂ states in the neat solids, establish a unique molecular skeleton for achieving multi-colored emissions from upper excited states by "suppressing" Kasha's rule.

Experimental and Theoretical Investigation of Excited-State Intramolecular Proton Transfer Processes of Benzothiazole Derivatives in Amino-polydimethylsiloxanes before and after Cross-Linking by CO2

The changes in the ability of three fluorescent derivatives of 2-(2'-hydroxyphenyl)benzothiazole to undergo excited-state intramolecular proton transfer (ESIPT) processes have been correlated with the rheological properties of four amino-polydimethylsiloxanes with different molar masses and containing different amounts of monomer units with amino pendant groups, in the presence and absence of a cross-linking molecule, CO₂. The changes lead to a variety of species (keto, enol, and enolate forms) in both the ground and excited states. Calculations using the density-functional theory/time-dependent density-functional theory method at the CAM-B3LYP/6-311++G(d,p) level helped to identify how ESIPT is involved in the formation of the intermediates. The results demonstrate that proton transfer in 2-(2'-hydroxyphenyl)benzothiazoles is a powerful tool to identify local changes in the viscosity and micropolarity of the environment that are attributed to the structural differences of the amino-polydimethylsiloxanes and their cross-linking.

Poly(vinyl alcohol)-induced thixotropy of an l-carnosine-based cytocompatible, tripeptidic hydrogel

The generally poor mechanical stability of hydrogels limits their use as functional materials for many biomedical applications. In this work, a poly(vinyl alcohol) (PVA) embedded hybrid hydrogel of a β-amino acid-containing Fmoc-protected tripeptide was produced at physiological pH (7.4) and room temperature. The hydrogel system was characterized by a number of techniques, including UV-vis, fluorescence, circular dichroism, FT-IR spectroscopy, electron microscopy, and rheology. While the tripeptide-based pure hydrogel was found to be unstable after ca. half an hour, addition of PVA, a water soluble polymer, increased the temporal and mechanical stability of the hydrogel. A rheological step-strain experiment demonstrates that the peptide-polymer hydrogel is thixotropic. Results from a fluorescence probe study and transmission electron microscopy reveal that addition of PVA increases both the fibre diameter and entanglement. Circular dichroism spectra of the hydrogels confirm the formation of aggregates with supramolecular chirality. The thixotropic nature of the hydrogel has been exploited to entrap and release doxorubicin, an anticancer drug, under physiological conditions. Furthermore, an MTT assay of the Fmoc-tripeptide using AH927 cells confirmed its cytocompatibility, which broadens the utility of the hybrid gel for biomedical applications.

Controlling Variables in Molecular Gel Science: How Can We Improve the State of the Art?

By design, no references are included in this article. It is intended to be a series of recommendations in which the focus is on lab practices for investigating substances rather than on the substances being investigated. Thus, it discusses some specific areas of concern identified by the author. Other scientists are encouraged to add to or amend the contents. This article should be read as a “living” document, like a blog in which many gel scientists work, over time, to achieve a consensus about reporting everything from acronyms and definitions to procedures and methods. For those entering the field and seeking compendia on the subject, the author suggests “Googling” the words “molecular gels” or “molecular gels books”.

Long Range Polymer Chain Dynamics of Highly Flexible Polysiloxane in Solution Probed by Pyrene Excimer Fluorescence

A poly(dimethylsiloxane-co-(3-aminopropyl)methylsiloxane) polymer (PDMS with 20.3 mol % of (3-aminopropyl)methyl siloxane monomer) has been labeled randomly with 1-pyreneacetyl groups to generate a series of polysiloxanes (Py-PDMS) with pyrenyl contents ranging from 0.7 mol % to 5.2 mol % of the total number of structural units. The remainder of the amino groups were acetylated to avoid intra-chain quenching of the excited singlet states of pyrene via exciplex formation with free amino groups while allowing the formation of excimers to proceed. The fluorescence spectra and temporal decays of the Py-PDMS samples were acquired in tetrahydrofuran (THF), N,N-dimethylformamide (DMF), and dioxane. <k^MF>^blob, the average rate constant for intra-chain pyrene excimer formation, was determined from the analysis of the fluorescence decays. <k^MF>^blob was found to equal 1.16 (±0.13) × 10⁹, 1.14 (±0.12) × 10⁹, and 0.99 (±0.10) × 10⁹ s⁻¹ in THF, DMF, and dioxane, respectively, at room temperature. They are the largest values found to date for any polymeric backbone in these solvents. The qualitative relationship found here between <k^MF>^blob and the chemical structures of the polymers indicates that the luminescence characteristics of randomly labeled polymers is a very useful method to probe the long range dynamics of chains of almost any polymer that is amenable to substitution by a lumophore.

