Bathochromic solvent effect

A bathochromic shift of about 5 nm results for the 320-nm band when a methyl substituent is introduced either in the 4- or 5-posiiion, The reverse is observed when the methyl is attached to nitrogen (56). Solvent effects on this 320-nm band suggest that in the first excited state A-4-thiazoline-2-thione is less basic than in the ground state (61). Ultraviolet spectra of a large series of A-4-thiazoline-2-thiones have been reported (60. 73). 

The extinction coefficients of carotenoids have been listed completely bnt solvent effects can shift the absorption patterns. If a colorant molecnle is transferred into a more polar environment, then the absorption will be snbjected to a bathochro-mic (red) shift. If the colorant molecnle is transferred into a more apolar enviromnent, the absorption will be subjected to a hypsochromic (blue) shift. If a carotenoid molecule is transferred from a hexane or ethanol solution into a chloroform solution, the bathochromic shift will be 10 to 20 nm. 

When a nonpolar solute is in solution in any solvent, either nonpolar or polar, then mainly dispersive forces operate between them, and any solvent effects are very small and bathochromic (Reichardt, 1988), increasing with the polarizability of the solvent. If the solute is dipolar in a nonpolar solvent, then both hypso- and bathochromic shifts, increasing with solvent polarizability, are possible, depending on the dipole moments of the ground and excited states. The situation becomes more complicated for a dipolar solute in a dipolar solvent. 

For less polar compounds, the solvent effect is weak. However, if the dipole moment of the chromophore increases during the transition, the final state will be more solvated. This is the case for n — n transitions in ethylenic hydrocarbons with a slightly polar double bond. A polar solvent has the effect of stabilising the excited state, which favours the transition. A shift towards greater wavelengths is observed unlike the spectrum obtained in a nonpolar solvent. This is the bathochromic effect. 

When a nonpolar solute is in solution in any solvent, either nonpolar or polar, then mainly dispersive forces operate between them, and any solvent effects are very small and bathochromic (Reichardt 1988), increasing with the polarizability... 

The solvent effects on the absorption spectra of ion pairs were studied by many authors and the direction of the observed shift depends on the change (increase or decrease) of dipole moment upon the electronic transition [25]. Generally a bathochromic shift is observed with an increase of solvent polarity. When going from a polar solvent to a less polar one, the association in the ground state increases more strongly than in the excited state this may be understood if the ion pair switches progressively from SSIP to CIP status. Observations of this type were often made, together with cation effects, as for instance in the case of alkali phenolates and enolates [7], fluorenyl and other carbanion salts [22] or even for aromatic radical anions [26, 27],... 

Bathochromic shift the displacement of absorption to a longer wavelength due to substituent or solvent effects. 

The rate of addition decreases moderately with increasing solvent polarity there is a 35-fold rate deceleration in going from cyclohexane to dimethyl sulfoxide. In polar solvents, the dipolar reactant thiyl radical is more stabilized than the less dipolar activated complex. The stabilization of the thiyl radical by solvation has been proven by its strong positive solvatochromism [i. e. bathochromic shift of Imax with increasing solvent polarity) [576]. Similar solvent effects on rate have been observed in the addition of the 4-aminobenzenethiyl radical to styrene [577]. 

As the steric hindrance around the OH group decreases, solute-solvent complexation increases, resulting in the bathochromic shift of Xmax and an increase in p-emission intensity. The general solvent effect on the Xmax and the emission composition of 2 (Figure 2) suggests that the complexation process is very general and complexation becomes very pronounced in solvents of TT > 0.65. Accordingly, a-band is the Franck-Condon emission of the excited state of the solute and P-band is the Franck-Condon emission of the excited state of the solute-solvent complex. 

Structure-Property Relationships, The studies aimed at construction of the (8 scale and related investigations have uncovered some interesting relationships between indicator structures and solvatochromic effects. It was found (78c, 134) for example, that 4-nitroaniline (1) forms two hydrogen bonds to HBA solvents, that the ratio of the hydrogen bond strengths is about 1.5/1, and that the ratio of the bathochromic spectral effects is 1 /(0.93 0.13). Comparable effects have also been observed with 3-nitroaniline (12). 

The solvents in class (b) give similar bathochromic shift with S. The spectral shifts of pure -donor solvents can be correlated only with zl Vj). These solvents bring about a blue shift vs their polarity and differ considerably from the solvents of class (a) and (b). This anomaly can easily be understood in terms of the competition existing between general and specific solvent effects [32]. Therefore it can be concluded that specific effects dominate the overall solvent effect in the case of complexes whose ground state is less polar than the excited state. 

The ultraviolet spectrum of T (solvent MeOH) shows bands at 281 nm (log = 4.12) and 369 nm (log e = 2.65) characteristic of the thioamide chromophore (14). In comparison the ultraviolet spectrum of TO (solvent MeOH absorption maxima at 328, 290, 276 and 245 nm with, respectively, log e = 3.86, 3.89, 3.92 and 3.92) shows an intense band at 328 nm, with a small solvent effect, which may indicate a 7r-conjugation enhancement in this compound. This bathochromic effect is in agreement with an S-oxide structure, in which the tt-electrons are delocalized throughout the thioamide S-oxide group. 

Solvent effects are important, both in considering the position of the absorption maximum and also the nature of the spectral transition involved. For n—n transitions, the excited state is more polar than the ground state, so it will tend to form dipole-dipole bonds with a polar solvent, such as water or ethanol. This will lower the transition energy and raise the absorption peak wavelength. This is called a red shift (or a bathochromic shift). Tables of solvent corrections are available in specialist texts. 

Donoi—acceptoi chromogens in solution are often strongly affected by the nature of the solvent or the resinous substrate in which they are dissolved. The more polar the solvent or resin, the longer the wavelength of the fluorescent light emitted. Progressing from less polar to more polar solvents, the bathochromic, or reddening, effect of the solvents on the dye increases in the order of aUphatics < aromatics < esters < alcohols < amides. 

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