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We describe the synthesis of spin-labeled bis-ureas which coassemble with bis-urea gelators and report on self-assembly as detected using electron paramagnetic resonance spectroscopy (EPR). Specifically, EPR detects the gel-sol transition and allows us to quantify how much spin-label is immobilized within the gel fibers and how much is present in mobile solvent pools-as controlled by temperature, gelator structure, and thermal history. EPR is also able to report on the initial self-assembly processes below the gelation threshold which are not macroscopically visible and appears to be more sensitive than NMR to intermediate-sized nongelating oligomeric species. By studying dilute solutions of gelator molecules and using either single or double spin-labels, EPR allows quantification of the initial steps of the hierarchical self-assembly process in terms of cooperativity and association constant. Finally, EPR enables us to estimate the degree of gel-fiber solvation by probing the distances between spin-labels. Comparison of experimental data against the predicted distances assuming the nanofibers are only composed of gelator molecules indicates a significant difference, which can be assigned to the presence of a quantifiable number of explicit solvent molecules. In summary, EPR provides unique data and yields powerful insight into how molecular-scale mobility and solvation impact on assembly of supramolecular gels.

Electron paramagnetic resonance (EPR) spectroscopy and spin trapping were used to explore the mechanism of alcohol oxidation over gold catalysts. Reaction of secondary alcohols with supported and unsupported gold catalysts (e.g., Au/CeO(2), polymer-incarcerated Au nanoparticles, PPh(3)-protected Au nanoparticles) in the presence of spin traps led to the formation of a hydrogen spin adduct. Using isotope labeling, we confirmed that the hydrogen in the spin adduct originates from the cleavage of the C-H bond in the alcohol molecule. The formation of the hydrogen spin adduct most likely results from the abstraction of hydrogen from the Au surface by a spin trap. These results thus strongly suggest intermediate formation of Au-H species during alcohol oxidation. The role of oxygen in this mechanism is to restore the catalytic activity rather than oxidize alcohol. This was further confirmed by carrying out gold-catalyzed alcohol oxidation in the absence of oxygen, with nitroxides as hydrogen abstractors. The support (e.g., metal oxides) can activate oxygen and act as an H abstractor from the gold surface and hence lead to a faster recovery of the activity. Peroxyl radicals were also observed during alcohol oxidation, consistent with a free-radical autoxidation mechanism. However, this mechanism is likely to be a minor side reaction, which does not lead to the formation of an appreciable amount of oxidation products.

Electron spin-echo envelope modulation (ESEEM) spectroscopy was used to investigate intramolecular and intermolecular complexes of cyclodextrins (CDs) with a nitroxide group. The interaction with solvent molecules (D(2)O) was followed through the (2)H modulation depth. Competition experiments with adamantane-type guests were used to confirm complexation. The shielding of the nitroxide group from solvent upon inclusion into a CD cavity made this technique more sensitive to complexation than cw EPR spectroscopy. ESEEM analysis of a series of CDs mono and bis spin-labeled on the primary rim of the cavity showed that only one compound formed a self-inclusion complex. This suggests that significant linker length/flexibility is required for formation of inclusion complexes in functionalized CDs. DEER (double electron-electron resonance) experiments confirmed that the self-inclusion complex of the spin-labeled CD was intra- rather than intermolecular.

New nanocomposites containing functionalized acrylate monomers and Au nanoparticles (NPs, 1.5−3 nm core diameter, 1−2 wt %) have been developed for an all-optical fabrication of periodic bulk structures by holographic photopolymerization. The Au NPs were coated with ethyl 11-mercaptoundecanoate to ensure good solubility in low-polarity organic media. The materials with only 1.5 wt % Au NPs show unusually high amplitude of the refractive index modulation in the diffraction volume gratings (0.0073). Both volume and surface relief gratings are formed during the holographic exposure. The proposed mechanism of the refractive index contrast amplification (as compared to monomer mixture without NPs) includes interception of free radicals by Au nanoparticles. This slows down the free-radical photopolymerization, thus promoting the increase in lateral periodic redistribution of the components in the interference pattern. The redistribution of both NP and monomers provides high efficient grating formation with the ...

Optically active polyaniline nanorods and hierarchically structured gold microspheres are synthesized using tetrachloroaurate as an efficient oxidant of aniline in the presence of a chiral inducing agent camphorsulfonic acid. The chiral organization of the resultant polyaniline nanorods is strongly affected by the ratio of aniline to AuCl4−, with the highest optical activity being observed for the ratio around 5:2. The particle size and size distributions of the gold microspheres are influenced by the concentration of the oxidant tetrachloroaurate.

