Solvatochromic and Thermochromic Shifts

The equations of solvatochromic shifts represent the difference between the solvation energies of the initial and final states as a function of solvent polarity. For an absorption spectrum the initial state is of course the ground state and the final state is an excited state this could be Sj or S2 or S3, etc. For an emission spectrum the initial state is an excited state (almost always Sj or Tj) and the final state is the ground state. The two main terms are 

These equations represent only the dipole-dipole interactions the first one applies to an absorption spectrum, the second one to an emission spectrum (g for ground state, e for excited state). 

The dipole-induced dipole solvatochromic shift is given by 

For a complete description of the solvatochromic shift the dispersion term should be added, but in many cases all terms in f(n2) can be neglected if the 

Finally we should mention the electrochromic shifts which are the changes in the energy of electronic transitions when an external electric field is applied to the sample. These effects are quite small but have proved useful for the measurements of the dipole moments and polarizabilities of excited molecules. 

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