Filling Helical Nanotubes with C₆₀

Abstract

We report herein the complexation of a string of buckminsterfullerenes in chloroform by our recently discovered selfassembled, helical nanotubes.[1] The poorly soluble C 60 molecules are solubilized by the nanotubes and they also experience the helicity of the environment, as demonstrated by induced circular dichroism (ICD). Previously reported C 60 receptors-mostly aza-crown ethers, calixarenes, and porphyrins [2] -require lengthy syntheses and only bind isolated fullerenes. Our polymeric, dynamic receptor is prepared in one step and represents an entirely new class of C 60 -complexing structures; it also allows the hitherto unknown formation of a one-dimensional C 60 array.We recently reported the synthesis [3] of a-amino acid functionalized naphthalenediimides (Scheme 1) and their self-assembly into hydrogen-bonded nanotubes.[1] The crystal structure of these helical superstructures shows the presence of tubular cavities with a mean diameter of 12.4 , which is well-suited to accommodate C 60 molecules with a van der Waals (vdW) radius of about 10.3 . [4] Colorimetric studies were carried out to test the C 60 complexation ability of the helical nanotubes. Solutions of l-and d-1-3 as well as 5 in chloroform were left to stand over solid C 60 . A drastic color change from pale yellow to dark orange or brown was seen within a few minutes. This color change is quite different from that observed upon mere addition of a saturated solution of C 60 in chloroform (pale purple) to the initial solution (Figure 1, inset). The visible region of the absorption spectrum (Figure 1, inset) is marked by the appearance of the absorbance bands characteristic of C 60 , as well as an additional, broad band centered around 452 nm (identified with * ). This band, best visualized by subtracting the naphthalenediimide (NDI) spectrum from the spectrum of the complex (Figure 1, blue trace), is very similar to an absorbance normally only seen in C 60 films [6] and aggregates; [7] it has been attributed to interactions between fullerenes.[8] This proximity effect, which has not previously been reported [4] for C 60 constrained within channels, indicates that the fullerenes are tightly packed within our system, although we cannot rule out a contribution from NDI-C 60 interactions.The uptake of C 60 corresponds to the appearance of the bands at 258 and 328 nm in the UV region of the absorption spectrum of 1 + C 60 . Comparison of a solution of 1 + C 60 in chloroform with known NDI concentration and a saturated solution of C 60 in chloroform allows calculation of the C 60 uptake. We found that depending on NDI concentration, and, to a lesser degree, on the amino acid side chain, the C 60 concentration increased up to 16-fold in the presence of NDI nanotubes relative to the solubility of C 60 in chloroform.The amount of C 60 taken up into the nanotubes is in good agreement with C 60 being contained inside the nanotubular core rather than interacting with the outside of the tubes. To quantify the ratio of NDI to C 60 molecules, we recorded the a...

Nanofibers of Fullerene C60 through Interplay of Ball‐and‐Socket Supermolecules

“…Fullerenes, as the only form of molecular carbon, are in a unique position to be used for these processes. A recent example of this is the formation of a one-dimensional C 60 array in solution templated by helical nanotubes [15] An area that has attracted significant attention in recent years is the supramolecular chemistry of fullerenes involving cavitands, with particular interest being devoted to calixarenes and related molecules. Atwood et al [16] and Suzuki et al [17] initially developed a convenient and efficient purification method for obtaining C 60 from "crude" fullerene soot, which is based on the formation of a discrete complex with p-tBu-calix [8]arene.…”

Proton‐Driven Switching Between Receptors for C60 and C70

“…[3][4][5] The NDI nanotubes are held together by classical intermolecular COOH-HOOC hydrogen bonds supplemented by weak CH···O = C bonding (Figure 1 a, c).…”

Templated Amplification of an Unexpected Receptor for C70

“…[3] We now show that C 70 templates the formation of a new discrete receptor at the expense of preformed nanotubes.[4] This behavior, which resembles that of a dynamic combinatorial system, [5] was unexpected. As C 60 and C 70 share the same small diameter (Figure 1 b), we had anticipated that the same complexation mode would be possible (Figure 1 c).…”