Energy, supramolecular chemistry, fullerenes, and the sky* - IMDEA ...

Energy, chemistry, fullerenes, and organic solar cells 203important implications when it comes to the possible noncovalent interactions that we can utilize toassociate fullerenes in solution. The impossibility of utilizing strong directional forces, like metal–ligandcoordination or hydrogen bonding, to recognize fullerenes, leaves us with π–π, van der Waals, andsolvophobic interactions as key factors to consider. Fortunately, optimizing all of them requires thesame strategy: increasing the surface of the receptor in short contact with the fullerene guest. Thus, thenuts and bolts of the design of hosts for fullerenes is the construction of a nonpolar cavity—preferentiallybut not necessarily featuring aromatic recognizing units—of the appropriate size to fit thefullerene guest.The search for molecules capable of associating fullerenes in solution was initiated just sevenyears after the discovery of C 60 and immediately after it became available in sufficient quantities bycontact-arc vaporization of graphite [12]. In 1992, the first designed host for fullerenes was reported byDiederich, Ringsdorf, and co-workers [13]. The receptor was based on a macrocyclic azacrown ether inwhich the nitrogen groups were alkylated or acylated with aromatic groups, further substituted withlong alkane chains, forming a lipophilic “cup”. Through a combination of surface pressure-area diagrams,UV–vis spectroscopy, and atomic force microscopy (AFM), the authors showed that thefullerenes preferentially dwelled inside the lipophilic cavities.In the same year, Wennerström’s group described the encapsulation of C 60 by two units ofγ-cyclodextrin to obtain water-soluble complexes [14], extending the same concept to C 70 two yearslater [15]. The positive cyclodextrin–fullerene interaction, particularly in water, has been exploited tobuild organized fullerene nanostructures [16–20]. At approximately the same time, the groups of Rastonand Shinkai, working independently, reported the selective purification of C 60 from fullerene soot byits selective association with p-tert-butylcalix[8]arene [21,22]. Drawing from these seminal investigations,several receptors based on calixarenes—mainly calix[5]arenes—have been described [23–34].Cyclotriveratrylenes (CTVs) have also been extensively investigated for the molecular recognitionof fullerenes. CTV was first found to interact with C 60 forming 1:1 complexes in the solid state[35]. Later, mainly by extension of the cavity of CTV or by the construction of dimeric capsules, a richcollection of hosts that work in solution and on surfaces have been conceived and synthesized [36–44].Porphyrins are well known to interact favorably with fullerenes both in solution and in the solidstate, which has been thoroughly exploited in the design of receptors for fullerenes [45,46]. In the para -digmatic example, Aida reported the formation of very stable inclusion complexes between fullerenesand a dimeric construct in which two metalloporphyrins are linked by flexible alkyl spacers, forming amacrocycle [47]. Structural variations on this design have led to what is probably the richest collectionof receptors for fullerenes [48], including the world-record holder in complex stability, with log K a =8.1 in 1,2-dichlorobenzene at room temperature [49], and the first chiral sensors for the inherently chiralhigher fullerenes [50,51]. Besides Aida’s work on macrocyclic structures, several molecular tweezersfeaturing porphyrins as binding motifs have also been reported [52,53], spearheaded by the work byBoyd and Reed on their “jaws” receptors [54–56]. More recently, concave recognition motifs [57,58],complementary to the convex surface of the fullerenes, like corannulene [59–61], or cyclo -phenyleneacetylenes, have been explored [62–65].© 2010, IUPAC Pure Appl. Chem., Vol. 83, No. 1, pp. 201–211, 2011