Reichardt's Dye Research Articles

Dihydrogen-bonding deep eutectic solvents as reaction media for lipase-catalyzed transesterification

Choline chloride (ChCl)-based deep eutectic solvents (DESs) were successfully used as reaction media for the Candida antarctica lipase B-catalyzed transesterification. The activity of lipase in the DESs was highly influenced by the water activity of the DESs, with optimal lipase activity at a water activity of 0.1. Seventeen DESs containing one or two hydrogen bond donors (HBDs) were used for lipase-catalyzed transesterification at a selected water activity. The highest lipase activity was obtained in ChCl:urea:formamide (molar ratio of 1:1:1), whereas the highest lipase stability was achieved in ChCl:glycerol:ethylene glycol (1:1:1). In general, higher lipase activity was achieved in DESs containing amide HBDs, whereas higher lipase stability was obtained in DESs containing hydroxyl HBDs. The effects of the DESs on the activity and stability of the lipase were evaluated by determining the solvatochromic parameters. Correlation of the solvent parameters of the DESs with the lipase stability showed that the lipase stability increased with an increase in polarity based on Reichardt's dye and the hydrogen bond acidity of the DESs.

Quantifying Surface Properties of Silica Particles by Combining Hansen Parameters and Reichardt's Dye Indicator Data

AbstractTo obtain quantitative understanding of the effects of a chemisorbed organic modification on the surface of particles, the use of Reichardt's dye (RD) and Hansen solubility parameter (HSP) is discussed, whereby the S should be understood in terms of “similarity” rather than solubility as dispersibility is in focus. Silica nanoparticles modified to different extents with a medium chain silane including completely hydrophilic and hydrophobic particles are chosen. During spray‐drying such particles form fully redispersible micro‐raspberry superstructures. After qualitative estimations of the particles' polarity based on measuring both immersion time and ability of modified particles to stabilize oil–water emulsions, surface properties are quantified by HSP and RD. With increasing hydrophobicity, i.e., increasing amount of silane at the surface, all three contributions to HSP change. At the same time, RD analysis reveals that the normalized solvent polarity parameter decreases progressively. HSP and RD analysis are in good agreement, giving strong confidence on each method applied individually. This work demonstrates that after noticeable attempts for combined solubility parameters in case of molecules, carbon allotropes, and gelators, such studies can be extended toward functional (nano)particles and that a full picture of particle surface properties is possible via the combination of different, quantitative techniques.

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 [ET(30)][Tf2N] and [ET(33)][Tf2N], respectively. When dissolved in 95vol% 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 pKb 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.

Combined optical and electronic paper-nose for detection of volatile gases

In this work, a paper-based optoelectronic sensor (paper-nose) is presented for sensing volatile gases in air. The proposed optoelectronic sensor is a combination of both colorimetric (optical) and chemiresistive (electronic) sensor arrays in order to improve the selectivity of the paper-nose in the complex air background. The optical sensors are based on chemoresponsive dyes, namely Reichardt's dye (2,6-diphenyl-4-(2,4,6-triphenyl-1-pyridinio)phenolate), bromocresol purple, methyl red, bromothymol blue, brilliant yellow and manganese tetraphenylporphyrin (Mn-TPP). The chemiresistive sensors are based on nanomaterials, such as carbon nanotubes (CNT), PEDOT:PSS, graphite, and an ionic liquid, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMI TFSI). Sensor is fabricated through direct handwriting of sensing materials using a pen on paper without the need of expensive cleanroom facilities. The optoelectronic sensor is tested in ambient air with different volatile gases such as methanol, ammonia, toluene, acetone and ethanol and their mixtures of varying concentrations. The detected electrical and optical responses together form a unique signature for each volatile gas and its mixture. Support-vector machine (SVM) is applied for target classification and detection. From the SVM result, it is found that better discriminative power is achieved by combining optical and electrical responses.

Solvatomorphism of Reichardt's dye

Six different crystal structures are obtained depending on the crystallization solvent.

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.

Character of Localization and Microenvironment of Solvatochromic Reichardt's Betaine Dye in Sodium n-Dodecyl Sulfate and Cetyltrimethylammonium Bromide Micelles: Molecular Dynamics Simulation Study.

