Co-reporter:Mei-Jin Li, Michael J. Bertocchi, and Richard G. Weiss
Macromolecules March 14, 2017 Volume 50(Issue 5) pp:1919-1919
Publication Date(Web):March 2, 2017
DOI:10.1021/acs.macromol.6b02434
A series of polymers derived from poly(isobutylene-alt-maleic anhydride) (PIMA) with ca. 1% to >90% of the anhydride units randomly substituted with 1-pyrenylmethylimido groups (Py-PIMA) has been synthesized and characterized. The remaining anhydride units in the Py-PIMA with 10% pyrenyl substitution have been converted to N-hexyl and N-decyl imides. The photophysical properties of these polymers, as obtained from steady-state fluorescence intensities, time-correlated single photon counting experiments, and time-resolved emission spectra, have provided insights into the dependence of the polymer conformations and their labilities on the degree of pyrenyl substitution, the nature of the appended alkyl chains, temperature, and solvent properties according to the Flory interaction coefficient and Hansen solubility parameters. None of the solvent characteristics, alone, can account for all aspects of the polymer behavior. The interactions of the medium with the polymer chains and the excited singlet states of the pyrenyl units must be considered somewhat differently. The results also indicate that PIMA is a potentially very useful and versatile platform for other investigations of polymer chain and probe dynamics.
Co-reporter:Teresa T. Duncan, Barbara H. Berrie, and Richard G. Weiss
ACS Applied Materials & Interfaces August 23, 2017 Volume 9(Issue 33) pp:28069-28069
Publication Date(Web):August 8, 2017
DOI:10.1021/acsami.7b09473
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.Keywords: boron chemistry; cleaning; cultural heritage; organogel; polymer;
Co-reporter:Janine L. Thoma and Jean Duhamel, Mei-Jin Li, Michael J. Bertocchi, and Richard G. Weiss
Macromolecules April 25, 2017 Volume 50(Issue 8) pp:3396-3396
Publication Date(Web):April 14, 2017
DOI:10.1021/acs.macromol.7b00527
The steady-state fluorescence (SSF) spectra and time-resolved fluorescence (TRF) decays of five poly(isobutylene-alt-maleic anhydride) copolymers (PIMA), randomly labeled with N-(1-pyrenylmethyl)succinimide groups (Py-PIMA), have been analyzed in order to describe their long-range, polymer chain dynamics (LRPCD) and to assess the stiffness of the PIMA backbone compared to that of a series of poly(alkyl methacrylate)s whose LRPCD had already been investigated. The SSF spectra, recorded under dilute concentration conditions where only intrachain events are important, show that the intensity ratio of pyrenyl excimer to pyrenyl monomer emission (i.e., (IE/IM)SSF) increases linearly with increasing pyrenyl content as a result of increased pyrenyl–pyrenyl contacts and diffusive encounters. The slope of the straight line, m(IE/IM), that reflected the efficiency of pyrenyl excimer formation for Py-PIMA, was found to be 60% smaller than for even poly(octadecyl methacrylate) (PC18MA), a polymer exhibiting slow LRPCD due to its long, bulky side chain. Analyses of the SSF spectra allowed separation of the averaged behavior of five different pyrenyl species in the Py-PIMA polymers, although only two of them were reflected in the LRPCD of PIMA. In addition, the fluorescence decays of the pyrenyl monomer and excimer, when fitted globally according to the model free analysis (MFA), yielded the average rate constant ⟨kMF⟩blob of pyrenyl excimer formation that directly represents the LRPCD of Py-PIMA. After having conducted two tests that confirmed the validity of the parameters retrieved from the MFA of the TRF decays, ⟨kMF⟩blob for Py-PIMA was compared to ⟨kMF⟩blob of nine other polymers. The result of this comparison leads us to conclude that the PIMA backbone is extremely stiff in THF. This study demonstrates the ability of the MFA of fluorescence decays acquired from polymers randomly labeled with pyrenyl groups to characterize quantitatively the LRPCD of even extremely stiff polymers.
Co-reporter:Yan Zhang, Richard G. Weiss
Journal of Colloid and Interface Science 2017 Volume 486() pp:359-371
Publication Date(Web):15 January 2017
DOI:10.1016/j.jcis.2016.10.008
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.
Co-reporter:Trevor M. Safko;Shenlong Jiang;Lei Zhang;Qun Zhang;Richard G. Weiss
Photochemical & Photobiological Sciences (2002-Present) 2017 vol. 16(Issue 6) pp:972-984
Publication Date(Web):2017/06/14
DOI:10.1039/C7PP00044H
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 pKa 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.
Co-reporter:Trevor M. Safko;Miklos Kertesz;Richard G. Weiss
Photochemical & Photobiological Sciences (2002-Present) 2017 vol. 16(Issue 10) pp:1546-1555
Publication Date(Web):2017/10/11
DOI:10.1039/C7PP00199A
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.
Co-reporter:V. Ajay Mallia and Richard G. Weiss
Soft Matter 2016 vol. 12(Issue 16) pp:3665-3676
Publication Date(Web):07 Apr 2016
DOI:10.1039/C6SM00377J
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.
Co-reporter:Mohan Zhang and Richard G. Weiss
Physical Chemistry Chemical Physics 2016 vol. 18(Issue 30) pp:20399-20409
Publication Date(Web):12 Jul 2016
DOI:10.1039/C6CP03435G
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.
Co-reporter:V. Ajay Mallia, Daniel L. Blair, and Richard G. Weiss
Industrial & Engineering Chemistry Research 2016 Volume 55(Issue 4) pp:954-960
Publication Date(Web):January 25, 2016
DOI:10.1021/acs.iecr.5b04267
The mechanical properties of films of gels composed of a crude oil or corn oil and (R)-12-hydroxystearic acid are described using oscillatory rheology and water-surface waves. The integrity of these gel films are contrasted with those of neat oil films subjected to the same types of mechanical testing. The oil-based gels are thixotropic, and we quantify their post-recovery yield. A simple model is proposed to describe the loss of integrity when the gels are subjected to high amplitude surface waves. Our technique provides a novel method for quantifying the role of dynamic surface perturbations on the mechanics of viscoelastic films, providing an illustrative model system for testing coagulants and dispersants designed to mitigate oil spills.
Co-reporter:Teresa T. Duncan;Dr. Barbara H. Berrie; Richard G. Weiss
ChemPhysChem 2016 Volume 17( Issue 16) pp:2535-2544
Publication Date(Web):
DOI:10.1002/cphc.201600266
Abstract
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 11B and 13C 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.
Co-reporter:KIZHMURI P DIVYA;MICHAEL J BERTOCCHI;RICHARD G WEISS
Journal of Chemical Sciences 2016 Volume 128( Issue 1) pp:119-132
Publication Date(Web):2016 January
DOI:10.1007/s12039-015-1009-5
Static and dynamic fluorescence measurements have been made on four N,N′−dialkylated perylene diimides in films of poly(alkyl methacrylate)s (PAMAs) with 5 different alkyl groups and in a ‘model solvent’, n-butyl acetate, over wide temperature ranges. The results indicate that the excited singlet states of the perylene guest molecules are controlled primarily by chain relaxations rather than hole free volumes in the polymer matrixes. The short singlet lifetimes of the perylene molecules require that the guest molecules respond primarily to the environments experienced by their ground states within the PAMA matrixes; each of the PAMAs offers slightly different locations in which the guest molecules can reside as a result of interactions between the N−alkyl substituents on the imide groups of the perylenes and the alkyl groups on the PAMA side chains. PAMAs with branched side chains were found to have a larger influence than PAMAs with linear side chains on the fluorescence properties of the guest molecules. The results are compared to those employing pyrenyl derivatives (with much longer excited singlet lifetimes) in the same PAMA films. The overall results indicate that the perylenes can be used as a complementary probe of local polymer chain dynamics, but they are less sensitive to their environments than are pyrenyl groups. However, they offer some distinct advantages: (1) a much wider range of N,N′−disubstituted perylene diimides can be synthesized easily; (2) those substituents can be designed to allow a greater or lesser interaction with an anisotropic host matrix. Also, rapid conformational changes of a bis-perylene derivative appear to be restricted in the polymer matrixes. Those restrictions appear reduced when the polymer films are placed under high pressures of the plasticizing gas, CO2, but not when they are under equal pressures of a much less intervening gas, N2.
