New Zealand Federation of Graduate Women 2012 Mid-Tenure Report
Anthea Blackburn
Biomimetics, that is, scientific endeavors inspired by nature, has shown itself to be an
important tool in identifying solutions to the various challenges that scientists face today.
Nature can be invoked to model analogous functions.1 One such natural phenomenon, which
has been borrowed as an inspiration for some time in the development of new technologies, is
electron transfer (eT). This process is commonly associated with photosynthesis in plants, as
well as respiration in mitochondria, to name two examples. In all cases, eT is vital in the
formation and transport of electrical energy between distant components within a cell, such
that chemical energy can be generated for use as fuel.2,3 Solar cells, which carry out the
harvesting of solar energy and its conversion into electrical energy,4 are one such technology
that is inspired by photosynthetic eT. Similarly, biological eT has acted as the inspiration for
the research proposed here – the development of a chemical system capable of undergoing eT
as efficiently as that achieved biologically.5
Possibly the most impressive aspect of biological eT is the great distance over which it
occurs. This directional long-range eT process is imperative in both natural and synthetic
systems and has posed a challenge to scientists for some time now. Biologically, the protein
cytochrome initiates this long-distance eT by converting photons from the sun into electrical
energy in Photosystem II. It then expedites the transfer of these electrons to other parts of the
photosynthetic cycle via quinone molecules in a multi-component process.6 Most importantly,
in order for this eT to be controlled, efficient and directional, charge recombination must be
prevented, so the electron donor and acceptor must be located as far apart as possible. Herein
is the success of biological systems – a long-lived charge transfer state.7 In this research, we
expect to take this process one step further and generate a one-component system that can
facilitate long-range eT efficiently via a precisely aligned array of porphyrin moieties.
In order to achieve efficient eT, I am exploiting the properties of
inherently rigid and linear mechanically interlocked polyrotaxanes.8 A
rotaxane consists of a thread, stoppered with bulky functional groups at
either end (a dumbbell), and encircled by a controlled number of
Figure 1: An
oligorotaxane made
up of a stoppered
thread and m
macrocycles.
macrocycles; these molecules are named according to the convention [n]rotaxane, where n is
the number of components that make up the molecule (Figure 1).9 In my research design,
zinc (II) porphyrins have been attached to the macrocyclic portions of the oligorotaxane
(Figure 2). It has been shown very recently that the parent
oligorotaxanes can be easily self- assembled in quantitative yields.10,11
This protocol, known as clipping, relies on templation, provided by
hydrogen-bonding and aromatic-aromatic interactions, to produce a
homologous series of preordained oligorotaxanes easily and efficiently.12
Figure 2: A schematic
of a porphyrinated
oligorotaxane system,
which facilitates eT via
redox.
Over the past seven months, I have focused on the synthesis of
a model rotaxane complex, which has enabled me to determine the
most successful and efficient strategy to attach the desired porphyrin
moiety to the molecule (Figure 3). Despite some initial synthetic
setbacks, the desired model rotaxane has been successfully
synthesised and characterised. With this goal achieved, the electronic
properties of the porphyrinic rotaxane can now be studied, that is, the
molecule’s propensity to harvest light energy and transform it into
electrical energy as an electron. In order for the transport of the
Figure 3: Chemical structure of
the porphyrinic rotaxane
synthesized in this work.
electron generated and the formation of the desired long-lived charge separated state,
directionality needs to be introduced into the rotaxane. This will be achieved through the
introduction of electron-donating and electron-accepting groups to either end of the molecule,
as a way of introducing bias into the system, and thus directing the movement of the electron
along the molecule. This is the basis of the photosynthetic process on which this research is
focused.
Outside of the lab, I have spent a large portion of my time volunteering at a local
Chicago organisation, Family Matters. This is a not-for-profit company, which exists to build
and strengthen the community through “programs that support personal growth and
leadership.” In particular, I have been involved in the Community Tutoring component of
Family Matters, which aims to encourage and support students in both their academic
pursuits, as well as in their everyday lives. I have worked each week with a young Burmese
refugee student, who arrived in Chicago from Thailand almost five years ago, where as a
non-native English speaker, we spent a large portion of our time together working on her
vocabulary, grammar and reading skills. One of the most rewarding aspects of this work is
being able to make a difference in a student’s life and knowing that the hard work we carry
out is appreciated – my tutee was honoured as the top student in her ESOL class this year!
References
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Macromolecule”. J. Am. Chem. Soc. 134: 5243–5261. 2012.
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Bravo, J.A., Raymo, F.M., Stoddart, J.F. “Molecular Meccano. Part 45. High Yielding
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