Receptors for Pristine Fullerenes Based on Concave–Convex π–πInteractions

Abstract

In 1990, Kr€ atchmer and coworkers reported the extraction of fullerenes (C 60 and C 70 ) from carbon soot using aromatic solvents (Figure 3.1) [1]. After this discovery, receptor molecules for fullerenes have been extensively studied for separation and purification of fullerenes. Supramlecular chemists have found that traditional host molecules composed of electron-rich aromatic rings such as calix[n]arenes 1-4, oxacalix[3]arenes 5, and cyclotriveratrylenes (CTV) 6 ( Figure 3.2) can be employed for the purpose. The crystallographic analyses of these complexes with C 60 clearly indicated the importance of the p-p interactions between the convex p surface of C 60 and the concave p surface of the traditional host molecules. On the other hand, except a few examples, the traditional hosts bound fullerenes only in solid state. In order to bind with fullerenes more tightly, two strategies have been presented to construct the well preorganized p cavity so far. First, electron-rich aromatic units are introduced to the host molecule as appendants (Figure 3.2a). Second, two host molecules are linked by appropriate tether(s) to form a dimeric structure with a creft-, ring-, or ball-shaped cavity (Figure 3.2b). The binding abilities of the modified receptors fairly increased in comparison with those of simple traditional hosts. On the other hand, new receptors based on curved conjugated systems such as corannulene 7, extended TTF (exTTF) 8, and carbon nanorings (Figure 3.3) have been developed recently. These compounds possess a concave p surface matching to the convex surface of C 60 . Cyclic [6]paraphenyleneacetylene ([6]CPPA) 9 and the related compounds have smooth belt-shaped structure similar to a cut piece of carbon nanotube, and thus may be termed carbon nanorings. These receptors form stable inclusion complexes with fullerenes both in solution and in the solid state, and are good model compounds to explore the nature of the concave-convex p-p interactions. This chapter discusses the fullerene receptors bearing a cavity surrounded by p-orbitals, the so-called p cavity, the concept of molecular designs, and the nature of p-p interactions. The survey of the fullerene receptors should provide an insight into the supramolecular properties of curved conjugated systems. Although the receptor Supramolecular Chemistry of Fullerenes and Carbon Nanotubes, First Edition. Edited by Nazario Martin and Jean-Francois Nierengarten.

Azabuckybowl-Based Molecular Tweezers as C60 and C70 Receptors

“…The solid-state structures of 2b·C 60 and 2b·C 70 were unambiguously determined by single-crystal X-ray diffraction analysis (Figure ), which revealed 1:1 compositions similar to those observed in solution. In the crystalline state, C 60 and the azabuckybowl units mutually interact in a concave–convex fashion. , The penetration depth of C 60 into 2b measured from the centroid of the pyrrole ring of 2b to the centroid of C 60 is 6.83 Å, which is consistent with that of 1 . The closest distance between C 60 and an azabuckybowl unit is 3.14 Å.…”

Buckyballs

“…There are several ways to obtain such supramolecular assemblies; among them several systems may be obtained by interactions between functionalized polymers and C 60 derivatives or fullerene itself or through the assembly of self-complementary C 60 derivatives. More recently, assemblies between ditopic concave guests and [60]fullerene by means of concave-convex complementary interactions have also been reported [89,90].…”

Selective recognition and enrichment of C70 over C60 using an anthracene-based nanotube

“…Since Krätschmer and coworkers reported the extraction of C 60 and C 70 from carbon soot by using aromatic solvents in 1990, 1 a number of articial receptors have been extensively developed for the recognition and separation of fullerenes. 2,3 Most traditional macrocyclic hosts and their derivatives, such as g-cyclodextrin (g-CD), [4][5][6][7] calix[n]arenes, [8][9][10][11][12][13] pillar [10]arene, 14 and theoretical cucurbit [9]uril (CB [9]), 15 have shown similar binding affinities to fullerene compounds including C 60 and C 70 . There are still many challenges in efficiently distinguishing C 60 and C 70 through molecular recognition.…”