Multiprobe Spectroscopic Investigation of Molecular-level Behavior within Aqueous 1-Butyl-3-methylimidazolium Tetrafluoroborate

Isomerization, Protonation, and Hydrolysis Properties of Naphthalimide-Containing Spiropyran in Aqueous Media

Synthesis of spiropyrans exhibiting ring-opening/closing isomerization driven by external stimuli is a challenging subject for the development of molecular sensors. A napthalimide-containing spiropyran (1) promotes rapid isomerization between the spirocyclic (SP) form and ring-opened merocyanine (MC) form by the change in solvent polarity even under the dark condition at room temperature. In this work, the effect of water on the isomerization behavior of 1 was studied. The addition of water caused an MC-water H-bonding interaction and shifted the resonance MC structure to the zwitterionic form with a lower ground-state energy. The stabilized MC form promoted spontaneous SP → MC isomerization and increased the equilibrium MC concentration. The effect of pH on the behavior of 1 was also studied. In acidic-neutral media, protonation/deprotonation of the naphthalimide moiety led to rapid and reversible changes in the absorption spectra. In contrast, strongly basic media (pH > 12) promoted irreversible base-catalyzed hydrolysis of the alkene moiety.

Deep eutectic solvents comprising creatine and citric acid and their hydrated mixtures

We report the phase diagram for the binary creatine-citric acid mixture which features a stable and broad eutectic region. Combinations containing 10-60 mol% creatine yield a deep eutectic solvent with a glass transition temperature at 270 K. Addition of up to 70 mol% water to the binary mixture affords retention of the eutectic nature and a handle to vary solvent viscosity and polarity.

Ionic liquid inspired alkalinochromic salts based on Reichardt's dyes for the solution phase and vapochromic detection of amines

Chromogenic salts based on the negatively solvatochromic pyridinium N-phenolate betaines 2,6-diphenyl-4-(2,4,6-triphenyl-N-pyridino)-phenolate (Reichardt's dye 30) and 2,6-dichloro-4-(2,4,6-triphenyl-N-pyridino)-phenolate (Reichardt's dye 33) proved to be promising probes for the colorimetric detection of bases, including hydroxide ion, ammonia, and aliphatic amines. Specifically, the protonated halide forms of these two dyes were ion exchanged to generate lipophilic bis(trifluoromethylsulfonyl)imide derivatives, denoted [E_T(30)][Tf₂N] and [E_T(33)][Tf₂N], respectively. When dissolved in 95 vol% EtOH, these essentially colorless solutions displayed dramatic "alkalinochromic" color-on switching due to phenolic deprotonation to generate the zwitterionic form of the dyes with their characteristic charge-transfer absorption. The extent of the colorimetric response varied with the base strength for the aliphatic amines tested (i.e., propylamine, ethanolamine, ethylenediamine, diethylenetriamine, triethylamine, triethanolamine), being loosely correlated with the pK_b of the amine. In addition, we demonstrated proof of concept for the vapochromic detection of ammonia and aliphatic amines by dissolution of the chromogenic probes in the ionic liquid 1-propyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide. We also showed that the dyed ionic liquid can be successfully immobilized within silica sol-gel ionogels to generate more practical and robust sensory platforms. This strategy represents a useful addition to existing colorimetric sensor arrays targeting amines and other basic species. In particular, the differential response of the two different probes offers a measure of chemical selectivity which will be of interest for detecting biogenic amines in food safety applications, among other areas.

Alternative probe for the determination of the hydrogen-bond acidity of ionic liquids and their aqueous solutions

Although highly relevant to a priori select adequate solvents for a given application, the determination of the hydrogen-bond acidity or proton donor ability of aqueous solutions of ionic liquids is a difficult task due to the poor solubility of the commonly used probes in aqueous media. In this work, we demonstrate the applicability of the pyridine-N-oxide probe to determine the hydrogen-bond acidity of both neat ionic liquids and their aqueous solutions, based on ¹³C NMR chemical shifts, and the suitability of these values to appraise the ability of ionic liquids to form aqueous two-phase systems.