Mechano-switchable, luminescent gels derived from salts of a long-chained, fatty-acid gelator

Stimulus-responsive molecular gel systems, based on metal salts of a luminescent gelator, 9,10-dioxooctadecanoic acid (DODA), are reported. These salts are structurally the simplest metallo-gelators of which we are aware that exhibit controllable mechano-responsive and luminescent properties. Aggregation is more favored by the metal salts than for DODA itself. However, gelation ability differs dramatically depending on the metal ion: whereas the salts with zinc(ii) and calcium(ii) are inefficient gelators, those with nickel(ii) and copper(ii) can gelate various aromatic liquids, alkanes, and long-chained alcohols. Unlike the DODA gels, no aggregation-induced shift in the positions of the emission spectra of the metal salts could be observed as the sols were transformed to their gel phases. Gels of both nickel(ii) and copper(ii) salts in benzonitrile are among the few known examples with crystalline networks and exhibiting thixotropic behavior. However, there are significant differences in their abilities to recover the initial viscoelastic properties. Structural data for the solid and gel states lead us to conclude that differences among the gelating abilities can be attributed principally to the specific nature of interactions of the salts at their head groups. They appear to control the mechanical and emissive properties of the gels as well as whether the initial aggregation of the salts in the sol phases will support the growth of 1D objects that are capable of maintaining strong contacts, leading to 3D networks and gel formation. Overall, the results provide a facile strategy for the design of luminescent materials with controllable mechano-responsiveness by modifying the metal ions within fibrillar assemblies.

l-Carnosine-Derived Fmoc-Tripeptides Forming pH-Sensitive and Proteolytically Stable Supramolecular Hydrogels

A series of β-amino acid containing tripeptides has been designed and synthesized in order to develop oligopeptide-based, thermoreversible, pH-sensitive, and proteolytically stable hydrogels. The Fmoc [N-(fluorenyl-9-methoxycarbonyl)]-protected tripeptides were found to produce hydrogels in both pH 7 and 2 buffers at a very low concentration (<0.2% w/v). It has been shown that the Fmoc group plays an important role in the gelation process. Also a dependence of gelation ability on hydrophobicity of the side chain of the Fmoc-protected α-amino acid was observed. The effect of the addition of inorganic salts on the gelation process was investigated as well. Spectroscopic studies indicated formation of J-aggregates through π-π stacking interactions between Fmoc groups in solution as well as in the gel state. In the gel phase, these self-assembling tripeptides form long interconnected nanofibrils leading to the formation of 3-dimensional network structure. The hydrogels were characterized by various techniques, including field emission electron microscopy, transmission electron microscopy, atomic force microscopy, rheology, Fourier transform IR, circular dichroism (CD), and wide-angle X-ray diffraction (WAXD) spectroscopy. The CD studies and WAXD analyses show an antiparallel β-sheet structure in the gel state. l-Phenylalanine and l-tyrosine containing tripeptides formed helical aggregates with handedness opposite to those containing l-valine and l-leucine residues. The mechanical stability of the hydrogels was found to depend on the hydrophobicity of the side chain of the tripeptide as well as on the pH of the solution. Also, the tripeptides exhibit in vitro proteolytic stability against proteinase K enzyme.

