Showing papers in "ChemPhysChem in 2003"
TL;DR: A new powerful tool to study the electron lifetime in dye solar cells as a function of the photovoltage (Voc) ; the open-circuit voltage-decay (OCVD) technique is developed and has certain advantages over frequenIn summary, the temperature effect on the arrangement of stilbenoid dendrimers on HOPG is presented in this work.
Abstract: Recently, a new class of photoelectrochemical cells based on nanoscaled porous metal oxide semiconductors (dye-sensitized solar cell) has promoted intense research due to the prospects of cheap and efficient conversion of visible light into electricity and of new applications such as transparent solar cells. It is widely agreed that the electron-transfer kinetics play a major role in determining the energy conversion efficiency of dye-sensitized solar cells. 3] Herein, we develop a new powerful tool to study the electron lifetime in dye solar cells as a function of the photovoltage (Voc) ; the open-circuit voltage-decay (OCVD) technique. This technique has certain advantages over frequenIn summary, the temperature effect on the arrangement of stilbenoid dendrimers on HOPG is presented in this work. It is seen that SD12 molecules form well-ordered hexagonal nanostructures at 16 C. However, if the adlayer is annealed at 65 C, the adlayer structure is changed into a well-ordered parallelogram nanostructure in a close-packed arrangement with a higher surface coverage. The phenomenon described here supports the earlier reports on two liquid-crystalline phases for SD12. The results in this research are useful in understanding the phase transition of SD12 as well as metastable complex systems with temperature.
1,156 citations
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405 citations
TL;DR: The bulk of the review highlights recent applications of Raman spectroscopy, including structural investigations of a hormone receptor, testing the biocompatibility of dental implants, probing soil components and plant tissue alkaloids, and localization of single bacteria.
Abstract: Although the physics of Raman spectroscopy and its application to purely chemical problems is long established, it offers a noninvasive, nondestructive, and water-insensitive probe to problems in the life sciences. Starting from the principles of Raman spectroscopy, its advantages, and methods for signal enhancement, the bulk of the review highlights recent applications. Structural investigations of a hormone receptor, testing the biocompatibility of dental implants, probing soil components and plant tissue alkaloids, and localization of single bacteria are just four problems in which Raman spectroscopy offers a solution or complements existing methods.
347 citations
TL;DR: The theoretical and experimental background for many of the recent experiments treated in the literature are discussed, and examples from the own research are used to illustrate several fundamental and practical aspects in gas-phase as well as liquid-phase quantum control.
Abstract: Active control of chemical reactions on a microscopic (molecular) level, that is, the selective breaking or making of chemical bonds, is an old dream. However, conventional control agents used in chemical synthesis are macroscopic variables such as temperature, pressure or concentration, which gives no direct access to the quantum-mechanical reaction pathway. In quantum control, by contrast, molecular dynamics are guided with specifically designed light fields. Thus it is possible to efficiently and selectively reach user-defined reaction channels. In the last years, experimental techniques were developed by which many breakthroughs in this field were achieved. Femtosecond laser pulses are manipulated in so-called pulse shapers to generate electric field profiles which are specifically adapted to a given quantum system and control objective. The search for optimal fields is guided by an automated learning loop, which employs direct feedback from experimental output. Thereby quantum control over gas-phase as well as liquid-phase femtochemical processes has become possible. In this review, we first discuss the theoretical and experimental background for many of the recent experiments treated in the literature. Examples from our own research are then used to illustrate several fundamental and practical aspects in gas-phase as well as liquid-phase quantum control. Some additional technological applications and developments are also described, such as the automated optimization of the output from commercial femtosecond laser systems, or the control over the polarization state of light on an ultrashort timescale. The increasing number of successful implementations of adaptive learning techniques points at the great versatility of computer-guided optimization methods. The general approach to active control of light–matter interaction has also applications in many other areas of modern physics and related disciplines.
330 citations
TL;DR: This research--although far from being transferred into technology--is attracting interest, as the nanometer realm seems to be out of reach for the "top-down" techniques currently available to microelectronics industry.