Co-reporter:Trevor M. Safko, Marcelo M. Faleiros, Teresa D. Z. Atvars, and Richard G. Weiss
The Journal of Physical Chemistry A 2016 Volume 120(Issue 23) pp:3983-3991
Publication Date(Web):June 7, 2016
DOI:10.1021/acs.jpca.6b01519
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.
Co-reporter:Mohan Zhang; Richard G. Weiss
Chemistry - A European Journal 2016 Volume 22( Issue 24) pp:8262-8272
Publication Date(Web):
DOI:10.1002/chem.201600225
Abstract
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.
Co-reporter:V. Ajay Mallia and Richard G. Weiss
Soft Matter 2015 vol. 11(Issue 25) pp:5168-5168
Publication Date(Web):05 Jun 2015
DOI:10.1039/C5SM90093J
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.
Co-reporter:V. Ajay Mallia and Richard G. Weiss
Soft Matter 2015 vol. 11(Issue 25) pp:5010-5022
Publication Date(Web):26 May 2015
DOI:10.1039/C5SM00353A
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.
Co-reporter:Michael A. Rogers and Richard G. Weiss
New Journal of Chemistry 2015 vol. 39(Issue 2) pp:785-799
Publication Date(Web):22 Oct 2014
DOI:10.1039/C4NJ01439A
A systematic structural development of molecular gelators based on n-alkanes is presented. The properties of the resultant molecular gels with a wide range of liquids can, in some cases, be correlated with the structural modifications to the gelators and the natures of their self-assembled fibrillar networks as analyzed at different distance scales. A special emphasis is placed on 12-hydroxystearic acid and its derivatives in which the chirality, placement, and functionality of the hydroxyl group center have been varied and the carboxylic head group has been modified systematically. The resulting correlations between molecular gelator structure and the various properties of their gels provide a basis for the design of structurally more complex gelators. However, the nuances of the systems explored demonstrate that additional factors, some of which are currently not well understood, must be considered before a priori design of molecular gelators with specific gelating abilities will be possible.
Co-reporter:Kizhmuri P. Divya;Richard G. Weiss
The Journal of Physical Chemistry Letters 2015 Volume 6(Issue 5) pp:893-897
Publication Date(Web):February 25, 2015
DOI:10.1021/acs.jpclett.5b00091
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.
Co-reporter:Mohan Zhang;Dr. Sermadurai Selvakumar; Xinran Zhang; Mukund P. Sibi; Richard G. Weiss
Chemistry - A European Journal 2015 Volume 21( Issue 23) pp:8530-8543
Publication Date(Web):
DOI:10.1002/chem.201500096
Abstract
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.
Co-reporter:V. Ajay Mallia;Richard G. Weiss
Journal of Physical Organic Chemistry 2014 Volume 27( Issue 4) pp:310-315
Publication Date(Web):
DOI:10.1002/poc.3193
Self-assembly of small molecules can lead to many different types of objects and phases. This mini-review concentrates on the formation of one-dimensional objects and their transformation into gels from solutions/sols of 12-hydroxystearic acid, a molecule derived from the castor bean, and its derivatives in a wide variety of liquids. It is shown how the molecular packing within the one-dimensional objects and the nature of the derived gels can be correlated with systematic variations of the gelator structure and the type of liquid. Copyright © 2013 John Wiley & Sons, Ltd.
Co-reporter:Ni Yan ; Zhiyan Xu ; Kevin K. Diehn ; Srinivasa R. Raghavan ; Yu Fang ;Richard G. Weiss
Journal of the American Chemical Society 2013 Volume 135(Issue 24) pp:8989-8999
Publication Date(Web):June 4, 2013
DOI:10.1021/ja402560n
The self-assembly behavior of a series of glucono-appended 1-pyrenesulfonyl derivatives containing α,ω-diaminoalkane spacers (Pn, where n, the number of methylene units separating the amino groups, is 2, 3, 4, 6, 7, and 8) in v:v tetrahydrofuran (THF):water mixtures is examined at room temperature. The Pn at 2 w/v % concentrations do not dissolve in either THF or water at room temperature. However, the Pn can be dissolved in some THF:water mixtures, and they form gels spontaneously in other compositions without dissolving completely. The self-assembly of the Pn in the liquid mixtures has been investigated using a variety of techniques. The particle sizes of the Pn in their solutions/sols, critical gelation concentrations, microstructures, thermal and mechanical stabilities of the gels, and molecular packing modes of Pn molecules in their gel networks are found to be very dependent on the composition of the liquid mixtures. Correlations between the self-assembly behavior of the Pn and the polarity of the liquid mixtures, as probed by ET(30) and Hansen solubility parameters, yield both qualitative and quantitative insights into why self-assembly of the Pn can or cannot be achieved in different liquid compositions. As revealed by UV–vis and fluorescence spectroscopy studies, π–π stacking of the pyrenyl groups occurs as part of the aggregation process. Correlations between the rheological properties of the gels and the Hansen solubility parameters of the Pn and the solvent mixtures indicate that hydrogen-bonding interactions are a major contributor to the mechanical stability. Overall, the results of this study offer a new strategy to investigate the balance between dissolution and aggregation of molecular gelators. To the best of our knowledge, this is the first example of the spontaneous formation of molecular gels without heating by placing gelators in mixtures of liquids in which they are insoluble in the neat components.
Co-reporter:Teresa D. Z. Atvars;Shibu Abraham;Anita J. Hill;Steven J. Pas;Carlos Chesta;Richard G. Weiss
Photochemistry and Photobiology 2013 Volume 89( Issue 6) pp:1346-1353
Publication Date(Web):
DOI:10.1111/php.12103
Abstract
Pyrene fluorescence spectra have been recorded in five poly(alkyl methacrylate)s (where alkyl is ethyl butyl, isobutyl, cyclohexyl and hexadecyl) over a 20–400 K temperature range. The changes in the position and the full width at half maximum (FWHM) of the 0–0 emission band (peak I) have been correlated with the structural characteristics of the alkyl groups in the different relaxation regimes of the polymers to assess the degree of coupling of the excited singlet states with the polymer cybotactic regions. Data treatment of the peak I positions using an electron–phonon model indicates that longitudinal optical modes are involved, and that the magnitude of coupling depends on the polymer structure and follows the same trend as the glass transition temperatures. The same spectral parameters have been correlated also with “hole” free volumes from positron annihilation spectroscopy over temperature ranges which span the glass or melting transitions of the polymers. Reasons why free volume and FWHM measurements follow the same trends, and other aspects of the systems, are discussed.
Co-reporter:Braja G. Bag, Rakhi Majumdar, Shaishab K. Dinda, Partha P. Dey, Gopal C. Maity, V. Ajay Mallia, and Richard G. Weiss
Langmuir 2013 Volume 29(Issue 6) pp:1766-1778
Publication Date(Web):January 10, 2013
DOI:10.1021/la304485e
Ten aliphatic and aromatic ketals of arjunolic acid, a renewable, nanosized triterpenic acid which is obtainable from Terminalia arjuna, have been synthesized upon condensation with aldehydes. Self-assembly properties of the ketals have been studied in a wide range of organic liquids. With the exception of the p-nitrobenzylidene derivative, low concentrations of the ketals self-assemble and form gel-like dispersions in many of the organic liquids examined. The morphologies of the assemblies, studied at different distance scales by optical, electron, and atomic-force microscopies, consisted of fibrillar networks and vesicles which were able to entrap 5(6)-carboxyfluorescein as a guest molecule. X-ray diffractograms indicate that the fibrillar objects are crystalline. A charge-transfer complex was formed from a 1:1 mixture of ketal derivatives with electron-donating and electron-accepting groups, and the 9-anthrylidene derivative in its fibrillar network dimerized upon irradiation. Results demonstrate that subtle changes in the ketal structures can lead to very different aggregation pathways.
