Preferential solvation within hydrophilic nanocavities and its effect on the folding of cholate foldamers

The conformations of three cholate foldamers and one molecular basket were studied by fluorescence and NMR spectroscopy. In nonpolar solvents (e.g., hexane/ethyl acetate or ethyl acetate) mixed with a small amount of a polar solvent (e.g., alcohol or DMSO), the cholate oligomer folded into a helix with the hydrophilic faces of the cholates turned inward. Folding created a hydrophilic nanocavity preferentially solvated by the entrapped polar solvent concentrated from the bulk. This microphase separation of the polar solvent was critical to the folding process. Folding was favored by larger-sized polar solvent molecules, as fewer such molecules could occupy and solvate the nanocavity, thus requiring a smaller extent of phase separation during folding. Folding was also favored by smaller/acyclic nonpolar solvent molecules, probably because they could avoid contact with the OH/NH groups within the nanocavity better than larger/cyclic nonpolar solvent molecules.     

Solvent-induced amphiphilic molecular baskets: unimolecular reversed micelles with different size, shape, and flexibility

Amphiphilic molecular baskets were obtained by attaching facially amphiphilic cholate groups to a covalent scaffold (calix[4]arene or 1,3,5-2,4,6-hexasubstituted benzene). In a solvent mixture consisting of mostly a nonpolar solvent (i.e., CCl4) and a polar solvent (i.e., DMSO), the hydrophilic faces of cholates turned inward to form a reversed-micelle-like conformer whose stability was strongly influenced by the number of the cholates and the topology of the scaffold. Preferential solvation of the hydrophilic faces of cholates within the molecule by the polar solvent was cooperative and gave the fundamental driving force to the conformational change. The reversed-micelle-like conformer was most stable in structures that allowed multiple cholates to form a microenvironment that could efficiently enrich the polar solvent molecules from the bulk solvent mixture.     

Solvent effect on the self-assembled structure of an amphiphilic perylene diimide derivative

An amphiphilic electron-deficient (n-type) perylene diimide has been synthesized and characterized. The diimide contains a hydrophobic long chain on one end and a hydrophilic ethoxy chain on the other. The self-assembly of this molecule in polar and nonpolar solvents has been demonstrated by concentration- and temperature-dependent absorption and fluorescence spectroscopies. Analysis of the spectral change for the aggregates shows typical J-aggregates for structures precipitated from polar solvents and H-aggregates for structures precipitated from nonpolar solvents. SEM and TEM micrographs and a suggested packing scheme, compatible with the formation of nanostrips in nonpolar solvents and nanofibers in polar solvents, are presented.     

含偶氮苯基团的三枝状凝胶因子的合成和凝胶性质研究

设计合成了一种中心为三乙基氨基,酰胺基作为氢键连接基团,柔性的烷基链连接偶氮苯基团的含多种分子间弱相互作用的三枝状有机凝胶因子1.由于偶氮苯基团处于分子的外缘,在THF溶液中,凝胶因子1表现出良好的光致变色行为.凝胶性能测试中,分子间存在氢键作用、π-π相互作用等使得该化合物在醇类、有机酸类和乙腈等极性溶剂中极易形成稳定的有机凝胶.在少数的非极性溶剂,如正己烷和环己烷中也可以形成稳定凝胶,并且随着溶剂极性的不同,凝胶形貌呈现出规则的纤维状或带状结构.     

Solvent-tunable binding of hydrophilic and hydrophobic guests by amphiphilic molecular baskets

[reaction: see text] Responsive amphiphilic molecular baskets were obtained by attaching four facially amphiphilic cholate groups to a tetraaminocalixarene scaffold. Their binding properties can be switched by solvent changes. In nonpolar solvents, the molecules utilize the hydrophilic faces of the cholates to bind hydrophilic molecules such as glucose derivatives. In polar solvents, the molecules employ the hydrophobic faces of the cholates to bind hydrophobic guests. A water-soluble basket can bind polycyclic aromatic hydrocarbons including anthracene, pyrene, and perylene. The binding free energy (-deltaG) ranges from 5 to 8 kcal/mol and is directly proportional to the surface area of the aromatic hosts. Binding of both hydrophilic and hydrophobic guests is driven by solvophobic interactions.     

