Sol–gel immobilization of octaethylporphine and hematoporphyrin for biomimetic photooxidation of α-pinene

The nature and aggregation of octaethylporphine (OEP) and hematoporphyrin (HmP–IX) immobilized in silica during the sol–gel process were evaluated using UV–Vis absorption, excitation and emission spectroscopies. An essential difference in biomimetic photooxidation of α-pinene was observed between the two obtained intensive luminescent materials. Biocatalytic activity was only found for OEP/SiO2, and the lack of such activity for HmP–IX/SiO2 was related to the interaction of its carboxyl groups with the silica structure. Myrtenol was the only primary and pinocarvone and pinocarveol the only secondary products of the reaction of α-pinene oxidation. A cooperative mechanism of photooxidation based on proton transfer between protonated porphyrin and silanol/siloxane groups followed by electron abstraction from substrate by the light-excited OEP/SiO2 matrix or photosensitized generation of singlet oxygen (1O2) was proposed. The correlation between the biocatalytic efficiency and spectral properties of the porphyrins was also examined. The reusability and stability of OEP/SiO2 were demonstrated in four catalytic cycles.