Co-reporter:Ni Yan, Zhiyan Xu, Kevin K. Diehn, Srinivasa R. Raghavan, Yu Fang, and Richard G. Weiss
Langmuir 2013 Volume 29(Issue 2) pp:793-805
Publication Date(Web):December 19, 2012
DOI:10.1021/la304957n
A series of glucono-appended 1-pyrenesulfonyl derivatives containing α,ω-diaminoalkane spacers (Pn, where n, the number of methylene units separating the amino groups, is 2, 3, 4, 6, 7, and 8) have been prepared. Careful analyses of correlations between the structures of these molecules and their gels have provided important insights into the factors responsible for one-dimensional aggregation of small molecules containing both lipophilic and hydrophilic parts. The gelation behavior has been examined in 30 liquids of diverse structure and polarity, and the properties of their gels and the gelation mechanisms have been investigated using a variety of techniques. Possible reasons are discussed regarding why the Pn are better gelators than the corresponding naphthyl analogues (Nn) which had been investigated previously. P2 and P3 are ambidextrous gelators (i.e., they gelate both water and some organic liquids), and P4–P8 gelate some organic liquids which are protic and aprotic, but not water. In at least one of the liquids examined, P3, P4, P6, P7, and P8 form gels at less than 1 w/v % concentrations, and some of the gels in 1-decanol are thixotropic. Analyses of the gelation abilities using Hansen solubility parameters yield both qualitative and quantitative insights into the role of liquid-gelator interactions. For example, the critical gelation concentrations increase generally with increasing polar and hydrogen bonding interactions between the gelators and their liquid components. As revealed by FT-IR, 1H NMR, UV–vis, and fluorescence spectra, hydrogen-bonding between glucono units and π–π stacking between pyrenyl groups are important in the formation and maintenance of the gel networks. The results from this study, especially those relating the aggregation modes and liquid properties, offer insights for the design of new surfactant-containing low-molecular-mass gelators with predefined gelating abilities.
Co-reporter:V. Ajay Mallia, Hyae-In Seo, and Richard G. Weiss
Langmuir 2013 Volume 29(Issue 21) pp:6476-6484
Publication Date(Web):May 15, 2013
DOI:10.1021/la400748q
The self-assembly and gelating characteristics of a set of N-alkyl-(R)-12-hydroxyoctadecylammonium salts (n-HOA-X, where n = 0–6, 18 is the length of the alkyl chain on nitrogen, X = Cl, n = 3, and X = Br, NO3, and BF4) are described. Solid–solid phase transitions were observed for powders of n-HOA-Cl, and orthorhombic-type crystal packing arrangements and lattice spacings were calculated from X-ray diffractograms at 22 °C. The diffractogram of 3-HOA-Br indicates the presence of more than one morph at room temperature, and that of 3-HOA-I corresponds to a lamellar packing arrangement. Differences in the molecular packing arrangements of 3-HOA-X are reflected in their gelation abilities. The melting temperatures (Tgel) of the hydrogels of 3-HOA-Br are higher than those of 3-HOA-Cl at the same concentrations, and 3-HOA-I failed to gelate any of the investigated liquids. 3-HOA-NO3 gelated only water and CCl4 and 3-HOA-BF4 formed only hydrogels. Plots of changes in conductivities of the 3-HOA-X salts (where X = Cl, Br, NO3 and BF4) as a function of temperature were used to calculate the critical aggregation concentrations (CGCs). Because the CGCs from the ‘falling drop’ method are nearly the same as those from the conductivity measurements, aggregation, nucleation, and gelation must occur within a very narrow 3-HOA-X concentration range. Tgel values of 2 wt % 3-HOA-Cl hydrogels (prepared by fast cooling of the sol phase) increased upon adding KCl up to 0.1 M. The effects can be attributed principally to the chloride anion rather than its cation partners. The properties of the hydrogels of 3-HOA-X do not follow the Hofmeister ranking rule. The variations in the counterions afford detailed insight into the behavior of 3-HOA-X in their neat solids and assemblies in gels as well as the processes accompanying gel formation in water and organic liquids.
Co-reporter:Amrita Pal, Shibu Abraham, Michael A. Rogers, Joykrishna Dey, and Richard G. Weiss
Langmuir 2013 Volume 29(Issue 21) pp:6467-6475
Publication Date(Web):May 14, 2013
DOI:10.1021/la400664q
A systematic study of the importance of functional group position and type on the gelator efficiencies of structurally simple, low molecular-mass gelators is reported. Thus, the gelation abilities of a series of positional isomers of ketooctadecanoic acid (n-KSA) are compared in a wide range of liquids. The gelation abilities of the n-KSA as a function of n, the keto group position along the chain, are characterized by several structural, thermal, and rheological techniques and are compared with those of the corresponding hydroxyoctadecanoic acid isomers (n-HSA) and the parent molecule, octadecanoic acid (SA). Analyses of the gels according to the strengths of functional group interactions along the alkyl chain in terms of group position and type are made. The conclusions derived from the study indicate that gel stability is enhanced when the functional group is located relatively far from the carboxylic headgroup and when group–group interactions are stronger (i.e., hydrogen-bonding interactions are stronger in the n-HSA than dipole interactions in the n-KSA, which are stronger than the London dispersion interactions in SA). Co-crystals of the keto- and hydroxy-substituted octadecanoic acids are found to be less efficient gelators than even the ketooctadecanoic acids, due to molecular packing and limited group interactions within the gelator networks.
Co-reporter:Tao Yu and Richard G. Weiss
Green Chemistry 2012 vol. 14(Issue 1) pp:209-216
Publication Date(Web):18 Nov 2011
DOI:10.1039/C1GC16027C
In situ-prepared, reversible, room-temperature ionic liquids (RTILs), amidinium carbamates, have been used as media for the syntheses of cyclic carbonates by cycloaddition of CO2 to epoxides. The amidinium carbamates were prepared by exposing equimolar mixtures of an easily synthesized amidine and a primary amine to CO2 gas. For comparison purposes, amidinium dithiocarbamates were also employed as the RTILs in some experiments. Reaction between CO2 and four epoxides in the RTILs, occurs in good yields (> 90% in many cases) at room temperature or 50 °C in the presence of an amidinium halide catalyst. Product and any unreacted epoxide were extracted easily upon addition of an immiscible, lower density liquid to an RTIL reaction mixture. This process was repeated three times with the same RTIL without any obvious decrease in catalytic activity; presumably, additional transformations could have been conducted. The influences of the type of catalyst, CO2 pressure, reaction time, and temperature on the reaction yields have been investigated. The relatively mild reaction conditions and ease of separation of products, as well as the ability of the amidinium carbamates to be reused in the presence or absence of water make this an attractive alternative to other procedures for the efficient syntheses of cyclic carbonates.
Co-reporter:Shibu Abraham and Richard G. Weiss
Photochemical & Photobiological Sciences 2012 vol. 11(Issue 11) pp:1642-1644
Publication Date(Web):24 Nov 2011
DOI:10.1039/C1PP05312D
Addition of CO2 to an equimolar amidine/amine mixture leads to formation of a switchable ionic liquid, an amidinium carbamate, and appearance of strong fluorescence by a solute, pyrene. Passing a ‘displacement gas’, N2, through the ionic liquid or heating it regenerates the amidine/amine mixture and quenches the fluorescence almost totally. No switching or appreciable fluorescence is found when an amidinium dithiocarbamate is the medium.
