Synthesis and Cooperative Emergent Properties of Porphyrin Nanostructures

Porphyrins are heterocyclic molecules composed of four pyrrole units joined via four methylene bridges. Molecules containing porphyrin moieties attract attention for various reasons such as:
- Porphyrins play key roles in photosynthesis; they harvest sunlight and efficiently convert the energy into chemical potential.
- Porphyrin rings exhibit aromaticity with 26 pi electrons, and they can be used as components of larger aromatic systems.
The group of Prof. Anderson has developed the synthesis of large porphyrin oligomers, both linear and cyclic. These systems are of great interest for their photophysical and supramolecular chemistry. Understanding of the collective photophysical behaviour of porphyrin-based nanostructures may lead to a deeper understanding of the nature of light harvesting in chloroplasts. Porphyrin nanorings are unique chemical species for their size and cooperative phenomena.
Aims and Objectives
- Macro-aromaticity of porphyrin nanorings
Recently, it was discovered in our group that oxidized nanorings (containing 6 porphyrin units) possess macro (anti)aromaticity around the whole nanoring. The oxidized six-member nanoring, with a diameter of 2.4 nm, is the largest known aromatic ring.
I plan to extend this study by synthesizing and investigating anions and radical anions/cations of 6-porphyrin nanorings. Furthermore, I plan to expand the scope of these experiments to larger system. For these studies, it will be necessary to synthesize nanorings as well as find conditions under which the redox chemistry and all necessary investigations will be practicable. I will carry out EPR spectroscopy investigations of radical anions of nanorings in collaboration with Prof. Christiane Timmel (Department of Chemistry, University of Oxford).
- Transient absorption anisotropy
I plan to investigate the anisotropy of transient absorptions of excited nanorings at extremely short times (below 200 fs) for a series of nanorings containing 6, 10, 20 and 40 porphyrin units. For this purpose, I will synthesize a series of nanorings with bis(octyloxy)phenyl sidechains and I will collaborate with the group of Ismael Heisler (School of Chemistry, University of East Anglia) to carry our ultrafast photophysical measurements. These experiments will lead to better understanding of energy flow in molecular nanostructures.
- Magneto-optical effects in nanorings
Based on preliminary results from Prof. Peter Beton (School of Physics & Astronomy, University of Nottingham) which showed highly oriented organization of deposited monolayer of nanorings on graphene surface, we intend to investigate the magneto-optical behaviour of such layers. With Prof. Robin Nicholas and Prof. Laura Herz (Department of Physics, University of Oxford), we plan to study Aharonov-Bohm oscillation and absorptions in very high magnetic field at low temperatures of nanorings layers deposited on graphene or BN surfaces. I plan to work closely with our collaborators in the Department of Physics on this project, and to synthesize suitable nanorings for these studies. The first step will be the synthesis of 10 and 12-porphyrin nanorings with bis(octyloxy)phenyl sidechains.
We are studying the largest aromatic and antiaromatic rings yet reported. It will be fascinating to compare the global ring currents at such large aromatic rings with the behaviour of macroscopic conductive rings with diameters of 20-1000 nm. This study could bridge the gap between experimental results obtained from small aromatic molecules and macroscopic rings, and answer the question of whether aromaticity is confined to small molecules or whether it is universal phenomenon. As this project develops, we plan to compare the ring currents in porphyrin-based nanorings with their charge-transport behaviour in single-molecule devices, through collaboration with Prof. Andrew Briggs.
This Project falls within the EPSRC research area of Physical Sciences.