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Structural studies of supramolecular photochemical beta-cyclodextrin inclusion complexes

Thomas John Brett, University of Nebraska - Lincoln

Abstract

X-ray crystallography has played an essential role in our understanding of the factors controlling the outcomes of solid-state photochemical reactions. The detailed and systematic study of supramolecular photochemical systems is not very common. The dissertation research described here was designed to help fill this deficit. ^ β-cyclodextrin (β-CD) is an example of a host molecule which has been used as a host to photochemical reactions. An important influence on the outcome of the solid state reaction is the surrounding crystalline environment. Structural studies of β-CD inclusion complexes with derivatized biphenyl molecules, biphenyl and p-amino-p-nitrobiphenyl, characterize the β-CD dimer environment as non-constraining. Both molecules exhibit twisted conformations within the β-CD dimer, identical to their conformations displayed in the gas phase. ^ The photodimerization of various coumarins in crystalline β-CD complexes was studied in detail. The β-CD/coumarin complex was found to be a 2:3 host:guest (H:G) complex in contradiction to previous literature reports. The β-CD dimers stack in long channels with the coumarin molecules stacked one on top of another inside creating a reaction nano-tube in which the theoretical yield is limited to 67%. ^ The photodimerization of 7-hydroxy-4-methylcoumarin in its crystalline β-CD inclusion complex was directly observed by X-ray diffraction. Examination of the structure of an unreacted crystal and one that was irradiated for 6 days revealed that the reaction proceeds in a topochemical fashion within the β-CD dimer cavity. The β-CD dimers arrange in a manner which creates isolated reaction nano-vessels throughout the crystal. The structure of the β-CD/7-hydroxycoumarin complex shows that this complex is nearly identical the β-CD/7-hydroxy-4-methylcoumarin complex despite the looser spatial fit of the guest to the cavity. ^ The studies of the β-CD/4,7-dimethylcoumarin complex produced unexpected results. For one, the β-CD dimers pack in a fashion which has not been observed before in the literature. Also, the observed photoproduct cannot be produced without unexpected rearrangement of the guest molecules within the β-CD dimer cavity. The reaction outcome seems to be controlled by a preferential fit of the photoproduct to the reaction cavity. Examining a series of coumarin complexes, the guest influences on β-CD dimer packing arrangements are suggested. ^

Subject Area

Chemistry, Organic|Chemistry, Physical|Physics, Condensed Matter

Recommended Citation

Brett, Thomas John, "Structural studies of supramolecular photochemical beta-cyclodextrin inclusion complexes" (1999). ETD collection for University of Nebraska - Lincoln. AAI9942114.
http://digitalcommons.unl.edu/dissertations/AAI9942114

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