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

Abstract

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.

Carboxylic groups in mesoporous silica and ethane-bridged organosilica: effect of the surface on the reactivity

“…It is worth noting that this model assumes that each carboxylic group is accompanied by four silanols, which is in fair agreement with the relative amount of the two species obtained by 29 Si MAS NMR spectroscopy for a -COOH functionalized SBA-15 22 prepared with the same procedure used in this work.…”

“…The component at 1747 cm À1 is attributed to CQO stretching mode of -COOH species H-bonded, through the -OH, to the oxygen atom of a siloxane bridge (Scheme 2b). The similar structure involving the carboxylic groups on SBA-15-COOH outgassed at 573 K exhibits a slight higher n(CQO) value (1752 cm À1 ): this difference might be ascribed to the higher basic character of the oxygen atom on PMO, 29 which can thus engage in a stronger interaction with -OH site of -COOH than on the silica surface.…”

Charge Transfer-Induced Molecular Hole Doping into Thin Film of Metal–Organic Frameworks

“…In addition to the characteristic peak from iodine at about 512.8 nm, a new peak in deep UV region (i.e., at about 241.5 nm) can be observed. This additional absorption peak of the doped materials can be assigned to the donor-acceptor charge transfer complex formation 33,34 (i.e.,  electrons from aromatic ring donor and I 2 acceptor interaction). These findings clarifies the above observed hole doping characteristic of the iodine doped Co 3 (NDC) 3 films in Hall Effect measurements.…”

Periodic mesoporous organosilicas for advanced applications

“…6,19 In addition, PMOs with crystal-like frameworks are interesting materials for specific molecular adsorption because their hybrid surfaces consist of regular arrays of aromatic rings and silanols, which exhibit Lewis base and weak Lewis acid characteristics, respectively. 20 More advanced PMO materials are those with bridges containing hetero atoms such as nitrogen or sulfur. 13,15,21 Long and flexible functionalities are difficult to incorporate into PMOs.…”