Rotational Diffusion of Coumarins in Alcohols: A Dielectric Friction Study

The rotational dynamics of three structurally similar polar molecules viz., coumarin 440, coumarin 151 and coumarin 450 has been studied in alcohols at room temperature using steady-state fluorescence depolarization method and time correlated single photon counting technique. The observed reorientation times of all the three probes are found to be longer than those predicted by Stokes–Einstein–Debye (SED) hydrodynamic theory with stick boundary condition and a deviation towards super-stick behavior is noted. Dielectric friction theories of Nee–Zwanzig and van der Zwan–Hynes, which treat the solute as a point dipole, overestimate the dielectric friction contribution exhibited by all the three coumarins in alcohols. Results are discussed in the light of theoretical models and the possibility of hydrogen bonding between the amino group of the probe molecules and the hydroxyl group of the alcohols.     

A New Emissive Chalcone-Based Chemosensor Armed by Coumarin and Naphthol with Fluorescence “Turn-on” Properties for Selective Detection of F<Superscript>−</Superscript> Ions

A new chalcone-based probe containing coumarin and naphthol at both ends has been synthesized via aldol condensation. The uniqueness of the newly derived probe can be ascribed to the presence of naphthol and coumarin units acting as binding site and signaling element, respectively. The fluorogenic behaviors toward various anions were investigated. The probe was characterized by various spectroscopic techniques and the in-depth study led to show excellent selectivity and sensitivity for fluoride ions. The hydrogen bonding thus formed with fluoride anion provides remarkable fluorometric responses. The interactions of the probe with fluoride ions were determined by fluorescence, FT-IR and NMR spectroscopic techniques. The exploratory studies by fluorescent spectral changes augur well for the naked-eye sensing applications.     

Rotational Diffusion of Coumarins: A Dielectric Friction Study

The rotational diffusion of three probes: coumarin 522B (C522B), coumarin 307 (C307) and coumarin 138 (C138) with nearly identical size was studied at room temperature employing steady-state and time-resolved fluorescence anisotropy techniques in series of alcohols and alkanes. Experimental observations indicate faster rotation of C138 compared to the other two dyes in alcohols and a faster rotation of C522B than C307 in alkanes. The dielectric friction theories of Nee-Zwanzig (NZ) and van der Zwan-Hynes (ZH) were employed to estimate the friction experienced by the probes in alcohols, in addition to the mechanical friction calculated using Stokes-Einstein-Debye (SED) hydrodynamic with slip boundary condition and Dote-Kivelson-Schwartz (DKS) quasihydrodynamic theories. The observed reorientation times for the three probes do not follow the trend predicted by dielectric friction theories of NZ and ZH. The dipole moments determined from solvatochromic techniques were found to be different for the three probes.     

Nonstationary rotational diffusion in room temperature liquids measured by femtosecond three-pulse transient anisotropy

A femtosecond three-pulse chi((5)) polarization anisotropy experiment is used to examine the time dependence of the rotational diffusion of coumarin 153 in polar liquids. By probing the polarization anisotropy decay at various points during the molecule's excited state lifetime, a time-dependent diffusion coefficient is found in several solvents. This anomalous behavior is consistent with the relaxation of the solvent friction to accommodate the solute's excited state charge distribution. Rotational diffusion times measured immediately after photoexcitation by two-pulse chi((3)) experiments may reflect a nonstationary bath dynamics, rather than the equilibrium friction of the solvent.     

Particular features of incorporation of coumarin 30 into nanoparticles from metal complexes and the intensity of its columinescence

We have studied the sensitized fluorescence of coumarin 30 incorporated into nanoparticles from complexes of p-phenylbenzoyltrifluoroacetone and 1,10-phenanthroline with Y, La, Lu, Gd, Al, and Sc ions in 10% alcoholic-aqueous solutions. We have shown that, upon formation of nanoparticles from complexes of Y(III) and Ln(III) ions, coumarin 30 molecules are completely incorporated from the solution into nanoparticles from complexes up to dye concentrations in the solution comparable with the concentration complexes. For the nanoparticles under study, in the whole range of the examined dye concentrations, concentration quenching of the coumarin 30 cofluorescence has not been observed. Our results show that coumarin 30 is incorporated into lanthanide and yttrium complexes as a synergistic bidentate ligand. The possibility of creating brightly luminescent markers that absorb not only in the range of 360?C370 nm, but also in the range of 440?C450 nm, and have a narrow fluorescence spectrum with ??_max = 520 nm has been demonstrated.     

