Showing papers by "J. Fraser Stoddart published in 2018"

Selective Extraction of C70 by a Tetragonal Prismatic Porphyrin Cage.

Abstract: Along with the advent of supramolecular chemistry, research on fullerene receptors based on noncovalent bonding interactions has attracted a lot of attention. Here, we present the design and synthesis of a cationic molecular cage: a cyclophane composed of two tetraphenylporphyrins, bridged face-to-face by four viologen units in a rhomboid prismatic manner. The large cavity inside the cage, as well as the favorable donor–acceptor interactions between the porphyrin panels and the fullerene guests, enables the cage to be an excellent fullerene receptor. The 1:1 host–guest complexes formed between the cage and both C60 and C70 were characterized in solution by HRMS and NMR, UV–vis and fluorescence spectroscopies, and confirmed in the solid state by single-crystal X-ray diffraction analyses. The results from solution studies reveal that the cage has a much stronger binding for C70 than for C60, resulting in a selective extraction of C70 from a C60-enriched fullerene mixture (C60/C70 = 10/1), demonstrating the ...

ExTzBox: A Glowing Cyclophane for Live-Cell Imaging

Abstract: The ideal fluorescent probe for live-cell imaging is bright and non-cytotoxic and can be delivered easily into the living cells in an efficient manner. The design of synthetic fluorophores having all three of these properties, however, has proved to be challenging. Here, we introduce a simple, yet effective, strategy based on well-established chemistry for designing a new class of fluorescent probes for live-cell imaging. A box-like hybrid cyclophane, namely ExTzBox·4X (6·4X, X = PF6−, Cl−), has been synthesized by connecting an extended viologen (ExBIPY) and a dipyridyl thiazolothiazole (TzBIPY) unit in an end-to-end fashion with two p-xylylene linkers. Photophysical studies show that 6·4Cl has a quantum yield ΦF = 1.00. Furthermore, unlike its ExBIPY2+ and TzBIPY2+ building units, 6·4Cl is non-cytotoxic to RAW 264.7 macrophages, even with a loading concentration as high as 100 μM, presumably on account of its rigid box-like structure which prevents its intercalation into DNA and may inhibit other intera...

Controlling Dual Molecular Pumps Electrochemically.

Abstract: Artificial molecular machines can be operated using either physical or chemical inputs. Light-powered motors display clean and autonomous operations, whereas chemically driven machines generate waste products and are intermittent in their motions. Herein, we show that controlled changes in applied electrochemical potentials can drive the operation of artificial molecular pumps in a semi-autonomous manner-that is, without the need for consecutive additions of chemical fuel(s). The electroanalytical approach described in this Communication promotes the assembly of cyclobis(paraquat-p-phenylene) rings along a positively charged oligomeric chain, providing easy access to the formation of multiple mechanical bonds by means of a controlled supply of electricity.

Molecular Russian dolls

Abstract: The host-guest recognition between two macrocycles to form hierarchical non-intertwined ring-in-ring assemblies remains an interesting and challenging target in noncovalent synthesis. Herein, we report the design and characterization of a box-in-box assembly on the basis of host-guest radical-pairing interactions between two rigid diradical dicationic cyclophanes. One striking feature of the box-in-box complex is its ability to host various 1,4-disubstituted benzene derivatives inside as a third component in the cavity of the smaller of the two diradical dicationic cyclophanes to produce hierarchical Russian doll like assemblies. These results highlight the utility of matching the dimensions of two different cyclophanes as an efficient approach for developing new hybrid supramolecular assemblies with radical-paired ring-in-ring complexes and smaller neutral guest molecules.

Synthetic oligorotaxanes exert high forces when folding under mechanical load

Abstract: Folding is a ubiquitous process that nature uses to control the conformations of its molecular machines, allowing them to perform chemical and mechanical tasks. Over the years, chemists have synthesized foldamers that adopt well-defined and stable folded architectures, mimicking the control expressed by natural systems 1,2 . Mechanically interlocked molecules, such as rotaxanes and catenanes, are prototypical molecular machines that enable the controlled movement and positioning of their component parts 3–5 . Recently, combining the exquisite complexity of these two classes of molecules, donor–acceptor oligorotaxane foldamers have been synthesized, in which interactions between the mechanically interlocked component parts dictate the single-molecule assembly into a folded secondary structure 6–8 . Here we report on the mechanochemical properties of these molecules. We use atomic force microscopy-based single-molecule force spectroscopy to mechanically unfold oligorotaxanes, made of oligomeric dumbbells incorporating 1,5-dioxynaphthalene units encircled by cyclobis(paraquat-p-phenylene) rings. Real-time capture of fluctuations between unfolded and folded states reveals that the molecules exert forces of up to 50 pN against a mechanical load of up to 150 pN, and displays transition times of less than 10 μs. While the folding is at least as fast as that observed in proteins, it is remarkably more robust, thanks to the mechanically interlocked structure. Our results show that synthetic oligorotaxanes have the potential to exceed the performance of natural folding proteins.

