In the compact lecture theatre of St. Catherine’s college, complex molecular frameworks depicting chlorophyll (the green pigment present in plant cells) and haem (the red-coloured compound in red blood cells) are projected onto a whiteboard emblazoned with a vibrant title: ‘Building Molecular Wires from the Colours of Life’.
This describes the field of research of Harry Anderson; a Professor of Chemistry at the University of Oxford with a particular interest in functional molecular materials. In this talk, delivered to Oxford University Scientific Society in October 2022, Anderson provided a glimpse into his field of expertise.
Anderson opened by addressing the elaborate title. A ‘molecular wire’, he explained, is simply a long chain of atoms which can conduct electricity, whilst the ‘colours of life’ (such as haem and chlorophyll) are porphyrins—naturally occurring rings of carbon, hydrogen and nitrogen atoms arranged into loosely square units, bound around a central metal atom.
These porphyrins are the molecules which conduct; hence conducting chains (‘molecular wires’) can be built from porphyrins (the ‘colours of life’).
There are many examples of conducting species within nature, including a protein essential for photosynthesis in plants, Light Harvesting Complex II (LHCII).
LHCII consists of two rings of chlorophyll units: a B800 ring composed of nine units (which absorbs light of 800 nanometre wavelength), and a B850 ring composed of 18 (which absorbs 850 nanometre light).
The relatively long wavelengths of light absorbed are most prevalent in dim conditions, where other plants have removed most of the other wavelengths of light. These are the environments, therefore, in which LHCII functions most efficiently.
Upon absorption of light, the light excitation energy can be delocalised around the B850 ring of chlorophyll molecules, thanks to their ability to conduct charge. The excitation can then migrate across the surface of a mosaic of LHCII rings to larger Light Harvesting Complex I proteins, where electronic excitation can be converted into chemical energy.
One arm of Anderson’s research is the laboratorial synthesis of these porphyrin units.
Through a series of chemical transformations, simple starting units based on carbon, hydrogen and nitrogen can be assembled into rings, bound around a central metal ion. These rings can subsequently be decorated with different groups to achieve a range of properties—for example, solubility can be varied in this way. The synthesised rings can then be linked together to form a molecular wire.
But complications in the syntheses can arise as twisting of the chain can lead to decreased conductivity, hence twisting must be minimised in order to produce a functional wire. This can be achieved through the binding of bidentate ligands to the chain. Bidentate ligands are molecules which can bind to two sites on a species – this property is harnessed by Anderson to hold the molecular chain flat, hence achieving efficient charge mobility.
It is not only molecular wires which Anderson interrogates through his research—the chains can be assembled into rings and their properties can be probed in a similar way.
He explains that the formation of such rings involves the use of templates to arrange the molecular components such that ring closing is favoured, rather than a straight chain.
In particular, a technique known as “Vernier templating” can be used to form the rings in the simplest way. This process allows large rings to be formed using smaller, simpler templates, which is more straightforward than having to make a large template to synthesise each ring.
For example, two smaller templates can be used to bind chains of porphyrin units to form structures known as ‘Vernier complexes’. The chains can then be linked covalently to give a ring which is wrapped around the two smaller templates in a figure-of-eight structure. The templates can finally be removed, revealing that a larger ring has been formed.
Once these rings of porphyrin units have been formed, Anderson described how he probes their properties through investigating how charge is delocalised around the ring and the degree of aromaticity of the systems (a property which provides the structure with additional stabilisation).
Anderson closed the talk with a description of the utility of his research. He explaind that synthesising molecules with unusual properties allows him to learn as much about that structure as possible, thus elucidating structure-property relationships which can be used to engineer molecules with desirable qualities in the future.
Yet he finished with a quote from the 19th century French chemist Marcellin Berthelot, which reads ’chemistry creates its subject’.
Hence, although the aim of his research may seem unclear to some, it can be said to create its own as it progresses, and as greater understanding of the topic is developed.
More information about the research undertaken by Prof. Anderson can be found at the Anderson Group.