The problem of using surfactant micellar aqueous solutions as reaction media centers on estimating the polarity of the micellar pseudophase. The most popular approach is the utilization of solvatochromic dyes. Among the last, the strongest ones are the dipolar pyridinium N-phenolate dyes. The complication of such approach, however, consists in the nonuniform character of the environment of the indicator fixed in the micellar pseudophase. The aim of this study is to reveal the character of localization and orientation of the standard solvatochromic pyridinium N-phenolate dye, 4-(2,4,6-triphenylpyridinium-1-yl)-2,6-diphenylphenolate, the so-called Reichardt's dye, within the micellar pseudophase of an anionic (sodium n-dodecyl sulfate, SDS) and cationic (cetyltrimethylammonium bromide, CTAB) surfactants using MD simulations. The locus and hydration of the dye are found to be dependent on the surfactant nature. New approaches are proposed to quantitatively describe the state of the dye within the pseudophase. The results confirm the experimental data, which indicate the higher polarity of the interfacial region in the case of the SDS micelles. Because this dye is also used as an interfacial acid-base probe, the corresponding study is simultaneously performed for its protonated, i.e., cationic form. The neutral and protonated forms of the dye are found to be localized and hydrated in a different way in both SDS and CTAB micelles. This should be taken into account when using the Reichardt's dye as an acid-base indicator for estimating the electrical surface potential of micelles. The presented approach may be recommended to shed light upon the locus of other solvatochromic and acid-base indicators in micelles and micellar-like aggregates.

Propylene Carbonate Based‐Organic Electrolytes for Cellulose Dissolution Processing and Derivatization

AbstractThe design of green and facile dissolution technologies to allow homogeneous processing and derivatization of cellulose towards a controllable facile preparation of cellulosic materials is prime important. Herein, in combination with [Emim]OAc ionic liquids, propylene carbonate (PC) is reported to dissolve up to 10wt% of cellulose at 80 oC in 20 mins when XILs in the designed “organic electrolyte solutions (OESs)” is larger than 0.3. Rheological study of the obtained cellulose solution was performed and then the polarity of the PC, [Emim]OAc and the OESs was assessed from the solvatochromic behaviour of Reichardt's dye. The results show that the addition of [Emim]OAc ionic liquids into PC can considerably increase the polarity of PC, as evidenced by the fact that ET(30) values increases from 47.501 to 50.132 when XILs is 0.4. Finally, the potential of the obtained cellulose solution in OESs for cellulose homogeneous processing into films and derivatization was primary demonstrated by the successful preparation of transparent cellulosic films and effective transesterification of cellulose with vinyl pivalate and benzoate. In a word, the study provides a new green solvent system and insights the future design of new solvents for cellulose homogeneous processing and derivatization.

Remarkable Solvatofluorochromism of a [2.2]Paracyclophane‐Containing Organoboron Complex: A Large Stokes Shift Promoted by Excited State Intramolecular Charge Transfer

AbstractAn organoboron complex containing a [2.2]paracyclophanyl group (2 BF2) exhibits remarkable solvatofluorochromism associated with a large Stokes shift. With increased solvent polarity, the fluorescence (FL) color of 2 BF2 changes from sky blue (λFL=485 nm in cyclohexane (c‐C6H12)) to orange (λFL=576 nm in CH3CN). This behavior derives from the intramolecular charge transfer (ICT) character of the excited state 2 BF2* in which the [2.2]paracyclophanyl group serves as the electron donor while the remaining 2 BF2 structure (apart from the sub‐benzene ring) functions as the electron acceptor. Wave deconvolution of FL spectra in a solvent mixture of CH2Cl2/c‐C6H12, and a plot of the FL emission energy versus the CH2Cl2/c‐C6H12 ratio, suggest that FL of 2 BF2 occurs from two types of 2 BF2* excited states. The results of DFT calculations support this explanation and further emphasize that the ICT character of 2 BF2* is derived from a large conformational change associated with the electron‐donating character of the [2.2]paracyclophanyl group. Moreover, the fact that 2 BF2 is more sensitive than Reichardt's dye to polarity changes in a low polarity CH2Cl2/c‐C6H12 solvent mixture is also of importance.

