Molecular movement in confined spaces is of broad scientific and technological importance in areas ranging from molecular sieving and membrane separation to active transport through ion channels. Whereas measurements of ensemble diffusion provide information about the overall behaviour of the guest in a porous host, tracking individual molecules provides insight into both the heterogeneity and the mechanistic details of molecular diffusion as well as into the structure of the host. Here, we show how single dye molecules can be used as nanoscale probes to map out the structure of mesoporous silica channel systems prepared as thin films via cooperative self-assembly of surfactant molecules with polymerizable silicate species. The dye molecules act as beacons while they diffuse through the different structural phases of the host: the structure of the trajectories, the diffusivities and the orientation of single molecules are distinctive for molecules travelling in the lamellar and the hexagonal mesophases. These experiments reveal unprecedented details of the host structure, its domains and the accessibility as well as the connectivity of the channel system.

Single dye molecules incorporated into a mesoporous matrix can act as highly sensitive reporters of their environment. Here, we use single TDI molecules incorporated as guests into hexagonal mesoporous films containing highly structured domains. The dye molecules allow us to map the size of these domains which can extend to over 100 microm. Investigation of the translational and orientational dynamics via single molecule fluorescence techniques gives structural as well as dynamical information about the host material. In an air atmosphere, the guest molecules show no movement but perfect orientation along the pore direction. The diffusion of the TDI molecules can be induced by placing the mesoporous film in a saturated atmosphere of chloroform. In single molecule measurements with very high positioning accuracy (down to 2-3 nm) the movement of molecules could be observed even between neighboring channels. This reveals the presence of defects like dead ends closing the pores or small openings in the silica walls between neighboring channels, where molecules can change from one channel to the next. A statistical analysis demonstrates that the diffusion of TDI in the mesoporous film cannot be described with a 1D-random diffusion but is more complicated due to the presence of adsorption sites in which the TDI molecules can be occasionally trapped.

Periodic mesoporous materials have attracted considerable attention during the last decade because of their promising applications as catalyst supports and nanoreactors, or as hosts for nanostructured materials with appealing optoelectronic properties. [1,2] Many of these applications will benefit from arrangements of preferentially aligned, ordered arrays of certain mesostructures. The evaporation-induced self-assembly (EISA) method has been established as an efficient process for the preparation of thin films with mono-oriented mesostructured domains. [3,4] However, the most frequently obtained films display hexagonally ordered channels that are aligned parallel to the surface of the substrate. [5] Recently, the synthesis of mesoporous materials within the regular, larger channels of anodic alumina membranes (AAMs) has been explored, with the aim of attaining greater control over the morphology of the mesoporous system.[6] A first approach, through a sol-gel synthesis route using the triblock copolymer poly(ethylene oxide) 100 -b-poly(propylene oxide) 65 -b-poly(ethylene oxide) 100 (PEO 100 PPO 65 PEO 100 or Pluronic F-127) as a structure-directing agent, resulted in 2D hexagonal mesostructures with two different orientations that were found to coexist at different ratios depending on the concentration of the surfactant. [7] In one case, the long axes of the mesopores were aligned with the long axes of the AAM channels (columnar orientation). In another case, a circular orientation of the mesostructure was observed. Similar (freestanding) unusual mesophase structures are known to exist in cetyltrimethylammonium bromide (CTAB)-templated materials prepared by solvothermal methods and have been named "circulites" or circular crystals. [8,9] The efficient EISA method can also be used to prepare AAM mesoporous composite materials by applying coating solutions that are typically used for the deposition of mesoporous silica films. When using cationic CTAB as a template, partially ordered mesoporous materials with aligned, columnar mesopores only in the vicinity of the alumina walls were obtained that showed promising behavior as molecular separators.[10] Use of the triblock copolymer PEO 20 PPO 70 PEO 20 (Pluronic123 or P123) as a template resulted in striking mesostructures with concentric or helical mesopores and single chains of spherical mesopores, depending on the confinement conditions imposed by alumina nanochannels with diameters of less than 100 nm. [11] In contrast, columnar mesopores were reported when the same template (P123) was used in the sol-gel approach in larger Anopore channels.[12] However, when a slightly different protocol at the same surfactant/silica ratio was used in the solgel synthesis route, hexagonal mesophases with mixed orientations resulted.[13] The presence of water vapor in the ageing process was investigated in a related study using the P123 template. [14,15] In this case, the circular orientation was favored over the columnar one at higher water pressure; this selectivity was attrib...