Preparation and characterization of zirconium phosphonate azobenzene intercalation compounds. A structural, photophysical and photochemical study

The physical and chemical photo-responses of azobenzene were employed to investigate the 'nanoscopic' properties of a layered zirconium phosphonate of formula ZrF(O3PCH2)(2)NHC8H17 (ZrC8, interlayer distance 1.82 nm), a solid belonging to a new family of zirconium diphosphonates with a poorly hindered interlayer region. Two samples were prepared: an intercalation compound containing 0.8 mol of dye per mol of zirconium, having an interlayer distance of 2.82 nm; and a compound in which the azobenzene was adsorbed on the surface of the host. The photobehaviour of the samples was examined by absorption, fluorescence and photochemical techniques and compared with those of the dye in the solid state and in solution. Intercalated or adsorbed azobenzene is able to fluoresce not only from the lowest excited singlet state (1)(n,pi*) but also from the upper one (1)(pi,pi*), as observed in the solid dye. However, differently from what happens in the pure solid dye, the trans-->cis photoisomerization of azo-groups occurred in both intercalated and adsorbed samples upon UV irradiation. The structural changes associated with the photoisomerization process in the interlamellar region induced an irreversible deintercalation of the dye, which was detected as adsorbed on the host surface or present in an amorphous phase. Space-resolved properties of the ZrC8-azobenzene materials were investigated by confocal fluorescence microscopy.