3.3. Shape changes
One of the first reported examples of an artificial molecular machine is system 1 shown in Fig. 18. It consists of two ring molecules, called crown ethers (A), connected to a central –N=N–unit (B) that can change its structure (in the chemical nomenclature, passing from the linear trans form to the bent cis and vice versa) due to the absorption of light of appropriate color. When a solution containing this system in the trans form is irradiated with ultraviolet light, the change in the structure of the central unit B causes the two side rings A to approach, which can thus capture a potassium ion (K+). Using visible light, or leaving the system in the dark, the reverse process occurs with the consequent release of the K+ ion. This mechanical action is comparable to that of nanometer-sized tweezers, which could form the basis for the construction of systems capable, within an organism, of transporting and releasing drugs, or eliminating harmful substances.