Insulated molecular wire with highly conductive pi-conjugated polymer core

Bidirectional Molecular Motors by Controlling Threading and Dethreading Pathways of a Linked Rotaxane

Artificial molecular motors have been presented as models for biological molecular motors. In contrast to the conventional artificial molecular motors that rely on covalent bond rotation, molecular motors with mechanically interlocked molecules (MIMs) have attracted considerable attention owing to their ability to generate significant rotational motion by dynamically shuttling macrocyclic components. The topology of MIM-type rotational molecular motors is currently limited to catenane structures, which require intricate synthetic procedures that typically produce a low synthetic yield. In this study, we develop a novel class of MIM-type molecular motors with a rotaxane-type topology. The switching of the threading/dethreading pathways of the linked rotaxane by protecting/deprotecting the bulky stopper group and changing the solvent polarity enables a net unidirectional rotation of the molecular motor. The threading/dethreading reaction rates were quantitatively evaluated through detailed spectroscopic investigations. Repeated net unidirectional rotation and switching of the direction of rotation were also achieved. Our findings demonstrate that linked rotaxanes can serve as MIM-type molecular motors with reversible rotational direction controlled by threading/dethreading reactions. These motors hold potential as components of molecular machinery.

Insulated π-conjugated 2,2'-bipyridine transition-metal complexes: enhanced photoproperties in luminescence and catalysis

2,2'-Bipyridine has been identified as a privileged ligand scaffold for photofunctional transition metal complexes. We herein report on the synthesis and photoproperties of an insulated π-conjugated 2,2'-bipyridine with a linked rotaxane structure consisting of permethylated α-cyclodextrin (PM α-CD) and oligo(p-phenylene ethynylene). The insulated π-conjugated 2,2'-bipyridine exhibited enhanced ligand performance in the solid-state emitting biscyclometalated Ir complexes and visible-light-driven Ni catalysts owing to π-extension and remote steric effects based on the linked rotaxane structure.

Complexation study of a 1,3-phenylene-bridged cyclic hexa-naphthalene with fullerenes C60 and C70 in solution and 1D-alignment of fullerenes in the crystals

To investigate the host ability of a simple macrocycle, 1,3-phenylene-bridged naphthalene hexamer N6, we evaluated the complexation of N6 with fullerenes in toluene and in the crystals. The complexes in the solid-state demonstrate the one-dimensional alignment of fullerenes. The single-crystals of the C60@N6 composite have semiconductive properties revealed by photoconductivity measurements.

Mechanically interlocked polymers based on rotaxanes

The nature of mechanically interlocked molecules (MIMs) has continued to encourage researchers to design and construct a variety of high-performance materials. Introducing mechanically interlocked structures into polymers has led to novel polymeric materials, called mechanically interlocked polymers (MIPs). Rotaxane-based MIPs are an important class, where the mechanically interlocked characteristic retains a high degree of structural freedom and mobility of their components, such as the rotation and sliding motions of rotaxane units. Therefore, these MIP materials are known to possess a unique set of properties, including mechanical robustness, adaptability and responsiveness, which endow them with potential applications in many emerging fields, such as protective materials, intelligent actuators, and mechanisorption. In this review, we outline the synthetic strategies, structure-property relationships, and application explorations of various polyrotaxanes, including linear polyrotaxanes, polyrotaxane networks, and rotaxane dendrimers.

2D and 3D metal-organic frameworks constructed with a mechanically rigidified [3]rotaxane ligand

A mechanically interlocked [3]rotaxane was newly designed, synthesized, and employed as a ligand for constructing metal-organic frameworks (MOFs). The nano-confinement by macrocycles forces the soft bis-isophthalate axle into a pseudo-rigid conformation and coordinates to zinc(II) ions, affording a two- or three-dimensional MOF under controlled conditions. The 2D MOF exhibits a neutral framework with a periodic puckering sheet structure, while an anionic framework with a pts topology was observed for the 3D MOF. The phase purity of both bulk materials was confirmed by powder X-ray diffraction. Thermogravimetric analysis reveals that both materials are stable up to 250 °C. The success of applying mechanical bonds to rigidify flexible ligands provides new insights for the design of metal-organic frameworks.

