Helical Wrapping of Single-Walled Carbon Nanotubes by Water Soluble Poly(p-phenyleneethynylene) Youn K. Kang, †,‡ One-Sun Lee, † Pravas Deria, †,| Sang Hoon Kim, § Tae-Hong Park, † Dawn A. Bonnell,* ,§ Jeffery G. Saven,* ,† and Michael J. Therien* ,| Department of Chemistry, UniVersity of PennsylVania, 231 South 34th Street, Philadelphia, PennsylVania 19104, DiVision of Chemistry and Molecular Engineering, Department of Chemistry, College of Natural Sciences, Seoul National UniVersity, Seoul 151-747, Korea, Department of Materials Science and Engineering, UniVersity of PennsylVania, 3231 Walnut Street, Philadelphia, PennsylVania 19104, and Department of Chemistry, French Family Science Center, 124 Science DriVe, Duke UniVersity, Durham, North Carolina 27708 Received October 26, 2008; Revised Manuscript Received January 13, 2009 ABSTRACT Amphiphilic, linear conjugated poly[p-{2,5-bis(3-propoxysulfonicacidsodiumsalt)}phenylene]ethynylene (PPES) efficiently disperses single- walled carbon nanotubes (SWNTs) under ultrasonication conditions into the aqueous phase. Vis-NIR absorption spectroscopy, atomic force microscopy (AFM), and transmission electron microscopy (TEM) demonstrate that these solubilized SWNTs are highly individualized. AFM and TEM data reveal that the interaction of PPES with SWNTs gives rise to a self-assembled superstructure in which a polymer monolayer helically wraps the nanotube surface; the observed PPES pitch length (13 ( 2 nm) confirms structural predictions made via molecular dynamics simulations. This work underscores design elements important for engineering well-defined nanotube-semiconducting polymer hybrid structures. Despite the extraordinary optical, 1-5 electronic, 6,7 electri- cal, 8-12 and mechanical 13-17 properties of single-walled carbon nanotubes (SWNTs), 18,19 the notorious insolubility of these structures that derives from strong intertube van der Waals interactions 20,21 limits their full utilization in materials and devices. Perhaps the most exploited method to overcome this obstacle relies upon ultrasonication enforced segregation of nanotube strands out of entangled bundles, followed by SWNT surface noncovalent 22 or covalent chemical function- alization. Preserving key optical, electrical, and structural properties of SWNTs requires noncovalent solubilization strategies; a wide-range of surfactants, small molecules, and polymers have been utilized for this purpose and have been the subject of a recent review by Tasis and Prato. 23 Among these agents, polymers offer the potential to evolve new functional materials through well-defined SWNT-polymer interactions. Flexible polymers, congruent with intuition, provide structures capable of wrapping the SWNT surface, facilitating dispersion. 24-28 Conjugated polymer-SWNT blends often exhibit synergistic optoelectronic effects; for example, it has been established that such compositions can manifest dramatically enhanced conductivities and charge mobili- ties. 29-33 Of these materials, it has been established that poly(m-phenylenevinylene) (PmPV) helically wraps SWNTs. 27,34 Related work reports that poly(p-phenylene- ethynylene) (PPE) interacts with the nanotube surface in a parallel mode and plays a role in assembling a ribbon-type super structure of SWNTs; 35 in contrast, in situ polymeri- zation of phenylacetylene in the presence of nanotubes suggests the formation of helical polymer-SWNT structures. 24 Further, recent works have established that regardless of morphology, linear, conformationally restricted conjugated polymers can function as dispersing agents that solubilize SWNTs in the nonaqueous phase. 36-38 While it is widely accepted that conjugated polymers bind SWNT surfaces through π-π interactions, there remains a general need to establish rules and principles that guide the assembly of individualized SWNT-semiconducting polymer structures having a well-defined morphology. Water-soluble poly[p-{2,5-bis(3-propoxysulfonicacid- sodiumsalt)}phenylene]ethynylene (PPES, shown in Chart * To whom correspondence should be addressed. (D.A.B.) Tel: +215- 898-6231. Fax: +215-746-3204. E-mail: bonnell@lrsm.upenn.edu. (J.G.S.) Tel: +215-573-6062. Fax: +215-573-0980. E-mail: saven@sas.upenn.edu. (M.J.T.)Tel:+919-660-1670.Fax:+919-660-1605.E-mail:michael.therien@duke.edu. † Department of Chemistry, University of Pennsylvania. ‡ Seoul National University. § Department of Materials Science and Engineering, University of Pennsylvania. | Duke University, Durham. NANO LETTERS 2009 Vol. 9, No. 4 1414-1418 10.1021/nl8032334 CCC: $40.75  2009 American Chemical Society Published on Web 03/12/2009