Wessig, Martin
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Localization of Guest Molecules in Nanopores by Pulsed EPR Spectroscopy
The localization of guest molecules at the molecular scale in mesoporous host materials is crucial for applications in heterogeneous catalysis, chromatography, drug delivery, and in different biomedical applications. Here, we present for the first time the precise localization of different guest molecules inside the mesoporous organosilica material UKON2a with a pore size of 6 nm. We exploited paramagnetic probe molecules 4-amino-2,2,6,6-tetramethylpiperidine-1-oxyl (4-amino-TEMPO) and 4-carboxy-TEMPO in combination with a deuteration strategy. Applying a complementary set of different pulsed electron paramagnetic resonance methods, we obtained information about the dimensionality of the spatial distribution and local concentration via double electron–electron resonance experiments, orientation of the guest molecules with respect to the pore walls via electron spin echo envelope modulation spectroscopy, and about the distance between guest molecules and pore walls via electron nuclear double resonance spectroscopy. This allowed localizing the guest molecules and shows that their spatial distribution in nanopores strongly depends on their polarity.
Directional Materials : Nanoporous Organosilica Monoliths with Multiple Gradients Prepared Using Click Chemistry
The existence of more than one functional entity is fundamental for materials, which are desired of fulfilling complementary or succeeding tasks. Whereas it is feasible to make materials with a homogeneous distribution of two different, functional groups, cases are extremely rare exhibiting a smooth transition from one property to the next along a defined distance. We present a new approach leading to high-surface area solids with functional gradients at the microstructural level. Periodically ordered mesoporous organosilicas (PMOs) and aerogel-like monolithic bodies with a maximum density of azide groups were prepared from a novel sol-gel precursor. The controlled and fast conversion of the azide into numerous functions by click chemistry is the prerequisite for the implementation of manifold gradient profiles. Herein we discuss materials with chemical, optical and structural gradients, which are interesting for all applications requiring directionality, for example, chromatography.
Intelligente anorganische Tenside : mehr als Oberflächenspannung
Tenside mit anorganischen Kopfgruppen, bestehend aus einem rutheniumsubstituierten Polyoxowolframatcluster, demonstrieren das außergewöhnliche Potenzial einer synergistischen Kombination von Selbstorganisationseigenschaften und stimuliresponsivem Verhalten. Das System antwortet dynamisch auf chemische Reize (etwa durch eine Änderung der Micellform; siehe Schema) und könnte neue Anwendungen für Tenside im Bereich der Lichtsammlung erschließen.
Negative and Positive Confinement Effects in Chiral Separation Chromatography Monitored with Molecular-Scale Precision by In-Situ Electron Paramagnetic Resonance Techniques
Separation of compounds using liquid chromatography is a process of enormous technological importance. This is true in particular for chiral substances, when one enantiomer has the desired set of properties and the other one may be harmful. The degree of development in liquid chromatography is extremely high, but still there is a lack in understanding based on experimental data how selectivity works on a molecular level directly at the surfaces of a porous host material. We have prepared amino-acid containing organosilica as such host materials. Watching the rotational dynamics of chiral spin probes using electron paramagnetic resonance spectroscopy allows us to differentiate between surface adsorbed and free guest species. Diastereotopic selectivity factors were determined, and the influence of chiral surface group density, chemical character of the surface groups, pore-size, and temperature was investigated. We found higher selectivity values in macroporous solids with a rather rigid organosilica network and at lower temperature, indicating the significant effect of confinement effects. In mesoporous materials features are opposed with regards to the T-dependent behavior. From EPR imaging techniques and the resulting (macroscopic) diffusion coefficients, we could confirm that the correlations found on the microscopic level transform also to the macroscopic behavior. Thus, our study is of value for the development of future chromatography materials by design.
Simultaneous Monitoring of Macroscopic and Microscopic Diffusion of Guest Molecules in Silica and Organosilica Aerogels by Spatially and Time-Resolved Electron Paramagnetic Resonance Spectroscopy
We used spatially and time-resolved electron paramagnetic resonance (EPR) spectroscopy to study diffusion of guest molecules within solvent filled aerogel monoliths. We experimentally obtained the time-dependent spin density of EPR active guest molecules ρ1d(y,t), numerically solved the diffusion equation to simulate ρ1d(y,t), and determined the macroscopic translational diffusion coefficients for different aerogels and guest molecules. Simultaneously, we determined the microscopic diffusion coefficient by spectral simulation. We show that diffusion in the aerogels under study is dominated by the tortuosity of the pore system but not by surface effects.
