Discrete Open-Shell Tris(bipyridinium radical cationic) Inclusion Complexes in the Solid State.

The solid-state properties of organic radicals depend on radical-radical interactions that are influenced by the superstructure of the crystalline phase. Here, we report the synthesis and characterization of a substituted tetracationic cyclophane, cyclobis(paraquat-p-1,4-dimethoxyphenylene), which associates in its bisradical dicationic redox state with the methyl viologen radical cation (MV•+) to give a 1:1 inclusion complex. The (super)structures of the reduced cyclophane and this 1:1 complex in the solid state deviate from the analogous (super)structures observed for the reduced state of cyclobis(paraquat-p-phenylene) and that of its trisradical tricationic complex. Titration experiments reveal that the methoxy substituents on the p-phenylene linkers do not influence binding of the cyclophane toward small neutral guests-such as dimethoxybenzene and tetrathiafulvalene-whereas binding of larger radical cationic guests such as MV•+ by the reduced cyclophane decreases 10-fold. X-ray diffraction analysis reveals that the solid-state superstructure of the 1:1 complex constitutes a discrete entity with weak intermolecular orbital overlap between neighboring complexes. Transient nutation EPR experiments and DFT calculations confirm that the complex has a doublet spin configuration in the ground state as a result of the strong orbital overlap, while the quartet-state spin configuration is higher in energy and inaccessible at ambient temperature. Superconducting quantum interference device (SQUID) measurements reveal that the trisradical tricationic complexes interact antiferromagnetically and form a one-dimensional Heisenberg antiferromagnetic chain along the a-axis of the crystal. These results offer insights into the design and synthesis of organic magnetic materials based on host-guest complexes.

A Dynamic Tetracationic Macrocycle Exhibiting Photoswitchable Molecular Encapsulation.

Designing macrocycles with appropriate molecular recognition features that allow for the integration of suitable external stimuli to control host-guest processes is a challenging endeavor which enables molecular containers to solubilize, stabilize, and separate chemical entities in an externally controllable manner. Herein, we introduce photo- and thermal-responsive elements into a semi-rigid tetracationic cyclophane, OPVEx2Box4+, that is composed of oligo( p-phenylenevinylene) pyridinium units and the biphenylene-bridged 4,4-bipyridinium extended viologens and adopts a rectangle-like geometry. It transpires that when the photoactive oligo( p-phenylenevinylene) pyridinium unit is incorporated in a macrocyclic scaffold, its reversibility is dramatically improved, and the configurations of the cyclophane can go back and forth between ( EE)- and ( EZ)-isomers upon alternating blue light irradiation and heating. When the macrocycle is found in its ( EE)-configuration, it is capable of binding various π-electron-rich guests-e.g., anthracene and perylene-as well as π-electron-deficient guests-e.g., 9,10-anthraquinone and 5,12-tetracenequinone-through charge-transfer and van der Waals interactions. When irradiated with blue light, the ( EE)-isomer of the cyclophane can be transformed successfully to the ( EZ)-isomer, resulting in the switching off of the binding affinity for guest molecules, which are bound once again upon heating. The use of light and heat as external stimuli to control host-guest interactions involving a multi-responsive host and various guests provides us with a new opportunity to design and construct more-advanced molecular switches and machines.

A Supramolecular Approach for Modulated Photoprotection, Lysosomal Delivery, and Photodynamic Activity of a Photosensitizer.

Prompted by a knowledge of the photoprotective mechanism operating in photosystem supercomplexes and bacterial antenna complexes by pigment binding proteins, we have appealed to a boxlike synthetic receptor (ExBox·4Cl) that binds a photosensitizer, 5,15-diphenylporphyrin (DPP), to provide photoprotection by regulating light energy. The hydrophilic ExBox4+ renders DPP soluble in water and modulates the phototoxicity of DPP by trapping it in its cavity and releasing it when required. While trapping removes access to the DPP triplet state, a pH-dependent release of diprotonated DPP (DPPH22+) restores the triplet deactivation pathway, thereby activating its ability to generate reactive oxygen species. We have employed the ExBox4+-bound DPP complex (ExBox4+⊃DPP) for the safe delivery of DPP into the lysosomes of cancer cells, imaging the cells by utilizing the fluorescence of the released DPPH22+ and regulating photodynamic therapy to kill cancer cells with high efficiency.

Combining Intra- and Intermolecular Charge Transfer with Polycationic Cyclophanes To Design 2D Tessellations.

A series of donor-acceptor (D-A) naphthalene-viologen-based cyclophanes of different shapes, sizes, and symmetries have been synthesized and characterized. Solution optical studies on these cyclophanes reveal the existence of photoinduced intramolecular charge transfer (CT) at 465 nm from naphthalene (D) to viologen (A) units, resulting in a conformational change in the viologen units and the emergence of an emission at 540 nm. The D-A cyclophanes with box-like and hexagon-like shapes offer an opportunity to control the arrangement within 2D layers where D-A interactions direct the superstructures. While a box-like 2,6-disubstituted naphthalene-based tetracationic cyclophane does not form square tiling patterns, a truncated hexagon-like congener self-assembles to form a hexagonal superstructure which, in turn, adopts a hexagonal tiling pattern. Tessellation of the more rigid and highly symmetrical 2,7-disubstituted naphthalene-based cyclophanes leads to the formation of 2D square and honeycomb tiling patterns with the box-like and hexagon-like cyclophanes, respectively. Co-crystallization of the box-like cyclophanes with tetrathiafulvalene (TTF) results in the formation of D-A CT interactions between TTF and viologen units, leading to tubular superstructures. Co-crystallization of the hexagon-like cyclophane with TTF generates well-ordered and uniform tubular superstructures in which the TTF-viologen CT interactions and naphthalene-naphthalene [π···π] interactions propagate with 2D topology. In the solid state, the TTF-cyclophane co-crystals are paramagnetic and display dual intra- and intermolecular CT behavior at ∼470 and ∼1000 nm, respectively, offering multi-responsive materials with potential pathways for electron transport.

