Chelation of spironaphthoxazine with zinc ions during the sol–gel–xerogel transitions in silicon alkoxide systems

Chelation of spironaphthoxazine (SNO) with zinc ions in the course of the sol–gel–xerogel transitions in the tetraethylorthosilicate (TEOS) systems has been investigated by monitoring the fluorescence and excitation spectra as a function of time. A fluorescence spectrum was observed at around 430 nm, just after the mixing SNO with zinc ions in TEOS, which was similar to that of SNO observed in protic high-polar solvents due to the existence of the considerable amounts of ethanol content in the TEOS systems. The fluorescence maxima were gradually red shifted to around 450 nm with the progress of the sol–gel reaction. This result indicates that SNO (ring-closed form) can interact by hydrogen bonding with SiOH groups generated by the hydrolysis of TEOS. Some of these SNO molecules formed the non-planar zwitterion as the CO bond was cleaved. A spectrum originating from the zinc complex of the SNO-derivative, merocyanine (MC, ring-open form), also appeared at around 540 nm near the gelation point. This complexation occurred because SNO molecules and zinc ions were concentrated in the prestructural pores and easily interact with each other. The hydrogen-bonding species and the chelate species decreased during the drying process of the wet gel due to growth of the silica networks including zinc along with the condensation of SiOH groups. As the gel converted to the xerogel, the SNO molecules were more strongly adsorbed on the surface of the pores in the xerogel than in the wet gel due to the evaporation of solvent and a contraction of the space. The SNO molecules, therefore, form not only the MC–Zn complex again, but also the MC–SiOH complex. The photochromic behavior including the ring opening and association of the MC–Zn or MC–SiOH complex, and its dissociation and ring-closure were observed by UV and visible light irradiations in the xerogel systems.