Solid-state electronic absorption, fluorescence and 13C 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.