Controlling Multivalent Interactions in Triply-Threaded Two-Component Superbundles

We have investigated the ¹H NMR spectra, the absorption spectra, the fluorescence spectra and decays, and the electrochemical properties of i) a tritopic receptor in which three benzo[24]crown-8 macrorings are fused onto a triphenylene core, ii) a trifurcated trication wherein three dibenzylammonium ions are linked 1,3,5 to a central benzenoid core, and iii) their 1:1 adduct which constitutes a triply-threaded, two-component supramolecular bundle. X-Ray crystallography has established the precise geometry of this paucivalent recognition motif in the solid state. In addition to [N⁺-H ··· O] hydrogen bonding and [C-H ··· O] interactions between the NH₂⁺ centers on the three dibenzyl-ammonium ion containing arms of the trication and the three crown ether rings in the tritopic receptor, there is a stabilizing [π ··· π] stacking interaction between the two aromatic cores. Mass spectrometry and ¹H NMR spectroscopy have confirmed the integrity of the 1:1 adduct beyond the solid state, provided the solvents are relatively apolar (e.g., chloroform and acetonitrile). The intense fluorescence emissions of the two recognition components are quenched upon association with the concomitant appearance of a lower energy, broad fluorescence band originating from the π - π stacking in the 1:1 adduct of the aromatic cores in the two matching components. Titration experiments, including Job plots, establish the 1:1 stoichiometry of the adduct, an observation which is also confirmed by electrochemical experiments. The electrochemical results show that, both in the tritopic receptor and in the superbundle itself, the first oxidation process is associated with the hexaalkoxytriphenylene core. The successive oxidation processes of the peripheral dioxybenzene units are affected by charge-transfer interactions in the tritopic receptor, whereas, in the superbundle, such units are not interacting. In acetonitrile solution, dethreading/rethreading of the 1:1 adduct can be controlled quantitatively by addition of base and acid. Dethreading and rethreading is also observed by ¹H NMR spectroscopy when dimethylsulfoxide is added to a solution of the 1:1 adduct in equal volumes of acetonitrile and chloroform. A trifurcated trication where methyl groups are located on the para positions of the three dibenzyl-ammonium ions, which are linked 1,3,5 to the neutral benzenoid core, has been employed to demonstrate that dethreading of the 1:1 adduct involves doubly-threaded and singly-threaded species, that is, the paucivalent site is dismembered in a sequence of logical steps involving stable intermediates. This molecular recognition system is a rare example of a supramolecular entity based on a cooperative binding motif that can be switched on and off by chemical means.