Co-reporter:Shibu Abraham, Indrajit Ghosh, Werner M. Nau, Carlos Chesta, Steven J. Pas, Anita J. Hill and Richard G. Weiss
Photochemical & Photobiological Sciences 2012 vol. 11(Issue 6) pp:914-924
Publication Date(Web):03 Jan 2012
DOI:10.1039/C1PP05282A
Norrish type 1 reactions of 1-(4-methylphenyl)-3-phenyl-2-propanone (ACOB1) have been used to probe structural and morphological properties of a series of poly(alkyl methacrylate)s (PAMAs, where the alkyl is ethyl, butyl, isobutyl, cyclohexyl, and hexadecyl) below and above their glass transition (or melting) temperatures. The PAMAs investigated cover a wide range of glass transition temperatures and structure types. The ratio of in-cage to the sum of in-cage and out-of-cage recombinations of the triplet benzylic radical-pairs generated upon irradiation of ACOB1 (Fc) have been calculated from relative photoproduct yields at different temperatures and are compared with the free hole volumes within the polymers as calculated from positronium annihilation lifetime spectroscopy. Laser flash photolysis experiments to follow the growth and decay of the radicals have also been conducted in order to correlate the steady-state irradiation results with the radical pair recombination processes (i.e., in-cage and out-of-cage). The changes in Fc as a function of PAMA type and phase (temperature) can be correlated with chain relaxation rates and the nature of the polymer side chains, but not hole free volumes. These results are compared with those from our previous work, conducted in polyethylenes with differing degrees of crystallinity, where hole free volume was the primary factor in controlling Fc.
Co-reporter:V. Ajay Mallia ; Paul D. Butler ; Bijay Sarkar ; K. Travis Holman ;Richard G. Weiss
Journal of the American Chemical Society 2011 Volume 133(Issue 38) pp:15045-15054
Publication Date(Web):July 6, 2011
DOI:10.1021/ja204371b
The CCl4 gel phases of a series of low-molecular-mass organogelators, (R)-18-(n-alkylamino)octadecan-7-ols (HSN-n, where n = 0–5, 18 is the alkyl chain length), appear to be unprecedented in that the fibrillar networks of some of the homologues undergo thermally reversible, gel-to-gel phase transitions, and some of those transitions are evident as opaque–transparent changes in the appearance of the samples. The gels have been examined at different concentrations and temperatures by a wide variety of spectroscopic, diffraction, thermal, and rheological techniques. Analyses of those data and data from the neat gelators have led to an understanding of the source of the gel-to-gel transitions. IR and SANS data implicate the expulsion (on heating the lower-temperature gel) or the inclusion (on cooling the higher-temperature gel) of molecules of CCl4 that are interspersed between fibers in bundles. However, the root cause of the transitions is a consequence of changes in the molecular packing of the HSN-n within the fibers. This study offers opportunities to design new gelators that are capable of behaving in multiple fashions without entering the sol/solution phase, and it identifies a heretofore unknown transformation of organogels.
Co-reporter:Shibu Abraham ;Richard G. Weiss
Journal of the American Chemical Society 2011 Volume 133(Issue 47) pp:19250-19256
Publication Date(Web):November 3, 2011
DOI:10.1021/ja208674s
The absorption and emission properties of a series of amphiphilic N-alkyl-N-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 exciplex 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 N-alkyl groups, or in selected solvents, electronic excitation of the salts led to dynamic and static excimeric emissions. For example, whereas the emission spectrum of 10–3 M N-hexyl-N-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 N-dodecyl-N-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 N-dodecyl-N-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.
Co-reporter:V. Ajay Mallia, Pierre Terech, and Richard G. Weiss
The Journal of Physical Chemistry B 2011 Volume 115(Issue 43) pp:12401-12414
Publication Date(Web):October 6, 2011
DOI:10.1021/jp207182p
The self-assembly and gelating ability of a set of N-alkyl-(R)-12-hydroxyoctadecylammonium chlorides (NCl-n, where n = 0–6, 18 is the length of the alkyl chain on nitrogen) are described. Several are found to be ambidextrous (gelating both water and a variety of organic liquids) and very efficient (needing less than ca. 0.5 wt % at room temperature). Structure–property correlations at different distance scales of the NCl-n in their hydro- and organo-gels and neat, solid states have been made using X-ray diffraction, neutron scattering, thermal, optical, cryo-SEM and rheological techniques. The self-assembled fibrillar networks consist of spherulitic objects with fibers whose diameters and degrees of twisting differ in the hydro- and organo-gels. Increasing n (and, thus, the molecular length) increases the width of the fibers in their hydrogels; an irregular, less pronounced trend between n and fiber width is observed in the corresponding toluene gels. Time-dependent, small angle neutron scattering data for the isothermal sol-to-gel transformation of sols of NCl-18/toluene to their gels, treated according to Avrami theory, indicate heterogeneous nucleation involving rodlike growth. Rheological studies of gels of NCl-3 in water and toluene confirm their viscoelastic nature and show that the hydrogel is mechanically stronger than the toluene gel. Models for the different molecular packing arrangements within the fibrillar gel networks of the hydro- and organogels have been inferred from X-ray diffraction. The variations in the fibrillar networks provide a comprehensive picture and detailed insights into why seemingly very similar NCl-n behave very differently during their self-assembly processes in water and organic liquids. It is shown that the NCl-n provide a versatile platform for interrogating fundamental questions regarding the links between molecular structure and one-dimensional self-aggregation, leading to gelation.
Co-reporter:Lora V. Angelova, Pierre Terech, Irene Natali, Luigi Dei, Emiliano Carretti, and Richard G. Weiss
Langmuir 2011 Volume 27(Issue 18) pp:11671-11682
Publication Date(Web):August 17, 2011
DOI:10.1021/la202179e
A gel-like, high-viscosity polymeric dispersion (HVPD) based on cross-linked borate, partially hydrolyzed poly(vinyl acetate) (xPVAc, where x is the percent hydrolysis) is described. Unlike hydro-HVPDs prepared from poly(vinyl alcohol) (PVA) and borate, the liquid portion of these materials can be composed of up to 75% of an organic cosolvent because of the influence of residual acetate groups on the polymer backbone. The effects of the degree of hydrolysis, molecular weight, polymer and cross-linker concentrations, and type and amount of organic cosolvent on the rheological and structural properties of the materials are investigated. The stability of the systems is explored through rheological and melting-range studies. 11B NMR and small-angle neutron scattering (SANS) are used to probe the structure of the dispersions. The addition of an organic liquid to the xPVAc-borate HVPDs results in a drastic increase in the number of cross-linked borate species as well as the agglomeration of the polymer into bundles. These effects result in an increase in the relaxation time and thermal stability of the networks. The ability to make xPVAc-borate HVPDs with very large amounts of and rather different organic liquids, with very different rheological properties that can be controlled easily, opens new possibilities for applications of PVAc-based dispersions.
Co-reporter:Tao Yu, Rodrigo Cristiano and Richard G. Weiss
Chemical Society Reviews 2010 vol. 39(Issue 5) pp:1435-1447
Publication Date(Web):06 Jan 2010
DOI:10.1039/B821320H
The simple structures of neutral triatomic molecules mask the complexity of their many reactions. Several of them are involved with our life cycle, and an example of their importance is witnessed by the now recognized effect of CO2 on our climate. In this tutorial review, we provide a basis for understanding why these triatomics react with other species, and show examples of how their chemistry has been exploited to synthesize complex molecules and aggregates, to change drastically the properties of materials, and to devise new processes.
Co-reporter:Tao Yu, Taisuke Yamada, and Richard G. Weiss
Chemistry of Materials 2010 Volume 22(Issue 19) pp:5492
Publication Date(Web):September 17, 2010
DOI:10.1021/cm101316h
Amidinium dithiocarbamates salts with diverse structures are prepared in situ by adding one equivalent of CS2 to an equimolar mixture of two nonionic molecules, an amidine and an amine. Many of the salts made in this way are room temperature ionic liquids (RTILs) and the others (ILs) melt well below the decomposition temperature of the salts, ca. 80 °C. Unlike the analogous amidinium carbamate RTILs, which are made by adding CO2 to amidine/amine mixtures and decompose near 50 °C, the amidinium dithiocarbamates do not revert to their amidine/amine mixtures when they are heated. The thermal, rheological, conductance, and spectroscopic properties of representative examples from a total of 50 of these ILs and RTILs are reported, comparisons between them and their nonionic phases (as well as with their amidinium carbamates analogues) are made, and the thermolysis pathways of the ammonium dithiocarbamates are investigated.
