Scott research Group

Most Recent Areas of Research

The directed laboratory synthesis of carbon nanotubes, from the ground up, by chemical methods, stands as one of the great unmet challenges in chemistry today. Far more than just an academic exercise, the successful development of versatile chemical methods for building single-walled nanotubes (SWNTs) will someday deliver useful supplies of uniform diameter nanotubes of preselected structural type for advanced applications in nanoscale electronics and other fields that require homogeneous samples of these remarkable materials.

The various empirical methods discovered and developed since the early 1990s for preparing carbon nanotubes, unfortunately, all yield mixtures of armchair, zig-zag, and chiral nanotubes. Techniques to separate these mixtures into their individual components have been vigorously sought by many research groups, and varying degrees of success on a sub-micromolar scale have been reported for some very clever strategies, but usable quantities of pure, single-index [n,m] nanotubes remain unobtainable. The logical alternative to separating SWNTs from mixtures is to synthesize them by controlled chemical methods so that all the nanotubes will have the same diameter and the same sidewall structure by design. Our approach to solving this problem relies heavily on lessons learned from our 12-step chemical synthesis of Buckminsterfullerene, C60 (Science 2002, 295, 1500). We plan first to synthesize a small hydrocarbon template, such as a short, hemispherical nanotube end-cap, by bottom-up chemical synthesis, and then to elongate it; the diameter and rim structure encoded in the template will dictate the diameter and sidewall structure of the resulting nanotubes. The following scheme outlines this approach for the case of [5,5] SWNTs:

Nanotube end-caps of different diameters represent distinct target molecules that will each require a different multi-step synthesis; however, a single, universal process is envisioned for “growing” armchair nanotubes from all such templates. The lengths of the nanotubes synthesized in this manner will not be controlled, other than by varying the time allowed for the polymerization, but length is not a critical parameter. Armchair nanotubes were chosen as our initial objectives, because they will all be electrically conductive (metallic) and can therefore serve as nanowires in molecular scale devices.

Our synthesis of pentaindenocorannulene (2) in three steps from corannulene (1) and the X-ray structure of 2 were first reported in J. Am. Chem. Soc. 2007, 129, 484. Simultaneously, we began exploring new C2 addition chemistry to transform bay regions of polycyclic aromatic hydrocarbons into new unsubstituted benzene rings. A reaction that can effect such benzannulations under fixed conditions, without requiring any intervention along the way, will be the key to elongating small hydrocarbon templates into SWNTs by this polymerization strategy, because the growing nanotube can never run out of bay regions. We have focused initially on an iterative Diels-Alder cycloaddition/rearomatization sequence, depicted in the following scheme formally with acetylene as the dienophile (J. Am. Chem. Soc. 2009, 131, 16006):

To address the problem posed by the low dienophilicity of acetylene, we developed a new “masked acetylene” that can serve as the reagent for metal-free growth of short hydrocarbon templates. Thus, we have found that nitroethylene will transform the bay regions of polycyclic aromatic hydrocarbons such as the soluble bisanthene derivative shown below into new, unsubstituted benzene rings in a single operation, and the reaction continues until it runs out of bay regions (Angew. Chem., Int. Ed. 2010, 49, 6626). Nitroethylene is a sufficiently potent dienophile to temporarily destroy the aromaticity of two benzene rings in a Diels-Alder cycloaddition. Thermal loss of dihydrogen then rearomatizes the original benzene rings, and a subsequent thermal loss of HNO2 aromatizes the new ring. Semiconducting chiral SWNTs should also be accessible by elongation of suitable chiral templates using the Diels-Alder cycloaddition/rearomatization strategy.

Progress has been made in these laboratories to synthesize geodesic polyarenes that can serve as templates from which to grow [5,5], [6,6], and [10,10] carbon nanotubes. In this connection, a short, rigid, structurally pure [5,5] SWNT has been synthesized from the ground up by stepwise chemical methods
(J. Am. Chem. Soc. 2012, 134, 107-110).

This C50H10 geodesic polyarene has been isolated, purified, crystallized, and fully characterized by NMR spectroscopy, UV-vis absorption spectroscopy, high resolution mass spectrometry, and X-ray crystallography.

I thank the National Science Foundation and the Department of Energy for generous, long-term financial support.