Exploring the properties and optical sensing capability of sol-gel materials containing a covalently bonded binucleating cryptand

A large octaaza cryptand containing two tris(2-aminoethyl)amine (tren) donor subunits and six -Si(OEt)(3) pendant groups has been synthesized and used to prepare monolithic samples of sol-gel materials containing the cryptand covalently bonded to the silica network. Despite the constrains imposed by the multiple bonds to the network, the cryptand maintains its ability to form a binuclear Cu(II) complex and to interact with an ancillary azide ligand. A large number of reaction cycles involving the formation of the Cu(II) complex, the interaction with azide, and decomposition by treatment with an excess of acid can be carried out and easily monitored through the spectral changes in the Vis region. Adsorption studies indicate a mesoporous nature for these materials, with the pore size increasing with the cryptand content. The formation of the Cu(II) complex causes a significant decrease in the intensity of the Q(3) band in the Si-29 NMR spectra, which suggests that the cryptand is placed preferentially in Q(3)-rich sites. The capability of these materials for optical sensing of the azide anion has been explored. Although the absorbance changes show a linear dependence with the azide concentration up to 1 x 10(-3) M, kinetic studies indicate a long response time caused by slow diffusion of the reagents through the cavity network of the monoliths.