Co-reporter:Emiliano Carretti, Irene Natali, Caterina Matarrese, Paola Bracco, Richard G. Weiss, Piero Baglioni, Antonella Salvini, Luigi Dei
Journal of Cultural Heritage 2010 Volume 11(Issue 4) pp:373-380
Publication Date(Web):October–December 2010
DOI:10.1016/j.culher.2010.04.002
The procedures for making and applying a new family of high viscosity aqueous polymeric dispersions based on poly(vinyl alcohol)-borax (PVA-borax) matrices are presented. A specific system of this type has been used to remove an oxidized varnish coating from the surface of “Coronation of the Virgin with Saints”, a 15th century egg tempera painting on wood by Neri di Bicci (Florence, 1418–1492). FTIR spectra showed that the oxidized varnish was constituted of highly aged shellac resin. Good cleaning performance was attained when the liquid portion of the dispersion consisted of a mixture of water and acetone. Rheological investigations indicate that the acetone content does not affect the mechanical properties of the polymeric dispersion. Those mechanical properties permit easy removal of the cleaning agent simply by peeling it from the surface by means of a forceps or spatula once it has carried out its cleaning function. Optical microscopic and FTIR investigations show that the cleaning agent is able to remove the oxidized varnish coating from the surface of the Neri di Bicci painting without leaving detectable residues.
Co-reporter:Rodrigo Cristiano;Andrew D. Walls ;Richard G. Weiss
Journal of Physical Organic Chemistry 2010 Volume 23( Issue 10) pp:904-909
Publication Date(Web):
DOI:10.1002/poc.1694
Abstract
A reusable bromination reagent based on polystyrene beads with covalently appended methyltriphenylphosphonium tribromide groups has been developed. The results from bromination reactions of several structurally diverse unsaturated substrates with the beads and with solutions of a non-polymeric model brominating reagent, methyltriphenylphosphonium tribromide, are described. It is shown that the reactions are highly regio- and stereo-selective and can be conducted easily. Copyright © 2010 John Wiley & Sons, Ltd.
Co-reporter:Yu-Zhe Chen, Yong-Hui Tian, Miklos Kertesz and Richard G. Weiss
Photochemical & Photobiological Sciences 2010 vol. 9(Issue 9) pp:1203-1211
Publication Date(Web):22 Jul 2010
DOI:10.1039/C0PP00113A
A combined experimental and theoretical study of the mechanisms and energies associated with intramolecular H-atom transfers from methyl groups with varying numbers of phenyl substituents to oxygen atoms of aryloxy radicals is reported. It is shown that the transfers within the six aryloxy radicals investigated would have high activation energies and, in all but one case, are endothermic. A detailed analysis of the calculated reaction coordinates indicates proton-coupled electron transfers as the favored mechanisms.
Co-reporter:Shibu Abraham, Teresa D. Z. Atvars, and Richard G. Weiss
The Journal of Physical Chemistry B 2010 Volume 114(Issue 38) pp:12221-12233
Publication Date(Web):September 8, 2010
DOI:10.1021/jp105461r
Temperature-induced changes in the static and dynamic characteristics of the fluorescence from pyrene and N,N-dimethyl-3-(pyren-1-yl)propan-1-amine (PyC3NMe2) have been used to determine the locations and mobilities of these probes in the anisotropic environments provided by films of 5 poly(alkyl methacrylate) (PAMA) polymers in which alkyl is ethyl, butyl, isobutyl, cyclohexyl, and hexadecyl. Whereas emission from pyrene reports on the polarity of the guest sites and the ability of molecules to diffuse translationally between sites, emission from PyC3NMe2 yields information about the fluidity and the shape of the guest sites. Data have been obtained from 20 to >400 K, a range that spans the onsets of several relaxation processes in the hosts. Those data indicate that the pyrenyl groups reside near to ester functionalities in most of the PAMAs, although the distance from them (and the main chains) depends upon the bulkiness of the alkyl groups. Among the most important conclusions derived from this research is that the rates of segmental relaxation phenomena near the probe molecules—and not free volume, as was concluded previously from fluorescence measurements in polyethylene films—are the dominant contributors to the fluorescence changes. Of practical importance, changes in those rates have permitted the onset temperatures of many of the relaxation phenomena occurring in the vicinity of the probes to be located.
Co-reporter:Yu-Zhe Chen and Richard G. Weiss
Photochemical & Photobiological Sciences 2009 vol. 8(Issue 7) pp:916-925
Publication Date(Web):26 Mar 2009
DOI:10.1039/B902109D
Photo-Fries rearrangements and associated photoreactions of four o-cresyl acylates were investigated in cyclohexane and in unstretched and stretched polyethylene (PE) films with different degrees of crystallinity. The esters differ in the number of phenyl substituents attached to the methyl group of the cresyl part and the length of the acyl chain. The influences of intramolecular structural factors and intermolecular environmental effects on the fates of the excited singlet states and the singlet state aryloxy/acyl radical pairs generated subsequently from them are explored. The results indicate that there are definable limits to the selectivity of the photoreactions of aryl esters that are dependent on their shapes and the ability of their environments to interact with them. Quantum yields for the reactions of the o-cresyl acylates were measured in cyclohexane and their values can be understood on the bases of a combination of conformational and electronic factors. The PE cavities holding the less globularly shaped o-cresyl acylates act as templates for the formation of the photoproducts: the ratios of the 2- and 4-position photo-Fries rearrangement products from o-cresyl acetate and 2-benzylphenyl acetate are higher when irradiated in films that are stretched or have higher film crystallinity; control by the PE cavities over the fates of the radical pairs is diminished for the o-cresyl acylate with two phenyl groups appended to the methyl of the cresyl part. The cavities of the PE films exert an interesting influence on the ability of the excited singlet states to undergo concerted decarboxylation reactions as well.
Co-reporter:Tao Yu, Koji Wakuda, Daniel L. Blair and Richard G. Weiss
The Journal of Physical Chemistry C 2009 Volume 113(Issue 27) pp:11546-11553
Publication Date(Web):March 23, 2009
DOI:10.1021/jp900115g
The physical properties of five siloxane polymers with different types and frequencies of amino functional groups along the polymer side chains have been manipulated from flowing liquids to gels and to rubber-like materials by the simple addition or subtraction of a neutral triatomic molecule, CO2 or CS2, at room temperature. The chemical changes, formation of ammonium carbamates and ammonium dithiocarbamates, create materials whose properties are totally different from those of the parent polymers as a result of the introduction of ionic cross-links. These materials can be returned to their original forms by heating (in the case of the CO2 adducts) or to their protonated original forms by treatment with an acid (in the case of the CO2 and CS2 adducts). Heating the ammonium dithiocarbamates leads to loss of H2S and permanent (covalent) thiourea cross-links between the polymer chains. The new materials adhere strongly to other surfaces and can be swelled to several times their original volumes by different liquids. The rheological, swelling, and physical properties of the new materials have been correlated with the structures of the original polymers to provide a comprehensive picture of how changes at the nanometric length scale are translated to macroscopic changes. At least for the polysiloxanes examined here, the properties of the adducts do not correlate with the molecular weights of the original polymers, but do with the frequency of amino groups. The results demonstrate a simple, new method to cross-link polysiloxanes (and, in principle, a wide range of other polymers), transforming them into materials with totally different and potentially commercially useful properties.
