Immobilisation of Zinc porphyrins on mesoporous SBA-15: Effect of bulky substituents on the surface interaction

Abstract

Two Zn porphyrin complexes, Zn-5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)porphyrin (ZnTBPP) and Zn-tetraphenylporphyrin (ZnTPP) were prepared and physically adsorbed on the surface of mesoporous SBA-15. The resulting materials were tested about the stability of the adsorbed dyes to leaching, and characterized by X-ray diffraction analysis (XRD), low temperature nitrogen adsorption/desorption isotherms, infrared and electronic (Diffuse Reflectance UV–visible adsorption and emission) spectroscopies. Thermogravimetric and spectrophotometric analyses indicate a surface concentration around 1/40molecule/nm2, on an average. The high dispersion of the complexes within the silica pores was suggested by the negligible modifications of the ordered mesoporous structure and textural properties (specific surface area and porosity). A detailed spectroscopic analysis allowed investigating the molecular interaction with silica surface, taking place through hydrogen bonding with Si–OH groups. In the case of ZnTBPP this interaction was mediated by the bulky tert-butyl substituents. This resulted in a higher stability to leaching and in a consistent blue-shift of the adsorption and emission bands typical of porphyrins, suggesting a more efficient orbital overlapping between porphyrin and phenyl groups as a result of hindered rotation. This effect was less evident in the ZnTPP complex, which was shown to directly interact with surface Si–OH through the aromatic rings.