Polar Molecular Solvents Research Articles

A metal-free blue chromophore derived from plant pigments.

Blue natural pigments are rare, especially among plants. However, flowering species that evolved to attract Hymenoptera pollinators are colored by blue anthocyanin-metal complexes. Plants lacking anthocyanins are pigmented by betalains but are unable to produce blue hues. By extending the π-system of betalains, we designed a photostable and metal-free blue dye named BeetBlue that did not show toxicity to human hepatic and retinal pigment epithelial cells and does not affect zebrafish embryonal development. This chiral dye can be conveniently synthesized from betalamic acid obtained from hydrolyzed red beetroot juice or by enzymatic oxidation of l-dopa. BeetBlue is blue in the solid form and in solution of acidified polar molecular solvents, including water. Its capacity to dye natural matrices makes BeetBlue the prototype of a new class of low-cost bioinspired chromophores suitable for a myriad of applications requiring a blue hue.

Product as Reaction Solvent: An Unconventional Approach for Ionic Liquid Synthesis

Ionic liquids (ILs) are considered a promising class of potentially environmentally friendly solvents for use in a variety of applications. However, they are generally synthesized by batch in nonsustainable solvents. ILs to be really “green” compounds for widespread use should be also synthesized in a benign manner. Here, we report an unconventional procedure to prepare [Bmim]Cl, as a typical IL, using the same product as solvent. This approach guarantees a high reaction rate assuring at the same time a high product purity degree. The comparison with three sustainable polar molecular solvents, having a sufficiently high boiling point (1-butanol, methylisobutylketone, and water), was also performed showing all of the potentialities of the proposed procedure.

Photo- and Radiation-Chemistry of Halide Anions in Ionic Liquids

One- and two- photon excitation of halide anions (X(-)) in polar molecular solvents results in electron detachment from the dissociative charge-transfer-to-solvent state; this reaction yields a solvated halide atom and a solvated electron. How do such photoreactions proceed in ionic liquid (IL) solvents? Matrix isolation electron paramagnetic resonance (EPR) spectroscopy has been used to answer this question for photoreactions of bromide in aliphatic (1-butyl-1-methylpyrrolidinium) and aromatic (1-alkyl-3-methyl-imidazolium) ionic liquids. In both classes of ILs, the photoreaction (both 1- and 2-photon) yields bromine atoms that promptly abstract hydrogen from the alkyl chains of the IL cation; only in concentrated bromide solutions (containing >5-10 mol % bromide) does Br2(-•) formation compete with this reaction. In two-photon excitation, the 2-imidazolyl radical generated via the charge transfer promptly eliminates the alkyl arm. These photolytic reactions can be contrasted with radiolysis of the same ILs, in which large yield of BrA(-•) radicals was observed (where A(-) is a matrix anion), suggesting that solvated Br(•) atoms do not occur in the ILs, as such a species would form three-electron σ(2)σ(*1) bonds with anions present in the IL. It is suggested that chlorine and bromine atoms abstract hydrogen faster than they form such radicals, even at cryogenic temperatures, whereas iodine mainly forms such bound radicals. These XA(-•) radicals convert to X2(•-) radicals in a reaction with the parent halide anion. Ramifications of these observations for photodegradation of ionic liquids are discussed.

Nanostructure and amphiphile self-assembly in polar molecular solvents: amides and the “solvophobic effect”

The ability of low molecular weight amides to support amphiphile self-assembly is shown to be a general feature for this class of solvents. This report extends the number of known polar solvents which can support amphiphile self-assembly by five new amides; more than doubling the number of known amides able to serve as amphiphile self-assembly media. The formation of lyotropic liquid crystalline phases by cationic and non-ionic surfactants in these liquid amides is reported. The ability of a solvent to promote amphiphile self-assembly is governed by the "solvophobic effect" and is linked to the solvent cohesiveness. The Gordon parameter which is a measure of the solvent cohesiveness was found to provide a guide to an amides capacity to support lyotropic liquid crystalline phase diversity and thermal stability ranges of those phases. The "solvophobic effect" and steric hindrance factors were compared between amide's and protic ionic liquids possessing analogous chemical structures and also being able to promote amphiphile self-assembly.

Solvation Dynamics in Ionic Liquids: What We Have Learned from the Dynamic Fluorescence Stokes Shift Studies

Ionic liquids have generated tremendous excitement as a new class of solvent systems having the potential to serve as an environmentally friendly alternative to the traditionally used volatile organic compounds and have emerged as specialty media or materials for many advanced processes and devices. Solvent reorganization in ionic liquids is a complex process, which differs significantly from that in conventional polar molecular solvents. This Perspective highlights the present understanding of the mechanism and time scale of the dynamics of solvation in ionic liquids obtained from the dynamic fluorescence Stokes shift studies on dipolar molecules taking the related theoretical results into consideration.

