Kinetics of 5alpha-cholestan-3beta-yl N-(2-naphthyl)carbamate/n-alkane organogel formation and its influence on the fibrillar networks

The kinetics and mode of nucleation and growth of fibers by 5alpha-cholestan-3beta-yl N-(2-naphthyl)carbamate (CNC), a low-molecular-mass organogelator (LMOG), in n-octane and n-dodecane have been investigated as their sols were transformed isothermally to organogels. The kinetics has been followed in detail by circular dichroism, fluorescence, small-angle neutron scattering, and rheological methods. When treated according to Avrami theory, kinetic data from the four methods are self-consistent and describe a gelation process involving one-dimensional growth and "instantaneous nucleation". As expected from this growth model, polarized optical micrographs of the self-assembled fibrillar networks (SAFINs) show fibrous aggregates. However, their size and appearance change abruptly from spherulitic to rodlike as temperature is increased. This morphological change is attended by corresponding excursions in static and kinetic CD, fluorescence and rheological data. Furthermore, the rheological measurements reveal an unusual linear increase in viscoelastic moduli in the initial stages of self-assembly. Each of the methods employed becomes sensitive to changes of the system at different stages of the transformation from single molecules of the LMOG to their eventual SAFINs. This study also provides a methodology for investigating aggregation phenomena of some other self-assembling systems, including those of biological and physiological importance.

Urea and Thiourea Derivatives as Low Molecular-Mass Organogelators

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 (M(W) 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.

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

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

Reaction cavities of liquid and solid phases of a long n-alkane, n-nonadecane, as probed by the regio- and stereo-chemical fates of two singlet radical pairs

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 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.

Combinations of chiral and prochiral singlet radical-pairs in reaction cavities of polyethylene films. Control and analysis of radical tumbling and translation

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 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 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 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 4 BN from (R)-2 indicate different influences of temperature on translational and tumbling motions of the radicals of radical-pairs B within their polyethylene cages.

Rheoreversible polymeric organogels: the art of science for art conservation

A new category of gels where gelification and breaking of the gels are chemically induced is presented. In particular, the latent gellant polyallylamine produced stable gels with some organic solvents after reaction with CO2 at room temperature, giving the gellant polyallylammonium carbamate. The rheological behavior switches from solution-type to gel-type. After weak acid-catalyzed displacement of CO2, the gel character disappears in a few seconds, making these polymeric organogels rheoreversible by a simple chemical action. This "intelligent" chemical switch between solution-type and gel-type rheological behavior has been exploited to clean pictorial surfaces in art conservation. In fact, during the cleaning procedure, there is a need for the gel supporting the cleaning solvent to have a very high viscosity. After cleaning has been successful, there is a strong necessity to reduce the viscosity, to better eliminate traces of the gellant that must be completely removed from the work of art. In the present study, we show that the art of science, in the sense of designing new physicochemical systems exploiting the "science palette", can lead to an improvement in the techniques used to protect and conserve the results of the "artists' palette".

Mediation of Conformationally Controlled Photodecarboxylations of Chiral and Cyclic Aryl Esters by Substrate Structure, Temperature, Pressure, and Medium Constraints

An aryl alkanoate, 2,4,6-trimethylphenyl (S)-(+)-2-methylbutyrate, whose ester group has a chiral center alpha to the carbonyl carbon and in which photo-Fries rearrangements are blocked by methyl substituents, undergoes facile photodecarboxylation under a variety of conditions and with complete retention of configuration. In fact, the decarboxylation process has many of the attributes of a symmetry-allowed suprafacial [1,3]sigmatropic rearrangement. The process requires concerted extrusion of carbon dioxide in a spiro-lactonic transition state, which has been investigated using high level DFT and CIS calculations: thermally less stable s-cis conformers in the ground and excited singlet states play an important role in determining the competitive efficiency of the process. Conformational control has also been imposed by substrate structure, solvent interactions, temperature, and applying external pressure, as well as using constraining media such as cyclodextrins and polyethylene films. The results are correlated with steady-state and dynamic fluorescence measurements at various temperatures in order to investigate further degrees to which ground and excited singlet state conformations affect the different photoreactivity channels available to the aryl esters.

