A collaboration of French scientists have shown that intramolecular fullerene-fullerene interactions significantly increase the stability of self-assembled structures of pyridine-functionalised fullerenes with zinc porphyrins. Jean-François Nierengarten spoke to NJC about this research. 

How did you get interested in this research project? 

During the last decade, our group has been involved in fullerene chemistry for the development of new photoactive materials. On one hand, donor-fullerene systems have shown excited state interactions, making them excellent candidates for fundamental photophysical studies. On the other hand, such derivatives have been used for various applications (photovoltaic cells, optical limiting). Whereas research focused on the use of C₆₀ as the acceptor in covalently bound donor-acceptor pairs has received considerable attention, only a few related examples of fullerene-containing non-covalent assemblies have been described so far. As part of our research on compounds combining C₆₀ with donors, we have decided to use a non-covalent approach for their preparation. The assembly of the two molecular components by using supramolecular interactions rather than covalent chemistry appears particularly attractive since the range of systems that can be investigated is not severely limited by the synthetic route. However, self-assembled systems are not always very stable in solution and the design of supramolecular ensembles with high binding constants remains an important challenge. The work described in this paper is a part of this research program.

What is the most important result in the paper?

This paper describes multicomponent supramolecular ensembles resulting from the self-assembly of fullerene-pyridine substrates onto multi-Zn(II)-porphyrin receptors. The binding studies revealed a higher stability when the number of Zn(II)-porphyrin subunits (as an example, see Chart 1) is increased. We explain this result by the positive cooperative effects of intramolecular C₆₀ -C₆₀ interactions between the different fullerene-pyridine guests assembled onto the multi-Zn(II)-porphyrin receptors . We have thus shown that an increase of the number of components within supramolecular fullerene-porphyrin conjugates is not only interesting from the photophysical point of view, but it improves also the overall thermodynamic stability of the assembly. 

What are the implications of the results you present in this paper? 

Increasing the number of building blocks does not constitute a severe limitation for the self-assembly of large supramolecular architectures. Furthermore, the largest assemblies are strengthened due to the increasing number of secondary interactions (pi-pi stacking, hydrophobic interactions) within the self-assembled structure. Thus, multiple binding of several guest molecules onto a polytopic host moiety is an attractive design principle to produce stable supramolecular photoactive devices. 

Are there any particular challenges facing future research in this area?

The stability of large proteins, constituted of several polypeptidic units, results from a large number of weak interactions and is quite remarkable. Multi-component supramolecular systems appear to be ideal model systems for a better understanding of non-covalently bound biological macromolecules. We are, however, far from exercising the structural and functional control of nature.