Co-reporter:V. Ajay Mallia, Mathew George, Daniel L. Blair and Richard G. Weiss
Langmuir 2009 Volume 25(Issue 15) pp:8615-8625
Publication Date(Web):March 11, 2009
DOI:10.1021/la8042439
Thirteen members of a new class of low molecular-mass organogelators (LMOGs), amides, and amines based on (R)-12-hydroxystearic acid (HSA; i.e., (R)-12-hydroxyoctadecanoic acid) and the properties of their gels have been investigated by a variety of structural and thermal techniques. The abilities of these LMOGs, molecules with primary and secondary amide and amine groups and the ammonium carbamate salt of 1-aminooctadecan-12-ol, to gelate a wide range of organic liquids have been ascertained. Their gelating efficiencies are compared with those of HSA and the corresponding nitrogen-containing molecules derived from stearic acid (i.e., HSA that lacks a 12-hydroxyl group). Several of the HSA-derived molecules are exceedingly efficient LMOGs, with much less than 1 wt % being necessary to gelate several organic liquids at room temperature. Generally, the self-assembled fibrillar networks of the gels consist of spherulitic objects whose dimensions depend on the protocol employed to cool the precursor sol phases. X-ray studies indicate that the LMOG molecules are packed in lamellae within the fibers that constitute the spherulites. In addition, some of the organogels exhibit unusual thixotropic properties: they recover a large part of their viscoelasticity within seconds of being destroyed by excessive strain shearing. This recovery is at least an order of magnitude faster than for any other organogel with a crystalline fibrillar network reported to date. Correlations of these LMOG structures (as well as with those that lack a hydroxyl group along the n-alkyl chain, a headgroup at its end, or both) with the properties of their gels, coupled with the unusual rheological properties of these systems, point to new directions for designing LMOGs and organogels.
Co-reporter:Braja G. Bag, Shaishab K. Dinda, Partha P. Dey, V. Ajay Mallia and Richard G. Weiss
Langmuir 2009 Volume 25(Issue 15) pp:8663-8671
Publication Date(Web):April 24, 2009
DOI:10.1021/la8042796
Nine esters of a naturally occurring triterpenoid, arjunolic acid (from Terminalia arjuna), with alkyl chains have been synthesized, and their self-assembly has been studied in organic liquids. All of the esters examined were found to be excellent gelators. No birefringence was detected in optical micrographs of the transparent toluene gels with 5% (w/w) ethyl arjunolate or 5% (w/w) p-nitrobenzyl arjunolate as the gelator, but a spherulitic-type pattern was seen for a gel of 1.2% (w/w) p-nitrobenzyl arjunolate in 1/1 (w/w) chloroform/cyclohexane. Electron microscope images revealed self-assembled fibrillar network (SAFIN) structures with right-handed helical ribbons in some gels. With increasing concentration of the gelators, the gel-to-sol transition temperature (Tgel) increased and then approached plateau values. Differential scanning thermograms demonstrated that the heats for transition from transparent gels to sols of ethyl arjunolate or p-nitrobenzyl arjunolate in toluene are very small. Powder X-ray diffractograms revealed that the molecular packing in the SAFIN of the 5% (w/w) ethyl arjunoate in the toluene gel was amorphous and similar to the diffractogram recorded for the neat gelator. Although the diffractogram of neat p-nitrobenzyl arjunolate consisted of broad peaks, suggesting disordered packing, the low-angle peaks of the corresponding toluene gel were much sharper; these results indicate more crystalline packing in the SAFIN than in the neat gelator. The kinetics and growth of the transformation of sols of p-nitrobenzyl arjunolate in 1/1 (w/w) chloroform/cyclohexane to their gels have been investigated at different incubation temperatures by circular dichroism spectroscopy. The data have been analyzed to probe the mechanism of SAFIN formation and the relationship between the molecular structures of the esters of arjunolic acid and their abilities to function as gelators of a wide variety of organic liquids.
Co-reporter:Astghik A. Shahkhatuni, Kefeng Ma and Richard G. Weiss
The Journal of Physical Chemistry B 2009 Volume 113(Issue 13) pp:4209-4217
Publication Date(Web):March 3, 2009
DOI:10.1021/jp810793n
The ordering parameters of selected solutes from NMR spectroscopic measurements have been assessed in the thermotropic and amphotropic smectic liquid-crystalline phases of a wide structural range of phosphonium salts with three equivalent long n-alkyl chains, one shorter chain, and various anions. The nature of the added liquids that convert the salts to amphotropic phases—alcohols and other small organic molecules—and their concentrations have been determined. These factors are correlated with the NMR-derived parameters in order to understand how the phases can be optimized to maximize information about the solutes. The various salts cover a range of liquid crystallinity from −40 to 100 °C. The phosphonium salts are easily aligned in the strong magnetic fields of the spectrometers. In several of the systems, a coexistence of isotropic and anisotropic phases is observed over a wide range of temperatures. The order parameters of the amphotropic liquid-crystalline phases vary from high to very low values, and some of the systems provide good spectral resolution for the solute molecules. Also, structural and orientational parameters of a model molecule, 13C-enriched acetonitrile, have been calculated in various systems in order to evaluate more precisely the applicability of the host systems for determining solute structures by NMR. The results, in toto, indicate that several of the phosphonium salts are very promising as hosts to determine solute structures.
Co-reporter:Tao Yu, Taisuke Yamada, Gabriel C. Gaviola and Richard G. Weiss
Chemistry of Materials 2008 Volume 20(Issue 16) pp:5337
Publication Date(Web):July 26, 2008
DOI:10.1021/cm801169c
The properties of reversible, room-temperature, chiral, ionic liquids (L-A-C) are reported. They are easily prepared by passing CO2 gas through equimolar mixtures of a simple amidine (L) and a chiral amino alcohol (A), L/A, derived from a naturally occurring amino acid, and they can be returned to their L/A states by passing a displacing gas, N2, through the ionic liquid; the process of passing from uncharged to charged states can be repeated several times without discernible degradation of each phase. All of the 40 L/A combinations examined form room-temperature ionic liquids (most to ca. 50 °C under 1 atm of CO2) and they remain liquids to at least −20 °C. The L-A-C phases are more viscous than their corresponding L/A phases, the conductivities are much higher in the L-A-C phases than in the L/A phases, and the solubility characteristics of the liquids can be modulated significantly by exposing them to either CO2 or N2 gas. The spectroscopic characteristics of the L/A and L-A-C phases have been compared also. Their reversibility, chirality, broad temperature ranges, tolerance to water, and ease of preparation should make the combination of L/A and L-A-C phases useful as solvents for several “green” applications.
Co-reporter:Kefeng Ma, Astghik A. Shahkhatuni, B. S. Somashekhar, G. A. Nagana Gowda, YuYe Tong, C. L. Khetrapal and Richard G. Weiss
Langmuir 2008 Volume 24(Issue 17) pp:9843-9854
Publication Date(Web):August 8, 2008
DOI:10.1021/la801594q
Tri-n-decylmethylphosphonium chloride and bromide (1P10X) salts are not liquid crystalline. However, mesophases are induced by adding very small amounts of an alcohol or water. The temperature ranges of the induced smectic A2 (SmA2) liquid-crystalline phases can be very broad and the onset temperatures can be below room temperature depending upon the concentration of the alcohol or water and the structure of the alcohol. At least one molar equivalent of hydroxyl groups is necessary to convert the 1P10X completely into a liquid crystal. Strong association between the hydroxyl groups of an alcohol or water and the head groups of the 1P10X is indicated by spectroscopic, diffraction, and thermochemical data. Unlike many other smectic phases, those of the 1P10X/alcohol complexes are easily aligned in strong magnetic fields and the order parameters of selectively deuterated alcohols as measured by 2H NMR spectroscopy, ∼10−2, are much lower than the values found when the host is a commonly employed thermotropic liquid crystal. The dependence of the specific values of the order parameters on temperature, the nature of the halide anion, and the structure and concentration of the alcohol are reported. In sum, a detailed picture is presented to explain how and why an alcohol or water induces liquid crystallinity in the 1P10X salts. The data also provide a blueprint for designing media with even lower order parameters that can be hosts to determine the conformations and shapes of guest molecules.
