Total reflection X-ray fluorescence study of organic nanostructures

X-ray scattering techniques based on total external fluorescence study and development of X-ray standing wave method are presented and used for characterization of organic nanostructure Langmuir–Blodgett films of fatty acid salts and phospholipids. Spectral selectivity of data obtained allows us to detect structure position of different ions in organic systems, to obtain information about interdiffusion at the interfaces, about ion permeation through organic bilayers, thus allowing us to develop models of biomembranes.     

Modulation of protein-protein interactions with small organic molecules

Many proteins exert their biological roles as components of complexes, and the functions of proteins are often determined by their specific interactions with other proteins. Because of the central importance of protein-protein interactions for cellular processes, the ability to interfere with specific protein-protein interactions provides a powerful means of influencing the function of selected proteins within the cell. Cell-permeable small organic modulators of protein-protein interactions are thus highly desirable tools both for the study of physiological cellular processes and for the treatment of numerous diseased states. Herein a number of protein-protein interactions that are considered to be pharmaceutical targets are presented, which will familiarize the reader with the strategies that have been employed for the successful identification of small molecule modulators of these protein-protein interactions. These encouraging examples suggest that combined research efforts in the areas of functional proteomics, assay development, and organic synthesis will open up novel possibilities for the treatment of human diseases in the future.     

Power-law blinking in the fluorescence of single organic molecules.

The blinking behavior of perylene diïmide molecules is investigated at the single‐molecule level. We observe long‐time scale blinking of individual multi‐chromophoric complexes embedded in a poly(methylmethacrylate) matrix, as well as for the monomeric dye absorbed on a glass substrate at ambient conditions. In both these different systems, the blinking of single molecules is found to obey analogous power‐law statistics for both the on and off periods. The observed range for single‐molecular power‐law blinking extends over the full experimental time window, covering four orders of magnitude in time and six orders of magnitude in probability density. From molecule to molecule, we observe a large spread in off‐time power‐law exponents. The distributions of off‐exponents in both systems are markedly different whereas both on‐exponent distributions appear similar. Our results are consistent with models that ascribe the power‐law behavior to charge separation and (environment‐dependent) recombination by electron tunneling to a dynamic distribution of charge acceptors. As a consequence of power‐law statistics, single molecule properties like the total number of emitted photons display non‐ergodicity.     

Theoretical analysis of electron transport through organic molecules

We present a theoretical study of electron transport through a variety of organic molecules. The analysis uses the Landauer formalism in combination with complex bandstructure and projected densities of states calculations to reveal the main aspects of coherent electronic transport through alkanes, benzene-dithiol, and phenylene-ethynylene oligomers. We examine the dependence of the current on molecule length, the effects of molecule-molecule interactions from film packing, differences in contact geometry, and the influence of phenyl ring rotation on the conductances of phenylene-ethynylene oligomers such as 1,4-bis-phenylethynyl-benzene.     

Self-assembled nanostructures of oligopyridine molecules

The high potential of self-assembly processes of molecular building blocks is reflected in the vast variety of different functional nanostructures reported in the literature. The constituting units must fulfill several requirements like synthetic accessibility, presence of functional groups for appropriate intermolecular interactions and depending on the type of self-assembly processsignificant chemical and thermal stability. It is shown that oligopyridines are versatile building blocks for two- and three-dimensional (2D and 3D) self-assembly. They can be employed for building up different architectures like gridlike metal complexes in solution. By the appropriate tailoring of the heterocycles, further metal coordinating and/or hydrogen bonding capabilities to the heteroaromatic molecules can be added. Thus, the above-mentioned architectures can be extended in one-step processes to larger entities, or in a hierarchical fashion to infinite assemblies in the solid state, respectively. Besides the organizational properties of small molecules in solution, 2D assemblies on surfaces offer certain advantages over 3D arrays. By precise tailoring of the molecular structures, the intermolecular interactions can be fine-tuned expressed by a large variety of resulting 2D patterns. Oligopyridines prove to be ideal candidates for 2D assemblies on graphite and metal sufaces, respectively, expressing highly ordered structures. A slight structural variation in the periphery of the molecules leads to strongly changed 2D packing motifs based on weak hydrogen bonding interactions. Such 2D assemblies can be exploited for building up host-guest networks which are attractive candidates for manipulation experiments on the single-molecule level. Thus, "erasing" and "writing" processes by the scanning tunneling microscopy (STM) tip at the liquid/solid interface are shown. The 2D networks are also employed for performing coordination chemistry experiments at surfaces.     

One-pot tethering of organic molecules through non-symmetric malonate derivatives

A new method for one-pot chemoselective heterobifunctional cross-linking of organic molecules is described. The method is based on tert-butyldiphenylsilyl malonate and involves two sequential carbodiimide couplings with two different molecules possessing a hydroxy or an amino functionality with one intermediate one-pot fluoride deprotection.     

