STM switches chlorophyll molecules

"To my knowledge, this is the first time that a single chlorophyll-a molecule could be imaged," Saw-Wai Hla of Ohio University told nanotechweb.org. "But what I'm really satisfied with about this work is the controlled switching of four molecular conformations with atomic scale precision. We can also change the switching frequency by changing the tunnelling current." Chlorophyll-a molecules are responsible for the green colour of most plants and play a vital role in photosynthesis – the process whereby plants convert carbon dioxide and water into carbohydrates and oxygen, using light as an energy source. The molecules consist of a porphyrin unit, the "head" and a tail made up of a phytyl group containing a long carbon chain. "The conformation changes of this molecule are supposed to play a vital role [in photosynthesis]," said Hla. "Thus, understanding its mechanical and electronic properties will be an initial stage to studying photosynthesis at a single molecule level. The natural plant molecules may also be useful in developing solar energy conversion devices." Hla and colleagues imaged chlorophyll-a molecules that were weakly bound to a gold (111) surface at 4.6 K. Some of the molecules assembled into clusters while others remained as single molecules. The researchers were able to change the conformation of a molecule by bringing the STM tip close to one of two particular regions of the molecule and applying a 1.5 V bias. This caused a 60° bending in the phytyl chain. The researchers believe that the transfer of a single tunnelling electron to the molecule provided energy for the switching process. "Single molecule switches using STM manipulation have been demonstrated before by us and by other groups but mostly these switches operate in two levels," said Hla. "To our knowledge, this is the first multi-step switching process realized by STM manipulation on a single molecule. Experimentally, we could also determine the energy barrier to switch the conformation of the molecule." "Source":[ http://nanotechweb.org/articles/news/5/9/6?alert=1]

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