Solvatochromic probes solvent mixtures

Spectroscopic measurements of solvatochromic and fluorescent probe molecules in room temperature ILs provide an insight into solvent inter-molecular interactions, although the interpretation of the different and generally uncorrelated polarity scales is sometimes ambiguous [23]. It appears that the same solvatochromic probes work in ILs as well [24], but up to now only limited data are available on the behavior of electronic absorption and fluorescence solvatochromic probes within ILs and IL-organic solvent mixtures. 

The polarities of binary solvent mixtures with limited mutual miscibility such as zz-BuOH/H20 and c-CeH /DMF [191] as well as of solid polymer mixtures (organic glasses) [195] have also been studied using solvatochromic probes such as the betaine dye (44). 

In 1994, a review on the further development and improvement of the n scale was given by Laurence, Abboud et al. [227], They redetermined n values for a total of 229 solvents, this time using only two (instead of seven) solvatochromic nitroaromatics as indicator compounds, i.e. 4-nitroanisole and A,A-dimethylamino-4-nitroaniline, for good reasons see later and reference [227] for a more detailed discussion. A thermodynamic analysis of the n scale [and the t(30) scale] has been reported by Matyushov et al. [228]. Using six novel diaza merocyanine dyes of the type R-N=N-R (R = N-methylpyridinium-4-yl or A-methylbenzothiazolium-2-yl, and R = 2,6-disubstituted 4-phenolates or 2-naphtholate) instead of nitroaromatics as positively solvatochromic probe compounds, an analogous n azo scale was developed by Buncel et al., which correlates reasonable well with the n scale, but has some advantages for a detailed discussion, see references [333], Another n scale, based solely on naphthalene, anthracene, and y9-carotene, was constructed by Abe [338], n values are mixed solvent parameters, measuring the solvent dipolarity and polarizability. The differences in the various n scales are caused by the different mixture of dipolarity and polarizability measured by the respective indicator. The n scale of Abe is practically independent of the solvent dipolarity, whereas Kamlet-Taft s n and Buncel s n azo reflect different contributions of both solvent dipolarity and polarizability. 

In solvent mixtures the phenomenon of preferential solvation of the solvatochromic probe by one component of the mixed solvent often takes place. Care must therefore be taken when such combinations of mixed solvents and probes are employed for the characterization of the mixed solvent as regards its solvation properties for any arbitrary solute other than the particular probe studied5. It has been argued that, contrary to the case of neat solvents, where the use of a single probe can be justified18,27, it ought to be more... 

Bosch, Roses and coworkers explored the use of solvatochromic probes in solvent mixtures involving various alcohols with water as well as with other alcohols, hexane and benzene in a series of papers106-110. The aniline derivatives they employed were 5 for the polarity/polarizability and the 8/5 probe pair for the EPD/HBA properties of the mixtures, in conjunction with other suitable non-aniline probes. In order to ensure that what is being... 

Kauffman and coworkers118 119 tried to fit the solvatochromic shifts of l-(9-anthryl)-3-(4-/V,/V-dimcthylanilino)propanc (83), relative to the hydrocarbon homomorph with the dimethylamino group replaced by H, to the dielectric non-ideality of solvent mixtures involving hexane with ethanol, tetrahydrofuran and dichloromethane. The shifts were not linear with the mole fraction of the polar component, and Suppan s theory of dielectric enrichment was applied to the data. It was found that the dielectric enrichment that can be calculated from the relative permittivities of the components and of the mixtures is not sufficient to account for the observed solvatochromic shifts, but that preferential solvation of the probe by the polar component is superimposed on this dielectric effect. Earlier,... 

Another attempt to relate the results from the use of a solvatochromic probe (Phenol Blue (66)) to the inherent properties of solvent mixtures was made by Phillips and Brennecke122. They obtained the interaction energies (required for the application of the non-random two-liquid (NRTL) approach) of 66 with each of the solvent components from its solubility in the neat solvent. The mixtures studied contained cyclohexane as one component and acetone, triethylamine, ethyl butyrate, cyclohexanone, toluene and acetophenone as the other. Then the local compositions deduced from the solvatochromism of 66 were compared with those calculated by the NRTL equation and reasonable agreement was found. 

Wetzler and coworkers123 employed 4-aminophthalimide (63) and 4-amino-lV-methyl-phthalimide (64) as solvatochromic (and thermochromic) fluorescent probes in solvent mixtures. A bathochromic shift of the emission spectra was found in mixtures of toluene with ethanol and with acetonitrile123 when the more polar solvent was added to toluene, but raising the temperature causes a relative hypsochromic effect. Mixtures of benzene and acetonitrile were studied by Nevecna and coworkers124 for their polarity by means of the probes 46 and 47 and with respect to the correlation of this with the rate constants of the reaction of triethylamine with ethyl iodide. The fluorescence of the ammonium salt of 4-(l-naphthylsulfonate)aniline (84) in dioxane and water mixtures was studied by Hiittenhain and Balzer125. 

Solvatochromic probes based on aniline derivatives have been used in a host of environments other than liquid solvents and their mixtures, supercritical fluids and room-temperature liquid salts. All these uses cannot be reviewed here and only examples of them are provided. 

Because of their sensitivities to environmental changes, wide applications for solvatochromic compounds were found in the study of solute-solvent interactions, mainly in the characterization of bulk or microenvironments. Various polarity scales employing solvatochromic dyes as solvent probes were proposed. Because these empirical scales may be used to characterize any solvent or solvent mixture, solvatochromism played an important role in the study of a wide variety of solvent-dependent processes. 

The use of solvatochromic compounds as polarity indicators may be extended to solvent mixtures and micellar systems. In this case, an additional difficulty is introduced in the assessment of systems that are not homogeneous from a microscopic point of view The microenvironment actaally seen by the sensor does not correspond to the bulk characteristics of the medium. In a binary solvent mixture, a solvatochromic probe may be more solvated by one of the components, thus reflecting through its spectrum a solvent composition that may be different from that of the bulk mixture. In micellar systems, the solvatochromic response of a probe reflects the nature of its microenvironment and is dependent on the relative solubility of the sensor in the aqueous or the organic pseudophases, or in the micellar interphase. 

A brief review has been presented of the correlation analysis of solvolysis rates 50 years later, i.e. since Grunwald and Winstein proposed their eponymous equation in 1948.111 -pije authors then propose a method of correlation analysis involving multiple regression on solvent scales SPP (polarity-polarizability), SA (acidity) and SB (basicity). These scales are based on the solvatochromism of suitable probes and were initially for pure (i.e. one-component) solvents, but have now been extended to binary solvent mixtures. This enabled the authors to present a correlation for the solvolysis rate constants of r-butyl chloride in 27 pure solvents and 147 binary solvent mixtures, having a correlation coefficient r = 0.990 and a standard error of the estimate s = 0.40. The most important term in the equation is that involving SPP next comes... 

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