Joint spectroscopic and theoretical investigation of cationic cyanine dye Astrazon Orange-R: solvent viscosity controlled relaxation of excited states.

In this paper, the first study of cationic cyanine dye Astrazon Orange-R by combined spectroscopic and theoretical investigation is presented. It is shown that molecular modeling of Astrazon Orange-R is in very good agreement with experiment, allowing us to gain insight into its complicated photophysics. A solvent viscosity controlled relaxation of excited states, involving cyanine isomerization, is also outlined.

Selective, direct detection of acetylcholine in PBS solution, with self-assembled fluorescent nano-particles: experiment and modelling.

We report synthesis, characterisation and molecular modelling of a new fluorescent cyclotriveratrylene probe for acetylcholine in aqueous media, with emission around 430 nm thanks to extended conjugation. The probe discriminates acetylcholine from choline, with respective binding constants 540 and 240 M(-1) in PBS buffered saline solution, an order of magnitude improvement over the previous best performance. Dynamic light scattering and transmission electron microscopy show the new probe self-assembles in ca. 5 nm diameter particles in PBS medium. Molecular modelling suggests that the high fluorescence quantum yield of the probe, 20% in aqueous media, is due to features of the molecular arrangement in the nano-particles, contributing both to exposure of the complexation site and to shielding of the fluorescent π system from quenching by water. Titration data for other quaternary ammoniums and modelling indicate that recognition of acetylcholine vs. choline depends on specific electrostatic interactions, and to a lesser extent on exclusion of water by hydrophobic-hydrophilic segregation. Probe-substrate interactions enhance the fluorescence of the probe by shielding against water and by flattening the π system.

Efficient cyanoaromatic photosensitizers for singlet oxygen production: synthesis and characterization of the transient reactive species.

In order to graft cyanoaromatic molecules onto various inert supports, we designed two new cyanoanthracene derivatives of benzo[b]triphenylene-9,14-dicarbonitrile (DBTP, 1), which already demonstrated good photosensitizing properties. We synthesized 3-(N-hydroxypropyl)carboxamido-9,14-dicyanobenzo[b]triphenylene, 3 and 3-(N-N0-Boc-aminohexyl)carboxamido-9,14-dicyanobenzo[b]triphenylene, 4 and compared their photophysical properties in acetonitrile relative to those of the parent compound 1 and its carboxylic derivative 9,14-dicyanobenzo[b]triphenylene-3-carboxylic acid, 2. The transient species were analysed and the quantum yields of singlet oxygen production (ΦΔ) determined in acetonitrile. The effect of chemical functionalization can be considered negligible, since absorption spectra, fluorescence emission spectra and fluorescence lifetimes do not significantly change with the substituent. The triplet-triplet absorption spectra and the triplet excited state lifetimes are similar for the whole series. For compounds 1-4 high values of ΦΔ, close to that of the standard sensitizer 1H-phenalen-1-one (PN, ΦΔ ≈ 1), and higher than that of the well-known photosensitizer 9,10-dicyanoanthracene (DCA), are due to very efficient intersystem crossing from the singlet to the triplet excited state and subsequent energy transfer to ground state oxygen ((3)O2). They belong to a class of very efficient photosensitizers, absorbing visible light and stable under irradiation, they may be functionalized without significant changes to their photophysical behaviour, and grafted onto various supports.

Host and adsorbate dynamics in silicates with flexible frameworks: empirical force field simulation of water in silicalite.

Molecular dynamics simulations are performed on the pure silica zeolite silicalite (MFI framework code), maintaining via a new force field both framework flexibility and realistic account of electrostatic interactions with adsorbed water. The force field is similar to the well-known "BKS" model [B. W. H. van Beest et al., Phys. Rev. Lett. 64, 1955 (1990)], but with reduced partial atomic charges and reoptimized covalent bond potential wells. The present force field reproduces the monoclinic to orthorhombic transition of silicalite. The force field correctly represents the hydrophobicity of pure silica silicalite, both the adsorption energy, and the molecular diffusion constants of water. Two types of adsorption, specific and weak unspecific, are predicted on the channel walls and at the channel intersection. We discuss molecular diffusion of water in silicalite, deducing a barrier to crossing between the straight and the zigzag channels. Analysis of the thermal motion shows that at room temperature, framework oxygen atoms incurring into the zeolite channels significantly influence the dynamics of adsorbed water.

Structure and molecular dynamics of crystalline and liquid anthracene and naphthalene: possible transient rotator phase of naphthalene.

We examine the structural and dynamical properties of the crystal and liquid states of anthracene and naphthalene, with special attention to melting. This molecular dynamics study is based on an all-atom force field, which we optimized for simulations of solid and liquid anthracene, over wide ranges of temperature and pressure. The force field is shown to be transferable to naphthalene. Local ordering of the simulated liquids is in fair agreement with structures deduced in the literature from X-ray scattering, while providing a much more detailed picture. In analogy with the rich polymorphism of substituted benzene and naphthalene complexes, we find for naphthalene (but not anthracene) a two-step melting process with a transient rotator phase in which rotational jumps precede the onset of full melting with translational diffusion.

Influence of homologous disaccharides on the hydrogen-bond network of water: complementary Raman scattering experiments and molecular dynamics simulations.

