Credit: © 2008 RSC

Covalently linked arrays of porphyrin molecules have shown potential as photonic materials —particularly as light-harvesting systems. Cyclic arrangements have been made previously by connecting porphyrins with small linking groups, such as alkynyl or phenyl spacers, but only small excitonic interactions between neighbouring chromophores have been observed. Linking porphyrins directly through a covalent bond results in a much smaller centre-to-centre distance, larger excitonic interactions and, therefore, faster energy and electron transfer.

Previous approaches for making directly linked porphyrins relied upon homo-coupling reactions mediated by silver(I) ions, but this limited the type of structures that could be formed. With this in mind, Naoki Aratani and Atsuhiro Osuka at Kyoto University1 have used a palladium-catalysed Suzuki–Miyaura coupling reaction to link together different types of porphyrins. In a stepwise process, they synthesized a square-shaped array made up of eight zinc-porphyrins, which were all connected through direct meso–meso linkages. This porphyrin square has a central cavity large enough to bind a molecule of dipyridylanthracene, with the pyridyl nitrogens coordinating to two zinc atoms on opposite sides of the square.

This new strategy, which enables different porphyrins to be coupled directly to one another in a stepwise fashion, offers a simple route to hybrid arrays with unique shapes and large numbers of porphyrin units. The degree of control in this molecular construction could lead to porphyrin materials with useful photonic and host–guest properties.