Molecular Switches: Optoelectronic components based on a dye-sensitized TiO2 solar cell

Electronic components must continue to get smaller: Miniaturization has now reached the nanometer scale (10^-9 m). In this tiny world, classic semiconductor technology is reaching its limits. We now need switches and other devices whose dimensions are on the scale of individual molecules. The difficulty with this is in the addressability and compatibility of molecular systems with the available nanoelectronic components. Until now, all molecular systems require at least one step in which a solution must be injected into the system and then rinsed out again, which is time-consuming.

L. Furtado, K. Araki, H. E. Toma, and co-workers at the University of São Paulo in Brazil describe for the first time an optoelectronic molecular gate that directly absorbs light and gives off electrical impulses.

The gate consists of a glass electrode onto which a thin, nanocrystalline film of TiO₂ is deposited. A dye, in this case a cluster of three ruthenium–pyrazinecarboxylate complexes, is adsorbed to this surface. A platinum counter electrode is used, and the space between the electrodes is filled by an electrolyte solution of I₃^-/I₂ in CH₃CN.

When this gate is irradiated with light, electrons are excited, which leads to charge separation and a flow of current. The direction of the current changes depending on the wavelength of the light irradiating the system: at 350 nm, the electrons flow from the Pt electrode to the glass electrode; at 420 nm, they flow the other way.

At 350 nm, the TiO₂ layer absorbs the light and gives off electrons to the underlying glass electrode. To compensate, the corresponding number of electrons is removed from the ruthenium cluster, which replaces them with electrons from the Pt electrode. At 420 nm, however, the ruthenium complexes are induced to give off electrons to the Pt electrode, which are re-supplied from the TiO₂ layer.

The result is a switch that is not only turned on and off by light, but whose signal can change direction on the basis of the wavelength of light used.

Citation: Angewandte Chemie International Edition 2006, 45, No. 19, 3143–3146, doi: 10.1002/anie.200600076

Source: Angewandte Chemie

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