Bathochromic shift 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. 

In addition to benzenoid diazo components, diazotised heterocyclic amines in which the amino group is attached to a nitrogen- or sulphur-containing ring figure prominently in the preparation of disperse dyes [87,88], since these can produce marked bathochromic shifts. The most commonly used of these are the 6-substituted 2-aminobenzothiazoles, prepared by the reaction of a suitable arylamine with bromine and potassium thiocyanate (Scheme 4.31). Intermediates of this type, such as the 6-nitro derivative (4.79), are the source of red dyes, as in Cl Disperse Red 145 (4.80). It has been found that dichloroacetic acid is an effective solvent for the diazotisation of 2-amino-6-nitrobenzothiazole [89]. Subsequent coupling reactions can be carried out in the same solvent system. Monoazo disperse dyes have also been synthesised from other isomeric nitro derivatives of 2-aminobenzothiazole [90]. Various dichloronitro derivatives of this amine can be used to generate reddish blue dyes for polyester [91]. 

The marked changes in the carbonyl IR bands accompanying the solvent variation from tetrahydrofuran to MeCN coincide with the pronounced differences in colour of the solutions. For example, the charge-transfer salt Q+ Co(CO)F is coloured intensely violet in tetrahydrofuran but imperceptibly orange in MeCN at the same concentration. The quantitative effects of such a solvatochromism are indicated by (a) the shifts in the absorption maxima and (b) the diminution in the absorbances at ACT. The concomitant bathochromic shift and hyperchromic increase in the charge-transfer bands follow the sizeable decrease in solvent polarity from acetonitrile to tetrahydrofuran as evaluated by the dielectric constants D = 37.5 and 7.6, respectively (Reichardt, 1988). The same but even more pronounced trend is apparent in passing from butyronitrile, dichloromethane to diethyl ether with D = 26, 9.1 and 4.3, respectively. The marked variation in ACT with solvent polarity parallels the behaviour of the carbonyl IR bands vide supra), and the solvatochromism is thus readily ascribed to the same displacement of the CIP equilibrium (13) and its associated charge-transfer band. As such, the reversible equilibrium between CIP and SSIP is described by (14), where the dissociation constant Kcip applies to a... 

The dramatic bathochromic shifts found for ligands such as 6 and various more elaborated probes in the presence of basic anions in organic solvents are mainly due to deprotonation effects [43,44]. 

Shifts in absorption spectra due to the effect of substitution or a change in environment (e.g. solvent) will be discussed in Chapter 3, together with the effects on emission spectra. Note that a shift to longer wavelengths is called a bathochromic shift (informally referred to as a red-shift). A shift to shorter wavelengths is called a hypsochromic shift (informally referred to as a blue-shift). An increase in the molar absorption coefficient is called the hyperchromic effect, whereas the opposite is the hypochromic effect. 

A shift (also known as a red shift ) in a substance s electronic absorption spectrum toward longer wavelengths, as a consequence of a substituent, solvent, environment, or other effect. The opposite of a bathochromic shift is referred to as a hypsochromic shift. 

An effect observed in the spectrum of a chemical species in which a substituent, solvent, change in environment, or other effect causes the electronic absorption spectrum to shift to shorter wavelengths. The opposite effect is referred to as a bathochromic shift. The hypsochromic shift is also known as the blue shift. 

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. 

Pyrazoles with nonconjugating substituents show only one region of UV absorption (200-230 nm, log e= 3.1-3.8).14,15,30 Salt formation results in a bathochromic shift in and a small increase in log .8,47 With aryl conjugation there is a large bathochromic shift, and a second band appears.10,15,42,43,45,47 The illustrated examples 57-59,15 60,45 61, 62,47 and 13,8 show the effect of substituents and salt formation. A change of solvent from ethanol to hexane causes a small hypsochromic shift.45,47... 

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. 

Solvent Influence. Solvent nature has been found to influence absorption spectra, but fluorescence is substantially less sensitive (9,58). Sensitivity to solvent media is one of the main characteristics of unsymmetrical dyes, especially the merocyanines (59). Some dyes manifest positive solvatochromic effects (60) the band maximum is bathochromically shifted as solvent polarity increases. Other dyes, eg, highly unsymmetrical ones, exhibit negative solvatochromicity, and the absorption band is blue-shifted on passing from nonpolar to highly polar solvent (59). In addition, solvents can lead to changes in intensity and shape of spectral bands (58). 

Fluorescence is affected by the medium, such as the solvent or surface. Polar media in general tend to enhance fluorescence and pH has a drastic effect creating hypsochromic or bathochromic shifts as well as... 

It is shown that the characteristic absorption bands of the substances dissolved in supercritical carbon dioxide were subjected to show intense hypsochromic shifts compared with conventional organic solvents. In addition, bathochromic shifts of the absorption bands were effected by increasing C02 density. 

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]. 

The solvent often exerts a profound influence on the quality and shape of the spectrum. For example, many aromatic chromophores display vibrational fine structure in non-polar solvents, whereas in more polar solvents this fine structure is absent due to solute-solvent interaction effects. A classic case is phenol and related compounds which have different spectra in cyclohexane and in neutral aqueous solution. In aqueous solutions, the pH exerts a profound effect on ionisable chromophores due to the differing extent of conjugation in the ionised and the non-ionised chromophore. In phenolic compounds, for example, addition of alkali to two pH units above the pKa leads to the classical red or bathochromic shift to longer wavelength, a loss of any fine structure, and an increase in molar absorptivity (hyper chromic... 

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