Carbon Nanotubes

Establishing the Supramolecular Chemistry of Carbon Nanotubes

More than a decade has passed since the discovery¹ of single-walled carbon nanotubes (SWNTs) in 1993. Nowadays they are prized on account of their unique structural and remarkable physical properties.² They have the potential of being transformed into new materials that will traverse a wide range of applications.³ Despite their obvious potential, SWNTs have, unfortunately, not yet been fully integrated, with a high degree of control over their constitutional isomers and spatial placements, into highly sophisticated SWNT devices. This situation has probably arisen because of the difficulty of transforming them into soluble materials that can be easily manipulated in either organic or aqueous solutions. Although the solubilization of SWNTs can be achieved⁴ by their covalent sidewall functionalization, leading to the improved manipulation of nanotubes, all these covalent functionalizations destroy the extended þ-networks on their surfaces, diminishing both their desirable mechanical and electronic properties. On the other hand, noncovalent supramolecular modifications that involve polymer wrapping^5-8 of the nanotubes’ surfaces preserves these desired properties, while remarkably improving their solubilities. The main focus of this paper is to present our progress toward forming soluble polymer/SWNT composite materials and to highlight the emergent properties of these new materials in prototypical SWNT-device settings.

So far, we and others have reported the solubilization of SWNT ropes and bundles in both organic^8d-e and aqueous^7c,8b,9 solutions as a result of their noncovalent association with polymers. The types of polymers employed in our solubilization studies range from the water-soluble helical polymer amylose^8b (1) found in starch, to organic soluble polymers such as poly(m-phenylenevinylene)-co-(2,5-dioctoxy-p-phenylene)vinylene^8c (2a, PmPV) and a variety of derivatives, including poly(2,6-pyridinlenevinylene)-co-(2,5-dioctoxy-p-phenylene)vinylene^8c (2b, PPyPV), and poly(5-alkoxy-m-phenylenevinylene)-co-(2,5-dioctoxy-p-phenylene)vinylene^8a (3, PAmPV) (Figure 1).

Previous experiments in our laboratory with starch^8b have revealed that, although carbon nanotubes are not soluble in an aqueous solution of starch, they are soluble in an aqueous solution of the starch-iodine complex (Figure 2). The reversible water solubilization of SWNTs in water using starch may provide the means for developing fully integrated biological nanotube devices. These observations suggest that iodine preorganizes the backbone of the amylose in starch into a helical conformation and makes its hydrophobic cavity accessible to a single carbon nanotube or bundles thereof. The formation (Figure 2) of such starch-wrapped SWNT complexes is driven by simultaneous enthalpic and entropic gains that result from creating favorable van der Waals interactions and from expelling the many small iodine molecules located inside the helix out into the solvent by a "pea-shooting" type of mechanism. This result has led to a simple protocol for the cleaning up of SWNTs with starch. The reversible solubilization of starch-wrapped SWNTs in water was further investigated by the action of enzymatic hydrolysis, using the commercially available amyloglucosidase from Rhizopus mold which removes the starch polymer-coat under very mild conditions. Addition of this enzyme to an aqueous solution of the starch-wrapped SWNTs resulted in the precipitation of all the nanotubes within 10 minutes, as indicated by light-scattering measurements and also by changes that are clearly visible to the naked eye. The fact that the supramolecular chemistry operating between SWNTs and amylose can be conducted under physical, chemical, or biological control constitutes an important scientific development with far-reaching implications for both carbon nanotube and starch research. It is now a simple matter to purify SWNTs cheaply, under ambient conditions, using the readily available starch-iodine complex.

A simple procedure for dispersing SWNTs in aqueous solutions of gum arabic has also been published recently.^9,10b In an earlier report by Chen et al.,^10a SWNTs were shown to form stable aqueous suspensions when milled with beta- and gamma-cyclodextrins. Recently Dodziuk et al.^10b reported that subsequent milling of beta- and gamma-cyclodextrin treated nanotubes with nu-cyclodextrin results in the formation of stable SWNT suspensions and present evidence that supports the partial sorting of SWNTs with respect to their diameters. The authors suggest that one or more of these nu-cyclodextrin can effectively thread around the surfaces of single SWNTs, forming a polypseudorotaxane and thus solubilizing nanotubes while discriminating their diameters. It is also obvious that opportunities now exist to form complexes between SWNTs and a range of different amylose derivatives^7c and peptides^9a, so enabling their deeper integration with other biosystems.