Resistance-switchable conjugated polyrotaxane for flexible high-performance RRAMs

A representative closely packed conjugated polyrotaxane (CPR1) is synthesized by threading polyaniline (PAN) into β-cyclodextrin (CD) macrocycles and utilized for the first time to construct an RRAM device that exhibits an outstanding resistive switching capability. The CPR1 RRAM device displays remarkable nonvolatile memory performance with an extremely high ON/OFF ratio of 10⁸, the ultra-fast response of 29 ns, excellent reliability and reproducibility, and long-term stability (more than 1 year). The mechanism underlying this resistive switching behavior is understood according to the electric-field-induced proton doping of the PAN core by the CD sheath through hydrogen bonding interactions. More impressively, the favorable solubility and intrinsic flexibility of CPR1 allow for large-scale fabrication of flexible CPR1 RRAM device arrays by full-printing technology with endurance of 1000 bending cycles at the minimum bending radius of 3 mm, higher ON/OFF ratio of 10⁸, and relatively lower operating voltage of 1.8 V. This work shows the potential of CPR materials in highly stable memory devices for next-generation flexible and wearable electronics.

Precision synthesis of linear oligorotaxanes and polyrotaxanes achieving well-defined positions and numbers of cyclic components on the axle

Interest in macromolecules has increased because of their functional properties, which can be tuned using precise organic synthetic methods. For example, desired functions have been imparted by controlling the nanoscale structures of such macromolecules. In particular, compounds with interlocked structures, including rotaxanes, have attracted attention because of their unique supramolecular structures. In such supramolecular structures, the mobility and freedom of the macrocycles are restricted by an axle and dependent on those of other macrocycles, which imparts unique functions to these threaded structures. Recently, methods for the ultrafine engineering and synthesis, as well as functions, of "defined" rotaxane structures that are not statistically dispersed on the axle (i.e., control over the number and position of cyclic molecules) have been reported. Various synthetic strategies allow access to such well-defined linear oligo- and polyrotaxanes, including [1]rotaxanes and [n]rotaxanes (mostly n > 3). These state-of-the-art synthetic methods have resulted in unique functions of these oligo-and polyrotaxane materials. Herein, we review the effective synthetic protocols and functions of precisely constructed one-dimensional oligomers and polymers bearing defined threaded structures, and discuss the latest reports and trends.

Ultralong π-Conjugated Bis(terpyridine)metal Polymer Wires Covalently Bound to a Carbon Electrode: Fast Redox Conduction and Redox Diode Characteristics

We developed an efficient and convenient electrochemical method to synthesize π-conjugated redox metal-complex linear polymer wires composed of azobenzene-bridged bis(terpyridine)metal (2-M, M = Fe, Ru) units covalently immobilized on glassy carbon (GC). Polymerization proceeds by electrochemical oxidation of bis(4′-(4-anilino)-2,2′:6′,2″-terpyridine)metal (1-M) in a water–acetonitrile–HClO₄ solution, affording ultralong wires up to 7400 mers (corresponding to ca. 15 μm). Both 2-Fe and 2-Ru undergo reversible redox reactions, and their redox behaviors indicate remarkably fast redox conduction. Anisotropic hetero-metal-complex polymer wires with Fe and Ru centers are constructed via stepwise electropolymerization. The cyclic voltammograms of two hetero-metal-complex polymer wires, GC/[2-Fe]–[2-Ru] (3) and GC/[2-Ru]–[2-Fe] (4), show irreversible redox reactions with opposite electron transfer characteristics, indicating redox diodelike behavior. In short, the present electrochemical method is useful to synthesize polymer wire arrays and to integrate functional molecules on carbon.

Spin-labelled mechanically interlocked molecules as models for the interpretation of biradical EPR spectra

Biradical spin probes can provide detailed information about the distances between molecules/regions of molecules because the through-space coupling of radical centres, characterised by J, is strongly distance dependent. However, if the system can adopt multiple configurations, as is common in supramolecular complexes, the shape of the EPR spectrum is influenced not only by J but also the rate of exchange between different states. In practice, it is often hard to separate these variables and as a result, the effect of the latter is sometimes overlooked. To demonstrate this challenge unequivocally we synthesised rotaxane biradicals containing nitronyl nitroxide units at the termini of their axles. The rotaxanes exchange between the available biradical conformations more slowly than the corresponding non-interlocked axles but, despite this, in some cases, the EPR spectra of the axle and rotaxane remain remarkably similar. Detailed analysis allowed us to demonstrate that the similar EPR spectral shapes result from different combinations of J and rates of conformational interconversion, a phenomenon suggested theoretically more than 50 years ago. This work reinforces the idea that thorough analysis must be performed when interpreting the spectra of biradicals employed as spin probes in solution.