Temperature-dependent cage effects from triplet radical pairs generated upon irradiation of 1-(4-methylphenyl)-3-phenyl-2-propanone in polyethylene films

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 (F(c)) 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, F(c) 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.

Regulation of murine myocardial energy metabolism during adrenergic stress studied by in vivo 31P NMR spectroscopy

Image-guided, spatially localized 31P magnetic resonance spectroscopy (MRS) was used to study in vivo murine cardiac metabolism under resting and dobutamine-induced stress conditions. Intravenous dobutamine infusion (24 mug. min-1. kg body wt-1) increased the mean heart rate by approximately 39% from 482 +/- 46 per min at baseline to 669 +/- 77 per min in adult mice. The myocardial phosphocreatine (PCr)-to-ATP (PCr/ATP) ratio remained unchanged at 2.1 +/- 0.5 during dobutamine stress, compared with baseline conditions. Therefore, we conclude that a significant increase in heart rate does not result in a decline in the in vivo murine cardiac PCr/ATP ratio. These observations in very small mammals, viz., mice, at extremely high heart rates are consistent with studies in large animals demonstrating that global levels of high-energy phosphate metabolites do not regulate in vivo myocardial metabolism during physiologically relevant increases in cardiac work.

Enhanced Photodecarboxylation of an Aryl Ester in Polyethylene Films

[reaction: see text] Photodecarboxylation is the exclusive photoreaction of 2,4,6-trimethylphenyl (S)-2-methylbutanoate in unstretched high-density polyethylene films. The sole product, (S)-1-(2-methylpropyl)-2,4,6-trimethylbenzene, is formed with complete retention of stereochemistry. In other polyethylene films, organic solvents, and beta-cyclodextrin cavities, cage-escaped products derived from Fries-type bond scission are obtained as well. The results indicate the importance of the media in controlling the conformations of aryl esters and, thereby, their photoreactions.

Determination of Initial and Long-Term Microstructure Changes in Ultrahigh Molecular Weight Polyethylene Induced by Drawing Neat and Pyrenyl Modified Films

Deformation processes in gel-crystallized ultrahigh molecular weight polyethylene (UHMWPE) films with draw ratios (DR) as high as 96 have been investigated by X-ray diffraction (XRD), differential scanning calorimetry (DSC), and positron annihilation lifetime spectroscopy (PALS). In addition, low concentrations of pyrene molecules have been introduced at the time of film preparation from the gels or afterward by sorption after film preparation, and the polarization of their electronic absorption and fluorescence spectra at different draw ratios has been measured over a large temperature range extending to below the glass transition. The pyrene-doped films have been irradiated to introduce covalently attached 1-pyrenyl groups, and these films at two draw ratios have been employed to investigate over large temperature ranges (1) the steady-state fluorescence intensity and (2) the rates of diffusion of N,N-dimethylaniline (DMA). These data have been correlated with the XRD, DSC, and PALS information obtained on the unmodified films. On the basis of analyses of this body of information, a novel deformation model that explains the decreased crystallinity and increased mean free volumes in gel-crystallized UHMWPE at low draw ratios is proposed. It involves "stretch" and "flip" motions of microfibrils present in the undrawn films. The high crystallinity content and stiffer chains due to drawing UHMWPE films result in weak alpha- and beta-relaxation processes, slower diffusion of DMA than in undrawn films, and orientation factors for doped pyrene molecules that are constant over a large temperature range. The overall picture that emerges allows several aspects of the morphology of UHMWPE, a polymer of fundamental importance in materials research, to be understood.

