Empirical Scale Of Solvent Polarity Research Articles

The Preferential Solvation of Methyl Red in Various Binary Aqueous Solutions: Solvatochromism, Preferential Solvation Parameters, Microheterogeneity of Solvents and Polarity Scale.

The solvatochromic characteristics of methyl red were examined in several aqueous solutions from pure water, with methanol, ethanol, propanol, acetonitrile, and dioxane. In order to explain the preferred solvation of the probe azo dye in the binary mixed solvents, the solvent exchange model of Bosch and Roses was used to evaluate the association between the empirical solvent polarity scale (ET) values of MR and solvent composition. Non-linear solvatochromism of methyl red was noted in all aqueous mixtures containing methanol, ethanol, propanol, acetonitrile, and dioxane. In addition to calculating the local mole fraction of each solvent composition in the cybotactic area of the probe, the impact of the solvating shell composition on the preferential solvation of the solute dye was examined in terms of both solvent-solvent and solute-solvent interactions. The local mole fraction of each solvent composition in the cybotactic region of the probe was also calculated. The results indicated that the MR solvation shell was thoroughly saturated with the solvent complex S12 in the following order: dioxane > ethanol > methanol > acetonitrile > propanol. Data from the binary systems were analyzed with KAT parameters using a multi-model; in aqueous methanol and ethanol solutions, the hydrogen acidity was more responsible for the spectral shift, whereas in aqueous acetonitrile and dioxane solutions, the basicity has a greater influence.

The non-ideality in binary aqueous systems contributed to the different abilities of solvent entities incorporated in the solvation shell of methylene blue

The solvatochromic properties of methylene blue (MB) were investigated in neat water, methanol, ethanol, propanol, dioxane and their corresponding aqueous mixtures. The correlation of the empirical solvent polarity scale (ET) values of MB with solvent composition was analysed using the solvent exchange model of Bosch and Roses to explain the preferential solvation of the probe thiazine dye in the binary mixed solvents. Non-linear solvatochromism of MB was observed in aqueous mixtures of methanol, ethanol, propanol and dioxane. The influence of the composition of the solvating shell in preferential solvation of the solute dye was investigated in terms of both solvent?solvent and solute?solvent interactions, and the local mole fraction of each solvent composition in the cybotactic region of the probe was also calculated. Effective mole fraction variation can provide important physicochemical insights into the microscopic and molecular interactions between MB species and solvent components. The results showed that the MB solvation shell was thoroughly saturated with the solvent complex S12 for dioxane more than ethanol and propanol mixtures, and opposite trends for methanol mixtures, whereas the solvent complex S12 could not incorporate into the MB solvation shell. Data from the binary systems were analysed with KAT parameters using a dual model of basicity and polarity. The results showed that the polarity was better suited for spectral shift in aqueous methanol and ethanol solutions, while the basicity was better for aqueous propanol and dioxane solutions.

Fluorescence based studies on the interaction and characterization of surface-active ionic liquids with polarity sensitive intramolecular charge transfer probe

Surface-active ionic liquids (SAILs) are of tremendous interest in recent times due to their improved application in the field of food, detergent, and health. The modulated fluorescence behavior of an intramolecular charge transfer (ICT) probe, trans‐ethyl‐p‐(dimethylamino) cinnamate (EDAC), in presence of five different SAILs with varying head groups (viz. imidazolium, pyridinium, and morpholinium) and different alkyl chain length (decyl, dodecyl, and tetradecyl) was monitored by steady state and time-resolved fluorescence spectroscopy. Extreme sensitivity of EDAC fluorescence was used to characterize different physicochemical properties of the SAILs, including critical micelle concentration, static polarity, and empirical solvent polarity scale, ET(30) of the microenvironment. The estimated parameters are in good agreement with the literature reports and measured independently from other complimentary experiments discussed here. Time‐resolved fluorescence experiments show a significant retardation in different nonradiative decay channels of EDAC, when compared to that in aqueous phase, indicating a preferential association of the probe in presence of SAILs. The results indicate that the physicochemical properties of SAILs can be tuned by controlling the nature of both the cations and as well as the chain length of the alkyl group. These properties also show significant modulation in solutions with varying SAIL concentration, particularly in the pre- and post-micellar region. The results, particularly the surface-active properties and the self-assembly behavior presented in this study, are expected to provide new knowledge towards the design and development of novel SAILs with specific industrial and biological applications.