Solvatochromic Parameters and Preferential Solvation Behavior for Binary Mixtures of 1,3-Dialkylimidazolium Ionic Liquids with Water

Binary mixtures of 1,3-dialkylimidazolium based ionic liquids (ILs) and water were selected as solvent systems to investigate the solute-solvent and solvent-solvent interactions on the preferential solvation of solvatochromic indicators at 25 °C. Empirical solvatochromic parameters, dipolarity/polarizability (π*), hydrogen-bond donor acidity (α), hydrogen-bond acceptor basicity (β), and Reichardt's polarity parameters (ENT) were measured from the ultraviolet-visible spectral shifts of 4-nitroaniline, 4-nitroanisole, and Reichardt's dye. The solvent properties of the IL-water mixtures were found to be influenced by IL type and IL mole fraction (xIL). All these studied systems showed the non-ideal behavior. The maximum deviation to ideality for the solvatochromic parameters can be obtained in the xIL range from 0.1 to 0.3. For most of the binary mixtures, the π* values showed the synergistic effects instead of the ENT, α and β values. The observed synergy extent was dependent on the studied systems, such as the dye indicator and IL type. A preferential solvation model was utilized to gather information on the molecular interactions in the mixtures. The dye indicator was preferentially solvated on the following trend: IL>IL-water complex>water.

Explicit Solvent Modeling of Solvatochromic Probes in Ionic Liquids: Implications of Solvation Shell Structure.

Calculations of absorption spectra have been performed for three solvatochromic dyes (para-nitroaniline, N,N-diethyl-para-nitroaniline, and Reichardt's dye) in four ionic liquids based on 1-ethyl-3-methylimidazolium cation. Different variants of explicit solvent model have been used: fully explicit representation of solvent ions, charge embedding method, and mixed approach. Electrostatic solute-solvent interactions have been shown to induce structuring of the solvation shell around dipolar solute, causing significant and nonmonotonic dependence of calculated transition energies on the number of ion pairs included in calculations. A mixed approach with few fully explicit solvent ions embedded in electrostatic field of point charges appears as a promising method of calculations of solvatochromic response, provided that sufficiently large system sizes are used in explicit solvent modeling.

Self‐Assembly of Amphiphilic Block Copolypeptoids with C2‐C5 Side Chains in Aqueous Solution

Nowadays, amphiphilic molecules play an important role in our life. In medical applications, amphiphilic block copolymers have attracted much attention as excipients in drug delivery systems. Here, the polymers are used as emulsifiers, micelles, or polymersomes with a hydrophilic corona block and a hydrophobic core or membrane. The aggregation behavior in aqueous solutions of a series of different amphiphilic block copolypeptoids comprising polysarcosine as a hydrophilic part is here reported. The formation of aggregates is investigated with 1H NMR spectroscopy and dynamic light scattering, and the determination of the critical micelle concentration (cmc) is performed using pyrene fluorescence spectroscopy. For the different block copolypeptoids cmc values ranging from 0.6 × 10−6m to 0.1 × 10−3m are found. The tendency to form micelles increases with increasing hydrophobicity at the nitrogen side chain in the hydrophobic moiety. Furthermore, in the case of the same hydrophobic side chain, a decreasing hydrophilic/lipophilic balance leads to the formation of larger aggregates. The aggregates formed in the buffer are able to solubilize the hydrophobic model compounds Reichardt's dye and pyrene, and exhibit versatile microenvironments. Final investigations about the cytotoxicity reveal that the block copolypeptoids are well tolerated by mammalian cells up to high concentrations. image

Polymeric chain dependent anomalous solvatochromism of ionic liquid + poly(ethylene glycol) mixtures

To alter the physicochemical properties of ILs with green cosolvents, molecular level understanding of the resultant mixtures is hotbed of the current research. In the present study, changes in the physicochemical properties of the IL 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([bmim][Tf2N]) on the addition of different polyethylene glycols (PEGs) (average MW 200, 400, 600, and 1500) by means of solvatochromic probes are investigated. The addition of the PEGs to IL [bmim][Tf2N] changes the physicochemical properties of the solvent mixtures in unusual and favorable fashion. The response of the solvatochromic probes Reichardt dye 33 and Nile Red hints about the “hyperpolarity” of the IL+PEG mixtures which is rare in the literature. The Kamlet–Taft parameters, hydrogen bond donor ability (HBD) (α), hydrogen bond acceptor ability (HBA) (β), and dipolarity/polarizability (π*) display nonideality, and show strong and unusual “hyperpolarity” behavior on the addition of PEGs to [bmim][Tf2N]. A solvent–solvent along with solute–solvent interaction hypothesis is proposed to explain the “hyperpolarity” of the mixtures. Further, the solvation models are used to delineate the solute–solvent/solvent–solvent interactions present in the binary mixtures. Combined nearly ideal binary solvent/Redlich–Kister (CNIBS/R–K) equation was demonstrated to predict the solvatochromic parameters satisfactorily.

