Insertion of lanthanide porphyrins in silica gel

In order to encapsulate organic dyes in silica gels it is necessary to use the dye molecules in water-soluble forms. In the case of the metallic porphyrins and phthalocyanines these forms are obtained by placing sulpho, carboxyl, etc., groups in the periphery of the macrocycles. We report here an easy and reliable technique to properly insert the lanthanide acetate–tetraphenylphorphyrins, Ln(TPP)Ac.2S (where Ln = Ho, Er, Tm, Yb and Lu and S = solvent molecule) in silica gels. These molecules were prepared from the free tetraphenylporphyrin base and the respective lanthanide acetate. The dark violet compounds obtained are soluble in ethanol, slightly soluble in water and their solutions in these solvents exhibit a red fluorescence (λmax=650 nm, λex=370 nm). The correct insertion was made possible by the addition of small quantities of pyridine (0.7%) to the initial mixture to inhibit the protonation and demetallization of the complex caused by the acid used to catalyze the reaction. The technique described here has been successfully extended to other metallic porphyrinic systems. The gelation process was monitored by UV–Vis spectroscopy. The monolithic gels obtained are strong, transparent and keep the original color of the lanthanoid complex solutions. The final spectra of all the gels do not show evidence of protonation (band at 660 nm) but the Soret and Q bands show a decrease in intensity. In all the experiments the fluorescence disappears in the monolithic dried xerogels.