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Amphiphilic phospholipid-iodinated polymer conjugates were designed and synthesized as new macromolecular probes for a highly radiopaque and biocompatible imaging technology. Bioconjugation of PEG 2000-phospholipids and iodinated polyesters by click chemistry created amphiphilic moieties with hydrophobic polyesters and hydrophilic PEG units, which allowed their self-assemblies into vesicles or spiked vesicles. More importantly, the conjugates exhibited high radiopacity and biocompatibility in in vitro X-ray and cell viability measurements. This new type of bioimaging contrast agent with a Mn value of 11 289 g mol-1 was found to have a significant X-ray signal at 3.13 mg mL-1 of iodine equivalent than baseline and no cytotoxicity after 48 hours incubation of with HEK and 3T3 cells at 20 μM (20 picomoles) concentration of conjugates per well. The potential of adopting the described macromolecular probes for bioimaging was demonstrated, which could further promote the development of a field-friendly and highly sensitive bioimaging contrast agent for point-of-care diagnostic applications.

Sonication of gallium or gallium-based liquid metals in an aqueous solution of vinyl monomers leads to rapid free radical polymerization (FRP), without the need for conventional molecular initiators. Under ambient conditions, a passivating native oxide separates these metals from solution and renders the metal effectively inert. However, sonication generates liquid metal nanoparticles (LMNPs) of ∼100 nm diameter and thereby increases the surface area of the metal. The exposed metal initiates polymerization, which proceeds via a FRP mechanism and yields high molecular weight polymers that can form physical gels. Spin trapping EPR reveals the generation of free radicals. Time-of-flight secondary ion mass spectrometry measurements confirm direct polymer bonding to gallium, verifying the formation of surface-anchored polymer grafts. The grafted polymers can modify the interfacial properties, that is, the preference of the metal particles to disperse in aqueous versus organic phases. The polymer can also be degrafted and isolated from the particles using strong acid or base. The concept of physically disrupting passivated metal surfaces offers new routes for surface-initiated polymerization and has implications for surface modification, reduction reactions, and fabrication of mechanically responsive materials.

Cross-linked poly(vinyl alcohol) nanofibers were dipped in alternating solutions of aqueous lanthanum acetate and alkali metal carbonates (for up to 10 cycles) to precipitate lanthanum carbonate. Only sodium carbonate yielded Na–La double-metal carbonates and lanthanum carbonate hydroxide as the cycles of precipitation increased. Potassium and cesium carbonates yielded lanthanite phases that removed more P than inorganics derived from Na2CO3 (212 and 157 mg P g−1 nanofiber, respectively, at pH 7 after 110 h). The lanthanum carbonate from Na2CO3 and K2CO3 showed the greatest potential for P removal when exposed to groundwater for more than 24 h. Among the phase and pH-dependent mechanisms of P removal, lanthanite transformed to lanthanum phosphate at neutral pH and phosphate ions exchanged with carbonate ions. Hydration of crystalline phases (synthesized from Na2CO3 and Cs2CO3) was accompanied by phosphate ion sorption at pH 10. Within 24 h, phosphate ion sorption was higher at pH 10 than pH 7, indicating that ion exchange between phosphate and carbonate at pH 7 occurs relatively slower than diffusion of hydrated phosphate ions at pH 10. Selective P removal of the developed lanthanum carbonate mineralized nanofibers will help develop novel water purification technology that can be used in contaminated sites.

We synthesized a series of novel degradable alternating copolyesters composed of diglycolic anhydride (DGA) and two epoxides, epoxymethoxytriethylene glycol (ETEG) and a photoactive crosslinking agent epoxy benzophenone (EBP). After UV crosslinking, soaking the films in a good solvent (tetrahydrofuran) removed uncrosslinked material, and the resulting film gel fractions were calculated. These network films were then degraded in buffer solutions of varying pH values. The degradation of networks with lower gel fraction (fewer crosslinks) was faster and followed first-order kinetics. In contrast, the denser network degraded slower and followed zeroth-order kinetics. The lower gel fraction networks possess a higher swelling ratio and resist bovine serum albumin (BSA) adsorption better by entropic shielding and faster degradation. In comparison, higher gel fraction networks with higher EBP mole fractions adsorb more BSA due to hydrophobic interactions and slower degradation.

