The Past, Present, and Future of Molecular Gels. What Is the Status of the Field, and Where Is It Going?

A Perspective is presented on the history and current understanding of molecular gels and the factors that must be considered to characterize them. The abilities of the most important structural, dynamic, and rheological tools available currently to provide the information necessary to follow the formation of a molecular gel from its initial sol phase and then to define it at different distance and time scales are discussed. Approaches to determining a priori when a molecule will gelate a selected liquid, as well as possible methodologies for overcoming current limitations in understanding molecular gels, are presented. Finally, some of the many potential and realized applications for these materials are enumerated

Low Molecular-Mass Gelators with Diyne Functional Groups and Their Unpolymerized and Polymerized Gel Assemblies

A series of low molecular-mass organogelators (LMOGs) with conjugated diyne units, R−C⋮CC⋮C−R‘, has been synthesized from 10,12-pentacosadiynoic acid. R is a long alkyl chain and R‘ is a short or long alkyl chain containing an amide or ester group. The gelation efficiencies of these LMOGs and the parent acid (as assessed by the variety of liquids gelled, the amount of gelator needed for gelation, and the temporal and thermal stabilities of the gels) differ widely according to the nature of the substituents. An LMOG with an amide substituent is much more efficient than the corresponding molecule with an ester group, and LMOGs with longer R‘ chains are more efficient than those with shorter ones. When irradiated, some gel networks polymerize. In most cases, the polymerized aggregates phase-separate microscopically, but maintain the gel structure macroscopically. These gels are irreversibly photo- and thermo-chromic, and the thermal stabilities of some of the colored polymerized organogel networks are similar to those of the monomeric assemblies. The molecular packing of the LMOGs as neat powders and in gels before and after polymerization has been examined by X-ray diffraction techniques. This and analyses of IR, UV, and CD (in the case of a chiral diyne LMOG) data allow the nature of the aggregate assemblies before and after irradiation to be assessed. These monomeric organogels and their treatment with light and heat afford an approach to the synthesis of microheterogeneous polymerized networks from relatively simple molecules

Chemically Reversible Organogels: Aliphatic Amines as “Latent” Gelators with Carbon Dioxide

Chemically Reversible Organogels:  Aliphatic Amines as “Latent” Gelators with Carbon Dioxid

Analyses of In-Cage Singlet Radical-Pair Motions from Irradiations of 1-Naphthyl (<i>R</i>)-1-Phenylethyl Ether and 1-Naphthyl (<i>R</i>)-2-Phenylpropanoate in <i>n</i>-Alkanes

The regio- and stereochemistries of photo-Claisen reactions of 1-naphthyl (R)-1-phenylethyl ether ((R)-2), in combination with photo-Fries and photo-Claisen-type reactions of 1-naphthyl (R)-2-phenylpropanoate ((R)-1), have been investigated in n-alkanes of different viscosities and at several temperatures. Analyses of the results provide detailed information about the in-cage motions of the singlet prochiral 1-naphthoxy/1-phenylethyl radical pairs (radical-pair B) that are formed directly from (R)-2 and indirectly from (R)-1 via decarbonylation of singlet chiral 1-naphthoxy/2-phenylpropanoyl radical pairs (radical-pair A). In hexane at 23 °C, the photo-Claisen products from irradiations of (R)-2 retain up to 31% enantiomeric excess (ee), but the ees of the same photoproducts from (R)-1 are near 0%. This disparity is attributed to differences between the initial orientations of the constituent radicals of radical-pair B at the moment of their “birth”. The regio- and stereoselectivities reach plateau values as the solvent viscosity increases, indicating that the relationships between the rates of radical−radical bond formation and either translational or tumbling motions within a solvent cage reach an asymptotic limit. Detailed analyses are presented of the various motions that are in competition within a solvent cage during the very short lifetimes of the radical pairs. The data, in toto, present interesting insights into how radical pairs move during short periods and over short distances when their solvent cages have walls of varying flexibility

