Photophysical and photochemical properties of porphyrin and naphthalene diimide modified silica-gel particles

Silica-gel particles modified with photoactive molecules have applications in several fields including photocatalysis, chromatography, combinatorial chemistry, and chemical sensors. By using phosphonate/sulfonate zirconium chemistry, silica-gel particles were functionalized with two photoactive molecules, N, N ′-bis(2-phosphonoethyl)-1,4,5,8-naphthalene diimide (DPN) and meso-tetra(4-sulfonatophenyl)porphyrin sodium salt (TPPS4). The photophysical and photochemical properties of the bound chromophores were shown to differ from their solution properties. In the luminescence spectra of DPN bound to silica-gel particles (DPN–silica gel), a band centered at 500 nm, which was not observed in solution, appeared and it was assigned to the emission of DPN excimers. The ratio of monomer to excimer emission was shown to change with the solution pH. In TPPS4 modified silica, changes in the absorption and in the emission spectra were observed when compared with the emission of TPPS4 in solution. These changes were correlated with the presence of TPPS4 dimers in the silica surface. Alterations in the photochemical reaction routes of DPN and TPPS4 were also observed by laser flash photolysis experiments. We were able to demonstrate that there are two competing photochemical reactions, i.e., energy transfer between triplet species and oxygen and electron transfer between triplets and ground state chromophores. Due to the close proximity of the chromophores in the silica surface, electron transfer processes are favored as demonstrated by the increase in the photogeneration of reduced radical species of DPN ( ΔAbs max=480 nm) and of TPPS4 ( ΔAbs max=700 nm).