Cyclotris(paraquat-p-phenylenes).

Reported here is the synthesis, solid-state characterization, and redox properties of new triangular, threefold symmetric, viologen-containing macrocycles. Cyclotris(paraquat-p-phenylene) (CTPQT6+ ) and cyclotris(paraquat-p-1,4-dimethoxyphenylene) (MCTPQT6+ ) were prepared and their X-ray single-crystal (super)structures reveal intricate three-dimensional packing. MCTPQT6+ results in nanometer-sized channels, in contrast with its parent counterpart CTPQT6+ which crystallizes as a couple of polymorphs in the form of intercalated assemblies. In the solid state, MCTPQT3(.+) exhibits stacks between the 1,4-dimethoxyphenylene and bipyridinium radical cations, providing new opportunities for the manipulation and control of the recognition motif associated with viologen radical cations. These redox-active cyclophanes demonstrate that geometry-matching and weak intermolecular interactions are of paramount importance in dictating the formation of their intricate solid-state superstructures.

Controlling Dual Molecular Pumps Electrochemically.

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.

Noninvasive Substitution of K+ Sites in Cyclodextrin Metal-Organic Frameworks by Li+ Ions.

Co-crystallization of K+ and Li+ ions with γ-cyclodextrin (γ-CD) has been shown to substitute the K+ ion sites partially by Li+ ions, while retaining the structural integrity and accessible porosity of CD-MOF-1 (MOF, metal-organic framework). A series of experiments, in which the K+/Li+ ratio was varied with respect to that of γ-CD, have been conducted in order to achieve the highest possible proportion of Li+ ions in the framework. Attempts to obtain a CD-MOF containing only Li+ ions resulted in nonporous materials. The structural occupancy on the part of the Li+ ions in the new CD-MOF has been confirmed by single-crystal X-ray analysis by determining the vacancies of K+-ion sites and accounting for the cation/γ-CD ratio in CD-MOF-1. The proportion of Li+ ions has also been confirmed by elemental analysis, whereas powder X-ray diffraction has established the stability of the extended framework. This noninvasive synthetic approach to generating mixed-metal CD-MOFs is a promising method for obtaining porous framework unattainable de novo. Furthermore, the CO2 and H2 capture capacities of the Li+-ion-substituted CD-MOF have been shown to exceed the highest sorption capacities reported so far for CD-MOFs.

The photodecarboxylative addition of carboxylates to phthalimides as a key-step in the synthesis of biologically active 3-arylmethylene-2,3-dihydro-1H-isoindolin-1-ones.

The synthesis of various 3-arylmethylene-2,3-dihydro-1H-isoindolin-1-ones was realized following a simple three-step process. The protocol utilized the photodecarboxylative addition of readily available carboxylates to N-(bromoalkyl)phthalimides as a versatile and efficient key step. The initially obtained hydroxyphthalimidines were readily converted to the desired N-diaminoalkylated 3-arylmethylene-2,3-dihydro-1H-isoindolin-1-ones via acid-catalyzed dehydration and subsequent nucleophilic substitution with the corresponding secondary amines. The procedure was successfully applied to the synthesis of known local anesthetics (AL-12, AL-12B and AL-5) in their neutral forms.

Electronically Stabilized Nonplanar Phenalenyl Radical and Its Planar Isomer.

Stable phenalenyl radicals have great potential as the basis for new materials for applications in the field of molecular electronics. In particular, electronically stabilized phenalenyl species that do not require steric shielding are molecules of fundamental interest, but are notoriously difficult to synthesize. Herein, the synthesis and characterization of two phenalenyl-type cations is reported: planar benzo[i]naphtho[2,1,8-mna]xanthenium (8(+)) and helical benzo[a]naphtho[8,1,2-jkl]xanthenium (9(+)), which can be reduced to the corresponding radicals. Radical 9 represents the first stable, helical phenalenyl radical which does not rely on bulky substituents to ensure its stability. Both cations are water-soluble, and the radicals are stable for weeks at room temperature under air. These compounds were characterized crystallographically, and also by NMR, EPR, electrochemistry, and electronic spectra. The synthesis of the previously reported compound benzo[5,6]naphthaceno[1,12,11,10-jklmna]xanthylium (5(+)), the largest oxygen-containing polycyclic hydrocarbon, was undertaken for comparison with 8(+) and 9(+), allowing us to report its crystal structure here for the first time. The different properties of these compounds and their radicals are explained by considering their differing aromaticities using in-depth computational methods.

Naphthoxanthenyl, a new stable phenalenyl type radical stabilized by electronic effects.

Naphthoxanthenyl 1 is a new stable phenalenyl-type radical. Electrochemical studies indicate that 1 has two reversible redox processes that occur on comparatively short time scales. Crystals containing 1 can be grown by electrocrystallization, suggesting that they are conductive.