Rotational Diffusion and Solvatochromic Correlation of Coumarin 6 Laser Dye

Rotational diffusion of coumarin 6 (C6) laser dye has been examined in n-decane and methanol as a function of temperature. The rotational reorientation of this probe has been measured in these solvents. It is observed that the decrease in viscosity of the solution is responsible for the decrease in the rotational relaxation time of the probe molecule. The molecule C6 has long reorientation times in n-decane solvent as compared to methanol over all temperatures. It is found that the coumarin 6 rotates slower in n-decane than in methanol especially at higher values of viscosity over temperature. Two methods are chosen to determine the ground state and excited state dipole moments. The change in dipole moments is estimated from Bakhshiev-Chamma-Viallet equations and, the ground and excited state dipole moments from Kawski et al. equations, by using the variations of the Stokes shifts with the dielectric constant and refractive index of the solvent. Our results are quite reliable which are solvatochromic correlation obtained using solvent polarity functions. The reported results show that excited state dipole moment is greater than ground state dipole moment, which indicates that the excited state is more polar than the ground state.     

Femtosecond solvation and charge transfer dynamics in liquid solution

Time-resolved fluorescence up-conversion experiments have been performed for a few fluorescent organic charge transfer molecules in liquid solution. Dynamic Stokes shifts have been measured, with a time resolution of 150 fs, for the probe molecules in alcoholic and ethereal solvents. The results reveal solvation dynamics determined by “inertial free streaming” motions of the solvent molecules in the solvent cages and rotational diffusion motions of the solvent molecules. Simulations of the temporal changes in the fluorescence spectra based on the Smoluchowski diffusion equation are also briefly discussed. Presented at the Czech-Israeli-German Symposium “Dynamical Processes in Condensed Molecular Systems”, Prague, Czech Republic, 26–30 May 1997. This work was supported in part by the Netherlands Foundation for Chemical Research (SON) with financial aid from the Netherlands Organization for Scientific Research (NWO).     

Diffusion relaxation of photoinduced gratings in polyvinyl acetate latex films

The features of the postexposure relaxation of holographic gratings recorded in inhomogeneous polyvinyl acetate latex films with photosensitive agents (photochromic molecules of fulgide dyes and phenanthrenequinone) have been considered. The diffusion coefficients and rms displacements of izomerized probe in polymer latex particles and aqueous environment are determined within the model of two diffusion states. The effective diffusion coefficient of the molecular probe, which is responsible for the relaxation of gratings, increases with an increase in their period in wet films, whereas in dry films, this parameter is independent of the grating period. In the films subjected to high-temperature treatment the effective diffusion coefficient decreases with an increase in the grating period. The successive stages of grating relaxation in latex films with phenanthrenequinone are related to the diffusion of free molecules, radicals, and polymer chains, as well as to the local displacement of macromolecular segments at distances of 5–25 nm.     

High Probe Intensity Photobleaching Measurement of Lateral Diffusion in Cell Membranes

Lateral diffusion measurements, most commonly accomplished through Fluorescence Photobleaching Recovery (FPR or FRAP), provide important information on cell membrane molecules' size, environment and participation in intermolecular interactions. However, serious difficulties arise when these techniques are applied to weakly expressed proteins of either of two types: fusions of membrane receptors with visible fluorescent proteins or membrane molecules on autofluorescent cells. To achieve adequate sensitivity in these cases, techniques such as interference fringe FPR are needed. However, in such measurements, cytoplasmic species contribute to the fluorescence recovery signal and thus yield diffusion parameters not properly representing the small number of surface molecules. A new method helps eliminate these difficulties. High Probe Intensity (HPI)-FPR measurements retain the intrinsic confocality of spot measurements to eliminate interference from fluorescent cytoplasmic species. However, HPI-FPR methods lift the previous requirement that FPR procedures be performed at probe beam intensities low enough to not induce bleaching in samples during measurements. The high probe intensities now employed provide much larger fluorescence signals and thus more information on molecular diffusion from each measurement. We report successful measurement of membrane dynamics by this technique.     

