Solvatochromism iodide

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 dramatic influence of solvent effects on the UV and visible spectra of certain pyridine compounds, known generally as a solvatochromic effect, has been much utilized in the expression of solvents effects. The polarity parameter. Z or ET is defined (58JA3253, B-68MI204002) from the longest wavelength charge transfer band of 1-ethyl-4-methoxycar-bonylpyridinium iodide (equation 3). 

The solvatochromic effects on UV/visible spectra of certain solutes are so large, that they can conveniently be employed as probes for certain solvating properties of the solvents. Those that have enjoyed widespread application in this capacity are discussed in Chapter 4. They include 2,6-diphenyl -4-(2,4,6-triphenyl-l-pyridino)-phenoxide, 4-methoxynitrobenzene, 4-(dimethylamino)-nitrobenzene, for the estimation of the polarity of solvents, acetylacetonato-N,N,N, N -tetramethylethylenediamino-copper(II) perchlorate, 4-nitrophenol, and 4-nitroaniline, for the estimation of the electron pair donicity of solvents, 4-carboxymethyl-l-ethylpyridinium iodide, 4-cyano-l-ethylpyridinium iodide, and bis-c/.v-1, lO-phenanthrolinodicyano-iron(II) for the estimation of the hydrogen bond donation abilities of solvents (Marcus 1993). 

Solvent polarity parameters — use solvatochromic dyes (dyes whose electronic transitions are strongly dependent on the nature of the solvent) as indicators of solvent polarity. A comprehensive solvent polarity scale was first proposed by Kosower who defined the polarity parameter, Z, as the molar transition energy, Ej, for the charge transfer band of 1-ethyl-(methoxycarbonyl)pyridynium iodide in a given solvent as... 

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]. 

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. 

The Z scale is based on the strong solvatochromism of the l-ethyl-4-(methoxycarbonyl)pyridinium iodide ion-pair (4) and defined by the position (in kcal mol ) of the maximum of its first absorption band, which has marked intermolecular charge-transfer connotations according to Scheme IV (the excited state of the chromophore is much less... 

Davis determined the solvatochromism of a charge-transfer complex formed by tetra-n-hexylammonium iodide-nitrobenzene in 23 different solvents and, using the Z and Et(30) scales, observed a bilinear behavior in scarcely polar and highly polar solvents. A... 

MPI24 is a structural analogue of l-ethyl-4-carboethoxypyridinium iodide (EPI) (SCHEME I) i.e., of the compound originally proposed by Kosower23 for semiempirical measurement of the solvent polarity. Very similar wavelengths were found for the maxima of the solvatochromic bands of these compounds. The polarity scale based on MPI may, therefore, be correlated with a similar scale proposed by Dimroth.25... 

Solvolysis kinetics of allyl bromide in several alcohols have been studied. Kinetics of the reaction of methyl iodide with a solvatochromic vinylogous a-pyridone have been studied in various solvents. There is a limited relationship of rate to the position of the CT band of the dye. 

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