光驱动分子梭

分子梭在分子开关、分子逻辑门、信息存储等领域有着潜在的应用价值,是超分子化学领域的研究热点之一。本文综述了光驱动分子梭的研究进展：重点举例介绍了荧光光谱识别法和圆二色光谱识别法这两种识别光驱动分子梭位置状态的方法;阐述了构建光驱动轮烷分子梭的新型方法学,包括光驱动环糊精[2]轮烷和[1]轮烷分子梭的定向合成,举例介绍了光间接驱动的轮烷分子梭,以及光驱动[3]轮烷型分子梭和分子梭聚合物;举例说明了光驱动分子梭的功能性应用,用光驱动分子梭来模拟分子水平的逻辑门,研究光驱动分子梭体系中的能量传递机理,以及非溶液态的光驱动分子梭;并对分子梭今后的发展做了展望。     

Biomimetic Synchronized Motion of Two Interacting Macrocycles in [3]Rotaxane‐Based Molecular Shuttles

Noncovalent interactions between all the neighboring components in biomolecular machines are responsible for their synchronized motion and thus complex functions. This strategy has rarely been used in multicomponent molecular machines. Here, we report four [3]rotaxane‐based molecular shuttles. Noncovalent interactions among the three components (two interacting macrocycles and one axle) not only cause a “systems‐level” effect on the relative positions of the two macrocycles along the axle, but also result in a synchronized motion of the two macrocycles when adding partial amount of stimuli. Moreover, the intermediate state with one shuttled macrocycle even exist predominantly in the solution during the titration of stimuli, which is theoretically unexpected for the [3]rotaxane with two non‐interacting rings. This biomimetic strategy may provide a method for constructing highly complex molecular machines.     

Fixed or invertible calixarene-based directional shuttles

The first examples of rotaxanes based on calixarenes threaded by dialkylammonium ions, which also represent the first examples of calixarene-based molecular shuttles, are reported. The base/acid treatment demonstrated that these systems act as molecular shuttles, which move between three sites on the axle. When small OMe groups are appended at the calix[6]arene lower rim an unprecedented inversion of its shuttling direction is observed, which occurs through a cone-to-cone inversion of the macrocycle.     

Acid‐Base Switchable [2]‐ and [3]Rotaxane Molecular Shuttles with Benzimidazolium and Bis(pyridinium) Recognition Sites

For the purpose of developing higher level mechanically interlocked molecules (MIMs), such as molecular switches and machines, a new rotaxane system was designed in which both the 1,2‐bis(pyridinium)ethane and benzimidazolium recognition templating motifs were combined. These two very different recognition sites were successfully incorporated into [2]rotaxane and [3]rotaxane molecular shuttles which were fully characterized by ¹H NMR, 2D EXSY, single‐crystal X‐ray diffraction and VT NMR analysis. By utilizing benzimidazolium as both a recognition site and stoppering group it was possible to create not only an acid/base switchable [2]rotaxane molecular shuttle (energy barrier 20.9 kcal?mol^?1) but also a [3]rotaxane molecular shuttle that displays unique dynamic behavior involving the simultaneous motion of two macrocyclic wheels on a single dumbbell. This study provides new insights into the design of switchable molecular shuttles. Due to the unique properties of benzimidazoles, such as fluorescence and metal coordination, this new type of molecular shuttle may find further applications in developing functional molecular machines and materials.     

Molecular shuttles by the protecting group approach

Two new [2]rotaxane-based molecular shuttles, in which a mechanically bound dibenzo[24]crown-8 (DB24C8) macroring shunts back and forth between two dialkylammonium recognition sites situated on a chemical dumbbell, have been constructed by a novel synthetic strategy that relies upon the use of the tert-butoxycarbonyl (Boc) protecting group. During the syntheses of both molecular shuttles, this protecting group masks a dialkylammonium recognition center which is liberated only after the [2]rotaxane constitution is established. In both cases, the molecular shuttles' other dialkylammonium center is essential for the rotaxane-forming reactions and it ensures that DB24C8 is interpenetrated by threadlike precursors, as a result of noncovalent bonding interactions, to produce [2]pseudorotaxanes which are stoppered subsequently through 1,3-dipolar cycloadditions between azides and bulky acetylenedicarboxylates. The new molecular shuttles have been examined by means of dynamic 1H NMR spectroscopy, which reveals that the movements of the DB24C8 macroring are very highly dependent both on solvent properties and on the nature of the spacer unit linking the two dialkylammonium centers. Thus, DB24C8 shunts facilely between the dialkylammonium centers when the shuttles are dissolved in solvents that readily donate their nonbonding electrons into noncovalent bonds, e.g., DMF, and when spacer units that do not offer much steric resistance to shuttling, e.g., hexamethylene, are used. On the other hand, shuttling is difficult in solvents that are less inclined to donate their electrons into noncovalent bonds, e.g., (CDCl2)2, and when relatively bulky benzenoid spacer units, e.g., p-xylylene, link the two dialkylammonium centers. It has been proposed that the DB24C8 might act as a "ferry" which carries a proton between dialkylammonium and dialkylamine moieties in a singly protonated [2]rotaxane by means of ion-dipole interactions.     

