Luminescence properties of the porphyrin/porous silicon composites

A study of luminescence properties of water-soluble and hydrophobic porphyrin and chlorin molecules immobilized in oxidized (i.e. non-luminescent) and as-prepared (i.e. luminescent) porous silicon (porSi) layers is carried out. It is found that the oxidized porSi matrix itself does not introduce any quenching effect on the porphyrin fluorescence, but the fluorescence is subjected to the concentration quenching. At maximum porphyrin/chlorin concentration used (ca. 10 -3 M in impregnation solutions) the quenching is strong in case of hydrophobic porphyrins and sufficiently moderate for water-soluble ones, especially for cationic porphyrin TMPyP4. A concentration of the latter in porSi layer is determined by direct measurements. Both hydrophobic and water-soluble porphyrins immobilized on the surface of as-prepared porSi are found to quench the luminescence of the latter. In case of hydrophobic porphyrins the quenched porSi luminescence is regained almost to the initial value on the time scale of tens of hours, whereas in case of water-soluble porphyrins the fluorescence spectrum of the latter builds up in course of time. Possible mechanisms responsible for these effects are discussed.