Structural characterization of artificial self-assembling porphyrins that mimic the natural chlorosomal bacteriochlorophylls c, d, and e

We report two crystal structures of a synthetic porphyrin molecule which was programmed for self-assembly. The same groups which ensure that bacteriochlorophylls c, d, and e can self-assemble into the chlorosomal nanorods, the photosynthetic antenna system of some green bacteria, have been engineered into desired positions of the tetrapyrrolic macrocycle. In the case of the 5,15-meso-substituted anchoring groups, depending upon the concentration, by using the same crystallization solvents, either a tetragonal or a layered structure of porphyrin stacks were encountered. Surprisingly, pi-pi interactions combined with extensive dispersive interactions, which also encompass cyclohexane, one of the crystallization solvents, win over putative hydrogen bonding. We are aware that our compounds differ considerably from the natural bacteriochlorophylls, but based upon our findings, we now question the hydrogen-bonding network, previously proposed to organize stacks of bacteriochlorophylls. Transmission electron microscopy (TEM), atomic force microscopy (AFM), and small-angle X-ray scattering (SAXS) on various isomeric compounds support our challenge of current models for the chlorosomal antenna as these show structures, astonishingly similar to those of chlorosomes.     

外缘烷基链长对共轭有机小分子聚集行为及光电性质影响

改变分子化学结构和调控分子结构聚集态行为从而影响或改变材料的化学和物理性质, 是开发新型高效有机光电功能材料的重要手段. 在共轭有机分子外缘引入烷基链一般是为了改进材料溶解性能, 但近来的一些研究表明, 烷基链长对一些共轭有机小分子固态聚集行为和光电性质具有重要影响, 烷基链扮演着显著调控材料光电性质的“功能基团”作用. 本文以聚集诱导发光（aggregation-induced emission, AIE）/聚集强化荧光（aggregation enhanced emission, AEE）发射共轭有机小分子为重点, 对近年来有关烷基链长对共轭有机分子聚集形态和光电性质影响的一些典型事例进行评述, 旨在使人们在进行共轭有机分子设计合成及其结构与性能关系研究中能够关注烷基链的因素, 使烷基链变化作为功能导向晶态共轭有机材料设计合成及其可控制备的一种手段.     

How the pi conjugation length affects the fluorescence emission efficiency

How the pi conjugation length affects the fluorescence emission efficiency is elucidated by examination of the theoretical and experimental relationship between absolute quantum yield (Phi(f)) and magnitude (Api) of the pi conjugation length in the excited singlet state, which provides a novel concept for molecular design for highly fluorescent organic compounds. As a tool to predict Phi(f) from a structural model, (nu(a) - nu(f))1/2 x a3/2 (nu(a): wavenumber of absorption maximum, nu(f): wavenumber of emission maximum, a: molecular radius) could be used instead of Api. The concept should be valuable for potential applications to (1) examination of an excited singlet state structure (for example, coplanarity of excited-state molecules) and (2) molecular design of novel materials, in which the excited singlet state plays an important role, such as highly efficient fluorophores, electroluminescent materials, photoconducting materials, and nonlinear optical materials. A remarkably intense green fluorophore (Phi(f) 0.88, log epsilon 4.72, lambda(em) 527 nm) is created based on this concept, which is of great interest in relation to a green fluorescent protein (Topaz, T203Y type, Phi(f) 0.60, log epsilon 4.98, lambda(em) 527 nm).     

Effect of head group polarity and spacer chain length on the aggregation properties of gemini surfactants in an aquatic environment

The aggregation behavior of cationic gemini surfactants with respect to variation in head group polarity and spacer length is studied through conductance, surface tension, viscosity, and small-angle neutron-scattering (SANS) measurements. The critical micellar concentration (cmc), average degree of micelle ionization (beta(ave)), minimum area per molecule of surfactant at the air-water interface (A(min)), surface excess concentration (gamma(max)), and Gibb's free energy of micellization (delta G(mic)) of the surfactants were determined from conductance and surface tension data. The aggregation numbers (N), dimensions of micelles (b/a), effective fractional charge per monomer (alpha), and hydration of micelles (h(E)) were determined from SANS and viscosity data, respectively. The increasing head group polarity of gemini surfactant with spacer chain length of 4 methylene units promotes micellar growth, leading to a decrease in cmc, beta(ave), and delta G(mic) and an increase in N and b/a. This is well supported by the observed increase in hydration (h(E)) of micelles with increase in aggregation number (N) and dimension (b/a) of micelle.     