Two‐Photon Solvatochromism II: Experimental and Theoretical Study of Solvent Effects on the Two‐Photon Absorption Spectrum of Reichardt’s Dye

In this study, we report on the influence of solvent on the two-photon absorption (2PA) spectra of Reichardt's dye (RD). The measurement of 2PA cross-sections is performed for three solvents (chloroform, dimethyl formamide, and dimethyl sulfoxide) using the Z-scan technique. The key finding of this study is the observation that the cross-section, corresponding to the 2PA of the intramolecular charge-transfer state, diminishes substantially upon increasing the solvent polarity. To unravel the solvent dependence of the 2PA cross-section, the electronic structure of RD is determined using a hybrid quantum mechanics/molecular mechanics (QM/MM) approach, in which polarization between the solute and solvent is taken into account by using a self-consistent scheme in the solvent polarization. The two-state approximation proves to be adequate for the studied system, and allowed the observed solvent-polarity-induced decrease of the 2PA cross-section to be related to the decrease of the transition moment and the increase in the excitation energy.

Solvatochromic solvent polarity parameters for the characterisation of some cyanobiphenyl nematic liquid crystals

In application of liquid crystals (LCs) as solvents, it is required to obtain the polarity and solvatochromic polarity parameters such as the Kamlet–Abboud–Taft polarity functions. In this study, solvatochromic polarity parameters of some cyanobiphenyl nematic liquid crystals were acquired in different temperatures and phases, isotropic and anisotropic phases, using the Reichardt's dye and 2,6-diphenyl-4-(2,4,6-triphenyl-1-pyridinio)phenolate standard probe. In addition, to decrease the overlapping of the absorption band of probe with the absorption bands of LCs and to increase the solubility of the probe in LCs, a new azo dye was used as a probe for obtaining some solvatochromic polarity parameters. Finally, a new polarity parameter, the LC anisotropic matrix, is introduced.

Determination of solvatochromic solvent polarity parameters for the characterisation of some nematic liquid crystal in anisotropic and isotropic phases

Characterisation of liquid crystals (LCs) as solvents is needed, to obtain the polarity and solvatochromic polarity parameters of these media. Polarity parameters demonstrate the effects of LC media on the photo-physical behaviour of solute molecules in an anisotropic medium. The practical limitations in determining solvent polarity scale parameters for LCs can overcome the overlapping absorption band of LCs and solvent-sensitive standard compounds or their insolubility in LCs. In this work, we report Kamlet–Abboud–Taft polarity functions of some nematic LCs in different temperatures and phases, isotropic and anisotropic, with the solvatochromic method, using the Reichardt's dye and 2,6-diphenyl-4-(2,4,6-triphenyl-1-pyridinio)-phenolate standard probe. In addition, a new azo and coumarin dye were used as probes to obtain some solvatochromic polarity parameters. Finally, a new polarity parameter, the LC anisotropic matrix, is introduced.

Identification and Molecular Interpretation of the Effects of Drug Incorporation on the Self-Emulsification Process Using Spectroscopic, Micropolarimetric and Microscopic Measurements

Addition of a drug to a self-emulsifying drug delivery system (SEDDS) can affect the emulsification process after administration, leading to variation in the emulsion droplet size formed and potentially its clinical behavior (Mercuri et al., Pharm. Res., 2011, 28, 1540-1551). However, the mechanisms involved and, in particular, the location of the drug within the system are poorly understood. Here, we have investigated the location of a model drug, ibuprofen, in the emulsions formed from a simple anhydrous SEDDS (soybean oil, Tween 80 and Span 80), using a range of physical characterization techniques. (1)H NMR studies showed an interaction between the drug and the polyoxyethylene chains of the surfactant Tween 80. Micropolarity assessment of the emulsion droplet interfacial region, using the chemical probes pyrene and Reichardt's dye, confirmed this interaction, and suggested that the drug was altering the microenvironment around the surfactants, and hence the behavior of the SEDDS with water during emulsification. Both dielectric spectroscopy and polarized light microscopy highlighted the differential behavior with water of placebo and drug-loaded SEDDS, also seen in the initial visual observational studies on the emulsification performance of the SEDDS. (1)H NMR studies with three other NSAIDs indicate that this effect is not confined to ibuprofen alone. The study has therefore indicated that the drug's influence on the emulsification process may be related to interactions within the microenvironment of the surfactant layer. Furthermore, such interactions may be usefully identified and characterized using a combination of micropolarity, spectroscopic and microscopic methods.

