Electronic Spectral Shifts of Nonpolar Molecules in Nonpolar Solvents

A theory of the red shift of low‐lying electronic bands of nonpolar molecules in nonpolar solvents is presented. The difference between the dispersion forces in the ground and excited states is investigated by perturbation theory and an approximate formula is derived according to which (in dilute solution) $$\mathrm{Red\; shift}=\frac{1}{6}{\alpha }_B{\mathrm{zR̄}}^{\text{−6}}\{\frac{1}{4}{\mathrm{E\alpha }}_A{\mathrm{+M}}^2\mathrm{\},}$$ where α_A and α_B are the molecular polarizabilities of the solute and solvent, respectively, M and E are the dipole moment and energy of the transition, and each solute molecule is supposed to be surrounded by z solvent molecules at a mean distance R̄.