Facile Convergent Route to Molecular Caltrops

The convergent syntheses of molecular caltrops are described starting from tetraethyl orthosilicate and using organolithium additions and Pd/Cu-catalyzed coupling methods. The caltrop core is based on a tetrahedral silicon atom, and there are three legs each bearing sulfur-tipped feet for adhesion to metallic surfaces. The forth prong (arm) is non-sulfur-bearing for projection upward from the surface. Rigid phenyleneethynylene segments are used for the legs and arms. These organosilicon caltrops may have utility as scanning probe microscopy tips

Synthesis of Imine-Bridged Phenylenepyridine Ladder Polymers. Optical Band Gap Widening through Intramolecular Charge Transfer in Planar Polymers

The syntheses of planar (ladder) poly(phenylenepyridine)s [(PPhPy)s] are described using Pd-catalyzed cross couplings of aryldistannanes and aryl dihalides. Imine bridges are utilized to effect the planarization of the rigid-rod polymer. In one set of (PPhPy)s, the phenyl rings bear the nitrogen portion of the bridging imines while the pyridine bears the carbon portion of the bridging imine. A second type of (PPhPy)s has the reverse imine-bridging mode. Surprisingly, the study here indicates that construction of alternating donor/acceptor repeat units for inducement of intramolecular charge-transfer resulted in an optical band widening; a result opposite to that obtained in nonplanar polymers possessing alternating donor/acceptor repeat units

Orthogonally Functionalized Oligomers for Controlled Self-Assembly

The synthesis of molecules terminated with complementary thiol-protecting groups is described. The target compounds contain functionalities on one end known to form self-assembled monolayers on metal surfaces while at the other end an intact thioacetate is present whereby self-assembly may again occur after an in situ deprotection. Self-assembly data is reported for selected compounds to assess their efficacy in surface adhesion

Self-Assembly of Supra-amphiphiles Based on Dual Charge-Transfer Interactions: From Nanosheets to Nanofibers

With the elaborate engineering of supra-amphiphiles based on dual charge-transfer interactions, the rational design and programmable transformation of well-defined 1D and 2D nanostructures have been demonstrated. First, H-shaped supra-amphiphiles are successfully obtained on the basis of the directional charge-transfer interactions of naphthalene diimide and naphthalene, which self-assemble in water to form 2D nanosheets. Second, by complexation of the H-shaped supra-amphiphiles with pyrene derivatives, the 2D nanosheets transform into ultralong 1D nanofibers. Therefore, this line of research represents a successful example of supramolecular engineering and has enriched its realm

Structure-Dependent Charge Transport and Storage in Self-Assembled Monolayers of Compounds of Interest in Molecular Electronics: Effects of Tip Material, Headgroup, and Surface Concentration

The electrical properties of self-assembled monolayers (SAMs) on a gold surface have been explored to address the relation between the conductance of a molecule and its electronic structure. We probe interfacial electron transfer processes, particularly those involving electroactive groups, of SAMs of thiolates on Au by using shear force-based scanning probe microscopy (SPM) combined with current−voltage (i−V) and current−distance (i−d) measurements. Peak-shaped i−V curves were obtained for the nitro- and amino-based SAMs studied here. Peak-shaped cathodic i−V curves for nitro-based SAMs were observed at negative potentials in both forward and reverse scans and were used to define the threshold tip bias, VTH, for electric conduction. For a SAM of 2‘,5‘-dinitro-4,4‘-bis(phenylethynyl)-1-benzenethiolate, VII, VTH was nearly independent of the tip material [Ir, Pt, Ir−Pt (20−80%), Pd, Ni, Au, Ag, In]. For all of the SAMs studied, the current decreased exponentially with increasing distance, d, between tip and substrate. The exponential attenuation factors (β values) were lower for the nitro-based SAMs studied here, as compared with alkylthiol-based SAMs. Both VTH and β of the nitro-based SAMs also depended strongly on the molecular headgroup on the end benzene ring addressed by the tip. Finally, we confirmed the “memory” effect observed for nitro-based SAMs. For mixed SAMs of VII and hexadecanethiol, I, the fraction of the charge collected in the negative tip bias region that can be read out at a positive tip bias on reverse scan (up to 38%) depended on the film composition and decreased with an increasing fraction of I, suggesting that lateral electron hopping among molecules of VII occurs in the vicinity of the tip