High-order multiphoton fluorescence of organic molecules in solution by intense femtosecond laser pulses

The five and possibly seven-photon fluorescence was observed for organic molecules in solution for the first time. A high-intensity laser enabled us to measure the properties of the high and any-order processes, and the emission could be directly visualized by the eye. These results showed that the common two-photon microscope could be upgraded to the higher order multiphoton microscope by choosing suitable excitation wavelengths. The multiphoton absorption cross sections differed by a factor of 10(33) as the order of the multiphoton process increased.     

Transport and separation of small organic molecules through nanotubules.

An electroless plating method was applied to deposit Au onto the surfaces and the walls of pores of polycarbonate membranes to prepare gold nanotubules. The nanotubules were modified with cysteine (Cys) or with carbamidine thiocyante (Gua). The effects of modifiers and of the fine structure of organic molecules on the transport properties of those molecules through the gold nanotubules were investigated. Studies show that the hydrophilicity of modifiers and the planar structure of permeating molecules clearly affect the transport of small organic molecules in gold nanotubules. Tryptophan (Try) and vitamin B(2) (VB(2)) was cleanly separated at pH 6.8.     

Molecular interactions in one-dimensional organic nanostructures

Intermolecular interactions involving pi-pi interaction and hydrogen bonding are used to create one-dimensional molecular nanostructures of hexasubstituted aromatics. Site-selective steady state fluorescence, time-resolved fluorescence, scanning electron microscopy, and atomic force microscopy measurements detail the intermolecular interactions that drive the aromatic molecules to self-assemble in solution to form well-ordered columnar stacks. These nanostructures, formed in solution, vary in their number, size, and structure depending on the solvent used. In addition, our results indicate that the substituents/ side groups and the proper choice of the solvent can be used to tune the intermolecular interactions. The 1D stacks and their aggregates can be easily transferred by solution casting, thus allowing a simple preparation of molecular nanostructures on different surfaces.     

Modeling permeation of volatile organic molecules through reverse osmosis spiral-wound membranes

Reverse osmosis is an interesting process to eliminate organic solutes from distillery condensates before recycling them into the fermentation step. However, organic solutes transport phenomena through reverse osmosis membranes are specific. Rejection and sorption of five compounds were studied on a brackish water membrane. Acetic acid and 2,3-butanediol were not sorbed on the membrane while furfural and 2-phenylethanol presented strong sorption following the Langmuir pattern. These sorption effects coupled with solute molecular weight (MW) led to low rejections of acetic acid and furfural (30–60%) and high rejections of 2,3-butanediol and 2-phenylethanol (80–98%). With intermediate sorption and MW, butyric acid showed rejections between 70 and 80%. A modified solution-diffusion model was developed to take into account the sorption pattern and predict the concentration profile along the membrane on the retentate and permeate sides. Equilibrium properties were determined experimentally while transport properties were identified with data obtained from a synthetic condensate. This model was validated for various operating conditions with the synthetic and the industrial condensates. It was then used to simulate the influence of the recovery rate on the retentate and permeate concentrations. It showed the behavior differences between solutes with a linear sorption and solutes with a saturating sorption.     

Fluorescent organic nanoparticles of benzofuran-naphthyridine linked molecules: formation and fluorescence enhancement in aqueous media

[structure: see text] Ethynyl-linked benzofuran-naphthyridine compounds show high-yield fluorescence with solvatochromic properties. One of the compounds, ABAN, has successfully formed fluorescent organic nanoparticles (FONs), for which the photophysical properties such as the spectral features and intensity are remarkably different from those at the molecular level (solution) and in bulk material. The results are tentatively rationalized by the FONs inducing coplanarization of the benzofuran-naphthyridine molecule to extend its effective conjugation length and hence increase the oscillator strength.     

Dynamical mean-field theory for molecules and nanostructures

Dynamical mean-field theory (DMFT) has established itself as a reliable and well-controlled approximation to study correlation effects in bulk solids and also two-dimensional systems. In combination with standard density-functional theory (DFT), it has been successfully applied to study materials in which localized electronic states play an important role. It was recently shown that this approach can also be successfully applied to study correlation effects in nanostructures. Here, we provide some details on our recently proposed DFT+DMFT approach to study the magnetic properties of nanosystems [V. Turkowski, A. Kabir, N. Nayyar, and T. S. Rahman, J. Phys.: Condens. Matter 22, 462202 (2010)] and apply it to examine the magnetic properties of small FePt clusters. We demonstrate that DMFT produces meaningful results even for such small systems. For benchmarking and better comparison with results obtained using DFT+U, we also include the case of small Fe clusters. As in the case of bulk systems, the latter approach tends to overestimate correlation effects in nanostructures. Finally, we discuss possible ways to further improve the nano-DFT+DMFT approximation and to extend its application to molecules and nanoparticles on substrates and to nonequilibrium phenomena.     

Permeation flux of organic molecules through silica-surfactant nanochannels in a porous alumina membrane.