A quinone based single-molecule switch as building block for molecular electronics

Using a model molecule, we show that it is possible to create molecules that show the required properties for use as elements in a molecular circuit or computer: two conformations with similar energy but different electric conductivity, and the possibility to switch between those by applying an external electric field.

Mechanically Interlocked Nitrogenated Nanographenes

Herein, we report the synthesis of mechanically interlocked nitrogenated nanographenes. These systems have been obtained by clipping different tetralactam macrocycles around a 1.9 nm dumbbell-shaped nitrogenated nanographene. Thermal, optoelectronic, and electrochemical characterization of the different mechanically interlocked nanographenes evidence enhanced thermal and photochemical stability, and also absorption and emission properties that vary with the structure of the macrocycle.

Suppression of Undesirable Isomerization and Intermolecular Reactions of Double Bonds by a Linked Rotaxane Structure

For luminescent materials, the isomerization and intermolecular reactions of their double bonds are often undesirable because they cause a reduction in the luminescence properties of the π-system. Herein, we report a new methodology to simultaneously prevent isomerization and intermolecular reactions by utilizing the steric effect of a linked rotaxane structure. The ring units are covalently linked in order to prevent any undesired shuttling effect from occurring during isomerization. In addition, the insulated structure provides robust optical properties by prevention of intermolecular reactions. Bulky linked rotaxane structures on both sides of the N=N and C=C double bonds suppress E/Z isomerization; photoluminescence quantum yield (PLQY) measurements reveal that this results in suppression of PLQY reduction caused by isomerization. Moreover, an improvement in the stability under light irradiation and air atmosphere is demonstrated.

A hydrogen-bonded polymer constructed from mechanically interlocked, suit[1]ane monomers

A T-shaped 2,4,7-substituted benzimidazolium “axle” with two ester functionalities and a 24-membered crown ether “wheel” with appendages containing terminal olefin groups were threaded — axle through wheel — to form a [2]pseudorotaxane. Grubbs’ ring-closing metathesis (RCM) was then used to form a third loop and create a bicyclic cage that fully encapsulates the axle and permanently interlocks the two molecular components creating a suit[1]ane. There are no bulky groups on the axle to prevent unthreading, but the axle is trapped due to the cage-like nature of the newly created polyether host. After hydrolysis of the esters groups to carboxylic acids, this novel mechanically interlocked molecule (MIM) polymerizes in the solid state. The structure of the resulting supramolecular polymer was determined by single-crystal X-ray diffraction and contains linear one-dimensional tapes of suit[1]ane monomers linked by intermolecular hydrogen bonding between the carboxylic acid groups.

Exploring and Exploiting the Symmetry-Breaking Effect of Cyclodextrins in Mechanomolecules

Cyclodextrins (CDs) are cone-shaped molecular rings that have been widely employed in supramolecular/host–guest chemistry because of their low cost, high biocompatibility, stability, wide availability in multiple sizes, and their promiscuity for binding a range of molecular guests in water. Consequently, CD-based host–guest complexes are often employed as templates for the synthesis of mechanically bonded molecules (mechanomolecules) such as catenanes, rotaxanes, and polyrotaxanes in particular. The conical shape and cyclodirectionality of the CD “bead” gives rise to a symmetry-breaking effect when it is threaded onto a molecular “string”; even symmetrical guests are rendered asymmetric by the presence of an encircling CD host. This review focuses on the stereochemical implications of this symmetry-breaking effect in mechanomolecules, including orientational isomerism, mechanically planar chirality, and topological chirality, as well as how they support applications in regioselective and stereoselective chemical synthesis, the design of molecular machine prototypes, and the development of advanced materials.

Interactions between shape-persistent macromolecules as probed by AFM

Water-soluble shape-persistent cyclodextrin (CD) polymers with amino-functionalized end groups were prepared starting from diacetylene-modified cyclodextrin monomers by a combined Glaser coupling/click chemistry approach. Structural perfection of the neutral CD polymers and inclusion complex formation with ditopic and monotopic guest molecules were proven by MALDI-TOF and UV-vis measurements. Small-angle neutron and X-ray (SANS/SAXS) scattering experiments confirm the stiffness of the polymer chains with an apparent contour length of about 130 Å. Surface modification of planar silicon wafers as well as AFM tips was realized by covalent bound formation between the terminal amino groups of the CD polymer and a reactive isothiocyanate-silane monolayer. Atomic force measurements of CD polymer decorated surfaces show enhanced supramolecular interaction energies which can be attributed to multiple inclusion complexes based on the rigidity of the polymer backbone and the regular configuration of the CD moieties. Depending on the geometrical configuration of attachment anisotropic adhesion characteristics of the polymer system can be distinguished between a peeling and a shearing mechanism.

