Molecules as Switches, Photochromic Effects

The possibility of modulating a particular physical properly of a molecular system with an external stimulus, e.g. with Hght or with an electric field, is of central importance when one wishes to design molecular or supermolecular devices. Therefore, molecular systems are being sought which can be switched reversibly between different states. They must permit the reversible selection of a physical property of the supermolecular or macroscopic system by means of a directed and rapid stimulus, for example a light pulse. 

Such an active system will be referred to here as a molecular switch. To recall the molecular physics, we give here a brief example of switches based on polychromic compounds. For more details, we refer to the original literature [6-10]. 

A class of substances which fulfills these conditions are the fulgides (Fig. 12.2). Equally useful and frequently investigated are the diaryl-ethenes and the dithienyl-ethenes (Fig. 12.3) and similar molecules. In both cases, the photochromism is based on a bonding isomerisation. Here, a tt bond is transformed into a a bond, accompanied by a ring closure. This leads in solution to a new absorption line 

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