Femtochemistry of Guest Molecules Hosted in Colloidal Zeolites (original) (raw)

Indirect Observation of Structured Incipient Zeolite Nanoparticles in Clear Precursor Solutions

Angewandte Chemie International Edition, 2008

The structure of the zeolite precursor particles present in clear synthesis solutions yielding colloidal silicalite-1 (purely siliceous zeolite with MFI-type structure) is one of the most controversial issues in modern zeolite science. Owing to the nature of the synthesis and the absence of Al, these systems are widely used as models to study the mechanism of zeolite crystallization in general. Ever since the "nanoslab" hypothesis was proposed by Martens et al. it has been the subject of a lot of criticism. The major disagreement is about the structure of the subcolloidal precursor particles present in the initial clear mixtures. Whereas Martens et al. suggest that these particles have MFI structural features, an amorphous nature of the particles has been proposed in numerous other studies. In some cases, the nanoparticles were extracted from the synthesis solutions, and concerns have been raised about possible changes in their structure or interference from the extraction procedure. Other authors base their arguments against the crystalline nature of the precursor particles on the results of solid-state NMR analysis after 29 Si enrichment. A recent NMR study on aqueous silicate solutions revealed the presence of structural units typical of the MFI structure. Other complementary techniques such as dynamic light scattering have also been used to further characterize the species present in the precursor solu-tions. Thus, despite the fact that a remarkable number of papers discussing the structure of the silicalite-1 precursor particles have already been published, the discussion is still ongoing and new publications regularly appear.

Optical spectroscopy of inorganic–organic host–guest nanocrystals organized as oriented monolayers

Inorganica Chimica Acta, 2007

Oriented and densely packed zeolite L monolayers were prepared on a glass support. The one-dimensional channels of zeolite L, being all oriented perpendicular to the glass and parallel to each other, were sequentially filled by ion exchange with two strongly fluorescent dye molecules. First N-methylacridine (MeAcr + ) was inserted followed by 3,3 0 -diethylthiacarbocyanine (DTC + ). The shorter MeAcr + is oriented perpendicular to the channel axis while the longer DTC + is parallel, due to the constraints imposed by the geometry of the zeolite L channels, as deduced from fluorescence anisotropy of single MeAcr + -zeolite L and DTC + -zeolite L crystals. The dye molecules can enter the channels only from the top side of the monolayer, since the entrances on the bottom are blocked by the glass support. The resulting ordering has been observed by fluorescence microscopy of single DTC + , MeAcr + -zeolite L crystals. Conditions were found to suppress the pronounced Rayleigh scattering of zeolite monolayers. Thus high quality absorption spectra of DTC + , MeAcr + -zeolite L monolayers on glass could be measured at different angles between the incident light and the layer. The results deliver a direct proof that microscopic ordering of the dyes in the channels of zeolite L as well as macroscopic organization of the dye-zeolite L monolayer on the glass support was achieved. Thus a high level of organization was obtained by controlled assembly of the zeolite L crystals into oriented structures followed by subsequent insertion of strongly luminescent dyes.

Molecular sieves as host materials for supramolecular organization

Microporous and Mesoporous Materials, 2004

The use of zeolites and mesoporous silicas (M41S family) as host materials for the supramolecular organization of organic dye molecules, metal sulfide clusters, and transition metal complexes is reviewed. For dye-zeolite systems, different stages of organization are discussed, ranging from the arrangement of the dyes in the zeolite channels to the specific adsorption of molecules at the channel entrances and finally to the coupling of the dye-zeolite crystals to an external device. The synthesis of metal sulfide nanoparticles in zeolites is illustrated by our studies of silver sulfide clusters in zeolite A. Starting from an activated Ag þ -loaded zeolite, the synthesis exploits the defined structure of the zeolite framework to achieve controlled cluster growth yielding composites with unique luminescence properties. The organization of guest species in mesoporous silicas of the M41S family is mostly related to the chemistry of the surface silanol groups. This aspect is discussed with particular emphasis on the functionalization with organic chromophores and the grafting of isolated transition metal centers through oxo-bridges.

