Photochromic behavior of spironaphthoxazine in metal ion-containing solutions

The isomerization and metal chelation of spironaphthoxazine (SNO) in ethanol solutions have been investigated by measuring the UV–vis absorption spectra as a function of time. The protonated species of merocyanine (MC), an isomer of SNO, was gradually formed in the solutions containing SNO and aluminum ions in the dark. The proton was provided by the reaction of the aluminum ions with a slight amount of water contained in the ethanol solvent. The protonated species was slowly transformed into the aluminum chelate complex of MC by ligand exchange of the ethanol-solvated aluminum for MC in the dark. UV irradiation promoted the isomerization and chelation without forming the protonated species. The spectroscopic analysis with various compositional ratios of SNO and aluminum ions revealed that one aluminum ion was coordinated to three bidentate MC ligands in the ethanol solution.

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► The isomerization and metal chelation of spironaphthoxazine in ethanol were studied.
► The protonated species of merocyanine was formed in the AlCl3 solutions in the dark.
► The protonated species was transformed into the Al chelate complex of merocyanine.
► UV irradiation promoted the complex formation without forming the protonated species.
► One aluminum ion was coordinated to three bidentate merocyanine ligands in ethanol.