Reversible mechanical protection: building a 3D "suit" around a T-shaped benzimidazole axle

The T-shaped benzimidazolium/crown ether recognition motif was used to prepare suit[1]anes. These novel mechanically interlocked molecules (MIMs) were fully characterized by 1H and 13C NMR spectroscopy, single-crystal X-ray diffraction, UV-vis absorption and fluorescence spectroscopy. By conversion to a suit[1]ane, a simple benzimidazole was shown to be protected from deprotonation by strong base. Moreover, it was demonstrated that this unique three-dimensional encapsulation can be made reversible, thus introducing the concept of "reversible mechanical protection"; a protecting methodology that may have potential applications in synthetic organic chemistry and the design of molecular machinery.

Preparation of Peapod Polymer via the Supramolecular Chain Formation by Tris(spiroborate) Twin Bowl

Successive guest-containing tubular polymer was prepared by the olefin metathesis polymerization of tris(spiroborate) twin bowl after the formation of supramolecular polymer. The cationic iridium(III) complexes were topologically fixed inside the polymer to form a peapod-like structure. The polymer was evaluated by SEC, ICP-AES, and DLS analyses, and string-like structures were found in the AFM observation of the peapod polymer.

Two-dimensional (2D) self-assembly of oligo(phenylene-ethynylene) molecules and their triangular platinum(ii) diimine complexes studied using STM

The self-assembly of a series of cyclic oligo(phenylene-ethynylene) (OPE) molecules and their triangular Pt(ii) diimine complexes were studied using scanning tunneling microscope (STM).

Polyyne Rotaxanes: Stabilization by Encapsulation

Active metal template Glaser coupling has been used to synthesize a series of rotaxanes consisting of a polyyne, with up to 24 contiguous sp-hybridized carbon atoms, threaded through a variety of macrocycles. Cadiot–Chodkiewicz cross-coupling affords higher yields of rotaxanes than homocoupling. This methodology has been used to prepare [3]rotaxanes with two polyyne chains locked through the same macrocycle. The crystal structure of one of these [3]rotaxanes shows that there is extremely close contact between the central carbon atoms of the threaded hexayne chains (C···C distance 3.29 Å vs 3.4 Å for the sum of van der Waals radii) and that the bond-length-alternation is perturbed in the vicinity of this contact. However, despite the close interaction between the hexayne chains, the [3]rotaxane is remarkably stable under ambient conditions, probably because the two polyynes adopt a crossed geometry. In the solid state, the angle between the two polyyne chains is 74°, and this crossed geometry appears to be dictated by the bulk of the “supertrityl” end groups. Several rotaxanes have been synthesized to explore gem-dibromoethene moieties as “masked” polyynes. However, the reductive Fritsch–Buttenberg–Wiechell rearrangement to form the desired polyyne rotaxanes has not yet been achieved. X-ray crystallographic analysis on six [2]rotaxanes and two [3]rotaxanes provides insight into the noncovalent interactions in these systems. Differential scanning calorimetry (DSC) reveals that the longer polyyne rotaxanes (C16, C18, and C24) decompose at higher temperatures than the corresponding unthreaded polyyne axles. The stability enhancement increases as the polyyne becomes longer, reaching 60 °C in the C24 rotaxane.

STM analysis of surface-adsorbed conjugated oligo(p-phenylene-ethynylene) (OPE) nanostructures

A series of conjugated oligomers with different backbone lengths and side chains displayed various self-assembled structures on HOPG surface.

