Pyridinium iodides, charge-transfer bands

In another approach to the estimation of solvent polarities the effect of a solvent on the absorbance maximum in the visible-ultraviolet region of the charge-transfer band of a salt such as 1 -ethyl-4-carbomethoxy pyridinium iodide is measured 147). A shift of the maximum to shorter wavelengths occurs as solvent polarity increases. The wavelength, expressed in kcal, is called the Z value of the solvent. This method provides a simple and rapid measure of solvent polarity at the molecular level. 

The spectra of substituted pyridinium iodides are characterized by charge transfer bands involving the interaction of pyridinium and iodide ions. Mukerjee and Rayt showed that this band is shifted about 90 nm toward the red for dodecyl pyridinium iodide, which forms micelles, compared to methyl pyridinium iodide, which does not. They measured max for the micelles in mixed solvents of variable relative dielectric constant and obtained the following results ... 

There are also other scales for characterization of solvents ionizing power. For instance, Kosower found that positions of certain absorption maxima ( charge transfer bands ) in the UV spectrum of l-methyl-4-carbomethoxy-pyridinium iodide depend very strongly on solvent polarity the better the ionizing power of a solvent, the better is the solvation of the ion pair ground state, which is, therefore, lower in energy relative to the ion pair in the gas phase. Thus, more energy is needed for formation of the excited state in solution... 

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... 

Merocyanines (5) undergo a red shift of their electronic absorption maximum on micellization, the degree of which increases with the hydrophobicity of the alkyl group.This implies solubilization in a water-poor region in both cationic and anionic co-micelles. Pyridinium iodide (6) has a charge-transfer absorption band in its u.v. spectrum, but only above the c.m.c., and the frequency suggests a polarity in the environment of the head-group similar to that of ethanol. ... 

The Z value was defined by Kosower as the energy of electronic transition corresponding to the charge-transfer absorption band of l-ethyl-4-carbo-methoxy pyridinium iodide, a substance which has an ultraviolet spectrum remarkably sensitive to solvent polarity. Application of this method involves the determination of the spectral characteristics of the solubilizate in a number of solvents of known Z values and the establishment of a relationship between Z values and the energy of transition which is readily obtained from the wavelength of maximum absorbance. From this relationship the Z value of the micellar micro-environment may be determined from the wavelength of maximum absorbance of the solubilizate when solubilized within the micelle. Utilizing this... 

Heteroaromatic salts are frequently coloured, both in the solid state and in solution, and the more polarizable the anion the deeper is the colour. Hantzsch showed the presence in the electronic absorption spectra of pyridinium salts, more particularly the iodides, of bands at longer wavelengths than those due to the heterocyclic cation itself—bands which do not obey Beer s law. Hantzsch attributed these to molecules formed by addition of the iodide ion to the 2- or 4-position of the methylpyridinium cation. Kosower confirmed these observations but concluded that the new bands are due to charge-transfer complexes. These are regarded as solvated ion pairs, represented in the ground state by (28), which by a charge-transfer transition give an excited state(29). This conclusion is supported by several... 

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