Photoactive graphene sheets prepared by "click" chemistry

Graphene sheets modified by phenylacetylene moieties provide a facile platform for attaching various photoactive functional molecules via"click" chemistry. The produced photoactive graphene materials are well-dispersed in various solvents and show dramatically improved photo-current responses.     

Facile synthesis of Janus "double-concave" tribenzo[a,g,m]coronenes

Hexabutoxytribenzo[a,g,m]coronenes have been prepared in three steps from readily accessible hexabutoxytriphenylene. Single-crystal X-ray diffraction analysis reveals that these molecules can adopt an extraordinary "double-concave" conformation that makes them ideal hosts for the binding of different guest molecules at each face.     

Biologically active molecules with a "light switch"

Biologically active compounds which are light-responsive offer experimental possibilities which are otherwise very difficult to achieve. Since light can be manipulated very precisely, for example, with lasers and microscopes rapid jumps in concentration of the active form of molecules are possible with exact control of the area, time, and dosage. The development of such strategies started in the 1970s. This review summarizes new developments of the last five years and deals with "small molecules", proteins, and nucleic acids which can either be irreversibly activated with light (these compounds are referred to as "caged compounds") or reversibly switched between an active and an inactive state.     

Self-associated, "distillable" ionic media

Liquid or low melting association products of carbon dioxide and a secondary amine, both neutral molecules that may be gaseous, are recognised as "distillable" ionic media.     

"Up" versus "down" alignment and hydration structures of solutes at the air/water interface revealed by heterodyne-detected electronic sum frequency generation with classical molecular dynamics simulation

We unambiguously demonstrate the "up" versus "down" alignment of a pair of prototypical solute molecules adsorbed at the air/water interface for the first time using heterodyne-detected electronic sum frequency generation spectroscopy. This molecular alignment is also reproduced by classical molecular dynamics (MD) simulation theoretically. Furthermore, the MD simulation indicates distinctly different interface-specific hydration structures around the two solute molecules, which dictate the molecular alignment at the interface. It is concluded that the hydrophilicity difference between the terminal functional groups of the solute governs the molecular orientation and surrounding hydration structures at the interface.     

Micelles for the self-assembly of "off-on-off" fluorescent sensors for pH windows

A micellar approach is proposed to build a series of systems featuring an "off-on-off" fluorescent window response with changes in pH. The solubilizing properties of micelles are used to self-assemble, in water, plain pyrene with lipophilized pyridine and tertiary amine moieties. Since these components are contained in the small volume of the same micelle, pyrene fluorescence is influenced by the basic moieties: protonated pyridines and free tertiary amines behave as quenchers. Accordingly, fluorescence transitions from the "off" to the "on" state, and viceversa, take place when the pH crosses the pK(a) values of the amine and pyridine fragments. To obtain an "off-on-off" fluorescent response in this investigation we use either a set of dibasic lipophilic molecules (containing covalently linked pyridine and tertiary amine groups) or combinations of separate, lipophilic pyridines and tertiary amines. The use of combinations of dibasic and monobasic lipophilic molecules also gives a window-shaped fluorescence response with changes in pH: it is the highest pyridine pK(a) and the lowest tertiary amine pK(a) that determine the window limits. The pK(a) values of all the examined lipophilic molecules were determined in micelles, and compared with the values found for the same molecules in solvent mixtures in which they are molecularly dispersed. The effect of micellization is to significantly lower the observed protonation constants of the lipophilized species. Moreover, the more lipophilic a molecule is, the lower the observed logK value is. Accordingly, changing the substituents on the basic moieties or modifying their structure, tuning the lipophilicity of the mono- or dibases, and choosing among a large set of possible combination of lipophilized mono- and dibases have allowed us to tune, almost at will, both the width and the position along the pH axis of the obtained fluorescent window.     

Complexity of "A-a" knob-hole fibrin interaction revealed by atomic force spectroscopy

During blood vessel injury, fibrinogen is converted to fibrin, a polymer that serves as the structural scaffold of a blood clot. The primary function of fibrin is to withstand the large shear forces in blood and provide mechanical stability to the clot, protecting the wound. Understanding the biophysical forces involved in maintaining fibrin structure is of great interest to the biomedical community. Previous reports have identified the "A-a" knob-hole interaction as the dominant force responsible for fibrin's structural integrity. Herein, biochemical force spectroscopy is used to study knob-hole interactions between fibrin fragments and variant fibrinogen molecules to identify the forces occurring between individual fibrin molecules. The rupture of the "A-a" knob-hole interaction results in a characteristic profile previously unreported in fibrin force spectroscopy with two distinct populations of specific forces: 110 +/- 34 and 224 +/- 31 pN. In the absence of a functional "A" knob or hole "a", these forces cease to exist. We propose that the characteristic pattern represents the deformation of the D region of fibrinogen prior to the rupture of the "A-a" knob-hole bond.     

