Abstract
In this work, a new method for the determination of a Franck‐Condon excited state moment in solution is described. This method is based both on the study of the solvatochromic effect and on the use of Onsager reaction field theory. The method we propose requires (1) calorimetric measurements of the heat of transfer from solvent S~1~ to solvent S~2~ for the solute in its ground state and for a homomorph; (2) the absorption frequencies of the solute in the two solvents and (3) experimental determination of the solute and homomorph ground state dipole moments.
It is easy to show that a very simple relationship exists between these various experimental quantities if the two solvents are such that the reaction field theory can be applied to describe the dipolar part of the solvent solute interactions. The use of the homomorph method permits one to take into account the variation of the cavity term from solvent to solvent and the variation of the solute solvent dispersive energy term with the nature of the solvent. Moreover, the method does not requires neither the estimation of the cavity radius nor the dangerous assumption that the dipole is centred in the cavity. This new method, which compared to older methods, must give more reliable results, is applied to the π* state of cyclohexanone and to the ^1^L~a~ state of indole.