Electron-Rich Sites at the Surface of Periodic Mesoporous Organosilicas: A UV-Visible Characterization of Adsorbed Iodine

Periodic mesoporous organosilicas (PMOs), a relatively new class of mesostructured organic-inorganic hybrid materials already thoroughly characterized as it concerns acidic sites, are studied in the present paper from the viewpoint of electron-rich, potentially basic sites. Iodine was adopted as a probe molecule, as it is known that the spectral shift suffered by the I-2 band in the visible region correlates with the donor strength of solvents and solids. Iodine adsorption on ethyl- and phenylene-containing PMOs was followed by means of UV-visible spectroscopy: the results show that the electron-donor ability of oxygen atoms at the PMO surface is higher than that reported for pure siliceous materials and comparable with that of some basic zeolites, in agreement with the previously reported weaker acidity of hydroxyls. In phenylene-bridged PMOs aromatic rings are the preferential adsorption sites for iodine. Comparison between phenylene-bridged PMOs with different degrees of order in the walls reveals a lower availability of aromatic sites on the ordered surface with respect to the amorphous one.