Amphiphilic compounds self-organise so that the incompatible parts of the molecule are separated from one another (e.g. micelles); the exact same principle holds for liquid crystals (LCs). In particular, perfluorinated (PFC) and hydrocarbon (HC) chains tend to be incompatible, and the inclusion of both in the same molecule directs the LC phase organisation strongly. For example, in tetracatenar liquid crystals, functionalising one end of the extended molecular core with two HC chains and the other with two PFC chains caused a major change in the LC behaviour to accommodate the incompatibility.
More recently, we have prepared a discotic complex of gold(III) containing both HC and PFC chains, and have observed an unprecedented LC phase sequence, namely: Crys • Colr • N • Colh • Iso. Thus, the extremely disordered nematic phase (N) is found out of thermodynamic sequence. While this observation requires much deeper investigation, it seems clear that it is driven by amphiphilicity. Furthermore, the amphiphilicity appears temperature dependent. Thus, effects of amphiphilicity are not seen in the Colh phase (there are reasons of symmetry that dictate this), whereas it drives the formation of the lower-symmetry Colr phase. As such, the N phase is ’frustrated’, meaning that it is a ’compromise’ between the hexagonal and rectangular phases that border it. This is a remarkable observation that requires understanding.
Objectives
First priorities are to prepare related materials and determine the extent to which this frustrated nematic phase can be dialled in and out as a function of HC and PFC chain lengths/volumes. Gold(III) complexes are based on CNC pincer ligands functionalised with phenylacetylenes, but use of NCN pincers, will allow access to analogous platinum(II) complexes. Likewise, the PFC chains can be bound to the pincer instead.
The generality of the structure/property relationship will also be tested by preparing low-symmetry triphenylenes. These normally have approximate D3h symmetry, but using cross-coupling and oxidative coupling methods, it is easy to lower the symmetry to C2v to generate motifs analogous to those in the gold complexes.
Experimental Approach and Training
The programme is heavily synthetic but based upon well-defined synthetic approaches. Chemical characterisation will use NMR spectroscopy, CHN analysis and mass spectrometry, while LC characterisation employs optical microscopy, DSC and small-angle X-ray scattering as well as more advanced optical experiments to determine the sign of the phase birefringence. The student will gain a broadly based training across a range of techniques and methodologies. 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/
Novelty
The observation of the frustrated nematic phase is unprecedented, but as a single-point observation is of limited value. The flexibility available in the synthetic chemistry will allow the problem to be tested thoroughly and, in addition, there will be ample opportunity to probe the idea of quadrupolar complexes as synthetic tools to control solubility. Furthermore, the gold(III) and platinum(II) complexes are also phosphorescent, giving an additional degree of novelty as well as a further dimension to student training.
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/.
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/
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
Continue with Facebook
