Photoexcitation free energies of solvated molecules from raw absorption spectra: Can a Jarzynski-like equality be employed?

A definition of photoexcitation free energy of solvated molecules, ΔAexc, is proposed by exploiting an analogy between optical transitions driven by the electromagnetic radiation and steered transformations in classical systems. We postulate the applicability of a ‘spectroscopic version’ of the Jarzynski equality (which was originally derived in the classical context) with a likely work distribution function obtainable from the experimental UV–vis spectrum, so to yield ΔAexc on empirical basis. Motivations that support such a postulate are provided, and tests of internal consistency are made on some model cases.