Theoretical prediction of spectral and optical properties of bacteriochlorophylls in thermally disordered LH2 antenna complexes

A general approach for calculating spectral and optical properties of pigment-protein complexes of known atomic structure is presented. The method, that combines molecular dynamics simulations, quantum chemistry calculations, and statistical mechanical modeling, is demonstrated by calculating the absorption and circular dichroism spectra of the B800-B850 bacteriochlorophylls of the LH2 antenna complex from Rs. molischianum at room temperature. The calculated spectra are found to be in good agreement with the available experimental results. The calculations reveal that the broadening of the B800 band is mainly caused by the interactions with the polar protein environment, while the broadening of the B850 band is due to the excitonic interactions. Since it contains no fitting parameters, in principle, the proposed method can be used to predict optical spectra of arbitrary pigment-protein complexes of known structure.     

The photophysics of monomeric bacteriochlorophylls c and d and their derivatives: properties of the triplet state and singlet oxygen photogeneration and quenching

Measurements of pigment triplet-triplet absorption, pigment phosphorescence and photosensitized singlet oxygen luminescence were carried out on solutions containing monomeric bacteriochlorophylls (Bchl) c and d, isolated from green photosynthetic bacteria, and their magnesium-free and farnesyl-free analogs. The energies of the pigment triplet states fell in the range 1.29-1.34 eV. The triplet lifetimes in aerobic solutions were 200-250 ns; they increased to 280 +/- 70 microseconds after nitrogen purging in liquid solutions and to 0.7-2.1 ms in a solid matrix at ambient or liquid nitrogen temperatures. Rate constants for quenching of the pigment triplet state by oxygen were (2.0-2.5) x 10(9) M-1 s-1, which is close to 1/9 of the rate constant for diffusion-controlled reactions. This quenching was accompanied by singlet oxygen formation. The quantum yields for the triplet state formation and singlet oxygen production were 55-75% in air-saturated solutions. Singlet oxygen quenching by ground-state pigment molecules was observed. Quenching was the most efficient for magnesium-containing pigments, kq = (0.31-1.2) x 10(9) M-1 s-1. It is caused mainly by a physical process of singlet oxygen (1O2) deactivation. Thus, Bchl c and d and their derivatives, as well as chlorophyll and Bchl a, combine a high efficiency of singlet oxygen production with the ability to protect photochemical and photobiological systems against damage by singlet oxygen.     

The properties of tetracyanocomplexes after contact with ethyl alcohol

The clathrates Cd(NH₃)₂ Ni(CN)₄·2C₆H₆ (1) and Cd(en) Ni(CN)₄·2C₆H₆ (2) have been studied as separative materials for the separation of mixtures of alkanes, of benzene and its derivatives, and of the chloroderivatives of methane and pyridine.By identifying the chromatographically active solid form we could observe on the basis of the IR spectra and the thermal analysis that the ethylalcohol after contact with clathrate (1) substituted one mole of benzene. Clathrate (2) was not affected by the action of ethylalcohol. The facts agree with the interactions existing between the tetracyanocomplexes and the sorbates.     

Rheological properties and structures of cationic surfactants and fatty alcohol emulsions: effect of surfactant chain length and concentration

 Rheological and optical properties of cetyltrimethylammonium chloride (CTAC)/fatty alcohol (FA), behenyltrimethylammonium chloride (BTAC)/FA and CTAC/FA/hydroxyethyl cellulose (HEC) emulsions have been studied with particular emphases on the effects of FA content, the difference in the chain length of the hydrophobic groups between CTAC and BTAC, and the addition of a water soluble polymer, HEC. The effects of the FA content are to accelerate the structure development during the aging period and to increase the storage modulus, the yield stress, and the zero-shear-rate viscosity in the three emulsion systems investigated. At a low FA content of 2% w/w, lamellar and vesicular aggregates and isolated multilamellar vesicles can be observed in the CTAC/FA and BTAC/FA emulsions, respectively. At a high FA content of 6% w/w or with an excess of FA present, networklike structures and sunflower-like structures form, respectively, instead, inducing a higher entanglement storage modulus and a higher yield stress relative to those emulsions with a low FA content. The effect of adding HEC to the CTAC/FA emulsion is to reduce the entanglement storage modulus and the yield stress, consistent with the optical observation that the presence of the polymer disrupts the formations of lamella and vesicular aggregates and network structures. Received: 27 July 2000 Accepted: 28 November 2000     

The length of the polymethine chain and the spectral-luminescent properties of symmetrical cyanine dyes

