Pyridinium solvatochromism

Donor strengths, taken from ref. 207b, based upon the solvent effect on the symmetric stretching frequency of the soft Lewis acid HgBr2. Gutmann s donor number taken from ref 207b, based upon AHr for the process of coordination of an isolated solvent molecule to the moderately hard SbCL molecule in dichioroethane. ° Bulk donor number calculated as described in ref 209 from the solvent effect on the adsorption spectrum of VO(acac)2. Taken from ref 58, based on the NMR chemical shift of triethylphosphine oxide in the respective pure solvent. Taken from ref 61, based on the solvatochromic shift of a pyridinium-A-phenoxide betaine dye. 

Reichardt, C. (1992). Solvatochromism, thermochromism, piezochromism, halochrom-ism, and chiro-solvatochromism of pyridinium IV-phenoxide betaine dyes. Chem. Soc. Rev. 21, 147-153... 

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. 

The triaryl compounds 290 (R = Ar) are prepared by condensation of 2-aminophenol with triarylpyrylium salts followed by treatment with alkali. The triphenyl betaine (290 R = Ph) is obtained as a purple solid, mp 165 C (decomp), which shows large thermo/solvatochromic effects. Oxidation of the betaine 290 (R = Ph) with hydrogen peroxide gives the triphenyl-pyridinium-3-olate 291 (R = Ph) (see Section III,A,2) and the pyrrole 292 (R = Ph). The mechanism of this unusual reaction has not yet been Established. 

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]). 

A striking example is the negatively solvatochromic effect observed for l-ethyl-4-(methoxycarbonyl)pyridinium iodide, the UV/Vis spectra of which in a variety of solvents are shown in Fig. 6-3 [65-67]. The longest-wavelength band of the ground-state ion-... 

Kosower [5, 55] has taken the longest-wavelength intermolecular charge-transfer (CT) transition of l-ethyl-4-(methoxycarbonyl)pyridinium iodide as a model process. It exhibits a marked negative solvatochromism cf. the formula of this dye and its UV/Vis... 

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. 

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]. ...

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. 

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],... 

Transition energies for the solvatochromic pyridinium betaine Et (30) [recalculated from the literature values given in kcal mol (Reichardt, 1994)]. 

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