Variable temperature electron paramagnetic resonance (EPR) spectra of spin-labeled Au nanoparticles in toluene solution were recorded at X-, Q- and W-bands. The chain length of the nitroxide-based spin-labeled ligand and the surrounding alkanethiol ligands was systematically varied. The spectra were analyzed using the NLSL program (Budil, D. E.; Lee, S.; Saxena, S.; Freed, J. H. J. Magn. Reson., Ser. A 1996, 120, 155) which simulates slow motion spectra using the stochastic Liouville approach. The simulation results show that the rotational diffusion parameters for particles (∼2.6 nm Au core diameter) are dominated by the local motion of the ligand. The length of the spin label has the biggest effect on the nitroxide dynamics followed by the chain length of the surrounding ligands. The results are consistent with the loose packing of ligands on the nanoparticle surface. The packing density of ligands further decreases with the increasing distance from the Au surface. The Arrhenius analysis of the dynamic ...

A series of Au nanoparticles modified with a nitroxide-functionalized ligand was prepared with a range of spin-label coverage. The X-band EPR spectra of frozen solutions of these nanoparticles showed coverage-dependent line-broadening due to dipole-dipole interactions between spin labels. We developed a methodology to analyze such spectra in terms of geometrical features of the nanoparticles (e.g., Au core size and the length of the spin-labeled ligand). Our method is based on the assumption that the spectral line shape is determined by the average distance between nearest-neighboring spin labels adsorbed on the Au particle. Geometrical and statistical analysis then relates this distance to the line shape parameter d1/d, which was calibrated using a model system. Application of this methodology to the experimental spectra provided information about the conformation of ligands on the Au surface. We found that, if the spin-labeled ligand is substantially longer than the surrounding protecting layer, it does not adopt a fully stretched conformation but wraps around the particle immediately above the layer of surrounding ligand. Our results also show that the ligands do not adsorb cooperatively on the Au surface.

The mechanism of a place-exchange reaction of ligand-protected Au nanoparticles was investigated using diradical disulfide spin labels. Analysis of reaction mixtures using a combination of GPC and EPR allowed us to determine concentration profile and propose a kinetic model for the reaction. In this model, only one branch of the disulfide ligand is adsorbed on the Au surface during exchange; the other branch forms mixed disulfide with the outgoing ligand. The two branches of the disulfide ligand therefore do not adsorb in adjacent positions on the surface of Au nanoparticles; this was ultimately proven by the powder EPR spectra of frozen exchange reaction mixtures. Our data also suggest the presence of different binding sites with different reactivity in the exchange reaction. The most-active sites are likely to be nanoparticle surface defects.

This paper describes the gelation of symmetric dendrimers based on building blocks constructed from L-lysine. These dendrimers form gel-phase materials in nonpolar organic solvents. The thermal properties and concentration dependence of the gelation were investigated, and it was found that there was a clear dendritic effect on the behavior of the soft materials formed, with higher generation dendrimers giving rise to more thermally stable gels. Variable temperature 1H NMR studies indicated that this behavior was probably a consequence of more extensive interdendrimer hydrogen bonding occurring between the peptidic groups in the higher generation dendrimers. The supramolecular aggregates were found to have a fibrillar structure, with the dimensions and alignment of the fibers being dependent on dendritic generation. Circular dichroism measurements confirmed that these fibers possessed chiral organization of the peptidic groups on the supramolecular (nano) scale, assigned as helicity. This paper indicates that dendritic functionalization provides a useful way of tuning gel-phase materials properties, with clear dendritic effects on gel formation being quantified for the first time, hence illustrating the way dendritic functionalization can play a positive role in the formation of highly functional organic materials with desirable properties.

The cooperative effect in reactions of multifunctional compounds has been probed by monitoring the kinetics of the Michael addition of a dendritic polyamine to acrylate-terminated Au nanoparticles. Comparison of the kinetic data for this reaction with those for model monofunctional compounds showed the absence of any cooperativity. This was tentatively attributed to the high flexibility of the dendrimer and the inability of the system to bring mutually reactive functional groups sufficiently close to each other to achieve the rate enhancement effect. The subsequent reaction, methanolysis of the Michael adduct, was found to follow different mechanistic pathways in model monofunctional compounds and Au nanoparticles: while the model reaction undergoes intramolecular general base catalysis, no anchimeric assistance was observed for the reaction of Au nanoparticles. This difference in reactivity, which was attributed to steric effects, resulted in substantially different product distribution patterns for rea...

