Duelling Dipoles: In Search of a New Theory of Photosynthetic Energy Transfer

Chemists of Ludwig-Maximilians-Universität (LMU) in Munich have refuted a basic postulate of Förster theory, which describes energy transfers between pigment molecules, such as those that underlie photosynthesis. A revised version of the theory could have an impact on the design of optical computers and improve the efficiency of solar cells.
Photosynthesis, the formation of energy-rich chemical compounds with the aid of sunlight, is fundamental to life on Earth. In plants, sunlight is collected by so-called antennal complexes, consisting of proteins bound to the green pigment chlorophyll. The chlorophyll captures the light energy and relays it, virtually without loss, via several intermediate molecules, to the reaction centers, where it is converted into stable forms of chemical energy.

The intermolecular transfer process is described by Förster theory. This postulates that pigments act as oscillating dipoles to electrically excite adjacent molecules, in much the same way as the elements of a dipole antenna pick up and feed radio signals to a receiver. Measurements carried out in the laboratory of LMU chemist Professor Heinz Langhals, in collaboration with the Department of Physics at LMU Munich, have now refuted this model.
“Energy transfer between dipoles depends on their orientation,” says Langhals. “When dipoles are orthogonally disposed, no energy transfer should occur. We have now tested this assumption experimentally and, to our surprise, we found that energy is rapidly and very efficiently transferred under these conditions.” In collaboration with international partners, the LMU team now wants to establish a firm experimental basis for the formulation of a new theory of energy transfer. This may well have repercussions for the development of optical computers and might help to enhance the performance of solar cells.

Chlorophylls and other pigment molecules, often in association with specialized proteins, can form complexes which act as efficient antennas that collect light energy and pass it on to the photosynthetic reaction centers or to the conducting layer of a solar cell. The energy is captured and transiently stored in the bonds between specific groups of atoms in the pigments, which are therefore referred to as chromophores. Different chromophores absorb light of different wavelengths, so a complex containing various types can harvest light over a large segment of the spectrum. Indeed, the original goal of the LMU researchers led by Langhals was to synthesize such a broadband light collector.

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