Behavior of Electrogenerated Bases in Room-Temperature Ionic Liquids

The reductive electrochemistry of substituted benzophenones in the aprotic room-temperature ionic liquid (RTIL) 1-butyl-1-methylpyrrolidinium bistriflimide occurs via two consecutive one-electron processes leading to the radical anion and dianion, respectively. The radical anion exhibited electrochemical reversibility at all time-scales whereas the dianion exhibited reversibility at potential sweep rates of >or=10 V s(-1), collectively indicating the absence of strong ion-paring with the RTIL cation. In contrast, reduction in 1-butyl-3-methylimidazolium bistriflimide is complicated by proton-transfer from the [Bmim] cation. At low potential sweep rates, reduction involves a single two-electron process characteristic of either an electrochemical, chemical, electrochemical (ECE) or disproportion-type (DISP1) mechanism. The rate of radical anion protonation in [Bmim] is governed by basicity and conforms to the Hammett free-energy relation. At higher potential sweep rates in [Bmim][NTf2], reduction occurs via two consecutive one-electron processes, giving rise to the partially reversible generation of the radical anion and the irreversible generation of the dianion, respectively. Also, the redox potentials for the reversible parent/radical anion couples were found to be a linear function of Hammett substituent constants in both RTIL media and exhibited effectively equivalent solvent-dependent reaction constants, which are similar to those for reduction in polar molecular solvents such as acetonitrile or alcohols.

Solvent reorganization energy of electron-transfer reactions in polar solvents.

A microscopic theory of solvent reorganization energy in polar molecular solvents is developed. The theory represents the solvent response as a combination of the density and polarization fluctuations of the solvent given in terms of the density and polarization structure factors. A fully analytical formulation of the theory is provided for a solute of arbitrary shape with an arbitrary distribution of charge. A good agreement between the analytical procedure and the results of Monte Carlo simulations of model systems is achieved. The reorganization energy splits into the contributions from density fluctuations and polarization fluctuations. The polarization part is dominated by longitudinal polarization response. The density part is inversely proportional to temperature. The dependence of the solvent reorganization energy on the solvent dipole moment and refractive index is discussed.

Dressed molecule theory for liquids and solutions: An exact charge renormalization formalism for molecules with arbitrary charge distributions

An exact statistical mechanical theory for fluid mixtures of rigid molecules with arbitrary charge distributions, sizes, and shapes is presented. It deals with many-body effects in electrostatic interactions between molecules in fluids and can, for example, be applied to mixtures of polar molecules and to solutions of electrolytes or colloidal dispersions in polar molecular solvents. All solute and solvent molecules are treated on the same fundamental level in statistical mechanics. The exact screened Coulomb potential φ0(r) for the solution is given a general definition. A renormalized charge distribution ρi0 for each molecule of any species i is uniquely defined such that the total electrostatic potential from each i molecule is exactly given by φ0 with ρi0 as the source. By using ρi0 when calculating the interaction between the molecule and the total electrostatic potential from any source, one includes the indirect effects from the surrounding polarizable molecular medium on the electrostatic part of the potential of mean force for the molecule. In general, all kinds of molecules (charged, polar, and apolar ones) acquire renormalized charges in electrolyte solutions. The dielectric function and other fundamental properties of the mixture can be expressed in terms of ρi0 for all species. The formally exact theory is expressed in a Poisson–Boltzmann (PB)-type manner by using the renormalized rather than actual (bare) charges and it is shown that the total electrostatic potential due to a molecule satisfies an equation that is the exact version of the linear PB equation. The decay behaviors of φ0, the pair potential of mean force and pair distribution functions are investigated.

Evidence for hydrogen bonding in solutions of 1-ethyl-3-methylimidazolium halides, and its implications for room-temperature halogenoaluminate(III) ionic liquids

Multinuclear NMR spectroscopy and conductivity measurements showed that the 1-ethyl-3-methyl-imidazolium cation, [emim]+, not only forms strong hydrogen bonds (using all three ring protons H2, H4 and H5) with halide ions in polar molecular solvents (e.g. ethanenitrile) and ionic liquids, but that it exists in a quasi-molecular state, [emim]X, in non-polar solvents (e.g. trichloro- and dichloro-methane), showing a conventional aromatic stacking phenomenon.

The potential of mean force between polyatomic molecules in polar molecular solvents

Theoretical results obtained with the extended reference interaction site (XRISM) formalism are presented for site–site solute solvent correlations and solute–solute potentials of mean force for infinitely dilute polar molecular solutes in various polar solvents. The standard RISM site–site Ornstein–Zernike like equations, in a Coulomb renormalized form, with a hypernetted chain (HNC) analog closure are used to derive results for polar molecular solutes in polar molecular solvents. For a dipolar diatomic solute the difference in the solvation behavior between atomic and molecular solvents is examined. Finite concentration results are compared with the infinite dilution intermolecular site–site potentials of mean force for diatomic molecules in a simple fluid solvent.