Construction of a photo-responsive supra-amphiphile based on a tetracationic cyclobis(paraquat-p-phenylene) and an azobenzene-containing guest in water.

A photo-responsive host-guest molecular recognition between a tetracation cyclophane cyclobis(paraquat-p-phenylene) host and an azobenzene-containing guest was constructed. Based on this novel recognition, a supra-amphiphile was constructed and the photo-responsive self-assembly behaviors were investigated.

Redox Control of the Binding Modes of an Organic Receptor.

The modulation of noncovalent bonding interactions by redox processes is a central theme in the fundamental understanding of biological systems as well as being ripe for exploitation in supramolecular science. In the context of host-guest systems, we demonstrate in this article how the formation of inclusion complexes can be controlled by manipulating the redox potential of a cyclophane. The four-electron reduction of cyclobis(paraquat-p-phenylene) to its neutral form results in altering its binding properties while heralding a significant change in its stereoelectronic behavior. Quantum mechanics calculations provide the energetics for the formation of the inclusion complexes between the cyclophane in its various redox states with a variety of guest molecules, ranging from electron-poor to electron-rich. The electron-donating properties displayed by the cyclophane were investigated by probing the interaction of this host with electron-poor guests, and the formation of inclusion complexes was confirmed by single-crystal X-ray diffraction analysis. The dramatic change in the binding mode depending on the redox state of the cyclophane leads to (i) aromatic donor-acceptor interactions in its fully oxidized form and (ii) van der Waals interactions when the cyclophane is fully reduced. These findings lay the foundation for the potential use of this class of cyclophane in various arenas, all the way from molecular electronics to catalysis, by virtue of its electronic properties. The extension of the concept presented herein into the realm of mechanically interlocked molecules will lead to the investigation of novel structures with redox control being expressed over the relative geometries of their components.

Pillar[10]arene-based size-selective host-guest complexation and its application in tuning the LCST behavior of a thermoresponsive polymer.

A new molecular recognition motif between a water soluble pillar[10]arene (WP10) and 1,10-phenanthrolinium guest (G) in water is established. Mainly driven by the cooperativity of multiple electrostatic interactions, hydrophobic interactions, and π-π stacking interactions between WP10 and G, this host-guest complex exhibits a high association constant in water, which is about 17 times higher than that between WP10 and paraquat (PQ). Furthermore, this size selective host-guest complexation is employed to tune the lower critical solution temperature behavior of a random copolymer with PQ derivative pendants.

Photoinduced electron transfer within a zinc porphyrin-cyclobis(paraquat-p-phenylene) donor-acceptor dyad.

Understanding the mechanism of efficient photoinduced electron-transfer processes is essential for developing molecular systems for artificial photosynthesis. Towards this goal, we describe the synthesis of a donor-acceptor dyad comprising a zinc porphyrin donor and a tetracationic cyclobis(paraquat-p-phenylene) (CBPQT(4+) ) acceptor. The X-ray crystal structure of the dyad reveals the formation of a dimeric motif through the intermolecular coordination between the triazole nitrogen and the central Zn metal of two adjacent units of the dyad. Photoinduced electron transfer within the dyad in MeCN was investigated by femtosecond and nanosecond transient absorption spectroscopy, as well as by transient EPR spectroscopy. Photoexcitation of the dyad produced a weakly coupled ZnP(+.) -CBPQT(3+.) spin-correlated radical-ion pair having a τ=146 ns lifetime and a spin-spin exchange interaction of only 0.23 mT. The long radical-ion-pair lifetime results from weak donor-acceptor electronic coupling as a consequence of having nine bonds between the donor and the acceptor, and the reduction in reorganization energy for electron transfer caused by charge dispersal over both paraquat units within CBPQT(3+.) .

A bioconjugated design for amino acid-modified mesoporous silicas as effective adsorbents for toxic chemicals.

A general synthetic method for functionalization of mesoporous silica with amino acid has been developed. The carboxylic acid functionalized SBA-15 was conjugated with l-phenylalanine (Phe) and l-tryptophan (Trp) to obtain nontoxic amino acid-conjugated functionalized mesoporous silica materials. The materials were used as adsorbents for the removal of the herbicide Paraquat (PQ) and its analog, ethyl viologen dibromide (EVB) from aqueous solutions. In comparison to the commercially available activated carbon adsorbents, the silica-based adsorbents prepared in this study exhibited relatively higher PQ removal efficiency in aqueous solutions at room temperature and pH 7.0. The silica-based adsorbents, pendant with amino acid moieties exhibited greater adsorption capacities toward PQ and EVB than the analogs but without the amino acid moiety, suggesting that there is a benefit for the enhanced π-π interaction between the aromatic groups of the conjugated amino acid moieties and the adsorbate. This bioconjugated method developed here provides a promising new tool to synthesize new materials for detoxification of herbicides in clinical trials.