Molecular Nuances Governing the Self-Assembly of 1,3:2,4-Dibenzylidene-d-sorbitol

1,3:2,4-Dibenzylidene-d-sorbitol (DBS) is the gold-standard for low-molecular-weight organogelators (LMOGs). DBS gels a wide array of solvents, as illustrated by the large Hansen sphere representing gels (2δ_d = 33.5 MPa^1/2, δ_p = 7.5 MPa^1/2, and δ_h = 8.7 MPa^1/2; radius = 11.2 MPa^1/2). Derivatives of DBS have been synthesized to isolate and determine molecular features essential for organogelation. In this work, π-π stacking and hydrogen bonding are the major noncovalent interactions examined. The importance of π-π stacking was studied using 1,3:2,4 dicyclohexanecarboxylidene-d-sorbitol (DCHS), which eliminates possible π-π stacking while still conserving the other structural aspects of DBS. The replacement of the benzyl groups with cyclohexyl groups led to a very a poor gelator; only one of the several solvents examined, carbon tetrachloride, formed a gel. 1,3:2,4-Diethylidene-d-sorbitol (DES), another DBS analogue incapable of π-π stacking but with very different polarity, gelated a large Hansen space (2δ_d = 34.0 MPa^1/2, δ_p = 10.9 MPa^1/2, and δ_h = 10.8 MPa^1/2; radius = 9.2 MPa^1/2). DES gels solvents with higher δ_p and δ_h values than DBS. To assess the role of hydrogen bonding, DBS was acetalated (A-DBS), and it was found that the Hansen space gelated by A-DBS shifted to less polar solvents with higher hydrogen-bonding Hansen solubility parameters (HSPs) (2δ_d = 33.8 MPa^1/2, δ_p = 6.3 MPa^1/2, and δ_h = 9.6 MPa^1/2; radius = 11.1 MPa^1/2) than for DBS. These systematic structural modifications are the first step in exploring how specific intermolecular features alter aspects of Hansen space corresponding to positive gelation outcomes.

Photophysics of N,N-dimethyl-3-(1-indolyl)propan-1-ammonium chloride and related derivatives

The photophysical properties of two new indole derivatives have been examined by steady-state and dynamic spectroscopic methods. The ground-state structures and conformations of 3-(1-indolyl)-N,N-dimethylpropan-1-ammonium chloride (InCl) and 3-(1-indolyl)-N,N,N-trimethylpropan-1-ammonium chloride (MeInCl) have been examined through density functional theory calculations. These calculations reveal a preference for a 'closed' conformation which places the cationic ammonium group in proximity to the π-electron cloud in low polarity environments. This interaction is best described as an intramolecular hydrogen-π bond in the case of InCl and a cation-π interaction for MeInCl. The ground-state conformational equilibria are influenced by changes in the dielectric constant of the solvent, resulting in a variety of photophysical behaviors. The excitation/emission spectra, fluorescence quantum yields, and excited-state lifetimes, are reported for InCl, MeInCl, and a reference compound, 1-methylindole, in 1,4-dioxane (ε = 2), acetonitrile (ε = 37), and water (ε = 78) where solubility allows. Data from these solvents provide evidence for independent fluorescence quenching pathways for InCl and MeInCl. In addition, they lead to insights into the complexities of indole photophysics by demonstrating the sensitivity of the locally-excited states to changes in charge-density and solvent environment.

Soft, Peelable Organogels from Partially Hydrolyzed Poly(vinyl acetate) and Benzene-1,4-diboronic Acid: Applications to Clean Works of Art

We have developed soft, peelable organogels from 40% hydrolyzed poly(vinyl acetate) (40PVAc) and benzene-1,4-diboronic acid (BDBA). The organic liquids gelated include dimethyl sulfoxide, dimethylformamide, tetrahydrofuran, 2-ethoxyethanol, and methanol. The rheology of these soft materials can be tuned by altering the concentration of the polymer and/or crosslinker. Insights into the mechanisms leading to gelation were obtained from ¹H NMR experiments, fluorescence measurements, and studies comparing properties of materials made from BDBA and phenylboronic acid, a molecule incapable of forming covalent crosslinks between the polymer chains. These organogels can be easily peeled off a surface, leaving no residue detectable by UV-vis spectroscopy. They are demonstrated to be effective at softening and removing deteriorated coatings from water-sensitive works of art and delicate surfaces. They have the needed characteristics to clean topographically complex surfaces: good contact with the surface, easy removal, and little to no residue after removal. A 2-ethoxyethanol organogel was used to remove oxidized varnish from a 16th century reliquary decorated with painted gold leaf, and an ethanol gel was used to remove solvent-resistant coatings from 16th and 18th century oil paintings.