Solvation dynamics in methanol: Experimental and molecular dynamics simulation studies

We have investigated the ultrafast dynamics of methanol by time dependent fluorescent shift experiments and molecular dynamic simulations. The experiments were performed with two different probe molecules, 1-aminonaphthalene and coumarin 153. The molecular dynamic simulations employed these probes as well as small atomic and diatomic solutes. We find a previously unobserved fast decay component in the solvation response of methanol. The molecular dynamics results are in good agreement with this experimental result. The origin of this fast response and the linearity of the solvent response are discussed.     

Special Features of Quenching of Fluorescence in the 1-Naphthol and Coumarin 1 Molecules in Binary Mixtures

The work studies the effect of intermolecular interactions on the relation between the rate constants of radiative processes and internal intercombination conversion in particlular molecule - solvent systems. The 1-naphthol complexes in binary mixtures with hydrogen bonding and those of coumarin 1 with nitrogen-nitrogen bonding are examined. The Stern - Volmer curves for the complexes with additions of nitromethane are shown to deviate from linearity.The interaction of coumarin 1 and 1-naphthol with nitromethane was simulated by the INDO/SP method to show that the lower singlet state is associated with nitromethane. The experimentally observed fluorescence band of the coumarin 1 and 1-naphthol molecules belongs to the third singlet state. The quantum yield of fluorescence from the S₃ state is decreased by an order of magnitude for the complexes under study. The quantum yield from the lower singlet state in the 1-naphthol - nitromethane complex is decreased by an order of magnitude, while in the coumarin 1 - nitromethane complex - by two orders. It is found that there is a possibility of photoprotonation in the S₁ and S₂ states for the 1-naphthol molecule along with the foregoing processes of quenching of fluorescence.__________Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 17–23, January, 2005.     

Studies of adsorption kinetics by means of the stochastic numerical simulation

This paper surveys the methods used in the theoretical studies of kinetics with a special emphasis on the works dealing with the stochastic methods and their application in studies of adsorption kinetics. One of the stochastic methods — Monte Carlo numerical simulation of the stochastic time evolution — is mainly discussed. Numerous studies show that this method, introduced by Gillespie [J. Comput. Phys. 22 (1976) 403], is very useful to investigate the adsorption kinetics. The systematic studies of adsorption kinetics of single gases and gas mixtures on solid surfaces are presented. The kinetic adsorption isotherms, involving the lateral interactions between molecules in the surface phase, energetic heterogeneity of the adsorbent surface and surface diffusion, are numerically simulated by means of the numerical program, which is presented in the appendix. These simulations show influence of the adsorbent heterogeneity, lateral interactions and surface diffusion on the adsorption kinetics.     

Coumarin 153 emission thermochromism studied in non-specifically and specifically interacting solvents

We report a study of the thermochromic shifts observed for coumarin 153 (C153) emission spectra in several selected chloroalkanes and several solvents potentially specifically interacting with the probe. It has been found that the shifts observed in five 1-chloroalkanes can be successfully predicted using the McRae molecular interaction model [E.G. McRae, J. Phys. Chem. 61 (1957) 562]. However, the shifts observed in two other chloroalkanes have been found not to correspond to the value, which can be derived using the same C153 molecular parameters (dipole moments, molecular radius and gas phase emission spectrum position) as in the analysis of shifts in 1-chloroalkanes. This observation suggests that 1-chloroalkanes may interact specifically, contrary to what was assumed in our previous papers. The temperature dependence of changes in the energy of these specific interactions, after C153 excitation, also in the solvents known to interact in this way, is discussed in view of the thermochromic results reported in this paper and literature data. We show that even for C153, one of the most often studied probe, one cannot definitively state in what kind of specific interactions it takes or does not take part.     

