Publications by authors named "Richard G Weiss"

100 Publications

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

Molecules 2021 Jun 1;26(11). Epub 2021 Jun 1.

Department of Chemistry, Georgetown University, Washington, DC 20057, USA.

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.
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http://dx.doi.org/10.3390/molecules26113332DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8199128PMC
June 2021

Luminescent Behavior of Gels and Sols Comprised of Molecular Gelators.

Gels 2021 Feb 17;7(1). Epub 2021 Feb 17.

Department of Chemistry and Institute for Soft Matter Synthesis and Metrology, Georgetown University, Washington, DC 20057, USA.

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.
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http://dx.doi.org/10.3390/gels7010019DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8005951PMC
February 2021

Triple bubble sign of jejunal atresia.

Abdom Radiol (NY) 2021 07 27;46(7):3512-3514. Epub 2021 Feb 27.

Division of Radiology, Connecticut Children's Medical Center, Hartford, CT, 06106, USA.

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http://dx.doi.org/10.1007/s00261-021-02974-4DOI Listing
July 2021

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.

Magn Reson Chem 2021 06 18;59(6):608-613. Epub 2021 Jan 18.

Departmento de Química Orgánica and Agrupación Estratéxica CITACA, Universidade de Vigo, Vigo, 36310, Spain.

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.
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http://dx.doi.org/10.1002/mrc.5124DOI Listing
June 2021

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

Angew Chem Int Ed Engl 2020 Jun 10;59(25):10173-10178. Epub 2020 Mar 10.

Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing, 100190, China.

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 (S ) and lowest (S ) singlet excited states, even at room temperature in air. Photophysical studies, single-crystal structures, and theoretical calculations indicate that large energy gaps between S and S states (T and T 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 S excited states (phosphorescence from T excited states). These results, including unprecedented fluorescence quantum yields (2.3-9.6 %) from the S states in the neat solids, establish a unique molecular skeleton for achieving multi-colored emissions from upper excited states by "suppressing" Kasha's rule.
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http://dx.doi.org/10.1002/anie.202000608DOI Listing
June 2020

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

J Phys Chem A 2020 Jan 3;124(2):288-299. Epub 2020 Jan 3.

Department of Chemistry and Institute for Soft Matter Synthesis and Metrology , Georgetown University , Washington , District of Columbia 20057-1227 , United States.

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, CO. 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.
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http://dx.doi.org/10.1021/acs.jpca.9b10325DOI Listing
January 2020

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

Polymers (Basel) 2018 Mar 21;10(4). Epub 2018 Mar 21.

Department of Chemistry, Georgetown University, Washington, DC 20057-1227, USA.

A poly(dimethylsiloxane--(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), ,-dimethylformamide (DMF), and dioxane. <>, the average rate constant for intra-chain pyrene excimer formation, was determined from the analysis of the fluorescence decays. <> was found to equal 1.16 (±0.13) × 10⁸, 1.14 (±0.12) × 10⁸, and 0.99 (±0.10) × 10⁸ s 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 <> 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.
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http://dx.doi.org/10.3390/polym10040345DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6414900PMC
March 2018

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

Authors:
Richard G Weiss

Gels 2018 Mar 22;4(2). Epub 2018 Mar 22.

Department of Chemistry and Institute for Soft Matter Synthesis and Metrology, Georgetown University, Washington, DC 20057-1227, USA.

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".
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http://dx.doi.org/10.3390/gels4020025DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6208976PMC
March 2018

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

Soft Matter 2019 Jan 20;15(3):433-441. Epub 2018 Dec 20.

Department of Chemistry and Institute for Soft Matter Synthesis and Metrology, Georgetown University, Washington, DC 20057-1227, USA.

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.
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http://dx.doi.org/10.1039/c8sm01766bDOI Listing
January 2019

Invited Commentary.

Authors:
Richard G Weiss

J Am Coll Surg 2018 06;226(6):1050-1051

Hartford, CT.

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http://dx.doi.org/10.1016/j.jamcollsurg.2018.03.009DOI Listing
June 2018

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

Langmuir 2017 11 3;33(45):12989-12999. Epub 2017 Nov 3.