Synthesis, Solvatochromism and Estimation of Ground and Excited State Dipole Moments of Silylated Benzothiazole Dyes.

Dyes derived from benzothiazoles are an important class of heterocycles which have remarkable photophysical properties. New photoluminescent 2-phenylbenzothiazole derivatives containing different functional groups were synthesized in high yields and used for silylated derivatives synthesis. The new photoactive compounds were fully characterized and their photophysical properties were investigated. The absorption and fluorescence spectra of the benzothiazoles and their silylated derivatives were evaluated in a series of organic solvents. The results showed that the benzothiazoles present absorption in the ultraviolet range and emission in the blue region with moderate quantum yields and large Stokes shift. The solvatochromism of these compounds was investigated by using Lippert and ET(30) Dimroth-Reichardt empirical solvent polarity scales. The dipole moments obtained by Bakshiev and Kawaski-Chamma-Viallet equations revealed that the excited states were more polar than the ground states.

Reappraisal of Empirical Solvent Polarity Scales for Organic Solvents

AbstractCorrelations of Reichardt'sET(30), the CatalánSdP(solvent dipolarity),SP(solvent polarizability),SA(solvent acidity),SB(solvent basicity), Kamlet‐Taftπ* (dipolarity/polarizability),α(hydrogen bond donating ability) andβ(hydrogen bond accepting ability) polarity parameters with the molar concentration of 161 pure organic solvents are presented. Mostly, linear relationships of the polarity parameter as a function of molar concentration are obtained as long as each individual solvent class is considered separately. A physically different interpretation of theET(30), Kamlet‐Taftπ* andαas well as CatalánSAandSBparameters has been proposed. Furthermore, the Hildebrand solubility parameter in combination with the diffraction index is used for the correlation analysis with the above‐mentioned solvent parameters. It can be concluded that empirical polarity parameters derived from solvatochromic UV/Vis measurements are inherently a function of the molecular structure of the probe.

Characterization of short-range molecular interaction by empirical solvent polarity scale to analyze the Kerr effect of nematic liquid crystals in the isotropic phase

It is well known that a relatively large Kerr effect in the isotropic phase of nematic liquid crystals (NLCs) is related to the existence of short-range ordering (SRO). Thus, the conversion of the long-range ordering (LRO) behavior in the nematic phase to the SRO in the isotropic phase of NLCs can characterize the nonlinear electro-optic Kerr effect. To show conversion from LRO to SRO, we employ an empirical solvent polarity scale (ETN). Interestingly, ETN originates from the solvatochromic polarity (SP) parameters, which demonstrates the quantitative value for describing the molecular interaction in each temperature point, even in the isotropic phase of NLCs. Our results show that the higher ETN value leads to a larger Kerr constant (B) owing to the larger short-range molecular interaction between rigid rod-like NLCs. To demonstrate a linear relationship of the B value with the ETN and SP parameters, a new correlation is introduced for the first time. Our investigations and newly introduced parameters strongly indicate that short-range molecular interactions still exist in the isotropic phase of NLCs, even when the macroscopically observable LRO behaviors disappear.

Structural and photophysical properties of alkylamino substituted 2-arylidene and 2,5-diarylidene cyclopentanone dyes

A series of alkylamino substituted 2-arylidene and 2,5-diarylidene cyclopentanone compounds were synthesized. Spectroscopic and photophysical properties of these compounds have been measured in a variety of solvents. Absorption and fluorescence maxima have been correlated with the Empirical Scale of Solvent Polarity (ET(30)). Theoretical TD-DFT spectral calculations and Lippert–Mataga analysis support the internal charge transfer (ICT) nature of the S0 → S1 excitation for these compounds, with higher degrees of ICT depicted for the alkylamino substituted 2,5-diarylidene cyclopentanones. Photophysical properties consisted of measuring the fluorescence quantum yields (Φf) and lifetimes (τf) in a variety of solvents. Radiative and nonradiative decay constants have been determined from the Φf and τf data. Variation with solvent in the nonradiative rate of decay is interpreted in terms of a competition between internal conversion and intersystem crossing. Lastly, two compounds presented have been shown to undergo excited state protonation in glacial acetic acid.