Fluorescent Alteration on a Bistable Molecular Shuttle

Solvent driven molecular shuttles containing a pyrene‐connected macrocycle and an intramolecular charge‐transfer (ICT) chromophore stopper are constructed. In one of the molecular shuttles, a long C‐10 chain is introduced in the thread to separate the peptide station and the ICT stopper. The macrocycle stays in the peptide station in apolar solvents and moves to the C10‐chain station in highly polar solvents. This moving process alters the electronic interaction between the pyrene unit in the macrocycle and the ICT stopper, which induces the change of the pyrene fluorescence emission. The molecular shuttle exhibits stronger emission when the macrocycle is adjacent to the ICT stopper.     

The Effect of Incorporating Fréchet Dendrons into Rotaxanes and Molecular Shuttles Containing the 1,2‐Bis(pyridinium)ethane⊂[24]Crown‐8 Templating Motif

Fréchet‐type dendrons (G0–G3) were added as both axle stoppering units and cyclic wheel appendages in a series of [2]rotaxanes, [3]rotaxanes, and molecular shuttles that employ 1,2‐bis(pyridinium)ethane axles and 24‐membered crown ethers wheels. The addition of dendrimer wedges as stoppering units dramatically increased the solubility of simple [2]rotaxanes in nonpolar solvents. The X‐ray structure of a G1‐stoppered [2]rotaxane shows how the dendritic units affect the structure of the interlocked components. Increased solubility allows observation of how the interaction of dendritic units on separate components in interlocked molecules influences switching properties and molecular size. In a series of [2]rotaxane molecular shuttles incorporating two recognition sites, it was demonstrated that an increase in generation on either the stoppering unit or cyclic wheel could influence both the rate of shuttling and the site preference of the wheel on the axle.     

Charge transfer chromophore-stopped [2]rotaxane through [2 + 2] cycloaddition

Three charge-transfer chromophore-terminated [2]rotaxanes were synthesized, using a high-yield [2 + 2]cycloaddition reaction in apolar solvent at room temperature. Two solvent-driving molecular shuttles were constructed, which exhibit distinct conformations in different solvent as a result of the shuttling movement of the macrocycle.     

Directional Shuttling of a Stimuli‐Responsive Cone‐Like Macrocycle on a Single‐State Symmetric Dumbbell Axle

Rotaxane‐based molecular shuttles are often operated using low‐symmetry axles and changing the states of the binding stations. A molecular shuttle capable of directional shuttling of an acid‐responsive cone‐like macrocycle on a single‐state symmetric dumbbell axle is now presented. The axle contains three binding stations: one symmetric di(quaternary ammonium) station and two nonsymmetric phenyl triazole stations arranged in opposite orientations. Upon addition of an acid, the protonated macrocycle shuttles from the di(quaternary ammonium) station to the phenyl triazole binding station closer to its butyl groups. This directional shuttling presumably originates from charge repulsion and an orientational binding preference between the cone‐like cavity and the nonsymmetric phenyl triazole station. This mechanism for achieving directional shuttling by manipulating only the wheels instead of the tracks is new for artificial molecular machines.     

