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 Royal Society of Chemistry (RSC). Using the tools of computational chemistry this work (Phys. Chem. Chem. Phys., 2015, 17, 6233-6241) 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 last years, our group has been devoted to the study of a series of concave-convex interactions between buckybowls and fullerenes in order to find the factors that maximize their interactions.
In this work a series of buckybowls with different sizes and structures have been tested as potential receptors of fullerenes C60, C70 and C40. Among these bowls are corannulene (C20H10), sumanene (C21H12), pinakene (C28H14), hemifullerene (C30H12), circumtrindene (C36H12), pentaindenocorannulene (C50H20) and bowl-shaped hexabenzocoronene derivatives. An exhaustive study, taking into account different orientations of fullerenes, was performed in order to obtain the most favourable arrangement for interacting with the bowls.
According to our calculations (performed at the B97-D2/TZVP level) dispersion plays a crucial role in the interaction energy in these complexes: it is responsible for the binding in them, showing an excellent correlation with the interaction energy (the minimal deviations can be explained by qualitative analysis based on the contribution of electrostatics to the interaction energy). As expected, an increase in dispersion energy shows a fairly good correlation with an increase in bowl size. However, there are some substantial deviations, which can be attributed to the shape of the bowl. In this way, compounds whose structure possesses flaps at the rim of the bowl show an enhanced ability to interact with fullerenes. Therefore, it seems that the addition of these flaps at the bowl rim by benzannelation is an effective strategy for increasing the interaction with fullerenes, providing enough flexibility to extend the contact surface with the fullerene moiety. Accordingly, a bowl-shaped hexabenzocoronene derivative (C72H24) showed the best ability among the buckybowls evaluated for catching the three fullerenes; it is noteworthy that, when interacting with C60, the interaction energy is three times that corresponding to the prototypical buckybowl, corannulene. On the contrary, the more rigid and compact is the structure of a buckybowl, the smaller its ability to interact with fullerenes.
It is also worth mentioning that the interaction energies for the complexes with C70 only show small increases with respect to those obtained with C60. Contrarily, complexes with C40 show a significant decrease in their interaction energies, with differences large enough to allow their selective separation of fullerene mixtures with C60 and C70.