An efficient synthesis via a precursor route to a new class of linear dialkyldiaminoazapentacenes is reported. The synthetic route involves the coupling of 4-substituted aniline derivatives to 2,5-dibromoterephthalonitrile via Buchwald-Hartwig amination followed by an acid-mediated cyclization to furnish the diazapentacenes. These reactions occur under short reaction times (

This paper describes surface functionalization of poly(ethylene terephthalate) (PET) films by transamidation of the ester groups with primary amines. The use of water as a solvent improves tremendously the reaction rate and yield compared to conventionally used alcohols. In this study, PET films were exposed to an aqueous solution of 3-aminopropyltriethoxysilane (APTES), which resulted in ester-to-amide reactions on the surface of the film. Hydrolysis of the resulting ethoxy moieties in APTES creates hydroxyl groups that can be used as anchoring points for further modification of PET films. This scheme offers an alternative approach to modify polyesters using water as the solvent.

The protein adsorption and degradation behaviors of poly(lactic acid), poly(glycolic acid) (PGA) and poly(ε-caprolactone) (PCL) brushes and their co-polymer brushes with oligo(ethylene glycol) (OEG) were studied. Both brush structure and relative amount of OEG and polyester were found to be important to the protein resistance of the brushes. A protein-resisting surface can be fabricated either by using OEG as the top layer of a copolymer brush or by increasing the amount of OEG relative to polyester when using a hydroxyl terminated OEG (OEG-OH) and a methoxy terminated OEG (OEG-OMe) mixture as the substrate layer. The degradation of single polyester brushes and their co-polymer brushes using OEG-OH as a substrate layer or using OEG as a top layer was hindered. This phenomenon was rationalized by the inhibition of the proposed back-biting process as the hydroxy end groups of polyester were blocked by OEG molecules. Among these brushes tested, PGA co-polymer brushes using the methoxy/hydroxyl OEG mixture as the substrate layer proved to be both protein-resistant and degradable due to the relatively large amount of OEG moieties and the good biodegradability of PGA.

In this work, the first synthesis of poly(amidoamine) (PAMAM) dendrimers whose branches are hybridized with peptide segments (DendriPeps) is reported. The intercalation of amino acids within the branches of PAMAMs provides supplementary internal functionalities to the coronal groups. Four DendriPep prototypes are synthesized with lysine or glutamic acid as "guest" amino acids, displaying, respectively, a primary amine or a carboxyl group, on generation (G)2 and G3 PAMAMs as host scaffolds. The precise control over the number, type, and topological placement of functional groups expands the functional behavior of DendriPeps beyond current PAMAM dendrimers toward new frontiers or colloids, drug delivery vectors, and catalysis.

The growth and degradation of poly(glycolic acid) (PGA) and poly(e-caprolactone) (PCL) brushes were compared. Using tin (octanoate) as the catalyst, optimal conditions were found for growth of each polyester brush from the hydroxy-terminated silicon surface via ring-opening polymerization. PCL brushes grew thicker at elevated temperatures but the thickest PGA brushes grew at room temperature. Unlike bulk polyesters that can degrade under both acidic and basic conditions, the confined surface polyester brushes only degraded under neutral or basic conditions. The degradation mechanism of grafted polyester brushes was probed through a blocking test. It was shown that the terminal hydroxy groups of these polyester brushes were essential to the degradation process indicating a preferential backbiting mechanism. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 4643–4649

A novel, ester-free initiator for surface-initiated free radical polymerization has been synthesized and tested. The structurally non-symmetrical azo-based initiator features a chemically stable alkane linker between the initiating group and the silane anchoring group, setting it apart from the majority of surface initiators that are linked by hydrolyzable moieties, such as esters. The novel design of the initiator is bolstered by an original synthetic approach, leading to a greater yield and a dramatic reduction in cyanide usage relative to previous methods. Here we demonstrate the capability of this novel initiator for surface-initiated free radical polymerization (SI-FRP), reverse ATRP, and RAFT, noting that Arrhenius behavior of SI-FRP differs significantly from that of FRP in the bulk. Furthermore, we show that polymer brushes formed from the novel initiator are more stable than those formed from ester-based initiators.