Dynamic force spectroscopy of synthetic oligorotaxane foldamers

Abstract: Wholly synthetic molecules involving both mechanical bonds and a folded secondary structure are one of the most promising architectures for the design of functional molecular machines with unprecedented properties. Here, we report dynamic single-molecule force spectroscopy experiments that explore the energetic details of donor–acceptor oligorotaxane foldamers, a class of molecular switches. The mechanical breaking of the donor–acceptor interactions responsible for the folded structure shows a high constant rupture force over a broad range of loading rates, covering three orders of magnitude. In comparison with dynamic force spectroscopy performed during the past 20 y on various (bio)molecules, the near-equilibrium regime of oligorotaxanes persists at much higher loading rates, at which biomolecules have reached their kinetic regime, illustrating the very fast dynamics and remarkable rebinding capabilities of the intramolecular donor–acceptor interactions. We focused on one single interaction at a time and probed the stochastic rupture and rebinding paths. Using the Crooks fluctuation theorem, we measured the mechanical work produced during the breaking and rebinding to determine a free-energy difference, Δ G , of 6 kcal·mol −1 between the two local conformations around a single bond.

Epitaxial Growth of γ-Cyclodextrin-Containing Metal-Organic Frameworks Based on a Host-Guest Strategy.

Abstract: A class of metal-organic frameworks (MOFs)-namely CD-MOFs-obtained from natural products has been grown in an epitaxial fashion as films on the surfaces of glass substrates, which are modified with self-assembled monolayers (SAMs) of γ-cyclodextrin (γ-CD) molecules. The SAMs are created by host-guest complexation of γ-CD molecules with surface-functionalized pyrene units. The CD-MOF films have continuous polycrystalline morphology with a structurally out-of-plane ( c-axial) orientation, covering an area of several square millimeters, with a thickness of ∼2 μm. Furthermore, this versatile host-guest strategy has been applied successfully in the growth of CD-MOFs as the shell on the curved surface of microparticles as well as in the integration of CD-MOF films into electrochemical devices for sensing carbon dioxide. In striking contrast to the control devices prepared from CD-MOF crystalline powders, these CD-MOF film-based devices display an enhancement in proton conductance of up to 300-fold. In addition, the CD-MOF film-based device exhibits more rapid and highly reversible CO2-sensing cycles under ambient conditions, with a 50-fold decrease in conductivity upon exposure to CO2 for 3 s which is recovered within 10 s upon re-exposure to air.

Proton Conduction in Tröger’s Base-Linked Poly(crown ether)s

Abstract: Exactly 50 years ago, the ground-breaking discovery of dibenzo[18]crown-6 (DB18C6) by Charles Pedersen led to the use of DB18C6 as a receptor in supramolecular chemistry and a host in host–guest chemistry. We have demonstrated proton conductivity in Troger’s base-linked polymers through hydrogen-bonded networks formed from adsorbed water molecules on the oxygen atoms of DB18C6 under humid conditions. Troger’s base-linked polymers—poly(TBL-DB18C6)-t and poly(TBL-DB18C6)-c—synthesized by the in situ alkylation and cyclization of either trans- or cis-di(aminobenzo) [18]crown-6 at room temperature have been isolated as high-molecular-weight polymers. The macromolecular structures of the isomeric poly(TBL-DB18C6)s have been established by spectroscopic techniques and size-exclusion chromatography. The excellent solubility of these polymers in chloroform allows the formation of freestanding membranes, which are thermally stable and also show stability under aqueous conditions. The hydrophilic nature of the DB18...

Growing community of artificial molecular machinists

Abstract: Over the past decades, chemists have been pursuing the creation of man-made molecular machines with either designed engineering-like operations or with higher performances compared with biological machines The promise of creating an artificial molecular world traces its origins in the well-known lecture of Richard Feynman, There’s plenty of room at the bottom (1) Feynman’s insights into the immense possibilities of such small artificial machines, assembled in a straightforward manner, were deeply inspiring for the scientific community The design of machines on the molecular scale is not an easy task to accomplish Instead of gravity and inertia, which are omnipresent in the macroscopic world, random thermal fluctuations are prevalent and dominate movements on the molecular scale Two main approaches are being considered for the construction of artificial molecular machines (AMMs): namely, bio-inspiration and miniaturization The former consists of integrating concepts from naturally occurring machines and unnatural building blocks into AMMs, while the latter involves engineering nano-devices based on the mechanical actions of macroscopic machines Perhaps a better way to design AMMs with unprecedented functions would be to follow neither of these routes, but rather to construct molecularly precise architectures based on recent advances in supramolecular chemistry: explore what has not been built by Nature This unnatural route would only share with the biological world its fundamental laws at small scales, but differentiates it from its working processes It is clear that the design of AMMs is a critical step in the process It requires novel chemical building blocks to assemble and produce functioning systems In this regard, mechanically interlocked molecules (MIMs) have paved the way for the synthesis of AMMs MIMs, introduced by Jean-Pierre Sauvage (2) more than 30 years ago, signaled a breakthrough in introducing a new type of bond—the mechanical bond (3)—into chemistry Examples of rotaxanes, catenanes, and other … [↵][1]1To whom correspondence should be addressed Email: stoddart{at}northwesternedu [1]: #xref-corresp-1-1