Co-reporter:Xiao Huang, Richard G. Weiss
Journal of Colloid and Interface Science 2007 Volume 313(Issue 2) pp:711-716
Publication Date(Web):15 September 2007
DOI:10.1016/j.jcis.2007.04.041
Simple methods for preparation of mesoporous rodlike silica and titania nanoobjects, some with exceedingly high aspect ratios, are described. They involve hydrolytic sol–gel processes using nanotubes from aqueous assemblies of very dilute (0.1 wt%) sodium lithocholate as templates. Shearing of the lithocholate nanotubes results in aligned, templated silica rods. The relative rates of lithocholate self-assembly and of polymerization of the titania precursors, especially, appear to be important factors in templating efficiency.Rodlike silica and titania nanoobjects are templated onto nanotubes of sodium lithocholate in very dilute aqueous media. The templated silica rods are easily aligned by shear.
Co-reporter:Chuping Luo;Philippe Passin;Richard G. Weiss
Photochemistry and Photobiology 2006 Volume 82(Issue 1) pp:163-170
Publication Date(Web):30 APR 2007
DOI:10.1562/2005-05-26-RA-546
Photo-Fries rearrangements of 4-dodecylphenyl phenylacetate have been investigated in polyethylene films with 0–71% crystallinity and in hexane over a range of temperatures. The results are compared to those reported previously from phenyl phenylacetate and 1-naphthyl tetradecanoate to assess the influence of a long alkyl chain on the in-cage motions of the intermediate singlet radical pairs. It is demonstrated that the reactivity and selectivity of intimate singlet radical pairs can be tuned by judicious placement of long-chain substituents and selection of a specific polyethylene type as the reaction matrix.
Co-reporter:Jarugu Narasimha Moorthy, Parthasarathy Venkatakrishnan, Govardhan Savitha and Richard G. Weiss
Photochemical & Photobiological Sciences 2006 vol. 5(Issue 10) pp:903-913
Publication Date(Web):13 Jul 2006
DOI:10.1039/B606027G
We have examined the photobehavior of a set of isomers of 2-pyranone-annulated stilbenes (6-styrylcoumarin 1, 7-styrylcoumarin 2, 4-methyl-6-styrylcoumarin 3, and 4-methyl-7-styrylcoumarin, 4) in their crystalline phases. While the cis isomers of 1–3 undergo cis
→
trans photoisomerizations in the solid state, cis-4 and the trans isomers of 1–3 do not; the trans isomer of 4 undergoes photo-induced intermolecular reactions. Solution-state irradiations of the trans isomers of 1–4 lead to the cis isomers quite readily, as does cis-4 lead to trans-4, which suggests that the absence of geometric isomerization of the trans isomers and the lack of reactivity of cis-4 in the solid state are due to molecular packing effects. X-Ray crystal structural analyses of 1–4 reveal interesting conformational preferences for the styrenic moieties and differences in the total ‘free’ volumes within the lattices, but neither factor explains satisfactorily why some of the molecules undergo geometric isomerizations in their single crystals and others do not. Using the PLATON program, we have located the sizes and positions of ‘void volumes’ within the crystal lattices, and identified trajectories necessary for atomic motions to lead to geometric isomerizations to understand the reactivities of 1–4. The voids in the reactive cis isomers of 1–3 crystals are located along the trajectories needed for geometric isomerization. The relevant voids in the crystals of cis-4 and the trans isomers of 1 and 2 (the non-isomerizing molecules for which suitable crystals could be grown for X-ray analyses) are located along a trajectory that does not permit isomerization. We hypothesize that the classical momentum gained from the initial motions that are facilitated due to the voids in the crystals of the cis isomers of 1–3, as well as the heat dissipated to the local environment by internal conversions and vibronic cascade of the Franck–Condon states, helps to drive the system over potential energy barriers that would not be possible otherwise. Cis-4 and the trans isomers of 1 and 2, as well as other examples from the literature in which geometric isomerizations do or do not occur in the solid state, also follow the predictions based upon the PLATON analyses. On these bases, it is suggested that the methodology described may be generally applicable for predicting when geometric isomerizations (and possibly other reactive processes) in crystalline materials will occur.
Co-reporter:Jarugu Narasimha Moorthy Dr.;Apurba L. Koner;Subhas Samanta;Nidhi Singhal;Werner M. Nau Dr.;Richard G. Weiss Dr.
Chemistry - A European Journal 2006 Volume 12(Issue 34) pp:
Publication Date(Web):25 AUG 2006
DOI:10.1002/chem.200600880
The stereochemistry at C2 and C3 carbons controls the partitioning of triplet 1,4-biradicals of ketones 2 among various pathways. Differences in the major reaction pathways, for example, cyclization (syn) and fragmentation (anti), adopted by the diastereomeric 1,4-radicals of ketones 2 have permitted unprecedented diastereomeric discrimination in their lifetimes to be observed by nanosecond laser flash photolysis. From quantum yield measurements and transient lifetime data, the absolute rate constants for cyclization and fragmentation of a pair of diastereomeric triplet 1,4-biradicals have been determined for the first time.
Co-reporter:Emiliano Carretti, Luigi Dei and Richard G. Weiss
Soft Matter 2005 vol. 1(Issue 1) pp:17-22
Publication Date(Web):29 Mar 2005
DOI:10.1039/B501033K
A form of soft matter, physical gels, has been exploited for many practical purposes for several centuries. During the last decade, several types of physical gels have been developed to clean surfaces for art conservation and restoration. A short history and an explanation of the structures and properties of these gels and recent modifications of them by the authors to allow “isothermal rheoreversibility” are described. Speculation about future developments and the factors that should be considered when formulating gels for art conservation and restoration is presented.
Co-reporter:Jinqi Xu and Richard G. Weiss
Photochemical & Photobiological Sciences 2005 vol. 4(Issue 4) pp:348-358
Publication Date(Web):03 Mar 2005
DOI:10.1039/B419493D
The regio- and stereo-chemistries of combination products from chiral 1-naphthoxy/(R)-2-phenylpropanoyl and prochiral 1-naphthoxy/1-phenylethyl singlet radical-pairs (radical-pairs A and B, respectively) have been studied at different temperatures in polyethylene (PE) films with different crystallinities. The radical-pairs have been generated as intermediates along the photo-Fries reaction course of 1-naphthyl (R)-2-phenylpropanoate ((R)-1) and the photo-Claisen reaction course of 1-naphthyl (R)-1-phenylethyl ether ((R)-2). Radical-pair B was produced directly upon lysis of the first excited singlet state of (R)-2 and indirectly after irradiation of (R)-1 and subsequent decarbonylation of the 2-phenylpropanoyl radical of radical-pair A. Comparison of the fates of the directly and indirectly formed radical-pairs B provides detailed information about the nature of the reaction cavities within the polyethylene hosts and how the combinations of the radical-pairs are influenced by their initial locations within a cavity. The results, especially when taken with those from irradiations in n-alkanes, indicate that the PE cavities are “templated” by the (R)-1 and (R)-2 guest molecules and that the templated shapes are retained in some form for periods that are at least as long as the time required for decarbonylation of a 2-phenylpropanoyl radical. In addition, the enantiomeric excesses of the decarbonylated photoproducts from (R)-1
(2, 2-(1-phenylethyl)-1-naphthol (2BN), and 4-(1-phenylethyl)-1-naphthol (4BN)) or 2BN and 4BN from (R)-2 indicate different influences of temperature on translational and tumbling motions of the radicals of radical-pairs B within their polyethylene cages.
Co-reporter:Jinqi Xu and Richard G. Weiss
Photochemical & Photobiological Sciences 2005 vol. 4(Issue 2) pp:210-215
Publication Date(Web):05 Jan 2005
DOI:10.1039/B416610H
The regio- and stereo-chemistries of photo-Fries reactions of 1-naphthyl (R)-2-phenylpropanoate ((R)-1) and photo-Claisen reactions of 1-naphthyl (R)-1-phenylethyl ether ((R)-2) have been investigated in the liquid and solid phases of n-nonadecane. The results are compared with those obtained in a lower and higher viscosity liquid n-alkane. Irradiation of (R)-1 yields a prochiral singlet 1-naphthoxy–1-phenylethyl radical pair (radical pair B)
indirectly, after decarbonylation of the initially formed chiral singlet 1-naphthoxy–2-phenylpropanoyl radical pair (radical pair A). Radical pair B is generated directly upon lysis of the excited singlet state of (R)-2. The restricted environments of the solid phases enable both the directly and indirectly formed radical pairs B to maintain much (but not all) of the memory of their initial orientations. Thus, the in-cage combination products from indirectly formed radical pair B contain up to 33% enantiomeric excess. However, the combinations of directly formed radical pair B lack complete stereo-specificity due to their disturbing influence on their local environments in the solid phases. A discussion of the dynamics of the radical pair motions in the different environments is presented.