Poly(lactic acid) (PLA) brushes were prepared through surface initiated ring-opening polymerization of l-lactide. Their degradation (depolymerization) was characterized by tracking their ellipsometric thickness as a function of time by repeated emersion from buffered, aqueous solutions. The pH and temperature both had a large effect on the rate of degradation. PLA brushes do not degrade in a manner similar to that of bulk PLA. Several experiments were performed, the results of which suggest that intramolecular transesterification (backbiting) was the pathway for the degradation of the PLA brush in basic, aqueous solution, similar to what has been observed for the depolymerization of PLA oligomers in basic solution. The PLA brushes degrade very slowly in acidic solution. This behavior is similar to the degradation of PLA oligomers in acidic solution.

Poly(lactic acid) (PLA) brushes prepared by the ring opening of lactide were thicker when polymerized at a lower temperature (25 °C) than was typically used for polymerization in solution. The molecular weight of solution polymerized lactide was also higher when lactide was polymerized at 25 °C compared with polymerization at higher temperatures. However, the yield of PLA was low at this temperature. These results highlight the different requirements for solution polymerization and brush growth. In the former case, both percentage of conversion and molecular weight are important considerations. In the latter case, however, percentage of conversion is unimportant as a brush represents a very small amount of polymer. It was also shown during the course of these studies that the native hydroxy groups on silicon substrates and silanols in solution were equally good initiators when compared with hydroxy terminated self-assembled monolayers on gold and alcohols, respectively.

Here it is shown that 3-phenyl-2-amino isoquinoline acts as a simple mercury sensor. It is simple to synthesize. The molecule requires base/buffer to bind in a 1:1 stoichiometry with mercury ion, however. Otherwise, it acts as a sacrificial base, presumably to pick up a proton liberated during binding. This binding is characterized quantitatively.

This work addresses a significant challenge in engineered molecular systems regarding both understanding and controlling the stability of molecule/nanoparticle nanostructures under ambient exposure. Results deal specifically with molecular electronic junctions, where electronic contacts and transport are known to be sensitive to sample history and ambient exposure. We demonstrate that low-temperature atomic layer deposition can gently encapsulate and stabilize molecular electronic junctions, making it feasible to handle and transport junctions in air for many days with minimal change in electronic conduction. These findings indicate the potential for long-term stability of advanced synthetic nanoparticle/molecule nanoconstructs. For this study, conductivity through nanoparticle/molecule/nanoparticle junctions is analyzed and found to be consistent with nonresonant charge tunneling through a single or a small number of oligomeric phenylene ethynylene molecules in the electrical junction. The conductivity w...

Poly(methyl methacrylate) (PMMA) anchored chains were grown on porous silicon (p-Si) and anodically etched aluminum oxide (AAO) substrates via surface-initiated atom transfer radical polymerization (ATRP). Using hydrogen fluoride, the chains could be cleaved from the substrates, as evidenced by infrared spectroscopy. The molecular weights and molecular weight distributions of PMMA could be analyzed directly on these substrates (after cleaving the chains from the support) using direct ionization mass spectrometry (DIOS-MS) and matrix-assisted laser desorption ionization mass spectrometry (MALDI-MS). Two principal conclusions were drawn from the study. First, matrix-free DIOS-MS was effective at direct analysis of the polymers up to a molecular weight of ≈6 kDa; the signal-to-noise ratio for heavier polymer chains diminished rapidly. Second, under the same polymerization conditions, PMMA grown on both p-Si and AAO substrates had a much lower molecular weight and a broader molecular weight distribution than ...

The reductive desorption of n-alkanethiolate (RS-) self-assembled monolayers (SAMs) on gold, platinum, and palladium substrates was investigated. The kinetics of this process were too slow to measure directly in the case of RS−Pt and RS−Pd SAMs. Thus, the faradaic response of potassium ferricyanide after partial or complete desorption was determined and used to quantify the relative electrochemically accessible surface area. Both the type of underlying metal substrate and the chain length of the n-alkanethiol were varied. The relative alkanethiol binding affinity and effective kinetics of binding for each of the substrates were discussed in light of these observations.

Several methodologies for the selective deprotection acetylenes have been reported previously. However, as is shown here, they are often not reliable or convenient. Here, an approach is reported that is efficient and general. Use of this approach to synthesize several two- and three-armed oligo(phenylene ethynylene) molecules with differentiated end groups is reported. In addition, preliminary characterization of the fluorescent properties of some of these molecules and their ability to form self-assembled monolayers (SAMs) is reported.

