Greater understanding of artificial photosynthesis

Plants photosynthesise sunlight into a direct source of fuel. Artificial nanotubes have been created which can partially imitate the process of photosynthesis. Research carried out by Groningen physicists together with their German and American colleagues has revealed exactly how the nanotubes work. This information is important to be able to design artificial photosynthetic systems that convert sunlight directly into engine-ready fuel.

In an article that was pre-published on the website of the journal Nature Chemistry, the researchers reveal how nanotubes consisting of stuck-together dye molecules can produce light energy. The nanotubes have a diameter of about ten nanometres (a nanometre is one millionth of a millimetre) and can be several thousandths of a millimetre long.

Two layers

‘We have known about these nanotubes for about ten years now,’ explains Jasper Knoester, Professor of Theoretical Physics at the University of Groningen. ‘One of their characteristics is that they consist of two layers. The dye molecules in the two layers absorb photons, which causes them to become “excited”. We have now discovered that the two layers function separately from each other.’

Tuning fork

Where photons in ordinary solar cells can release electrons and so generate electric power, the dyes in the nanotubes convert the photons themselves into energy. That energy can jump from molecule to molecule. ‘You can compare it with a tuning fork that vibrates when it is struck,’ explains Knoester. ‘If you line up a row of tuning forks and strike one so that it vibrates, other tuning forks will vibrate too. The captured light energy in the nanotube is passed on in much the same way: some molecules become excited and these cause other molecules to ‘vibrate’ too.

From light to fuel

The photosynthetic systems of green plants work exactly the same way. Large molecular aggregates absorb light and convert it into energy that eventually ends up in a photosynthetic reaction centre. This is where the system stores the energy, for example as sugar. ‘There is a lot of interest in building systems that can copy this ‘light-to-fuel’ process. You could store such fuel and put it right into your car’s fuel tank, something that is a lot more complex with the energy that solar cells produce.’

Coherence

Some years ago, researchers found strong evidence for the occurrence of quantum mechanical effects when natural photosynthetic systems capture energy. That was unexpected, because such effects are mainly seen in very small structures and at low temperatures. The energy spreads in such a system via ‘coherence’. Put simply, the energy finds the shortest route through the photosynthetic system by trying out all possible routes simultaneously and then ‘selecting’ the best one. This may be a reason for the extremely high efficiency of photosynthesis: 95 percent of the energy captured by plants is converted into fuel.

Decisive experiment

‘We wanted to know if coherence is limited to one cell wall, or whether it also occurs between the walls. The available experimental data were conflicting in that respect,’ explains Knoester. This knowledge is important in the design of artificial light-to-fuel systems. In the article that appeared today, the researchers describe a decisive experiment, in which the outer shell of the nanotubes is slowly disintegrated by the process of oxidation, while at the same time the change in light absorption is measured.

Greater understanding

The conclusion is that the interior and exterior of the nanotube are only very weakly connected. They act as two independent systems, in any case at room temperature. A second experiment confirmed this result. Knoester: ‘This has given us greater understanding of the relationship between the molecular structure and function of these nanotubes.’ This will help researchers to develop new, simpler light-harvesting structures. ‘Moreover, we now have more fundamental insight into how quantum mechanical effects occur in these types of molecules.

Quantum computers

The latter insight may be important for a very different application: the quantum computer. Coherence can cause a ‘quantum bit’ to assume several values at once. In principle, this will make it possible to do calculations much faster than ordinary computers can. The difficulty lies in making quantum bits that work at room temperature. ‘This is why it is important to gain a better understanding of how quantum effects occur in natural and artificial photosynthetic systems.’

Note for the press

- Contact: Prof. J. Knoester, j.knoester rug.nl; tel. 050-3634617. Jasper Knoester is Dean of the Faculty of Mathematics and Natural Sciences and Professor with the University of Groningen’s Institute for Theoretical Physics and the Zernike Institute for Advanced Materials (ZIAM).
- Reference: Utilizing redox-chemistry to elucidate the nature of exciton transitions in supramolecular dye nanotubes; D. M. Eisele^1,2, C. W. Cone³, E. A. Bloemsma⁴, S.M. Vlaming ^1,4, C. G. F. van der Kwaak⁴, R. J. Silbey^1,†,M. G. Bawendi¹, J. Knoester^4*, J. P. Rabe² and D. A. Vanden Bout^3*; Nature Chemistry; DOI: 10.1038/NCHEM.1380

¹Massachusetts Institute of Technology, Center for Excitonics and Department of Chemistry, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, USA, ²Humboldt-Universität zu Berlin, Department of Physics and IRIS Adlershof, Newtonstraße 15, D-12489 Berlin, Germany, ³University of Texas at Austin, Department of Biochemistry and Chemistry and Center for Nano and Molecular Science and Technology, 1 University Station A5300, Austin, Texas 78712-0165, USA, ⁴University of Groningen, Institute for Theoretical Physics and Zernike Institute for Advanced Materials, Groningen, The Netherlands; ^†Deceased.