Stability constants of lithium complexes with 15-crown-5 and 18-crown-6 in a hydrophobic ionic liquid 1-butyl-3-methylimidazolium bis[(trifluoromethyl)sulphonyl]imide

Thermodynamic data for lithium complexes formation with 15-crown-5 (15C5, L) and 18-crown-6 (18C6, L) in a hydrophobic room temperature ionic liquid (RTIL) 1-butyl-3-methylimidazolium bis[(trifluoromethyl)sulphonyl]imide ([BMIM][N(Tf)2]) are measured with NMR 7Li technique at 27–57°C. For 15C5 and 18C6 only LiL+ complexes are found. The stability of lithium complexes in RTIL is estimated to be in the range between water and acetonitrile. The following values for logK(Li+L) and ΔНo(Li+L) at 25°C are determined: 1.66 (0.03), ∼0kJ/mol for 15C5 and 2.0 (0.1), −17.0 (0.5)kJ/mol for 18C6, respectively. Unlike the situation with molecular solvents the lithium complex with 15C5 in [BMIM][N(Tf)2] is entropy stabilized. The polarity of [BMIM][N(Tf)2] fits well the existing empirical solvent polarity scale for molecular solvents characterizing the polarity of this RTIL as rather normal instead of “superpolar”.

Spectral Investigations of Preferential Solvation and Solute–Solvent Interactions of Free Base and Protonated 5,10,15,20-Tetrakis(4-trimethyl-ammonio-phenyl)-porphine Tetratosylate in Aqueous Organic Mixed Solvents

The solvatochromic properties of the free base and the protonated 5,10,15,20-tetrakis(4-trimethyl-ammonio-phenyl)-porphine tetratosylate (TTMAPP) were studied in pure water, methanol, ethanol, 2-propanol, and their corresponding aqueous mixtures. The correlation of the empirical solvent polarity scale (E T) values of TTMAPP with composition of the solvents were analyzed by the solvent exchange model of Bosch and Roses to clarify the preferential solvation of the probe dyes in the binary mixed solvents. The solvation shell composition effects in preferential solvation of the solute dyes were investigated in terms of both solvent–solvent and solute–solvent interactions and also the local mole fraction of each solvent composition was calculated in the cybotactic region of the probe. The effective mole fraction variation may provide significant physicochemical insights in the microscopic and molecular level of interactions between TTMAPP species and the solvent components and, therefore, can be used to interpret the solvent effect on kinetics and thermodynamics of TTMAPP.

Effect of Solvent Polarity on the Vibrational Dephasing Dynamics of the Nitrosyl Stretch in an FeII Complex Revealed by 2D IR Spectroscopy

The vibrational dephasing dynamics of the nitrosyl stretching vibration (ν(NO)) in sodium nitroprusside (SNP, Na2[Fe(CN)5NO]·2H2O) are investigated using two-dimensional infrared (2D IR) spectroscopy. The ν(NO) in SNP acts as a model system for the nitrosyl ligand found in metalloproteins which play an important role in the transportation and detection of nitric oxide (NO) in biological systems. We perform a 2D IR line shape study of the ν(NO) in the following solvents: water, deuterium oxide, methanol, ethanol, ethylene glycol, formamide, and dimethyl sulfoxide. The frequency of the ν(NO) exhibits a large vibrational solvatochromic shift of 52 cm(-1), ranging from 1884 cm(-1) in dimethyl sulfoxide to 1936 cm(-1) in water. The vibrational anharmonicity of the ν(NO) varies from 21 to 28 cm(-1) in the solvents used in this study. The frequency-frequency correlation functions (FFCFs) of the ν(NO) in SNP in each of the seven solvents are obtained by fitting the experimentally obtained 2D IR spectra using nonlinear response theory. The fits to the 2D IR line shape reveal that the spectral diffusion time scale of the ν(NO) in SNP varies from 0.8 to 4 ps and is negatively correlated with the empirical solvent polarity scales. We compare our results with the experimentally determined FFCFs of other charged vibrational probes in polar solvents and in the active sites of heme proteins. Our results suggest that the vibrational dephasing dynamics of the ν(NO) in SNP reflect the fluctuations of the nonhomogeneous electric field created by the polar solvents around the nitrosyl and cyanide ligands. The solute solvent interactions occurring at the trans-CN ligand are sensed through the π-back-bonding network along the Fe-NO bond in SNP.