Neighboring Component Effect in a Tri-stable [2]Rotaxane

Abstract: The redox properties of cyclobis(paraquat-p-phenylene)cyclophane (CBPQT4+) render it a uniquely variable source of recognition in the context of mechanically interlocked molecules, through aromatic donor–acceptor interactions in its fully oxidized state (CPBQT4+) and radical-pairing interactions in its partially reduced state (CBPQT2(•+)). Although it is expected that the fully reduced neutral state (CBPQT(0)) might behave as a π-donating recognition unit, resulting in a dramatic change in its binding properties when compared with the other two redox states, its role in rotaxanes has not yet been investigated. To address this challenge, we report herein the synthesis of a tri-stable [2]rotaxane in which a CBPQT4+ ring is mechanically interlocked with a dumbbell component containing five recognition sites—(i) a bipyridinium radical cation (BIPY(•+)) located centrally along the axis of the dumbbell, straddled by (ii) two tetrafluorophenylene units linked to (iii) two triazole rings. In addition to the selec...

Shuttling Rates, Electronic States, and Hysteresis in a Ring-in-Ring Rotaxane

Abstract: The trisradical recognition motif between a 4,4′-bipyridinium radical cation and a cyclo-bis-4,4′-bipyridinium diradical dication has been employed previously in rotaxanes to control their nanomechanical and electronic properties. Herein, we describe the synthesis and characterization of a redox-active ring-in-ring [2]rotaxane BBR·8PF6 that employs a tetraradical variant of this recognition motif. A square-shaped bis-4,4′-bipyridinium cyclophane is mechanically interlocked around the dumbbell component of this rotaxane, and the dumbbell itself incorporates a smaller bis-4,4′-bipyridinium cyclophane into its covalently bonded structure. This small cyclophane serves as a significant impediment to the shuttling of the larger ring across the dumbbell component of BBR8+, whereas reduction to the tetraradical tetracationic state BBR4(+•) results in strong association of the two cyclophanes driven by two radical-pairing interactions. In these respects, BBR·8PF6 exhibits qualitatively similar behavior to its pred...

Mixed-Valence Superstructure Assembled from a Mixed-Valence Host–Guest Complex

Abstract: Herein, we report an unprecedented mixed-valence crystal superstructure that consists of a 2:1 host-guest complex [MV⊂(CBPQT)2]2/3+ [MV = methyl viologen, CBPQT = cyclobis(paraquat- p-phenylene)]. One electron is distributed statistically between three [MV⊂(CBPQT)2]•+ composed of a total of 15 viologen units. The mixed-valence state is validated by single-crystal X-ray crystallography, which supports an empirical formula of [MV⊂(CBPQT)2]3·(PF6)2 for the body-centered cubic superstructure. Electron paramagnetic resonance provides further evidence of electron delocalization. Quantum chemistry calculations confirm the mixed-valence state in the crystal superstructure. Our findings demonstrate that precise tuning of the redox states in host-guest systems can lead to a promising supramolecular strategy for achieving long-range electron delocalization in solid-state devices.

Toward a Charged Homo[2]catenane Employing Diazaperopyrenium Homophilic Recognition

Abstract: An octacationic diazaperopyrenium (DAPP2+)-based homo[2]catenane (DAPPHC8+), wherein no fewer than eight positive charges are associated within a mechanically interlocked molecule, has been produced in 30% yield under ambient conditions as a result of favorable homophilic interactions, reflecting a delicate balance between strong π–π interactions and the destabilizing penalty arising from Coulombic repulsions between DAPP2+ units. This DAPPHC8+ catenane is composed of two identical mechanically interlocked tetracationic cyclophanes, namely DAPPBox4+, each of which contains one DAPP2+ unit and one extended viologen (ExBIPY2+) unit, linked together by two p-xylylene bridges. The solid-state structure of the homo[2]catenane demonstrates how homophilic interactions play an important role in the formation of DAPPHC8+, in which the mean ring planes of the two DAPPBox4+ cyclophanes are oriented at about 60° with respect to each other, with a centroid-to-centroid separation of 3.7 A between the mean planes of the...

X-Shaped Oligomeric Pyromellitimide Polyradicals

Abstract: The synthesis of stable organic polyradicals is important for the development of magnetic materials. Herein we report the synthesis, isolation, and characterization of a series of X-shaped pyromellitimide (PI) oligomers (Xn-R, n = 2–4, R = Hex or Ph) linked together by single C–C bonds between their benzenoid cores. We hypothesize that these oligomers might form high-spin states in their reduced forms because of the nearly orthogonal conformations adopted by their PI units. 1H and 13C nuclear magnetic resonance (NMR) spectroscopies confirmed the isolation of the dimeric, trimeric, and tetrameric homologues. X-ray crystallography shows that X2-Ph crystallizes into a densely packed superstructure, despite the criss-crossed conformations adopted by the molecules. Electrochemical experiments, carried out on the oligomers Xn-Hex, reveal that the reductions of the PI units occur at multiple distinct potentials, highlighting the weak electronic coupling between the adjacent redox centers. Finally, the chemically...