Mediating Stochastic Switching of Single Molecules Using Chemical Functionality

We have studied oligo(phenylene-ethynylene)s inserted into amide-containing alkanethiol self-assembled monolayers using scanning tunneling microscopy to demonstrate switching based on chemical functionality of the environment of the inserted molecules. The molecules show stability in two conductance states:  an ON and an OFF state. We demonstrate bias-dependent switching due to hydrogen bonding between the inserted oligo(phenylene-ethynylene) and the matrix molecules. In addition, the inserted molecules exhibit fewer switching events than previously reported for alkanethiol matrixes, which we attribute to the rigidity of the hydrogen-bonded matrix

Experimental and Theoretical Identification of Valence Energy Levels and Interface Dipole Trends for a Family of (Oligo)Phenylene-ethynylenethiols Adsorbed on Gold

Metal−molecule−metal junctions composed of organic molecular wires formed via self-assembly are of relevance in the empirical evaluation of single-molecule electronics. Key to understanding the effects of these monolayer structures on the transport through single molecules, however, is discerning how the molecular electronic levels evolve under the influence of the metal substrate and intermolecular interactions. We present a joint experimental and computational investigation of the electronic structure and electrostatic properties of a series of self-assembled donor- and acceptor-substituted (oligo)pheneylene-ethynylenethiols (OPEs) on gold. Photoemission spectroscopy is employed to determine the energy-level alignment for these monolayers. Isolated molecule and small cluster calculations are performed to assess changes in geometry, electronic structure, and charge distribution upon chemisorption. The calculated densities of electronic states allow assignment of the higher-lying occupied states mapped by experimental photoemission data. Calculated estimates of the surface, bond dipole, and image potential energies are used to estimate contributions to the measured work function changes; good correlations between the experimental and theoretical values are found. Importantly, these results point to a dependence of the dipole contributions on the orientational order of the SAM

Quantitative Real-Time Analysis of Living Materials by Stimulated Raman Scattering Microscopy

Composite materials built in part from living organisms have the potential to exhibit useful autonomous, adaptive, and self-healing behavior. The physicochemical, biological, and mechanical properties of such materials can be engineered through the genetic manipulation of their living components. Successful development of living materials will require not only new methods for design and preparation but also new analytical tools that are capable of real-time noninvasive mapping of chemical compositions. Here, we establish a strategy based on stimulated Raman scattering microscopy to monitor phosphatase-catalyzed mineralization of engineered bacterial films in situ. Real-time label-free imaging elucidates the mineralization process, quantifies both the organic and inorganic components of the material as functions of time, and reveals spatial heterogeneity at multiple scales. In addition, we correlate the mechanical performance of films with the extent of mineralization. This work introduces a promising strategy for quantitatively analyzing living materials, which should contribute to the accelerated development of such materials in the future

Cross-Step Place-Exchange of Oligo(phenylene−ethynylene) Molecules

We have observed nitro-functionalized oligo(phenylene−ethynylene) molecules exhibiting motion up and down Au{111} substrate monatomic step edges within host self-assembled monolayers of n-alkanethiols, independent of previously observed conductance switching. Single molecules have been imaged with scanning tunneling microscopy to place-exchange reversibly between the top and bottom of monatomic substrate step edges

Self-Assembled Oligo(phenylene-ethynylene) Molecular Electronic Switch Monolayers on Gold: Structures and Chemical Stability

Self-assembled monolayers (SAMs) of the nitro-substituted oligo(phenylene-ethynylene) (OPE) 4,4‘-(diethynylphenyl)-2‘-nitro-1-benzenethiolate on Au{111} were prepared, and the structures were characterized by multiple techniques, including infrared spectroscopy, ellipsometry, and X-ray photoelectron spectroscopy. Assembly of the nitro-OPE SAM, either via acidic hydrolysis of the thioacetate derivative or from the thiol in pure solvent, produces a well-ordered SAM with a (√3 × √3) superlattice structure and an average molecular tilt of 32−39° from the surface normal. In comparison, SAMs prepared from the unsubstituted OPE show the same lattice structure and a similar tilt of ∼33°. In contrast, when the nitro-OPE SAM is assembled by hydrolysis of the thioacetate derivative under basic conditions, extensive redox reactions arise in which oxidation of the S atoms occurs with accompanying reduction of −NO2 to −NH2, apparently via intermediates including −NH(OH), to form mixed composition SAMs typically containing ∼30% of the amino-substituted molecule. Further, the nitro-OPE SAM, regardless of the preparation method, shows significant chemical instability under storage in air and/or light exposure. Since the nitro-OPE molecule and molecules with related structures are of considerable interest for molecular electronics applications, these results indicate that extreme diligence must be used in designing conditions for the fabrication of devices utilizing these SAMs