This study presents new photofunctional materials producing singlet oxygen, 1O2, 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 μs 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.
We report the formation of host-guest complexes between water-soluble calix[n]arene-p-tetrasulfonates (n = 4, 6, 8) or 2-hydroxypropyl-cyclodextrins (alpha-, beta-, gamma-) and the tetratosylate salt of 5,10,15,20-tetrakis(4-N-methylpyridyl)porphyrin (TMPyP). The binding constants ranging between 10(2) and 10(5) M-1 were calculated from the absorption and fluorescence changes. Calix[4]arene-p-tetrasulfonate has a high binding affinity and forms with TMPyP a 1:1 complex, whereas other calixarenes bind two molecules of TMPyP. Electrostatic attraction is the dominating binding mode. Binding to calixarenes leads to a considerable decrease of the quantum yields of the triplet and excited singlet states and to shortening of the singlet and triplet lifetimes of TMPyP. The quenching mechanism is attributed to electron transfer between calixarene phenolates and excited TMPyP. Photoinduced electron transfer within a novel supramolecular complex calixarene/TMPyP (electron donor)/methyl viologen (electron acceptor) has been proven by absorption and fluorescence measurements. Electrostatic attraction between the cationic donor and cationic acceptor, on the one hand, and the anionic host, on the other, overcomes the electrostatic repulsion forces. In contrast, the interaction of cyclodextrin with TMPyP is hydrophobic in nature and only slightly influences the photophysical properties of TMPyP. The different behavior of TMPyP bound to either of the hosts has been assigned to the specific effects of the dominant binding modes, viz. the electrostatic attraction for calixarenes and the hydrophobic interactions for inclusion complexes with cyclodextrins.
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