Solvatochromic

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The Solvatochromic effect or solvatochromic shift refers to a strong dependence of absorption and emission spectra with the solvent polarity. Since polarities of the ground and excited state of a chromophore are different, a change in the solvent polarity will lead to differential stablization of the ground and excited states, and thus, a change in the energy gap between these electronic states. Consequently, variations in the position, intensity, and shape of the absorption spectra can be direct measures of the specific interactions between the solute and solvent molecules.

Due to the Franck-Condon principle (atoms do not change position during light absorption), the excited state solvent shell is not in equilibrium with the excited state molecule ("solute"). In fact, charge-transfer transitions of ground state ion-pairs give the largest changes in absorption spectra, and are thus, the most useful for measuring solvent polarity.

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