Theory of solvatochromic shifts in nonpolar solvents reveals a new spectroscopic rule.

An expression of unexpected simplicity is derived for the shift in optical transition energies of solute molecules in nonpolar solvents. The expression reveals a new spectroscopic rule that says: The higher the excited state of the solute, the larger the solvatochromic red shift. A puzzle formulated >50 years ago by Bayliss is solved. Bayliss, based on arguments from classical physics, assumed that the shift scales with the oscillator strength of the solute transition, but noted strong quantitative deviations from this rule in experiments. As the present expression shows, the shift does not depend on the oscillator strength of the transition, but reflects the change in dispersive solute-solvent interactions between the ground and excited states of the solute, that are determined by the anisotropy of intramolecular electron correlation. The theory is applied to explain the solvatochromic shifts of the two lowest electronic excitations of bacteriochlorophyll a and bacteriopheophytin a.