Co-reporter:Mathew George Dr.;Grace Tan Dr.;Vijay T. John Dr.;Richard G. Weiss Dr.
Chemistry - A European Journal 2005 Volume 11(Issue 11) pp:
Publication Date(Web):22 MAR 2005
DOI:10.1002/chem.200401066
The properties of a series of organogels consisting of a urea or thiourea derivative with one or two n-alkyl substitutuents at the nitrogen atoms (a low molecular-mass organogelator (LMOG)) and an organic liquid are described. They include N,N′-dimethylurea, the LMOG of lowest molecular mass (MW 88) we are aware of. The efficiencies of the LMOGs, based the diversity of liquids gelated, the minimum amount required for gelation of a liquid at room temperature, and the temporal and thermal stabilities of the gels formed, have been investigated as a function of the number, length, and substitution pattern of their n-alkyl chains. The gels are thermally reversible and require generally very low concentrations (<2 wt %) of an LMOG. Some of the LMOGs with shorter chains are more efficient than their longer chained analogues. The structural and thermodynamic properties of the gels have been examined by IR, DSC, and X-ray diffraction techniques. Polarizing optical microscopic analyses of the gels show that the nature of gelator aggregates depends mainly on the alkyl chain length. Changes in the aggregation ability have been examined systematically by perturbing the molecular structure.
Co-reporter:Mathew George;Chuping Luo;Caihua Wang;Emiliano Carretti;Luigi Dei;Richard G. Weiss
Macromolecular Symposia 2005 Volume 227(Issue 1) pp:173-182
Publication Date(Web):4 AUG 2005
DOI:10.1002/masy.200550917
The properties of several types of organogels that can undergo a chemical transition during gelation or while in the gel phase are described. The transitions can be physically induced by light or chemically triggered by the addition of an acid or a neutral molecule such as CO2 or CS2. In some cases, the gelation properties of the new species formed are markedly different from those of the precursors. The link between molecular structure and the nature of the gel networks as well as results obtained from the multidisciplinary tools used to study them are discussed.
Co-reporter:Urbashi Bhattacharjee, Carlos A. Chesta and Richard G. Weiss
Photochemical & Photobiological Sciences 2004 vol. 3(Issue 3) pp:287-295
Publication Date(Web):01 Dec 2003
DOI:10.1039/B312198D
The photochemistry of 1-(4-methylphenyl)-3-phenyl-2-propanone (MeDBK) in polyethylene (PE) films of varying crystallinity and at different temperatures has been investigated. Specifically, the selectivity of combination of the initially formed triplet pairs of benzylic radicals derived from irradiation of MeDBK has been used to determine the natures of the PE reaction cavities and how they influence the motions of the radicals. The results indicate that in-cage combinations are favored at lower temperatures and in PE of higher crystallinity. At temperatures significantly above the glass transition of PE, the fraction of radical pairs combining in-cage (Fc) is independent of the extent of conversion of MeDBK. The data are interpreted according to a structural model that considers the shape, free volume, wall stiffness, and permeability of the reaction cavities and a dynamic model that includes the competition between the rates of in-cage combination and cage escape by the radical pairs. Near the glass transition temperature, Fc decreases as the temperature is reduced and the data are interpreted using a stochastic model that considers the slow relaxation of the walls of the reaction cavities. The results demonstrate that the cavities afforded by PE can be subtly ‘tuned’ by temperature changes and microscopic variations in the polymer structure and morphology.
Co-reporter:S. B. Yamaki, T. D. Z. Atvars and Richard G. Weiss
Photochemical & Photobiological Sciences 2002 vol. 1(Issue 9) pp:649-655
Publication Date(Web):21 Aug 2002
DOI:10.1039/B202651C
Micromorphology is an important factor in determining polymer properties and uses. Here, steady-state and dynamic fluorescence from covalently attached 1-alkylpyrenyl groups are used to investigate the micromorphology of several random ethylene-co-vinyl acetate (EVA) copolymers with defined compositions of vinyl acetate monomer. The results are compared with those from homopolymers of high- and low-density polyethylenes and poly(vinyl acetate). Selective attachment of pyren-1-yl groups to polymer chains was accomplished by irradiation of pyren-1-yldiazomethane sorbed into polymer films. Steady-state fluorescence spectra and fluorescence decay rates of attached pyrenyl groups in films of the polymers have been compared with those from films with sorbed pyrene. I1/I3 intensity ratios from vibrational bands in the fluorescence spectra and the fluorescence decay rates of attached 1-alkylpyrenyl groups are much less sensitive to changes in the copolymer composition than are those of pyrene. These observations suggest that attachment occurs selectively to the olefinic segments (rather than to the acetate-rich regions) of polymer chains of EVA copolymers. This conclusion is consistent with the known preferences for reaction by pyren-1-ylcarbene in solution.
Co-reporter:Weiqiang Gu, Shuguang Bi and Richard G. Weiss
Photochemical & Photobiological Sciences 2002 vol. 1(Issue 1) pp:52-59
Publication Date(Web):02 Jan 2002
DOI:10.1039/B107011H
The photo-Fries and associated photoreactions of four 1-naphthyl acylates have been examined in two types of poly(vinyl acetate) (PVAc) films above and below their glass transition temperatures, Tg. Because of the ‘templating’ effect of the esters on their reaction cavities, especially below Tg, the distributions of photo-Fries products, as mandated by the intermediate acyl/1-naphthyloxyl singlet radical pairs, are determined largely by the initial conformations of the guest molecules. Even above Tg, at 50 °C, where segmental chain motions of PVAc are relatively rapid, the influence of the cages in directing product formation is apparent. The radical-pair recombination rates for formation of the keto precursor of 2-(2-phenylpropanoyl)-1-naphthol upon irradiation of 1-naphthyl 2-phenylpropanoate in PVAc are reduced drastically as the temperature is lowered from above to
below Tg. Comparison of results in PVAc with those in low-viscosity solvents (ethyl acetate and hexane) and low-polarity polymer films (polyethylene and polypropylene) indicate that interactions between the radicals produced from irradiation of 1 and the acetate pendant groups of PVAc, as well as the nature of its chain motions above and below Tg, influence enormously the course of the photo-Fries rearrangements.
Co-reporter:Tao Yu, Rodrigo Cristiano and Richard G. Weiss
Chemical Society Reviews 2010 - vol. 39(Issue 5) pp:NaN1447-1447
Publication Date(Web):2010/01/06
DOI:10.1039/B821320H
The simple structures of neutral triatomic molecules mask the complexity of their many reactions. Several of them are involved with our life cycle, and an example of their importance is witnessed by the now recognized effect of CO2 on our climate. In this tutorial review, we provide a basis for understanding why these triatomics react with other species, and show examples of how their chemistry has been exploited to synthesize complex molecules and aggregates, to change drastically the properties of materials, and to devise new processes.
Co-reporter:Mohan Zhang and Richard G. Weiss
Physical Chemistry Chemical Physics 2016 - vol. 18(Issue 30) pp:NaN20409-20409
Publication Date(Web):2016/07/12
DOI:10.1039/C6CP03435G
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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![2,9-Dibutylisoquinolino[4',5',6':6,5,10]anthra[2,1,9-def]isoquino line-1,3,8,10(2H,9H)-tetrone](http://img.cochemist.com/ccimg/52100/52000-75-6_b.png)
