4- phenolates solvatochromism

Another solvatochromic polarity measure, (30), is the transition energy for compound 8, which is 2,6-diphenyl-4-(2,4,6-triphenylpyridinio)phenolate, also referred to as Dimroth-Reichardt s betaine. 

Hydrogen bond formation between dissimilar molecules is an example of adduct formation, since the hydrogen atom that is bonded to an electronegative atom, such as oxygen or nitrogen, is a typical acceptor atom. The ability of molecules to donate a hydrogen bond is measured by their Taft-Kamlet solvatochromic parameter, a, (or a . for the monomer of self-associating solutes) (see Table 2.3). This is also a measure of their acidity (in the Lewis sense, see later, or the Brpnsted sense, if pro tic). Acetic acid, for instance, has a = 1.12, compared with 0.61 for phenol. However, this parameter is not necessarily correlated with the acid dissociation constant in aqueous solutions. 

Reichardt, C., Pyridinium N-phenoxide betaine dyes and their application to the determination of solvent polarities, part 29— Polarity of ionic liquids determined empirically by means of solvatochromic pyridinium N-phenolate betaine dyes. Green Chem., 7, 339-351, 2005. 

Dienes, 11 addition to, 194-198 cisoid conformation, 197, 350 conjugated, 11 Cope rearrangement, 354 cycUsation, 346 cycloaddition to, 348 Diels-Alder reaction, 197, 349 excited state, 13 heat of hydrogenation, 16,194 isolated, 11 m.o.s of, 12 polymerisation, 323 Dienone intermediates, 356 Dienone/phenol rearrangement, 115 Dienophiles, 198, 350 Digonal hybridisation, 5 Dimedone, 202 Dimroth s Et parameter, 391 solvatochromic shifts, 391 solvent polarity, 391 Y and,392 Dinitrofluorobenzene proteins and, 172... 

Solvatochromic shift data have been obtained for phenol blue in supercritical fluid carbon dioxide both with and without a co-solvent over a wide range in temperature and pressure. At 45°C, SF CO2 must be compressed to a pressure of over 2 kbar in order to obtain a transition energy, E, and likewise a polarizability per unit volume which is comparable to that of liquid n-hexane. The E,j, data can be used to predict that the solvent effect on rate constants of certain reactions is extremely pronounced in the near critical region where the magnitude of the activation volume approaches several liters/mole. 

Solvatochromic data, specifically absorption or transition energies (E s), have been obtained for the dye phenol blue in supercritical fluids as a function of both temperature and pressure. These data will be used to compare the "solvent strength" of these fluids with liquid solvents. He will use the terms "solvent strength" and "Et" synonymously in this paper such that they include the magnitude of the polarizability/volume as well as the dipole moment. The "solvent strength" has been characterized by the spectroscopic solvatochromic parameter, E, for numerous liquid solvents (9 JU, J7,JJ3). 

For phenol blue, the E, is zero for ethylene and CF3CI, but nonzero for the Lewis acids CF3H and CO2. One of the attractive features of solvatochromic scales is that the non-specific and specific interactions may be separated since the former can be calculated straightforwardly using Eq. 5. 

The final set of solvatochromic data are shown in Figure 6 for phenol blue in SF C02 doped with various amounts of the co-solvent or entrainer, methanol. Consider a pressure of 100 bar where the Et of phenol blue in C02 is 54 kcal/mol. The red shift is increased more by the addition of 3.5 mole percent methanol at constant pressure than by an increase in the pressure of pure C02 of over 200 bar. The large specific "solvent strength" of methanol causes this behavior. The red shift caused by the co-solvent is in... 

Likewise, Fiorilli et al.145 covalently anchored solvatochromic Reichardt s dye [2,6-diphenyl-4-(2,4,6-triphenyl-A-pyridinio)-phenolate] on both aniline- and... 

Figure 16.20 Examples of sensor molecules (a) tetraphenyl porphyrin, solvatochromic Rei-chardt s dye and pH indicator (phenol red) (b) image of the 36-dye matrix before exposure, after exposure, and the resulting difference map and (c) colour change profiles for common beverages. (Reprinted with permission from Zhang and Suslick [90], Suslick et at. [91], and Janzen et al. [147]. American Chemical Society 2006-2007)...

Then, Dimroth and Reichardt proposed a solvent polarity parameter, Ey(30), based on transition energy for the longest-wavelength solvatochromic absorption band of the pyridynium N-phenolate betaine dye, which is dye No. 30 in a table constructed by these authors. The x(30) values have been determined for more than 360 pure organic solvents and many binary solvent mixtures. 

Different methods for the study of selective solvation have been developed [118, 120] conductance and Hittorf transference measurements [119], NMR measurements (especially the effect of solvent composition on the chemical shift of a nucleus in the solute) [106-109], and optical spectra measurements like IR absorption shifts [111] or UV/Vis absorption shifts of solvatochromic dyes in binary solvent mixtures [124, 249, 371]. Recently, the preferential solvation of ionic (tetralkylammonium salts) and neutral solutes (phenol, nitroanilines) has been studied particularly successfully by H NMR spectroscopy through the analysis of the relative intensities of intermolecular H NOESY cross-peaks [372]. 

In contrast to these nonpolar compounds, very dramatic solvent effects on UV/ Vis spectra have been observed for dipolar meropolymethine dyes, especially mero-cyanines, due mainly to the change in their dipole moments on electronic transition. An example is the following negatively solvatochromic pyridinium V-phenolate betaine, which exhibits one of the largest solvatochromic effects ever observed cf. Fig. 6-2 [10, 29]). 

