Color detection using chromophore-nanotube hybrid devices.

We present a nanoscale color detector based on a single-walled carbon nanotube functionalized with azobenzene chromophores, where the chromophores serve as photoabsorbers and the nanotube as the electronic read-out. By synthesizing chromophores with specific absorption windows in the visible spectrum and anchoring them to the nanotube surface, we demonstrate the controlled detection of visible light of low intensity in narrow ranges of wavelengths. Our measurements suggest that upon photoabsorption, the chromophores isomerize from the ground state trans configuration to the excited state cis configuration, accompanied by a large change in dipole moment, changing the electrostatic environment of the nanotube. All-electron ab initio calculations are used to study the chromophore-nanotube hybrids and show that the chromophores bind strongly to the nanotubes without disturbing the electronic structure of either species. Calculated values of the dipole moments support the notion of dipole changes as the optical detection mechanism.

Near-edge X-ray absorption fine structure spectroscopy of diamondoid thiol monolayers on gold.

Diamondoids, hydrocarbon molecules with cubic-diamond-cage structures, have unique properties with potential value for nanotechnology. The availability and ability to selectively functionalize this special class of nanodiamond materials opens new possibilities for surface modification, for high-efficiency field emitters in molecular electronics, as seed crystals for diamond growth, or as robust mechanical coatings. The properties of self-assembled monolayers (SAMs) of diamondoids are thus of fundamental interest for a variety of emerging applications. This paper presents the effects of thiol substitution position and polymantane order on diamondoid SAMs on gold using near-edge X-ray absorption fine structure spectroscopy (NEXAFS) and X-ray photoelectron spectroscopy (XPS). A framework to determine both molecular tilt and twist through NEXAFS is presented and reveals highly ordered diamondoid SAMs, with the molecular orientation controlled by the thiol location. C 1s and S 2p binding energies are lower in adamantane thiol than alkane thiols on gold by 0.67 +/- 0.05 and 0.16 +/- 0.04 eV, respectively. These binding energies vary with diamondoid monolayer structure and thiol substitution position, consistent with different degrees of steric strain and electronic interaction with the substrate. This work demonstrates control over the assembly, in particular the orientational and electronic structure, providing a flexible design of surface properties with this exciting new class of diamond nanoparticles.

(E)-2-{Eth-yl[4-(4-nitro-phenyl-diazen-yl)phen-yl]amino}ethyl anthracene-9-carboxyl-ate.

The crystal structure of the title compound, C(31)H(26)N(4)O(4), displays a trans conformation for the nitro-phenyl-diazenyl portion of the mol-ecule. Packing diagrams indicate that weak C-H⋯O hydrogen bonds, likely associated with a strong dipole moment present in the mol-ecule, dictate the arrangement of mol-ecules in the crystal structure.

(E)-4-[(4-Nitro-phen-yl)diazen-yl]phenyl anthracene-9-carboxyl-ate.

In the title compound, C(27)H(17)N(3)O(4), the azo group displays a trans conformation and the dihedral angles between the central benzene ring and the pendant anthracene and nitro-benzene rings are 82.94 (7) and 7.30 (9)°, respectively. In the crystal structure, weak C-H⋯O hydrogen bonds, likely associated with a dipole moment present on the mol-ecule, help to consolidate the packing.

Investigating the affinities and persistence of VX nerve agent in environmental matrices.

Laboratory experiments were conducted to determine environmental variables that affect the affinities and persistence of the nerve agent O-ethyl S-(2-diisopropylaminoethyl) methylphosphonothiolate (VX) at dilute concentrations in environmental matrices. Quantitative analyses of VX and its degradation products were performed using LC-MS. Batch hydrolysis experiments demonstrated an increasing hydrolysis rate as pH increased, as shown in previous studies, but also indicated that dissolved aqueous constituents can cause significant differences in the absolute hydrolysis rate. Adsorption isotherms from batch aqueous experiments revealed that VX has a high affinity for hydrophobic organics, a moderate affinity for montmorillonite clay, and a very low affinity for an iron-oxyhydroxide soil mineral, goethite. The adsorption on goethite was increased with the presence of dissolved organic matter in solution. VX degraded rapidly when dried onto goethite, when specific adsorption was forced. No enhanced degradation occurred with goethite in small amounts of water. These results suggest that aqueous conditions have important controls on VX adsorption and degradation in the environment and a more mechanistic understanding of these controls is needed in order to enable accurate predictions of its long-term fate and persistence.

