Photoionization of Porphyrins in Mesoporous Siliceous MCM-41, AlMCM-41, and TiMCM-41 Molecular Sieves

Abstract

The photoionization of meso-tetraphenylporphyrin (H2TPP) within mesoporous MCM-41 silica tube molecular sieves is studied with visible light irradiation at room temperature and 77 K. Porphyrin π-cation radicals (H2TPP•+) are generated by photoionization within MCM-41 molecular sieves and are characterized by electron spin resonance (ESR) and diffuse reflectance UV−vis spectroscopy. Mesoporous MCM-41 molecular sieves are shown to be effective heterogeneous hosts to accomplish long-lived net photoinduced electron transfer of bulky porphyrin molecules at room temperature. Siliceous MCM-41 molecular sieves with different pore sizes designated as C n -MCM-41 (C n is the alkyl chain length of the surfactant used in the synthesis) demonstrate a significant pore size effect on the photoionization efficiency of H2TPP. The H2TPP•+ photoyield in C n -MCM-41 measured by ESR increases with increasing C n and is greatest in C16-MCM-41. A series of titanosilicate TiMCM-41 and aluminosilicate AlMCM-41 molecular sieves were synthesized and change the porphyrin photoionization efficiency in comparison with siliceous MCM-41. The photoionization efficiency increases in the order AlMCM-41 < MCM-41 < TiMCM-41. H2TPP•+ radicals are stable in MCM-41 and TiMCM-41 even at room temperature and decay after 30 min photoirradiation by only 5% after 48 h in TiMCM-41 at room temperature.

Incorporation of CdS Nanoparticles Formed in Reverse Micelles into Mesoporous Silica

“…A much greater photocatalytic property is obtained for the greater pore sizes of L-FM41 and FSG6, which probably results because the adsorption of water and the desorption of H 2 occur more easily in the greater mesopores. The pore size dependency is also consistent with previous reports concerning the photocatalytic CO 2 reduction on TiO 2 -MCM-41 and MCM-48 (4), and the photoionization of porphyrins in MCM-41 (21).…”

Cobalt ortho- and para-substituted tetraphenylporphyrins inserted in SiO2 gels

“…However, only slight attention has been paid to the problem of trapping [3][4][5][6][7] or fixing [8,9] the porphyrins into metal oxides networks such as SiO 2 , TiO 2 , Al 2 O 3 , etc. Clearly, it is not possible to insert porphyrins by thermal diffusion and the impregnation method would give a too low concentration and homogeneity to low for practical purposes in the final material.…”

Nonaggregated Zinc Phthalocyanine in Mesoporous Nanocrystalline TiO2 Thin Films

“…[12] It is known that the p electrons of embedded ZnPc monomer would interact with surface hydroxyl groups on the TiO 2 wall. [15] Such chemisorption of ZnPc monomers may promote the interface charge-separation from excited ZnPc to TiO 2 , and thus, making ZnPc a possible photosensitizer for the host anatase TiO 2 . The electron paramagnetic resonance (EPR) spectrum (not shown) of a visible light-irradiated ZnPc-TiO 2 film reveals a peak at a g-tensor value of 1.992, indicating the presence of Ti 3þ .…”