Abstract: Miniaturization has been an essential ingredient in the outstanding progress of information technology over the past fifty years The next, perhaps ultimate, limit of miniaturization is that of molecules, which are the smallest entities with definite size, shape, and properties Recently, great effort has been devoted to design and investigate molecular-level systems that are capable of transferring, processing, and storing information in binary form Some of these nanoscale devices can, in fact, perform logic operations of remarkable complexity This research--although far from being transferred into technology--is attracting interest, as the nanometer realm seems to be out of reach for the "top-down" techniques currently available to microelectronics industry Moreover, such studies introduce new concepts in the "old" field of chemistry and stimulate the ingenuity of researchers engaged in the "bottom-up" approach to nanotechnology
240 citations
TL;DR: In this paper, two-photon absorption cross sections delta and solvatochromic properties were determined for a series of quadrupolar and dipolar compounds by using femtosecond excitation in the spectral range between 710 and 960 nm.
Abstract: Two-photon absorption cross sections delta and solvatochromic properties were determined for a series of quadrupolar and dipolar compounds by using femtosecond excitation in the spectral range between 710 and 960 nm. The compounds investigated were distyrylbenzenes and polyenes bearing appropriate pi or sigma acceptors. The delta values for the centrosymmetric compounds trans, trans- 1,4-bis[2-(2',5'-dihexyloxy)phenylethenyl]-2,3,5,6-tetrafluorobenzene (6), trans, trans-1,4-bis[2-(4'-dibutylamino)phenylethenyl]- 2,3,5,6-tetrafluorobenzene (2), trans, trans-1,4-bis[2-(4'dimethylamino)phenylbutadienyl]- 2,3,5,6-tetrafluorobenzene (7), trans,-trans-1,4-bis[2-(4'-dimethylamino)phenylethenyl]2,5- dicyanobenzene (4) and trans,trans-1,4-bis[2-(4'-dimethylamino)phenylethenyl]-2- propylsulfonyl-5-(2-ethylhexyl)sulfonylbenzene (3) are on the order of 600, 1400, 1700, 3000, and 4100 x 10(-50) cm4 s photon-1, respectively. The corresponding dipolar compounds trans-2-(4'- dimethylaminophenyl)ethenyl-2,3,4,5,6-pentafluorobene (8), trans-4-(4'-dimethylaminophenyl)butadienyl-2,3,4,5,6-pentafluorobenzene (9), trans-6-(4'-dimethylaminophenyl)hexatrienyl-2,3,4,5,6- pentafluorobenzene (10) were additionally investigated. All centrosymmtric compounds are good fluorescent materials, while the dipolar chromophores 8-10 exhibit low fluorescence quantum yields. Solvatochromism was also observed for the fluorophores 2-10 as a result of intramolecular charge transfer (ICT). Furthermore, a reasonable correlation was obtained between measured and calculated delta. Quantum chemical calculations were performed by using the INDO Hamiltonian with a MRDCI scheme. The results show that the sum over states (SOS) expression for the second hyperpolarizability gamma is appropriate to describe the mechanism of two-photon absorption. Mechanistic investigations of quadrupolar compounds showed that the energy of the two-photon excited state is higher than S1.
200 citations
TL;DR: The polarization-dependent SERS from isolated aggregates of silver nanoparticles characterized by scanning electron microscopy (SEM) is found to be consistent with electrodynamic theory and strongly support the idea that nanogaps are a key ingredient of ultrasensitive SERS analysis.
