Fluorescence and Textural Characterization of Ortho-Amine Tetraphenylporphyrin Covalently Bonded to Organo-Modified Silica Xerogels.

Most of the studies performed with porphyrins involve these species functionalized with peripheral substituents lying on the same macrocyclic molecular plane. The main objective of this work deals with the successful preservation and optimization of the fluorescence of a uncommonly used porphyrin species, i.e. tetrakis-(ortho-amino-phenyl)-porphyrin; a molecule with substituents localized not only at one but at both sides of its molecular plane. In cases like this, it must be stressed that fluorescence can only be partially preserved; nevertheless, intense fluorescence can still be reached by following a twofold functionalization strategy involving: (i) the bonding of substituted macrocycles to the pore walls of (ii) organo-modified silica monoliths synthesized by the sol-gel method. The analysis of both absorption and emission UV spectra evidenced a radiation energy transfer taking place between the porphyrin and the host silica matrix. Our results showed that the adequate displaying of the optical properties of macrocyclic species trapped in SiO2 xerogels depend on the polarity existing inside the pores, a property which can be tuned up through the adequate selection of organic groups used to modify the surface of the pore cavities. Additionally, the pore widths attained in the final xerogels can vary depending on the identity of the organic groups attached to the network. All these facts finally demonstrated that, even if using inefficient surface functionalization species, such as ortho-substituted tetraphenylporphyrins, it is still possible to modulate the pore shape, pore size, and physicochemical environment created around the trapped macrocycles. The most important aspect related to this research deals with the fact that the developed methodology offers a real possibility of controlling both the textural and morphological characteristics of a new kind of hybrid porous materials and to optimize the physicochemical properties of diverse active molecules trapped inside the pores of these materials.