Layered double hydroxides with intercalated porphyrins as photofunctional materials:: Subtle structural changes modify singlet oxygen production

This study presents new photofunctional materials producing singlet oxygen, O-1(2), and investigates the interdependence between their structural and photophysical properties. These materials consist of Mg-Al layered double hydroxides ( LDH) with intercalated photosensitizers, 5,10,15,20-tetrakis( 4-sulfonatophenyl) porphyrin (TPPS) or Pd( II)-5,10,15,20-tetrakis( 4-carboxyphenyl) porphyrin (PdTPPC). Powder X-ray diffraction and X-ray photoelectron spectroscopies were employed to characterize the host structure and confirm intercalation of porphyrins into the interlayer space. Because the kinetic parameters of the sensitizer triplet states predetermine the formation of 1O2, the excited-state kinetics of intercalated porphyrins were investigated by means of time-resolved diffuse reflectance. Comparison of the decay rates in the presence and absence of oxygen confirms that the triplet states of PdTPPC and TPPS in LDHs are quenched by oxygen. Photoproduction of 1O2 was monitored by time-resolved measurement of its luminescence at 1270 nm. It was established that PdTPPC-doped LDHs are very effective producers of 1O2, regardless of whether the porphyrin molecules are intercalated or adsorbed on the surface. The measured lifetimes of 1O2 lie in the 6-64 As range, which means that the 1O2 molecules generated in the interior of LDHs can diffuse out of the matrix and react with a contiguous substrate. Dehydration of the LHD matrices enhances its singlet oxygen quenching capacity and inhibits the production of the long-lived 1O2 molecules, a process that can be reverted by exposing the material to atmospheric humidity. Consequently, we envisage LDHs with intercalated PdTPPC as efficient 1O2 sources whose oxidative activity can be modulated by successive dehydration-rehydration cycles.