Abstract: Surface-enhanced Raman scattering (SERS) is a vibrational spectroscopy technique based on surface plasmon enhanced optical interactions at noble-metal nanostructures. With a detection limit at the single-molecule level and a power to provide structural information through the Raman TMvibrational fingerprint∫, SERS has a unique potential for ultrasensitive molecular identification and analysis. A recent example is the detection of single-DNA and RNA strands labeled by SERS-active dye molecules. Nanostructured substrates for SERS are expected to be anisotropic in terms of the local surface plasmon resonances, which imply that the enhancement factor should extrinsically depend on the incident polarization. This effect is expected to be particularly strong for nanometric gaps between nanoparticles, which have been implicated as the most likely sites for single-molecule SERS. 7] However, most SERS studies have been performed at fixed polarization configuration and/or using macroscopic sampling volumes, for which a detailed comparison between local morphology, SERS activity and theory is difficult. Herein, we report on the polarization-dependent SERS from isolated aggregates of silver nanoparticles characterized by scanning electron microscopy (SEM). The results are found to be consistent with electrodynamic theory and strongly support the idea that nanogaps are a key ingredient of ultrasensitive SERS analysis. SERS-active structures were prepared by aggregating colloidal Ag particles (35 pM) through incubation with excess hemoglobin (5 nM), followed by immobilization on polymer-coated Si substrates. The role of the protein is twofold: to induce nanogaps through aggregation and to uniformly cover the nanoparticles with SERS-active species. The SERS structures soformed were well separated and allowed for polarizationdependent Raman studies of isolated nanoparticle aggregates using a Renishaw 2000 micro-Raman setup operated in either spectroscopy or imaging mode. Figure 1 gives examples of polarization-dependent SERS from two Ag dimers shown in the inset. As reported elsewhere, 8, 9] [13] DFT computations indicate that the TMligand band∫ lies below the TMmetal band∫ in silver fluorides; for example for AgF the energy difference is some 3 eV, similar to the experimental value of 2.5 eV. [14] J. J. Yeh, I. Lindau, Atomic Data and Nuclear Data Tables 1985, 32, 1. [15] The ionization cross-section of the F(2s) electrons is one order of magnitude smaller than that for the Ag(4d) ones, and the contribution from the former to the valence band is small, according to the calculations . [16] U. Gelius, in Electron Spectroscopy (Ed. D. Shirley) North Holland, Amsterdam, 1972. [17] This band is centered at 3.68 eV, and has a small half width of 0.74 eV and integrated intensity of 7% of the whole valence band. It has been erroneously assigned either to valence orbitals baring some O(2p) character. The same peak shows at 3.78 eV (half width of 0.70 eV, integrated intensity of 7% of the whole valence band) in the spectrum of comm-AgF2, and it certainly belongs to AgF (comm-AgF2 is partially reduced on the surface). [18] The TMligand band∫ progressively shifts to lower binding energies in the order AgF AgCl AgBr AgI, and simultaneously gains intensity, thus reflecting the increasing energy of the np orbitals of the nonmetal and their stronger mixing with the Ag(4d) orbitals. [19] A. Goldmann, J. Tejeda, N. J. Shevchik, M. Cardona, Phys. Rev. B 1974, 10, 4388. [20] J. Tejeda, N. J. Shevchik, W. Braun, A. Goldmann, M. Cardona, Phys. Rev. B 1975, 12, 1557. [21] R. Matzdorf, A. Goldmann, J. Electron Spectrosc. Relat. Phenom. 1993, 63, 167. [22] Note, also that the dispersion of the TMligand band∫ usually increases with the increase of the silver oxidation state (0.79 eV for Ag(I), 1.56 eV for KAgF3, and 1.71 ± 1.73 eV for AgF2 and KAgF4), due to the decreasing (shortest) F ± F distance in these compounds (3.493 ä, 3.026 ä, 2.843 ä, and 2.639 ä, respectively). [23] The values listed in Table 6 of ref.[4] were normalized so that the sum of contributions is 100%. Contributions from all silver valence orbitals (4d 5s 5p) was in fact taken in ref.[2] . However the contribution from Ag(5s 5p) states to the valence band is computed to be small and can be neglected in the comparison that follows. [24] In fact, the computation was for CsAgF3 and not for KAgF3, but it should not have significant impact on the quantitative comparison. [25] The corresponding value for the comm-AgF2 is 70:30, in between the values for AgF and for the freshly prepared AgF2; it suggests that commAgF2 is partially reduced, at least on the surface. [26] Thus it proves not to be only a (computational) artifact of the choice of the Weigner ± Seitz radii. [27] More detailed considerations should involve the difference of the lattice energies of substrate and products. [28] KAgF3 studied here, proves to be metallic : W. Grochala, P. P. Edwards, unpublished results. [29] http://www.dl.ac.uk/RUSTI/xps/esca300.htm [30] Practical Surface Analysis (Ed. D. Briggs, M. P. Seah), Wiley, Chichester 1990. [31] B. Zœemva, R. Hagiwara, W. J. Casteel, Jr. , K. Lutar, A. Jesih, N. Bartlett J. Am. Chem. Soc. 1990, 112, 4846. AgF2 was prepared by the reaction between AgNO3 and elemental fluorine at 250 C. [32] R. H. Odenthal, R. Hoppe, Monatsh. Chem. 1971, 102, 1340. The procedure was modified by use of AgF2 instead of AgF and F2 as substrates. [33] K. Lutar, S. Milic¬ev, B. Zœemva, B. G. Mueller, B. Bachmann, R. Hoppe, Eur. J. Solid State Inorg. Chem. 1991, 28, 1335. KAgF4 was prepared by the reaction between AgF2, KF and KrF2 in anhydrous HF as a solvent.
