From Cyclodextrin Assemblies to Porous Materials by Silica Templating We thank the Max-Planck society for funding.

Since the discovery by Beck, Vartuli et al.[1, 2] that ordered mesoporous silicas, such as MCM-41, can be obtained by surfactant templating, the development of new porous materials and porogens as well as the characterization of the resulting structures has become one of the most promising fields in modern materials chemistry.[3±5] Meanwhile, these observations have been extended to a number of techniques by which pore morphology, pore size, and wall material can be varied over a broad range.[6±14] One of the modifications of the original process relevant here is the TMnanocasting∫ introduced by Attard, Glyde, and Gˆltner.[15] This is a high-concentration synthesis in which the solidified inorganic compound is a copy of the original phase structure, that is its structure is predetermined by the selection of the template phase. They used lyotropic liquid-crystalline phases, derived from surfactants or–later–block copolymers,[15±17] as templates and obtained the material either as a homogeneous film or as a monolithic object. In addition, this technique also allows the structural characterization of fragile phase structures and transition states of polymers or surfactants by analyzing the well-contrasted solid replica instead of the original soft organic phases.[16, 18] We extend this approach to supramolecular aggregates of nonpolar cyclodextrins (CDs) in aqueous solution as templates. Our aim is to exploit the use of CDs to generate porous materials, focussing on the fine details of the pore structure and the self-organization of this template. The self-assembly of cyclodextrins has been assumed previously,[19±23] but their organization could–to the best of our knowledge–not be determined in detail. CDs are cyclic oligosaccharides consisting of covalently linked glucose units (6 units -CD, 7 units -CD, 8-units -CD); they are characterized by a hydrophilic exterior and a hydrophobic interior[24] the size of which is in the range of 1.5 ± 2 nm. The relatively large number of hydroxy groups per unit area should ideally mediate compatibility with an oxidic inorganic matrix through hydrogen bonding. In fact, this has been proven by three reports in which CDs were employed at low concentrations in a sol ± gel process to incorporate functional units such as dyes into a dense and nonporous silica.[25±27] So far, CD-containing solutions have been difficult to analyze by standard techniques, thus information about the assembly of CDs in water is still rather limited. We employ nanocasting to investigate the CD assemblies as templates for the silica pore in detail, and we also investigate the driving force for the self-assembly of the cyclodextrin molecules to ordered aggregates. In a solution of a hydrophilic sugar, such as a cyclodextrin, one would assume that it is moleculary dispersed in the aqueous phase. Nanocasting by a sol ± gel process should then result in a porous material in which each pore exhibits the size and shape of a CD molecule, and in which the pores are not mutually ordered. Depending on the concentration, mesoporosity would be achieved by a molecular percolation process at corresponding volume fractions of template to water, but still a disordered pore system would be obtained (as found for instance for polyethyleneglycol solutions[28]). When a 18 wt% solution of -CD was templated we observed a different behavior, namely evidence for an ordered aggregation of CDs instead of their molecular dispersion in the silica matrix (see experimental conditions in Table 1 and Supporting Information; all unmodified CDs