Three state redox-active molecular shuttle that switches in solution and on a surface

Although the desirability of developing synthetic molecular machine systems that can function on surfaces is widely recognized, to date the only well-characterized examples of electrochemically switchable rotaxane-based molecular shuttles which can do so are based on the tetracationic viologen macrocycle pioneered by Stoddart. Here, we report on a [2]rotaxane which features succinamide and naphthalene diimide hydrogen-bonding stations for a benzylic amide macrocycle that can shuttle and switch its net position both in solution and in a monolayer. Three oxidation states of the naphthalene diimide unit can be accessed electrochemically in solution, each one with a different binding affinity for the macrocycle and, hence, corresponding to a different distribution of the rings between the two stations in the molecular shuttle. Cyclic voltammetry experiments show the switching to be both reversible and cyclable and allow quantification of the translational isomer ratios (thermodynamics) and shuttling dynamics (kinetics) for their interconversion in each state. Overall, the binding affinity of the naphthalene diimide station can be changed by 6 orders of magnitude over the three states. Unlike previous electrochemically active amide-based molecular shuttles, the reduction potential of the naphthalene diimide unit is sufficiently positive (-0.68 V) for the process to be compatible with operation in self-assembled monolayers on gold. Incorporating pyridine units into the macrocycle allowed attachment of the shuttles to an acid-terminated self-assembled monolayer of alkane thiols on gold. The molecular shuttle monolayers were characterized by X-ray photoelectron spectroscopy and their electrochemical behavior probed by electrochemical impedance spectroscopy and double-potential step chronoamperometry, which demonstrated that the redox-switched shuttling was maintained in this environment, occurring on the millisecond time scale.     

Bis-p-xylyl[26]crown-6/pyridinium ion recognition: one-pot synthesis of molecular shuttles

Simple one-pot syntheses allow the preparation of [2]rotaxane-based degenerate molecular shuttles featuring the recognition of pyridinium ions by BPX26C6 macrocycles. Because of the weak interactions between the BPX26C6 and pyridinium units in the [2]rotaxanes in CD₃COCD₃, the rates of shuttling of the BPX26C6 moieties between the pyridinium stations are rapid, relative to those of DB24C8-based shuttles, on the NMR spectroscopic time scale at ambient temperature.     

Stereoprogrammed direct synthesis of calixarene-based [3]rotaxanes

Directional calixarene wheels were threaded along a bis(benzylalkylammonium) axle in a stereoprogrammed way to obtain the first examples of calixarene-based [3]rotaxanes. The base/acid treatment demonstrated that these systems act as nanosized molecular shuttles. An unprecedented switching between the tail-to-tail and head-to-head relative orientation of the calix-wheels was observed.     

Replacement of a proline with silaproline causes a 20-fold increase in the cellular uptake of a Pro-rich peptide

The results presented here show that elementary design enhancements have led to a 20-fold increase in the cellular uptake properties of a Pro-rich cell-penetrating peptide. These results are relevant not only due to the increasing interest in using CPPs as molecular shuttles for intracellular drug delivery but also because they illustrate the power of combining conformational analysis with rational design to modulate the behavior of biologically active compounds.     

Rotaxanes as ligands: from molecules to materials

This tutorial review documents the discovery and application of the supramolecular template (1,2-bis(pyridinium)ethane) subset (24-crown-8) for preparing interlocked molecules. Focus is on the supramolecular chemistry of the pseudorotaxanes formed with various pyridinium axles and crown ether wheels and how this particular class of mechanically linked molecule has been (i) used to construct rudimentary molecular machines such as molecular shuttles and flip switches, (ii) used as ligands for coordination chemistry and (iii) used to create metal-organic framework (MORF) materials.     

Dual modulation of a molecular switch with exceptional chiroptical properties

The ability to modulate the chiroptical properties of optically active molecules induced by external stimuli such as light, heat, and electrical fields allows for the design and development of molecular switches, memory devices, sensors, and photonic devices. A helical o-terphenyl compound functionalized with photoresponsive azobenzene and electroactive imide groups is designed as a dual-mode chiroptical molecular switch. Its exceptional optical activity (e.g., [alpha]436 = -9500) can be changed and modulated through photoisomerization of the azobenzene moiety using UV and visible light. Reversible modulation by electrochemical means was also achieved through the redox reaction occurring at the imide group. Large chiroptical read-out signals were observed during the redox cycles as indicated by the molar ellipticity values as high as 285,000 deg.cm2.dmol-1. Exceptionally high optical activity and large responses to both light and electrical bias make this chiral molecule suitable for the development of new molecular switches, sensors, and other optical devices.     

