An Electrochromic Tristable Molecular Switch

A tristable [2]­catenane, composed of a macro­cyclic poly­ether incorporating 1,5-dioxy­naphthalene (DNP) and tetra­thia­fulvalene (TTF) units along with a 4,4′-bi­pyridinium (BIPY^•+) radical cation as three very different potential recognition sites, inter­locked mechanically with the tetra­cationic cyclo­phane, cyclo­bis­(paraquat-<i>p</i>-phenylene) (CBPQT⁴⁺), was synthesized by donor–acceptor templation, employing a “threading-followed-by-cyclization” approach. In this catenane, movement of the CBPQT⁴⁺ ring in its different redox states among these three potential recognition sites, with corresponding color changes, is achieved by tuning external redox potentials. In the starting state, where no external potential is applied, the ring encircles the TTF unit and displays a green color. Upon oxidation of the TTF unit, the CBPQT⁴⁺ ring moves to the DNP unit, producing a red color. Finally, if all the BIPY²⁺ units are reduced to BIPY^•+ radical cations, the resulting CBPQT^2(•+) diradical dication will migrate to the BIPY^•+ unit, resulting in a purple color. These readily switchable electro­chromic properties render the [2]­catenane attractive for use in electro-optical devices

Redox Switchable Daisy Chain Rotaxanes Driven by Radical–Radical Interactions

We report the one-pot synthesis and electrochemical switching mechanism of a family of electrochemically bistable ‘daisy chain’ rotaxane switches based on a derivative of the so-called ‘blue box’ (BB⁴⁺) tetracationic cyclophane cyclobis­(paraquat-<i>p</i>-phenylene). These mechanically interlocked molecules are prepared by stoppering kinetically the solution-state assemblies of a self-complementary monomer comprising a BB⁴⁺ ring appended with viologen (V²⁺) and 1,5-dioxynaphthalene (DNP) recognition units using click chemistry. Six daisy chains are isolated from a single reaction: two monomers (which are not formally ‘chains’), two dimers, and two trimers, each pair of which contains a cyclic and an acyclic isomer. The products have been characterized in detail by high-field ¹H NMR spectroscopy in CD₃CNmade possible in large part by the high symmetry of the novel BB⁴⁺ functionalityand the energies associated with certain aspects of their dynamics in solution are quantified. Cyclic voltammetry and spectroelectrochemistry have been used to elucidate the electrochemical switching mechanism of the major cyclic daisy chain products, which relies on spin-pairing interactions between V^•+ and BB^2(•+) radical cations under reductive conditions. These daisy chains are of particular interest as electrochemically addressable molecular switches because, in contrast with more conventional bistable catenanes and rotaxanes, the mechanical movement of the ring between recognition units is accompanied by significant changes in molecular dimensions. Whereas the self-complexed cyclic monomerknown as a [<i>c</i>1]­daisy chain or molecular ‘ouroboros’conveys sphincter-like constriction and dilation of its ultramacrocyclic cavity, the cyclic dimer ([<i>c</i>2]­daisy chain) expresses muscle-like contraction and expansion along its molecular length

Self-Assembly of a [2]Pseudorota[3]catenane in Water

A donor–acceptor [3]­catenane incorporating two cyclobis­(paraquat-<i>p</i>-phenylene) rings linked together by a dinaphtho[50]­crown-14 macrocycle possesses a π-electron-deficient pocket. Contrary to expectation, negligible binding of a hexaethylene glycol chain interrupted in its midriff by a π-electron-rich 1,5-dioxynaphthalene unit was observed in acetonitrile. However, a fortuitous solid-state superstructure of the expected 1:1 complex revealed its inability to embrace any stabilizing [C–H···O] interactions between the clearly unwelcome guest and the host reluctantly accommodating it. By contrast, in aqueous solution, the 1:1 complex becomes very stable thanks to the intervention of hydrophobic bonding

γ‑Cyclodextrin Cuprate Sandwich-Type Complexes

Three structures, based on γ-cyclodextrin (γ-CD) and metal ions (Cu²⁺, Li⁺, Na⁺, and Rb⁺), have been prepared in aqueous and alkaline media and characterized structurally by single-crystal X-ray diffraction. Their dimeric assemblies adopt cylindrical channels along the <i>c</i> axes in the crystals. Coordinative and hydrogen bonding between the cylinders and the solvent molecules lead to the formation of two-dimensional sheets, with the identity of the alkali-metal ion strongly influencing the precise nature of the solid-state structures. In the case of the Rb⁺ complex, coordinative bonding involving the Rb⁺ ions leads to the formation of an extended two-dimensional structure. Nonbound solvent molecules can be removed, and gas isotherm analyses confirm the permanent porosity of these new complexes. Carbon dioxide (CO₂) adsorption studies show that the extended structure, obtained upon crystallization of the Rb⁺-based sandwich-type dimers, has the highest CO₂ sequestration ability of the three γ-CD complexes reported