荧光/表面增强拉曼散射双模式纳米光学探针的构建及性能研究

提出一种新型的荧光及表面增强拉曼散射（SERS）双模式光学纳米探针。首先,通过再沉淀-包覆法合成二氧化硅包覆香豆素6的纳米颗粒,再在二氧化硅表面静电吸附多聚赖氨酸分子形成包覆层,随后通过原位还原的方法在多聚赖氨酸壳层复合银纳米颗粒,最后在银纳米颗粒表面吸附拉曼分子即形成双模式纳米探针。该探针通过二氧化硅包覆的荧光分子产生荧光信号,以多聚赖氨酸表面的银纳米颗粒作为SERS增强基底,利用拉曼分子获得SERS信号,实现了荧光及SERS双模式成像。荧光与表面增强拉曼散射相结合的双模式分析技术可同时发挥二者的优点,提高成像的分辨率和灵敏度,在生物医学领域具有重要的应用价值。     

1H DNP at 1.4 T of water doped with a triarylmethyl-based radical

Recently a triarylmethyl-based (TAM) radical has been developed for research in biological and other aqueous systems, and in low magnetic fields, 10 mT or less, large (1)H dynamic nuclear polarization (DNP) enhancements have been reported. In this paper the DNP properties of this radical have been investigated in a considerably larger field of 1.4 T, corresponding to proton and electron Larmor frequencies of 60 MHz and 40 GHz, respectively. To avoid excessive microwave heating of the sample, an existing DNP NMR probe was modified with a screening coil, wound around the sample capillary and with its axis perpendicular to the electric component of the microwave field. It was found that with this probe the temperature increase in the sample after 4 s of microwave irradiation with an incident power of 10 W was only 16 degrees C. For the investigations, 10 mM of the TAM radical was dissolved in deionized, but not degassed, water and put into a 1-mm i.d. and 6-mm long capillary tube. At 26 degrees C the following results were obtained: (I) The relaxivity of the radical is 0.07 (mMs)(-1), in accordance with the value extrapolated from low-field results; (II) The leakage factor is 0.63, the saturation factor at maximum power is 0.85, and the coupling factor is -0.0187. It is shown that these results agree very well with an analysis where the electron-dipolar interactions are the dominant DNP mechanism, and where the relaxation transitions resulting from these interactions are governed by translational diffusion of the water molecules. Finally, the possibilities of combining DNP with magnetic resonance microscopy (MRM) are discussed. It is shown that at 26 degrees C the overall DNP-enhanced proton polarization should become maximal in an external field of 0.3 T and become comparable to the thermal equilibrium polarization in a field of 30 T, considerably larger than the largest high-resolution magnet available to date. It is concluded that DNP MRM in this field, which corresponds to a standard microwave frequency of 9 GHz, has the potential to significantly increase the sensitivity in NMR and MRI experiments of small aqueous samples doped with the TAM radical.     

Supramolecular self-assembly on the B-Si(111)-(√3x√3) R30° surface: From single molecules to multicomponent networks

Understanding the physical and chemical processes in which local interactions lead to ordered structures is of particular relevance to the realization of supramolecular architectures on surfaces. While spectacular patterns have been demonstrated on metal surfaces, there have been fewer studies of the spontaneous organization of supramolecular networks on semiconductor surfaces, where the formation of covalent bonds between organics and adatoms usually hamper the diffusion of molecules and their subsequent interactions with each other. However, the saturation of the dangling bonds at a semiconductor surface is known to make them inert and offers a unique way for the engineering of molecular patterns on these surfaces. This review describes the physicochemical properties of the passivated B-Si(111)-(√3x√3) R30° surface, that enable the self-assembly of molecules into a rich variety of extended and regular structures on silicon. Particular attention is given to computational methods based on multi-scale simulations that allow to rationalize the relative contribution of the dispersion forces involved in the self-assembled networks observed with scanning tunneling microscopy. A summary of state of the art studies, where a fine tuning of the molecular network topology has been achieved, sheds light on new frontiers for exploiting the construction of supramolecular structures on semiconductor surfaces.     

