Molecular rectification in a metal-insulator-metal junction based on self-assembled monolayers

An electrical junction formed by mechanical contact between two self-assembled monolayers (SAMs)--a SAM formed from an dialkyl disulfide with a covalently linked tetracyanoquinodimethane group that is supported by silver (or gold) and a SAM formed from an alkanethiolate SAM that is supported by mercury-rectifies current. The precursor to the SAM on silver (or gold) was bis(20-(2-((2,5-cyclohexadiene-1,4-diylidene)dimalonitrile))decyl)) disulfide and that for the SAM on mercury was HS(CH(2))(n-1)CH(3) (n = 14, 16, 18). The electrical properties of the junctions were characterized by current-voltage measurements. The ratio of the conductivity of the junction in the forward bias (Hg cathodic) to that in the reverse bias (Hg anodic), at a potential of 1 V, was 9 +/- 2 when the SAM on mercury was derived from HS(CH(2))(15)CH(3). The ratio of the conductivity in the forward bias to that in the reverse bias increased with decreasing chain length of the alkanethiol used to form the SAM on mercury. These results demonstrate that a single redox center asymmetrically placed in a metal-insulator-metal junction can cause the rectification of current and indicate that a fixed dipole in the insulating region of a metal-insulator-metal junction is not required for rectification.     

pH-Dependent rectification in self-assembled monolayers based on electrostatic interactions

Asymmetric electrostatic interactions dependent on pH between the redox molecules and the terminal group on the top of the self-assembled monolayer (SAM) afford control of the electron transfer property of the SAM having the imidazole terminal group.     

Self-addition of a sterically hindered alkynylselenolate

[structure: see text] The unprecedented formation of a complex macrocyclic selenium-carbon ring system, which is initiated by the action of coordinating reagents upon a sterically hindered alkynylselenolate, has been observed. The crystal structure of the product could be obtained and shows a rigid tricyclic arrangement consisting only of selenium and sp(2)-hybridized carbon atoms. This reactivity stands in contrast to the corresponding unsubstituted alkynylselenolates and is an example where a bulky substituent destabilizes an adjacent unsaturated pi-system.     

Ultrafast shock compression of self-assembled monolayers: a molecular picture

Simulations of self-assembled monolayers (SAMs) are performed to interpret experimental measurements of ultrafast approximately 1 GPa (volume compression deltaV approximately 0.1) planar shock compression dynamics probed by vibrational sum-frequency generation (SFG) spectroscopy (Lagutchev, A. S.; Patterson, J. E.; Huang, W.; Dlott, D. D. J. Phys. Chem. B 2005, 109, XXXX). The SAMs investigated are octadecanethiol (ODT) and pentadecanethiol (PDT) on Au(111) and Ag(111) substrates, and benzyl mercaptan (BMT) on Au(111). In the alkane SAMs, SFG is sensitive to the instantaneous orientation of the terminal methyl; in BMT it is sensitive to the phenyl orientation. Computed structures of alkane SAMs are in good agreement with experiment. In alkanes, the energies of gauche defects increase with increasing number and depth below the methyl plane, with the exception of ODT/Au where both single and double gauche defects at the two uppermost dihedrals have similar energies. Simulations of isothermal uniaxial compression of SAM lattices show that chain and methyl tilting is predominant in PDT/Au, ODT/Ag and PDT/Ag, whereas single and double gauche defect formation is predominant in ODT/Au. Time-resolved shock data showing transient SFG signal loss of ODT/Au and PDT/Au are fit by calculations of the terminal group orientations as a function of deltaV and their contributions to the SFG hyperpolarizability. The highly elastic response of PDT/Au results from shock-generated methyl and chain tilting. The viscoelastic response of ODT/Au results from shock generation of single and double gauche defects. Isothermal compression simulations help explain and fit the time dependence of shock spectra but generally underestimate the magnitude of SFG signal loss because they do not include effects of high-strain-rate dynamics and shock front and surface irregularities.     

Improved conditions for converting sterically hindered amides to 1,5-disubstituted tetrazoles

Improved conditions for converting amides into 1,5-disubstituted tetrazoles are described. The optimum reaction conditions [diisopropyl azodicarboxylate (DIAD), diphenylphosphoryl azide (DPPA), and diphenyl-2-pyridyl phosphine in THF at 45 °C] converted sterically hindered amides to their corresponding tetrazoles in good yield.     

Adsorption of sulfite oxidase on self-assembled monolayers from molecular dynamics simulations

Sulfite oxidase (SO) is an enzyme catalyzing the terminal step of the metabolism of sulfur-containing amino acids that is essential for almost all living organisms. The catalytic activity of SO in vertebrates strongly depends on the efficiency of the intramolecular electron transfer (IET) between the catalytic Moco domain and the cytochrome b5 (cyt b5) domain. The IET process is assumed to be mediated by large domain motions of the cyt b5 domains within the enzyme. Thus, the interaction of SO with charged surfaces may affect the mobility of the cyt b5 domain required for IET and consequently hinder SO activation. In this study, we present a molecular dynamics approach to investigating the ionic strength dependence of the initial surface adsorption of SO in two different conformations-the crystallographic structure and the model structure for an activated SO-onto mixed amino- and hydroxyl-terminated SAMs. The results show for both conformations at low ionic strengths a strong adsorption of the cyt b5 units onto the SAM, which inhibits the domain motion event required for IET. Under higher ion concentrations, however, the interaction with the surface is weakened by the negatively charged ions acting as a buffer and competing in adsorption with the cathodic cyt b5 domains. This competition prevents the immobilization of the cytochrome b5 units onto the surface, allowing the intramolecular domain motions favoring IET. Our predictions support the interpretation of recent experimental spectroelectrochemical studies on SO.     

