Preparative isolation of (+)-beta-eudesmol fromAmyris balsamifera

Summary Optically pure (+)-beta-eudesmol is a possible starting material for the synthesis of several termite defense compounds. A two step procedure for the isolation of gram quantities of (+)-beta-eudesmol from commercially availableAmyris balsamifera oil (syn. West Indian sandalwood oil), containing 8% beta-eudesmol, was developed. Step one consisted of an efficient vacuum distillation of the total oil. Step two was a medium pressure LC separation with an AgNO₃ impregnated silica gel stationary phase. Several other separation procedures failed due to the presence of many closely related sesquiterpene alcohols (75% of the oil).     

Prototype reflectivity analyses of hydrogen storage levels in single-walled carbon nanotubes

A prototype case study is presented that examines the level of hydrogen content in H-SWNTs using the Surface Plasmon Resonance technique. The damping effect and the angular shift in the resonance minimum of an SWNT-gold interface due to the presence of hydrogen is analyzed using a parametric model, which is based on the concept of an effective permittivity. The new approach provides for a non-invasive analysis of the level of hydrogen content in H-SWNTs and is potentially extendable to other carbon-based hydrogen storage materials.     

Formation of aromatic siloxane self-assembled monolayers

We describe reproducible protocols for the chemisorption of self-assembled monolayers (SAMs), useful as imaging layers for nanolithography applications, from p-chloromethylphenyltrichlorosilane (CMPS) and 1-(dimethylchlorosilyl)-2-(p,m-chloromethylphenyl)ethane on native oxide Si wafers. Film chemisorption was monitored and characterized using water contact angle, X-ray photoelectron spectroscopy, and ellipsometry measurements. Atomic force microscopy was used to monitor the onset of multilayer deposition for CMPS films, ultimately allowing film macroscopic properties to be correlated with their surface coverage and nanoscale morphologies. Although our results indicate the deposition of moderate coverage, disordered SAMs under our conditions, their quality is sufficient for the fabrication of sub-100-nm-resolution metal features. The significance of our observations on the design of future imaging layers capable of molecular scale resolution in nanolithography applications is briefly discussed.     

Phenylphosphonic acid functionalization of indium tin oxide: surface chemistry and work functions

The work function of indium tin oxide (ITO) substrates was modified with phosphonic acid molecular films. The ITO surfaces were treated prior to functionalization with a base cleaning procedure. The film growth and coverage were quantified by contact angle goniometry and XPS. Film orientation was determined by reflection/absorption infrared spectroscopy using ITO-on-Cr substrates. The absolute work functions of nitrophenyl- and cyanophenyl-phosphonic acid films in ITO were determined by Kelvin probe measurement to be 5.60 and 5.77 eV, respectively.     

Expression of molecular chirality and two-dimensional supramolecular self-assembly of chiral, racemic, and achiral monodendrons at the liquid-solid interface

We have investigated the two-dimensional ordering of chiral and achiral monodendrons at the liquid-solid interface. The chiral molecules self-assemble into extended arrays of dimers. As expected, the R enantiomer forms the mirror image type pattern of the chiral two-dimensional structure formed by the S enantiomer. A racemic mixture applied from solution onto the substrate undergoes spontaneous segregation: the enantiomers separate on the surface and appear in different domains. In contrast to the chiral molecules, the achiral analogue self-assembles into cyclic tetramers. Moreover, the pattern formed by the achiral molecule strongly depends on the solvent used. In the case of 1-phenyloctane, solvent molecules are coadsorbed in a 2:1 (dendron:solvent) ratio whereas in 1-octanol, no solvent molecules are coadsorbed. By the appropriate solvent choice, the distance between the potential "supramolecular containers" can be influenced.     

Hollow spherical supramolecular dendrimers

The synthesis of a library containing 12 conical dendrons that self-assemble into hollow spherical supramolecular dendrimers is reported. The design principles for this library were accessed by development of a method that allows the identification of hollow spheres, followed by structural and retrostructural analysis of their Pm3n cubic lattice. The first hollow spherical supramolecular dendrimer was made by replacing the tapered dendron, from the previously reported tapered dendritic dipeptide that self-assembled into helical pores, with its constitutional isomeric conical dendron. This strategy generated a conical dendritic dipeptide that self-assembled into a hollow spherical supramolecular dendrimer that self-organizes in a Pm3n cubic lattice. Other examples of hollow spheres were assembled from conical dendrons without a dipeptide at their apex. These are conical dendrons originated from tapered dendrons containing additional benzyl ether groups at their apex. The inner part of the hollow sphere assembled from the dipeptide resembles the path of a spherical helix or loxodrome and, therefore, is chiral. The spheres assembled from other conical dendrons are nonhelical, even when they contain stereocenters on the alkyl groups from their periphery. Functionalization of the apex of the conical dendrons with diethylene glycol allowed the encapsulation of LiOTf and RbOTf in the center of the hollow sphere. These experiments showed that hollow spheres function as supramolecular dendritic capsules and therefore are expected to display functions complementary to those of other related molecular and supramolecular structures.     

Helical pores self-assembled from homochiral dendritic dipeptides based on L-Tyr and nonpolar alpha-amino acids

The synthesis of dendritic dipeptides (4-3,4-3,5)12G2-CH2-Boc-L-Tyr-X-OMe where X = Gly, L-Val, L-Leu, L-Ile, L-Phe, and L-Pro is reported. Their self-assembly in bulk and in solution and the structural and retrostructural analysis of their periodic assemblies were compared to those of the previously reported and currently reinvestigated dendritic dipeptides with X = L-Ala. All dendritic dipeptides containing as X nonpolar alpha-amino acids self-assemble into helical porous columns. The substituent of X programs the structure of the helical pore and the resulting periodic array, in spite of the fact that its molar mass represents only between 0.05 and 4.77% from the molar mass of the dendritic dipeptide. In addition to the various 2-D columnar lattices, the dendritic dipeptides based on L-Ala, L-Leu, and L-Phe self-organize into 3-D hexagonal columnar crystals while those based on L-Val and L-Ile into an unknown columnar crystal. The principles via which the aliphatic and aromatic substituents of X program the structure of the helical pores indicate synthetic pathways to helical pores with bioinspired functions based on artificial nonpolar alpha-amino acids.     

Laser induced fluorescence in NO2: absolute fluorescence cross sections and quantum yields

Spectrally resolved collision-free absolute fluorescence cross sections have been measured, for NO₂ excited, by the second harmonic of the Nd: YAG laser. The total cross section into the 523.0–650.0 nm range was found to be 2.3 × 10^?20 cm². The measured absorption cross section of 1.46 × 10^?19 cm² implies a quantum yield of 16% over this range.