Guest molecules can be ordered by the self-assembly of a symmetrical cage into a 2-D network structure. 

Within a highly symmetrical cavity, a guest molecule tends to be disordered making it difficult to analyse the structure by X-ray crystallography. Makoto Fujita and colleagues at the University of Tokyo, Japan, have designed a host-guest system which overcomes this problem by preventing the disordering of guest molecules. 

The host-guest complex has two different guests - one guest molecule creates a network between the coordination cages and, as a result, the movement of the second guest is restricted.

The tetrahedral coordination cage, which has a highly symmetric cavity, self-assembles to make a 2-D network of cages in which the symmetry is lessened. This self-assembly is possible because one of the guest molecules has coordination sites which interact with metal centres to form a 2-D layered structure. The connections made between these guest molecules in adjoining cages forces the other guest molecules into one corner of the cage.

The second guest, which becomes restricted in the corner of the cage, is a manganese compound with a cyclopentadienyl ligand - this guest was chosen because the researchers are interested in studying its photoreactivity in the cavity. 

'We would like to use the cavity as a molecular flask for crystalline-state reactions,' said Fujita. 'By obtaining an ordered crystalline phase, we could directly observe photoreactions at the atomic level.' 

Alison Stoddart