Proton-coupled charge-transfer reactions and photoacidity of N,N-dimethyl-3-arylpropan-1-ammonium chloride salts

Excited-state, intermolecular proton-transfers of aromatics tethered to ammonium groups are solvent mediated and coupled to either the formation of an exciplex or a solvent-separated ion pair.

Intramolecular, Exciplex-Mediated, Proton-Coupled, Charge-Transfer Processes in N,N-Dimethyl-3-(1-pyrenyl)propan-1-ammonium Cations: Influence of Anion, Solvent Polarity, and Temperature

An intramolecular exciplex-mediated, proton-coupled, charge-transfer (PCCT) process has been investigated for a series of N,N-dimethyl-3-(1-pyrenyl)propan-1-ammonium cations with different anions (PyS) in solvents of low to intermediate polarity over a wide temperature range. Solvent mediates both the equilibrium between conformations of the cation that place the pyrenyl and ammonium groups in proximity (conformation C) or far from each other (conformation O) and the ability of the ammonium group to transfer a proton adiabatically in the PyS excited singlet state. Thus, exciplex emission, concurrent with the PCCT process, was observed only in hydrogen-bond accepting solvents of relatively low polarity (tetrahydrofuran, ethyl acetate, and 1,4-dioxane) and not in dichloromethane. From the exciplex emission and other spectroscopic and thermodynamic data, the acidity of the ammonium group in conformation C of the excited singlet state of PyS (pKa*) has been estimated to be ca. -3.4 in tetrahydrofuran. The ratios between the intensities of emission from the exciplex and the locally excited state (IEx/ILE) appear to be much more dependent on the nature of the anion than are the rates of exciplex formation and decay, although the excited state data do not provide a quantitative measure of the anion effect on the C-O equilibrium. The activation energies associated with exciplex formation in THF are calculated to be 0.08 to 0.15 eV lower than for the neutral amine, N,N-dimethyl-3-(1-pyrenyl)propan-1-amine. Decay of the exciplexes formed from the deprotonation of PyS is hypothesized to occur through charge-recombination processes. To our knowledge, this is the first example in which photoacidity and intramolecular exciplex formation (i.e., a PCCT reaction) are coupled.

Mechano-Responsive, Thermo-Reversible, Luminescent Organogels Derived from a Long-Chained, Naturally Occurring Fatty Acid

The gelating ability of an α-diketo derivative of oleic acid, 9,10-dioxooctadecanoic acid (DODA), is investigated. DODA can gelate aromatic liquids and many other organic liquids. By contrast, none of the liquids examined can be gelated by the methyl ester of DODA. DODA is a more efficient gelator than stearic acid and the monoketo derivative due to its more extensive intermolecular dipole-dipole interactions. Formation of organogels of DODA can be induced by both thermal and mechanical stimuli, during which the luminescent and mechanical properties can be modulated significantly. The emission from DODA in 1-octanol exhibits a large, reversible, hypsochromic shift (≈25 nm) between its thermally cycled gel and sol states. The emission changes have been exploited to probe the kinetics of the aggregation and deaggregation processes. DODA is the simplest gelator of which we are aware that exhibits a reversible shift in the emission. Although the self-assembled fibrillar networks of the DODA gels in 1-octanol, benzonitrile, or silicone oil are crystalline, isothermal mechanical cycling between the gel and the sol states is rapid and can be repeated several times (i.e., they are thixotropic). The single-crystal structure of DODA indicates that extended intermolecular dipole-dipole interactions are crucial to the thermal and mechanical formation of DODA gels and the consequential changes in emissive and mechanical properties. From analyses of structural information, gelator packing, and morphology differences, we hypothesize that the mechanical destruction and reformation of the gel networks involves interconversion between the 3D networks and 1D fiber bundles. The thermal processes allow the fibrillar 3D networks and their 0D components (i.e., isolated molecules or small aggregates of DODA) to be interconverted. These results describe a facile approach to the design of mechano-responsive, thermo-reversible gels with control over their emission wavelengths.