Directing Zeolite Structures into Hierarchically Nanoporous Architectures

Science, 2011

Crystalline mesoporous molecular sieves have long been sought as solid acid catalysts for organic reactions involving large molecules. We synthesized a series of mesoporous molecular sieves that possess crystalline microporous walls with zeolitelike frameworks, extending the application of zeolites to the mesoporous range of 2 to 50 nanometers. Hexagonally ordered or disordered mesopores are generated by surfactant aggregates, whereas multiple cationic moieties in the surfactant head groups direct the crystallization of microporous aluminosilicate frameworks. The wall thicknesses, framework topologies, and mesopore sizes can be controlled with different surfactants. The molecular sieves are highly active as catalysts for various acid-catalyzed reactions of bulky molecular substrates, compared with conventional zeolites and ordered mesoporous amorphous materials.

Playing with dye molecules at the inner and outer surface of zeolite L

Solid State Sciences, 2000

Plants are masters of transforming sunlight into chemical energy. In the ingenious antenna system of the leaf, the energy of the sunlight is transported by chlorophyll molecules for the purpose of energy transformation. We have succeeded in reproducing a similar light transport in an artificial system on a nano scale. In this artificial system, zeolite L cylinders adopt the antenna function. The light transport is made possible by specifically organized dye molecules, which mimic the natural function of chlorophyll. Zeolites are crystalline materials with different cavity structures. Some of them occur in nature as a component of the soil. We are using zeolite L crystals of cylindrical morphology which consist of a continuous one-dimensional tube system and we have succeeded in filling each individual tube with chains of joined but noninteracting dye molecules. Light shining on the cylinder is first absorbed and the energy is then transported by the dye molecules inside the tubes to the cylinder ends. We expect that our system can contribute to a better understanding of the important light harvesting process which plants use for the photochemical transformation and storage of solar energy. We have synthesized nanocrystalline zeolite L cylinders ranging in length from 300 to 3000 nm. A cylinder of 800 nm diameter, e.g. consists of about 150 000 parallel tubes. Single red emitting dye molecules (oxonine) were put at each end of the tubes filled with a green emitting dye (pyronine). This arrangement made the experimental proof of efficient light transport possible. Light of appropriate wavelength shining on the cylinder is only absorbed by the pyronine and the energy moves along these molecules until it reaches the oxonine. The oxonine absorbs the energy by a radiationless energy transfer process, but it is not able to send it back to the pyronine. Instead it emits the energy in the form of red light. The artificial light harvesting system makes it possible to realize a device in which different dye molecules inside the tubes are arranged in such a way that the whole visible spectrum can be used by conducting light from blue to green to red without significant loss. Such a material could conceivably be used in a dye laser of extremely small size. The light harvesting nanocrystals are also investigated as probes in near-field microscopy, as materials for new imaging techniques and as luminescent probes in biological systems. The extremely fast energy migration, the pronounced anisotropy, the geometrical constraints and the high concentration of monomers which can be realized, have great potential in leading to new photophysical phenomena. Attempts are being made to use the efficient zeolite-based light harvesting system for the development of a new type of thin-layer solar cell in which the absorption of light and the creation of an electron-hole pair are spatially separated as in the natural antenna system of green plants. Synthesis, characterization and applications of an artificial antenna for light harvesting within a certain volume and transport of the electronic excitation energy to a specific place of molecular dimension has been the target of research in many laboratories in which different approaches have been followed. To our knowledge, the system developed by us is the first artificial antenna which works well enough to deserve this name. Many other highly organized dye-zeolite materials of this type can be prepared by similar methods and are expected to show a wide variety of remarkable properties. The largely improved chemical and photochemical stability of dye molecules inserted : S 1 2 9 3 -2 5 5 8 ( 0 0 ) 0 0 1 2 9 -1 G. Calzaferri et al. / Solid State Sciences 2 (2000) 421-447 422 in an appropriate zeolite framework allows us to work with dyes which otherwise would be considered uninteresting because of their lack of stability. We have developed two methods for preparing well-defined dye -zeolite materials, one of them working at the solid-liquid and the other at the solid -gas interface. Different approaches for preparing similar materials are in situ synthesis (ship in a bottle) or different types of crystallization inclusion synthesis.