Chirality transfer from chiral solvents and its memory in an azobenzene derivative exhibiting photo-switchable racemization

The transfer and dynamic fixation of chirality in cyclic azobenzenes using R-(+)-1-phenylethylalcohol (R-PEA) and S-(-)-1-phenylethylalcohol (S-PEA) as solvents or additives are investigated. The cyclic azobenzenes used in this study carry a 1,5-dioxynaphthalene moiety as rotating unit, connected to the photoisomerizing (E-Z) azobenzene unit with spacers of varying lengths. With suitable lengths of the spacers the molecules exhibit stable enantiomers originated from the element of planar chirality in the E form due to the stopped rotation of the rotor, while in the Z form the allowed rotation results in racemization. The CD spectra of racemic compounds in the E form in chiral solvents were inert or almost negligible before irradiation, while 366 nm irradiation causing E-Z photoisomerization resulted in induction of clear CD bands. The thermal or photochemical reverse Z-E isomerization causes a change in the CD spectra to new ones which are reasonably matching with the spectra of the pure enantiomers recorded in non-chiral solvents. The obtained new CD spectra are maintained even in a racemic solvent system attained by the dilution with an equal amount of chiral solvent of opposite stereostructure. These results indicate that the chirality is transferred from the chiral solvents or additives to the racemizing Z form of cyclic azobenzene and it is fixed in the non-racemizing E form. The molecule without racemization in both E and Z forms did not show any significant induced CD bands irrespective of E-Z isomerizations. The molecule showing racemization in E and Z forms just shows the non-fixed induced CD. The property of photo-switchable racemization is necessary for the effective transfer and temporal fixation of the chirality in this type of chirality sensors.     

Spectroelectrochemical Study on Radical Intermediate of Coenzyme Q10 at Silver Electrode

The radical intermediate (ubisemiquinone) of CoenzymeQ₁₀ (CoQ₁₀) had been observed in nonpolar solvents by spectroscopic techniques.1 The present work investigates the radical speices, as well known, it is extremely unstable in polar solvents, in ethanol/water system through spectroelectrochemical techniques including in situ UV/Vis,EPR and NMR.     

Solvent extraction of ionic solutes in aqueous solutions

Extraction of ionic solutes in aqueous solutions into various organic solvents is reviewed by showing several examples. The extraction of strong acids into polar organic solvents and nonpolar solvents containing hydrogen-bonding extractants is described as the first example and the extraction of simple metal salts into strongly dielectric or solvating polarsolvents and nonpolar solvents containing solvating extractants is then reported. Finally, the solvent extraction of anionic metal complexes with bulky cations into nonpolar solvents as ion-paris is described and the statistical method for such extraction equilibria is considered.     

Photoluminescent behavior of poly(3‐hexylthiophene) derivatives with a high azobenzene content in the side chains

A series of polythiophene derivatives with substantially higher azobenzene contents in the side chains were prepared via copolymerization of 3‐hexylthiophene with four different types of 4‐((4‐(phenyl)azo)phenoxy)alkyl‐3‐thienylacetate. The alkyl spacers with different lengths, i.e. butyl, hexyl, octyl and undecyl groups were used between the azobenzene group and the thiophene ring. The compositions, structures and thermal properties of these polythiophene derivatives were characterized. The structural dependence of photoluminescent emission, photochromic behavior of these copolymers were systematically studied and compared with poly(3‐hexylthiophene). The results show that the azobenzene substitution renders the polythiophene some interesting optical properties that can be modulated by UV light irradiation. In the azobenzene modified polythiophene, the intensity of photoluminescent emission associated with the conjugated polythiophene main chain was found to decrease significantly upon UV irradiation. The finding suggests that the photo‐induced trans‐cis isomerization of the azobenzene pendant groups has a significant effect on photoluminescent emission, particularly when short spacers are used between azobenzene groups and the main chain. However, the effect becomes less prominent when longer spacers are used between the azobenzene group and the main chain. Furthermore, UV irradiation of the copolymers also resulted in an increase in intensity and broadening of bandwidth for the absorption peak associated with the polythiophene backbones. Again the magnitude of intensity changes upon UV irradiation were found to be dependent on the spacer length between the azobenzene group and polythiophene main chain. Copyright © 2005 John Wiley & Sons, Ltd.     

Photophysics of (1-butyl-4-(1H-inden-1-ylidene)-1,4-dihydropyridine (BIDP): an experimental test for conical intersections

Fluorescence experiments on (1-butyl-4-(1H-inden-1-ylidene)-1,4-dihydropyridine (BIDP) are reported in liquid and glassy solutions. The data indicate a fast decay in the fluid nonpolar, nonprotic solutions (decay times approximately 10(-12) s) and rapid but considerably slower decay in polar ones. In frozen solutions (polar and nonpolar), the fluorescence quantum yield is much higher (near 0.5 and around 0.1 in polar and nonpolar glasses, respectively). The rapid nonradiative transitions in fluid solutions are assigned to internal conversion in both solvent classes, as intersystem crossing is much slower and no net reaction is observed. These results are in agreement with predictions made for the closely related (in terms of electronic structure) but simpler molecule cyclopentadienyl-1,4-dihydropyridine (CPDHP) for which an S1/S0 conical intersection was recently proposed [Int. J. Quant. Chem. 2005, 102, 961]. The crossing of the two lowest singlet states is calculated to vanish in polar solvents such as methyl cyanide, leading to longer lifetime of S1 of CPDHP. As BIDP has a very similar electronic structure, the model predicts a corresponding change in this larger molecule. The strong fluorescence observed in the glassy environments is rationalized by the hindering of the internal torsion required to reach the geometry of the conical intersection.     

