The first dendrimers were created by making conventional covalent bonds between unconventional, branched building blocks. Self-assembly and non-covalent interactions have been used to modify existing dendrimers or dendritic building blocks. Now, German researchers have developed the first dendrimer that self-assembles from scratch, over several generations, using only hydrogen bonding to fit the pieces together.

Andreas Hirsch and colleagues at the University of Erlangen-Nürnberg used a two-pronged molecular claw, known as the Hamilton receptor, which can form six hydrogen bonds to a guest molecule such as cyanuric or barbituric acid. The core of their dendrimer is a benzene ring from which three such receptors emerge. The branching element has one cyanuric acid function and two further Hamilton receptors. The outermost layer is sealed off with cyanuric acid alone, serving as an end cap.

Although the researchers carried out the assembly as a 'one-pot reaction', without ensuring orderly layer-by-layer assembly of the dendrimer, the products appear homogeneous.

Hirsch credits thermodynamics for the success. 'Taking into account that the complete saturation of all sites that can be used for hydrogen bonding is thermodynamically favoured,' he says, 'the preferred formation of the expected perfect structures is the most likely case.'

So dendrimer development might be easier than expected, if the molecules are left to assemble by themselves.

Michael Gross