Electrical conductance through conjugated biphenyl and saturated alkane molecules on gold electrodes is characterized using a molecule/nanoparticle/molecule electrical test-bed assembly, and comparisons are made between molecules containing isocyanide (−NC) and thiol (−S) terminal groups bound to the gold. Current versus voltage analysis is consistent with charge tunneling through all systems studied. For molecules containing biphenyl, diphenyl acetylene, and alkane bridges, those containing an isocyanide terminal group show an order of magnitude increase in conductance as compared to those containing a thiol terminal group. Various theoretical predictions of the effect of isocyanide terminal groups on charge transfer through conjugated molecular systems are discussed and related to the results observed for charge transfer through conjugated and saturated molecules. The similar trends in charge transport observed for the different metal/linker interfaces suggest that the metal/linker contact plays an impo...

We have synthesized cysteamine-terminated ferrocene molecules and determined the dependence of the electron transport properties of the molecules on their redox states by measuring the current through the molecules as a function of the electrode potential. The current fluctuates over a large range, but its average value increases with the potential. We attribute the current fluctuation and its increase with the potential to the switching of the molecules from low-conductance reduced state to high-conductance oxidized state.

Heterogeneous electron-transfer rates in metal tris(bipyridine) core dendrimers were measured using Osteryoung square-wave voltammetry. Rates decreased with generation, and this decrease could be correlated with the molecular weight increase. These results indicate that the coordination number around the redox center did not play any special role in sterically encapsulating the redox center.

In this Letter, we report on the motion of water droplets on surfaces decorated with molecular gradients comprising semifluorinated (SF) organosilanes. SF molecular gradients deposited on flat silica substrates facilitate faster motion of water droplets relative to the specimens covered with an analogous hydrocarbon gradient. Further increase in the drop speed is achieved by advancing it along porous substrates coated with the SF wettability gradients. The results of our experiments are in quantitative agreement with a simple scaling theory that describes the faster liquid motion in terms of reduced friction at the liquid/substrate interface.

Performing logic and memory operations with one or a very small collection of molecules would be the ultimate in electronic-device miniaturization. For this reason, the field of molecular electronics has received attention that ranges from scientific curiosity to the generation of intellectual property and venture capital. While new paradigms and financial rewards in nanotechnology will probably come (although perhaps not as fast as an investor would like), answers to several key questions are a necessary first step in this evolution. In this regard, chemists (who might be regarded as molecular engineers) have an exciting task ahead of them—sorting out the fundamental precepts that will govern this field. A number of central questions have emerged. Some loom large and will probably require substantial shifts in our approaches for working with molecules. For example, what mix of lithography (top-down engineering) and self-assembly (bottom-up manufacturing) will be required to achieve the dense integration of components that allow us to truly exploit the size scale of single-molecule devices? How will nanotubes be used in these regards? To date, no realistic approach has addressed this issue. Other questions have proven to be more manageable and are equally important. They require us to question fundamentally how molecular science will work in nanometer-scale collections. For example, how does one make contact with a molecule? What is the electronic structure of a molecule when it is in contact with “wires”? Can molecular structure–property relationships be derived that relate the structure of a molecule to nonlinear current–voltage behaviors, switching, and, ultimately, gating? These latter questions have been addressed with some recent, plausible approaches. Such work is highlighted herein. In performing nanoscale electronic measurements, the issue at hand, first and foremost, is how to make electrical contact to these elements. In doing so, one must confront the issue that this contact is going to perturb the molecules under study. The first strategies for contact to small collections of molecules began with the mechanical break junction. A break junction is formed by attaching a metal wire onto a flexible substrate and then bending the substrate just until the wire has broken. The gap produced is then exposed to molecules designed to bind across it. Resistances are measured that are determined to be consistent with the resistance of a single molecule. A second top contact can be made to a collection of molecules (e.g., a selfassembled monolayer (SAM) or Langmuir–Blodgett (LB) film) by metal evaporation. In a nanopore configuration the area of the nanopore is designed to be smaller than the domain size of the SAM and the evaporated metal accumulates only on the top of the SAM. By using nanopores, Reed, Tour, et al. showed current–voltage measurements in molecules containing a nitroamine redox center that exhibited negative differential resistance. As these metal–molecule–metal assemblies must be made one at a time, it can be difficult to get a sense of how variable their behavior is. Furthermore, although evaporating a top contact makes a metal–molecule–metal sandwich that most naturally resembles a device, metals are strong reducing agents. Reduced molecules are typically quite chemically reactive. Thus, the molecule that is placed into the sandwich may not be the structure that is ultimately measured. This concern is exacerbated by the fact that the geometry of the sandwich precludes any spectroscopic characterization of the molecules in that device. To address this issue, a number of investigators have employed the tip of a scanning tunneling microscope (STM, or conducting atomic force microscope, AFM) as a second contact to a molecule (often organized into a self-assembled monolayer). Several examples are noted. Hipps and co-workers reported orbital-mediated tunneling through phthalocyanins and porphyrins that contain metal centers. Tour, Bard, and co-workers displayed peak shaped I–V curves in phenylene ethynylene oligomers (OPEs) by using a tuning-fork STM. We have studied negative differential resistance in patterned, electroactive SAMs by using STM. Weiss and co-workers inserted individual OPEs into an insulating n-alkanethiolate SAM background and determined that these molecules were more conducting than the background. By visualizing individual molecules over time, they observed changes in conductance. These variations in conductance (stochastic switching) were attributed to [*] R. A. Wassel, Prof. C. B. Gorman Department of Chemistry North Carolina State University Raleigh, NC 27695-8204 (USA) Fax: (+1)919-515-8920 E-mail: chris_gorman@ncsu.edu Highlights

