Thermochromism of N-Triphenylmethylsalicylideneimine Activity of the Trans-Open Enol Conformer at Low Temperature

Abstract

N-triphenylmethylsalicylideneimine (MS1) represents a reversible thermal and photo-coloration system. The initial stage of the system studied was defined by means of X-ray structure analysis. Crystal structures were done at 295 and 230 K, taking into account the results of differential scanning calorimetry; basing on the DSC measurements, a pseudo-first order phase transition at 241 K was identified. According to the analysis of the luminescence spectra of MS1, low temperature favors a benzenoid-type trans-enol open form of the molecule under study. However, after UV excitation, the photochemical reaction (proton transfer and/or photoisomerization) generates the quinoid form. So, thermochromy has been interpreted as due to geometry changes of a chelate ring, and creating an open enol molecular form rather than a shift of the tautomeric equilibrium benzenoid-to-quinoid form. Below 240 K, the light-induced generation of unstable photochromic species is manifested as the sample color change (colorless-to-orange). That newly created species is of a trans-keto-type isomer of MS1 molecule with breaking hydrogen bond, and the open trans-enol form existing in the ground state is a source of that orange species.

Keywords:

• N-triphenylmethylsalicylideneimine
• phase transition
• photochromism
• photoluminescence
• thermochromism

Acknowledgments

The author thanks Prof. Mirosław Soroka (Faculty of Chemistry, Wrocław University of Technology) for a gift of the sample used in the experiments and Dr. Hab. Ilona Turowska-Tyrk (Institute of Physical and Theoretical Chemistry, Wrocław University of Technology) for X-ray measurements.