Anthea K. Blackburn

Efficient syntheses of pillar[6]arene-based hetero[4]rotaxanes using a cooperative capture strategy

While a single pillar[6]arene ring, nestling between two cucurbit[6]uril rings in a series of three hetero[4]rotaxanes, is conformationally mobile in solution, it adopts the energetically most favourable conformation with local C3V symmetry in the solid state.

Electrochemically addressable trisradical rotaxanes organized within a metal–organic framework

Significance This research paper presents a strategy for the organization of artificial molecular switches based on mechanically interlocked molecules within a porous crystalline framework. Once arranged within the pores of the framework, the electronic state of the switches can be altered by the application of an electrochemical potential. This strategy is particularly useful when it comes to integrating dynamic, stimulus-responsive, mechanically interlocked molecules with the robustness and periodicity of porous solids. The findings of the research establish proof-of-concept for the application of postsynthetic transformations of porous crystalline frameworks in the creation of solid-state molecular switches and machines. The organization of trisradical rotaxanes within the channels of a Zr6-based metal–organic framework (NU-1000) has been achieved postsynthetically by solvent-assisted ligand incorporation. Robust ZrIV–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. Ultraviolet–visible–near-infrared (UV-Vis-NIR), electron paramagnetic resonance (EPR), and 1H 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.

Lock-arm supramolecular ordering: A molecular construction set for cocrystallizing organic charge transfer complexes

Organic charge transfer cocrystals are inexpensive, modular, and solution-processable materials that are able, in some instances, to exhibit properties such as optical nonlinearity, (semi)conductivity, ferroelectricity, and magnetism. Although the properties of these cocrystals have been investigated for decades, the principal challenge that researchers face currently is to devise an efficient approach which allows for the growth of high-quality crystalline materials, in anticipation of a host of different technological applications. The research reported here introduces an innovative design, termed LASO-lock-arm supramolecular ordering-in the form of a modular approach for the development of responsive organic cocrystals. The strategy relies on the use of aromatic electronic donor and acceptor building blocks, carrying complementary rigid and flexible arms, capable of forming hydrogen bonds to amplify the cocrystallization processes. The cooperativity of charge transfer and hydrogen-bonding interactions between the building blocks leads to binary cocrystals that have alternating donors and acceptors extending in one and two dimensions sustained by an intricate network of hydrogen bonds. A variety of air-stable, mechanically robust, centimeter-long, organic charge transfer cocrystals have been grown by liquid-liquid diffusion under ambient conditions inside 72 h. These cocrystals are of considerable interest because of their remarkable size and stability and the promise they hold when it comes to fabricating the next generation of innovative electronic and photonic devices.

Two-point halogen bonding between 3,6-dihalopyromellitic diimides

The syntheses of 3,6-dichloro-, -dibromo-, and -diiodopyromellitic diimides—ACl, ABr, and AI, respectively—have been achieved. X-Ray crystallography of single crystals of ACl and ABr unveils the formation of extensive halogen-bonding networks in the solid state as a consequence of interactions between the lone pairs on the carbonyl oxygen atoms with the σ-holes of the halogen atoms. Further, the solid-state superstructure of diiodopyromellitic diimide is characterised by the formation of associated halogen-π dimers. The co-crystallisation of ACl or ABr with a 1,5-diaminonaphthalene derivative DN yields co-crystals of a mixed-stack charge-transfer (CT) complex which are supported by an expansive hydrogen-bonded network in addition to halogen-bonded belts that bring adjacent mixed-stacks into association with each other. 2,6-Dimethoxynaphthalene (DO) proved to be an effective CT complement to AI, yielding solvent-free co-crystals with superstructures which are comprised of a 1:2 ratio of AI to DO. This dimeric halogen-bonding motif is reminiscent of the formation of hydrogen-bonded dimers between carboxylic acids.

Three-Dimensional Architectures Incorporating Stereoregular Donor-Acceptor Stacks

We report the synthesis of two [2]catenane-containing struts that are composed of a tetracationic cyclophane (TC(4+)) encircling a 1,5-dioxynaphthalene (DNP)-based crown ether, which bears two terphenylene arms. The TC(4+) rings comprise either 1) two bipyridinium (BIPY(2+)) units or 2) a BIPY(2+) and a diazapyrenium (DAP(2+)) unit. These degenerate and nondegenerate catenanes were reacted in the presence of Cu(NO3)2⋅2.5 H2O to yield Cu-paddlewheel-based MOF-1050 and MOF-1051. The solid-state structures of these MOFs reveal that the metal clusters serve to join the heptaphenylene struts into grid-like 2D networks. These 2D sheets are then held together by infinite donor-acceptor stacks involving the [2]catenanes to produce interpenetrated 3D architectures. As a consequence of the planar chirality associated with both the DNP and hydroquinone (HQ) units present in the crown ether, each catenane can exist as four stereoisomers. In the case of the nondegenerate (bistable) catenane, the situation is further complicated by the presence of translational isomers. Upon crystallization, however, only two of the four possible stereoisomers--namely, the enantiomeric RR and SS forms--are observed in the crystals. An additional element of co-conformational selectivity is present in MOF-1051 as a consequence of the substitution of one of the BIPY(2+) units by a DAP(2+) unit: only the translational isomer in which the DAP(2+) unit is encircled by the crown ether is observed. The overall topologies of MOF-1050 and MOF-1051, and the selective formation of stereoisomers and translational isomers during the kinetically driven crystallization, provide evidence that weak noncovalent bonding interactions play a significant role in the assembly of these extended (super)structures.