194 citations
TL;DR: These novel approaches, which are mostly computer simulations and approximate integral equation theories based on the so-called primitive model, have scarcely been applied in practice, and a critical survey on the relevance of ion size correlation in real systems is included.
Abstract: A broad range of manufactured products and biological fluids are colloids. The ability to understand and control the processes (of scientific, technological and industrial interest) in which such colloids are involved relies upon a precise knowledge of the electrical double layer. The traditional approach to describing this ion cloud around colloidal particles has been the Gouy-Chapman model, developed on the basis of the Poisson-Boltzmann equation. Since the early 1980s, however, more sophisticated theoretical treatment have revealed both quantitative and qualitative deficiencies in the Poisson-Boltzmann theory, particularly at high ionic strengths and/or high surface charge densities. This review deals with these novel approaches, which are mostly computer simulations and approximate integral equation theories based on the so-called primitive model. Special attention is paid to phenomena that cannot, be accounted for by the classic theory as a result of neglecting ion size correlations, such as overcharging, namely, the counterion concentration in the immediate neighborhood of the surface is so large that the particle surface is overcompensated. Other illustrative examples are the nonmonotonic behavior of the electrostatic potential and attractive interactions between equally charged surfaces. These predictions are certainly remarkable and, on paper, they can have an effect on experimentally measurable quantities (for instance, electrophoretic mobility). Even so, these new approaches have scarcely been applied in practice. Thus a critical survey on the relevance of ion size correlation in real systems is also included. Overcharging of macroions can also be brought about by adsorption of oppositely charged polyelectrolytes. Noteworthy examples and theoretical approaches for their are also briefly reviewed.
187 citations
TL;DR: The PtBi electrode exhibits superior properties when compared to polycrystalline platinum in terms of oxidation onset potential, current density, and a much diminished poisoning effect by CO.
Abstract: The electrocatalytic oxidation of formic acid at a PtBi ordered intermetallic electrode surface has been investigated using cyclic voltammetry, rotating disk electrode (RDE) voltammetry and differential electrochemical mass spectrometry (DEMS). The results are compared to those at a polycrystalline platinum electrode surface. The PtBi electrode exhibits superior properties when compared to polycrystalline platinum in terms of oxidation onset potential, current density, and a much diminished poisoning effect by CO. Using the RDE technique, a value of 1.4 x 10(-4) cm s-1 was obtained for the heterogeneous charge transfer rate constant. The PtBi surface did not appear to be poisoned when exposed to a CO saturated solution for periods exceeding 0.5 h. The results for PtBi are discussed within the framework of the dual-path mechanism for the electrocatalytic oxidation of formic acid, which involves formation of a reactive intermediate and a poisoning pathway.
TL;DR: Covalent sidewall functionalization of SWCNTs using addition chemistry has recently started to develop and theoretical and computational support is urgently needed to achieve control over chemoand regioselectivity of such additions.
Abstract: Covalent sidewall functionalization of SWCNTs using addition chemistry has recently started to develop. This approach is considered to be a very promising method for nanotube derivatization, since it leads to improved solubilities as well as processibilites and at the same time allows for the combination of the unique properties of SWCNTs with those of other compound classes. However, it is very difficult to achieve control over chemoand regioselectivity of such additions, which requires very 'hot' addends and drastic reaction conditions. This is one of the reasons why it is very difficult to characterize prepared functionalized SWCNTs as such. In particular, all experimental attempts to determine the precise location and mode of addition have failed to date. As a consequence, theoretical and computational support is urgently needed. The analysis of the principles of fullerene chemistry has shown that the reactivity of these spherical molecules towards addition reactions depends strongly on the curvature of their carbon framework. Among the stable fullerenes C60 is the smallest and as a consequence has the highest curvature. Because of the resulting pronounced pyramidalization of the C atoms within the rigid carbon framework, the outer convex surface is very susceptible towards addition reactions. 4] On the other hand, the inner concave surface of C60 is inert towards endohedral addition reactions. This is demonstrated, for example, by the remarkable stability of N@C60, in which a free nitrogen atom is encapsulated by the fullerene cage. In case of C60, the two sample. The probe light was provided by a 100 W tungsten lamp with wavelength selection being achieved by monochromators before and after the sample. Changes in optical density induced by the excitation pulse were monitored with a silicon photodiode and a home-built amplification/filtering system and digitised by a tetronix TDS220 digital storage oscilloscope.