Molecular "chaperones" guide the spontaneous formation of a 15-component hydrogen-bonded assembly

Chaperones are small molecules that assist in the folding of naturally occurring peptides. There are no examples of small molecules acting as chaperones in the self-assembly of synthetic noncovalent assemblies. In this communication we describe an unprecedented example of the "chaperone effect" in the noncovalent synthesis of organic nanostructures. Tetrarosette assemblies 2(3).(BuCYA)(12) form quantitatively in CHCl(3) at room temperature upon mixing tetramelamine 2 with N-butylcyanurate (BuCYA) in the presence of 5,5-diethylbarbituric acid (DEB). Without the DEB units present, only oligomeric assemblies are formed that cannot rearrange to the tetrarosettes by themselves. The DEB units act as molecular "chaperones" by preorganizing the tetramelamine units for the spontaneous assembly of the tetrarosette structure.     

Towards copper-free nanocapsules obtained by orthogonal interfacial "click" polymerization in miniemulsion

A facile method to produce nanocapsules by copper-free interfacial "click"-polymerization as orthogonal reaction for the encapsulation of functional molecules is successfully performed using stable miniemulsion droplets. Difunctional azides and alkynes have been used for polymerization around the miniemulsion droplets, leading to the formation of nanocapsules. The results were compared with copper-catalyzed systems.     

Single-molecule force spectroscopy measurements of "hydrophobic bond" between tethered hexadecane molecules

The hydrophobic effect is important for many biological and technological processes. Despite progress in theory, experimental data clarifying water structure and the interaction between hydrophobic solutes at the nanometer scale are scarce due to the very low solubility of hydrophobic species. This article describes an AFM single molecule force spectroscopy method to probe the interaction between molecules with low solubility and reports measurements of the strength and the length scale of the "hydrophobic bond" between hexadecane molecules. Hexadecane molecules are tethered by flexible poly(ethylene glycol) linkers to AFM probes and substrates, removing the aggregation state uncertainty of solution-based approaches as well as spurious surface effects. A shorter hydrophilic polymer layer is added to increase the accessibility of hydrophobic molecules for the force spectroscopy measurements. Statistical analysis of the rupture forces yields a barrier width of 0.24 nm, and a dissociation rate of 1.1 s(-1). The results of single molecule measurements are related to the theoretical predictions of the free energy of cavitation in water and to the empirical model of micellization of nonionic surfactants. It is estimated that approximately one-quarter of each molecule's surface is hydrated during forced dissociation, consistent with an extended (nonglobular) conformation of the hexadecane molecules in the dimer.     

"Non-evaporating" microdroplets on self-assembled monolayer surfaces under ambient conditions

A unique "non-evaporation" phenomenon, i.e., the unusually slow evaporation process of sessile microdroplets on self-assembled monolayer (SAM) surfaces, is reported. It has been observed that only droplets containing a certain proportion of a volatile and a less-volatile component undergo non-evaporation, which is characterized by hours-long existence of the droplets maintaining constant contact angle, contact area, and volume. We propose that for alcohol-water binary mixtures on SAM surfaces, the highly orientated and closely packed hydrophobic 1-decanethiol molecules induce a concentration gradient of alcohol in water, with a higher concentration of alcohol near the SAM surface. Initial evaporation of the alcohol (more volatile) increased the contact angle until the establishment of a new composition, which contains a strong hydrogen-bonding network among the water molecules in the presence of the alcohol alkyl chains. There is a lessened tendency for the alcohol to evaporate in the presence of a concentration gradient due to such interactions, which results in the observed "non-evaporating" phenomenon. This type of unusual evaporating profile was not observed on conventional substrates, such as polycarbonate sheets and microscope glass slides modified with alkyltrichlorosilanes.     

An iterative route to "decorated" ethylene glycol-based linkers

Iterative copper-catalyzed cycloadditions of azides to alkynes were used to join functionalized triethylene glycol molecules to give "linkers" of defined lengths equipped with several different end-group functionalities.     

Supramolecular pattern of fullerene on 2D bimolecular "chessboard" consisting of bottom-up assembly of porphyrin and phthalocyanine molecules

Two-component adlayers consisting of zinc(II) phthalocyanine (ZnPc) and a metalloporphyrin, such as zinc(II) octaethylporphyrin (ZnOEP) or zinc(II) tetraphenylporphyrin (ZnTPP), were prepared by immersing either an Au(111) or Au(100) substrate in a benzene solution containing those molecules. The bimolecular adlayers thus prepared were investigated in 0.1 M HClO4 by cyclic voltammetry (CV) and electrochemical scanning tunneling microscopy (EC-STM). A supramolecularly organized "chessboard" structure was formed for the ZnPc and ZnOEP bimolecular array on Au(111), while characteristic nanohexagons were found in the ZnTPP and ZnOEP bimolecular adlayer. EC-STM revealed that the surface mobility and the molecular re-organization of ZnPc and ZnOEP on Au(111) were tunable by manipulating the electrode potential, whereas the ZnTPP and ZnOEP bimolecular array was independent of the electrode potential. A "bottom-up" hybrid assembly of fullerene molecules was formed successfully on an alternate array of bimolecular ZnPc and ZnOEP molecules. The bimolecular "chessboard" served as a template to form the supramolecular assembly of C60 by selective trapping in the open spaces. A supramolecular organization of ZnPc and ZnOEP was also found on the reconstructed Au(100)-(hex) surface. A highly ordered, compositionally disordered but alternate array of ZnPc and ZnOEP was formed on the reconstructed Au(100)-(hex) surface, indicating that the bimolecular adlayer structure is dependent on the atomic arrangement of underlying Au in the formation of supramolecular nanostructures composed of those molecules. On the bimolecular array consisting of ZnPc and ZnOEP on the Au(100)-(hex), no highly ordered supramolecular assembly of C60 was found, suggesting that the supramolecular assembly of C60 molecules is strongly dependent upon the bimolecular packing arrangement of ZnPc and ZnOEP.     