A Statistical Analysis of Fluorescence Correlation Data

Fluorescence correlation spectroscopy (FCS) is a relatively recent technique in which the diffusion coefficient of fluorescently labeled molecules can be determined. The change in diffusion behavior when these molecules interact with others can also be used to study interactions in solution. A new statistical method is proposed to analyze FCS measurements that cannot be evaluated with a classical autocorrelation function, which is normally used to analyze FCS data. It applies to binding studies where one of the interacting particles has a much brighter fluorescence intensity with respect to the other, which causes high fluorescence bursts whenever these molecules are detected. This biases the autocorrelation function, making it in most cases impossible to use this function as a fitting equation. Here, a statistical approach is used to quantify the amount of fluorescence found in bursts, thereby enabling to perform binding studies in cases where the fluorescence per molecule of both interacting species differs greatly. The method is demonstrated on a system of known composition, making it a promising tool for future FCS measurements.     

Novel NMR techniques for porous media research

Diffusion in porous media has been used as a probe of pore geometry in various NMR techniques. We will examine the effect of time-dependent diffusion in CPMG by showing that the diffusion time in CPMG is approximately the echo time, even in grossly inhomogeneous magnetic fields. Extension of the diffusion time in modified CPMG sequences is discussed. Diffusion in the susceptibility-contrast induced internal field is discussed as a means to probe pore size and pore shape. Finally, we present the general concept of two-dimensional relaxation-type experiments for study of molecules, fluids, materials and their dynamics that are characterized by spin relaxation and diffusion.     

Rotational Diffusion of Coumarins in Aqueous DMSO

The rotational dynamics of four structurally similar polar molecules viz., coumarin 440, coumarin 450, coumarin 466 and coumarin 151 has been studied in binary mixtures comprising of dimethyl sulphoxide and water at room temperature using the steady state fluorescence depolarization method and time correlated single photon counting technique. The binary mixtures are characterized by the fact that at a particular composition the viscosity (η) of the solution reaches a maximum value that is higher than the viscosities of either of the two co-solvents. The dielectric properties of the solution change across the composition range and the qualitative features of the solvent relaxation dynamics in complex systems are known to differ from those in simple solutions. A hook type profile of rotational reorientation time (τ _r ) vs viscosity (η) is obtained for all the solutes in dipolar aprotic mixture of dimethyl sulphoxide-water, with the rotational reorientation times being longer in organic solvent-rich zone, compared to the corresponding isoviscous point in water-rich zone due to strong hydrogen bonding. Fluorescence lifetimes as well as rotational reorientation times are sensitive to the composition of the binary solvent system under study than to the viscosity suggesting the importance of local structure. The results are discussed in the light of hydrodynamic and dielectric friction models. Dedicated to Professor M.I. Savadatti on his 77^th Birthday.     

Corticospinal interaction during isometric compensation for modulated forces with different frequencies

Background

The zebrafish visual system is a good research model because the zebrafish retina is very similar to that of humans in terms of the morphologies and functions. Studies of the retina have been facilitated by improvements in imaging techniques. In vitro techniques such as immunohistochemistry and in vivo imaging using transgenic zebrafish have been proven useful for visualizing specific subtypes of retinal cells. In contrast, in vivo imaging using organic fluorescent molecules such as fluorescent sphingolipids allows non-invasive staining and visualization of retinal cells en masse. However, these fluorescent molecules also localize to the interstitial fluid and stain whole larvae.

Results

We screened fluorescent coumarin derivatives that might preferentially stain neuronal cells including retinal cells. We identified four coumarin derivatives that could be used for in vivo imaging of zebrafish retinal cells. The retinas of living zebrafish could be stained by simply immersing larvae in water containing 1 μg/ml of a coumarin derivative for 30 min. By using confocal laser scanning microscopy, the lamination of the zebrafish retina was clearly visualized. Using these coumarin derivatives, we were able to assess the development of the zebrafish retina and the morphological abnormalities induced by genetic or chemical interventions. The coumarin derivatives were also suitable for counter-staining of transgenic zebrafish expressing fluorescent proteins in specific subtypes of retinal cells.

Conclusions

The coumarin derivatives identified in this study can stain zebrafish retinal cells in a relatively short time and at low concentrations, making them suitable for in vivo imaging of the zebrafish retina. Therefore, they will be useful tools in genetic and chemical screenings using zebrafish to identify genes and chemicals that may have crucial functions in the retina.