Angewandte Chemie International Edition
© WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
For full article and contact information, see Angew. Chem. Int. Ed. 2000, 39 (8), 1458 - 1462
Molecular Chains as "Wires"
Packets of energy travel along
lined-up porphyrin molecules
Whether sensors or systems for the conversion of solar energy, modern photo-electronic components keep getting smaller. Tiny but highly conducting wires are needed to conduct electric and optical signals from one point to the other. Single rod-shaped molecules are well suited to take on this task.
The synthesis of such molecular "wires" poses a particular challenge to chemists, because they must make molecular rods of a uniform, defined length. At the same time, the optical and electronic characteristics of the tiny wires must meet specific demands. Japanese researchers working with Atsuhiro Osuka have taken up this challenge - successfully. They have been able to develop a highly promising approach to the construction of molecular wires that conduct light.
Once again, something already employed by Mother Nature has proven itself useful: the chemists have selected porphyrins to act as the building blocks for their wires. Porphyrins are an important component of enzymes, such as the hemoglobin that renders our blood red. As part of the chlorophyll in plants, porphyrins are fundamentally involved in photosynthesis. They help the plant make chemical use of the sun´s energy.
Porphyrins are stable, ring-shaped molecules with a strictly planar structure. Osuka and his colleagues have managed to get up to 128 of these building blocks hooked together. The planes of the rings in these chains are offset by 90° from each other. "These are the longest rod-shaped molecules of defined, uniform length yet synthesized," stresses Osuka. "But this length is far from the limit. We are already very successfully constructing much longer porphyrin chains."
Because of their favorable optical and electronic properties, these regularly arranged porphyrin chains are highly promising candidates for light-collecting and -conducting wires. The electrons in the individual building blocks are in a strong, long-ranging interaction with each other. If the electrons of one ring are excited by absorption of a photon, that is by light energy, they pass the excitation on to the next building block. This "packet" of energy - called an exciton - is thus quickly carried along the entire chain.