A Mechanically Interlocked Bundle Jovica D. Badjic ¬, [a] Vincenzo Balzani, [b] Alberto Credi,* [b] James N. Lowe, [a] Serena Silvi, [b] and J. Fraser Stoddart* [a] Introduction Nowadays, there seems to be an almost inexhaustible collec- tion of complex supramolecular assemblies known [1] that uti- lize hydrogen bonding as their main source of noncovalent interactions to hold together, often in highly cooperative ways, a number of molecular building blocks or tectons [2] with a remarkable degree of architectural control and preci- sion. The discovery [3] in the mid-90 s that suitably sized crown ether macrocycles will thread spontaneously onto sec- ondary dialkylammonium ions, forming interpenetrating complexes called pseudorotaxanes, [4] primarily as a result of N + H¥¥¥O hydrogen bonds and C H¥¥¥O interactions, [5] has evolved [6] from these early beginnings, where only one monotopic crown ether macrocycle was threaded onto one dialkylammonium ion center, to the construction of much more elaborate multiply threaded superstructures. This very simple recognition motif, in which the sources of the nonco- valent interactions are primarily hydrogen bonds, is aug- mented, not only by multivalency, [8] but also by the fact that one molecular structure (the cationic component) interpene- trates the other molecular structure. [9] In the limit, of course, post-assembly covalent modification can be employed to transport the multivalent supramolecular assembly into a mechanically interlocked molecular domain. It is this limit that will be addressed in this paper. [a] Dr. J. D. Badjic ¬, Dr. J. N. Lowe, Prof. J. F. Stoddart Department of Chemistry and Biochemistry University of California, Los Angeles 405 Hilgard Avenue, Los Angeles, CA 90095-1569 (USA) Fax: (+ 1) 310-206-1843 E-mail : stoddart@chem.ucla.edu [b] Prof. V. Balzani, Dr. A. Credi, S. Silvi Dipartimento di Chimica ™G. Ciamician∫ Universit‡ di Bologna Via Selmi 2, 40126 Bologna (Italy) Fax: (+ 39) 051-209-9456 E-mail: alberto.credi@unibo.it Abstract: The prototype of an artificial molecular machine consisting of a tris- ammonium tricationic component in- terlocked with a tris(crown ether) com- ponent to form a molecular bundle with averaged C 3v symmetry has been designed and synthesized. The system is based on noncovalent interactions, which include 1) N + H¥¥¥O hydrogen bonds; 2) C  H¥¥¥O interactions be- tween the CH 2 NH 2 + CH 2 protons on three dibenzylammonium-ion-contain- ing arms, which are attached symmetri- cally to a benzenoid core, and three di- benzo[24]crown-8 macrorings fused onto a triphenylene core; and 3) p¥¥¥p stacking interactions between the aro- matic cores. The template-directed syn- thesis of the mechanically interlocked, triply threaded bundle involves post- assembly covalent modification, that is, the efficient conversion of three azide functions at the ends of the arms of the bound and threaded trication into bulky triazole stoppers, after 1,3-dipo- lar cycloaddition with di-tert-butylacet- ylenedicarboxylate to the extremely strong 1:1 adduct that is formed in di- chloromethane/acetonitrile (3:2), on account of a cluster effect associated with the paucivalent adduct. Evidence for the averaged C 3v symmetry of the molecular bundle comes from absorp- tion and luminescence data, as well as from electrochemical experiments, 1 H NMR spectroscopy, and mass spec- trometry. The photophysical properties of the mechanically interlocked bundle are very similar to those of the super- bundle that precedes the formation of the bundle in the process of supra- molecular assistance to covalent syn- thesis. Although weak non-nucleophilic bases (e.g., nBu 3 N and iPr 2 NEt) fail to deprotonate the bundle, the strong tBuOK does, as indicated by both lumi- nescence and 1 H NMR spectroscopy. While deprotonation undoubtedly loos- ens up the interlocked structure of the molecular bundle by replacing relative- ly strong N + H¥¥¥O hydrogen bonds by much weaker NH¥¥¥O ones, the p¥¥¥p stacking interactions ensure that any structural changes are inconsequential, particularly when the temperature of the solution of the neutral molecular bundle in dichloromethane is cooled down to considerably below room tem- perature. Keywords: hydrogen bonds ¥ luminescence ¥ molecular recognition ¥ multivalency ¥ self-assembly ¹ 2004 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim DOI: 10.1002/chem.200305687 Chem. Eur. J. 2004, 10, 1926 ± 1935 1926 FULL PAPER