Operating Molecular Elevators

Abstract

Inspired by the concept of multivalency in living systems, two mechanically interlocked molecules have been conceived that incorporate not once or twice but thrice the features of a pH-switchable [2]rotaxane with two orthogonal recognition sites for dibenzo[24]crown-8 (DB24C8), and 2,3-dinaphtho[24]crown-8 (DN24C8)-one a dialkylammonium ion (CH(2)NH(2)(+)CH(2)) and the other a bipyridinium dication (BIPY(2+)). Whereas at low pH, the CH(2)NH(2)(+)CH(2) sites bind the DB24C8/DN24C8 macrocycles preferentially, at high pH, deprotonation occurs with loss of hydrogen bonding and the macrocycles will move to the BIPY(2+) sites, where they can acquire some stabilizing [pi-pi] stacking interactions. Such mechanically interlocked molecules have been assembled from a trifurcated rig-like component wherein the dumbbell-like components of three [2]rotaxanes have one of their ends fused onto alternate positions (1,3,5) around a benzenoid core. The rig is mechanically interlocked by a platform based on a tritopic receptor, wherein either three benzo[24]crown-8 or three 2,3-naphtho[24]crown-8 macrocycles are fused onto a hexaoxatriphenylene core. The synthesis of these molecular elevators involves 1:1 complexation, followed by stoppering, i.e., feet are added to the rig. (1)H NMR spectroscopy and cyclic voltammetry, aided and abetted by absorption spectroscopy, have been employed to unravel the details of the mechanism by which the rig and platform components move on the alternate addition of base and acid. For each molecular elevator, the platform operates by taking three distinct steps associated with each of the three deprotonation/reprotonation processes. Thus, molecular elevators are more reminiscent of a legged animal than they are of passengers on freight elevators.

Remote electrochemical modulation of pK a in a rotaxane by co-conformational allostery

“…Electrochemical techniques are useful not only to perform the redox switching but also to assess the position of the ring, because the reduction of the bipyridinium unit occurs at more negative potentials when it is surrounded by M (27,28,31). Thus, to investigate the interplay of the redox state of the bipyridinium unit and the apparent acidity of the ammonium site we performed cyclic and differential pulse voltammetric experiments in acetonitrile on the rotaxane alone and in the presence of different bases and compared the results with those observed for the axle component 2H 3+ .…”

Noble Metal Nanoparticles and Their (Bio) Conjugates. II. Preparation

“…Chak et al have discussed (Chak et al, 2009) the synthesis of composite nanoparticles consisting of small gold nanoparticles (2 nm), dibenzo(24)crown-8 (DB24C8) and dibenzylammonium (DBA). DB24C8 and DBA fragments were connected with the gold nanoparticle via the amine group links, acted as supramolecular recognition motifs and formed form a 1:1 supramolecular complex, namely, (DBADB24C8) pseudorotaxane (Badjic et al, 2006). The conjugate of amine monofunctional gold nanoparticles (1-AuNPs, decorated with dibenzyl ammonium (DBA)-disulfide ligand) together with a crown-ether-decorated polystyrene resin (PS-DB24C8) and crownether-decorated superparamagnetic iron oxide nanoparticles (SPIO-DB24C8) formed interesting recognition nanomotifs.…”

Recent Advances in the Synthesis of Ammonium-Based Rotaxanes

“…However, because of its interlocked nature, rotaxanes have been synthesized mostly as a nanomachine component in which a translational motion can be induced by various stimuli [71]. In order to respond to such stimuli and to accomplish complex tasks at the nanoscale, rotaxanes have to be modified with proper functional moieties, including photo-, electroand bioactive units [72].…”