New heterocyclic tetrathiafulvalene compounds with an azobenzene moiety: photomodulation of the electron-donating ability of the tetrathiafulvalene moiety

New heterocyclic TTF compounds 1a-c and 2 with an azobenzene moiety were described. The oxidation potential of 1a could be reversibly modulated by alternating UV and visible light irradiation. As a result, a molecular switch with UV/visible light as the inputs and the electrochemical signal as the output was achieved. Moreover, it was found that the influence of the azobenzene photoisomerization on the electronic property of the TTF unit became stronger with shorter spacers in compounds 1a-c.     

Environmentally responsive molecular baskets: unimolecular mimics of both micelles and reversed micelles

[structure: see text] When four facially amphiphilic cholate derivatives are attached to a tetraaminocalixarene scaffold, the resulting molecule responds to environmental changes by rotation of the cholate units. In polar solvents, the molecule adopts a micellelike conformation with the hydrophilic alpha-faces of the cholates pointing outward. In nonpolar solvents, it turns inside out, assuming a reversed micellelike conformation with the hydrophobic beta-faces pointing outward. Switching between the two conformations is driven by solvophobic interactions and is fully reversible.     

The role of the ring nitrogen and the amino group in the solvent dependence of the excited-state dynamics of 3-aminoquinoline

The nonradiative rate in 3-aminoquinoline is found to exhibit anomalous solvent dependence, being rather fast in nonpolar solvents and remarkably slower in more polar and especially, more protic ones. The cause of such behavior is investigated by studying the dependence of fluorescence spectral and temporal parameters on the solvent properties such as polarity and hydrogen bonding ability. Complementary quantum mechanical calculations have been performed and the picture that emerges from these studies is that of an excited state with a short radiative lifetime due to the flipping of the amino group. This state is selectively populated in nonpolar, nonhydrogen bonding solvents, but is destabilized with respect to the more polar intramolecular charge transfer (ICT) state in polar solvents and even more so in protic solvents and dimethylsulfoxide. The slower nonradiative rates in the ICT state is attributed to the more restricted motion of the amino group in this state. The role of hydrogen bonding of the amino group and the ring nitrogen in stabilization/destabilization of the ICT state and therefore on the nonradiative rate is also explored.     

Oligomeric cholates: amphiphilic foldamers with nanometer-sized hydrophilic cavities

The hydroxyl at the C-3 of cholic acid was converted to an amino group, and the resulting amino-functionalized cholic acid was used as a monomer to prepare amide-linked oligomeric cholates. These cholate oligomers fold into helical structures with nanometer-sized hydrophilic internal cavities in solvent mixtures consisting of mostly nonpolar solvents such as carbon tetrachloride or ethyl acetate/hexane and 2-5% of a polar solvent such as methanol or DMSO. The conformations of the foldamers were studied by UV, fluorescence, fluorescence quenching, and fluorescence resonance energy transfer. The nature of the polar/nonpolar solvents and their miscibility strongly influenced the folding reaction. Folding was cooperative, as evidenced by the sigmoidal curves in solvent denaturation experiments. The folded conformers became more stable with an increase in the chain length. The folding/unfolding equilibrium was highly sensitive toward the amount of polar solvent. One percent variation in the solvent composition could change the folding free energies by 0.5-1.4 kcal/mol.     

4-Dimethylaminochalcone as fluorescent probe: effect of the medium polarity on relaxation processes in the excited state

Relaxation processes in a 4-dimethylaminochalcone molecule after excitation with a light pulse of duration 70 fs were studied. During 0.4–1 ps after excitation, an absorbance of an excited state S₁ with a maximum at 460 nm is formed in both polar and nonpolar media. Subsequent relaxation processes depend on the polarity of the medium. In nonpolar hexane, the 4-dimethylaminochalcone molecule transits to the triplet state having an absorption maximum at 570 nm (lifetime longer than 600 ps) for 20 ps. In polar aprotic acetonitrile, the absorbance at 460 nm decreases slowly (during hundreds of picoseconds), indicating that the molecules return to the ground state. The induced emission from the level S₁ in a region of 520–550 nm and fluorescence from the same level with a maximum at 537 nm are also observed in acetonitrile. Thus, a reason for a sharp decrease in the fluorescence yield on going from polar to nonpolar media was found. The mechanism of fluorescence quenching of 4-dimethylaminochalcone in nonpolar media is confirmed by the data on phosphorescence. The phosphorescence of 4-dimethylaminochalcone is observed at–196 °C in nonpolar solvents, indicating a triplet excited state, while no phosphorescence is revealed in polar solvents.Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1607–1610, August, 2004.     

