Jonathan Faiz, Valérie Heitz and Jean-Pierre Sauvage, University of Strasbourg, France, outline recent advances in the construction of interlocked molecules inspired by photosynthesis  

Artificially recreating photosynthesis - in the quest to find environmentally friendly and renewable energy sources - is a hot topic across many scientific disciplines. It is now known that porphyrin-like units are key features of the reaction centre where photosynthesis occurs, and synthetically reproducing these molecules has become a very active research area. 

Porphyrins are planar and highly conjugated cyclic molecules that can complex a variety of metals. They are found in many natural systems, including blood (as hemoglobin in their iron-complexed forms) and in the photosynthetic reaction centre (as magnesium-complexed chlorins, chlorophylls - which are structurally very similar to porphyrins).

The electrochemical and photoactive properties of porphyrins make them ideal for performing energy and electron transfer in a similar fashion to photosynthesis. Importantly their ability to form noncovalent interactions, with a metal centre, can be exploited to form mechanically interlocked systems - such as catenanes and rotaxanes - that incorporate porphyrins in their structure.  

 

A transition metal ion can hold together the components needed to make a catenane

 

One of the most remarkable features of catenanes (two or several interlocked rings) and rotaxanes (two-component assemblies consisting of a central thread encapsulated by a ring and stoppered by two bulky units on each end of the thread to stop the ring slipping off) is their high flexibility, meaning they can undergo a very large number of different motions. These movements are important in photosynthesis as they facilitate electron transfer. The motions occur both naturally due to the molecules inherent energy (when all the components are not or only very weakly interacting), and when the molecule's most stable geometry is altered by an external stimulus. 

Although seemingly simple when sketched on paper, the construction of catenanes or rotaxanes is not trivial.  This is because attractive forces are needed to hold the components together, to template the reaction, before either the rings are closed (in the case of catenanes) or the stoppers are attached (rotaxanes). 

One construction method is the use of transition-metal templates, where the various components of the macrocycle contain 1,10-phenanthroline units that can coordinate to copper(I) ions - holding the components in place.  The rotaxanes and catenanes then form around the metal ion, that is removed once the macrocycle is constructed. This method has been used to make a comprehensive range of rotaxanes, with porphyrin stoppers, and catenanes, containing porphyrins rings. 

Other templating methods for rotaxanes include the use of hydrogen-bonding or -stacking interactions to either form a macrocycle around a thread already bearing stoppers or to hold the macrocycle around the thread whilst the stoppers are grafted.  The size and metal-binding properties of porphyrins make them ideal for stoppers for rotaxanes.  This route has given access to a wide variety of architectures in which electron transfer can occur between porphyrins and, for example, fullerenes and electron-deficient aromatic macrocycles.

Rotaxanes have also been made with a manganese porphyrin ring that has an olefin-containing backbone threaded through it.  The ring can zip along the backbone and catalyse the oxidation of the olefins in the backbone to epoxides - demonstrating the sheer breadth of application of these porphyrin-containing systems.  

Mechanically interlocked porphyrin-containing architectures are important synthetic analogues of natural systems as they contain subunits held at predetermined distances and geometries - but not through conventional covalent bonds. In this way, just like natural systems, any intercomponent process that occurs between subunits takes place through the shortest pathways, such as through hydrogen bonds or solvent.

Read Jean-Pierre Sauvage's tutorial review 'Design and Synthesis of Porphyrin-Containing Catenanes and Rotaxanes' in issue 2, 2009 of Chemical Society Reviews