Enantioselectivity of Prochiral Radical-Pair Recombinations. Reaction Cavity Differentiation in Polyethylene Films

Recombinations of prochiral radical pairs from irradiations of 1-naphthyl (R)-2-phenylpropanoate in polyethylene films occur with significant enantioselectivity due to templating effects in the reaction cavities. Photoreactions in PE films in their unstretched or stretched states and of different crystallinity have been employed to distinguish between the characteristics of reaction cavities in amorphous and interfacial regions of the polymer

Silica Structures Templated on Fibers of Tetraalkylphosphonium Salt Gelators in Organogels

Phosphonium cations (18nRP+) consisting of three or four n-octadecyl chains and R = PhCH2 or CmH2m+1 (m = 1−5 or 12) when n = 3 and with iodide, bromide, chloride, fluoride, or perchlorate anions are used to gelate and polymerize solutions of 2−10 wt % tetraethyl orthosilicate in ethanol, benzene, tetrahydrofuran, and dimethyl sulfoxide using acid or base catalysis and under hydrolytic or nonhydrolytic conditions. These are the simplest low-molecular-mass organic gelator structures of which we are aware that have been able to template silica. The silica objects that are obtained after the hydrolytic sol−gel process include porous, spherulitic, and tubular objects in the size range of several micrometers to tens of nanometers. Their specific shapes and sizes depend on the specific conditions of the hydrolytic sol−gel process, including the nature of the catalyst. The electrostatic interaction between silicate intermediates and gelator strands is the driving force for templating. The template effect is strongly influenced by several factors, including (1) the competition between silicate/solvent and silicate/template interactions, (2) the period of the sol−gel process, (3) the hardness of the anion of the gelator salt, (4) the surface tension of the solvent, (5) and the sequence of drying and template removal processes. The nature of the R group influences the stability of the molecular gels but appears to have little effect on the silica morph obtained. In addition, it is shown in one case, where a direct comparison is possible, that the fibers of one of our phosphonium salts are a much more efficient template for silica than those of the corresponding ammonium salt (with its “harder” cationic center). The specific nature of the objects and the conditions under which they can be formed are discussed

Chemically Reversible Organogels via “Latent” Gelators. Aliphatic Amines with Carbon Dioxide and Their Ammonium Carbamates^†

Rapid and isothermal (at room temperature) uptake of CO2 by solutions or, in some cases, organogels comprised of a primary or secondary aliphatic amine (1) and an organic liquid leads to in situ chemical transformation to the corresponding alkylammonium alkylcarbamate (2) based gels. Chemical reversibility is demonstrated by removal of CO2 from 2-based gels upon gentle heating in the presence of nitrogen. This is a general strategy for reversible self-assembly or disassembly of molecular aggregates relying on the initiation or termination of ionic interactions. The dependence of the amine structure and the nature of the liquid component on the formation and stability of the 1 and 2 organogels are examined by differential scanning calorimetry, optical microscopy, and X-ray diffraction methods. In most cases, the 2 gelators are more effective (based on the minimum gelator concentration required at room temperature, the gelation temperature, and the duration of time a gel persists without bulk phase separation) and more diverse (based on the classes of liquids gelled) than their corresponding amines. The differences are attributed to the presence of ionic interactions between molecular segments of the alkylammonium alkylcarbamates that are stronger than the hydrogen-bonding interactions available between molecules of amines. The initial stages of aggregation in the gel assemblies (i.e., changes in the degree of aggregation of sols of some 2 gelators) have been examined as a function of concentration and temperature by NMR techniques

Detection of Pre-Sol Aggregation and Carbon Dioxide Scrambling in Alkylammonium Alkylcarbamate Gelators by Nuclear Magnetic Resonance^†