TL;DR: After a brief review of the development of the high-speed AFM developed by the research group at Kanazawa University, its recent advancement and imaging data obtained with motor proteins are presented.
Abstract: The atomic force microscope (AFM), which was invented by Binnig et al. in 1986, can image at nanometer resolution individual biological macromolecules on a substrate in solution. This unique capability awoke an expectation of imaging processes occurring in biological macromolecules at work. However, this expectation was not met, because the imaging rate with available AFMs was too low to capture biological processes. This expectation has at last been realized by the high-speed AFM developed by our research group at Kanazawa University. In this article, after a brief review of the development of our apparatus, its recent advancement and imaging data obtained with motor proteins are presented.
TL;DR: Two related methods for preparing arrays of nanowires composed of molybdenum, copper, nickel, gold, and palladium electrodeposited are described, which could be lifted off the graphite surface thereby facilitating the incorporation of metal nanowire arrays into devices such as sensors.
Abstract: We describe two related methods for preparing arrays of nanowires composed of molybdenum, copper, nickel, gold, and palladium. Nanowires were obtained by selectively electrodepositing either a metal oxide or a metal at the step edges present on the basal plane of highly oriented pyrolytic graphite (HOPG) electrodes. If a metal oxide was electrodeposited, then nanowires of the parent metal were obtained by reduction at elevated temperature in hydrogen. The resulting nanowires were organized in parallel arrays of 100-1000 wires. These nanowires were long (some > 500 microns), polycrystalline, and approximately hemicylindrical in cross-section. The nanowire arrays prepared by electrodeposition were also "portable": After embedding the nanowires in a polymer or cyanoacrylate film, arrays of nanowires could be lifted off the graphite surface thereby facilitating the incorporation of metal nanowire arrays into devices such as sensors.
TL;DR: The observed dynamic features of the [BMIM]+ cation confirm quantum-chemical structures obtained in a former study, and were described by a Cole-Davidson spectral density with a Vogel-Fulcher-Tammann temperature dependence of the correlation times.
Abstract: The reorientational dynamics of the ionic liquid 1-butyl-3-methyl-imidazolium hexafluorophosphate ([BMIM]PF 6 ) were studied over a wide range of temperatures by measurement of 13 C spin-lattice relaxation rates and NOE factors. The reorientational dynamics were evaluated by performing fits to the experimental relaxation data. Thus, the overall reorientational motion was described by a Cole - Davidson spectral density with a Vogel - Fulcher - Tammann temperature dependence of the correlation times. The reorientational motion of the butyl chain was modelled by a combination of the latter model for the overall motion with a Bloembergen - Purcell - Pound spectral density and an Arrhenius temperature dependence for the internal motion. Except for C2 in the aromatic ring, an additional reduction of the spectral density by the Lipari -Szabo model had to be employed. This reduction is a consequence of fast molecular motions before the rotational diffusion process becomes effective. The C2 atom did not exhibit this reduction, because the librational motion of the corresponding C2-H vector is severely hindered due to hydrogen bonding with the hexafluorophosphate anion. The observed dynamic features of the [BMIM] - cation confirm quantum-chemical structures obtained in a former study.
TL;DR: The results demonstrate that combined temperature--pressure-dependent studies can help delineate the free-energy landscape of proteins and hence help elucidate which features and thermodynamic parameters are essential in determining the stability of the native conformational state of proteins.
