Azobenzene-Modified Betacyclodextrin: Molecules, Materials and Molecular Devices

Abstract

Cyclodextrins (CD) are naturally occurring macrocycles composed of alpha-1,4-linked D-glucopyranose units. CDs are common components in supramolecular chemistry owing to their ability to act as hosts to complex a wide variety of guests. Similarly, azobenzene has frequently been used in supramolecular chemistry as a photoswitchable moiety. We have explored the combined utility of CDs and azobenzene to explore supramolecular systems on the molecular, material and molecular device scale. Chapter 1 summarises CD and azobenzene chemistry. The applications of CDs are explored, including its use as components of polymeric hydrogels, mechanically interlocked molecules and molecular devices. Chapter 2 describes the attempted synthesis of three structural isomers of azobenzene-linked beta-CD dimers, in which the beta-CD substituent is attached to the para, meta or ortho position of both phenyl rings of azobenzene to give bis(6A-deoxy-beta-cyclodextrin-6A-yl)-4,4’-aminocarbonylazobenzene (p-beta-CD2az), bis(6A-deoxy-beta-cyclodextrin-6A-yl)-3,3’-aminocarbonylazobenzene (m-beta-CD2az) and bis(6A-deoxy-beta-cyclodextrin-6A-yl)-2,2’-aminocarbonylazobenzene (o-beta-CD2az), respectively. The aim of this research was to determine the effect of structural isomerisation on the photochemical properties of beta-CD dimers. The synthesis of p-beta-CD2az and m-beta-CD2az was successful, while the synthesis of o-beta-CD2az did not yield the desired compound. Instead, (6A-deoxy-beta-cyclodextrin-6A-yl)-2-aminocarbonyl-aminobenzene (beta-CDab) was produced. The photoisomerisation properties of p-beta-CD2az and m-beta-CD2az were examined by NMR and UV-vis spectroscopy. Chapter 3 investigated the host-guest complexation properties of beta-CD, beta-CDab and E-p-beta-CD2az. Three dyes were chosen as guests: cationic crystal violet (CV+), zwitterionic rhodamine B (RB) and anionic ethyl orange (EO-). The host-guest complexes were qualitatively studied by NMR spectroscopy. The complexation constants and thermodynamic parameters were determined by UV-vis spectroscopy. Chapter 4 explored the effect of guest modification on the host-guest complexation properties of beta-CD and beta-CDab. Four porphyrins were chosen as guests: meso-tetra(4-sulfonatophenyl)porphine (TSPP), meso-tetra(4-carboxyphenyl)porphine (TCPP), meso-tetra(4-N,N,N-trimethylanilinium)porphine (TMAP) and meso-tetra(N-methyl-4-pyridyl)porphine (TMPyP). The host-guest complexation properties were studied by NMR spectroscopy. The complexation constants and thermodynamic parameters were determined by UV-vis spectroscopy. Chapter 5 investigated the complexation between beta-CD, beta-CDab, E-p-beta-CD2az and E/Z-m-beta-CD2az and hydrophobe-substituted poly(acrylates) (PAA) in the formation of polymer hydrogels. The molecular-scale complexation properties were investigated to understand the factors that influence the bulk-material properties of the solutions. Three adamantane (AD) substituted PAAs were chosen for study, in which the length of the alkyl tether connecting the AD group to the PAA backbone was an ethyl, hexyl or dodecyl group (PAAADen, PAAADhn and PAAADddn, respectively). One dodecyl substituted PAA (PAAC12) was also chosen for study. The host-polymer complexation was qualitatively investigated by NMR spectroscopy. The complexation constants and thermodynamic parameters were investigated by isothermal titration calorimetry. The macroscopic properties of the host-polymer solutions were investigated by rheology. Chapter 6 describes the attempted synthesis of three azobenzene-modified beta-CD-based molecular muscles, differing by the choice of blocking groups as either adamantane, aza-18-crown-6 and 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA). The synthesis of an azobenzene-linked beta-CD compound was successful. However, the attachment of the blocking groups was not successful. Chapter 7 describes detailed procedures for experiments in Chapters 2 – 6. The research presented in this thesis hopes to extend our understanding of azobenzene-modified beta-CD compounds used on the molecular, material and molecular device scale, as summarised in Chapter 8.