N,N-dialkyl perfluoroalkanesulfonamides: Synthesis, characterization and properties

Twenty derivatives of N,N-dialkyl perfluoroalkanesulfonamides, n-CmF2m+1SO2NRR′ (m=1, 2, 4, 6, 8; R, R′=CH3, CH2CH3 or CH2CH2OCH3), have been prepared and characterized. Their fundamental physicochemical properties, including melting point, density, viscosity and dielectric constant, are determined. The influences of structural variations in both the perfluoroalkyl and alkyl chains on the above properties are extensively studied. Among these N,N-dialkyl perfluoroalkanesulfonamides, 14 compounds are liquid at room temperature, and some of them exhibit low melting points, even lower than −40°C. The empirical solvent polarity scales for these liquid perfluoroalkanesulfonamides, such as ‘energy of transition’ET(30) values and Kamlet–Taft solvent parameters (π*, α, β), are also determined by solvatochromic measurements, and show good linear temperature dependence. The ET(30) values are in the range of 45.36–55.19kcalmol−1 at 25°C, comparable to those of short chain alcohols. The π* parameters for the compounds containing alkyl chain are in the range of 0.550–0.651 at 25°C, being a little higher than those of short chain alcohols. All these prepared compounds show strong hydrogen-bond donating ability (α values ranging from 0.865 to 1.204 at 25°C), comparable to that of short chain alcohols. Meanwhile, those containing alkoxyalkyl chain display strong hydrogen-bond accepting ability (β values ranging from 0.71 to 0.82 at 25°C), comparable to that of short chain alcohols.

Solvent and substituent effects on aggregation constants of perylene bisimide π-stacks – a linear free energy relationship analysis

A series of six perylene bisimides (PBIs) with hydrophilic and hydrophobic side chains at the imide nitrogens were applied for a comparative study of the solvent and structural effects on the aggregation behaviour of this class of dyes. A comparison of the binding constants in tetrachloromethane at room temperature revealed the highest binding constant of about 10(5) M(-1) for a PBI bearing 3,4,5-tridodecyloxyphenyl substituents at the imide nitrogens, followed by 3,4,5-tridodecylphenyl and alkyl-substituted PBIs, whereas no aggregation could be observed in the accessible concentration range for PBIs equipped with bulky 2,6-diisopropylphenyl substituents at the imide nitrogens. The aggregation behaviour of three properly soluble compounds was investigated in 17 different solvents covering a broad polarity range from nonpolar n-hexane to highly polar DMSO and water. Linear free energy relationships (LFER) revealed a biphasic behaviour between Gibbs free energies of aggregation and common empirical solvent polarity scales indicating particularly strong π-π stacking interactions in nonpolar aliphatic and polar alcoholic solvents whilst the weakest binding is observed in dichloromethane and chloroform. Accordingly, PBI aggregation is dominated by electrostatic interactions in nonpolar solvents and by solvophobic interactions in protic solvents. In water, the aggregation constant is increased far beyond LFER expectations pointing at a pronounced hydrophobic effect.

Preferential solvation and solvation shell composition of free base and protonated 5, 10, 15, 20-tetrakis(4-sulfonatophenyl)porphyrin in aqueous organic mixed solvents