The general characteristics that relate the length of the polymethine chain of symmetrical cyanine dyes to their spectral-luminescent properties depending on the electron-donor character of the heterocycles and the nature of the solvent are formulated. For various types of symmetrical cyanines, the Stokes shifts decrease with the elongation of the polymethine chain due to weakening of the vibronic interactions. The vinylene shifts of the band maxima are essentially constant and fall within the range 100 to 130 nm depending on the nature of the heterocycles and the solvent. When the polymethine chain elongates the fluorescence quantum yields first increase and then decrease. The greater the effective length of the heterocycle the stronger the decrease. The fluorescence decay occurring when the polymethine chain gets longer is associated with intensification of the internal conversion. For symmetrical cyanines, the changes in the shapes of the electronic bands (their width, asymmetry, excess, and fine structure) as the chain elongates are governed by the competing effects of the vibronic and intermolecular interactions. The former decrease as the chain lengthens, causing the narrowing of the absorption bands for the lower vinylogs. On the other hand, the latter increase as chain lengthens, which leads to broadening of the bands for the higher vinylogs. The higher the solvent nucleophilicity and the greater the deviation of the electron-donor ability of the heterocycle from the average value the greater the broadening. Any elongation of the polymethine chain of symmetrical cyanines causes only narrowing of the bands and an increase in the asymmetry, excess, and structuring in the fluorescence spectra, which, unlike the absorption spectra, is independent of the electron-donor character of the heterocycles and the nature of the solvent. These effects are caused by the fact that, in contrast to absorption, changes in the shape of emission bands with increasing chain length are governed predominantly by vibronic rather than by intermolecular interactions.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1227–1239, July, 1994.     

Properties of poly(vinyl alcohol)-graft-polyacrylamide copolymers depending on the graft length. 2. Thermal properties in the bulk state

Thermal behavior of poly(vinyl alcohol)-graft-polyacrylamide copolymers (PVA-g-PAA), so-called intramolecular polymer-polymer complexes (intraPC), with variable M̄ _vPAA and constant average number N of grafts is considered in this report. Complete compatibility of PVA and PAA through hydrogen bonding is realized in the range of the graft lengths not exceeding some critical value. The content of adsorbed and trapped water in graft copolymers reflects some features of PVA-g-PAA_N structure depending on the graft lengths. The first thermal decomposition region in air for copolymers begins at higher temperatures with growing M̄ _vPAA, but the total destruction interval reduces. Formal kinetic decomposition parameters of the first decomposition stage appear to be the largest for the PVA-g-PAA with the largest quantity of H-bonds between the main and graft chains. Chemical transformations in graft copolymers, PVA and PAA during thermal decomposition are discussed.     

The effect of the composition on the properties ofγ-irradiated ethylenevinyl alcohol copolymers

Two ethylene-vinyl alcohol copolymers (Eval E and Eval F with about 45% and 25% of ethylene respectively) have been irradiated in air in the dose range 0–200 Mrad. The irradiated samples show both crosslinking and degradation; the former effect is more relevant in the copolymer with a larger content of polyethylene. Calorimetric results show that both the crystalline and the amorphous phases are involved in the reactions initiated by the irradiation.     

The effect of ion aggregation on some melt properties of styrene ionomers

The dynamic modulus, G′, and viscosity, η′, in the melt were determined for styrene ionomers containing up to 7.7 mole-% sodium methacrylate in the frequency range of 10^?2 to 40 rad/sec, at various temperatures. The same parameters were determined for the methyl ester of these ionomers, as well as for some acid samples. It was found that in the temperature range studied the time–temperature superposition principle was applicable to the salts as well as to the esters and acids. Furthermore, it was found that the shapes of the G′ versus ω plots for the three types of polymers were identical for a particular molecular weight and comonomer concentration, and independent of whether the sample was in the acid, ester, or salt form. The temperatures required to achieve superposition were, however, quite different. This temperature difference, ΔT (between the salt and the ester), was found to be a function of the ion concentration, c. The shape of the plot of ΔT versus c suggests that the structure of the ionic aggregates changes in the range of 4–5 mole-% of ions.     

The effect of the composition on the properties of γ-irradiated ethylenvinyl alcohol copolymers

Stress-strain and dynamic-mechanical tests were performed on two ethylene-vinyl alcohol copolymers (Eval E and Eval F, with about 45% and 25% of ethylene respectively) irradiated in air. The experimental results, in agreement with the solubility and calorimetric results previously reported [1], show that both crosslinking and chain scission are present, and that a decrease in the degree of crystallinity with the irradiation dose occurs.     

