Chelation of spironaphthoxazine with zinc ions and its photochromic behavior during the sol–gel–xerogel transitions of alkyl silicon alkoxide

The isomerization and zinc-chelation of spironaphthoxazine (SNO) during the course of the sol–gel–xerogel transitions of the tetraethylorthosilicate (TEOS) system and the mixture systems of TEOS and octyltriethoxysilane (OcTES) have been investigated by measuring the fluorescence and excitation spectra as a function of time. The influence of the hydrophobic octyl groups on the spectroscopic behavior of SNO in the TEOS and TEOS–OcTES systems containing zinc ions have been discussed. A fluorescence spectrum at around 430 nm, observed just after the preparation of the TEOS system, was similar to that of SNO observed in protic highly-polar solvents due to the presence of the ethanol solvent in the system. A weak fluorescence spectrum was observed at around 400 nm just after the preparation of the TEOS–OcTES systems. This spectrum corresponded to that of SNO in low-polar solvents due to the hydrophobicity of OcTES. The fluorescence spectra in both systems were gradually red-shifted to around 450 nm along with the progress of the sol–gel transition, indicating that SNO (ring-closed form) interacts with silanol groups generated by the hydrolysis of the alkoxides. A spectrum due to the zinc complex of the SNO-derivative, merocyanine (MC, ring-open form), also appeared at around 540 nm during the initial stage of the sol–gel–xerogel transitions (around the gelation point) for both systems. SNO molecules and zinc ions were concentrated in prestructural pores and easily interacted with each other. The fluorescence intensity of the complex in the TEOS–OcTES systems, however, was remarkably lower than that in the TEOS system due to the hydrophobicity of the system. Depending on changes in the polarity and volume of the space containing the species, the complex fluorescence intensity increased and decreased. In the TEOS–OcTES systems, the non-complex band was blue-shifted around the gelation point since the polymerization of the silanol groups preferentially occurred and the polarity decreased in the system containing hydrophobic groups. In the gel of the TEOS system and the sol of the TEOS–OcTES system, a photochromic reaction was observed, i.e., SNO formed the MC–Zn complex during UV irradiation, whereas a change was hardly observed during visible irradiation. In the xerogel of the TEOS system and in the gel and xerogel of the TEOS–OcTES system, a reversible photochromic reaction was observed, i.e., the MC–Zn complex was associated and dissociated by UV and visible irradiation, respectively.