The influence of pH over topography and spectroscopic properties of silica hybrid materials embedding meso-tetratolylporphyrin

Hybrid porphyrin–silica materials consisting in 5,10,15,20-tetratolyl-21H,23H-porphine, encapsulated in silica matrices were obtained by sol–gel method. The hydrolysis and condensation reactions were performed by using tetraethylorthosilicate as precursor, either in one-step acid or in two steps acid–base catalysis, assisted or not by an ultrasonic field. The porphyrin–silica hybrid materials were monitored and characterized by AFM, FT-IR, fluorescence and UV–vis spectroscopy. As a result of porphyrin entrapping into silica gels, a general phenomenon regarding hyperchromic effects of the Q bands, occurs. UV–vis study revealed that during the sol–gel process, major changes regarding porphyrin ring structure occur, especially at acidic pH, when the dicationic species of porphyrin are formed. During acid–base catalyzed method the dye molecules tend to aggregate by π−π and hydrophobic co-facial interactions of sandwich H-type. In acid catalyzed process, the protonation of the porphyrins prevent the formation of aggregates, due to increased electrostatic repulsion between the molecules.