Background. Aromaticity has been a fundamental principle of chemical bonding for over a century. It accounts for the unusually high stability of molecules such as benzene and its derivatives, which are found in all manner of natural and synthetic materials. We investigate tropylium compounds, which, like benzene, are Hückel aromatic. Their distinguishing features include (i) their permanent positive charge and (ii) the seven carbon atoms in their heptagonal aromatic rings.
We have developed methods (e.g., McGonigal et al., Angew. Chem. Int. Ed. 2022, 61, e202202193) to modify the structures of tropylium rings, allowing us to tune their steric overcrowding by introducing large groups around their periphery. Rather unexpectedly, we recently discovered that as the size of these groups around the tropylium ring is increased, the steric strain reaches a point where the aromaticity is ruptured! (McGonigal et al. Nature Chemistry, 2023, accepted preprint DOI:10.21203/rs.3.rs-1347057/v1). The tropylium ring ‘breaks’ to form a bicyclic ‘Dewar tropylium’ structure. By choosing appropriate groups, we can balance the steric strain and the aromatic stabilisation energy with one another, placing the aromatic tropylium and the Dewar tropylium in rapid exchange, i.e., creating an unprecedented aromatic-to-nonaromatic cyclisation equilibrium.
Objectives. Based on these preliminary results, this PhD project will answer two key open questions (below) about breaking aromaticity under strain. The project is ideal for a student who is interested in obtaining an all-rounder skillset in performing organic synthesis, calculations, and spectroscopic measurements. We ask:
●Can strained aromatic rings be ‘broken’ and ‘reformed’ on demand? In other words, can we apply a stimulus to switch the between the aromatic and nonaromatic structures?
●Are other aromatic systems (beyond tropylium) susceptible to strain-induced rupture?
Experimental Approach. The general methodology will be to (1) prepare strained tropyliums and other aromatic molecules by chemical synthesis, (2) apply stimuli (i.e., light irradiation or chemical reagents), and (3) use spectroscopic analyses to elucidate the resulting structural changes. The investigations will follow two parallel paths, investigating two different stimuli of interest. Knowledge gained about switching the structures of tropyliums will then be applied to impart similar structural changes to other aromatic ring systems, such as five- and six-membered heterocycles.
Training. The student will obtain key skills in synthetic organic chemistry, including air-sensitive manipulations and purification by column chromatography. They will become adept at compound characterisation using advanced NMR spectroscopy techniques, as well as X‑ray crystallography and optical spectroscopy. There will also be the opportunity for an interested student to learn to perform DFT modelling of their compounds and to train in time-resolved optical measurements.
All Chemistry research students have access to our innovative Doctoral Training in Chemistry (iDTC): cohort-based training to support the development of scientific, transferable and employability skills: https://www.york.ac.uk/chemistry/postgraduate/training/idtc/
The Department of Chemistry holds an Athena SWAN Gold Award and is committed to supporting equality and diversity for all staff and students. The Department strives to provide a working environment which allows all staff and students to contribute fully, to flourish, and to excel: https://www.york.ac.uk/chemistry/ed/
For more information about the project, click on the supervisor's name above to email the supervisor. For more information about the application process or funding, please click on email institution
To apply for this project, submit an online PhD in Chemistry application: https://www.york.ac.uk/study/postgraduate/courses/apply?course=DRPCHESCHE3
You should hold or expect to achieve the equivalent of at least a UK upper second class degree in Chemistry or a related subject. Please check the entry requirements for your country: https://www.york.ac.uk/study/international/your-country/
Continue with Facebook