The two-dimensional phase behavior of mixed-monolayers composed of poly(amidoamine) (PAMAM) dendrimers and n-alkylthiols adsorbed to Au(111) surfaces was studied using tapping mode atomic force microscopy (TM-AFM). Mixed-monolayers were prepared by sequential immersion of Au(111) substrates into ethanolic solutions of, first, dendrimers and then n-alkylthiols. Films were prepared by systematically varying the dendrimer generation (G4, G6, and G8) and the n-alkylthiol chain length (CH3(CH2)xSH; x = 4, 11, 15). TM-AFM was used to monitor time-dependent morphological changes in the mixed-monolayers. Dendrimer monolayers immersed in hexane solutions of n-alkylthiols do not phase separate to an appreciable extent, which demonstrates the important role of dendrimer solvation in these processes. In contrast to the amine-terminated dendrimer monolayers, thiol-terminated dendrimers do not phase separate when exposed to ethanolic n-alkylthiol solutions, suggesting that one of the driving forces for this process is ...

Monolayers containing a reactive p-nitrophenyl ester group at different levels with respect to the monolayer interface have been self-assembled on a gold surface. Analysis of grazing angle IR spectra, surface plasmon resonance (SPR), and wettability measurements suggests disordered organization of the alkane chains in the monolayers. Kinetics of monolayer reactions with external reagents (alkylamines) have been studied and compared with those of the same process in bulk medium. Burying of a reaction center under the surface and other structural changes of monolayers were shown to have only a minor effect on the rates of reaction, implying that these monolayers could be easily penetrated by guest molecules. The higher reaction rates with monolayers than in bulk solution are possibly due to a weak binding of the external reagent to the monolayer prior to reaction.

Monolayers possessing reactive amino and p-nitrophenyl ester functional groups at the same or different levels with respect to the monolayer interface have been self-assembled on a gold surface. Intramolecular reactions in these monolayers are at least 1000 times slower than the same processes in the bulk medium. Control experiments with external reagents showed that the monolayer p-nitrophenyl ester group reacts readily with amines from solution, whereas nucleophilicity of the monolayer amino functionality is significantly suppressed. This unusually low reactivity of the amino group was tentatively assigned to its interaction with the gold surface.

Solutions of alkanethiol-protected Au nanoparticles undergo a slow aging process over a period of several days, which dramatically reduces their reactivity in a place-exchange reaction with disulfides. This behavior was tentatively attributed to the reorganization of the most reactive binding sites (e.g., defect sites) on the nanoparticle surface.

An EPR study of a place-exchange reaction of a diradical disulfide with butanethiol-protected Au nanoparticles showed that the two branches of the disulfide molecule do not adsorb adjacent to each other on the Au surface. The most likely mechanism includes adsorption of only one branch of the disulfide molecule in the exchange process. The exchange reaction was found to be zeroth-order with respect to the diradical, indicative of a dissociative "SN1"-type mechanism.

