Chromophore hypsochromic effect

A shift in A,max to shorter wavelength is called a hypsochromic effect, or blue shift, and usually occurs when compounds with a basic auxochrome ionise and the lone pair is no longer able to interact with the electrons of the chromophore. Hypsochromic effects can also be seen when spectra are run in different solvents or at elevated temperatures. Spectral shifts of this type can be used to identify drugs that contain an aromatic amine functional group, e.g. the local anaesthetic benzocaine (see Figure 7.9). 

In some transitions, the polarity of the chromophore is weaker after absorption of radiation. One case of this is the n — jt absorption due to the carbonyl present in ketones in solution. Before absorption, the C+-0 polarisation stabilises in the presence of a polar solvent whose molecules will be clustered around the solute because of electrostatic effects. Thus, the n —> -rr electronic transition will require more energy and its maximum will be displaced towards a shorter wavelength, contrary to what would be observed in a nonpolar solvent. This is the hypsochromic effect. Because the excited state is readily formed, the solvent shell around the... 

If the chromophore responsible for the observed transition is more polar in its fundamental state than when it is excited, then a polar solvent will stabilize, by solvation, the form prior to absorption of the photon. Molecules of solvent will be clustered around the solute because of electrostatic effects. More energy would therefore be required to excite the electronic transition concerned, causing a displacement of the absorption maxima to shorter wavelengths than would occur in a non-polar solvent. This is the hypsochromic effect (Figure 9.7). 

Now, the crucial question is, how is the electron transition So Si of such a push-pull system changed, when the chromophores in 52 are extended ( = 1, 2, 3,...) Figure 11.15 reveals the expected red-shift in the simply donor-substituted series 52a n = 1-5). This effect is preserved for 52b with CN groups as weak acceptors. In the formyl series 52c, the absorption maidma are almost independent of the size of the conjugated chromophore and the nitro series 52d shows a distinct hypsochromic effect. The latter effect is even more pronounced for the dicyano-vinyl series [210, 211, 213). How can this unexpected behavior be explained We found that the blue-shift in the DAOPV series is restricted to measurements in normal organic solvents like CHCI3. As soon as a protonation occurs, the hypsochromic effect is reversed to a bathochromic effect (Fig. 11.16). 

Instead of preventing the dehydrochlorination process, which generates colored conjugated sequences, it may be more appropriate to use additives that react with the chromophoric polyene formed and reduce the length of the conjugated sequences. A hypsochromic effect is observed in this case, which decreases the absorption in the visible range. 

In general, auxochromic substitution of chromophores causes bathochromic shifts and increases in intensity for 7t —> k transitions, and hypsochromic or blue shifts (to shorter wavelengths) for n K transitions. The shifts are explainable in terms of mesomeric resonance) effects caused by interaction of lone pair electrons associated with such auxochromes as —OH, —Cl, —NH2 with the k system of the chromophore. This leads to... 

C/5 -isomerization of a chromophore s double bond causes a slight loss in color, small hypsochromic shift (usually 2-6 nm for mono-cw), and hypochromic effect, accompanied by the appearance of a cis peak in or near the ultraviolet region. The intensity of the cis band is greater as the cis double bond is nearer the center of the molecule. 

The Amux of absorption depends upon the microenvironment of the chromophore, as does the intensity of absorbance. For example, as just discussed, solvation can affect the energies of transitions. Binding of the chromophore to a receptor, natural or synthetic, can also often influence Amax/ as can a variety of other effects. When the shift in absorption is to shorter or longer wavelengths it is defined as a hypsochromic (blue shift) or bathochromic shift (red shift), respectively. When the intensity of absorption increases or decreases it is called a hyperchromic or hypochromic effect, respectively. 

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