Photon Factory Activity Report 2005 #23 Part B (2006)
ChemistryNW2/2005G153

Discrete Stacking of Large Aromatic Molecules
within Organic-Pillared Coordination Cages
Michito Yoshizawa1, Junichi Nakagawa1, Kazuhisa Kumazawa1, Muneki Nagao1,
Masaki Kawano1, Tomoji Ozeki2, and Makoto Fujita1*
1
Department of Applied Chemistry, The University of Tokyo, CREST, Japan Science andTechnology Corporation (JST), Bunkyo-ku, Tokyo 113-8656, Japan 2 Department of Chemistry andMaterials Science, Tokyo Institute of Technology, Meguro-ku, Tokyo 152-8551, Japan
Introduction
Aromatic stacking of π-conjugated planar molecules
leads to the exhibition of unique chemical and physicalproperties. Discotic liquid crystals are, for example,
columnar assembly of aromatic compounds with longalkyl chains. Organic electroconductive materials involve
alternative charge-transfer stacking of electron-donatingand -accepting π-conjugated compounds. Whereas such
infinite assemblies have been thoroughly studied,precisely controlled discrete assemblies composed ofmore than two π-conjugated molecules are much lessexplored. Here we report the self-assembly of a metalhinged organic-pillared cage 1 with a large cavity which
can accommodate two large π-conjugated molecules. The
cage consists of two large organic panels 2, three rod-like
2+pillars 3, and six metal-hinges 4 (Figure1a). The large
box-shaped
cavityaccommodates
π-conjugated[1]molecules in limited numbers.
Results
In the presence of coronene (5), we observed thequantitative self-assembly of a quadruple-stacking
12+2+structure [1⊃(5)2] . Four components 2, 3, 4 , and 5IIwere combined in a 2:3:6:2 ratio in D2O ([Pd ] = 60 mM).
After being stirred at 100 °C for 2 h, the suspension
1became clear and turned from pale yellow to deep red. H
NMR analysis of the solution indicated the formation of12+[1⊃(5)2] as a single product (Figure1b). The spectrum
12+agreed with the D3h-symmetry of 1 and with thestoichiometry of the components. The signal of 5 (He)was highly up-field shifted due to the encapsulation in the12+12+
cavity of 1 . Remarkably, the [1⊃(5)2] structure was[9]stable even under CSI-MS conditions. After anion
exchange with PF6 , CSI-MS showed a series of peaks form+12+was[1⊃(5)2+(12–m)•PF6+n•DMF] . Guest-free 1
hardly detected suggesting strong π-π interactions
between the host and the guest.
X-ray crystallographic analysis provided a reliable
evidence for the quadruple-stacking structure of[1⊃(5)2]12+. When pyrene (6) was used as a large planar
12+guest, a dark-reddish single crystal of [1⊃(6)2] wasobtained over a few weeks at room temperature by theslow diffusion of ethanol vapor into an aqueous solution
containing [1⊃(6)2]12+. Diffraction data were collected bysynchrotron X-ray radiation at –184 °C. Despite severe
solvents and counter ions disorder, the prism-likestructure of 112+ accommodating two molecules of 6 in thecavity was successfully solved (Figure2). The cage istwisted by 36° resulting in the efficient quadruple
aromatic stacking. The inter-plane distances 2···6 and6···6 are 3.4 and 3.3 Å, respectively.
In summary, we have achieved the quadruple stackingof large π-conjugated molecules by self-assembly.

Figure1. a) Schematic representation of the self-assembly of12+1[1⊃(5)2] and b) H NMR (500 MHz, D2O, RT) spectrum after
2+the combination of 2, 3, 4 , and 5 in a 2:3:6:2 ratio at 100 ºC for
2 h (en = ethylenediamine).

Figure2. X-ray crystal structure of [1⊃(6)2]12+ (6: pyrene).

References[1] M. Yoshizawa et al, Angew. Chem., Int. Ed., 44, 1810
(2005).
*mfujita@appchem.t.u-tokyo.ac.jp

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