[ch3mgr8trthanbio]

My understanding of the lippert equation is that it predicts the solvent mediated stokes shifts in the fluorescent and absorption spectra for a molecules.
? a ?? f = 2/hC[(e-1/2e+1)-(n^2-1)/(2n^2-1)]*(u1-u2)^2/a^3

where e is the dielectric constant, u is the dipole moment of the S1 and S0 states, and n is the the refractive index of the solvent.
I understand where e and u come into play, but why does the refractive index affect this stokes shift, and why is it subtracted in this equation?

[ampacmopac]

? a ?? f = 2/hC[(e-1/2e+1)-(n^2-1)/(2n^2-1)]*(u1-u2)^2/a^3
the term in the brackets of the above equation is called thr orientation polarizability. the first term of it accounts for spectral shifts due to both the reorientation of the solvent dipoles and to the redistribution of the electrons in the solvent molecules.
the second term accounts for only the redistribution of the electrons.
the differenece between these two terms accounts for spectral shifts due to  reorientation of the solvent molecules.
we are interested in the difference because that during the emission process there will be enough time for the solvent molecules to reorient.
the process of electron redistribution also takes place during the absorption process, however, there is no enough time during the absorption, or the excitation, for the solvent molecules to reorient.
the solvent molecules need relatively long time to reorient and this is possible during the emission not the excitation.
to me all the above discussion is nothing without the understanding of the reasons that make use the dielectric constant and the refractive index to account for the spectral shifts.
ampacmopac