Trans-keto* form detection in non photochromic N-salicylidene aminomethylpyridines

Abstract

Five N-(5-chloro)salicylidene aminomethylpyridine derivatives were successfully synthesized and their structural and solid state optical properties analyzed. Yellow crystalline powders of CH22L33 and CH2222L3–43–43–43–4Cl present both thermo- and photochromism whereas CH22L22 and CH222L444 do not show any switching properties, as probed by diffuse reflectance spectroscopy. Contrary to structural–optical expectations, the non-photochromic CH22L22 molecule (P21/n) shows an open crystal structure, and photochromic molecules CH2222L3–43–43–43–4Cl (P21/c) present a closed crystal packing, revealed by single crystal X-ray diffraction, which is typical of exclusively thermochromic molecules. After UV irradiation, trans-keto* emission observation in CH22L22 and CH222L444 indicates the unexpected formation of the trans-keto form in these non-photochromic anil molecules. Radiative relaxation of the trans-keto* form is in addition detected, for the first time, by fluorimetry for all N-salicylidene molecules of the series, whatever their switchable chromic properties.