The thermodynamic redox potentials of films of constitutional isomers of iron-sulfur cluster core dendrimers were measured and compared. It was determined that the primary structure of the dendrimer influences its reduction potential. Dendrimers containing so-called backfolded linkages were more difficult to reduce than their extended analogues. This behavior is rationalized by suggesting that the backfolded isomers pack more tightly around the iron-sulfur cluster, creating a more hydrophobic local microenvironment. Also, all of these molecules are easier to reduce in the film than in dimethyl formamide solution. The variation in redox potential between film and solution environment was compared to that of dendrimers of differing generations and correlated with the amount of hydrophobic dendron surrounding the cluster.

Excitation energy transfer dynamics in light-harvesting dendritic macromolecules has generated a great deal of interest due to the fact that such systems can be used in photovoltaic and light-emitting devices. Studies on phenylacetylene-based dendrimers played a key role in stimulating this interest and extensive theoretical as well as experimental investigations have been carried out to get a better insight into the phenomena behind the excitation energy transfer dynamics of these dendrimers. In this manuscript, time-resolved femtosecond fluorescence and fluorescence anisotropy dynamics as well as temperature dependent steady state spectral studies of a second generation homogeneous (compact) phenylacetylene dendrimer are reported. The low-temperature fluorescence spectrum of the dendrimer showed two distinctive emission peaks that can be related to the existence of two closely spaced electronic states of the chromophore. Time-resolved fluorescence results suggest delocalization of the excitation energy ...

The negative differential resistance (NDR) peak current observed in redox active self-assembled monolayer-based molecular junctions has been attenuated by controlling the composition of the molecular junction. Two approaches studied here include capping the electroactive ferrocenyl groups with beta-cyclodextrin and functionalizing the scanning tunneling microscope tip used to probe the self-assembled monolayer (SAM) with n-alkanethiols of different lengths. These are the first examples of systematic modification of the magnitude of the NDR response in a molecule-based system.

nism for the variation in the conductance in each of these systems likely differs, each has the common feature that a nanometer-scale collection of molecules conduct the current. Because the current used in STM feedback is based on tunneling, an increase in the dimensions of the tip substrate gap is expected to result in an exponential change in the current. This phenomenon translates into order-of-magnitude changes in the tunneling current for angstrom changes in the gap dimensions. Given that any metal-molecule-metal junction is likely to have enough variability on this length scale, conductance changes in these junctions (and thus stochastic switching) should be completely general in this type of system. In this paper, we show that stochastic switching can be observed in two types of electroactive thiol molecules inserted into an n-alkanethiolate SAM on gold. Previously we have observed negative differential resistance (NDR, decreasing current with increasing bias) in these types of molecules when in a SAM. 11 At the applied bias in which these molecules show NDR, they show an enhanced conductance compared to an n-alkanethiolate SAM background. Here, we show that this enhanced conductance behavior blinks on and off, presumably because of conformation and/ or orientation changes of the inserted molecules with the SAM over time.

Examples of new dendrimers are presented. In the first, the effect on electron transfer rate attenuation is investigated for two dendrimer isomers that differ only in the linkage (ortho- versus meta-linked) of the phenyl ether units within one generation of the structure. Second, the effect of encapsulation on electrochemical and luminescence behavior of a new type of rhenium selenide cluster core dendrimer is illustrated.