Microemulsions: Effect of alkyl chain length of alcohol and alkane

In microemulsions consisting of four components, i.e. detergent — water — oil — cosurfactant, the free energy of transfer from the continuous oil phase to the interfacial region for the cosurfactant is reported. From the effect of temperature on the free energy, the entropy and the enthalpy values are also reported. The effect of chain length of the alcohol (cosurfactant) is also described. It is further shown, that if the oil phase consists of hexadecane, then the free energy changes as a linear function of the number of carbon atoms in the cosurfactant. On the other hand, if the oil phase is benzene, the cosurfactant chain length has very little effect. These data are analyzed with respect to the microemulsion structure and stability.     

Dielectric properties of alcohol electrolyte solutions

Dielectric properties of ethanol and 1-hexanol solutions containing LiCl, CaCl₂·2H₂O and Ca(NO₃)₂·4H₂O, respectively, have been determined. It is found that LiCl reduces the static permittivity in ethanol, but CaCl₂·2H₂O and Ca(NO₃)₂·4H₂O both give an initial increase in _s. All the electrolytes studied increase the mean relaxation time of the ethanol solutions. In 1-hexanol the static permittivity is rather invariant for all studied electrolytes at low concentrations, while the same lengthening of the mean relaxation time is observed. When water is added in addition to the hydration water of the electrotyte, the static permittivity in hexanol is almost unaltered while the relaxation time is drastically shortened. The experimental result is discussed in terms of a formation of ion pairs, solvation sheaths, and kinetic depolarization, a partial release of hydration water and a structuring influence on the alcohol structure by the hydrated cation.     

Influence of aggregation of asphaltenes on the rheological properties of oil

Photon correlation spectroscopy was used to determine the threshold concentrations of n-heptane for phase transitions of asphaltenes in model systems for two types of oil. The rheological properties exhibited by high-paraffin oil in the asphaltene aggregation process were analyzed. It was shown that the presence of resin and paraffi n fractions in oil prevents phase transition of asphaltenes at the threshold concentration of n-heptane for the model system, so that a higher precipitant concentrations is required for aggregation of asphaltenes and formation of oil sludge.     

The effect of the alkyl chain length on the mesomorphic properties of new lactic acid derivatives

New series of lactic acid derivatives with alkyl terminal chain have been synthesised and their mesomorphic properties studied. We have varied the length of chiral and non-chiral terminal alkyl chains and found that prolonging both chains has a strong effect on the SmA*–SmC* phase transition. Most of the new materials exhibit only paraelectric SmA* phase; for homologues with a longer non-chiral chain (m ≥ 10), the ferroelectric (SmC*) phase appears below the SmA* on cooling and persists down to a room temperature. The role of the chiral terminal chain in the molecule is quite opposite – only its short length supports the existence of ferroelectric phase. Additionally, a hexatic phase appeared below the SmA*–SmC* phase sequence for several homologues at low temperatures. All materials have been studied using standard experimental techniques (differential scanning calorimetry (DSC), texture observations, polarisation and tilt angle measurements, etc.). Liquid crystalline properties of new materials have been compared with the previously prepared and studied lactic acid derivatives.     

Effect of ethanol on the aggregation properties of cetyltrimethylammonium bromide surfactant

Surfactant aggregation properties in aqueous and mixed organic/aqueous solutions have attracted considerable interests especially for the applications to the template synthesis of nanoporous inorganic materials. In this work, we study the aggregation behavior of cetyl trimethylammonium bromide surfactant in both aqueous and mixed water/ethanol solutions by the steady-state fluorescence probe technique. The critical micelle concentration (CMC) and the micelle aggregation number were determined in solutions with different ethanol contents. The CMC increases and the aggregation number decreases with increasing ethanol content in solutions. The effect of ethanol on the micelle formation can be treated as a structure breaking process. The density functional theory was used to calculate the charge density distribution over the surfactant molecules in different solvents. The results obtained reveal that the micelle size distribution in solutions can be adjusted by varying the content of ethanol in solvents.__________From Kolloidnyi Zhurnal, Vol. 67, No. 2, 2005, pp. 186–191.Original English Text Copyright © 2005 by Li, Han, Zhang, Wang.This article was submitted by the authors in English.     

Molecular aggregation and mechanical properties of Kapton H

Molecular aggregation of Kapton H (KH) was investigated by small-angle x-ray scattering (SAXS). Superstructure parameters were estimated using a one-dimensional model, taking into account that the SAXS from KH is anisotropic out of the film plane. The results show that KH has a two-phase structure with a volume ratio of the ordered to the less-ordered phase of about 1:1. The β dispersion in dynamic mechanical properties is reasonably ascribed to oscillations of p-phenylene groups in the main chain.