Large-scale molecular dynamics simulations of alkanethiol self-assembled monolayers

Large-scale molecular dynamics simulations of self-assembled alkanethiol monolayer systems have been carried out using an all-atom model involving a million atoms to investigate their structural properties as a function of temperature, lattice spacing, and molecular chain length. Our simulations show that the alkanethiol chains of 13-carbons tilt from the surface normal by a collective angle of 25 degrees along next-nearest-neighbor direction at 300 K. The tilt structure of 13-carbon alkanethiol system is found to depend strongly on temperature and exhibits hysteresis. At 350 K the 13-carbon alkanethiol system transforms to a disordered phase characterized by small collective tilt angle, flexible tilt direction, and random distribution of backbone planes. The tilt structure also depends on lattice spacing: With increasing lattice spacing a the tilt angle increases rapidly from a nearly zero value at a=4.7 A to as high as 34 degrees at a=5.3 A at 300 K for 13-carbon alkanethiol system. Finally, the effects of the molecular chain length on the tilt structure are significant at high temperatures.     

The molecular level modification of surfaces: from self-assembled monolayers to complex molecular assemblies

The modification of surfaces with self-assembled monolayers (SAMs) containing multiple different molecules, or containing molecules with multiple different functional components, or both, has become increasingly popular over the last two decades. This explosion of interest is primarily related to the ability to control the modification of interfaces with something approaching molecular level control and to the ability to characterise the molecular constructs by which the surface is modified. Over this time the level of sophistication of molecular constructs, and the level of knowledge related to how to fabricate molecular constructs on surfaces have advanced enormously. This critical review aims to guide researchers interested in modifying surfaces with a high degree of control to the use of organic layers. Highlighted are some of the issues to consider when working with SAMs, as well as some of the lessons learnt (169 references).     

High-pressure transesterification of sterically hindered esters

A mild, rapid, and efficient method for the solvolysis of sterically hindered esters under high pressure is described. Transesterification is carried out in the presence of DBU at room temperature and at a pressure of 10 kbar to give quantitative conversions within short reaction times. The substrates examined included aromatic and aliphatic esters of sterically hindered alcohols and phenols. An optically pure benzyl ester of phenylalanine was chosen to study racemization of the amino acid esters under high-pressure reaction conditions.     

Novel synthesis of sterically hindered N-substituted lactams from imides

An efficient and practical synthesis of sterically hindered N-substituted lactams has been developed starting from simple starting materials. The stereochemistry of the synthetically useful N,N acetal intermediate has been established.     

Aquametric microdetermination of sterically hindered ketones

Conclusions The previously developed aquametric micromethod for determining carbonyl compounds also gives reliable results when most of the sterically hindered ketones studied by us are analyzed. The determination error is ±0.3%.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 4, pp. 942–944, April, 1972.     

Photoisomerization of sterically hindered polymethine dyes

Conclusions In sterically hindered polymethine dyes the photoisomerization process, with the formation of sterically hindered trans and di-cis forms, successfully competes with the nonradiative degradation of the energy on the higher vibrational sublevels of the ground state.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 8, pp. 1926–1930, August, 1977.     

Claisen rearrangement of sterically hindered N-alkenylindolines

Conclusions Claisen rearrangement of N-(1-methyl-2-butenyl)-2-methyl-2-ethylindoline occurs under mild conditions upon treatment with acids at 20°C. Tricyclic compounds, namely, 6,6-dimethylperhydroeyclopent[g]-2-methyl-2-ethylindoline and 8-methyl-8-ethylpyrrolidino[h, i]-3-methyl-2-ethylindole, were obtained from 7-(1-methyl-2-butenyl)-2-methyl-2-ethylindoline.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 3, pp. 613–618, March, 1987.     

Synthesis of sterically hindered aromatic dialdehydes

A study was carried out on the formylation of a series of aromatic compounds containing two mesitylene or durene residues [dimesityl (I), dimesitylmethane (II), 1,2-dimesitylethane (III), 1,6-dimesitylhexane (IV), dimesityl sulfide (V), 1,1-dimesitylethylene (VI), 1,1-dimesityl-1-butene (VII), and didurylmethane (VIII)] by the action of dichloromethyl methyl ether (DCM) in the presence of A1C1₃ and TiCl₄. The corresponding dialdehydes are the major products. The formylation products when the reaction is carried out in the presence of A1C1₃ in the case of (I) and (V) contain significant amounts of monoaldehydes, while partial cleavage of the substrates with the formation of products containing only one benzene ring is observed in the case of (II) and (VIII) in addition to formylation.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 7, pp. 1700–1703, July, 1991.     

Synthesis of sterically hindered aromatic aldehydes

Formylation of mesitylene, durene, and m-xylene derivatives containing electron-donor and electron-acceptor substituents by dichloromethyl methyl ether was studied in the presence of TiCl₄. A series of functionally substituted sterically hindered benzaldehydes was prepared from the products of these reactions.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 7, pp. 1609–1615, July, 1991.     

Dipole moments of sterically hindered p-hydroxystyrylpyridines

On the basis of measurements of dipole moments and a comparison of the results obtained with those calculated by a vectorial additive scheme, the structure of a series of sterically hindered p-hydroxystyrylpyridines have been determined.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 11, pp. 1528–1531, November, 1973.