Role of solvophilic moieties of gelators in the thermal stability of organogels.

Affinity analysis using Hansen solubility parameters (HSP) reveals that the gel-sol transition point of binary organogels is correlated with preferential interactions between the solvophilic moiety of gelators and solvents. The decomposition of gelators in HSP analysis can be effectively used for predicting the thermal stability of gels.

Solvent-Dependent Properties and Higher-Order Structures of Aryl Alcohol + Surfactant Molecular Gels.

Molecular organogels, comprising small organic gelators in solvents, can be applied for dispersal of optical devices, such as emitters. Phenolic compounds and the surfactant bis(2-ethylhexyl) sulfosuccinate (AOT) are known examples of self-assembly organogels. However, conventional phenol + AOT gels in aromatic and acyclic alkane solvents are optically turbid, which is an obstacle for use as host materials in optical devices. In this study, a variety of aryl alcohol-AOT-solvent sets have been investigated systematically, and the correlation between the molecular architecture and optical transparency of the gels was considered. Accordingly, p-chlorophenol + AOT gels in cyclic alkane solvents were shown to form optically transparent gels. In contrast, aromatic and acyclic alkane solvents gave rise to turbid or opaque gels, even when utilizing the same gelators. AFM, NMR, SAXS, and FTIR were employed to determine the organogel structures. Consequently, we found that the gel transparency strongly depends on the size of the fibrous network of the gel, the structure of which is attributed to higher-order aggregates of the gelators. The average contour length and diameter of the fibrous network, lav and dav, respectively, were determined from AFM images. The transparent gels were shown to have lav = 4-9 µm and dav ≤ 0.3 µm, whereas the turbid gels had lav = 15 µm and dav = 0.4-0.6 µm. Such differences in the size of the fibrous network significantly affected the mechanical response of the gels, as shown by stress-strain measurements.

Solvent dependence of Stokes shift for organic solute-solvent systems: A comparative study by spectroscopy and reference interaction-site model-self-consistent-field theory.

The Stokes shift magnitudes for coumarin 153 (C153) in 13 organic solvents with various polarities have been determined by means of steady-state spectroscopy and reference interaction-site model-self-consistent-field (RISM-SCF) theory. RISM-SCF calculations have reproduced experimental results fairly well, including individual solvent characteristics. It is empirically known that in some solvents, larger Stokes shift magnitudes are detected than anticipated on the basis of the solvent relative permittivity, ɛr. In practice, 1,4-dioxane (ɛr = 2.21) provides almost identical Stokes shift magnitudes to that of tetrahydrofuran (THF, ɛr = 7.58), for C153 and other typical organic solutes. In this work, RISM-SCF theory has been used to estimate the energetics of C153-solvent systems involved in the absorption and fluorescence processes. The Stokes shift magnitudes estimated by RISM-SCF theory are ∼5 kJ mol(-1) (400 cm(-1)) less than those determined by spectroscopy; however, the results obtained are still adequate for dipole moment comparisons, in a qualitative sense. We have also calculated the solute-solvent site-site radial distributions by this theory. It is shown that solvation structures with respect to the C-O-C framework, which is common to dioxane and THF, in the near vicinity (∼0.4 nm) of specific solute sites can largely account for their similar Stokes shift magnitudes. In previous works, such solute-solvent short-range interactions have been explained in terms of the higher-order multipole moments of the solvents. Our present study shows that along with the short-range interactions that contribute most significantly to the energetics, long-range electrostatic interactions are also important. Such long-range interactions are effective up to 2 nm from the solute site, as in the case of a typical polar solvent, acetonitrile.

Nona-coordinated chiral Eu(III) complexes with stereoselective ligand-ligand noncovalent interactions for enhanced circularly polarized luminescence.

Circularly polarized luminescence (CPL) of chiral Eu(III) complexes with nona- and octa-coordinated structures, [Eu(R/S-iPr-Pybox)(D-facam)(3)] (1-R/1-S; R/S-iPr-Pybox, 2,6-bis(4R/4S-isopropyl-2-oxazolin-2-yl)pyridine; D-facam, 3-trifluoroacetyl-d-camphor), [Eu(S,S-Me-Ph-Pybox)(D-facam)(3)] (2-SS; S,S-Me-Ph-Pybox, 2,6-bis(4S-methyl-5S-phenyl-2-oxazolin-2-yl)pyridine), and [Eu(Phen)(D-facam)(3)] (3; Phen, 1,10-phenanthroline) are reported, and their structural features are discussed on the basis of X-ray crystallographic analyses. These chiral Eu(III) complexes showed relatively intense photoluminescence due to their (5)D(0) → (7)F(1) (magnetic-dipole) and (5)D(0) → (7)F(2) (electric-dipole) transition. The dissymmetry factors of CPL (g(CPL)) at the former band of 1-R and 1-S were as large as -1.0 and -0.8, respectively, while the g(CPL) of 3 at the (5)D(0) → (7)F(1) transition was relatively small (g(CPL) = -0.46). X-ray crystallographic data indicated specific ligand-ligand hydrogen bonding in these compounds which was expected to stabilize their chiral structures even in solution phase. CPL properties of 1-R and 1-S were discussed in terms of transition nature of lanthanide luminescence.

