DOI: 10.1002/chem.201101297 Encapsulation of Acenaphthene within C-Propan-3-ol-pyrogallol[4]arene Dimeric Nanocapsules Katrina K. Kline, DrewA. Fowler, Sheryl A. Tucker, and Jerry L. Atwood* [a] Nanometer-scale structures have been formed in a variety of ways including covalent, non-covalent, and metal-directed assembly. [1–16] Self-assembling, noncovalent molecular struc- tures have been shown to form a wide range of unique host structures. [17] Previously investigated hexameric C-alkylpyro- gallol[4]arene (PgC n ) nanocapsules have an interior volume of  1250  3 , are seamed together with 72 hydrogen bonds, and are stable in nonpolar and moderately polar sol- vents. [18–23] These hexameric nanocapsules were shown to act as a host to multiple guest molecules, mainly pyrene deriva- tives. The dimeric nanocapsules that are the focus of this work are seamed together with crystallisation solvent mole- cules (methanol) and are capable of encapsulating a single acenaphthene molecule. Investigation into the behaviour of these entities in solu- tion has been fairly limited, although our previous re- ports [24–27] have provided insight through the use of molecu- lar spectroscopy techniques, particularly steady-state and dy- namic fluorescence measurements. This ground-breaking work illustrated the potential of the PgC n nanocapsules for wide-ranging applications including molecular transport, sorting and catalysis. [20, 23–38] Previous studies of pyrene butyric acid (PBA) and pyrene butanol (PBOH) encapsulated within the hexameric C-hex- ylpyrogallol[4]arene (PgC 6 ) nanocapsules remarkably showed that the guest remained entrapped within the hex- americ nanocapsules in solution for both systems. [24, 25] Single-crystal X-ray crystallographic data revealed that the nanocapsules were mainly doubly occupied by the guest. The fluorophores inside the cavity were separated by sand- wiched assembly solvent, acetonitrile. Spectroscopic and crystallographic investigations determined that both guests interacted with the interior walls of the capsule both in solid and solution states. In order to elucidate the role of guest size in encapsula- tion and stability of the resulting complex, the fluorescence reporter acenaphthene (AN) was investigated. AN differs from previously investigated guest molecules in both its smaller size and lack of a polar functional group within the structure. The PgC 3 OH host forms a dimeric capsule seamed with crystallisation solvent molecules to accommo- date a single AN guest molecule. This varies from the hex- americ capsules formed when PgC 6 building blocks were uti- lised to entrap pyrene derivatives. The resulting supramolec- ular assembly is shown in Figure 1. Alternate positions of the guest within the dimeric nanocapsule have been mod- elled and additonal images are included in the Supporting Information. Single-crystal X-ray diffraction studies of the pale pink crystals show that the PgC 3 OH self-assembled into a dimeric capsule containing a single AN guest molecule. Each dimer- ic capsule is seamed together by hydrogen-bonding mediat- ed through four methanol solvent molecules. The interior volume created by this capsule was calculated to be approxi- mately 218  3 , shown in Figure 1. A single molecule of ace- napthene, disordered over two positions, is centred within the void space of each capsule. For each acenapthene, the presence of four C  H···p interactions between the fluoro- phore and macrocycle, two to each PgC 3 OH, assist in the as- sociation of the host–guest complex. The tails of each PgC 3 OH in the dimers form hydrogen bonds to the tails of the capsule above or below itself, again mediated through methanol molecules. There is a highly disordered acenap- thene within the region between each capsule that is en- shrouded by the hydrogen-bonded tails. Due to the high amount of translational and rotational disorder, a reason- ACHTUNGTRENNUNGable model in the crystal structure was not obtained. This conclusion was reached based on the size and planarity of the disordered electron density. In order to gain information about the behaviour of these complexes in solution, molecular spectroscopic techniques were employed. Such studies reveal information about the nanoenvironment and interactions between the host and the guest. Initial fluorescence spectra of the AN-containing nanocapsules and free AN are shown in Figure 2. With ap- proximately a twofold increase in the fluorescence emission intensity relative to free AN, it is immediately evident that AN remains in a protected microenvironment while in solu- tion. This intensity increase is due to the guest being in a protected environment within the nanocapsule, decreasing the incidence of collisional deactivation, which results in a radiationless, excited-state decay pathway. There is also a wavelength shift of  15 nm between the free and encapsu- lated AN, indicating the guest is experiencing a different [a] Dr. K. K. Kline, D. A. Fowler, Prof.Dr. S. A. Tucker, Prof. Dr. J. L. Atwood Department of Chemistry, University of Missouri 125 Chemistry Building, Columbia, MO 65211 (US) Fax: (+ 1) 573-882-2754 E-mail : AtwoodJ@missouri.edu Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201101297.  2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Chem. Eur. J. 2011, 17, 10848 – 10851 10848