For the first time scientists have engineered a series of molecules that show quantum effects similar to that observed in the light-harvesting complexes. Greg Engel’s group in University of Chicago have been able to both understand as well as mimic the efficient mechanism of light transfer happening in plants.
Aside from other benefits, this would lead to the production of artificial energy-transfer devices which could use the mechanism efficiently.
Photosynthetic antennae are arrays of proteins and chlorophyll which transfer absorbed light energy to the reaction centres where light energy is converted to chemical energy. This enhances the efficiency of light transfer compared to the process when light is absorbed directly by the reaction centres themselves. The secret of the efficiency of the transfer process lies in quantum electronic coherence that stretches over some femtoseconds (a femtosecond is a millionth of a billionth of a second). When there is coherence, energy from the incoming photon can simultaneously explore every possible cholorophyll route from the protein’s surface to the reaction centre at its core and then settle for the shortest route. Compare this with the time and energy wasted if the photon had to sequentially try out every path before reaching the reaction centre. Thus the efficiency of the process is increased manifold.
The researchers have engineered a series of molecules that show quantum effects similar to that observed in the light-harvesting complexes. Biological light-harvesting systems are so complex that they obscure the design principles involved. However, the model systems engineered by the group are simpler yet manage to capture the physics involved, according to the report published on April 18 in Science Express.
The main actor in this is a dye-like material called fluorescein. The researchers modified fluorescein and linked parts of these together rigidly to form a series of compounds. The resulting molecules were able to mimic the behaviour of light-harvesting centres in plants that use photosynthesis, especially the coherences which persists for over tens of femtoseconds. They infer the presence of this coherence using two-dimensional spectroscopy.
To observe the quantum coherence in the system, the team shone laser light into the system and recorded the emitted light by means of a camera and recorded it in movies.
Every frame of the movie was a two-dimensional spectrum. The movie showed quantum beats, or oscillations, in a particular region, which is evidence of quantum coherence. It is an exciting thought for the future that discovery of this molecule series and the mechanism of energy transfer may initiate the development of synthetic light harvesters which could lead to highly efficient and green energy manufacturing units.