Ex situ and in situ monitoring of the syntheses of thermochromic Schiff bases

Abstract

Schiff bases derived from o-hydroxy aldehydes were obtained by mechanochemical synthetic methods with a liquid or a paste present as an intermediate phase. The effectiveness of this procedure was monitored by means of ex situ PXRD and DSC, respectively. In the case of four Schiff bases, in situ PXRD studies were also conducted in order to monitor the synthesis of the product formed by merely putting the amine and the aldehyde in close contact. One of the o-hydroxy Schiff bases appears in two polymorphic forms. The monoclinic polymorph has been reported previously in the literature (herein obtained by the solution based method using methanol) while the synthesis and characterisation of the triclinic form is now reported (obtained by neat grinding or using acetonitrile during recrystallization). The reversible thermochromic properties in the solid state were also investigated in the temperature range from 298 to 110 K. The structural reasons of such behaviour were thoroughly reasoned.