Acid/Base and H2PO4(-) Controllable High-Contrast Optical Molecular Switches with a Novel BODIPY Functionalized [2]Rotaxane

A novel multifunctional mechanically interlocked switchable [2]rotaxane R4 containing two molecular stations and rotaxane arms terminated with boron-dipyrromethene (BODIPY) fluorophores and its derivatives were synthesized for the first time by CuAAC click reaction. The shuttling motion of macrocycle between the dibenzylammonium and triazolium recognition sites and the distance dependent photoinduced electron transfer process of R4 is demonstrated by utilizing external chemical stimuli (acid/base). Interestingly, the reversible self-assembly process of R4 was recognized by the acid-base molecular switch strategy. Notably, two symmetrical triazolium groups acted as molecular stations, H2PO4(-) receptors, and H-bonded donors. Both [2]rotaxane R4 and thread R2 demonstrated excellent optical responses and high selectivity toward H2PO4(-) ion. The specific motion and guest-host interactions of mechanically interlocked machines (MIMs) were also further explored by quantum mechanical calculations. The thread R2 also demonstrated to enable the detection of H2PO4(-) in RAW 264.7 cells successfully.

Analysis of carbohydrates and glycoconjugates by matrix-assisted laser desorption/ionization mass spectrometry: an update for 2009-2010

This review is the sixth update of the original article published in 1999 on the application of MALDI mass spectrometry to the analysis of carbohydrates and glycoconjugates and brings coverage of the literature to the end of 2010. General aspects such as theory of the MALDI process, matrices, derivatization, MALDI imaging, arrays and fragmentation are covered in the first part of the review and applications to various structural typed constitutes the remainder. The main groups of compound that are discussed in this section are oligo and polysaccharides, glycoproteins, glycolipids, glycosides and biopharmaceuticals. Many of these applications are presented in tabular form. Also discussed are medical and industrial applications of the technique, studies of enzyme reactions and applications to chemical synthesis.

Charge carrier mobility in organic molecular materials probed by electromagnetic waves

Charge carrier mobility is an essential parameter providing control over the performance of semiconductor devices fabricated using a variety of organic molecular materials. Recent design strategies toward molecular materials have been directed at the substitution of amorphous silicon-based semiconductors; accordingly, numerous measurement techniques have been designed and developed to probe the electronic conducting nature of organic materials bearing extremely wide structural variations in comparison with inorganic and/or metal-oxide semiconductor materials. The present perspective highlights the evaluation methodologies of charge carrier mobility in organic materials, as well as the merits and demerits of techniques examining the feasibility of organic molecules, crystals, and supramolecular assemblies in semiconductor applications. Beyond the simple substitution of amorphous silicon, we have attempted to address in this perspective the systematic use of measurement techniques for future development of organic molecular semiconductors.

Hierarchical assembly of a dual-responsive macroscopic insulated molecular wire bundle in a gradient system

Here, we report the hierarchical self-assembly of a cationic gemini amphiphile, Azo 1, in a composition gradient solution generated using solvent evaporation. As the gradient solution is formed, Azo 1 forms nanorods in the lower region of the solution. Depending on solvent composition, these nanorods can further develop into nanofibres, which can then intertwine to form double helices and other types of nanohelices in the upper region of the solution. Finally, a macroscopic wire bundle is formed via the fusion of nanohelices; this ribbon-like bundle exhibits elasticity and linear ohmic resistance properties. More intriguingly, this bundle exhibits photoresponsive properties that affect its deformation and conductivity, as well as a rapid electroresponse that affects its conductivity, indicating that it is feasible to control the charge pathway.

Modulation and evaluation of the charge carrier mobility in a polymer alloy of polythiophene and an insulating matrix with an electron accepting molecule

Non-contact measurements with time-resolved microwave conductivity and absorption spectroscopy offer direct access to the charge carrier mobility in conjugated backbones.

A significant impact of host–guest stoichiometry on the extensibility of polyrotaxane gels

Host–guest stoichiometry in polyrotaxane gels has a significant impact on extensibility through the regulated “pressure” of the confined rings.

Synthesis of an organic-soluble π-conjugated [3]rotaxane via rotation of glucopyranose units in permethylated β-cyclodextrin

We synthesized symmetrically insulated oligo(para-phenylene) and oligothiophene with a pseudo-linked [3]rotaxane structure by full rotation of glucopyranose units via a flipping (tumbling) mechanism in the π-conjugated guest having two permethylated β-cyclodextrin units at both ends. We also succeeded in the synthesis of an organic-soluble fixed [3]rotaxane by a cross-coupling or complexation reaction of thus formed pseudo linked [3]rotaxane. Oligo(para-phenylene), oligothiophene, and porphyrin derivatives were used as π-conjugated guests with stopper groups.