Phase-selective solubility of poly(N-alkylacrylamide)s

The extent of the phase-selective solubility of poly(N-alkylacrylamide)s was studied by UV-vis and fluorescence spectroscopy using poly(N-isopropylacrylamide) and poly(N-octadecylacrylamide) as representative polar and nonpolar poly(N-alkylacrylamide)s in a mixture of polar and nonpolar thermomorphic solvents. Phase-selective solubilities of greater than 10000:1 were seen with each labeled polymer in polar and nonpolar solvents such as heptane and DMF or heptane and 90% EtOH-H(2)O. Using a poly(N-acryloxysuccinimide) as a common precursor, a pool-split synthesis was devised to prepare a library of poly(N-alkylacrylamide)s whose members varied only in the size of their N-alkyl substituent. The solubilities of these library members were measured in both the polar and nonpolar phases of a thermomorphic heptane/90% EtOH-H(2)O mixture at 25 degrees C. Such solvent mixtures are miscible hot (70 degrees C) and biphasic cold (25 degrees C). The results show that poly(N-pentylacrylamide) is selectively soluble (>99.5%) in the polar EtOH-rich phase at rest. Poly(N-alkylacrylamide)s with larger N-alkyl groups are predominantly (C(6), 85%; C(7), 95%) or exclusively (>C(8), >99.5%) in the heptane-rich phase at rest.     

Effect of surfactant and solvent properties on the stacking behavior of non-aqueous suspensions of organically modified clays

Montmorillonite clay was treated with quaternary ammonium surfactants with 1-3 long chains of 10-18 carbons to form organoclays which can be suspended in non-aqueous solvents. The effects of surfactant chain length, number of long chains, and the properties of the solvent on the colloidal behavior of the surfactant coated clay plates were studied using small-angle X-ray scattering. The scattering data were modeled using a one-dimensional aggregation theory to describe the stacking of the clay plates. The plates self-organize into stacks with a reproducible basal spacing in the range of 30-50 A, and for each surfactant, the basal spacing falls into one of two preferred distances. We interpret this by considering that the surfactant layer on the clay has two strata, one being the polar near-clay headgroup region and the other the nonpolar alkane chain region. Polar solvents will swell the polar stratum preferentially while nonpolar solvents will swell the nonpolar stratum of the surfactant. As the nonpolar stratum is larger than the polar one, the nonpolar solvents increase the basal spacing between the clay plates more than the polar solvents. The number of long chains on the surfactant does not have an effect on the basal spacing, as the density of surfactant molecules on the surface is low enough to allow the unimpeded swelling of the chains. The one-dimensional aggregation theory can be used to determine the number of plates in a stack, but the effect of changing clay particle size or concentration is not as great as would be expected from this theory. This may be due to the formation of large-scale structures in the suspensions which prevent a true equilibrium stack size being attained.     

Estimation of Solubility of Carbon Dioxide in Polar Solvents

The solubility of CO₂ in polar solvents is poorly predicted by all estimation methods that use only properties of the pure components. It is thought that this is because CO₂ molecules, although they do not have a permanent dipole moment, behave like electrical multipoles as a consequence of their strongly polar bonds. An equation, proposed in a previous paper, for estimating the activity coefficient of a nonpolar gas dissolved in a polar liquid is modified by adding a term containing the quadrupole moment of the gas molecule. Errors in estimating gas solubilities on that basis are less than 20%, except for the solvents in which specific interactions (e.g., acid–base interactions) are present.     

Spectrophotometric Studies on Some Arylazo Barbituric Acids and Arylazo Pyrimidine in Organic Solvents and in Buffer Solutions

The UV and visible spectra of some arylazo barbituric acids and arylazo pyrimidine in pure and mixed organic solvents of varying polarities have been studied. The different absorption bands displayed in the electronic spectra of these compounds are assigned and the possibility of the existence of these compounds in tautomeric equilibrium as well as in intra‐ and intermolecular H‐bonding were examined. The solute‐solvents molecular complexes liable to exist in polar‐nonpolar mixed solvents are investigated. The structure of the azo compounds were confirmed by ¹H NMR and IR spectroscopy. The ionization processes of the compounds were investigated and the possible ionization constants were determined and discussed in terms of molecular structure of the azo compounds.     

Exciplex formation between pyrene and guanine in highly polar solvents

Guanine derivatives connected to pyrene with methylene spacers exhibited exciplex emission in highly polar solvents such as N,N-dimethylformamide, acetonitrile, and water. The result will open up a novel approach to synthesis of fluorescent nucleic bases.