A comparative investigation of trehalose, sucrose, and maltose in water solution has been performed using Raman scattering experiments and Molecular Dynamics simulations. From the analysis of the O-H stretching region in the [2500,4000] cm(-1) Raman spectral range, which includes for the first time the contribution of 'free' water, and the statistical distribution of water HB probabilities from MD simulations, this study confirms the privileged interaction of trehalose with water above a peculiar threshold weight concentration of about 30%. The role of the hydration number of sugars--found higher for trehalose--on the destructuring effect of the water hydrogen bond network is also addressed. The analysis of the water O-H-O bending spectral range [1500,1800] cm(-1) reveals a change of the homogeneity of water molecules influenced by sugars, but the three investigated sugars are found to behave similarly.

Watching the Photo-Oxidation of a Single Aromatic Hydrocarbon Molecule.

The photooxidation of single dye molecules can be followed by confocal fluorescence microscopy. The self-sensitized reaction with singlet oxygen leads to a suite of products, which may be differentiated spectrally. Tentative structures for certain photoproducts have been obtained from quantum-chemical calculations.

Association of indigo with zeolites for improved color stabilization.

The durability of an organic color and its resistance against external chemical agents and exposure to light can be significantly enhanced by hybridizing the natural dye with a mineral. In search for stable natural pigments, the present work focuses on the association of indigo blue with several zeolitic matrices (LTA zeolite, mordenite, MFI zeolite). The manufacturing of the hybrid pigment is tested under varying oxidizing conditions, using Raman and ultraviolet-visible (UV-Vis) spectrometric techniques. Blending indigo with MFI is shown to yield the most stable composite in all of our artificial indigo pigments. In the absence of defects and substituted cations such as aluminum in the framework of the MFI zeolite matrix, we show that matching the pore size with the dimensions of the guest indigo molecule is the key factor. The evidence for the high color stability of indigo@MFI opens a new path for modeling the stability of indigo in various alumino-silicate substrates such as in the historical Maya Blue pigment.

Exploring conformational energy landscape of glassy disaccharides by cross polarization magic angle spinning 13C NMR and numerical simulations. I. Methodological aspects.

This article demonstrates the ability of chemical shift surfaces to provide information on distributions of various conformations of disaccharides in the glassy, solid state. The validity of the general method leading to a simulation of inhomogeneous (13)C chemical shift distributions is discussed in detail. In particular, a proper consideration of extrema and saddle points of the chemical shift map correctly accounts for the observed discontinuities in the experimental cross polarization magic angle spinning spectra. Provided that these basic requirements are met, density functional theory/gauge-independent atomic orbital chemical shift maps calculated on relaxed conformations lead to a very satisfactory description of the experimental line shapes. Using amorphous trehalose as a model disaccharide, the simulation unambiguously defines the range of most populated conformations in the glass.

Exploring the conformational energy landscape of glassy disaccharides by cross polarization magic angle spinning 13C nuclear magnetic resonance and numerical simulations. II. Enhanced molecular flexibility in amorphous trehalose.

This paper uses chemical shift surfaces to simulate experimental (13)C cross polarization magic angle spinning spectra for amorphous solid state disaccharides, paying particular attention to the glycosidic linkage atoms in trehalose, sucrose, and lactose. The combination of molecular mechanics with density functional theory/gauge invariant atomic orbital ab initio methods provides reliable structural information on the conformational distribution in the glass. The results are interpreted in terms of an enhanced flexibility that trehalose possesses in the amorphous solid state, at least on the time scale of (13)C nuclear magnetic resonance measurements. Implications of these findings for the fragility of trehalose glass and bioprotectant action are discussed.

Single dibenzoterrylene molecules in an anthracene crystal: main insertion sites.

We present a spectroscopic study of the properties of the two principal insertion sites (at 785.1 and 794.3 nm) of single dibenzoterrylene molecules in anthracene single crystals at cryogenic temperatures. We measured the temperature dependence of the line width, the orientation of the transition dipole moments, and the Stark effect. We performed molecular dynamics simulations, which show that one dibenzoterrylene molecule preferably replaces three anthracene molecules. From simulated annealing, we derive the molecular conformations in the most stable insertion sites and the orientations of the transition dipole moments. The good agreement between the spectroscopic results and the simulations allows us to propose unambiguous structures for the two principal spectroscopic sites.

Does the interaction potential determine both the fragility of a liquid and the vibrational properties of its glassy state?

By performing molecular dynamics simulations of binary Lennard-Jones systems with three different potentials, we show that the increase of anharmonicity and capacity for intermolecular coupling of the potential is the cause of (i) the increase of kinetic fragility and nonexponentiality in the liquid state, and (ii) the T(g)-scaled temperature dependence of the nonergodicity parameter determined by the vibrations at low temperatures in the glassy state. Naturally, these parameters correlate with each other, as observed experimentally by T. Scopigno et al. [Science 302, 849 (2003)]

Coherent electronic coupling versus localization in individual molecular dimers.

We have investigated electronic excitation transfer in individual molecular dimers by time and spectrally resolved confocal fluorescence microscopy. The single molecule measurements allow for directly probing the distribution of the electronic coupling strengths due to static disorder in the polymer host. We find dimers where the excitation is delocalized (superradiant emission) while for others emission originates from a localized state. Transitions between delocalized and localized states as observed for a given dimer are attributed to structural fluctuations of the guest-host system.