engleski

On keto-enol equilibrium in novel thermochromic o-vanillin Schiff bases

Schiff bases are a long time known group of photochromic and thermochromic organic compounds. The structural reason for a colour change upon UV-irradiation or the change of temperature is the electron density alteration in the region of the C=N imino group followed occasionally by the change of molecular planarity. The state of electron density in the six-membered pseudo-aromatic chelate ring determines the position of the hydrogen atom in the intramolecular O-H∙∙∙N or N-H∙∙∙O hydrogen bond and the ratio of the tautomers. A recent study has revealed that the fluorescence plays a crucial role in the solid-state thermochromism of salicylideneanilines on the temperatures below the room temperature when the fluorescence is much stronger than on the temperatures above room temperature. To obtain a better insight into chromic properties of salycilideneanilines, four novel thermochromic Schiff bases (I, II, III, IV) derived from o-vanillin and different aromatic amines have been synthesized by neat grinding (NG) and characterized by means of differential scanning calorimetry and IR- and NMR-spectroscopy. The crystal and molecular structures of four Schiff bases were obtained using the single crystal X-ray diffraction performed at RT and 110 K. The quality and purity of the solid products obtained by NG was characterized by DSC and X-ray powder diffraction and compared with the data of recrystallized material. The thermochromic properties of I to IV were studied using XRD giving quantitative insight into the diverse colorization upon intramolecular proton transfer changes, tautomer equilibriums and molecular planarity.

o-vanillin Schiff bases; mechanochemical synthesis; tautomerism; thermochromism

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