Synthesis of a water-soluble bis(m-phenylene)-32-crown-10-based cryptand and its pH-responsive binding to a paraquat derivative.

By introducing a third arm with two anionic carboxylate groups, a water-soluble bis(m-phenylene)-32-crown-10-based cryptand was synthesized. Its pH-controlled host-guest complexation with a paraquat derivative in water was studied.

Formation of 1:2 host-guest complexes based on triptycene-derived macrotricycle and paraquat derivatives: anion-π interactions between PF6(-) and bipyridinium rings in the solid state.

A triptycene-derived macrotricyclic host containing two dibenzo-[30]-crown-10 moieties forms stable 1:2 host-guest complexes with paraquat derivatives in both solution and the solid state, in which anion-π interactions between PF(6)(-) and the bipyridinium rings play an important role. Moreover, it was found that binding and release of the guest molecules in the complexes could be easily controlled by the addition and removal of potassium ions.

Complexation between pentiptycene-based mono(crown ether)s and tetracationic cyclobis(paraquat-p-phenylene): who is the host or the guest?

Pentiptycene-based mono(crown ether)s and tetracationic cyclobis(paraquat-p-phenylene) can form 1 : 1 stable complexes in solution and in the solid state, in which both of the components act as the host as well as the guest.

Complexation of a pentiptycene-based tweezer-like receptor with paraquat derivatives: ion-controlled binding and release of the guests.

A new tweezer-like receptor containing a pentiptycene unit and a benzene ring linked by two crown ether chains has been synthesized. It could form stable complexes with paraquat derivatives with different functional groups in solution and in the solid state. It was found that the complexes dissociated upon two one-electron reduction of the bipyridinium ring. Moreover, binding and release of the guest molecules could also be easily controlled by the addition and removal of potassium ion.

First-principles studies of the dynamics of [2]rotaxane molecular switches.

Realization of controlled binary switching in individual molecules is of fundamental importance for nanoscale electronics where the use of molecular components promises the flexibility of engineering performance through controlled organic synthesis. The active component of the [2]rotaxane molecule consists of a cyclobis-(paraquat-p-phenylene) ring-shaped structure [(CBPQT(4+))(PF(6)(-))(4)], proposed to switch between two stations, tetrathiafulvalene (TTF) and 1,5-dioxynapthalene (DNP), that lie along a common molecular backbone. However, there are still several open questions regarding their operation and performance, particularly in a device geometry. In this work, the switching speed of crossbar array devices based on [2]rotaxane arrays is studied with first principles density functional theory (DFT). The energetics of a likely configurational pathway for the CBPQT-ring shuttling along the molecular backbone between stations is computed and analyzed, as are ionization potentials and electrostatic screening properties. From these quantities, a new switching mechanism is identified. The applied bias at the cathode alters the energy landscape, making the OFF-state configuration energetically unfavorable relative to the ON-state without involving charging, as previously suggested. (1) For a crossbar memory array of reasonable size, the calculations predict that the switching speed is dominated by the shuttling time of the CBPQT-ring, which is estimated to be a few microseconds. The applicability of this technology is discussed in light of this result.

Complexation between methyl viologen (paraquat) bis(hexafluorophosphate) and dibenzo[24]crown-8 revisited.

Paraquat bis(hexafluorophosphate) undergoes stepwise dissociation in acetone. All three species-the neutral molecule, and the mono- and dications-are represented significantly under the experimental conditions typically used in host-guest binding studies. Paraquat forms at least four host-guest complexes with dibenzo[24]crown-8. They are characterized by both 1:1 and 1:2 stoichiometries, and an overall charge of either zero (neutral molecule) or one (monocation). The monocationic 1:1 host-guest complex is the most abundant species under typical (0.5-20 mM) experimental conditions. The presence of the dicationic 1:1 host-guest complex cannot be excluded on the basis of our experimental data, but neither is it unambiguously confirmed to be present. The two confirmed forms of paraquat that do undergo complexation-the neutral molecule and the monocation-exhibit approximately identical binding affinities toward dibenzo[24]crown-8. Thus, the relative abundance of neutral, singly, and doubly charged pseudorotaxanes is identical to the relative abundance of neutral, singly, and doubly charged paraquat unbound with respect to the crown ether in acetone. In the specific case of paraquat/dibenzo[24]crown-8, ion-pairing does not contribute to host-guest complex formation, as has been suggested previously in the literature.

Guest-dependent complexation of triptycene-based macrotricyclic host with paraquat derivatives and secondary ammonium salts: a chemically controlled complexation process.