Here we describe the spontaneous assembly of fatty acids onto the surface of amine-terminated, poly(amidoamine) (PAMAM) dendrimers (Scheme 1). This process, which is driven by acidbase chemistry and ion pairing, results in the extraction of dendrimers from aqueous solutions to nonpolar phases. Dendrimer-encapsulated guest molecules, such as the hydrophilic dyes and catalytically active metal nanoparticles described here, can thus be easily solubilized in organic solvents. Dendrimers are monodisperse, hyperbranched polymers possessing a very high concentration of surface functional groups.1 Many of the properties of dendrimers, including their solubility, are strongly influenced by the nature of these terminal functionalities.2,3 For example, dendrimers terminated in hydrophobic groups are soluble in nonpolar solvents, while those having hydrophilic groups are soluble in polar solvents such as water and low-molecular-weight alcohols.4-6 Terminal groups are normally covalently bonded to the body of dendrimers, but there have been a few reports of electrostatic binding of charged molecules to the surface of dendrimer polyions.1,7-10 We now demonstrate that complete surface modification of dendrimers can be accomplished by spontaneous, acid-base self-assembly, which eliminates the need for chemical synthesis and purification. A similar approach has previously been reported for solubilization of ionic polymers11,12 and DNA13,14 in organic solvents. The electrostatic self-assembly process we report is reversible, and therefore the dendrimers and whatever guests they may contain can be easily shuttled between hydrophilic and hydrophobic phases by adjustment of the pH of the aqueous phase. Fourth-generation, amine-terminated PAMAM dendrimers (G4NH2) readily dissolve in toluene or heptane containing dodecanoic acid. The amount of G4-NH2 that can be dissolved in 1% dodecanoic acid/toluene corresponds to about 1 dendrimer per 70-80 molecules of acid15 which suggests an approximately 1:1 stoichiometry between the fatty acid and each of the 64 terminal amine groups present on the dendrimers. Transmission FT-IR spectroscopy of this toluene solution (Figure 1) indicates that solubilization is accompanied by proton transfer from the acid to the terminal amine groups of the dendrimers and that ionization is essentially complete. Evidence for this comes from the nearly complete disappearance of the dodecanoic acid carboxyl peak at 1710 cm-1 (part a of Figure 1) and the appearance of the asymmetric carboxylate peak at 1557 cm-1 (part c of Figure 1) upon addition of dendrimer.16 We conclude that the acid molecules arrange themselves around the dendrimer in a composite structure that resembles an inverted micelle having a hydrophilic dendritic interior and a hydrophobic alkyl-chain-dominated exterior that lends solubility to the ensemble (Scheme 1). In the presence of large excesses of dodecanoic acid, proton transfer extends to the tertiary amine groups within the dendrimer interior (see Supporting Information). Dendrimers with covalently grafted hydrophobic terminal groups have previously been shown to dissolve in nonpolar solvents and solubilize encapsulated guest molecules.3,4,6 The present system, however, offers two significant advantages. First, commercially available dendrimers can be used directly to prepare solutions in nonpolar solvents without the need for chemical synthesis or separation. Second, since acid-base interactions are reversible, addition of HCl to the aqueous phase or simple dilution of the organic phase with pure solvent leads to the transfer of G4-NH2 back into aqueous layer. We report here two examples * To whom correspondence should be addressed. Telephone: (409) 8455629. Fax: (409) 845-1399. E-mail: crooks@tamu.edu. (1) For a recent review, see Zeng, F. W.; Zimmerman, S. C. Chem. ReV. 1997, 97, 1681. (2) Hawker, C. J.; Chu, F. Macromolecules 1996, 29, 4370. (3) Cooper, A. I.; Londono, J. D.; Wignall, G.; McClain, J. B.; Samulski, E. T.; Lin, J. S.; Dobrynin, A.; Rubinstein, M.; Burke, A. L. C.; Frechet, J. M. J.; DeSimone, J. M. Nature 1997, 389, 368. (4) Jansen, J. F. G. A.; Meijer, E. W. Macromol. Symp. 1996, 27. (5) Schenning, A. P. H. J.; Elissen-Roman, C.; Weener, J.-W.; Baars, M. W. P. L.; van der Gaast, S. J.; Meijer, E. W. J. Am. Chem. Soc. 1998, 120, 8199. (6) Sayed-Sweet, Y.; Hedstrand, D. M.; Spinder, R.; Tomalia, D. A. J. Mater. Chem. 1997, 7, 1199. (7) Li, Y.; Dubin, P. L.; Spindler, R.; Tomalia, D. A. Macromolecules 1995, 28, 8426. (8) Jockusch, S.; Turro, N. J.; Tomalia, D. A. Macromolecules 1995, 28, 8, 7416. (9) Caminati, G.; Turro, N. J.; Tomalia, D. A. J. Am. Chem. Soc. 1990, 112, 8515. (10) Watkins, D. M.; Sayed-Sweet, Y.; Klimash, J. W.; Turro, N. J.; Tomalia, D. A. Langmuir 1997, 13, 3136. (11) Kabanov, A. V.; Sergeev, V. G.; Foster, M. S.; Kasaikin, V. A.; Levashov, A. V.; Kabanov, V. A. Macromolecules 1995, 28, 3657. (12) Bakeev, K. N.; Shu, Y. M.; Zezin, A. B.; Kabanov, V. A.; Lezov, A. V.; Mel’nikov, A. B.; Kolomiets, I. P.; Rjumtsev, E. I.; MacKnight, W. J. Macromolecules 1996, 29, 1320. (13) Sergeyev, V. G.; Mikhailenko, S. V.; Pyshkina, O. A.; Yaminsky, I. V.; Yoshikawa, K. J. Am. Chem. Soc. 1999, 121, 1780. (14) Mel’nikov, S. M.; Lindman, B. Langmuir 1999, 15, 1923. (15) This value was calculated from the maximum amount of neat G4NH2 which completely dissolves in 1% dodecanoic acid/toluene mixture after 5 min of sonication. (16) The νa(CO2) peak at 1557 cm-1 overlaps with the amide II band. In a similar system with the PAMAM dendrimer replaced by a poly(propyleneimine) Cascade dendrimer which does not contain any amide bonds, we have clearly observed the appearance of the νa(CO2) peak. Scheme 1 4910 J. Am. Chem. Soc. 1999, 121, 4910-4911

A series of physical organic chemistry experiments suitable for second- or third-year undergraduate students is presented. 1H NMR spectroscopy is used to monitor the enolization of butanone and calculate rate constants, activation parameters, and primary and secondary isotope effects for this reaction. The experiments teach students to handle and interpret NMR spectra and offer a broad range of topics for discussion, including isotope effects, kinetics, activation parameters, regiochemistry, and reaction mechanisms.