N-alkyl perfluoroalkanamides as low molecular-mass organogelators

A new class of low molecular-mass organogelators (LMOGs), N-alkyl perfluoroalkanamides, F(CF(2))(n)CONH(CH(2))(m)H, is described. The molecules are designed to exploit the incompatibilities of their three molecular parts, and the results demonstrate that this strategy can be used to tune molecular aggregation and gel stability. The gelating properties of these LMOGs have been examined in a wide variety of organic liquids (including alkanes, alcohols, toluene, n-perfluorooctane, CCl(4), and DMSO) as a function of the N-alkyl and perfluoroalkyl chain lengths by X-ray diffraction, polarizing optical microscopy, infrared spectroscopy, differential scanning calorimetry, and small-angle neutron scattering (SANS). The gels are thermally reversible and require generally very low concentrations (<2 wt %) of LMOG. Several of the gels are stable for very long periods at room temperature. The incompatibility of the fluorocarbon and hydrocarbon segments causes the LMOGs to aggregate, probably into lamellae within the fibrils that constitute the basic unit of the gel networks. The SANS studies show that the cross-sections of fibers in the gel networks of LMOGs with shorter perfluoroalkyl chains are much larger than those with longer ones. Comparisons with the gelating properties of some analogous esters (F(CF(2))(n)CO(2)(CH(2))(m)H) and diblock perfluoroalkylalkanes (F(CF(2))(n)(CH(2))(m)H) indicate that additional ordering within the aggregate units is enforced by the intermolecular H bonding among amide groups that is evidenced by IR spectroscopy. Analyses of these results and structure/solvent correlations are provided.

Enantioselectivity of prochiral radical-pair recombinations. Reaction cavity differentiation in polyethylene films

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

Synthesis and characterization of gels from polyallylamine and carbon dioxide as gellant

The synthesis and characterization of a novel series of polymer gels are reported. They are formed at ambient temperatures by bubbling CO(2) through solutions of polyallylamine (PAA; a latent gellant) in several organic liquids, including aliphatic alcohols and 1-methyl-2-pyrrolidone. The stabilities of the alcohol gels, as indicated by the (irreversible) degelation temperature, T(g)(dt), were strongly dependent on the number of carbon atoms (C(n)) in the alkyl chains of the alcohol liquids. The mechanism of formation and the microenvironmental properties of PAA-based gels containing a small amount of a pH-sensitive probe, 2,6-naphthalenedicarboxylic acid (2,6-NDCA), have been probed using static and dynamic fluorescence measurements. A measurable pH change and significant alterations to the fluorescence spectra were coincident with gelation of PAA solutions in 1-butanol as CO(2) was bubbled through, and the fluorescence spectra were monitored over several hours until no further changes were detected. Analyses of dynamic fluorescence decay histograms indicate the presence of three decay times due to different microenvironments where the 2,6-NDCA molecules are located.

Soft tissue perineuriomas in children: report of three cases and review of the literature [corrected]

Perineuriomas (PN) are uncommon, slowly growing, usually benign tumors composed of well-differentiated perineural cells. Two variants are recognized: intraneural perineuriomas and soft tissue perineurioma, which includes a sclerosing subset of tumors. They are usually reported in the adult population. We present three cases of soft tissue perineuriomas in children. One was located in the deep soft tissue of the retroperitoneum in a 14-year-old girl, the second one in the left thumb of a 14-year-old boy, and the third one in the index finger of a 16-year-old boy. This report, which describes the clinicopathologic, immunohistochemical, and ultrastructural features of these tumors, should alert pathologists to the occurrence of perineuriomas in children. A review of the English language literature on perineuriomas in children is also included.

Selectivity and efficiency of pyrene attachment to alkanes induced by broadband X-rays

Bombardment of pyrene-doped n-heneicosane (C21H44) in its orthorhombic solid phase with <1.3> MeV broadband X-rays results in the formation of both mono- and di-heneicosylpyrenes, whereas the same dose in liquid cyclohexane yields only monosubstituted pyrene. In both cases, the reaction efficiency decreases as pyrene concentration is increased from 10-5 to 10-2 M. Qualitatively, the overall attachment efficiency is higher in orthorhombic n-heneicosane than in liquid cyclohexane, but the selectivity of attachment is greater in cyclohexane. Differences between these results and those from irradiations of the same samples with eV range photons are discussed.

Selective attachment of pyrenyl groups to ethylene-co-vinyl acetate copolymers: dynamic and static fluorescence studies

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.