An article by a research group belonging to the CIQUS which is led by Professor Jesús Rodríguez Otero has been selected as the cover of the latest issue of the journal Physical Chemistry Chemical Physics of the RSC. Using the tools of computational chemistry this work (Phys. Chem. Chem. Phys., 2015, 17, 13206-13214) suggests how to design concave receptors for the fullerene recognition.
Nearly 30 years after their discovery, fullerenes still attract the attention of many researches worldwide due to their unique properties and their applications in a wide range of fields as material science or medicine. Nowadays, one of the most active fields in fullerene chemistry is the search for molecular receptors capable to form stable associates with them. These receptors not only are useful for isolating fullerenes of the soot, but also for developing new materials for solar energy conversion, optoelectronics, catalysis or switching.
Since dispersion forces are predominant in the stabilization of fullerene complexes, a key strategy to design new molecular receptors is to make use of concave-convex complementarity to maximize these forces. Nevertheless, the examples of concave fullerene receptors are relatively scarce given that curved molecules are not always an easy synthetic target due to their tensioned structures. In this context, bowl-shaped polycyclic aromatic hydrocarbons, commonly known as buckybowls or fullerene fragments, seem very attractive because several buckybowls have been synthetized in the last years, being their concave surface highly suitable for fitting to the convex surface of fullerenes through concave-convex "ball-and-socket" π···π interactions.
In this work a series of molecular tweezers have been tested as potential receptors of fullerene C60. In order to design these tweezers three different strategies have been tested: (1) changing the corannulene pincers to other buckybowls, (2) replacing the tetrabenzocyclooctatetraene tether by a buckybowl (for this purpose, a bowl-shaped hexabenzocoronene was employed), and (3) adding methyl groups on the molecular tweezers to favour the development of C-H···π contacts. According to the results (performed at the B97-D2/TZVP level), all the three approaches are effective, in such a way that a combination of the three strategies results in buckycatchers with complexation energies (with C60) up to 2.6 times larger than that of the original buckycatcher, reaching almost -100 kcal mol-1.
On the other hand we have verified that the B97-D2/TZVP//SCC-DFTB-D approach can be a rapid screening tool for testing new molecular tweezers. However, since this approach does not reproduce correctly the deformation energy and this energy represents an important contribution to the total complexation energy of complexes, subsequent higher-level re-optimization is compulsory to achieve reliable results. This re-optimization could be superfluous when quite rigid buckycatchers are studied.
In summary, our calculations lead to the fact that tweezers synthesised so far have much room for improvement. Some possible strategies for achieving this improvement are the three strategies analysed here, which represent promising possibilities.