Die molekulare Elektronik ist zwar eine noch junge Disziplin, dennoch konnten bereits grose Fortschritte erzielt werden, und wir erkennen heute sehr deutlich, welche Materialien geeignet sind und welche Eigenschaften sie aufweisen mussen. Dieser Aufsatz soll auf breiter Basis die derzeit vielversprechendsten Ansatze fur eine molekulare Elektronik vorstellen und in ihre Teildisziplinen einfuhren. Wir behandeln zunachst die historische Entwicklung der konventionellen Elektronik und zeigen auf, welche Faktoren den Beginn der molekularen Elektronik ausgelost haben. Die Probleme, die bei der Miniaturisierung konventioneller Bauelemente auftreten, werden ebenso diskutiert wie einige Losungsansatze. Daran anschliesend stellen wir die wichtigsten Ergebnisse auf diesem Gebiet vor, die einen tiefen Einblick in das Verhalten molekularer Systeme geben.

A long-established area of scientific excellence in Europe, solid state chemistry has emerged in the US in the past two decades as a field experiencing rapid growth and development. At its core, it is an interdisciplinary melding of chemistry, physics, engineering, and materials science, as it focuses on the design, synthesis and structural characterization of new chemical compounds and characterization of their physical properties. As a consequence of this inherently interdisciplinary character, the solid state chemistry community is highly open to the influx of new ideas and directions. The inclusionary character of the field’s culture has been a significant factor in its continuing growth and vitality. This report presents an elaboration of discussions held during an NSF-sponsored workshop on Future Directions in Solid State Chemistry , held on the UC Davis Campus in October 2001. That workshop was the second of a series of workshops planned in this topical area. The first, held at NSF headquarters in Arlington, Virginia, in January of 1998, was designed to address the core of the field, describing how it has developed in the US and worldwide in the past decade, and how the members of the community saw the central thrusts of research and education in solid state chemistry proceeding in the next several years. A report was published on that workshop (J.M. Honig, chair, “Proceedings of the Workshop on the Present Status and Future Developments of Solid State Chemistry and Materials”, Arlington, VA, January 15–16, 1998) describing the state of the field and recommendations for future development of the core discipline. In the spirit of continuing to expand the scope of the solid state chemistry community into new areas of scientific inquiry, the workshop elaborated in this document was designed to address the interfaces between our field and fields where we thought there would be significant opportunity for the development of new scientific advancements through increased interaction. The 7 topic areas, described in detail in this report, ranged from those with established ties to solid state chemistry such as Earth and planetary sciences, and energy storage and conversion, to those such as condensed matter physics, where the connections are in their infancy, to biology, where the opportunities for connections are largely unexplored. Exciting ties to materials chemistry were explored in discussions on molecular materials and nanoscale science, and a session on the importance of improving the ties between solid state chemists and experts in characterization at national experimental facilities was included. The full report elaborates these ideas extensively.

The hybridization of nanoparticle-labeled DNA targets to surface-attached DNA probes has been investigated. Scanning tunneling microscopy (STM) and Raman and Fourier transform infrared (FTIR) spectroscopy were used to elucidate surface morphology, coverage, and the presence of aggregates. The factors that affect surface coverage, such as probe density, labeled target concentration, and particle size, were systematically investigated by STM in order to determine the best set of experimental conditions allowing the formation of dense monolayers with a minimal number of surface defects for both 5(′1) nm and 10(′2) nm gold nanoparticle labels on the target strand. Grazing-angle FTIR spectroscopy demonstrates that DNA is largely oriented once the labeled targets hybridized to the probes. Raman microscopy was used to probe the surface for the presence of large aggregates that would give rise to large scattering signals. Both STM and optical experiments provide evidence that dense surface layers can be formed without extensive aggregation. Nonselective binding was shown to be a function of the target concentration and nanoparticle size. Propensity for both aggregation and nonspecific binding is greater for 10(′2) nm than for 5(′1) nm gold nanoparticles.

