Solid-state electronic absorption, fluorescence and13C CPMAS NMR spectroscopic study of thermo- and photo-chromic aromatic Schiff bases

Abstract

Solid-state electronic absorption, fluorescence emission and 13C CPMAS spectroscopies have been applied to a series of aromatic Schiff bases displaying both ground and excited state intramolecular proton transfer phenomena. All the results can be explained on the basis of a thermal equilibrium between enolimine and keto–enamine tautomeric forms in the crystalline state. Most of the studied compounds are thermochromic. However, a few are photochromic. The carbon-13 NMR data in the solid state show, in general, residual (13C, 14N) dipolar coupling effects. In certain cases, however, where fast proton transfer occurs in the ground state, these effects are shown to be self-decoupled.