The "JK-only" approximation in density matrix functional and wave function theory

Various energy functionals applying the "JK-only" approximation which leads to two-index two-electron integrals instead of four-index two-electron integrals in the electron-electron interaction term of the electronic energy are presented. Numerical results of multiconfiguration self-consistent field calculations for the best possible "JK-only" wave function are compared to those obtained from the pair excitation multiconfiguration self-consistent (PEMCSCF) method and two versions of density matrix functional theory. One of these is derived making explicit use of some necessary conditions for N representability of the second-order density matrix. It is shown that this method models the energy functional based on the best possible "JK-only" wave function with good accuracy. The calculations also indicate that only a minor fraction of the total correlation energy is incorporated by "JK-only" approaches for larger molecules.     

Crystalline-state guest-exchange and gas-adsorption phenomenon for a "soft" supramolecular porous framework stacking by a rigid linear coordination polymer

A 1D rigid, linear coordination polymer, (4,4'-bipyridine)(2-pyridylsulfonate)copper, has been applied for the controlled-assembly of a new porous host that generates 1D channels by an interdigitated packing through the recognition of hydrogen bonding and pi-pi stacking interactions. The porous structure is architecturally robust when it reversibly uptakes water molecules and exchanges guest small molecules (MeOH, i-PrOH) from solution as determined by single-crystal-to-single-crystal transformation studies. Moreover, the open-channel solid displays irreversible benzene and toluene vapor sorption behaviors attributed to a widening of the channel cross-section that fetters the larger guest molecules, resulting from the dynamic, "soft" supramolecular framework.     

“推-拉”型巯基偶氮苯衍生物的合成

分子电子器件的研究是近期基础研究中受到极大关注的前沿热点[1-4]。带巯基的有机分子在金电极表面形成的自组装单分子膜(SAMs)及其电活性,是分子电子学领域具有代表性和研究最多的体系之一[5-13]。由于结构的特殊性和特征的光致异构化,偶氮苯衍生物具有对光和电的双重活性[14]     

Deformation of a "rigid" molecule in self-assembled nanostructures

A simple spirobifluorene molecule with pseudotetrahedral structure was investigated for its supposed conformational resilience upon adsorption. Through deposition at room temperature of this molecule on a Cu(111) surface and subsequent observation at 5 K with an ultrahigh vacuum scanning tunneling microscope, this "rigidity" upon physisorption is confirmed. However, an unexpected chemisorbed state was also found with the molecules arranged in trimers. The unique coexistence of physisorbed and chemisorbed states on the same substrate is thus demonstrated at the early stage of self-assembly.     

A programmable "build-couple" approach to the synthesis of heterofunctionalized polyvalent molecules

A maximally divergent "build-couple" synthesis of heterofunctionalized polyvalent molecules is described. This strategic approach enables the synthesis of highly diverse polyvalent structures from a pre-programmed combinatorial set of modules.     

Trace of the interesting "V"-shaped dynamic mechanism of interactions between water and ionic liquids

The mixture of ionic liquid (1-butyl-3-methylimidazolium tetrafluoroborate, bmimBF4) and water (2.5%, molar fraction) under isothermal conditions at 80 degrees C was investigated by FTIR spectroscopy and two-dimensional correlation infrared spectroscopy (2D-IR) methods. Three regions were focused: the OH stretching band of water (3755-3300 cm (-1)), the stretching band of CH on the imidazole ring (3300-3020 cm (-1)), and the BF stretching band of anions (1310-1260 cm (-1)). During this process, water was gradually evaporated as time passed, which produced influences on the interactions among cations, anions, and water molecules. In the FTIR analysis, we found an interesting "V"-shaped changing trend in peak areas of the C-H on the imidazole ring and the B-F stretching band; the inflection of the system was 913 s, gained through the "moving window" method. A two-step variation was accordingly found during this process. Hydrogen bonds formed by water molecules with cations or water molecules with anions were destroyed by the reduction of water, making a fall in the former period of "V" process, while electrostatic interactions newly formed between anions and cations leading to a rise during the latter period of this course. In this paper, various conformations formed among cations, anions, and water molecules were clearly assigned, and we managed to trace the whole dynamic mechanism of this isothermal process by 2D-IR techniques.