Keto forms of salicylaldehyde Schiff bases: structural and theoretical aspects.

Twelve Schiff bases of methoxy-substituted salicylaldehyde have been examined by crystallographic and spectroscopic methods, as well as by DFT theoretical calculations in order to investigate the effect of the substituent's position on the keto-enol equilibrium in the crystalline state. Four out of the 10 structurally characterized compounds with methoxy substitution on the para and/or ortho positions with respect to the aldimine bridge and deriving from aliphatic amines or alkylarylamines are found as cis-keto tautomers and form dimers. In contrast, the five pure enol tautomers derive either from aliphatic or alkylarylamines and are meta substituted or from aniline or benzylamine and are para and/or ortho methoxy substituted.

Photochromism and thermochromism of Schiff bases in the solid state: structural aspects.

This tutorial review describes in a brief historical perspective the most important organic compounds that exhibit photochromism in the crystalline state since its discovery in 1867 up to now and considers in detail Schiff bases of salicylaldehyde with amines (anils). The latter comprise a chemical system undergoing hydrogen-atom tautomerism between enol and keto forms and show the phenomena of solid state photochromism and thermochromism. The system has been investigated extensively.