Guidance of actin filament elongation on filament-binding tracks

Biomolecular motors, which convert chemical energy into mechanical work in intracellular processes, have high potential in bionanotechnology in vitro as molecular shuttles or nanoscale actuators. In this context, guided elongation of actin filaments in vitro could be used to lay tracks for myosin motor-based shuttles or to direct nanoscale actuators based on actin filament end-tracking motors. To guide the direction of filament polymerization on surfaces, microcontact printing was used to create tracks of chemically modified myosin, which binds to, but cannot exert force on, filaments. These filament-binding tracks captured nascent filaments from solution and guided the direction of their subsequent elongation. The effect of track width and protein surface density on filament alignment and elongation rate was quantified. These results indicate that microcontact printing is a useful method for guiding actin filament polymerization in vitro for biomolecular motor-based applications.     

The magic effect of endocyclic but non-sterically hindering biisoquinoline chelates: From fast-moving molecular shuttles to [3]rotaxanes

The use of 8,8′-diaryl-substituted 3,3′-biisoquinolines allows the construction of new multi-component assemblies that are inaccessible with the 2,9-diaryl-substituted 1,10-phenanthroline ligands previously developed by the Sauvage group. This is due to the sterically non-hindering nature of the new chelates, which makes three-component entanglements around octahedral metal centres such as iron(II), cobalt(II) and ruthenium(II) readily possible. Among the newly synthesized molecular assemblies are [3]rotaxanes and [3]pseudorotaxanes in which two molecular strings pass through a single macrocycle, as well as molecular shuttles that exhibit greatly improved shuttling kinetics when compared to previously investigated molecular machines that are based on copper(I)/copper(II) redox chemistry.     

Optically sensed, molecular shuttles driven by acid-base chemistry

A pair of bistable [2]rotaxane, molecular shuttles were prepared that combine 1,2-bis(pyridinium)ethane and benzylanilinium recognition sites; acid-base controlled shuttling of DB24C8 was accompanied by a change in colour and/or fluorescence intensity.     

New Concepts in Tetrathiafulvalene Chemistry

Tetrathiafulvalene (TTF) and its derivatives were originally prepared as strong electron-donor molecules for the development of electrically conducting materials. This Review emphasizes how TTF and its derivatives offer new and in some cases little-exploited possibilities at the molecular to the supramolecular levels, as well as in macromolecular aspects. TTF is a well-established molecule whose interest goes beyond the field of materials chemistry to be considered an important building block in supramolecular chemistry, crystal engineering, and in systems able to operate as machines. At the molecular level, TTF is a readily available molecule which displays a strong electron-donor ability. However, its use as a catalyst for radical-polar crossover reactions, thus mimicking samarium iodide chemistry, has only recently been addressed. Important goals have been achieved in the use of TTF at the macromolecular level where TTF-containing oligomers, polymers, and dendrimers have allowed the preparation of new materials that integrate the unique properties of TTF with the processability and stability that macromolecules display. The TTF molecule has also been successfully used in the construction of redox-active supramolecular systems. Thus, chemical sensors and redox-switchable ligands have been prepared from TTF while molecular shuttles and molecular switches have been prepared from TTF-containing rotaxanes and catenanes. A large synthetic effort has been devoted to the preparation of the so-called organic ferromagnets, many of which are derived from TTF. The main task in these systems is the introduction of ferromagnetic coupling between the conduction electrons and localized electrons. TTF has also played a prominent role in molecular electronics where TTF-containing D-sigma-A molecules have allowed the preparation of the first confirmed unimolecular rectifier. Recently, it has been confirmed that TTF can display efficient nonlinear optic (NLO) responses in the second and third harmonic generation as well as a good thermal stability. These findings can be combined with the redox ability of TTF as an external stimuli to provide a promising strategy for the molecular engineering of switchable NLO materials. Fullerenes endowed with TTF exhibit outstanding photophysical properties leading to charge-separated (CS) states that show remarkable lifetimes.     

Hybrid Organic–Inorganic Rotaxanes,Including a Hetero‐Hybrid [3]Rotaxane Featuring Two Distinct Heterometallic Rings and a Molecular Shuttle

[2] and [3] hybrid rotaxanes are reported based on {Ti₇M} rings (M is a trivalent metal such as Fe^III or Ga^III). NMR studies show that [2]rotaxanes can act as molecular shuttles, while EPR studies of [3]rotaxanes show weak interactions between the paramagnetic components of the supramolecular assemblies.