Department of Chemistry and Institute for Soft Matter Synthesis and Metrology, Georgetown University , Washington, D.C. 20057-1227, United States.

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.
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http://dx.doi.org/10.1021/acs.langmuir.7b03018DOI Listing
November 2017

Influence of Cations on the Fluorescence Quenching of an Ionic, Sterically Congested Pyrenyl Moiety by Iodide in Water.

J Phys Chem A 2017 Oct 27;121(40):7588-7596. Epub 2017 Sep 27.

Department of Chemistry, Indian Institute of Technology Kanpur , Kanpur 208 016, India.

Quenching of the excited singlet states of a water-soluble, sterically congested tetraarylpyrene, 1,3,6,8-tetrakis(2,6-dimethyl-4-(α-carboxy)methoxyphenyl)pyrene (Py4C), by a series of iodide salts has been investigated by steady-state and time-resolved fluorescence measurements. Access to the pyrenyl group of Py4C is restricted sterically as a result of the four flanking (2,6-dimethylphenoxy)acetic acid groups and the energy costs associated with their rotation. Deprotonation of the carboxylic acid groups of Py4C permits examination of ion-ion electrostatic interactions on the rates of quenching by iodide salts in which different steric and electrostatic factors are introduced by varying the cationic portions. At the same concentrations and with the same cations, chloride anions are ineffective quenchers. The quenching rate constants of Py4C by iodide are found to correlate linearly with the ionic radii of the cations and their enthalpies of hydration. These correlations are discussed in terms of the Hofmeister series. Furthermore, the results indicate that the cations that flank Py4C decrease the quenching efficiency of iodide through polarization and shielding effects (i.e., lowering the effective charge), which isolate to varying degrees the π-system. The effects of the different cations on quenching the fluorescence of a simpler and sterically unencumbered pyrenyl derivative, 1-pyrenylbutyric acid (PyBu), by iodide are much smaller. Overall, the results with Py4C indicate that the fluorescence quenching efficiency by iodide is influenced by direct interactions with the cations associated with the carboxylate groups of Py4C and not the solvation of water molecules. This observation is germane to a topic of current debate: Are the effects of the cations more closely related to bulk water properties or to direct ion-ion interactions? The conclusions obtained from these studies are applicable clearly to a wide variety of other systems in which ion pairing influences cooperative or inhibitory interactions.
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http://dx.doi.org/10.1021/acs.jpca.7b07853DOI Listing
October 2017

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

Langmuir 2017 10 3;33(41):10907-10916. Epub 2017 Oct 3.

Department of Chemistry, Georgetown University , Washington, District of Columbia 20057-1227, United States.

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 = 33.5 MPa, δ = 7.5 MPa, and δ = 8.7 MPa; radius = 11.2 MPa). 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 = 34.0 MPa, δ = 10.9 MPa, and δ = 10.8 MPa; radius = 9.2 MPa). DES gels solvents with higher δ and δ 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 = 33.8 MPa, δ = 6.3 MPa, and δ = 9.6 MPa; radius = 11.1 MPa) 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.
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http://dx.doi.org/10.1021/acs.langmuir.7b02191DOI Listing
October 2017

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

Photochem Photobiol Sci 2017 Oct;16(10):1546-1555

Department of Chemistry, Georgetown University, 37th and O Streets, NW, Washington, DC 20057-1227, USA.

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.
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http://dx.doi.org/10.1039/c7pp00199aDOI Listing
October 2017

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

ACS Appl Mater Interfaces 2017 Aug 8;9(33):28069-28078. Epub 2017 Aug 8.

Conservation Division, National Gallery of Art , Washington, DC 20565, United States.

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 H 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.
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http://dx.doi.org/10.1021/acsami.7b09473DOI Listing
August 2017

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

Photochem Photobiol Sci 2017 Jun;16(6):972-984

Department of Chemistry, Georgetown University, 37th and O Streets, NW, Washington, DC 20057-1227, USA.