Synthesis and fluorescence spectroscopy of tris(pyrenyl)pnictogen compounds

A homologous series of tris(pyrenyl)pnictogen compounds has been prepared and studied, revealing the dramatic impact of pnictogen choice on fluorescence quantum yield and photophysical properties.

Water Nanocluster Formation in the Ionic Liquid 1-Butyl-3-methylimidazolium Tetrafluoroborate ([C4mim][BF4])-D2O Mixtures

The microstructure of mixtures of deuterated water in the ionic liquid [C4mim][BF4] is investigated by small-angle neutron scattering (SANS) measurement. In the salt-rich region, water dissolves in the ionic liquid up to 0.7 mole fraction, whereupon distinct, nanometer-sized water clusters are observed. These water nanoclusters increase in size with increasing water addition while the mole ratio of water dissolved into the ionic liquid nanostructure increases from 2 to 4. These results provide direct confirmation for recent simulations as well insight into the source of nonidealities in some thermophysical and transport properties (e.g., density and viscosity) of salt-rich aqueous mixtures reported in the literature.

Photophysics and Rotational Dynamics of a Hydrophilic Molecule in a Room Temperature Ionic Liquid

We have studied the photophysics and rotational diffusion of hydrophilic solute 7-(N, N'-diethylamino)coumarin-3-carboxylic acid (7-DCCA) in a room temperature ionic liquid methyltrioctylammonium bis(trifluoromethylsulfonyl) imide ([N1888 ][NTf2 ]). Comparison of activation energies of viscous flow and nonradiative decay shows that the photophysical properties of 7-DCCA are not guided by the bulk viscosity of the medium but are dependent on the specific solute solvent interaction and structural heterogeneity of the medium. The rotational relaxation behaviour of 7-DCCA in [N1888 ][NTf2] shows significant deviation from the Stokes Einstein Debye hydrodynamic model of rotational diffusion. This is indicative of the influence of specific solute solvent interaction on the rotational relaxation behaviour of 7-DCCA. Comparison of activation energy of rotational relaxation with activation energy of viscous flow clearly reinforces our assumption that the structural heterogeneity of the medium and specific solute solvent interaction plays a dominant role on the rotational diffusion instead of bulk viscosity.

Investigation of the fluorescence quenching of 1-aminoanthracene by dissolved oxygen in cyclohexane

This study provides a detailed investigation of the fluorescence quenching mechanisms of the fluorophore, 1-aminoanthracene, by dissolved oxygen in cyclohexane. Dynamic/collisional quenching dominates in the system studied, but there is also a small component of static quenching. Stern-Volmer plots revealed that the dynamic quenching constant is 0.445 ± 0.014 mM(-1) and represents ∼95% of total quenching in the system. The static quenching rate constant is 0.024 ± 0.001 mM(-1), and mechanisms by complex formation and "sphere of action" static quenching were examined. Compensation of steady-state fluorescence data for solvent loss during the gradual deoxygenation period of the experiment was found to be important in order to conduct a thorough evaluation of the different quenching mechanisms of the system. The enhancement factors, (F(o)/F) and (τ(o)/τ), for 1-aminoanthracene were determined to be 2.20 ± 0.01 and 2.08 ± 0.01, respectively, and the diffusion-controlled bimolecular rate constant was found to be 2.1 × 10(10) ± 0.2 × 10(10) M(-1) s(-1). The work involved the development of a novel instrumental setup that simultaneously measures several important spectroscopic parameters (steady-state fluorescence intensity, absorbance, fluorescence lifetime, and dissolved oxygen concentration) for the careful study of oxygen quenching mechanisms of 1-aminoanthracene in a cyclohexane solution.

