Water-soluble anionic N-confused porphyrin for sensitive and selective detection of heavy metal pollutants in aqueous environment.

Efficient detection and quantification of metal ions in real time and in a cost-effective manner is a critical step in combating the increasing danger of heavy metal contamination of our biosphere. The potential of water-soluble anionic derivative of N-confused tetraphenylporphyrin (WS-NCTPP) has been investigated for quantitative detection of heavy metal ions. The results show that the photophysical properties of WS-NCTPP differ significantly in the presence of four metal ions, namely Hg(II), Zn(II), Co(II) and Cu(II). The variation in the spectral behaviour is driven by the formation of 1:1 complexes with all the four cations with varied degree of complexation. The selectivity of the sensing is studied through interference studies, indicating maximum selectivity for Hg(II) cations. Computational studies of the structural features of the metal complexes with WS-NCTPP help in establishing the geometry and binding interactions between the metal ions and the porphyrin nucleus. The results demonstrate the promising potential of the NCTPP probe which should be utilized for detection of heavy metal ions, especially mercury, in the near future.

Spectroscopic investigation of preferential solvation of N-confused tetraphenylporphyrinin binary mixtures of dichloromethane with organic cosolvents.

Spectroscopic behaviour of porphyrin is known to be sensitive to the choice of the solvent and has been used to probe solvation phenomena. The UV-visible spectra of N-confused tetraphenylporphyrin are observed in a series of binary solvent mixtures - (dichloromethane + diethyl ether), (dichloromethane + chlorobenzene) and (dichloromethane + ethyl acetate). The resulting ET scale for NCTPP solvation indicates nonlinear behaviour and preferential solvation by a solvent-solvent complex in all the three solvents - with the dichloromethane + ethyl acetate system showing the largest deviation from ideality. The data is fitted to the model based on solvent exchange equilibria for determining the preferential solvation parameters which are specific to the probe as well as the identity of solvents in the binary mixture. Further analysis using polarity parameters from literature indicate that the solvation of NCTPP in the (dichloromethane + ethyl acetate) mixture is dependent on the hydrogen bond accepting capacity and polarizability of the medium. Excess spectra derived from ATR-FTIR measurement of dichloromethane + ethyl acetate solutions lead to important inferences about the structure and role of solvent-solvent interactions responsible for the preferential solvation.

Temperature-Dependent Empirical Parameters for Polarity in Choline Chloride Based Deep Eutectic Solvents.

Deep eutectic solvents have emerged as a promising and economically viable alternative to address our need for environmentally benign media. Efficient utilization of deep eutectic solvents in various fields will require adequate knowledge about their physicochemical properties. In this work, independent polarity scales pertaining to solvent polarizability, dipolarity, acidity and basicity have been determined for some commonly used deep eutectic solvents based on choline chloride. The three eutectic mixtures selected for the study include reline (1:2 mixture of choline chloride and urea), ethaline (1:2 mixture of choline chloride and ethylene glycol), and glyceline (1:2 mixture of choline chloride and glycerol). The variation of the probe response as a function of temperature revealed some interesting details about the polarity of neoteric solvents being investigated. Our observations indicate that while the polarity parameters for ethaline and glyceline are easily determined, the instability of probe molecules in reline poses a potential challenge for the determination of its polarity parameters. Attempts to correlate the empirical polarity parameters with solvatochromic response of Reichardt's dye (ETN parameter) also led to surprising observations. Use of alternative probes to measure the polarity in reline indicated that the polarity scales using different probes need to be normalized for DESs. The results emphasize the extent of applicability of currently employed polarity scales to determine the polarity of DES, the need to develop appropriate probe molecules, and the factors limiting our understanding of these extremely interesting class of solvents.

Nonlinear effects at the soft interface of an emulsion in the List-Lerner-Barbas aldol reaction.

Nonlinear effects have been observed for the List-Lerner-Barbas aldol reaction catalyzed by a proline-based surfactant catalyst at the aqueous-organic interface under emulsifying conditions. The differential partitioning of the amphiphilic catalyst enantiomers at the interfacial assembly leads to deracemization at the soft interface of an emulsion. The effect cascades to the interfacial chemical processes, which reflect the asymmetry of the interacting chiral assembly. The observations establish that soft interfaces such as those present in an emulsion can present a platform for inducing asymmetric partitioning of catalysts and hence, lead to an apparently nonlinear trend in the induced chirality for interfacial processes.

Viscosity dependence of intra- and intermolecular Diels-Alder reactions.

The kinetics of the bimolecular Diels-Alder reaction between anthracene-9-carbinol and N-ethyl maleimide have been studied in a series of pyridinium-based ionic liquids with the tetrafluoroborate ([BF(4)](-)) and the bis(trifluorosulfonimide) ([NTf(2)](-)) anions and the viscosity dependence of the rate constants have been compared with the results for the intramolecular Diels-Alder reaction of (E)-1-phenyl-4-[2-(3-methyl-2-butenyloxy)benzylidene]-5-pyrazolone. The comparison leads to the surprising observation that the intermolecular reaction is less susceptible to viscosity variations of the ionic liquids as compared to the intramolecular reaction. The observed similarities and differences emphasize the complicated nature of solvent friction on the kinetics of cycloaddition reactions. The results are explained by considering the bimolecular and intramolecular processes as cases of translational and rotational diffusion, respectively. Plausible indicators of microviscosity consistent with the kinetic data for the Diels-Alder reactions are briefly discussed along with the constraints involved in designing a general microviscosity scale.

Interfacial reactivity of "on water" reactions in the presence of alcoholic cosolvents.

A detailed study of the variation in the interfacial reactivity and selectivity of "on water" reactions in the presence of increasing amounts of alcoholic cosolvents is discussed in the present work. The initial increase in the rates of "on water" reactions on the addition of alcoholic cosolvents is contrary to the sharp decrease in rates observed for homogeneous aqueous reactions. The existing theoretical framework is presented in terms of a typical "on water" process to enable a discussion of the experimental observations with reference to changing composition of the reaction medium. On the basis of the interesting observations of reactivity and selectivity variations with composition of the reaction medium, a simple "signature" characteristic for "on water" reactions is proposed.

Intramolecular Diels-Alder reaction in ionic liquids: effect of ion-specific solvent friction.

The present work aims at understanding the role of viscosity or solvent friction in ionic liquids for an intramolecular Diels-Alder (IMDA) reaction of (E)-1-phenyl-4-[2-(3-methyl-2-butenyloxy)benzylidene]-5-pyrazolone (1). The results have been analyzed on the basis of the current theoretical models, and their failure to account for the observed trends is discussed in terms of "effective" viscosity or microviscosity. The rates of the reaction decrease with the increasing viscosity of the ionic liquids. As evident from the anionic effect, the solute-solvent specific interactions play a role in governing the kinetics of the reaction. The lower viscosities of the bistrifluoromethanesulfonimide [NTf2](-) based ionic liquids as compared to those based on tetrafluoroborate [BF4](-) anion fail to result in a corresponding acceleration in the rates of the reaction. These contradictory results indicate that solvent microviscosity, rather than the bulk macroscopic viscosity, should be the criteria for selecting the ionic liquids as reaction media.

Unusual temperature dependence of salt effects for "on water" Wittig reaction: hydrophobicity at the interface.

An unusual variation with temperature of the salt effects in aqueous Wittig reaction is observed, suggesting that hydrophobic acceleration of reactions comprising "on water" reactants is fundamentally different from that for reactions with small non-polar solutes.