The initial stages of aggregation of a series of organogelator salts, prepared from n-alkylamines by the rapid in situ and isothermal (at room temperature) uptake of the neutral triatomic molecule, CO2, have been probed by NMR spectroscopy in the nongelled liquid, chloroform-d. Evidence for specific interactions of the ionic headgroups in the aggregates is presented. The influences of concentration and temperature on the processes leading to pre-sol aggregates of decylammonium decylcarbamate (2b) have been investigated in detail. NMR spectra of selectively deuterated (at the α-methylene group) and selectively 13C-enriched (at the carbonyl carbon) 2b demonstrate that CO2 is scrambled rapidly between the ammonium and carbamate parts of the molecule in chloroform solution. No scrambling of CS2 was detected in alkylammonium alkyldithiocarbamates under the same experimental conditions

Primary Alkyl Amines as Latent Gelators and Their Organogel Adducts with Neutral Triatomic Molecules^†

A series of organogelator salts has been prepared from n-alkylamines by the rapid in situ and isothermal (at room temperature) uptake of a neutral triatomic molecule, CO2, NO2, SO2, or CS2. The organogels have been examined by differential scanning calorimetry, optical microscopy, and X-ray diffraction methods. The efficiency of each gelator has been assessed on the bases of the diversity of liquids it gelled, the minimum amount of it required for gelation, and the temporal and thermal stabilities of its gels. Thus, alkylammonium alkylcarbamates, amine−CO2 adducts, are the most effective gelators and the amine−NO2 adducts are the least efficient. Salts from longer n-alkylamines are better gelators than those from shorter homologues. Some of the salts are reconverted to their amine and triatomic constituents by heating, while others are transformed into new compounds. In the case of the CS2 adducts, H2S is expelled and the new species formed, N,N‘-dialkylthioureas, are also gelators

‘Remote’ Adiabatic Photoinduced Deprotonation and Aggregate Formation of Amphiphilic <i>N</i>-Alkyl-<i>N</i>-methyl-3-(pyren-1-yl)propan-1-ammonium Chloride Salts

The absorption and emission properties of a series of amphiphilic <i>N</i>-alkyl-<i>N</i>-methyl-3-(pyren-1-yl)propan-1-ammonium chloride salts were investigated in solvents of different polarities and over a wide concentration range. For example, at 10^–5 M concentrations in tetrahydrofuran (THF), salts with at least one N–H bond exhibited broad, structureless emissions even though time-correlated single photon counting (TCSPC) experiments indicated negligible static or dynamic intermolecular interactions. Salts with a butylene spacer or lacking an N–H bond showed no discernible structureless emission; their emission spectra were dominated by the normal monomeric fluorescence of a pyrenyl group and the TCSPC histograms could be interpreted on the basis of intramolecular photophysics. The broad, structureless emission is attributed to an unprecedented, rapid, adiabatic proton-transfer to the medium, followed by the formation of an intramolecular <i>exciplex</i> consisting of amine and pyrenyl groups. The proposed mechanism involves excitation of a ground-state conformer of the salts in which the ammonium group sits over the pyrenyl ring due to electrostatic stabilization. At higher concentrations, with longer <i>N</i>-alkyl groups, or in selected solvents, electronic excitation of the salts led to dynamic and static <i>excimeric</i> emissions. For example, whereas the emission spectrum of 10^–3 M <i>N</i>-hexyl-<i>N</i>-methyl-3-(pyren-1-yl)propan-1-ammonium chloride in THF consisted of comparable amounts of monomeric and excimeric emission, the emission from 10^–5 M <i>N</i>-dodecyl-<i>N</i>-methyl-3-(pyren-1-yl)propan-1-ammonium chloride in 1:9 (v:v) ethanol/water solutions was dominated by excimeric emission, and discrete particles near micrometer size were discernible from confocal microscopy and dynamic light scattering experiments. Comparison of the static and dynamic emission characteristics of the particles and of the neat solid of <i>N</i>-dodecyl-<i>N</i>-methyl-3-(pyren-1-yl)propan-1-ammonium chloride indicate that molecular packing in the microparticles and in the single crystal are very similar if not the same. It is suggested that other examples of the adiabatic proton transfer found in the dilute concentration regime with the pyrenyl salts may be occurring in very different systems, such as in proteins where conformational constraints hold ammonium groups over aromatic rings of peptide units