The influence of supramolecular directors bound to surfaces of porous hosts with chiral walls on the dynamic of enantiomers as guests

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An important application of porous solids is for separation of (similar) chemical compounds by chromatography. Enantiomers represent the case of ultimate similarity between molecules and, thus, purification is in general demanding. Chromatographic separation also relies on statistics, a high number of contacts of the dissolved guest species with the surfaces via diffusion ensures, some events occur in proper orientation to induce stereochemical differentiation in intermolecular interaction. Maximum mobility discrimination of the enantiomers to be separated with a minimum amount of a chiral selector necessary at the surfaces of the solid phase is desired. For the identification of new concepts in stereoselective chromatography, it is helpful referring to a different area, asymmetric catalysis. There, the sole presence of a chiral ligand is not enough for achieving best enantiomeric excess. A precise control over the orientation of the starting compounds towards the active center is pivotal. The transfer of the latter concept to host-guest chemistry in porous materials leads to our long-term vision: The design of surfaces to become capable of orienting chiral guests for maximizing the effect on their mobility and reaching optimum enantioselective separation. For the target-oriented synthesis of such a complex host material, one has to be able to watch separation with molecular precision and quantify the molecular dynamics and mobility of guests confined to the pores of the material close to chromatographic process conditions. Our group has shown in preliminary work, this is possible using electron spin resonance spectroscopy (ESR) techniques. The methodology is now developed sufficiently, it can be applied to gather detailed information about separation using chiral, amino-acid modified organosilica materials as model systems. Our first work-package is concerned with gaining control over confinement conditions. We present new templating approaches for the synthesis of organosilica materials with narrow size distribution of pores in the 50-150 nm range. A systematic variation of the interaction of chiral nitroxides as paramagnetic spin-probes with the surfaces is realized in the second work-package. A particularly interesting question is, how non-chiral neighboring groups on the surface can influence the separation process and ultimately act as supramolecular directors for orienting the nitroxide guests in a specific way. Our main observable is the enantiomer selectivity factor alphaT obtained from cw-ESR experiments, and also coefficients for nanoscopic and macroscopic transport will be determined, for instance from imaging ESR spectroscopy. Finally, we want to perform a HPLC separation experiment using a monolithic organosilica material, to learn how the fundamental host-guest study translate to application.