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 (J. Phys. Chem. B, 2017, 121, 8817).
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 (i.e. below a much more ordered columnar hexagonal phase). While this observation requires much deeper investigation (is it a ’conventional’ discotic nematic or a columnar nematic variant?), 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 precise reasons of symmetry that dictate this – these complexes have approximate C2v symmetry (PFC chain along the C2 axis) whereas a columnar phase requires effective molecular three-fold symmetry; hexagonal phase formation shows no discrimination between HC and PFC chains), whereas it drives the formation of the lower-symmetry Colr phase (threefold tessellation no longer possible). 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.
First priorities are to prepare closely 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. The present example has –OCH2CH2CnF2n+1 PFC chains, but –O(CH2)4CnF2n+1 chains are readily accessible. 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 lower-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. A challenge is the low solubility resulting from introduction of PFC chains, but recently we have observed a possible general strategy to resolve this question relying on solubilisation through the formation of small aggregates stabilised by quadrupolar interactions. We will probe this idea further through this chemistry. 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.
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.
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/idtc/
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. This PhD project is available to study full-time or part-time (50%).
This PhD will formally start on 1 October 2020. Induction activities will start on 28 September.