Photochromic behavior of spironaphthoxazine in metal ion-containing solutions

Abstract

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.

Reaction of spironaphthoxazine with acid

“…The band assigned to MCH + was observed at 530 nm and its intensity increased with time in the acidic SNO ethanol solution in which the molar ratio of SNO to H + was 3/1 as previously reported [24]. Fig.…”

“…Under this condition, the reaction rates were faster and the amounts of products were greater than in the dark although the spectral peak positions and the patterns of increase and decrease in their intensities were similar to those in the dark. It was previously confirmed that SNO directly formed the MC-Al 3+ complex without forming the protonated species during the UV irradiation [24]. The protonation and aluminum chelation of SNO easily proceeded under the present conditions.…”

“…In addition, the spectrum observed just after the preparation (at time 0) exhibited a band at 530 nm assigned to the species protonated at the naphtholate part of the MC (MCH + ) [24,32,33]. ( Fig.…”

“…On the other hand, a small number of protons assisted the MC-Al 3+ complex formation in the ethanol solution containing Al 3+ [24].…”

“…The xerogel system also contained the MC-SiOH complex, like the MC-Zn 2+ , because there were many acidic SiOH groups on the surface of the gel pores in which the organic molecules were encapsulated [16-18,25−28]. The complex formation is expected to depend on the type of metal ions involving a specific electron affinity [18,24]. MC can interact not only with the metal ions, but also with the protons on the silica gel surface.…”