Abstract: We studied the thermodynamic stability of a small monomeric protein, staphylococcal nuclease (Snase), as a function of both temperature and pressure, and expressed it as a 3D free-energy surface on the p,T-plane using a second-order Taylor expansion of the Gibbs free-energy change delta G upon unfolding. We took advantage of a series of different techniques (small-angle X-ray scattering, Fourier-transform infrared spectroscopy, differential thermal analysis, pressure perturbation calorimetry and densitometry) in the evaluation of the conformation of the protein and in evaluating the changes in the thermodynamic parameters upon unfolding, such as the heat capacity, enthalpy, entropy, volume, isothermal compressibility and expansivity. The calculated results of the free-energy landscape of the protein are in good agreement with experimental data of the p,T-stability diagram of the protein over a temperature range from 200 to 400 K and at pressures from ambient pressure to 4000 bar. The results demonstrate that combined temperature--pressure-dependent studies can help delineate the free-energy landscape of proteins and hence help elucidate which features and thermodynamic parameters are essential in determining the stability of the native conformational state of proteins. The approach presented may also be used for studying other systems with so-called re-entrant or Tamman loop-shaped phase diagrams.
TL;DR: Spectroscopic, computational, redox, and photochemical behavior of a self-assembled donor-acceptor dyad formed by axial coordination of zinc naphthalocyanine, ZnNc, and fulleropyrrolidine bearing an imidazole coordinating ligand was investigated in toluene and o-dichlorobenzene, and the results were compared to the intermolecular electron transfer processes in a coordinating solvent, benzonitrile.
Abstract: Spectroscopic, computational, redox, and photochemical behavior of a self-assembled donor-acceptor dyad formed by axial coordination of zinc naphthalocyanine, ZnNc, and fulleropyrrolidine bearing an imidazole coordinating ligand (2-(4′-imidazolylphenyl)fulleropyrrolidine, C60Im) was investigated in noncoordinating solvents, toluene and o-dichlorobenzene, and the results were compared to the intermolecular electron transfer processes in a coordinating solvent, benzonitrile. The optical absorption and ab initio B3 LYP/3–21G(*) computational studies revealed self-assembled supramolecular 1:1 dyad formation between the ZnNc and C60Im entities. In the optimized structure, the HOMO was found to be entirely located on the ZnNc entity while the LUMO was found to be entirely on the fullerene entity. Cyclic voltammetry studies of the dyad exhibited a total of seven one-electron redox processes in o-dichlorobenzene, with 0.1 M tetrabutylammonium perchlorate. The excited-state electron-transfer processes were monitored by both optical-emission and transient-absorption techniques. Direct evidence for the radical-ion-pair (C60Im.−:ZnNc.+) formation was obtained from picosecond transient-absorption spectral studies, which indicated charge separation from the singlet-excited ZnNc to the C60Im moiety. The calculated rates of charge separation and charge recombination were 1.4×1010 s−1and 5.3×107 s−1in toluene and 8.9×109 s−1and 9.2×107 s−1in o-dichlorobenzene, respectively. In benzonitrile, intermolecular electron transfer from the excited triplet state of ZnNc to C60Im occurs and the second-order rate constant (kqtriplet) for this quenching process was 5.3×108 M−1 s−1.
TL;DR: The various techniques of SMS, such as confocal epifluorescence detection and wide-field imaging are presented and discussed, together with the different methods of data analysis such as fluorescence correlation spectroscopy and burst-by-burst analysis.
Abstract: This review presents an overview of the fluorescence detection and spectroscopy of single molecules (SMS) in liquids and on surfaces under ambient conditions. The various techniques of SMS, such as confocal epifluorescence detection and wide-field imaging are presented and discussed, together with the different methods of data analysis such as fluorescence correlation spectroscopy and burst-by-burst analysis. Selected applications of the various techniques in physics, chemistry, and biology are described.
TL;DR: The results demonstrate that for this system a purely classical description of the solvent is sufficient, since inclusion of the first solvent shell of 12 water molecules into the quantum system does not show a significant effect on this transition.