Solvatochromic probe behavior within choline chloride-based deep eutectic solvents: effect of temperature and water

Deep eutectic solvents (DESs) have shown potential as promising environmentally friendly alternatives to conventional solvents. Many common and popular DESs are obtained by simply mixing a salt and a H-bond donor. Properties of such a DES depend on its constituents. Change in temperature and addition of water, a benign cosolvent, can change the physicochemical properties of DESs. The effect of changing temperature and addition of water on solvatochromic probe behavior within three DESs formed from choline chloride combined with 1,2-ethanediol, glycerol, and urea, respectively, in 1:2 mol ratios termed ethaline, glyceline, and reline is presented. Increase in temperature results in reduced H-bond donating acidity of the DESs. Dipolarity/polarizability and H-bond accepting basicity do not change with changing temperature of the DESs. The response of the fluorescence probe pyrene also indicates a decrease in the polarity of the DESs as temperature is increased. Addition of water to DES results in increased dipolarity/polarizability and a decrease in H-bond accepting basicity. Except for pyrene, solvatochromic probes exhibit responses close to those predicted from ideal-additive behavior with slight preferential solvation by DES within the aqueous mixtures. Pyrene response reveals significant preferential solvation by DES and/or the presence of solvent-solvent interactions, especially within aqueous mixtures of ethaline and glyceline, the DESs constituted of H-bond donors with hydroxyl functionalities. FTIR absorbance and Raman spectroscopic measurements of aqueous DES mixtures support the outcomes from solvatochromic probe responses. Aqueous mixtures of ethaline and glyceline possess relatively more interspecies H-bonds as compared to aqueous mixtures of reline, where interstitial accommodation of water within the reline molecular network appears to dominate.

Unusual solvatochromic absorbance probe behaviour within mixtures of poly(ethylene glycol)-400+ionic liquid, [bmim][Tf2N]

The potentially green solvents made up of ionic liquids (ILs) and poly(ethylene glycols) may have wide range of the applications in many chemical and biochemical fields. In the present work, solvatochromic absorbance probe behaviour is used to assess the physicochemical properties of the mixtures composed of PEG-400+IL, 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, [bmim][Tf2N]. Lowest energy intramolecular charge-transfer absorbance maxima of a betaine dye, i.e., E(T)(N), indicates the dipolarity/polarizability and/or hydrogen-bond donating (HBD) acidity of the [bmim][Tf2N]+PEG-400 mixtures to be even higher than that of neat [bmim][Tf2N], the solution component with higher dipolarity/polarizability and/or HBD acidity. Dipolarity/polarizability (π(∗)) obtained separately from the electronic absorbance response of probe N,N-diethyl-4-nitroaniline, and the HBD acidity (α) of PEG-400+[bmim][Tf2N] mixtures are also observed to be anomalously high. A comparative study of the PEG+IL mixtures has also been done with PEG-400+molecular organic solvents (protic polar [methanol], aprotic polar [N,N-dimethylformamide], and non polar, [benzene]) mixtures, but these mixtures do not show this type of unusual behaviour. A four-parameter simplified combined nearly ideal binary solvent/Redlich-Kister (CNIBS/R-K) equation is shown to satisfactorily predict the solvatochromic parameters within PEG-400+different solvent mixtures.

The solvation dynamics at millisecond scale of Pd(II)-meso-tetra(4-carboxyphenyl)porphine in solid imidazolium-sulfonate-based ionic liquids

The phosphorescence behavior and solvation dynamics of Pd(II)-meso-tetra(4-carboxyphenyl)porphine (Pd-TCPP) in three imidazolium ionic liquids (ILs) has been studied. The phosphorescence excitation and emission position of Pd-TCPP are dependent on the polarity, constituent ions and structure of the ILs. The phosphorescence decay of Pd-TCPP in these ILs is bi-exponential, and the average lifetimes decrease in the following order: 0.844 ms in 1-butylimidazolium p-toluenesulfonate (HBIMTS)>0.472 ms in 1-butylimidazolium trifluoromethane sulfonate (HBIMTfO)>0.412 ms in 1-butyl-3-methylimidazolium p-toluenesulfonate (BMIMTS). The time-resolved phosphorescence emission spectra (TRPES) of Pd-TCPP display apparently blue-shifts with increasing time. The normalized Stokes shift correlation functions C(t) of Pd-TCPP in these ILs are mono-exponential, and the solvation relaxation times are 2.97, 47.7 and 12.1 ms in HBIMTfO, HBIMTS and BMIMTS, respectively. The solvation dynamics may be attributed to the collective motion of the ion pairs composed of imidazolium cations and counter anions to the area surrounding Pd-TCPP.