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

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 1/40 molecule/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.