Correlations between thixotropic and structural properties of molecular gels with crystalline networks

This review focuses on correlations between the thixotropic and structural properties of molecular gels having crystalline fibrillar networks (SAFINs). Formation of thixotropic molecular gels and their recovery after the application of destructive strain depends on the strength and type of intermolecular interactions in the SAFINs of the gelator molecules. Here, we limit our discussion to gelator molecules with simple structures in order to dissect more easily the important contributors to the thixotropic behaviors. Possible mechanisms to explain the thixotropic phenomena, involving the transformation of the SAFINs into unattached objects, and their reassembly into 3-dimensional networks, are advanced. The data are analyzed to provide insights into the rational design of thixotropic molecular gelators.

Structural bases for mechano-responsive properties in molecular gels of (R)-12-hydroxy-N-(ω-hydroxyalkyl)octadecanamides. Rates of formation and responses to destructive strain

The self-assembly and gelation behavior of a series of (R)-12-hydroxy-N-(ω-hydroxyalkyl)octadecanamides (HS-n-OH, where n = 2, 3, 4 and 5 is the length of the alkyl chain on nitrogen), as well as those of two ‘model’ compounds, N-(3-hydroxypropyl)octadecanamide (S-3-OH) and (R)-12-hydroxy-N-propyloctadecanamide (HS-3), have been investigated in a wide range of liquids. A unique aspect of some of the HS-n-OH gels is the degree and velocity of their recovery of viscoelasticity after the cessation of destructive shear. The recovery times vary from less than one second to hundreds of seconds, depending on the length of the ω-hydroxyalkyl group on nitrogen. The data indicate that the modes and dynamics of aggregation of the gelator molecules from incubation of a sol phase below the gel melting temperature, as analyzed by Avrami and fractal equations, cannot be used to explain the degree and dynamics of the thixotropy: sol-to-gel transformations involve assembly of 0-dimensional objects (i.e., individual gelator molecules) into 1-dimensional fibrils and then into 3-dimensional networks; recovery after mechano-destruction of gels requires only 1-dimensional to 3-dimensional re-assembly or re-association of 3-dimensional spherulitic objects. A model to understand the extreme sensitivity of the thixotropy on the length of the ω-hydroxyalkyl group in the HS-n-OH (which is based upon detailed comparisons among the dynamic properties of the gels, the morphologies of the neat gelators, and the fibrillar networks of the gels) invokes the importance of the cleavage and reformation of H-bonds between fibers at ‘junction zones’ or between spherulitic objects.

Selective formation of organo, organo-aqueous, and hydro gel-like materials from partially hydrolysed poly(vinyl acetate)s based on different boron-containing crosslinkers

Viscoelastic, gel-like, polymeric dispersions (HVPDs) can be prepared by crosslinking polyols with borax or boric acid in water under alkaline conditions. Rheologically similar HVPDs have been prepared in organic liquids containing no water or hydroxylic groups through crosslinking partially or fully hydrolysed poly(vinyl acetate)s with trimethyl borate, boric acid, or borax. The organo-HVPDs are water-sensitive and rheoreversible on exposure to water. They were characterised rheologically and by solution and solid-state (11)B NMR spectroscopy. Spectroscopic analyses show the presence of mono- and di-diol crosslinks, as well as non-crosslinked boron species in HVPDs prepared with trimethyl borate or boric acid. The number of crosslinks in organo-HVPDs prepared with borax increased over the course of several days. Results from solution and solid-state (11)B NMR spectroscopy are comparable; no solid-like component was detectable. We demonstrate that hydro, organo, or organo-aqueous HVPDs can be obtained from partially hydrolysed poly(vinyl acetate)s by 'tuning' the structure of the boron-based crosslinker.

Correction: Structural bases for mechano-responsive properties in molecular gels of (R)-12-hydroxy-N-(ω-hydroxyalkyl)octadecanamides. Rates of formation and responses to destructive strain

Correction for "Structural bases for mechano-responsive properties in molecular gels of (R)-12-hydroxy-N-(ω-hydroxyalkyl)octadecanamides. Rates of formation and responses to destructive strain" by V. Ajay Mallia and Richard G. Weiss, Soft Matter, 2015, DOI: 10.1039/C5SM00353A.