[19] T. Cassagneau, J. H. Fendler, T. E. Mallouk, Langmuir 2000, 16, 241.Y. Sun, E. Hao, X. Zhang, B. Yang, J. Shen, L. Chi, H. Fuchs, Langmuir1997, 13, 5168.[20] Y. Liu, A. Wang, R. Claus, J. Phys. Chem. B 1997, 101, 1385.[21] F. Caruso, H. Lichtenfeld, M. Giersig, H. Mohwald, J. Am. Chem. Soc.1998, 120, 8523. Y. M. Lvov, J. F. Rusling, D. L. Thomsen, F. Papadimitra-kopoulos, T. Kawakami, T. J. Kunitake, Chem. Commun. 1998, 1229.Y. Lvov, K. Ariga, M. Onda, I. Ichinose, T. Kunitake, Langmuir 1997,13, 6195.[22] F. G. Aliev, M. A. Correa-Duarte, J. W. Ostrander, M. Giersig, L. M. Liz-Marzan, N. A. Kotov, Adv. Mater. 1999, 11, 1006. I. Ichinose, H. Tagawa,S. Mizuki, Y. Lvov, T. Kunitake, Langmuir 1998, 14, 187.[23] A. Rosidian, Y. Liu, R. O. Claus, Adv. Mater. 1998, 10, 1087. Y. Liu,A. Wang, R. O. Claus, Appl. Phys. Lett. 71, 16, 2265.[24] For a recent review of stepwise multilayer self-assembly techniques, see:G. Decher, Science 1997, 277, 1232.[25] Ellipsometric measurements were obtained on a Rudolph AutoEL-IIIautomatic ellipsometer assuming a refractive index for the multilayersbefore and after calcination of 1.80 and 2.10, respectively. These indexeswere representative of values obtained experimentally on films greaterthan 60 nm in thickness.[26] To ensure that both sides of the quartz slide were coated evenly, the sub-strate was coated by dipping into the component solutions, rather than bydripping.[27] The approximate escape depth of photoemission, d, is given by 3k(sinh),where k is the inelastic mean free path of the photoelectron in the filmand h is the takeoff angle. We approximate k as 3 nm, a value betweenthat of TiO

The phenomenon of negative differential resistance (NDR) is potentially very useful in molecular electronics device schemes. Here, it is shown that NDR can be observed in self-assembled monolayers composed of electroactive thiols on gold. Furthermore, these monolayers can be patterned using a scanning probe lithography technique described earlier to form a basis for potential molecular electronic device construction.

Molecular resolution scanning tunneling microscopy images are shown of alkanethiol self-assembled monolayers (SAMs) containing alkene, cyano, and carboxylic acid headgroups. The alkene-terminated thiolate SAM displayed a lattice structure indistinguishable from those of methyl-terminated thiol. The cyano-terminated thiolate SAM showed hexagonal, square, and double-row lattice structures, indicative of polymorphism and/or multiple chemical species on the surface. The carboxylic acid-terminated thiolate SAM showed a double-row lattice structure very similar to that of the cyano-terminated thiolate SAM suggesting that perhaps the cyano groups had undergone hydrolysis to form amide and/or carboxylic acid groups. This hypothesis was supported by the results of a friction-force microscopy experiment on micro-contact printed patterns of these molecules.

An efficient synthesis of fully aromatic, fused-ring, monoaza-acenes through a nucleophile initiated cascading cyclization is illustrated. Photophysical properties of the resulting molecules are reported.

Several molecular structure-property relationships are presented and compared to illustrate our current understanding of macromolecular encapsulation using dendrimers. Specifically, the effect that dendrimer architectures have on encapsulating photoactive and redox-active units fixed at the molecular core is considered.

A method of chemically well-defined, scanning tunneling microscope-based lithography is presented in which one thiolate in a self-assembled monolayer is removed and replaced with a second thiol. This method is distinguishable from other lithographic replacement processes on SAMs in that a nonpolar solution and an uncoated tip can be employed. Elevated relative humidity was important in the facility of this process, suggesting an electrochemical mechanism for replacement. The resolution of features written with this process is ca. 10−15 nm. In nonpolar solution, the apparent heights of self-assembled decanethiolate and dodecanethiolate monolayers are reversed compared to those observed in images obtained in air. When the thiol solution was exchanged after the first replacement, writing with two different thiols was demonstrated.

The effect of repeat unit structure on the shape and internal organization of various dendrimers was probed using atomistic molecular dynamics simulations. In this technique, care was taken to ensure complete structural equilibration by implementing a high temperature dynamics/simulated annealing protocol prior to evaluation of the molecular structure and dynamics. Both flexible and stiff repeat units that have been employed previously in the synthesis of dendrimers were considered. Flexible-unit dendrimers were found to be globular but not completely spherical. In contrast, stiff-unit dendrimers had a more eccentric, disk-like shape. For all dendrimers, the different generations within each molecule were found to be radially distributed throughout its interior. This appearance could be attributed to back-folding of some of the repeat units in the flexible case and to a branching angle effect in the stiff case. This distribution, however, did not preclude a molecular surface composed of a substantial portion of the topologically terminal groups.