Catenation through a Combination of Radical Templation and Ring-Closing Metathesis

Synthesis of an electrochemically addressable [2]catenane has been achieved following formation by templation of a [2]pseudorotaxane employing radically enhanced molecular recognition between the bisradical dication obtained on reduction of the tetracationic cyclophane, cyclobis(paraquat-p-phenylene), and the radical cation generated on reduction of a viologen disubstituted with p-xylylene units, both carrying tetraethylene glycol chains terminated by allyl groups. This inclusion complex was subjected to olefin ring-closing metathesis, which was observed to proceed under reduced conditions, to mechanically interlock the two components. Upon oxidation, Coulombic repulsion between the positively charged and mechanically interlocked components results in the adoption of a co-conformation where the newly formed alkene resides inside the cavity of the tetracationic cyclophane. (1)H NMR spectroscopic analysis of this hexacationic [2]catenane shows a dramatic upfield shift of the resonances associated with the olefinic and allylic protons as a result of them residing inside the tetracationic component. Further analysis shows high diastereoselectivity during catenation, as only a single (Z)-isomer is formed.

Stereochemistry of Molecular Figures-of-Eight

A trans isomer of a figure-of-eight (Fo8) compound was prepared from an electron-withdrawing cyclobis(paraquat-p-phenylene) derivative carrying trans-disposed azide functions between its two phenylene rings. Copper(I)-catalyzed azide-alkyne cycloadditions with a bispropargyl derivative of a polyether chain, interrupted in its midriff by an electron-donating 1,5-dioxynaphthalene unit acting as the template to organize the reactants prior to the onset of two click reactions, afforded the Fo8 compound with C(i) symmetry. Exactly the same chemistry is performed on the cis-bisazide of the tetracationic cyclophane to give a Fo8 compound with C(2) symmetry. Both of these Fo8 compounds exist as major and very minor conformational isomers in solution. The major conformation in the trans series, which has been characterized by X-ray crystallography, adopts a geometry which maximizes its C-H···O interactions, while maintaining its π···π stacking and C-H···π interactions. Ab initio calculations at the M06L level support the conformational assignments to the major and minor isomers in the trans series. Dynamic (1)H NMR spectroscopy, supported by 2D (1)H NMR experiments, indicates that the major and minor isomers in both the cis and trans series equilibrate in solution on the (1)H NMR timescale rapidly above and slowly below room temperature.

Topological isomerism in a chiral handcuff catenane

The self-assembly of two topological isomers of a handcuff catenane has been achieved by utilizing the template-directed synthesis between the π-electron-rich bis-1,5-dioxynaphtho[50]crown-14 and the precursors to two fused π-electron-deficient cyclobis(paraquat-p-phenylene) cyclophanes. Characterization of the product using 1H NMR spectroscopy and single-crystal X-ray diffraction, with supporting density functional theory (DFT) calculations, suggests that the 1,5-dioxynaphthalene units in the major topological isomer align themselves with the same relative orientations inside the cyclophanes on account of restrictions imposed by the lengths of the polyether loops. The DFT calculations also reveal that the energies of the two topological isomers are similar to each other, supporting the experimental observation that both isomers can be isolated as a mixture from a one-pot reaction. The two isomers – designated as the meta–meta and ortho–ortho isomers with different topologies that are not interconvertible – only differ in the manner in which the polyether loops wind their way around the central 1,2,4,5-tetrasubstituted benzenoid ring in the ditopic host. X-Ray crystallography proves that by far the major topological isomer in the solid state is the meta–meta one. 1H NMR spectroscopy confirms that it is also the major isomer in solution, whilst also revealing the presence of a minor isomer, which is assumed, for the time-being, to have the ortho–ortho topology. The free fused ditopic host has been obtained using a protocol similar to that employed in the template-directed synthesis of the handcuff catenane, except that the crown ether is replaced with an acyclic template which can be removed post-synthesis. The results of isothermal titration calorimetry studies shed some light on the mechanism of binding of π-electron-rich guests: they lead us to believe that when two electron-rich guests bind to the ditopic host, they do so with allosteric negative cooperativity.

Allosteric Modulation of Substrate Binding within a Tetracationic Molecular Receptor

The synthesis and recognition phenomena of a tetracationic molecular receptor that possesses a nanometer-sized molecular cavity are described. The host-guest properties of the molecular receptor can be tuned and modulated allosterically, where the association of a heterotropic effector at the periphery of the molecule serves to modulate its affinity for the globular, electron-rich guest that resides within its molecular cavity. This stimuli-responsive host-guest behavior was observed in both the solution phase and the crystalline solid state, and can be reversed with high fidelity by sequestration of the effector molecule.

Ferroelectric Polarization and Second Harmonic Generation in Supramolecular Cocrystals with Two Axes of Charge-Transfer

Ferroelectricity in organic materials remains a subject of great interest, given its potential impact as lightweight information storage media. Here we report supramolecular charge-transfer cocrystals formed by electron acceptor and donor molecules that exhibit ferroelectric behavior along two distinct crystallographic axes. The solid-state superstructure of the cocrystals reveals that a 2:1 ratio of acceptor to donor molecules assemble into nearly orthogonal mixed stacks in which the molecules are positioned for charge-transfer in face-to-face and edge-to-face orientations, held together by an extended hydrogen-bonding network. Polarization hysteresis was observed along the face-to-face and edge-to-face axes at room temperature. The noncentrosymmetric nature of the cocrystals, required to observe ferroelectric behavior, is demonstrated using second harmonic generation measurements. This finding suggests the possibility of designing supramolecular arrays in which organic molecules support multidimensional information storage.