Photoionization of metalloporphyrins in silica gel

Abstract

The photoionization of metalloporphyrins (MTPP) adsorbed in 40 Å pore silica gel is studied with visible light irradiation at room temperature and 77 K. MTPPs, such as zinc(II) meso-tetraphenylporphyrin (ZnTPP), nickel(II) meso-tetraphenylporphyrin (NiTPP), and copper(II) meso-tetraphenylporphyrin (CuTPP) are used to study the effects of the central metal ions on the efficiency of MTPP photoionization in silica gel in comparison with metal-free meso-tetraphenylporphyrin (H₂TPP). Porphyrin π-cation radicals are generated in silica gel by photoirradiation and are characterized by electron paramagnetic resonance (EPR) and diffuse reflectance (DR) UV–VIS spectroscopy. The photoionization efficiency of such porphyrins decreases in the order ZnTPP > H₂TPP > NiTPP. For CuTPP/silica gel, no CuTPP⁺ radical is detected by EPR or by DR UV–VIS spectroscopy. Upon photoirradiation, porphyrin π-cation radicals Zn^IITPP⁺ and Ni^IITPP⁺ are produced by oxidation of the porphyrin ring instead of the central metal ion. Thus, (Ni^IIITPP)⁺ is not observed in NiTPP/silica gel by photoirradiation.