As shown by the NMR chemical shifts of negatively solvatochromic mero-polymethine dyes e.g. phenol blue), the electronic ground-state structure of these dyes changes from a polymethine-like state (b) in non-polar solvents to a polyene-like state (c) in polar solvents [50, 78]. 

Z values have been widely used to correlate other solvent-sensitive processes with solvent polarity, e.g. the a absorption of haloalkanes [61], the n n and n n absorption of 4-methyl-3-penten-2-one [62], the n n absorption of phenol blue [62], the CT absorption of tropylium iodide [63], as well as many kinetic data (Menschutkin reactions, Finkelstein reactions, etc. [62]). Copol5mierized pyridinium iodides, embedded in the polymer chain, have also been used as solvatochromic reporter molecules for the determination of microenvironment polarities in synthetic polymers [173]. No correlation was observed between Z values and the relative permittivity e, or functions thereof [317]. Measurement of solvent polarities using empirical parameters such as Z values has already found favour in textbooks for practical courses in physical organic chemistry [64]. 

The practical limitations in the Z-value approach can be overcome by using pyridinium A -phenolate betaine dyes such as (44) as the standard probe molecule. They exhibit a negatively solvatochromic n n absorption band with intramolecular charge-transfer character cf. discussion of this dye in Section 6.2.1, its UV/Vis spectrum in Fig. 6-2, and its dipole moment in the electronic ground and excited states mentioned in Table 6-1, dye no. 12. 

The primary standard betaine dye (44) is only sparingly soluble in water and less polar solvents it is insoluble in nonpolar solvents such as aliphatic hydrocarbons. In order to overcome the solubility problems in nonpolar solvents, the more lipophilic penta-t-butyl-substituted betaine dye (45) has additionally been used as a secondary reference probe [174]. The excellent Hnear correlation between the Ej values of the two dyes allows the calculation of t(30) values for solvents in which the solvatochromic indicator dye (44) is not soluble. Introduction of electron-withdrawing substituents e.g. Cl [323], F, CF3, C6F13 [324]) in the betaine molecule reduces the basicity of its phenolate moiety, which allows the direct determination of x(30) values for somewhat more acidic solvents. Moreover, the Hpophilic and fluorophilic penta(trifluoromethyl)-substituted betaine dye (46) is more soluble in nonpolar solvents e.g. hexafluoro-benzene) than the standard dye (44) [324]. Conversely, the solubility in aqueous media can be improved through replacement of some of the peripheral hydrophobic phenyl groups in (44) by more hydrophilic pyridyl groups, to yield the more water-soluble betaine dye (47) [325]. The Ej values of these new secondary standard betaine dyes correlate linearly with the x(30) values of (44), which allows the calculation of x(30) values for solvents in which only betaine dyes (45)-(47) are sufficiently stable and soluble for the UV/Vis spectroscopic measurements [324, 325]. 

Table 7-3. Empirical parameters of solvent polarity, t(30) [cf. Eq. (7-27)] and normalized Ej values [cf. Eq. (7-29)], derived from the long-wavelength UV/Vis charge-transfer absorption band of the negatively solvatochromic pyridinium IV-phenolate betaine dyes (44) and (45), measured at 25 °C and 1013 hPa, for a selection of 288 solvents, taken from reference [293]. ...

The major limitation of the x(30) values is the fact that they cannot be measured for acidic solvents such as carboxylic acids. Addition of traces of an acid to solutions of (44) or (45) immediately changes the colour to pale yellow due to protonation at the phenolic oxygen atom of the dye. The protonated form no longer exhibits the long-wavelength solvatochromic absorption band. The excellent linear correlation between iix(30) and Kosower s Z values, which are available for acidic solvents, allows the calculation of t(30) values for such solvents [174]. A further limitation has been the fact that it has not been possible to measure the absorption maximum of the standard betaine dye (44) in the gas phase as a reference state. 

It should be mentioned that the pyridinium A-phenolate betaine dye (44) is not only very sensitive to changes in solvent polarity, but in addition its longest-wavelength solvatochromic absorption band also depends on changes in temperature [73, 175, 180, 208] and pressure [74, 182, 208], on the addition of electrolytes (ionophores) [209-213], as well as on the introduction of substituents in the peripheral phenyl groups cf. Fig. 7-2 in Section 7.1 and reference [332] for a review. 

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. 

The structure of the open-chain form was assigned on the basis of its negative solvatochromic behaviour, which is similar to that of other meropolymethines such as the pyridinium A-phenolate betaines [108]. The correlation shown in Fig. 7-4 allows one to calculate absorption maxima of the merocyanine dye in other solvents for which x(30) values are known. 

Another UV/Vis spectroscopic method for the determination of water in organic solvents involves the use of solvatochromic dyes (such as the pyridinium A-phenolate betaine dye (44) m Chapter 7.4), and is based on the observation that water has a very high polarity compared with most organic solvents [142-145]. Even small amounts of water cause a strong hypsochromic band shift of the dissolved solvatochromic dye, which can be related to the water content by a cahbration curve. A typical detection hmit of this method is of the order of 1 mg water in 1 mL solvent for routine spectrophotometers [142], An analogous solvatochromic method has been developed for the determination of aqueous ethanol mixtures [146],... 

Figure 1.8 Negatively solvatochromic standard pyridinium-A-phenolate betaine dye, known as Reichardt s dye.

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