Prototropic tautomerism and solid-state photochromism of N-phenyl-2-aminotropones

N-(4-X-phenyl)-2-aminotropones (X = methoxy, chloro, or nitro substituent) and their clathrate crystals with deoxycholic acid (DCA) were prepared and their prototropic phenomena were determined. Each 2-aminotropone derivative existed as a keto–amine form in the crystal state irrespective of its substituent, which influenced the stability of keto–amine or enol–imine structure in solution. X-Ray crystal structure analysis of N-(4-methoxyphenyl)-2-aminotropone revealed the existence of bimolecular hydrogen bonds and close molecular packing, which would inhibit structural change necessary to photo-induced prototropic tautomerization. Thus, N-phenyl-2-aminotropones did not show photocoloration in the crystal state, while their clathrate crystals with DCA were found to exhibit photochromism.