Substituent effects on the intramolecular proton transfer in the ground and lowest-lying singlet excited states of salicylaldimine

The effects of chemical substitution on the photophysical and photochemical properties of salicylaldimine (2-iminomethylphenol) have been studied by replacing the hydrogen atom of the imino group in salicylaldimine by SiH3, CH3, NH2, COH, NO, CF3, and CN. For the different derivatives of salicylaldimine, we have computed geometries and potential energy curves for the intramolecular proton transfer in their ground states along with the Franck-Condon energy curves for their three lowest-lying singlet excited states. Our results show that the intramolecular hydrogen bond in the enol form is weakened by the presence of the NH2, NO, CF3, and CN substituents. This weaker bond causes an increase of the energy barrier for the ground state proton transfer that converts the enol form to the keto tautomer. Moreover, the keto form becomes unstable in the ground state when the CH3, NH2, CF3, or CN substituents are present in the molecule. Finally, whereas the 1 pi pi* state remains almost unaltered by the effect of the substituent, the npi(*) excited state is stabilized in the SiH3, COH, and NO derivatives. This fact is especially true for NO, where the pi pi* state becomes the first excited state. The results have been rationalized by taking into account the different electronic effects of each substituent. (C) 2000 Published by Elsevier Science B.V.