The absorption spectrum depends on the solvent in which the absorbing substance is dissolved. The choice of solvent is important as it may shift peaks to shorter or longer wavelengths. This depends on the nature of the interaction of the particular solvent with the environment of the chromophore in a molecule.
Criteria of good solvents
1. It should not absorb uv-radiation in the region under investigation.
2. It should be transparent within the wavelength range being examined.
3. The solvent, itself should not have a conjugated system.
4. It should be inert to the compound under study.
5. The dielectric constant of the solvent should not be high. If it is so, there will be stronger solute-solvent interactions and due to this vibrational and rotational energy states of molecule increases and thus fineness of the absorption band falls.
Why polar solvents cannot be used-
The position and the intensity of absorption maxima are shifted for a particular chromophore by changing the polarity of the solvent.
since the polarities of the ground and excited state of a chromophore are different hence a change in the solvent polarity will stabilize the ground and excited states to different extend resulting change in the energy gap between these electronic states.
The non-polar solvents such as saturated hydrocarbons (hexane, cyclohexane, etc ) have the least interaction with the molecules either in the ground or excited state and hence the absorption spectrum of a compound in these solvents is similar to the one in a pure gaseous state.
However polar solvents such as H2O, ROH, etc may stabilize or destabilize the molecular orbital of a molecule either in the ground state or in the excited state. Therefore the spectrum of a compound in these solvents may significantly vary from the one recorded in a non- polar solvent.
