Cyclodextrin-based rotaxanes for material science applications

Abstract

In the work described in this thesis, CD-based rotaxanes were prepared and their solid-state structures/packing, morphology and crystal growth behaviour were explored. Encapsulation of profluorescent nitroxides through rotaxane formation demonstrated the use of CD-based rotaxanes to change the reactivity of CD guests and therefore preserve their properties. In Chapter 2, the rotaxane 1.17 having a stilbene-based axle and 3,5-dimethylaniline blocking groups, as well as the rotaxane 1.18 having an azobenzene-based axle and 2,6-dimethylaniline blocking groups were prepared to complete the set of rotaxanes 1.9, 1.10, 1.17 and 1.18 with 2,6-dimethylaniline and 3,5-dimethylaniline blocking groups, with azobenzene- and stilbene-based axles. Chapter 3 describes the solid-state structures/packing, morphology and crystal growth of the set of rotaxanes 1.9, 1.10, 1.17 and 1.18. Observation of crystal morphology using SEM and microscope images, suggests not only that crystal shape and size can be controlled depending on the choice of blocking groups and varying the conditions of crystal growth, but also that crystal packing is reflected in the crystal growth and the final shape of the crystals. Thus, crystal engineering can be bottomed-up to make nano-molecular architecture to micrometre scale or beyond. In Chapter 4, the unsymmetrically capped rotaxanes 4.1 and 4.2 were purposely designed and prepared using a crystal engineering approach such that they can be used to regulate the crystal growth of the self-assembling rotaxane 1.9 through co-crystallisation. 2D NMR spectroscopy was used to determine the orientation of the CDs with respect to the dumbbells and assign the structures of the rotaxane isomers 4.1 and 4.2. Co-crystallisation experiments showed that the rotaxanes 4.1 and 4.2 act as crystal growth inhibitors of the rotaxane 1.9. In addition, a method to prepare supersaturated solutions from a mixture of amorphous and crystalline rotaxane was developed for crystal growth studies. The unsymmetrically capped rotaxane 5.5 with 3,5-dimethylaniline and trinitrophenyl blocking groups on either end that can form pi-pi interactions was prepared in Chapter 5. Co-crystallisation experiments with the rotaxanes 1.9 and 5.5 illustrate that blocking groups of rotaxanes can be used as synthons to guide intermolecular self-assembly to regulate the formation of fibrous structures. Therefore crystal engineering using CD-based rotaxanes is a versatile approach towards regulating self-assembly and understanding molecular level interactions in the solid-state. These interactions may be used to fine-tune physical, chemical, photochemical or electrical properties of the end product that is fabricated through a bottom-up approach. New paramagnetic profluorescent nitroxides, the rotaxanes 1.33, 1.35 and 1.36, as well as the previously reported rotaxane 1.34 were prepared and their photoisomerisation and decomposition behaviour in comparison to the corresponding dumbbells 1.37 and 1.38 were examined in Chapters 6 and 7. UV/visible spectroscopy and HPLC analysis illustrated that the profluorescent probes of the dumbbells 1.37 and 1.38 were preserved through formation of the CD-rotaxanes 1.33, 1.34, 1.35 and 1.36 making them advantageous for fluorescence detection. Show more