The solvatochromic properties of the free base and the protonated 5, 10, 15, 20-tetrakis(4-sulfonatophenyl)porphyrin (TPPS) were studied in pure water, methanol, ethanol (protic solvents), dimethylsulfoxide, DMSO, (non-protic solvent), and their corresponding aqueous-organic binary mixed solvents. The correlation of the empirical solvent polarity scale ( E T) values of TPPS with composition of the solvents was analyzed by the solvent exchange model of Bosch and Roses to clarify the preferential solvation of the probe dyes in the binary mixed solvents. The solvation shell composition and the synergistic effects in preferential solvation of the solute dyes were investigated in terms of both solvent–solvent and solute–solvent interactions and also, the local mole fraction of each solvent composition was calculated in cybotactic region of the probe. The effective mole fraction variation may provide significant physico-chemical insights in the microscopic and molecular level of interactions between TPPS species and the solvent components and therefore, can be used to interpret the solvent effect on kinetics and thermodynamics of TPPS. The obtained results from the preferential solvation and solvent–solvent interactions have been successfully applied to explain the variation of equilibrium behavior of protonation of TPPS occurring in aqueous organic mixed solvents of methanol, ethanol and DMSO.

On the Molecular Structure and UV/vis Spectroscopic Properties of the Solvatochromic and Thermochromic Pyridinium-N-Phenolate Betaine Dye B30

Quantum chemical calculations as well as vis absorption and fluorescence measurements of the pyridinium-N-phenolate betaine dye B30, dissolved in 1-chlorobutane at temperatures between 343 and 77 K, shed more light on the solvatochromism, thermosolvatochromism, and photophysical behavior of this probe dye, formerly used to establish an empirical scale of solvent polarity, called E(T)(30) or E(T)(N) scale. A new calculated gas-phase E(T)(30) value is reported. Complementary to recent work of Kharlanov and Rettig (J. Phys. Chem. A 2009, 113, 10693-10703), it is shown that fluorescence of B30 in 1-chlorobutane solution is observable already at temperatures just below the solvent's melting point and not only at 77 K. Analogous to increasing solvent polarity, decreasing solvent temperature leads to a large hypsochromic shift of the vis absorption band of B30, dissolved in 1-chlorobutane (Deltalambda = -245 nm from 797 nm at 343 K to 552 nm at 77 K). This thermosolvatochromism can be easily seen: the solution color changes from greenish yellow (343 K) to magenta-violet (77 K).

Solvation in Pure Liquids: What Can Be Learned from the Use of Pairs of Indicators?

The solvation of six solvatochromic probes in a large number of solvents (33-68) was examined at 25 degrees C. The probes employed were the following: 2,6-diphenyl-4-(2,4,6-triphenylpyridinium-1-yl) phenolate (RB); 4-[(E)2-(1-methylpyridinium-4-yl)ethenyl] phenolate, MePM; 1-methylquinolinium-8-olate, QB; 2-bromo-4-[(E)-2-(1-methylpyridinium-4-yl)ethenyl] phenolate, MePMBr, 2,6-dichloro-4-(2,4,6-triphenyl pyridinium-1-yl) phenolate (WB); and 2,6-dibromo-4-[(E)-2-(1-methylpyridinium-4-yl)ethenyl] phenolate, MePMBr(2), respectively. Of these, MePMBr is a novel compound. They can be grouped in three pairs, each with similar pK(a) in water but with different molecular properties, for example, lipophilicity and dipole moment. These pairs are formed by RB and MePM; QB and MePMBr; WB and MePMBr(2), respectively. Theoretical calculations were carried out in order to calculate their physicochemical properties including bond lengths, dihedral angles, dipole moments, and wavelength of absorption of the intramolecular charge-transfer band in four solvents, water, methanol, acetone, and DMSO, respectively. The data calculated were in excellent agreement with available experimental data, for example, bond length and dihedral angles. This gives credence to the use of the calculated properties in explaining the solvatochromic behaviors observed. The dependence of an empirical solvent polarity scale E(T)(probe) in kcal/mol on the physicochemical properties of the solvent (acidity, basicity, and dipolarity/polarizability) and those of the probes (pK(a), and dipole moment) was analyzed by using known multiparameter solvation equations. For each pair of probes, values of E(T)(probe) (for example, E(T)(MePM) versus E(T)(RB)) were found to be linearly correlated with correlation coefficients, r, between 0.9548 and 0.9860. For the mercyanine series, the values of E(T)(probe) also correlated linearly, with (r) of 0.9772 (MePMBr versus MePM) and 0.9919 (MePMBr(2) versus MePM). The response of each pair of probes (of similar pK(a)) to solvent acidity is the same, provided that solute-solvent hydrogen-bonding is not seriously affected by steric crowding (as in case of RB). We show, for the first time, that the response to solvent dipolarity/polarizability is linearly correlated to the dipole moment of the probes. The successive introduction of bromine atoms in MePM (to give MePMBr, then MePMBr(2)) leads to the following linear decrease: pK(a) in water, length of the phenolate oxygen-carbon bond, length of the central ethylenic bond, susceptibility to solvent acidity, and susceptibility to solvent dipolarity/polarizability. Thus studying the solvation of probes whose molecular structures are varied systematically produces a wealth of information on the effect of solute structure on its solvation. The results of solvation of the present probes were employed in order to test the goodness of fit of two independent sets of solvent solvatochromic parameters.

