Photochromism and fluorescence modulation of pyrazolone derivatives in the solid state

Two novel photochromic compounds derived from pyrazolones, which undergo a photochromic reaction in the solid state, have been synthesized and their photochromic and fluorescence properties were investigated. Their fluorescence emission can be efficiently modulated by photoisomerization of pyrazolones almost up to 90% upon UV light irradiation. Importantly, the fluorescence emission of 1 can be reversibly switched “on” and “off” upon alternative UV light irradiation and heating. These compounds exhibited photochromic coloration from white E -form to yellow brown K -form. On the basis of the molecular structure determined by X-ray single crystal analysis, there are intermolecular hydrogen bonds which provide suitable proton transfer channels for the enol–keto transformation. The colored K -form isomers, which are very stable at ambient temperature, may be due to the reverse reaction occurring at a high temperature.