Betaine dye

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. 

Solvatochromic pareuaeters, so called because they were Initially derived from solvent effects on UV/visible spectra, have been applied subsequently with success to a wide variety of solvent-dependent phenomena and have demonstrated good predictive ability. The B jo) scale of solvent polarity is based on the position of the intermolecular charge transfer absorption band of Reichardt s betaine dye [506]. Et(io> values are available for over 200 common solvents and have been used by Dorsey and co-%rarkers to study solvent interactions in reversed-phase liquid chromatography (section 4.5.4) [305,306]. For hydrogen-bonding solvents the... 

There are two important drawbacks of such an approach (1) a polarity scale based on a particular class of probes, in principle, does not account, for example, sizes of probes, which should strongly effect the interactions (2) betain dyes do not fluoresce, which restrict essentially the field of application of this approach, because in many cases, absorption spectrum could not be measured accurately (small volumes of samples, study of cells, and single molecules spectroscopy). Therefore, polarity-sensitive fluorescent dyes offer distinct advantage in many applications. 

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. 

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

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-... 

The sensitivity of betain dyes to solvent polarity is exceptionally high, but unfortunately they are not fluorescent. Yet polarity-sensitive fluorescent dyes offer... 

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

Figure 1.12 Reichardt s betaine dye in its zwitterionic ground state (a) and first excited state (b). The ground state has a larger dipole moment (15 D) than the excited state (6 D). Measurement of the energy of the transition between these two states (n - n ) is the basis for the Ej scale of solvent polarity...

Figure 1.15 t-Butyl- (a) and perfluoroalkyl- (b) substituted betaine dyes for polarity measurements in less polar or highly fluorous solvents... 

In 1963, Dimroth6 introduced the Et parameter based on the spectroscopic properties of an intramolecular charge transfer of the pyridinium phenoxide betaine dyes 2, to define the solvent polarity for interpreting solvent effects on the reactivities. 

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. 

Figure 3.57 Solvatochromic plot of the first absorption band of a betaine dye. The transition corresponds to a CT from 0 to the aromatic system. Triangles are protic solvents. E is in kcal moE1 and f(D) is the Onsager function...

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. 

A great variety of models for a quantitative description of the composition dependence of the physicochemical properties of solutes dissolved in binary solvent mixtures have been developed [257-261, 373-378]. For example, using a rather simple two-step solvent-exchange model [374, 377], the behaviour of seventy binary solvent mixtures towards a solvatochromic betaine dye (structure see Fig. 6-2 in Section 6.2.1) can be quite precisely described, even for so-called synergetic solvent mixtures [377]. 

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