Dimroth-Reichardt betaines

The pyridine ylide of la is the parent compound of the Dimroth-Reichardt betaine dyes used to probe solvent polarity.71,72 These dyes show a very pronounced solvatochromism, and the shifts of the absorption maxima as a function of the solvent polarity has been used to establish the quantitative ET30 scale. 

The Dimroth-Reichardt betaines are synthesized by reactions of p-aminophenols with pyrylium salts bearing aryl substituents in ortho position. 

The phenomenological theory has been applied by Skwierczynski to the Et values of the Dimroth-Reichardt betaine, a quantity sensitive to the polarity of the medium. The approach is analogous to the earher development. We need only consider the solvation effect. The solute is already in solution at extremely low concentration, so solute-solute interactions need not be accoxmted for. The solvent cavity does not alter its size or shape dining an electronic transition (the Franck-Condon principle), so the general medium effect does not come into play. We write Ej. of the mixed solvent as a weighted average of contributions from the three states ... 

Compounds 5-7 are indicators that have successfully been used in this way (Eigure 11.2). For phenyl methyl sulfoxide 5, four equations have been derived, of which the most impressive is Eq. (11.7), in which the shift of its 4-carbon atom is referenced (AS) to that of the 3-carbon atom in trifluoromethylbenzene [5]. For 6, the comparison involves its F resonance versus that of fluorobenzene itself [9] to give Eq. (11.8), whereas similarly for 7, its F resonance relative to internal fluorobenzene [10] gives Eq. (11.9). In the same way, the Dimroth-Reichardt betaine (8, R = Ph) [11] may be characterized [5] as Eq. (11.10) ... 

Figure 11.2 Probes used for a (5-7) plus the Dimroth-Reichardt betaine 8.

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. 

Solvatochromic effects occur then the color of a solute depends on the nature of the solvent in which it is dissolved. Two such systems were discussed in section 4.9. The first is the betaine dye, 4-(2,4,6-triphenylpyridinium)-2,6-diphenylphen-oxide, which is used to define the Dimroth-Reichardt parameter Ej. This molecule undergoes a ti ti transition in the visible region whose wavelength is very solvent dependent. In the strongly acidic solvent water it is equal to 453 nm which corresponds to an energy gap of 264kJmoH. In the very weakly acidic solvent diphenylether, the wavelength for the same transition is 810 nm. This corresponds... 

Dimroth-Reichardt values (kcal/mol) for the longest-wavelength solvatochromic absorption based on a pyridinium- -phenoxide betaine dye no. 30. 

Fig. 1. Solvent dependent intramolecular charge-transfer absorption of 2,4,6 t ripheny1-N-(2,6-dipheny1-4-phenoxide)-pyridinium betaine, proposed as solvent polarity indicator by Dimroth, Reichardt et al.(9).

Kosower in 1958 was the first to use solvatochromism as a probe of solvent polarity. The relevant Z-scale is based on the solvatochromic shift of 4-methoxycarbonyl-1-ethylpyridinium iodide (1). Later, Dimroth and Reichardt suggested using betain dyes, whose negative solvatochromism is exceptionally large. In particular, 2,6-... 

Compound 2 was the betain dye no. 30 in the original paper of Dimroth and Reichardt, which explains the notation Ej(30). 

Dimroth and Reichardt defined the Ej parameter on the basis of the solvatochro-mism of the pyridinium betaine 17 as equation 44 ... 

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. 

Dimroth and Reichardt devised a scale based on the solvatochromic behavior of the pyridinium-V-phenoxide betaine dye shown in Figure 1(a).5 The UV spectrum varies over several hundred nanometers according to the solvent in which it is dissolved. The wavenumber observed relates to T(30) or normalized f TN values. At one end of the TN scale is cyclohexane with a value of 0.006, and water lies at the other end with a value of 1.000. 

Descriptor 6, is the Reichardt-Dimroth (30) parameter[24] expressed in SI units. (30) defined as the transition energy (kcal) at 25 C of the long wave absorption band of standard betain dyes when dissolved in the solvent. Descriptor 8, log P is the logarithm of the equihbrium constant of the distribution of the solvent between 1-octanol and water at 25 °C. 

For an example of a pairwise correlation, consider the following two quantities that purport to measure the polarity of the solvent. Kosower (1958,1968) proposed a parameter, Z, based on the change in frequency of a charge-transfer transition of 1-ethyl-4-methoxycarbonyl-pyridinium iodide (1), which shows a pronounced increase of frequency when the polarity of the solvent is increased, because the ionic ground state is stabilized by a polar medium relative to the nonionic excited state. Dimroth and Reichardt (1969 Reichardt and Harbusch-Gomert, 1983) proposed another, E (30), or in normalized form, E, based on a visible transition of a pyridinium-V-phenoxide betaine dye (8, p, 40). (The label (30) is because the dye was the 30th of a number tried.) Eigure 4.5 shows the correlation of these two parameters for 40 solvents. 

Dimroth, K., Reichardt, Ch., Siepmann, T. and Bohlmann, R, Uber Pyridinium-n-phenol-betaine und ihre Verwendung zur Charakterisierung der Polaritat von Losungsmittel, Liebigs Ann. Chem., 1963, 661, 1-37. 

Electronic spectra of molecules are often strongly dependent upon the nature of the solvent. For example, the n- rc transition for C=0 groups shifts considerably to the red on going from protic to aprotic media. Such shifts have been used, in particular, in the construction of empirical solvent scales. Use has been made of systems with strongly dipolar transitions, as for example, the betaine (2) used by Dimroth and Reichardt [3] to construct the , scale and (3) used by Kosower [4] to give the Z-value scale. Much has been written about these scales and their utility [5]. However, they are empirical and do not provide direct structural information. 

The polarity of ILs can be compared by the use of empirical scales. The most widely apvplied scale has been based on changes in the charge transfer n-n absorption band for the betaine dye, 2,6-diphenyl-4 (2,4,6-t tiiphenylpyridinium-l-yl) phenolate also known as Reichardt s dye (Dimroth et al., 1963). The Et(30) value describing solvent polarity is calculated on the basis of the following equation ... 

Dimroth, K. Reichardt, C. Siepmann, T. Bohlmann, F. (1963). Uberpyiidinium-N-phenol-betaine und ihre verwendung zur charakterisierung der p>olaritat von losungsmitteln. Justus Liebigs Ann.Chem. Vol. 661 1-37. 

Of the spectroscopic solvent parameters, the Ej value, originated in 1963 by Dimroth and co-workers and subsequently developed greatly by Reichardt, is much used. This is based on the large solvent-induced shift for the long-wave absorption band of a series of A -phenol-pyridinium betaine dyes. Values of Ej (transition energies in kcal mol ) are available for several hundred one-component solvents and also for many mixed solvents. ... 

One of the most popular and successful scales has been developed by Dimroth and Reichardt. It is based on the pyridlnlum-N-phenoxide betaine [3], which exhibits one of the largest solvatochromic effects ever observed. The solvatochromism of this dye is negative since its ground state is considerably more polar than the excited state and is stabilized by polar solvents. Thus, in diphenylether the dye absorbs at 810 ran and appears blue-green, whereas in water the absorption is centred at 453 nm, giving an orange impression. The transition energy, expressed in kcal mol, is the so-called Ej(30) value of the solvent. Ex(30) values have been determined and tabulated for more than 270 pure solvents and many different solvent mixtures. Protonation converts the dye (Scheme 3) into a phenol as a consequence, Et(30) values cannot be measured for acidic solvents, such as carboxylic acids. 

---