Estimation of the solvent reorganization energy and the absolute energy of solvation of charge-transfer states from their emission spectra

We report herein the solvent and temperature effects on the emission of the intermolecular exciplexes 1-cyanonaphthalene/triethylamine and 1-methylnaphthalene/triethylamine and the intramolecular exciplexes formed by the bichromophoric compounds diethyl-(3-naphthalen-1-yl-propyl)-amine (I), diethyl-(2-naphthalen-1-yl-ethyl)-amine (II), 3-[ethyl-(2-naphthalen-1-yl-ethyl)-amino]-propionitrile (III) and 3-[(2-cyano-ethyl)-(2-naphthalen-1-yl-ethyl)-amino]-propionitrile (IV). The results are interpreted within the formalism of the semiclassical Marcus theory for radiative back electron transfer (BET) reactions. We show that, following a few simple assumptions, reliable values of the Gibbs free energy changes (Delta G(-et)(epsilon)) and the solvent reorganization energies (lambda(epsilon)(s)) associated to the BET reactions (and their corresponding enthalpic and entropic contributions) can be estimated directly from the emission of the CT states. We also show that for the 1-cyanonaphthalene/triethylamine exciplex, which exhibits emission in the vapour phase, the experimental calculation of the absolute energy of solvation of the CT state (Delta G(s)(epsilon)) is also possible. The calculated Delta G(-et)(epsilon) correlate quite satisfactorily with the corresponding values obtained from independent electrochemical and kinetics experiments. The temperature effects on Delta G(-et)(epsilon), lambda(epsilon)(s) and Delta G(s)(epsilon) are discussed qualitatively using different solvation models. The limitations of the present approach for the estimation of Delta G(-et)(epsilon) and lambda(epsilon)(s) and its possible application to more complex A/D systems are also examined.