Effects of chemical substitution upon excited state proton transfer. Fluoroderivatives of salicylaldimine

Abstract

Ab initio methods are used to probe the effect of fluorosubstitution upon the excited state proton transfer process in the OH⋯N hydrogen bond contained in the model system salicylaldimine. Two different sites are chosen for substitution, one nearby to the nitrogen on the cycle that contains the intramolecular H-bond, while the other position is located on the aromatic ring close to the oxygen. Many of the computed effects on the proton transfer potential can be considered inductive in nature; the electronegative F makes the proximate N or O atom a stronger acid/weaker base and thereby moderates the preferred position of the bridging proton. The magnitude of the perturbation diminishes as the site of substitution is further removed from the H-bond. This principle also controls the manner in which F affects the geometry and strength of the intramolecular H-bond in both the enol and keto tautomers. There are a number of stretches and contractions that occur in bonds that do not border on the H-bond, patterns that can be understood via the bonding patterns in the orbitals involved in the excitation. Whereas these notions apply fairly consistently to the ground state and excited ππ ∗ singlet and triplet, there are a number of anomalous patterns that emerge in the 1 nπ ∗ state. In general, the effects of fluorosubstitution are smaller in magnitude than the changes that occur in the proton transfer properties as a result of electronic excitation.