Controlling Hydrolysis Reaction Rates with Binary Ionic Liquid Mixtures by Tuning Hydrogen-Bonding Interactions

The ability of a binary ionic liquid (IL) system consisting of a phosphonium transition state analogue (TSA) and 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([BMIM][NTf(2)]) to accelerate the rate of the well-studied hydrolysis of a tert-alkyl picolinium salt by influencing the solvent structure was investigated. A significant rate enhancement was observed in the presence of the TSA; however, comparison with other cations illustrated that this enhancement was not unique to the chosen TSA. Instead, the rate enhancements were correlated with the dilution of hydrogen bonding by the added cations. This phenomenon was further examined by the use of 1-butyl-2,3-dimethylimidazolium bis(trifluoromethanesulfonyl)imide ([BMMIM][NTf(2)]) as a cosolvent and the use of Reichardt's dye to measure the extent of hydrogen bonding on solutes in these systems. The rate increases are rationalized in terms of weaker hydrogen bonding from the solvent system to water.

Distinct influence of the anion and ether group on the polarity of ammonium and imidazolium ionic liquids

The polarity of ionic liquids (ILs), usually denoted as ET(30) by the solvatochromic probe Reichardt’s dye, is one of the most fundamental properties that remarkably affect the solvation and chemical reaction in ILs. It was generally accepted that the ET(30) of ILs was dominated by the nature of the cation. However, in this work, it was found that the common ammonium-based ILs showed strongly anion-dependent ET(30). For example, the ET(30) value for [N1124][DCA] and [N1124][NTf2] is 49.0 and 59.0 kcal mol−1, respectively, while the corresponding imidazolium ILs bearing the same anions possess nearly identical ET(30), the ET(30) value for [BMIm][DCA] and [BMIm][NTf2] is 51.4 and 51.6 kcal mol−1, respectively. Moreover, introduction of an ether group was found to increase the ET(30) of imidazolium ILs while having no obvious effect on that of ammonium-based ILs. The Kamlet–Taft parameters and density functional theory (DFT) calculations indicated that the distinct result is related to different stabilization of the ground state of Reichardt's dye 30. In imidazolium ILs, the main interactions between ILs and zwitterionic dye involve both coulombic interaction (between the cation and the phenolate oxygen atom) and H-bonding interaction (between the acidic hydrogen on imidazolium ring and the phenolate oxygen atom). However, with the ammonium ILs lack of active hydrogen, the dye is only stabilized by the coulombic interaction between the cation and the phenolate oxygen atom. Interestingly, in both imidazolium and ammonium-based ILs, the spiropyran–merocyanine equilibrium exhibit obvious anion-dependent photochromism, solvatochromism, and thermal relaxation.

Solvent induced channel interference in the two-photon absorption process—a theoretical study with a generalized few-state-model in three dimensions

For the first time, we report the effect of interference between different optical channels on the two-photon absorption (TPA) process in three dimensions. We have employed response theory as well as a sum-over-states (SOS) approach involving few intermediate states to calculate the TPA parameters like transition probabilities (δ(TP)) and TPA tensor elements. In order to use the limited SOS approach, we have derived a new formula for a generalized few-state-model (GFSM) in three dimensions. Due to the presence of additional terms related to the angle between different transition moment vectors, the channel interference associated with the TPA process in 3D is significantly different and much more complicated than that in 1D and 2D cases. The entire study has been carried out on the two simplest Reichardt's dyes, namely 2- and 4-(pyridinium-1-yl)-phenolate (ortho- and para-betain) in gas phase, THF, CH(3)CN and water solvents. We have meticulously inspected the effect of the additional angle related terms on the overall TPA transition probabilities of the two 3D isomeric molecules studied and found that the interfering terms involved in the δ(TP) expression contribute both constructively and destructively as well to the overall δ(TP) value. Moreover, the interfering term has a more conspicuous role in determining the net δ(TP) associated with charge transfer transition in comparison to that of π-π* transition of the studied systems. Interestingly, our model calculations suggest that, for o- and p-betain, the quenching of destructive interference associated with a particular two-photon process can be done with high polarity solvents while the enhancement of constructive interference will be achieved in solvents having relatively small polarity. All the one- and two-photon parameters are evaluated using a range separated CAMB3LYP functional.