Tarnowska, Marysia
(2023)
The supramolecular chemistry of handcuff rotaxanes and
deteriorating leather.
PhD thesis, University of Nottingham.
Abstract
Supramolecular chemistry deals with systems made of distinct chemical species which, when put together, have different properties than each individual component. These species are often not bonded in a covalent or ionic sense, but are based on host-guest chemistry. Mechanically interlocked molecules (MIMs) and handcuff rotaxanes in particular, which are systems containing multiple covalently bonded macrocycles, through which a rod is threaded then stoppered, are an example of such species. MIMs have wide ranging applications in nanoscale electronics, systems which mimic biology, and many others. Since its recent discovery, pillar[5]arene has been used extensively in MIMs due to its interesting host guest properties, and this work deals with handcuff rotaxanes containing it, as well as perylene diimide (PDI), tetrazine, and imidazole groups.
The feasibility of the synthesis of a three-membered PDI cofacial stack or a ‘triple decker handcuff rotaxane’, using a templated, clipping method, utilising interactions between PDI cores and between pillar[5]arenes and imidazole groups, is investigated. The difficulty in this lies in the synthesis of a ‘linker’ that can connect two pillar[5]arene macrocycles and also the PDI core of the macrocycle based component of the handcuff rotaxane. Various methods of amine alkylation, amide coupling and ether synthesis were explored to connect the functionalised pillar[5]arenes together, but due to the difficulties with pillar[5]arene syntheses and purifications they were all unsuccessful in either of those steps. However, novel pillar[5]arene-based species were successfully synthesised in this part of the project and fully or partially characterised.
Next, a simpler cofacial stack of a PDI and a tetrazine based on handcuff rotaxane topology was attempted but was unsuccessful due to the low solubility of pillar[5]arenebased species in solvents required for the tetrazine forming reaction. However, during this part of the project two new functionalised pillar[5]arenes were synthesised and characterised and attempts at optimising these syntheses were made.
Finally, a handcuff rotaxane containing only one PDI species was successfully synthesised and characterised. Its properties were probed using cyclic voltammetry, spectroelectrochemistry and electron paramagnetic resonance spectroscopy and compared with similar handcuff rotaxanes previously synthesised in the group. The lack of stacking species in this handcuff proved a previously postulated hypothesis that the synthesis method here described only requires templating between the host’s macrocycles and the threading guest. This does however, lower the yields relative to species with additional molecular recognition sites.
The thus far described part of this work explored and celebrated the organic synthesis of supramolecular structures, while the second part focuses on spectroscopic analysis of supramolecular systems. The systems in question are the collagen-tannin matrices found in leather, and more specifically in deteriorated leather book bindings from the Natural History Museum. As such the two parts of this thesis are only related due to dealing with supramolecular structures and should be interpreted separately.
This second project explored a leather acidic deterioration process known to conservators as red rot. Red rot affects leather book bindings from the late 19th and early 20th centuries and causes them to become dry and reddish, or in advanced stages crumble into a dark powder.
This project set out to explore health and safety aspects of red rot dust using various spectroscopic techniques and suggest further necessary tests. This was evaluated using Inductively Coupled Plasma analysis (Optical Emission Spectroscopy and Mass Spectrometry), in comparison with permissible heavy metal content in samples as defined by a commercial certification body, OEKO TEX. For the tested samples, it was found that arsenic, cadmium and lead were not within the permissible limits and hence caution when handling these items is recommended.
Furthermore, this work aimed to understand more about the red rot deterioration process by comparing undeteriorated and deteriorated leather samples with Fourier Transform Infrared spectroscopy and Scanning Electron Microscopy. No features present in red rot but absent in other types of deterioration were found, however, this work recommends gas chromatography as another technique that might achieve this.
Finally, this work attempts to quantify the degree of deterioration of bindings suffering from red rot through an FTIR peak ratios. Such a ratio is explored and found to relate to visually assessed levels of degradation. However it is not certain that this is a red rot specific ratio and not a general degradation ratio.
This work allowed for health and safety recommendations to be made to the museum, as well as for a deeper understanding of the red rot deterioration process to be obtained.
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