A Mechanically Interlocked Bundle

Abstract

The prototype of an artificial molecular machine consisting of a trisammonium tricationic component interlocked with a tris(crown ether) component to form a molecular bundle with averaged C3ν 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 between the CH2NH2+CH2 protons on three dibenzylammonium-ion-containing arms, which are attached symmetrically to a benzenoid core, and three dibenzo[24]crown-8 macrorings fused onto a triphenylene core; and 3) π⋯π stacking interactions between the aromatic cores. The template-directed synthesis 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-dipolar cycloaddition with di-tert-butylacetylenedicarboxylate to the extremely strong 1:1 adduct that is formed in dichloromethane/acetonitrile (3:2), on account of a cluster effect associated with the paucivalent adduct. Evidence for the averaged C3ν symmetry of the molecular bundle comes from absorption and luminescence data, as well as from electrochemical experiments, 1H NMR spectroscopy, and mass spectrometry. 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 synthesis. Although weak non-nucleophilic bases (e.g., nBu3N and iPr2NEt) fail to deprotonate the bundle, the strong tBuOK does, as indicated by both luminescence and 1H NMR spectroscopy. While deprotonation undoubtedly loosens up the interlocked structure of the molecular bundle by replacing relatively strong N+-H⋯O hydrogen bonds by much weaker N-H⋯O ones, the π⋯π 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 temperature.