Unlike amylose (1), PmPV (2a) behaves as a photonic material, a property which means that it has distinctive UV/Vis absorption and fluorescence spectra, and can harvest light. When appropriately doped, PmPV devices exhibit photovoltaic responses. Add on to these virtues of PmPV, the fact that SWNTs have high aspect ratios, making them ideal candidates for bridging across electrodes for charge-transport measurements. We have obtained results^8c,e from two experiments that indicate that the PmPV polymer is in intimate electrical contact with the nanotubes and that the polymer wraps itself around bundles of tubes, rather than around individual tubes which then aggregate to form ropes. The two experiments relate to (1) the photoconductivity response of a single polymer-wrapped SWNT bundle and (2) some two-photon fluorescence (2PF) measurements of single SWNT/PmPV bundles which correlate with structural measurements obtained by atomic force microscopy (AFM). Control experiments^8e on unwrapped SWNT devices exhibited no optically modulated response and only PmPV-wrapped tubes show the on/off difference for negatively and positively biased junctions. The magnitude of the change in current is similar and corresponds to ca. 15-20% of the total current. Thus, the current passing through the SWNT/PmPV device is photoamplified for a positive bias, but is photorectified for a negative applied bias. This feature is an intrinsic property of PmPV-wrapped SWNTs and is not reversed by changing the connections to the two electrodes. The wavelength dependence of the optically gated conductivity in PmPV-wrapped SWNT ropes was recorded from 375-805 nm in 5-10 nm measurements. The absolute value of the wavelength dependence of the illumination on the current response, along with the polymer absorption spectrum, was shown to have a correlation between the polymer absorption spectrum and the wavelength of the photomodulated conductivity of these devices.

The interaction between SWNTs and an analogue of PmPV, namely the pH-sensitive polymer, poly{(2,6-pyridinylenevinylene)-co-[(2,5-dioctyl-oxyparaphenylene) vinylene]} (2b) (PPyPV) has also been investigated.^8c The fundamental difference between PmPV and PPyPV is that the latter is a base and is readily protonated by addition of acid (Scheme 1). We have found that the SWNT/PPyPV interaction lowers the pKa of PPyPV. Also, for PPyPV, the wavelength dependence correlates with the absorption spectrum of the protonated PPyPV, indicating that the SWNTs assist in charge stabilization. This observation is an important one in relation to future research projects we have in mind. All of these complexes are formed on account of stabilizing noncovalent bonding interactions, presumably as a result of þ-þ stacking and van der Waals interactions between PmPV and the surfaces of the SWNTs. Molecular modeling studies on the PmPV/SWNT complex have revealed that the polymer's backbone is very flexible, allowing the polymer to wrap itself around SWNT bundles of different diameters without any discrimination.

Stilbene-like dendrimers, which may be regarded as a hyperbranched analogue of PmPV, possess well-defined cavities.8d Molecular modeling studies suggest (Figure 4) that only single nanotubes will fit inside these cavities. We have synthesized such a hyperbranched polymer. It was found to be more efficient at breaking up nanotube bundles, provided it is employed at higher polymer-to-nanotube ratios, than the parent PmPV polymer. Introducing a certain degree of branching into the PmPV polymer makes it more rigid and less efficient when it comes to wrapping bundles of SWNTs and so more polymeric material is required to achieve a surface coverage for nanotube dispersion and solubility. However, the pockets provided by the hyperbranched polymer offer a better fit for SWNTs. The outcome is that more single strands of SWNTs are observed on a mica wafer by atomic force microscopy than was the case when the PmPV polymer was used.

We have also recently^8a synthesized and characterized a variety of PmPV derivatives functionalized in the synthetically accessible C-5 position of the meta-disubstituted phenylene ring, affording poly{(5-alkoxymetaphenylenevinylene)-co-[(2,5-dioctyloxy-p-phenylene)-vinylene]} (PAmPV) derivatives. PAmPV Derivatives which bear tethers or rings form pseudorotaxanes with rings and threads, respectively.

The noncovalent functionalization of bundles of carbon nanotubes with conducting polymers that have the capacity to form pseudorotaxanes represents (Figure 5) the operation of supramolecular phenomena at three different levels of superstructure, viz., (1) the aggregation of the nanotubes into bundles, (2) the wrapping of the bundles by the polymer, and (3) the formation, through the side-arms attached to the polymer, of threaded complexes. These results conjure up the prospect of developing arrays of molecular actuators and switches in the future.

Even though these approaches significantly enhance the solubility and the manipulation of SWNTs, they lack the ability to discriminate and sort single tubes based on their diameters and lengths. In light of this deficiency, we are pursuing11 novel solubilization and purification methods that rely on the dynamic noncovalent encirclement of SWNTs with linear divergent ligands and charges complexes of palladium (Figure 6).

The cavity size of the cyclic and acyclic products can be designed to accommodate and discriminate between a range of SWNT diameters by employing readily available linear organic compounds and transition metal complexes of suitable geometries (Figure 7). The formation of these organized charged complexes can offer alternative routes toward future diameter- and length-selective separations and manipulations of SWNTs.

References

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