Seminar: Dr Martin Peeks - Unusual magnetic properties of closed-shell pi-conjugated molecules

Wednesday, 19 September 2018 - 10:00am – Wednesday, 19 September 2018 - 11:00am  |  OMB 230

Speaker: Scientia Seminar

Closed shell molecules lack unpaired electrons, and so do not tend to exhibit strong interactions with magnetic fields (e.g. Zeeman splitting). However, if ‘magnetic properties’ are defined more broadly, then a whole gamut of interactions become apparent. Prominent among these, to an organic chemist, is aromaticity – generally defined as the cyclic delocalization of electrons with an accompanying increase in stability. This cyclic delocalization of electrons leads to persistent ‘ring currents’ in an applied magnetic field. In this first part of this talk, I will discuss the preparation of the largest (anti-)aromatic molecules yet synthesised – 2.4 nm oxidised porphyrin nanorings.[1] These materials are interesting because they show that there isn’t necessarily a “size-limit” for aromaticity.

All materials exhibit certain magnetooptical effects, including magnetic circular dichroism and Faraday rotation. Such effects could be useful for sensing weak magnetic fields using light. Usually these effects are small in diamagnetic materials, but it was recently discovered that some polythiophenes exhibit Faraday rotations which are competitive with the leading ferrimagnetic inorganic materials.[2] The benefit of ‘plastic materials’ comes from their flexibility and tunability, which enhances their utility. However, there is little understanding of the structure-property relationships underpinning the exceptional Faraday rotation in some organic materials. I will thoroughly introduce the Faraday effect, discuss our efforts towards developing techniques for measuring it, and our progress so far in understanding its origin in organic polymers and small molecules.[3]

 

[1] Peeks, Claridge, Anderson, Nature 2017, 541, 200.

[2] Gangopadhyay, Koeckelberghs, Persoons, Chem. Mater. 2011, 23, 516

[3] Wang, Jeon, Lin, Peeks, Savagatrup, Kooi, Van Voorhis, Swager, J. Am. Chem. Soc. 2018, 140, 6501; Wang, Lin, Lin, Peeks, Van Voorhis, Swager, J. Am. Chem. Soc. 2018, 140, 10881.