Patel, Arjun
(2021)
Pillar[5]arene based molecular machines.
PhD thesis, University of Nottingham.
Abstract
Scientists have been trying to shrink large scale technologies into smaller, molecular scales. One such pursuit is that of molecular machines. By utilising a mechanical bond, molecules can be made to perform a task. Examples such as molecular shuttles, motors and switches have been produced and are usually composed of mechanically interlocked molecules (MIMs) such as rotaxanes and catenanes. Due to the lack of chemical bonds between the components, these molecules possess translational and rotational motions with respect to one another, a key aspect as to why MIMs are ideal candidates for molecular machines. A relatively new class of macrocycle, pillararenes have been of interest for the past decade in the construction of mechanically interlocked molecules. This thesis aims to bridge the gap between pillararene based MIMs and their corresponding machine-like behaviour.
On the nanoscale, molecules are constantly in motion and inputting energy does not necessarily equate to fuel being consumed. From the impact of Brownian motion on each individual molecule, controlling the relative positions of components on a MIM is crucial in order for a molecule to perform a task. A series of [3]rotaxanes, when two rings are locked onto a single axle, were synthesised for this purpose using pillar[5]arene as macrocycles through their interactions with imidazolium moieties. Shuttling studies have been conducted on this series and their shuttling motions have been identified in polar and non-polar solvents. Non-polar solvents allow the pillar[5]arene-imidazolium interactions to be enhanced, where pillar[5]arene are found to be situated over these favourable stations on average. Polar solvents disrupt these favourable interactions and the macrocycles are forced into close proximity to each other.
This has been taken a step further with regards to controlling the ring’s motion across an axle. A series of [2]rotaxane analogues, when only one ring is locked onto an axle, of the [3]rotaxane series which uses the same pillar[5]arene macrocycles have also been synthesised and their shuttling motions analysed. By freeing up space along the axle, a larger proportion of the track is accessible to the macrocycle. This however, it not necessarily the case when a steric barrier is introduced into the centre of the axle, meaning the macrocycle has to be pushed energetically uphill to travel along this track. When in non-polar solvents, such as chloroform, the macrocycle is locked onto one side of the axle via the steric barrier. When in polar solvents such as DMSO, the macrocycle is forced closer to the steric barrier via the disruption of the pillar[5]arene-imidazolium interactions and is allowed to hop over the steric barrier without any additional fuel.
A threading and stoppering approach has been taken to assemble the rotaxanes reported in this thesis. By doing so, the number of components required to synthesise a rotaxane is three: a pillar[5]arene macrocycle, an axle and a stopper group. By swapping out components, a toolkit can be adopted to fabricate rotaxanes in order to be photopolymerised. A series of shorter, anthracene stoppered [2]rotaxanes have additionally been synthesised utilising the same pillar[5]arene-imidazole interactions to assemble. The terminal anthracene moieties possess the ability to undergo a [4 + 4] cycloaddition via photoirradiation and thus a polyrotaxanation of the [2]rotaxane species was attempted. In addition, the shuttling ability of the macrocycle was evaluated electrochemically across the series of [2]rotaxanes and the fluorescence of the products probed.
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