Abstract: The authors present a hybrid Car-Parrinello quantum mech./mol. mech. (QM/MM) approach that is capable of treating the dynamics of mol. systems in electronically excited states in complex environments. The potential energy surface in the excited state is described either within the restricted open-shell Kohn-Sham (ROKS) formalism or within time-dependent d. functional theory (TDDFT). As a test case, the authors apply this technique to the study of the solvent effects on the ground state and on the first excited singlet state of acetone in water. Results demonstrate that for this system a purely classical description of the solvent is sufficient, since inclusion of the first solvent shell of 12 water mols. into the quantum system does not show a significant effect on this transition. The excited-state energies calcd. with ROKS are red shifted by a const. value compared to the TDDFT results, while the relative variations of the excitation energy for different configurations are in very good agreement. The exptl. obsd. blue shift of the excitation energy in going from gas phase to condensed phase is well reproduced. Excited-state dynamics carried out with ROKS yield the relaxation of the solute and the rearrangement of the solvent structure on a picosecond timescale. The calcd. Stokes shift is in reasonable agreement with exptl. data. [on SciFinder (R)]
TL;DR: Temporary wetting of porous templates provides customized nanotubes and allows us to investigate how the wall curvature affects the structure formation.
Abstract: Nanotubes have an outstanding potential both for applications in nanotechnology and as the subject of basic research. Wetting of porous templates is a simple technique that overcomes many limitations of established preparation methods. It extends the range of processable materials, for example, by a broad range of multicomponent mixtures or by high-performance polymers such as poly(oxy-1,4-phenyleneoxy-1,4-phenylenecarbonyl-1,4-phenylene) (PEEK) and polytetrafluoroethylene (PTFE). Inducing controlled phase transitions generates a large specific surface, a specific nanoporosity, or oriented crystalline domains within the nanotube walls. Template wetting provides customized nanotubes and allows us to investigate how the wall curvature affects the structure formation.
TL;DR: Two-terminal molecular switch tunnel junctions (MSTJs) which incorporate a semiconducting, single-walled carbon nanotube (SWNT) as the bottom electrode are described which can be reconfigurably switched and repeatedly cycled between high and low current states under ambient conditions.
Abstract: This article describes two-terminal molecular switch tunnel junctions (MSTJs) which incorporate a semiconducting, single-walled carbon nanotube (SWNT) as the bottom electrode. The nanotube interacts noncovalently with a monolayer of bistable, nondegenerate [2]catenane tetracations, self-organized by their supporting amphiphilic dimyristoylphosphatidyl anions which shield the mechanically switchable tetracations from a two-micrometer wide metallic top electrode. The resulting 0.002 micron 2 area tunnel junction addresses a nanometer wide row of approximately 2000 molecules. Active and remnant current-voltage measurements demonstrated that these devices can be reconfigurably switched and repeatedly cycled between high and low current states under ambient conditions. Control compounds, including a degenerate [2]catenane, were explored in support of the mechanical origin of the switching signature. These SWNT-based MSTJs operate like previously reported silicon-based MSTJs, but differently from similar devices incorporating bottom metal electrodes. The relevance of these results with respect to the choice of electrode materials for molecular electronics devices is discussed.
TL;DR: The Watson-Crick A-T base pair is not the most stable base-pair structure at different levels of ab initio theory, and its vibrational spectrum is not in agreement with the observed experimental spectrum.
Abstract: The vibronic spectrum of the adenine ± thymine (A ± T) base pair was obtained byone-color resonant two-photon ionization (R2PI) spectroscopyin a free jet of thermallyevaporated A and T under conditions favorable for formation of small clusters. The onset of the spectrum at 35 064 cm 1 exhibits a large red shift relative to the ±* origin of 9H-adenine at 36 105 cm 1 . The IR ± UV spectrum was assigned to cluster structures with HNH ¥¥¥ OC/N ¥¥¥ HN hydrogen bonding bycomparison with the IR spectra of A and T monomers and with ab initio calculated vibrational spectra of the most stable A ± T isomers. The Watson ± Crick A ± T base pair is not the most stable base-pair structure at different levels of ab initio theory, and its vibrational spectrum is not in agreement with the observed experimental spectrum. Experiments with methylated A and T were performed to further support the structural assignment.
TL;DR: Recent trends in the characterization of minute quantities of DNA or protein samples requires highly integrated, automated, and miniaturized "total analysis systems" (mu-TAS) on nano- and picoliter scale.