Photoinduced intermolecular proton-transfer processes from N,N-dimethyl-3-arylpropan-1-ammonium chloride salts (ArCl, with aryl as 1-pyrenyl, 9-anthryl, and 2-naphtyl) to a solvent molecule have been investigated by steady-state and dynamic spectroscopic methods. The intermolecular proton-transfers are coupled either to the formation of an exciplex or to a solvent-separated ion pair in what we have termed a 'proton-coupled charge-transfer reaction'. A range of solvents has been observed to mediate both the ground-state conformations of the ArCl and the extent of electron transfer. Unlike typical photoacids, in which through-bond interactions control photoacidity, through-space charge-transfer interactions are responsible in the excited singlet states of the ArCl. Transient absorption experiments reveal a range of electronic comportments in the excited-states of the ArCl. Excited-state pK values of -3.4, 1.3, and -3.3 in THF were calculated using a Förster-like approach for the 1-pyrenyl, 9-anthryl, and 2-naphthyl salts, respectively. The observed rate of proton-transfer was found to be independent of the thermodynamic driving force and the short-term reversibility of these reactions has been approximated. The data suggest how other systems may be designed to facilitate this novel process.
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http://dx.doi.org/10.1039/c7pp00044hDOI Listing
June 2017

New Insights into an Old Problem. Fluorescence Quenching of Sterically-Graded Pyrenes by Tertiary Aliphatic Amines.

J Phys Chem A 2017 Jan 9;121(2):458-470. Epub 2017 Jan 9.

Department of Chemistry, Indian Institute of Technology Kanpur , Kanpur 208 016, India.

Although the quenching of singlet-excited states of aromatic molecules by amines has been studied for several decades, important aspects of the mechanism(s) remain nebulous. To address some of the unknowns, steric, and electronic factors associated with the quenching of the singlet-excited states of three electronically related aromatic molecules, pyrene, 1,3,6,8-tetraphenylpyrene (TPPy), and 1,3,6,8-tetrakis(4-methoxy-2,6-dimethylphenyl)pyrene (PyOMe), by a wide range of tertiary aliphatic amines have been assessed quantitatively. Correlations among the steric and electronic properties of the amines and the pyrenes (e.g., sizes, shapes, conformational labilities, excitation energies, and oxidation or reduction potentials) have been used in conjunction with the steady-state and dynamic fluorescence quenching data and DFT calculations on the ground and excited state complexes to make quantitative assessments of the steric and electronic factors controlling the quenching processes. PyOMe is a rather rigid bowl-like molecule that, in its electronic ground state, does not make stable complexes with amines in solution. TPPy has a shallower bowl-like shape that is much more flexible. Experiments conducted with a crystalline ground-state complex of an amine and PyOMe demonstrate (as assumed in many other studies but not shown conclusively heretofore) that the geometry needed for quenching the excited singlet state of PyOMe must place the lone-pair of electrons of the amines over the π-system of the pyrenyl group. Furthermore, there is a significant dependence on the shape and size of the amine on its ability to quench PyOMe, but not on the less conformationally constrained TPPy. The conclusions obtained from these studies are clearly applicable to a wide variety of other systems in which fluorescence from an aromatic moiety is being quenched, and they provide insights into how weak host-guest pairs interact.
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http://dx.doi.org/10.1021/acs.jpca.6b11382DOI Listing
January 2017

Insights into the Gelating Abilities of Ricinelaidic Acid and its Ammonium Salts: How do Stereochemistry, Charge, and Chain Lengths Control Gelation of a Long-Chain Alkenoic Acid?

Chemphyschem 2016 Dec 15;17(24):4059-4067. Epub 2016 Nov 15.

Department of Chemistry, Georgetown University, 37 & O Streets, NW, Washington, DC, 20057-1227, USA.