The permeation fluxes of phenol, benzene sulfonate (BS) and benzene disulfonate (BDS) through a porous anodic alumina membrane with the perpendicularly oriented silica-surfactant nanochannel assembly membrane (NAM) were measured in water-ethanol mixture media. The permeation flux depended on solute charges and on solvent composition. As the ethanol ratio increased, the fluxes of BS and BDS increased and the flux of phenol decreased. The results of extraction/elution experiments also depended on the solute charges and the solvent composition. Chromatographic experiments in n-hexane showed that dipole and hydrophobic interactions affect the retention of solutes. Permeation of the solute across the NAM in water-ethanol mixture is likely to be determined by various factors such as dipole interaction, hydrophobic interaction, solvation, and anion-exchange efficiencies.     

Rotational tunnelling in organic molecules

Experimental investigations of molecular tunnelling in symmetrical molecular groups using the techniques of neutron scattering are reviewed. Particular emphasis is laid upon the tunnelling spectra of methyl groups. The relationship between the dynamics of the rotors and their “static” molecular environment is discussed with examples from published work.     

Self-assembly of 1-D organic semiconductor nanostructures

This review focuses on the molecular design and self-assembly of a new class of crowded aromatics that form 1-D nanostructures via hydrogen bonding and pi-pi interactions. These molecules have a permanent dipole moment that sums as the subunits self assemble into molecular stacks. The assembly of these molecular stacks can be directed with electric fields. Depending on the nature of the side-chains, molecules can obtain the face-on or edge-on orientation upon the deposition onto a surface via spin cast technique. Site-selective steady state fluorescence, time-resolved fluorescence, and various types of scanning probe microscopy measurements detail the intermolecular interactions that drive the aromatic molecules to self-assemble in solution to form well-ordered columnar stacks. These nanostructures, formed in solution, vary in their number, size, and structure depending on the functional groups, solvent, and concentration used. Thus, the substituents/side-groups and the proper choice of the solvent can be used to tune the intermolecular interactions. The 1-D stacks and their aggregates can be easily transferred by solution casting, thus allowing a simple preparation of molecular nanostructures on different surfaces.     

Saturable absorption dynamics in the triplet system and triplet excitation induced singlet fluorescence of some organic molecules

The triplet saturable absorption behaviour of the xanthene dyes eosin Y, erythrosin B, and rose bengal and of the fullerene molecule C₇₀ is studied. The molecules are excited to the S₁-state by intense picosecond pulses (wavelength λ_P=527 nm). They relax dominantly to the triplet system by intersystem crossing. The triplet–triplet saturable absorption is investigated with time-delayed intense picosecond pulses (wavelength λ_L=1054 nm) in the transparency region of the molecules in the singlet ground state. Higher excited-state triplet absorption cross-sections and higher excited-state triplet relaxation times are determined by numerical simulation of the experimental results. Time-resolved fluorescence measurements reveal higher excited-state triplet to singlet back-intersystem-crossing and multi-step triplet photoionization. Additionally the two-photon absorption cross-sections at λ_L=1054 nm are determined by measurement of the fundamental pulse two-photon induced fluorescence relative to the second-harmonic pulse single-photon induced fluorescence.     

X-ray fluorescence investigation of ordered intermetallic phases as electrocatalysts towards the oxidation of small organic molecules

The composition of ordered intermetallic nanoparticles (PtBi and PtPb) has been quantitatively studied by in situ X-ray fluorescence (XRF) during active electrochemical control in solutions of supporting electrolyte and small organic molecules (SOMs). Because the Pt L(β1,2) lines and the Bi L(α1,2) lines are only separated by 200 eV, an energy-dispersive detector and a multiple-channel analyzer (MCA) were used to record the major fluorescent emission lines from these two elements. The molar ratios of platinum to the less-noble elements (Bi, Pb) in the nanoparticles dramatically changed as a function of the applied upper limit potentials (E(ulp)) in cyclic voltammetric (CV) characterization. Similar to previous investigations for bulk intermetallic surfaces, the less-noble elements leached out from the surfaces of the intermetallic nanoparticles. For PtBi nanoparticles, the ratios of fluorescence intensities of Pt/Bi in the samples were 0.42, 0.96, and 1.36 for E(ulp)=+0.40, +0.80, and 1.20 V, respectively, while cycling the potential from -0.20 V to the E(ulp) value for 10 cycles. The leaching-out process of the less-noble elements occurred at more negative E(ulp) values than expected. After cycling to relatively positive E(ulp) values, nonuniform PtM (M=Bi of Pb) nanoparticles formed with a Pt-rich shell and intermetallic PtM core. When the supporting solutions contained active fuel molecules in addition to the intermetallic nanoparticles (formic acid for PtBi, formic acid and methanol for PtPb), kinetic stabilization effects were observed for E(ulp)=+0.80 V, in a way similar to the response of the bulk materials. It was of great importance to quantitatively explore the change in composition and structure of the intermetallic nanoparticles under active electrochemical control. More importantly, this approach represents a simple, universal, and multifunctional method for the study of multi-element nanoparticles as electrocatalysts. This is, to our knowledge, the first report of nondestructive, quantitative characterization of bimetallic or multi-elemental nanoparticles electrocatalysts under active electrochemical control.