Nanoarchitecture Effects on Persistent Room Temperature Photoconductivity and Thermal Conductivity in Ceramic Semiconductors: Mesoporous, Yolk–Shell, and Hollow ZnO Spheres
A gas-phase approach is applied to synthesize a set of spherical particles with mesoporous, yolk−shell, or hollow character. A special arrangement of the ZnO lattice results in a polar character of the particle shell, and this facilitates effective separation of electrons and holes on different sides of the interface.
Magneto-Adaptive Surfactants Showing Anti-Curie Behavior and Tunable Surface Tension as Porogens for Mesoporous Particles with 12-Fold Symmetry
Gaining external control over self-organization is of vital importance for future smart materials. Surfactants are extremely valuable for the synthesis of diverse nanomaterials. Their self-assembly is dictated by microphase separation, the hydrophobic effect, and head-group repulsion. It is desirable to supplement surfactants with an added mode of long-range and directional interaction. Magnetic forces are ideal, as they are not shielded in water. We report on surfactants with heads containing tightly bound transition-metal centers. The magnetic moment of the head was varied systematically while keeping shape and charge constant. Changes in the magnetic moment of the head led to notable differences in surface tension, aggregate size, and contact angle, which could also be altered by an external magnetic field. The most astonishing result was that the use of magnetic surfactants as structure-directing agents enabled the formation of porous solids with 12-fold rotational symmetry.
Multiple scale investigation of molecular diffusion inside functionalized porous hosts using a combination of magnetic resonance methods
Mass transport of molecular compounds through porous solids is a decisive step in numerous, important applications like chromatography or heterogeneous catalysis. It is a multi-scale, hierarchical phenomenon: macrodiffusion (>μm) is influenced, in addition to parameters like grain boundaries and particle packing, by meso-scale (>10 nm, <μm) factors like particle size and the connectivity of pores. More importantly, meso-scale diffusion and macro-scale diffusion are first and foremost determined directly by processes on the molecular scale (<10 nm), which depend on numerous factors like pore-size, interactions of the host with the solid surfaces and with the solvent. Due to the high complexity of the latter and the fact that current analytical techniques enable only limited insights into solvent-filled pores with sufficient spatial and temporal resolution, the knowledge about the molecular origins of diffusive processes in porous materials is still restricted. The main focus of the current paper is on the development of continuous wave (CW) electron paramagnetic resonance (EPR) spectroscopy into a tool shedding some new light on molecular diffusion inside mesoporous silica materials differing systematically in pore size and surface functionalities. The advantages of CW-EPR are that its spatial resolution fits ideally to the size of mesopores (2–10 nm), it is fast enough for spotting molecular processes, and any conventional solvent and the porous matrix are EPR silent. Diffusion coefficients have been calculated considering spin exchange occurring from the diffusive collision of radicals, and are compared to complementary analytical techniques like MAS PFG NMR (sensitive for meso-scale) and EPR-imaging (sensitive to macroscale diffusion). Our results show that the choice of surface bound functional groups influences diffusion much stronger than pore-size. There are indications that this is not only due to different guest–surface interactions but also due to an altered mobility within the solvent under confinement.
Probing Functional Group Specific Surface Interactions in Porous Solids Using ESR Spectroscopy as a Sensitive and Quantitative Tool
The interactions between molecular compounds and functionalized surfaces are omnipresent in many areas. For example, they are of crucial importance in all chromatographic processes using porous hosts. Developing a thorough understanding about the complex interplay of molecular scale processes involved in surface interactions is an issue of current scientific activity. In the present paper we discuss ESR spectroscopy as a powerful and easy to apply tool for examining host–guest effects within mesoporous materials. The method is tested for a series of modified, mesoporous materials comprising different surface functionalities like amine groups, carboxy groups, and others. There is a characteristic influence on the rotational characteristics of a molecular nitroxide spin probe. From temperature dependent data one can derive key thermodynamic parameters like interaction enthalpies. It is possible to differentiate (qualitatively and quantitatively) between mobile spin probes interacting mainly with the solvent and immobile guests interacting mainly with the surfaces. The polarity difference Δpv between solvent and pore-surface determines if molecular guests are in a free and highly mobile, interacting but still mobile, or interacting and immobilized state. It is interesting to note that immobilization takes place even for moderate Δpv values no matter if the surface is polar or unpolar. However, the behavior changes if there is a chemically specific interaction between the functional groups of the guests with a functionalized surface. Based on the systematic effect of the pore size on the rotational dynamics of the spin probe a qualitative model is proposed that includes a characteristic distance between confined guest species and pore surface as a function of the interaction strength.