DOI: 10.1002/chem.200700310 Vibrational Study on the Local Structure of Post-Synthesis and Hybrid Mesoporous Materials: Are There Fundamental Distinctions? Pedro D. Vaz,* [a] Carla D. Nunes, [a] Maria Vasconcellos-Dias, [a] Mariela M. Nolasco, [b] Paulo J. A. Ribeiro-Claro, [b] and Maria JosØ Calhorda* [a] Introduction Inorganic materials based on silica matrices have been used in many fields for miscellaneous applications for some time, though lack of technological and architectural features has restricted their variety. In 1992 a major leap forward oc- curred with the discovery of the M41s mesoporous material family. [1,2] Since then, numerous papers have contributed de- cisively to the development of this field, opening new routes for functionalization. [3,4] The main approach for derivatizing such materials was by post-synthesis grafting or tethering procedures of the silica walls. Whereas the former is based on the direct reaction of a suitable silicon derivative with the OH groups on the walls of the material, in the latter, the chosen molecule is introduced with the reagents when syn- thesizing the material. The tethering approach has enabled the introduction of organic moieties into the inorganic frameworks by the one-pot preparation of hybrid organic– inorganic materials, [5–13] which also offers the advantage of a better distribution of the organic moieties in the walls than achieved with the grafting approach. Despite these available approaches to engineer new mate- rials and all the improvements introduced, some problems still remain. A relevant question is related to the structural similarities or differences that arise when different synthetic procedures are used, namely whether organic moieties lie at the outer or inner side of the walls. For instance, the grafting process only allows the introduction of the organic moieties at the surface, but the same cannot be said of the tethering process. It is well documented that in silica gel, the SiO 4 tetrahe- dral units are arranged not only in the thermodynamically favored siloxane 4-rings, but also in the form of kinetically favored and less strained 6-rings. [14] Recently, state-of-the-art studies addressing the local structure of silica gels showed that the infrared-active Si-O-Si asymmetric stretch appears as a broad band and its frequency is a function of the Si-O- Abstract: Organic–inorganic mesopo- rous materials of the MCM-41 type are important materials that can be pre- pared by either post-synthesis or one- pot synthesis procedures. A complete control of the characteristics at a local level is of the utmost importance in view of the applications of such materi- als. However, there are not many stud- ies relating such features with synthetic approaches. In this work, we prepared samples by post-synthesis derivatiza- tion of materials from Si-based MCM- 41, with bidentate nitrogen ligands bearing one or two silylated arms, and by one-pot synthesis of organic–inor- ganic hybrid materials. The bulk prop- erties of the two kinds of materials were comparable. Diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy and Raman spectroscopy were used to investigate the local envi- ronment, namely, the number of OH groups and distribution of SiO 4 units (large and small ring units). Hydrophi- licity correlates with both the type of organic moiety used (mono- or disily- lated), as well as with the synthetic procedure. The same vibrational stud- ies showed how the structure in the channels changes as a function of pres- sure, reflecting the low mechanical sta- bility of the mesoporous materials. Keywords: IR spectroscopy · meso- porous materials · organic-inorganic hybrid composites · Raman spec- troscopy · sol–gel process [a] Dr. P.D. Vaz, Dr. C.D. Nunes, M. Vasconcellos-Dias, Prof. M. J. Calhorda Department of Chemistry and Biochemistry CQB, Faculty of Science, University of Lisbon C8 Campo Grande, 1749–016 Lisboa (Portugal) Fax: (+ 351)217-500-088 E-mail: pmvaz@fc.ul.pt [b] M. M. Nolasco, Dr. P. J. A. Ribeiro-Claro Department of Chemistry, CICECO University of Aveiro Campus de Santiago, 3810–193 Aveiro (Portugal) Supporting information for this article is available on the WWW under http://www.chemeurj.org/ or from the author. # 2007 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim Chem. Eur. J. 2007, 13, 7874–7882 7874