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

An octacationic diazaperopyrenium (DAPP²⁺)-based homo­[2]­catenane (<b>DAPPHC</b>^<b>8+</b>), 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 <b>DAPPHC</b>^<b>8+</b> 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 <i>p</i>-xylylene bridges. The solid-state structure of the homo­[2]­catenane demonstrates how homophilic interactions play an important role in the formation of <b>DAPPHC</b>^<b>8</b>+, 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 Å between the mean planes of the outer ExBIPY²⁺ and inner DAPP²⁺ units, and 3.6 Å between the mean planes of the two inner DAPP²⁺ units. We show that irradiation of the <b>DAPPHC</b>^<b>8+</b> catenane at 330 nm in acetonitrile solution results in simultaneous energy and electron transfer. The latter occurs from the inner DAPP²⁺ dimer to the outer ExBIPY²⁺ unit, leading to the generation of a temporary charge-separated state within a rigid and robust homo­[2]­catenane. Compared to <b>DAPPBox</b>^<b>4+</b>, both forward- and back-electron transfer in <b>DAPPHC</b>^<b>8+</b> occur with faster rates, owing to the closer proximity between the electron donor and acceptor in the homo­[2]­catenane than in the separated cyclophane

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

An octacationic diazaperopyrenium (DAPP²⁺)-based homo­[2]­catenane (<b>DAPPHC</b>^<b>8+</b>), 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 <b>DAPPHC</b>^<b>8+</b> 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 <i>p</i>-xylylene bridges. The solid-state structure of the homo­[2]­catenane demonstrates how homophilic interactions play an important role in the formation of <b>DAPPHC</b>^<b>8</b>+, 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 Å between the mean planes of the outer ExBIPY²⁺ and inner DAPP²⁺ units, and 3.6 Å between the mean planes of the two inner DAPP²⁺ units. We show that irradiation of the <b>DAPPHC</b>^<b>8+</b> catenane at 330 nm in acetonitrile solution results in simultaneous energy and electron transfer. The latter occurs from the inner DAPP²⁺ dimer to the outer ExBIPY²⁺ unit, leading to the generation of a temporary charge-separated state within a rigid and robust homo­[2]­catenane. Compared to <b>DAPPBox</b>^<b>4+</b>, both forward- and back-electron transfer in <b>DAPPHC</b>^<b>8+</b> occur with faster rates, owing to the closer proximity between the electron donor and acceptor in the homo­[2]­catenane than in the separated cyclophane

Anticancer Activity Expressed by a Library of 2,9-Diazaperopyrenium Dications

Polyaromatic compounds are well-known to intercalate DNA. Numerous anticancer chemotherapeutics have been developed upon the basis of this recognition motif. The compounds have been designed such that they interfere with the role of the topoisomerases, which control the topology of DNA during the cell-division cycle. Although many promising chemotherapeutics have been developed upon the basis of polyaromatic DNA intercalating systems, these candidates did not proceed past clinical trials on account of their dose-limiting toxicity. Herein, we discuss an alternative, water-soluble class of polyaromatic compounds, the 2,9-diazaperopyrenium dications, and report <i>in vitro</i> cell studies for a library of these dications. These investigations reveal that a number of 2,9-diazaperopyrenium dications show similar activities as doxorubicin toward a variety of cancer cell lines. Additionally, we report the solid-state structures of these dications, and we relate their tendency to aggregate in solution to their toxicity profiles. The addition of bulky substituents to these polyaromatic dications decreases their tendency to aggregate in solution. The derivative substituted with 2,6-diisopropylphenyl groups proved to be the most cytotoxic against the majority of the cell lines tested. In the solid state, the 2,6-diisopropylphenyl-functionalized derivative does not undergo π···π stacking, while in aqueous solution, dynamic light scattering reveals that this derivative forms very small (50–100 nm) aggregates, in contrast with the larger ones formed by dications with less bulky substituents. Alteration of the aromaticitiy in the terminal heterocycles of selected dications reveals a drastic change in the toxicity of these polyaromatic species toward specific cell lines

Carbohydrate-Mediated Purification of Petrochemicals

Metal–organic frameworks (MOFs) are known to facilitate energy-efficient separations of important industrial chemical feedstocks. Here, we report how a class of green MOFsnamely CD-MOFsexhibits high shape selectivity toward aromatic hydrocarbons. CD-MOFs, which consist of an extended porous network of γ-cyclodextrins (γ-CDs) and alkali metal cations, can separate a wide range of benzenoid compounds as a result of their relative orientation and packing within the transverse channels formed from linking (γ-CD)₆ body-centered cuboids in three dimensions. Adsorption isotherms and liquid-phase chromatographic measurements indicate a retention order of <i>ortho-</i> > <i>meta-</i> > <i>para</i>-xylene. The persistence of this regioselectivity is also observed during the liquid-phase chromatography of the ethyltoluene and cymene regioisomers. In addition, molecular shape-sorting within CD-MOFs facilitates the separation of the industrially relevant BTEX (benzene, toluene, ethylbenzene, and xylene isomers) mixture. The high resolution and large separation factors exhibited by CD-MOFs for benzene and these alkylaromatics provide an efficient, reliable, and green alternative to current isolation protocols. Furthermore, the isolation of the regioisomers of (i) ethyltoluene and (ii) cymene, together with the purification of (iii) cumene from its major impurities (benzene, <i>n</i>-propylbenzene, and diisopropylbenzene) highlight the specificity of the shape selectivity exhibited by CD-MOFs. Grand canonical Monte Carlo simulations and single component static vapor adsorption isotherms and kinetics reveal the origin of the shape selectivity and provide insight into the capability of CD-MOFs to serve as versatile separation platforms derived from renewable sources