Thermo‐Solvatochromism of Merocyanine Polarity Probes – What Are the Consequences of Increasing Probe Lipophilicity through Annelation?

AbstractThe question raised in the title has been answered by comparing the solvatochromism of two series of polarity probes, the lipophilicities of which were increased either by increasing the length of an alkyl group (R) attached to a fixed pyridine‐based structure or through annelation (i.e., by fusing benzene rings onto a central pyridine‐based structure). The following novel solvatochromic probes were synthesized:2,6‐dibromo‐4‐[(E)‐2‐(1‐methylquinolinium‐4‐yl)ethenyl]phenolate (MeQMBr2) and 2,6‐dibromo‐4‐[(E)‐2‐(1‐methylacridinium‐4‐yl)ethenyl)]phenolate (MeAMBr2). The solvatochromic behavior of these probes, along with that of 2,6‐dibromo‐4‐[(E)‐2‐(1‐methylpyridinium‐4‐yl)ethenyl]phenolate (MePMBr2) was analyzed in terms of increasing probe lipophilicity, through annelation. Values of the empirical solvent polarity scale [ET(MePMBr2)] in kcal mol–1 correlated linearly with ET(30), the corresponding values for the extensively employed probe 2,6‐diphenyl‐4‐(2,4,6‐triphenylpyridinium‐1‐yl)phenolate (RB). On the other hand, the nonlinear correlations of ET(MeQMBr2) or ET(MeAMBr2) with ET(30) are described by second‐order polynomials. Possible reasons for this behavior include: i) self‐aggregation of the probe, ii) photoinduced cis/trans isomerization of the dye, and iii) probe structure‐ and solvent‐dependent contributions of the quinonoid and zwitterionic limiting formulas to the ground and excited states of the probe. We show that mechanisms (i) and (ii) are not operative under the experimental conditions employed; experimental evidence (NMR) and theoretical calculations are presented to support the conjecture that the length of the central ethenylic bond in the dye increases in the order MeAMBr2 > MeQMBr2 > MePMBr2. That is, the contribution of the zwitterionic limiting formula predominates for the latter probe, as is also the case for RB, this being the reason for the observed linear correlation between the ET(MePMBr2) and the ET(30) scales. The effect of increasing probe lipophilicity on solvatochromic behavior therefore depends on the strategy employed. Increasing the length of R affects solvatochromism much less than annelation, because the former structural change hardly perturbs the energy of the intramolecular charge‐transfer transition responsible for solvatochromism. The thermo‐solvatochromic behavior (effect of temperature on solvatochromism) of the three probes was studied in mixtures of water with propanol and/or with DMSO. The solvation model used explicitly considers the presence of three “species” in the system: bulk solution and probe solvation shell [namely, water (W), organic solvent (Solv)], and solvent–water hydrogen‐bonded aggregate (Solv‐W). For aqueous propanol, the probe is efficiently solvated by Solv‐W; the strong interaction of DMSO with W drastically decreases the efficiency of Solv‐W in solvating the probe, relative to its precursor solvents. Temperature increases resulted in desolvation of the probes, due to the concomitant reduction in the structured characters of the components of the binary mixtures.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)

Use of Microdevices To Determine the Diffusion Coefficient of Electrochemically Generated Species: Application to Binary Solvent Mixtures and Micellar Solutions

