Supramolecular Daisy Chains Based On Cyclodextrins

Abstract

The work in this thesis describes the development of a range of supramolecular daisy chains [cn]-rotaxanes based on substituted cyclodextrins (CDs).

The background of supramolecular chemistry and CDs is introduced in Chapter 1. A brief summary of recent progress and development of supramolecular daisy chains based on CDs is presented with relevant examples from the literature. To investigate the effects of different substituted positions of CD on the formation of supramolecular daisy chains, an amide-linked C6-cinnamoyl-a-CD and its C3-substituted analogue were prepared in Chapter 2. Isothermal titration calorimetry (ITC) and 2D NMR spectroscopy were used to study their self assembly behaviors in aqueous solution and the results showed that inclusion complexes other than monomers had formed. Then, to mechanically interlock the complexes formed, a one-pot capping reaction with TNBS was conducted. The amide-linked C6-substituted-a-CD formed [c3]-rotaxane, whilst the C3-position substituted CD mainly formed a [c2]-complex in aqueous solution instead of linear supramolecular complexes. It suggests that C6-substituation of CD is more advantageous for the formation of longer supramolecular daisy chains.

In the continued pursuit of preparing a larger supramolecular daisy chain, an ester-linked C6-azobenzene-substituted-permethylated-a-CD was studied in Chapter 3. In particular, the ITC experiment showed that more than one self-assembly process occurred during the titration and in any event, the titration curve did not fit a dimer dissociation model. Next, the complexes formed were mechanically interlocked by a one-pot capping reaction with 2-naphthol. Apart from an [a1]-species and a [c5]-rotaxane, an [a2]-rotaxane, a trimer rotaxane and a tetramer rotaxane were also obtained from the crude reaction mixture. These results suggest that the azobenzene moiety and/or the O-permethylateda-CD are important to the formation of a larger supramolecular daisy chain.

Therefore, to study the effects of O-permethylation of a-CD on the formation of supramolecular daisy chains, a non-methylated-a-CD with an azobenzene substituent was designed and synthesized in Chapter 4. Both ITC experiments and the isolation of a [c2]-rotaxane from a one-pot capping reaction of the non-methylated-a-CD with 2-naphthol showed that only a [c2]-complex formed in aqueous solution. For comparison, an amide-linked C6-azobenzene-substituted-permethylated-a-CD was also designed and synthesized in Chapter 4. ITC experiments indicated that more than one self-assembly process happened during the titration and in any event, the titration curve did not correspond to a dimer dissociation model. Moreover, four compounds including an [a1]-species, a [c3]-rotaxane, a [c5]-rotaxane and an [a2]-rotaxane were isolated from a onepot capping reaction of amide-linked C6-azobenzene-substituted-permethylated-a-CD with 2-naphthol. All compounds were characterized by MALDI-TOF mass spectrometry and NMR spectroscopy. These results demonstrate that O-permethylation of a-CD is decisive in the formation of a larger supramolecular daisy chain.

The conclusion and future work are presented in Chapter 5, and the experimental methods of the work described in this thesis are in Chapter 6. Show more