1 © 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim wileyonlinelibrary.com 1. Introduction Biological cells can change morphologies in response to envi- ronmental conditions or their internal stage, such as in the cell cycle or in development. Similar to biological systems, the 2D graphite building material can be modulated into 0D bucky balls, 1D carbon nanotubes or 3D graphite sheets. [1] Such structural transformations and molecular recognition provides an opportunity to synthesize molecular hosts with varying architectures and huge internal cavities that can be utilized for guest encapsulation, gas storage or other poten- tial applications. [1,2] The complexity in building large intricate Investigating Structural Alterations in Pyrogallol[4] arene-Pyrene Nanotubular Frameworks Harshita Kumari, Steven R. Kline, Wei G. Wycoff, and Jerry L. Atwood* architectures can be addressed by achieving control over inter- molecular interactions, in particular hydrogen bonding, that assist in the formation of stable supramolecular entities. [2c,3] Such control has been accomplished in dendrisosomes, [4] p-sulfonatocalix[4]arene [5] and pyrogallol[4]arene-based self- assemblies that is evidenced by their huge library of varying architectures. [6] Supramolecular scaffolds range from zeo- lite-like helixes of Co(III) sepulchurate [Co(diHOsar)] 3 + and Na p-sulfonatocalix[4]arene [7] to molecular tweezers of oxacalix[2]arene[2]naphthalene [8] to molecular squares of Pt based cis-(dppf)M(OTf) 2 . [9] Few examples of organic nano- tubes include self-assembled tubular octapeptides, [10] zeolites or engineered porous crystals, [11] and the tetrameric/hexam- eric C-alkylpyrogallol[4]arenes with pyrene (exo-)/ferrocene (endo-) guests. [12] Most of these structural studies have focused on exploring the solid-state properties of nanoassemblies. Solution-phase behavior of nanoassemblies has also been investigated using techniques, such as NMR spectroscopy and light scattering; [13] however, investigation of paramagnetic species (NMR) or non-spherical entities (light scattering/NMR) with traditional methods has been a major limitation in the area of solution- phase chemistry. More recently, we have utilized small-angle Small-angle neutron scattering (SANS) and diffusion NMR studies are performed to investigate the stability and geometry of hydrogen-bonded pyrene-guest-containing C-hexylpyrogallol[4]arene (PgC 6 -pyrene) nanotubular frameworks in solution. In the solid state, hydrogen-bonded pyrogallol[4]arene tubes are formed; however, the scattering data for PgC 6 -pyrene assemblies in acetone are best modeled as dimeric spheres of PgC 6 with no pyrene guest. The result of diffusion NMR study also indicates the rearrangement of tubular entity into spherical framework in acetone. This is the first example of structural transformation of pyrogallol[4]arene nanotubes (guest-exo) in solution. Individual hydrogen-bonded spheres of PgC 6 exhibits a uniform radius of ca. 8.6 Å and a diffusion coefficient of 9.12 × 10 -10 m 2 s -1 in acetone. The diffusion NMR measurements further gave, for the first time, insights into how the type of solvent (acetone vs. methanol vs. acetontitrile/D 2 O) governs the structural differences in these nanoassemblies. Solution-phase structural alteration observed for PgC 6 -pyrene gives evidence of enhanced stability of pyrogallol[4]arene nanocapsules over nanotubes. Supramolecular Chemistry DOI: 10.1002/smll.201201384 Dr. H. Kumari, Dr. W. G. Wycoff, Prof. J. L. Atwood Department of Chemistry University of Missouri-Columbia 601 S. College Avenue, Columbia, MO 65211, USA E-mail: atwoodj@missouri.edu Dr. S. R. Kline National Institute of Standards and Technology 100 Bureau Drive, Stop 6102, Gaithersburg, MD 20899, USA small 2012, DOI: 10.1002/smll.201201384