A new approach for the determination of diffusion coefficient, D, of redox species is presented. It is based on the use of a home-constructed twin electrode within a thin-layered cell (TETLC) filled with a solution of electroactive species. Values of D are readily calculated, provided that the time required for the electrochemically generated species (produced at the generator electrode) to reach the collector electrode and the distance between both electrodes are known. Other parameters typically required to calculate D, e.g., concentration of the redox species, area of the electrode, and number of electrons transferred, are not needed. Diffusion coefficients of Fe(CN)(6)(3-), Ru(NH3)(6)(2+), and quinone were determined in water and, for Fe(CN)(6)(4-), in binary mixtures with glycerol. The results obtained were in good agreement with literature values. Aqueous glycerol solutions are microheterogeneous, as shown by the dependence on medium composition of the empirical solvent polarity scale, ET(30), determined by the solvatochromic probe RB. The responses of RB and the electrochemically generated species (Fe(CN)(6)(4-)) to the composition of aqueous glycerol mixtures were found to be remarkably similar. Measurements of D of ferrocene in micellar solutions of the cationic surfactant CTABr were also performed. Values of D for ferrocene and the ferrocenium cation are very different, in agreement with the chemical affinity of both species for the cationic micelle.

Excited State Intramolecular Proton Transfer in 2-(2‘-Arylsulfonamidophenyl)benzimidazole Derivatives: Insights into the Origin of Donor Substituent-Induced Emission Energy Shifts

Donor-substituted 2-(2'-arylsulfonamidophenyl)benzimidazoles undergo efficient excited-state intramolecular proton transfer (ESIPT) upon photoexcitation. The tautomer emission energy depends strongly on the substituent attachment position on the fluorophore pi-system. While substitution with a donor group in the para-position relative to the sulfonamide moiety yields an emission energy that is red-shifted relative to the unsubstituted fluorophore, fluorescence of the meta-substituted derivative appears blue-shifted. To elucidate the origin of the surprisingly divergent emission shifts, we performed detailed photophysical and quantum chemical studies with a series of methoxy- and pyrrole-substituted derivatives. The nature and contribution of solvent-solute interactions on the emission properties were analyzed on the basis of solvatochromic shift data using Onsager's reaction field model, Reichardt's empirical solvent polarity scale ET(30), as well as Kamlet-Abboud-Taft's empirical solvent index. The studies revealed that all ESIPT tautomers emit from a moderately polarized excited-state whose dipole moment is not strongly influenced by the donor-attachment position. Furthermore, the negative solvatochromic shift behavior was most pronounced in protic solvents presumably due to specific hydrogen-bonding interactions. The extrapolated gas-phase emission energies correlated qualitatively well with the trends in Stokes shifts, suggesting that solute-solvent interactions do not play a significant role in explaining the divergent emission energy shifts. Detailed quantum chemical calculations not only confirmed the moderately polarized nature of the ESIPT tautomers but also provided a rational for the observed emission shifts based on the differential change in the HOMO and LUMO energies. The results gained from this study should provide guidelines for tuning the emission properties of this class of ESIPT fluorophores with potential applications in analytical chemistry, biochemistry, or materials science.

Polarity of Ionic Liquids Determined Empirically by Means of Solvatochromic Pyridinium N‐Phenolate Betaine Dyes

AbstractFor Abstract see ChemInform Abstract in Full Text.

Polarity of ionic liquids determined empirically by means of solvatochromic pyridinium N-phenolate betaine dyes

Solutions of the zwitterionic betaine dye 2,6-diphenyl-4-(2,4,6-triphenylpyridinium-1-yl)phenolate are solvatochromic, thermochromic, piezochromic, and halochromic. That is, the position of the longest-wavelength intramolecular CT absorption band depends on the solvent polarity, solution temperature, external pressure, and the nature and concentration of added salts. The outstanding large negative solvatochromism of this standard betaine dye has been used to introduce spectroscopically an empirical scale of solvent polarity, called the ET(30) or ENT scale, which includes meanwhile a large number of molecular solvents and solvent mixtures. In this review, efforts to determine empirically the polarity of room-temperature ionic liquids by means of this betaine dye are summarized and discussed , because ionic liquids have gained importance as environmentally more benign new reaction media in the framework of Green Chemistry.