Abstract: The rapid developments in biotechnology create a great demand for fluid handling systems on the nano- and picoliter scale. The characterization of minute quantities of DNA or protein samples requires highly integrated, automated, and miniaturized total analysis systems (-TAS). The small scales necessitate new concepts for devices both from a technological and from a fundamental physical point of view. Here, we describe recent trends in both areas. New technologies include soft lithography, chemical, and topographical structuring of surfaces in order to define pathways for liquids, as well as electrowetting for manipulation purposes. Fundamentally, the interplay between geometric confinement and the size of biological macromolecules gives rise to complex dynamic behavior. The combination of both fluorescence imaging and scattering techniques allows for detailed insight into the dynamics of individual molecules and into their self-assembly into supramolecular aggregates.
TL;DR: The regular and tunable pore size together with the large mesoporous volume and the nontoxic nature of amorphous silica have led the group to investigate the potential of these materials for the storage and release of drugs.
Abstract: Formulations that control drug release are now common and have become complex. Solubility, stability and toxicity of the drugs, and routes of administration are factors that should be considered during the development of an appropriate drug delivery system. The design of such systems requires the use of excipients such as polymers, lipids, and to a lesser extent the use of minerals. Some time ago Unger envisaged the possibility of adsorbing drugs on a silica surface. The discovery of ordered mesoporous silicas of the MCM-41S family in 1992 opened the way to a host of new developments in chemistry, catalysis, separation sciences and nanotechnology. 4] Proteins could be adsorbed on these materials 6] and some maintained their activity. However, the exact location of the proteins is not always known. In these materials, which now include many other metal oxides, the pore size may be tuned between 2 and 10 nm and the pore size distribution is very narrow. They have a large specific surface area; up to 1200 mg , and a large mesoporous volume; up to 0.9 cmg . Furthermore, for silica-based mesoporous materials, the bulk of the silica forming the walls separating the channels may be amorphous. The regular and tunable pore size together with the large mesoporous volume and the nontoxic nature of amorphous silica have led our group to investigate the potential of these materials for the storage and release of drugs. Work initiated in 1999 showed that it is possible to encapsulate several drugs belonging to the nonsteroidal anti-inflammatory drug (NSAID) tetragonal zirconia nanocrystals at high temperatures by refluxdigestion in basic solutions are induced by the incorporation of Si impurity from the glass vessel. These findings also lead to a practical alternative for the synthesis of tetragonal zirconia nanocrystals for advanced applications, that is, by doping with a small amount of Si ions during the synthesis of zirconia.
TL;DR: The geometries, electronic structures, dipole moments, and intra- and intermolecular charge transfers of the dyes in different solvents are discussed, showing that including explicit H-bonding solvent molecule(s) in the calculations is important to obtain the correct order of the excitation and emission energies.
Abstract: Using the density-functional vertical self-consistent reaction field (VSCRF) solvation model, incorporated with the conductor-like screening model (COSMO) and the self-consistent reaction field (SCRF) methods, we have studied the solvatochromic shifts of both the absorption and emission bands of four solvent-sensitive dyes in different solutions. The dye molecules studied here are: S-TBA merocyanine, Abdel-Halim's merocyanine, the rigidified aminocoumarin C153, and Nile red. These dyes were selected because they exemplify different structural features likely to impact the solvent-sensitive fluorescence of push-pull, or merocyanine, fluorophores. All trends of the blue or red shifts were correctly predicted, comparing with the experimental observations. Explicit H-bonding interactions were also considered in several protic solutions like H 2 O, methanol and ethanol, showing that including explicit H-bonding solvent molecule(s) in the, calculations is important to obtain the correct order of the excitation and emission energies. The geometries, electronic structures, dipole moments, and intra- and intermolecular charge transfers of the dyes in different solvents are also discussed.
TL;DR: It is found that the PCH model using a three-dimensional Gaussian observation volume profile is inadequate for fitting experimental data obtained from a confocal setup with one-photon excitation, and an improved model is proposed, based on the correction to the observation volume profiles for the out-of-focus emission.
Abstract: The photon counting histogram (PCH) analysis is a fluorescence fluctuation method that is able to characterize the brightness and concentration of different fluorescent species present in a liquid sample. We find that the PCH model using a three-dimensional Gaussian observation volume profile is inadequate for fitting experimental data obtained from a confocal setup with one-photon excitation. We propose an improved model, which is based on the correction to the observation volume profile for the out-of-focus emission. We demonstrate that this model is able to resolve different species present under a wide range of conditions. Attention is given to how this model allows the examination of the effects of different instrumental setups on the resolvability.