Photoionization of porphyrins in mesoporous siliceous MCM-41, AlMCM-41, and TiMCM-41 molecular sieves

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 pi-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 C-16-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.