Electrochemically addressable trisradical rotaxanes organized within a metal–organic framework

Abstract

The organization of trisradical rotaxanes within the channels of a Zr 6 -based metal-organic framework (NU-1000) has been achieved postsynthetically by solvent-assisted ligand incorporation. Robust Zr IV -carboxylate bonds are forged between the Zr clusters of NU-1000 and carboxylic acid groups of rotaxane precursors (semirotaxanes) as part of this building block replacement strategy. Ultravioletvisible-near-infrared (UV-Vis-NIR), electron paramagnetic resonance (EPR), and 1 H nuclear magnetic resonance (NMR) spectroscopies all confirm the capture of redox-active rotaxanes within the mesoscale hexagonal channels of NU-1000. Cyclic voltammetry measurements performed on electroactive thin films of the resulting material indicate that redox-active viologen subunits located on the rotaxane components can be accessed electrochemically in the solid state. In contradistinction to previous methods, this strategy for the incorporation of mechanically interlocked molecules within porous materials circumvents the need for de novo synthesis of a metal-organic framework, making it a particularly convenient approach for the design and creation of solid-state molecular switches and machines. The results presented here provide proof-of-concept for the application of postsynthetic transformations in the integration of dynamic molecular machines with robust porous frameworks.mechanically interlocked molecules | metal-organic framework | molecular switches | rotaxanes | radicals T he predictability and reliability with which metal-organic frameworks (MOFs) are assembled (1-11) has accelerated the rate at which porous materials can be developed for applications as diverse as gas storage and separation (7), sensing (8), catalysis (9), and light harvesting (10, 11). Metal oxide joints and organic struts are arranged regularly within MOFs, giving rise to hybrid materials with permanent porosities. It has been proposed (12, 13) that integrating the rigidity and periodicity of MOFs with the addressability and workings of molecular switches and machines, such as bistable mechanically interlocked molecules (MIMs) (14-16), stands a good chance of giving rise to a new class of functional materials that are simultaneously both robust and dynamic. Most switchable MIMs that have been developed operate in solution where they are stochastically oriented and the net movement of a population of switches averages to zero. By integrating such rudimentary molecular switches within highly ordered MOFs, however, they can be organized periodically and precisely in 3D space, allowing their otherwise incoherent motions to be rectified. Although steady progress has been made (17-26) toward this goal in recent years, it still remains a considerable challenge to design such systems that can be addressed by stimuli in the solid state. Electrochemical potential and light would be particularly appealing stimuli on account of their ease of interfacing with current technologies. The avenues of investigation that have been explored up to this point have entailed t...

Symbiotic Control in Mechanical Bond Formation

“…For several decades,m echanically interlocked molecules [1] (MIMs), such as bistable catenanes and rotaxanes,h ave attracted [2] much attention from the broader community of scientists and engineers.A pplications in directing the construction of molecular switches [3] and machines, [4] as well as in drug delivery [5] and molecular electronic devices, [6] are already well established. With our rapidly growing insight into molecular recognition and its use in aiding self-assembly processes,M IMs can now be synthesized in high yields by employing template-directed approaches [7] that rely on noncovalent bonding interactions.A mong the redox-based approaches to templation, radical-pairing interactions [8] have been shown to provide au nique form of covalent assistance in the formation of mechanical bonds.Subsequent eradication of the radical-pairing interactions is more often than not followed by the formation of repulsive electrostatic interactions.I np articular, recognition [9] between BIPYC + units (the radical state of 4,4'-bipyridinium BIPY 2+ )h as captured our attention because of our interest in the syntheses of MIMs where the component parts repel each other in the final analysis.…”

Immobilizing Organic‐Based Molecular Switches into Metal–Organic Frameworks: A Promising Strategy for Switching in Solid State

“…Redox processes related to the gain and loss of electrons are very common in nature and have been employed to build assorted electroactive materials for various technological applications. Several examples of redox‐responsive OMS incorporated into MOFs have also been reported . For example, Dincaˇ and co‐workers synthesized NDI‐based redox active organic linkers to build Zn(NDI‐X) (X = H, SEt, NHEt) MOF thin films on fluorine‐doped tin oxide (FTO)‐coated glass substrates .…”

“…Recently, by using a postsynthetic solvent‐assisted ligand incorporation (SALI) method, Stoddart and co‐workers successfully incorporated MIMs‐based switches into a robust zirconium‐based MOF (NU‐1000, Figure ) . For example, A semirotaxane consisted of a viologen‐containing thread and a tetracationic cyclobis(paraquat‐ p ‐phenylene) macrocycle was incorporated in activated NU‐1000 through coordination in an inert atmosphere . Later on, they reported the immobilization of bistable [2]catenane‐based molecular switches into NU‐1000 again by the SALI method (Figure ) .…”

Switching in Metal–Organic Frameworks

“…Progress in supramolecular chemistry has led to switchable molecules capturing widespread attention. Although the denotation of “switching” has been variably employed in the literature, for the purposes of limiting the scope of this Review the term “switch” will signify any system that, upon exposure to a particular, external stimulus, such as light, temperature, or electrical field, undergoes a reversible and controllable transformation between two or more distinct molecular states, commonly involving isomerization, bond breaking or formation, the gain or loss of electrons, or conformational changes. Under ideal conditions, it is assumed that every stage in the transformation of a reversible process is in equilibrium with the next state and the prior state, so that infinitesimally small reversal steps along the reaction coordinate will yield a state indistinguishable to that from before the process began.…”