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Issue Two 2012 - Issue #16

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Leaf matter – New understanding of photosynthesis

Story by James Hutson

View articles in related topics: Optical Physics, Physics, Sustainability & The Environment

Understanding the intricacies of nature’s highly efficient energy transfer has long been the focus of researchers around the world.

The more we learn about this energy transfer mechanism, the more feasible it becomes to mimic nature in the development of highly efficient photovoltaic solar cells, says Dr Jeff Davis, Research Fellow with Swinburne’s Centre for Atom Optics and Ultrafast Spectroscopy (CAOUS).

He is leading a research group that has developed a technique that provides new insights into the photosynthesis process at the atomic level.

The key to high energy efficiency
Classical physics was long deemed to be sufficient to explain the passing of energy from sunlight to where the biochemistry can take place.

However, in 2007 experiments from the Fleming group at the University of California, Berkeley, showed that during photosynthesis, energy doesn’t hop between the different light absorbing plant molecules (chromophores) as classical physics would suggest. Instead, the chromophores are momentarily coupled in a quantum superposition, effectively sharing energy for relatively long times (more than 200 femtoseconds or 0.0000000000002 seconds).

It is speculated that these quantum-superpositions allow the energy absorbed from the sun to rapidly and reversibly explore all possible pathways before taking the most efficient one and provide the key to extremely high efficiency energy transfer in photosynthesis.

Only a handful of research groups around the world have now seen these quantum effects exist in photosynthesis but until now they have not been able to clearly confirm or refute any of the predictions regarding their role.

Observing quantum effects
The Swinburne team has developed a technique to isolate these quantum effects. This has led to the first clear measure of how long these quantum superpositions last and the first observation of strong coupling between the electronic transitions of the chromophores and the vibrational energy states of the protein matrix that holds the chromophores in place.

This observation provides clear experimental evidence that classical treatment of photosynthesis isn’t good enough. Better models, which include these newly observed quantum details are required. Initial theoretical models suggest that quantum coupling on its own would actually reduce the system’s efficiency, and that only by factoring in dephasing, quantum tunnelling and noise due to vibration and motion processes does highly efficient energy transfer occur.

As Dr Davis points out, the more we understand about this energy transfer mechanism and how the environment affects that mechanism, the more feasible it becomes to use these findings to make more efficient photovoltaic solar cells.

More efficient photovoltaics
Current organic photovoltaics are a thin layer mixture of two types of light-harvesting molecule: simple, but with only single digit efficiencies. Dr Davis admits applying these findings would require a change in approach. Additional structures would be required to position the chromophores in a similar manner to which they are held in place by the protein matrix in the plant cells.

“If we can understand how nature has designed these structures we can apply those to organic photovoltaic cells or some other form of biomimetic light harvesting so that energy from the sun can be stored and used with virtually no loss.” l

The project team
Dr Davis is a Senior Research Fellow in the ultrafast spectroscopy group at the Centre for Atom Optics and Ultrafast Spectroscopy (CAOUS), and ARC Centre of Excellence for Coherent X-ray Science hosted at Swinburne. CAOUS was founded in 1999 by Emeritus Professor Peter Hannaford.
PhD student Gethin Richards performed the experiments and much of the analysis that led to these observations.
The team collaborates with external researchers Professor Paul Curmi at the University of NSW (chromophore samples) and Dr Harry Quiney at the University of Melbourne (modelling).

This work was supported by a Discovery Project grant from the Australian Research Council.


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