Protic properties of water confined in the pores of oxidized porous silicon studied by excited-state proton transfer from a photoacid

The properties of water confined in the pores of oxidized porous silicon (PSi) were studied using the wellknown photoacidic 8-hydroxy-1,3,6- pyrenetrisulfonate trisodium salt. We used porous silicon samples, whose pores have an average diameter of 10 nm and a depth of 2 m. We found that the rate of the excitedstate proton transfer to water in PSi pores is slightly enhanced with respect to bulk water. We found that the transferred proton reaches the pores walls and is subsequently reflected back with no appreciable loss. Since the proton diffusion occurs inside the elongated space of the cylindrical pore the proton photoacid molecule pair distribution function tends to deviate from spherical symmetry at long times. The detailed analysis of the time-resolved emission of the protonated species of the photoacid (ROH*) shows that at long-times after excitation the diffusion space is nearly two-dimensional, and consequently it does not have a spherical symmetry.