The gelating abilities of ricinelaidic acid (d-REA), the trans-isomer of ricinoleic acid (d-RA), and a series of its alkylammonium and alkane-α,ω-diammonium salts have been examined in a wide range of organic liquids. The gelation efficiency of the trans acid is much better than that of the cis, although neither is as efficient as is the completely saturated molecular gelator analogue, (R)-12-hydroxystearic acid (d-12HSA). The formation of ammonium salts also improves the gelation ability of d-REA in high polarity liquids. The gelating properties are highly dependent upon the chain length of the alkyl group of the alkylammonium salts, but not very dependent on the chain length of the alkane-α,ω-diammonium salts. Structural insights from Fourier transform infrared spectroscopy and powder X-ray diffraction indicate that the absence or presence of unsaturation, the incorporation of (charged) ammonium centers, and the different chain lengths of the alkylammonium salts lead to different packing arrangements and different strengths of H-bonding interactions within the gel assemblies of the d-REA derivatives. Insights into the relationships among the various systematic structural changes to d-REA and the properties of their aggregated structures, including the gel states, are provided.
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http://dx.doi.org/10.1002/cphc.201600902DOI Listing
December 2016

(R)-12-Hydroxystearic Acid Hydrazides as Very Efficient Gelators: Diffusion, Partial Thixotropy, and Self-Healing in Self-Standing Gels.

Chem Asian J 2016 Dec 27;11(23):3414-3422. Epub 2016 Oct 27.

Department of Chemistry, Georgetown University, 37 & O Streets, NW, Washington DC, 20057-1227, USA.

The gelation properties of derivatives of N-alkylated (R)-12-hydroxystearic acid hydrazide (n-HSAH, n=0, 2, 6, 10; n is the length of an n-alkyl chain on the terminal nitrogen atom) in a wide variety of liquids is reported. The n-HSAH compounds were derived from a naturally occurring alkanoic acid, (R)-12-hydroxystearic acid (R-12HSA), and although they differ from the analogous N-alkyl (R)-12-hydroxystearamides (n-HSAA) only by the presence of one N-H group, their behavior as gelators is very different. For example, the parent molecule (0-HSAH) is a supergelator in ethylene glycol, in which it forms self-standing gels that are self-healing, partially thixotropic, moldable, and load-bearing; gels of 0-HSAA are not self-standing. 0-HSAH is structurally the simplest molecular gelator of which we are aware that is capable of forming both self-standing and partially thixotropic gels. Also, diffusion of the cationic dye erythrosine B and the anionic dye methylene blue in 0-HSAH/ethylene glycol gel blocks is much slower than the self-diffusion of ethylene glycol. Polarizing optical microscopy, X-ray diffraction, and FTIR studies revealed that the self-assembled fibrillar networks (SAFINs) of the gels are crystalline, and that 0-HSAH molecules may be arranged in a triclinic subcell with bilayer stacking. The SAFINs are stabilized by strong hydrogen-bonding interactions between the hydrazide groups of adjacent molecules and a perpendicular hydrogen-bonding network between the pendent hydroxyl groups of 0-HSAH. The other n-HSAH (n=2, 6, 10) molecules appear to be arranged in orthorhombic subcells with monolayers and strong hydrogen-bonding interactions between the hydrazide group of one gelator molecule and the hydroxyl group of a neighboring one. These results show how small structural modifications of structurally simple gelator molecules can be exploited to form gels with novel properties that can lead potentially to valuable applications, such as in drug delivery.
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http://dx.doi.org/10.1002/asia.201601163DOI Listing
December 2016

How do H-bonding interactions control viscoelasticity and thixotropy of molecular gels? Insights from mono-, di- and tri-hydroxymethylated alkanamide gelators.

J Colloid Interface Sci 2017 Jan 5;486:359-371. Epub 2016 Oct 5.

Department of Chemistry, Georgetown University, Washington, DC 20057-1227, USA; Institute for Soft Matter Synthesis and Metrology, Georgetown University, Washington, DC 20057-1227, USA. Electronic address:

