A novel solid-state photochromic compound containing double heterocycles

• Pyrazolone containing thiophene has good solid-state photochromic properties.
• Pyraolone phenylsemicarbazone exhibits excellent thermal stability.
• The photochromic mechanism is the double-proton transfer by hydrogen bonds.
• Tautomerization between enol form and keto form occurs upon the light irradiation.

A new compound containing double heterocycles of pyrazole and thiophene, i.e. 1,3-diphenyl-4-(2-thienylmethylene)-5-hydroxypyrazole 4-phenylsemicarbazone (I), was synthesized and characterized by elemental analysis, MS, IR, 1HNMR, 13CNMR and X-ray single crystal diffraction. In solid state, it exhibits good photochromic properties under the UV and visible light irradiation. Upon the irradiation of UV light, the keto isomer (II) shows three IR absorption bands at 1697, 3302, 3404 cm−1, which are not observed for the enol isomer. To understand the IR spectral changes, theoretical calculations were carried out using Gaussian 03 program. Based on analysis of the single crystal structure and the results of theoretical calculation, the photochromic mechanism is due to the double-proton transfer by intramolecular and intermolecular hydrogen bonds, leading to the isomerization between the enol and the keto form.

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