Uptake of Metal (Zn, Y, Ti) Oxide Nanoparticles by Poaceae and Cucurbitaceae Plants Based On Metal Properties and Surface Conditions.

Metal nanoparticles (NPs) may serve as biomarkers, as the surfaces can be chemically modified to enable an analysis of several biosystems, including plant pathogenesis. We supplied metal oxide NPs including those of ZnO, TiO2, Y2O3, and Y2O3 doped with europium to plants of eight species of the Poaceae and Cucurbitaceae families. The plants were grown using hydroponics, where NPs were incorporated into the cultivation media. Energy-dispersive X-ray spectroscopy was used to detect the uptake of NPs by the plant in regions of the root, stem, and leaf. Results show that ZnO NPs were taken up more readily by the plants compared to other NPs. Unmodified NPs were only delivered up till the stems and not the leaves; however, when the surfaces were modified using photoinduced hydrophilization supplemented with poly(ethylene glycol), NPs were delivered to the leaves of plants. It is suggested that plants readily take up metals such as zinc that function as nutrients. Additionally, hydrophilization of NP surfaces using UV irradiation enhances uptake, where modified ZnO and TiO2 NPs may be delivered to the leaves. These findings may be used to design biomarker systems for detecting tissue damage and infections in various crops.

Solvation dynamics in polar solvents studied by means of RISM/mode-coupling theory.

The extended reference interaction site model (RISM) theory coupled with the generalized Langevin/mode-coupling theory (MCT) is applied to the investigation of solvation dynamics in polar solvents. The RISM/ MCT framework used in this paper significantly upgrades the previous report by Nishiyama and co-workers [Nishiyama, K.; et al. J. Chem. Phys. 2003, 118, 2279.] for the calculation of the solvation response function, Ss(t). This function is experimentally observable from dynamic Stokes shift measurements, for example. Ss(t) obtained by RISM/MCT relaxes with an initial Gaussian decay followed by damped oscillation, which is in accordance with experimental results or molecular dynamics simulations published elsewhere. Ss(t) is then decoupled into the acoustic and optical modes of solvent, which indicate the translational and rotational motions of solvent, respectively. The majority (> 90%) of Ss(t) is explained by the optical mode, whereas the slower acoustic mode also plays an important role. Resultingly, RISM/MCT is shown to be an appropriate theoretical methodology to capture a molecular view of solvation dynamics, without assuming any empirical parameters.

Relation between Anharmonicity of Free-Energy Profile and Spectroscopy in Solvation Dynamics: Differences in Spectral Broadening and Peak Shift in Transient Hole-Burning Spectroscopy Studied by Equilibrium Molecular Dynamics Simulation.

Solvation dynamics is used to monitor the time-dependent fluctuation of solvents, which plays an essential role in chemical reactions in solution. Transient hole-burning spectroscopy, in which a ground-state depletion (hole) formed by a laser pulse is observed, can be used to monitor solvation dynamics. Previous experiments demonstrated that the hole bandwidth relaxes an order of magnitude slower than the hole peak shift in organic solute-solvent systems. However, the detailed mechanisms behind this are still unclear. In this study, we developed a methodology to calculate transient hole spectra using equilibrium molecular dynamics simulation, in which a series of time-dependent system ensembles is accumulated to derive the appropriate dynamic properties. The simulated transient hole spectra adequately reproduced previous spectroscopic results. The different hole bandwidth and peak shift dynamics are ascribed to a non-Gaussian property or anharmonicity of the free energy profile with respect to the solvation coordinate.

Reorientational Relaxation of Small Solutes in Viscoelastic Liquids.

The reorientational relaxation times of some small aromatic solutes are determined with nuclear magnetic resonance spectroscopy and time-resolved fluorescence anisotropy measurements in various solvents that exhibit viscoelasticity in the megahertz region. All the reorientational relaxation times in viscoelastic liquids are shorter than those predicted by the hydrodynamic Stokes-Einstein-Debye (SED) relation using the steady-state shear viscosity. The deviation from the SED relation becomes larger in solvents whose shear relaxation is slower. When the reorientational relaxation times in a solvent are compared, the deviation from the SED relation tends to decrease when the reorientational relaxation time increases. From a comparison with the shear relaxation spectra, it is demonstrated that the deviation from the SED relation can be ascribed to the effective reduction of the viscous friction on fast reorientation, reflecting the decrease in shear viscosity with increasing frequency.