12298 Chem. Commun., 2011, 47, 12298–12300 This journal is c The Royal Society of Chemistry 2011 Cite this: Chem. Commun., 2011, 47, 12298–12300 Solution structure of copper-seamed C-alkylpyrogallol[4]arene nanocapsules with varying chain lengthsw Harshita Kumari, a Steven R. Kline,* b Nathaniel J. Schuster a and Jerry L. Atwood* a Received 10th September 2011, Accepted 3rd October 2011 DOI: 10.1039/c1cc15615b The stability of copper-seamed C-alkylpyrogallol[4]arene hexamers with varying chain lengths in solution has been studied using small-angle neutron scattering (SANS). The progression in diameter of spherical capsules with increasing alkyl chain lengths of copper-seamed hexamers in solution suggests both robustness as well as a close correlation between the solid phase and solution phase structures. Naturally occurring macromolecules like DNA serve as excellent examples of self-assembled molecular superstructures. 1 Their self-assembly process can be described as bringing together complementary molecular entities that have the propensity to self-orient into functional molecular devices. Self-assembled hosts are an especially important class of molecular superstruc- tures. They include bilayers, 2 molecular channels, 3 cylindrical tubes 4 and spherical nanocapsules and have the capacity to mimic viral geometries. 5 Amongst these hosts, metal–organic nanocapsules (MONCs) capture extensive attention owing to their potential to accommodate guests, 6 show magnetic inter- actions, 7 and self-assemble into various conformations. 5 The spontaneous formation of a spherical molecular assembly from six C-methylresorcin[4]arene macrocycles and eight water molecules focused attention on nanocapsule research. This assembly is seamed together by 60 hydrogen bonds and possesses a cavity of approximately 1375 A ˚ 3 . 8 Similar to C-alkylresorcin[4]arenes, C-alkylpyrogallol[4]arenes (PgC n , where n = alkyl chain length) self-assemble into hexameric nanocapsules. Seamed together by 72 hydrogen bonds, pyrogallol[4]arene hexamers are more robust than their resorcin[4]arene counterparts. Indeed, the incorporation of water molecules into their hydrogen-bonded frameworks has not been reported. The cavities of the PgC n hexameric nano- capsules are large enough to accommodate fluorescent mole- cular probes. 6,9–12 Crystallographic studies later revealed that the replacement of hydrogen ions in the hydrogen-bonded framework of the PgC n nanocapsules with metal ions produced especially robust nanocapsules with dimensions similar to hydrogen-bonded hexamers. 13–16 In the formation of these metal–organic assemblies, both metal ions and solvent mole- cules were observed to direct the shape and size of the MONCs. For example, the pyridine solvent favours the formation of dimeric zinc- or nickel-seamed nanocapsules while methanol or acetone provide solvent environment pre- ferable for the self-assembly of copper- or gallium-seamed hexamers. 13–20 However, the rationale for the preferred metal- solvent selectivity, their mechanisms of formation and the parameters governing their architectures are yet to be deter- mined. Our initial investigations explored the stability, shape, and size of MONCs in solution. It is clear that knowledge of MONC cavity volume and stability are key for the use of MONCs for applications such as drug delivery or guest entrapment. 21 In this contribution, we investigate the shape and size of copper-seamed C-alkylpyrogallol[4]arene nanocapsules PgC n Cu (n = 3, 6, 9) upon change of the alkyl chain lengths using small-angle neutron scattering (SANS). Unlike diffusion NMR or DLS, SANS, without assumption, can measure the size, shape, and internal arrangement of nanostructures. PgC n Cu MONCs were prepared by adding a methanol solution of PgC n (0.01 mol L À1 ) to a methanol solution of Fig. 1 Single crystal XRD structure for C-propylpyrogallol[4]arene metal-seamed hexamer illustrating the approximate radius of the capsule. For clarity only one of the pyridine ligands attached to metal ion framework is shown. O: red, M (metal): turquoise, C: green, N: blue. a Department of Chemistry, University of Missouri-Columbia, 601 S. College Avenue, Columbia, MO 65211, USA. E-mail: AtwoodJ@missouri.edu b NIST Center for Neutron Research, Gaithersburg, MD 20899-6102, USA. E-mail: Steven.kline@nist.gov; Fax: +1 573 882 2754; Tel: +1 573 882 8374 w Electronic supplementary information (ESI) available. See DOI: 10.1039/c1cc15615b ChemComm Dynamic Article Links www.rsc.org/chemcomm COMMUNICATION Downloaded by National Institutes of Standards & Technology on 27 June 2012 Published on 25 October 2011 on http://pubs.rsc.org | doi:10.1039/C1CC15615B View Online / Journal Homepage / Table of Contents for this issue