Intramolecular Energy and Electron Transfer within a Diazaperopyrenium-Based Cyclophane

Molecules capable of performing highly efficient energy transfer and ultrafast photoinduced electron transfer in well-defined multichromophoric structures are indispensable to the development of artificial photofunctional systems. Herein, we report on the synthesis, characterization, and photophysical properties of a rationally designed multichromophoric tetracationic cyclophane, <b>DAPP­Box⁴⁺</b>, containing a diaza­pero­pyrenium (DAPP²⁺) unit and an extended viologen (Ex­BIPY²⁺) unit, which are linked together by two <i>p</i>-xylylene bridges. Both ¹H NMR spectroscopy and single-crystal X-ray diffraction analysis confirm the formation of an asymmetric, rigid, box-like cyclophane, <b>DAPP­Box⁴⁺</b>. The solid-state superstructure of this cyclophane reveals a herringbone-type packing motif, leading to two types of π···π interactions: (i) between the Ex­BIPY²⁺ unit and the DAPP²⁺ unit (π···π distance of 3.7 Å) in the adjacent parallel cyclophane, as well as (ii) between the Ex­BIPY²⁺ unit (π···π distance of 3.2 Å) and phenylene ring in the closest orthogonal cyclophane. Moreover, the solution-phase photophysical properties of this cyclophane have been investigated by both steady-state and time-resolved absorption and emission spectroscopies. Upon photoexcitation of <b>DAPP­Box⁴⁺</b> at 330 nm, rapid and quantitative intramolecular energy transfer occurs from the ^1*Ex­BIPY²⁺ unit to the DAPP²⁺ unit in 0.5 ps to yield ^1*DAPP²⁺. The same excitation wavelength simultaneously populates a higher excited state of ^1*DAPP²⁺ which then undergoes ultrafast intramolecular electron transfer from ^1*DAPP²⁺ to Ex­BIPY²⁺ to yield the DAPP^3+•–Ex­BIPY^+• radical ion pair in τ = 1.5 ps. Selective excitation of DAPP²⁺ at 505 nm populates a lower excited state where electron transfer is kinetically unfavorable

Intramolecular Energy and Electron Transfer within a Diazaperopyrenium-Based Cyclophane

Molecules capable of performing highly efficient energy transfer and ultrafast photoinduced electron transfer in well-defined multichromophoric structures are indispensable to the development of artificial photofunctional systems. Herein, we report on the synthesis, characterization, and photophysical properties of a rationally designed multichromophoric tetracationic cyclophane, <b>DAPP­Box⁴⁺</b>, containing a diaza­pero­pyrenium (DAPP²⁺) unit and an extended viologen (Ex­BIPY²⁺) unit, which are linked together by two <i>p</i>-xylylene bridges. Both ¹H NMR spectroscopy and single-crystal X-ray diffraction analysis confirm the formation of an asymmetric, rigid, box-like cyclophane, <b>DAPP­Box⁴⁺</b>. The solid-state superstructure of this cyclophane reveals a herringbone-type packing motif, leading to two types of π···π interactions: (i) between the Ex­BIPY²⁺ unit and the DAPP²⁺ unit (π···π distance of 3.7 Å) in the adjacent parallel cyclophane, as well as (ii) between the Ex­BIPY²⁺ unit (π···π distance of 3.2 Å) and phenylene ring in the closest orthogonal cyclophane. Moreover, the solution-phase photophysical properties of this cyclophane have been investigated by both steady-state and time-resolved absorption and emission spectroscopies. Upon photoexcitation of <b>DAPP­Box⁴⁺</b> at 330 nm, rapid and quantitative intramolecular energy transfer occurs from the ^1*Ex­BIPY²⁺ unit to the DAPP²⁺ unit in 0.5 ps to yield ^1*DAPP²⁺. The same excitation wavelength simultaneously populates a higher excited state of ^1*DAPP²⁺ which then undergoes ultrafast intramolecular electron transfer from ^1*DAPP²⁺ to Ex­BIPY²⁺ to yield the DAPP^3+•–Ex­BIPY^+• radical ion pair in τ = 1.5 ps. Selective excitation of DAPP²⁺ at 505 nm populates a lower excited state where electron transfer is kinetically unfavorable