Surface structure and chemical switching of thioctic acid adsorbed on Au(111) as observed using near-edge X-ray absorption fine structure.

Thioctic acid (alpha-lipoic acid) is a molecule with a large disulfide-containing base, a short alkyl chain with four CH2 units, and a carboxyl termination. Self-assembled monolayer (SAM) films ofthioctic acid adsorbed on Au(111) have been investigated with near-edge X-ray absorption fine structure (NEXAFS) spectroscopy and X-ray photoelectron spectroscopy (XPS) to determine film quality, bonding, and morphology. Using standard preparation protocols for SAMs, that is, dissolving thioctic acid in ethanol and exposing gold to the solution, results in poor films. These films are highly disordered, contain a mixture of carboxyl and carboxylate terminations, have more than monolayer coverage, and exhibit unbound disulfide. Conversely, forming films by dissolving 1 mmol thioctic acid into 5% acetic acid in ethanol (as previously reported with carboxyl-terminated alkanethiols) forms ordered monolayers with small amounts of unbound sulfur. NEXAFS indicates tilted over endgroups with the carboxyl group normal on average 38 degrees from the surface normal. Slight angle-dependent intensity modulations in other features indicate alkyl chains statistically more upright than prostrate on the surface. Reflection-absorption Fourier transform infrared (RA-FTIR) spectra indicate hydrogen bonding between neighboring molecules. In such well-formed monolayers, a stark reorientation occurs upon deprotonation of the endgroup by rinsing in a KOH solution. The carboxylate plane normal is now about 66 degrees from sample normal, a much more upright orientation. Data indicate this reorientation may also cause a more upright orientation to the alkyl portion of the molecules.

Chemically transformable configurations of mercaptohexadecanoic acid self-assembled monolayers adsorbed on Au(111).

Carboxyl-terminated self-assembled monolayers (SAMs) are commonly used in a variety of applications, with the assumption that the molecules form well-ordered monolayers. In this work, near-edge X-ray absorption fine structure measurements verify that well-ordered monolayers can be formed using acetic acid in the solvent. Disordered monolayers with unbound molecules present in the film result using only ethanol. A stark reorientation occurs upon deprotonation of the end group by rinsing in a KOH solution. This reorientation of the end group is reversible with tilted-over, hydrogen-bound carboxyl groups while the carboxylate ion end groups are upright. C(1s) photoemission shows that SAMs formed and rinsed with acetic acid in ethanol have protonated end groups, while SAMs formed without acetic acid have a large fraction of carboxylate-terminated molecules.

An Octahedral Template Based on a New Molecular Turn: Synthesis and Structure of a Model Complex and a Reactive, Diphenolic Ligand and Its Metal Complexes.

The design and synthesis of a new family of tridentate Schiff base ligands for use in octahedral molecular templates has been successfully demonstrated. As anticipated, two molecules of the ligand, 2,6-pyridinedicarboxaldhydebis(p-hydroxyphenylimine), coordinate equatorially to six-coordinate octahedral metal ions to give orthogonally oriented molecular turns around the anchoring metal ion. This new template offers the advantages that (1) syntheses of the ligand and its complexes are straightforward, giving high yields in simple, one-pot reactions, and (2) structural variations are easily accomplished. X-ray structural analysis has shown that the ligand will only function as a turn when its spatial organization is controlled by coordination to a metal ion. Upon chelation, the phenolic groups are directed across and beyond the metal ion center, and, unlike earlier ligands of this general type which lacked reactive moieties, substitution reactions may be carried out at the free phenolic groups of the octahedral complex. These reactions remain to be optimized in order to achieve the long-term goal of proving their value as synthons for interlocked molecules.