The structural and dynamic properties of molecular gels, made from 9 structurally-related mono-, di, and tri-hydroxymethylated alkanamide gelators, have been examined at different distance scales. The subtle changes in the gelator structures, in terms of the number of hydroxymethyl groups and the length of the alkanamide chain, have been correlated with the type of the self-assembled fibrillar networks and the viscoelasticity of the gels as well as the characteristics of the liquid as indicated by Hansen solubility parameters. Some of the gels exhibit high degrees of thixotropy and very rapid recovery after the cessation of destructive strain. Gelation efficiencies-based upon the range of liquids gelated, the critical gelator concentrations, and the gel-sol transition temperatures-depend upon both the length of the fatty acid chain and the number of hydroxymethyl groups: the best gelator of the series contains the longest alkyl chain examined (hexadecyl) and two hydroxymethyl groups (i.e., better than the gelators with one or three groups). FT-IR and powder X-ray diffraction data indicate that hydrogen-bonding and molecular packing modes in the gels and the neat gelator powders are very similar. Polarizing optical microscopy images of the gels show that the morphology of the gel networks can be tuned by changing the cooling processes used to transform the sols into gels. In total, the observations and conclusions derived provide useful insights into the relationship between gelator structure and gel properties. These data will be useful to those interested in the a priori design of new gelators or other molecules undergoing a variety of self-assembly processes that lead to robust thermal- and mechano-reversible materials.
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http://dx.doi.org/10.1016/j.jcis.2016.10.008DOI Listing
January 2017

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

Phys Chem Chem Phys 2016 Jul;18(30):20399-409

Department of Chemistry, Georgetown University, Washington, DC 20057-1227, USA. and Institute for Soft Matter Synthesis and Metrology, Georgetown University, Washington, DC 20057-1227, USA.

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.
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http://dx.doi.org/10.1039/c6cp03435gDOI Listing
July 2016

Colloidal Properties of Aqueous Poly(vinyl acetate)-Borate Dispersions with Short-Chain Glycol Ethers.

Chemphyschem 2016 Aug 7;17(16):2535-44. Epub 2016 Jul 7.

Department of Chemistry, Georgetown University, Washington, DC, 20057-1227, USA.

We report the influence of adding five short-chain glycol ethers (SCGEs) on the structure, stability, and viscoelastic properties of aqueous dispersions of partially hydrolyzed poly(vinyl acetate) and borax. The properties of these gel-like materials have been investigated as a function of the structure of the added SCGE both below and above the critical aggregation (or micellar) concentrations using (11) B and (13) C NMR, rheology, and small-angle neutron scattering. The results indicate that the SCGE aggregation behavior is not affected by incorporation into the gel-like network. However, changes in the viscoelasticity and structural properties of the dispersions were detected that can be correlated to the nature of the solvent system. Also, the ability of these materials to clean an unvarnished acrylic paint surface coated with synthetic soil has been evaluated using colorimetery, and the surface of the dispersion after cleaning was visualized with scanning electron microscopy.
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http://dx.doi.org/10.1002/cphc.201600266DOI Listing
August 2016

Insights into the Formation and Structures of Molecular Gels by Diimidazolium Salt Gelators in Ionic Liquids or "Normal" Solvents.

Chemistry 2016 Aug 30;22(32):11269-82. Epub 2016 Jun 30.

Department of Chemistry, Georgetown University, Washington, DC, 20057-1227, USA.

Insights are provided into the properties of molecular gels formed by diimidazolium salts both in "normal" solvents and ionic liquids. These materials can be interesting for applications in green and sustainable chemistry in which ionic liquids play a significant role, like catalysis and energy. In particular, two positional isomers of a diimidazolium cation have been examined with a wide range of anions for their ability to form gel phases. In particular, di-, tri-, and tetravalent anions bearing aliphatic or aromatic spacers were paired with the divalent cations. The properties of the organo- and ionogels formed have been analyzed by means of several different techniques, including calorimetry, rheology, resonance light scattering, UV/Vis absorption, polarizing optical microscopy, and powder X-ray diffraction measurements. The investigations performed enabled us to obtain a wide range of conductive materials characterized by a high thermal stability and a low corrosiveness of the gelator (organogels) or of both gelator and solvent (ionogels). The information gained should be useful in the broader quest to identify and promote their applications.
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http://dx.doi.org/10.1002/chem.201600670DOI Listing
August 2016

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.

J Phys Chem A 2016 Jun 7;120(23):3983-91. Epub 2016 Jun 7.

Department of Chemistry and cInstitute for Soft Matter Synthesis and Metrology, Georgetown University , 37th and O Streets NW, Washington, DC 20057-1227, United States.

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.
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http://dx.doi.org/10.1021/acs.jpca.6b01519DOI Listing
June 2016

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

Chemistry 2016 06 2;22(24):8262-72. Epub 2016 May 2.

