Rapid development of different electronic devices was initiated by the discovery of semiconductor-based switch - a transistor in 1948 by J. Bardeen, W.H. Brattain and W.B. Shockley. All electronic devices are based on semiconductor components ever since. Growing demand for bigger memories and faster processors requires smaller and smaller transistors and other components. Soon the integration scale of electronic components will reach the physical limits and further speeding up will not be possible. The only solution of the crisis is application of single molecules and molecular systems for data acquisition, storage, transfer and processing.
There are many chemical systems capable of digital data processing. The most important are: fullerenes, carbon nanotubes, DNA processing systems and Aviram-Ratner type devices. All these systems, except of DNA, use however classical electronics approach. Logic gates and switches are in principle based on the same phenomena as semiconductor devices. Chemical logic gates present other approach. These systems consist of molecules, which can exist in at least two different states (isomers, rotamers etc.) and there are defined physical or chemical stimuli capable of switching the system from one state to the other (for example light, redox potential, temperature, pressure, pH, metal ions etc.).
Usually the chemical logic systems are based on extremely complicated supramolecular complexes, like Balzani's "molecular meccano" systems (rotaxanes, catenanes and dendrimers) or large organic ligands encompassing receptor sites and chromo- or fluorophores, like de Silva's systems.
There are, however, other systems, much simpler from chemical point of view, but their logic structure is even more complicated as compared with previously mentioned systems. One of the simplest chemical systems capable of performing complex logical operations consists of two pentacyanoferrates in equilibrium. Reactivity and photoreactivity of he system strongly depends on different chemical and physical stimuli and perfectly mimics the behaviour of different logic gates.