Two series of redox-active, iron−sulfur core dendrimers of the general structure (nBu4N)2[Fe4S4(S-Dend)4] (Dend = dendrons of generations 1 through 4) were prepared. Heterogeneous electron-transfer rate constants indicated that the rigid series of dendrimers were more effective at attenuating the rate of electron transfer than were the flexible series of dendrimers. These results were rationalized using computationally derived models which indicated an offset and mobile iron−sulfur core in the flexible series of molecules and a more central and relatively immobile iron−sulfur core in the rigid series of molecules. Further consideration of these data indicated that, while the dendrimers containing rigid ligands had better encapsulated redox cores for a given molecular weight, these molecules had higher electron-transfer rates for a given molecular radius.

Poly(cyanoacetylene) has been prepared by polymerization of cyanoacetylene using a variety of late-transition-metal (Pd- and Ni-based) catalysts. The effect of reaction conditions on molecular weight and polymer yield was explored. Significant catalyst residues remain in the polymer samples. However, spectra are consistent with the proposed poly(cyanoacetylene) structure. Notable differences in molecular weight, effective conjugation length, and ability to undergo thermal cyclization to a supposed aromatic ladder polymer were observed between these polymers and those prepared using conventional anionic or radical initiators.

Self-assembled monolayers of dodecanethiol on gold, palladium, and platinum can be locally replaced with a different thiol under the action of a scanning tunneling microscopy tip at elevated bias. This process is characterized and the bias dependence of the degree of replacement for each metal substrate is illustrated. Replacement on gold substrates occurs at lower applied potentials than on platinum substrates, provided care has been taken to remove surface oxides.

A series of focally substituted organothiol dendrons of the first through third generation were used to construct adlayers on a gold surface. The presence and structural features of these adlayers were confirmed by X-ray photoelectron spectroscopy, infrared spectroscopy, and ellipsometry. The relative coverage and/or permeability of these adlayers was studied using capacitance and electrochemical blocking experiments. It was found that as the number of hyperbranches in the dendron increased from one to three, the dendron adlayers became initially less and then more permeable. This result indicated a tradeoff between size and packing efficiency when using these molecules to cover a surface. These data also suggest that the dendrons have formed homogeneous but permeable adlayers on the gold surface rather than adlayers consisting of islands of material. These adlayers showed large differences in their ability to trap and hold a small molecule, trans-cyclohexanediol, within them.

The longitudinal relaxation time constants (T 1 ) of the protons in a series of dendrimers that alternatively had paramagnetic ([Fe 4 S 4 (SR) 4 ] 2- , R = dendron) and diamagnetic (tetraphenylmethane) cores were compared. The T 1 values of the phenyl and benzyl protons in the paramagnetic core dendrimers were attenuated compared to those of analogous protons in the diamagnetic core dendrimers. This observation indicated that protons in each set of topologically different repeat units (generation) of the dendrimer approach the core of the molecule closely in space. This conclusion is consistent with the computed radial density distributions of the different generations calculated from molecular dynamics simulations. In addition, by comparing T 1 values of protons at two slightly different temperatures, the terminal groups in both sets of dendrimers were concluded to be, on average, more mobile than the other generations within the dendrimers. This conclusion is consistent with the computed mean square displacement correlation functions for the different generations also calculated from molecular dynamics simulations.

Hydrogen sulfide was observed by scanning tunneling microscopy to easily corrode a Au(111) surface, as suggested by an increase in surface roughness upon H2S exposure to the surface. However, this process could be tempered by moderating the concentration of H2S in the vapor above the gold. When corrosion was minimized, corroded regions were observed to exist predominantly at step edges. At a sufficiently low concentration of H2S, linear triple-row structures were observed. These are not the same as those observed when aqueous sulfide was reacted with a gold surface. A model that accounts for all the features of the adlayer structure is presented that involves a packing density of hydrosulfide groups of 28.5 A2/molecule, which is lower than that found in a √3 x √3 R30 overlayer (21.5 A2/molecule) found for long-chain alkanethiolate self-assembled monolayers on Au(111).