The triptycene-based macrotricyclic host containing two dibenzo-[24]-crown-8 moieties has been found to form stable 1:1 or 1:2 complexes in different complexation modes with different functional paraquat derivatives and secondary ammonium salts in solution and in the solid state. Consequently, the alkyl-substituted paraquat derivatives thread the lateral crown cavities of the host to form 1:1 complexes. It was interestingly found that the paraquat derivatives containing two beta-hydroxyethyl or gamma-hydroxypropyl groups form 1:2 complexes, in which two guests thread the central cavity of the host. Other paraquat derivatives containing terminal hydroxy, methoxy, 9-anthracylmethyl, and amide groups were included in the cavity of the host to form 1:1 complexes. Moreover, the host also forms a 1:2 complex with two 9-anthracylmethylbenzylammonium salts, in which the 9-anthracyl groups were selectively positioned outside the lateral crown cavities. The competition complexation process between the host and two different guests (the propyl-substituted paraquat derivative and a dibenzylammonium salt) could be chemically controlled.

Computational models for the shuttling motion of the macrocycle in rotaxane-based molecular switches.

The shuttling motion of a macrocycle in rotaxane-based molecular switching devices has been studied by computational density functional methods. In the test case, energy profiles corresponding to the dethreading process of different types of guest molecules in a cyclobis(paraquat-p-phenylene) host verified the experimental preference of the tetrathiafulvalene recognition site over the dioxynaphthalene site in a Stoddart-Heath type molecular device. Furthermore, modification of the redox state of either the macrocycle or the guest molecule resulted in considerable changes in the computational energy profiles, which can be utilized in explaining the behavior of the host-guest system. In order to study the effect of chemical oxidation/reduction in the guest molecule, we have investigated a prototypical shaft including two octahedral ruthenium complexes linked by a conjugated C(14) carbon chain, where the shuttling motion can be triggered by changing the electronic environment of the active complexes with ligand exchange reactions. The computational results also indicated effective communication between the macrocycle and the conjugated carbon chain, therefore showing the importance of non-covalent host-guest interactions in the control of the motion.

Tuneable pseudorotaxane formation between a biotin-avidin bioconjugate and CBPQT4+.

A biotinylated 1,5-dialkoxynaphthalene derivative has been shown to have the ability to bind strongly to avidin and thus act as an artificial binding site for cyclobis(paraquat-p-phenylene) thereby facilitating the formation of a tuneable pseudorotaxane-based bioconjugate.

Organogel formation by a cholesterol-stoppered bistable [2]rotaxane and its dumbbell precursor.

The switching properties, gelation behavior, and self-organization of a cholesterol-stoppered bistable [2]rotaxane containing a cyclobis(paraquat-p-phenylene) ring and tetrathiafulvalene/1,5-dioxynaphthalene recognition units situated in the rod portion of the dumbbell component have been investigated by electrochemical, spectroscopic, and microscopic means. The cyclobis(paraquat-p-phenylene) ring in the [2]rotaxane can be switched between the tetrathiafulvalene and 1,5-dioxynaphthalene recognition units by addressing the redox properties of the tetrathiafulvalene unit. The organogels can be prepared by dissolving the [2]rotaxane and its dumbbell precursor in a CH2Cl2/MeOH (3:2) mixed solvent and liquified by adding the oxidant Fe(ClO4)3. Direct evidence for the self-organization was obtained from AFM investigations which have shown that both of the [2]rotaxane and its dumbbell precursor form linear superstructures which we propose are helical in nature.

Molecular modelling and enzymatic studies of acetylcholinesterase and butyrylcholinesterase recognition with paraquat and related compounds.

The potent herbicide paraquat and three other analogues MPP+, MPDP+ and MPTP have a known toxicological profile linked to the ability to damage dopaminergic neurons. Other biological effects were recently addressed to this class of compounds, including the ability to interact with enzymatic targets involved in the Central Nervous System, such as the acetylcholinesterase (AChE) and the butyrylcholinesterase (BuChE). A combined molecular modelling and enzymatic study focusing onto their interaction against the AChE and BuChE is reported. The former study was performed by docking techniques using target known co-crystallographic models. The latter study was carried out by the widely adopted Ellman's method. In both studies the anti-Alzheimer FDA approved drug tacrine was used as reference inhibitor. Our results indicate that paraquat, MPTP, MPDP+ and MPP+ recognize both enzymatic cleft in a similar fashion compared to the reference inhibitor. A structure-activity correlation was found with the net charge of the ligands, indicating a major role of the electrostatic term in the recognition and inhibition of these compounds. Our data completed their enzymatic profile, added new information on the molecular mechanisms underlying their neurotoxicity useful for the rational design of new cholinesterase inhibitors.

Novel triptycene-based cylindrical macrotricyclic host: synthesis and complexation with paraquat derivatives.

[reaction: see text] A novel triptycene-based cylindrical macrotricyclic polyether containing two dibenzo[24]crown-8 cavities has been synthesized and proved to be a highly efficient host for the complexation with paraquat derivatives. Consequently, a new kind of very stable pseudorotaxane-type complex was formed in solution and in the solid state.

Promotion of host folding during the formation of a taco complex.

Host folding for the formation of taco complexes can be promoted by introduction of additional interactions between the host and guest as shown by enhanced associations and X-ray crystal structures.