Department of Chemistry, Georgetown University, 37 & O Streets, NW, Washington DC, 20057-1227, USA.

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.
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http://dx.doi.org/10.1002/chem.201600225DOI Listing
June 2016

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

Soft Matter 2016 Apr;12(16):3665-76

Department of Chemistry, Georgetown University, Washington DC 20057-1227, USA. and Institute for Soft Matter Synthesis and Metrology, Georgetown University, Washington DC 20057-1227, USA.

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.
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http://dx.doi.org/10.1039/c6sm00377jDOI Listing
April 2016

Reversible Switching of Tb(III) Emission by Sensitization from 2,3-Dihydroxynaphthalene in an Isothermally Reversible Ionic Liquid.

J Phys Chem Lett 2015 Mar 25;6(5):893-7. Epub 2015 Feb 25.

†Department of Chemistry and Institute for Soft Matter Synthesis and Metrology, Georgetown University, Washington, District of Columbia 20057-1227, United States.

A reversible room-temperature ionic liquid (ILO) was prepared by the addition of CO2 to an equimolar mixture of hexylamidine (AD) and butylamine (AN). The ILO and AD/AN mixture were cycled repeatedly by alternating the passage of CO2 and N2 gases through the liquid. The ILO was utilized to sensitize very efficiently energy transfer to and emission by Tb(III) ions when 2,3-dihydroxynaphthalene (DHN) was irradiated. The emission was nearly completely quenched in the AD/AN mixture. The process described here is unique in its use of CO2 and N2 to "switch on and off" the emission by a lanthanide ion, Tb(III) in this case. In the corresponding amidinium dithiocarbamate ionic liquid (ILS), no appreciable Tb(III) emission was found due to quenching of the excited singlet state of DHN by thio groups. The ILS was not reconverted to the AD/AN mixture upon adding N2; N2 bubbling did not result in the displacement of CS2.
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http://dx.doi.org/10.1021/acs.jpclett.5b00091DOI Listing
March 2015

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.

Soft Matter 2015 Jul;11(25):5168

Department of Chemistry, Georgetown University, Washington, DC 20057-1227, USA.

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.
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http://dx.doi.org/10.1039/c5sm90093jDOI Listing
July 2015

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.

Soft Matter 2015 Jul;11(25):5010-22

Department of Chemistry, Georgetown University, Washington, DC 20057-1227, USA.

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.
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http://dx.doi.org/10.1039/c5sm00353aDOI Listing
July 2015

Structural and solubility parameter correlations of gelation abilities for dihydroxylated derivatives of long-chain, naturally occurring fatty acids.

Chemistry 2015 Jun 29;21(23):8530-43. Epub 2015 Apr 29.

Department of Chemistry, Georgetown University, 37 & O Streets, NW, Washington DC 20057-1227 (USA).

Creating structure-property correlations at different distance scales is one of the important challenges to the rational design of molecular gelators. Here, a series of dihydroxylated derivatives of long-chain fatty acids, derived from three naturally occurring molecules-oleic, erucic and ricinoleic acids-are investigated as gelators of a wide variety of liquids. Conclusions about what constitutes a more (or less!) efficient gelator are based upon analyses of a variety of thermal, structural, molecular modeling, and rheological results. Correlations between the manner of molecular packing in the neat solid or gel states of the gelators and Hansen solubility data from the liquids leads to the conclusion that diol stereochemistry, the number of carbon atoms separating the two hydroxyl groups, and the length of the alkanoic chains are the most important structural parameters controlling efficiency of gel formation for these gelators. Some of the diol gelators are as efficient or even more efficient than the well-known, excellent gelator, (R)-12-hydroxystearic acid; others are much worse. The ability to form extensive intermolecular H-bonding networks along the alkyl chains appears to play a key role in promoting fiber growth and, thus, gelation. In toto, the results demonstrate how the efficiency of gelation can be modulated by very small structural changes and also suggest how other structural modifications may be exploited to create efficient gelators.
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http://dx.doi.org/10.1002/chem.201500096DOI Listing
June 2015
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