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 CH₂222L^3–43–43–43–4Cl 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 CH₂222L^3–43–43–43–4Cl (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.