Phase Transfer Catalysts Drive Diverse Organic Solvent Solubility of Single-Walled Carbon Nanotubes Helically Wrapped by Ionic, Semiconducting Polymers Pravas Deria, † Louise E. Sinks, ‡,| Tae-Hong Park, ‡ Diana M. Tomezsko, ‡ Matthew J. Brukman, § Dawn A. Bonnell, § and Michael J. Therien* ,† † Department of Chemistry, French Family Science Center, 124 Science Drive, Duke University, Durham, North Carolina 27708, ‡ Department of Chemistry, University of Pennsylvania, 231 South 34th Street, Philadelphia, Pennsylvania 19104, and § Department of Materials Science and Engineering, University of Pennsylvania, 3231 Walnut Street, Philadelphia, Pennsylvania 19104 ABSTRACT Use of phase transfer catalysts such as 18-crown-6 enables ionic, linear conjugated poly[2,6-{1,5-bis(3-propoxy- sulfonicacidsodiumsalt)}naphthylene]ethynylene (PNES) to efficiently disperse single-walled carbon nanotubes (SWNTs) in multiple organic solvents under standard ultrasonication methods. Steady-state electronic absorption spectroscopy, atomic force microscopy (AFM), and transmission electron microscopy (TEM) reveal that these SWNT suspensions are composed almost exclusively of individualized tubes. High-resolution TEM and AFM data show that the interaction of PNES with SWNTs in both protic and aprotic organic solvents provides a self-assembled superstructure in which a PNES monolayer helically wraps the nanotube surface with periodic and constant morphology (observed helical pitch length ) 10 ( 2 nm); time-dependent examination of these suspensions indicates that these structures persist in solution over periods that span at least several months. Pump-probe transient absorption spectroscopy reveals that the excited state lifetimes and exciton binding energies of these well-defined nanotube-semiconducting polymer hybrid structures remain unchanged relative to analogous benchmark data acquired previously for standard sodium dodecylsulfate (SDS)-SWNT suspensions, regardless of solvent. These results demonstrate that the use of phase transfer catalysts with ionic semiconducting polymers that helically wrap SWNTs provide well-defined structures that solubulize SWNTs in a wide range of organic solvents while preserving critical nanotube semiconducting and conducting properties. KEYWORDS Single chain, helical wrapping, ionic poly(aryleneethynylene), phase transfer catalyst, SWNTs, organic solvent S ingle wall carbon nanotubes (SWNTs) 1-4 possess a wide range of uncommon mechanical, 5-9 opti- cal, 1,10-12 electrical, 13-17 magnetic, 18-21 and ther- mal 22,23 properties that fuel multidisciplinary efforts aimed at developing neoteric nanoscale, microscale, and bulk- phase materials. 2 Strong van der Waals interactions between SWNTs 24,25 are an anathema to SWNT solubilization. 13 Water-solubilized SWNTs dominate the literature; there exists no general method to disperse SWNTs in nonaqueous media. Potential dispersion approaches are truncated se- verely by the fact that preservation of significant SWNT electrooptic properties require solubilization via agents that noncovalently interact with the nanotube surface. 24,25 It has thus been a long-standing goal to develop a universal SWNT solubilization strategy that (i) relies on noncovalent interac- tions for nanotube dispersion, (ii) provides a high yield of individualized tubes, (iii) enables dispersion in a wide range of dielectric media, and yet (iv) gives rise to suspended SWNT structures having a constant morphology, regardless of solvent. Ultrasonication 11,26 and high-speed vibrational milling techniques 27 are commonly utilized to drive exfoliation of nanotube ropes and bundles in the presence of surfactants, small molecules, and polymers. 28 The huge library of estab- lished noncovalent nanotube solubilizing agents is domi- nated by species that only exfoliate tubes in an aqueous medium; surfactants such as SDS, 11 SDBS, 29,30 and sodium cholate, 31 and amphiphilic aromatic molecules, 32 typify these SWNT-solubilizing compositions. Flexible polymers such as polystyrene sulfonate (PSS), polyvinyl pyrrolidone (and their poly acrylic and maleic acid-containing copoly- mers), 33 R-helical amphiphilic peptides, 34 DNA, 35-37 sodium carboxymethylcellulose (Na-CMC), 38 and -1,3-glucans 39 also wrap SWNTs in water. Among these agents, only -1,3- glucans and DNA wrap individualized SWNTs with a fixed periodicity; notably, neither of these biological polymers disperse SWNTs in organic solvent to provide well-defined structures of similar morphology. Further, multiple studies * To whom correspondence should be addressed. Tel: 919-660-1670. Fax: 919- 660-1605. E-mail: michael.therien@duke.edu. | Present address: Department of Biochemistry and Biophysics, University of Pennsylvania, 317 Anatomy-Chemistry Bldg., 36th Street and Hamilton Walk, Philadelphia, PA 19104. Received for review: 07/20/2010 Published on Web: 09/01/2010 pubs.acs.org/NanoLett © 2010 American Chemical Society 4192 DOI: 10.1021/nl102540c | Nano Lett. 2010, 10, 4192–4199