Toward Electrochemically Controllable Tristable Three-Station [2]Catenanes

The Story of the Little Blue Box: A Tribute to Siegfried Hünig

The tetracationic cyclophane, cyclobis(paraquat-p-phenylene), also known as the little blue box, constitutes a modular receptor that has facilitated the discovery of many host-guest complexes and mechanically interlocked molecules during the past 35 years. Its versatility in binding small π-donors in its tetracationic state, as well as forming trisradical tricationic complexes with viologen radical cations in its doubly reduced bisradical dicationic state, renders it valuable for the construction of various stimuli-responsive materials. Since the first reports in 1988, the little blue box has been featured in over 500 publications in the literature. All this research activity would not have been possible without the seminal contributions carried out by Siegfried Hünig, who not only pioneered the syntheses of viologen-containing cyclophanes, but also revealed their rich redox chemistry in addition to their ability to undergo intramolecular π-dimerization. This Review describes how his pioneering research led to the design and synthesis of the little blue box, and how this redox-active host evolved into the key component of molecular shuttles, switches, and machines.

Probing the Electrostatic Barrier of Tetrathiafulvalene Dications using a Tetra-stable Donor-Acceptor [2]Rotaxane

A tetra-stable donor-acceptor [2]rotaxane 1⋅4PF₆ has been synthesized. The dumbbell component is comprised of an oxyphenylene (OP), a tetrathiafulvalene (TTF), a monopyrrolo-TTF (MPTTF), and a hydroquinone (HQ) unit, which can act as recognition sites (stations) for the tetra-cationic cyclophane cyclobis(paraquat-p-phenylene) (CBPQT⁴⁺ ). The TTF and the MPTTF stations are located in the middle of the dumbbell component and are connected by a triethylene glycol (TEG) chain in such a way that the pyrrole moiety of the MPTTF station points toward the TTF station, while the TTF and MPTTF stations are flanked by the OP and HQ stations on their left hand side and right hand side, respectively. The [2]rotaxane was characterized in solution by ¹ H NMR spectroscopy and cyclic voltammetry. The spectroscopic data revealed that the majority (77 %) of the tetra-stable [2]rotaxane 1⁴⁺ exist as the translational isomer 1⋅MPTTF⁴⁺ in which the CBPQT⁴⁺ ring encircles the MPTTF station. The electrochemical studies showed that CBPQT⁴⁺ in 1⋅MPTTF⁴⁺ undergoes ring translation as result of electrostatic repulsion from the oxidized MPTTF unit. Following tetra-oxidation of 1⋅MPTTF⁴⁺ , a high-energy state of 1⁸⁺ was obtained (i.e., 1⋅TEG⁸⁺ ) in which the CBPQT⁴⁺ ring was located on the TEG linker connecting the di-oxidized TTF²⁺ and MPTTF²⁺ units. ¹ H NMR spectroscopy carried out in CD₃ CN at 298 K on a chemically oxidized sample of 1⋅MPTTF⁴⁺ revealed that the metastable state 1⋅TEG⁸⁺ is only short-lived with a lifetime of a few minutes and it was found that 70 % of the positively charged CBPQT⁴⁺ ring moved from 1⋅TEG⁸⁺ to the HQ station, while 30 % moved to the much weaker OP station. These results clearly demonstrate that the CBPQT⁴⁺ ring can cross both an MPTTF²⁺ and a TTF²⁺ electrostatic barrier and that the free energy of activation required to cross MPTTF²⁺ is ca. 0.5 kcal mol^-1 smaller as compared to TTF²⁺ .

Through-the-annulus threading of the larger calix[8]arene macrocycle

A complete study of the through-the-annulus threading of the larger calix[8]arene macrocycle with di-n-alkylammonium cations has been performed in the presence of the "superweak" TFPB counterion. Thus, it was found that such threading occurs only upon partial preorganization of the calix[8]arene macroring by intramolecular bridging. In particular, 1,5-bridged calix[8]arenes with a meta- or para-xylylene bridge (2 and 3) gave pseudo[2]rotaxanes in which one dialkylammonium axle (4a-4e(+)) was threaded into one of the two subcavities of the calix[8]-wheel. Conformational studies by using chemical shift surface maps and DFT calculations evidenced a 3/4-cone geometry for these subcavities. Higher pseudorotaxane K(ass) values were obtained for calix[8]-wheels 2 and 3, with respect to calix[6]-host 1a, due to the cooperative effect of their two subcavities. Dynamic NMR studies on calix[8]-pseudorotaxanes evidenced a direct correlation between K(ass) (and ΔG(ass)) values and energy barriers for calix inversion due to the effectiveness of thread templation. In accordance with DFT calculations, an endo-alkyl preference, over the endo-benzyl one, was observed by threading calix[8]-wheel 3 with the directional n-butylbenzylammonium axle 4d(+).

Catenation of calixarene annulus

Through-the-annulus-catenated calixarenes have been obtained, for the first time, by exploiting the "superweak anion" approach that allows the threading of the calix cavity with functionalized dialkylammonium axles. In addition, the first example of a stereoprogrammed synthesis of a catenane orientational isomer (an oriented calix[2]catenane) has been obtained, after macrocyclization, by using a directional alkylbenzylammonium axle.

Hysteretic Tricolor Electrochromic Systems Based on the Dynamic Redox Properties of Unsymmetrically Substituted Dihydrophenanthrenes and Biphenyl-2,2'-Diyl Dications: Efficient Precursor Synthesis by a Flow Microreactor Method

A series of biphenyl-2,2'-diylbis(diarylmethanol)s 3, which have two kinds of aryl groups at the bay region, were efficiently obtained by integrated flow microreactor synthesis. The diols 3NO/NX are the precursors of unsymmetric biphenylic dications 2NO/NX²⁺, which are transformed into the corresponding dihydrophenanthrenes 1NO/NX via 2NO/NX^+• upon reduction, when they exhibit two-stage color changes. On the other hand, the steady-state concentration of the intermediate 2NO/NX^+• is negligible during the oxidation of 1NO/NX to 2NO/NX²⁺, which reflects unique tricolor electrochromicity with a hysteretic pattern of color change [color 1→color 2→color 3→color 1].

Redox control of molecular motion in switchable artificial nanoscale devices

The design, synthesis, and operation of molecular-scale systems that exhibit controllable motions of their component parts is a topic of great interest in nanoscience and a fascinating challenge of nanotechnology. The development of this kind of species constitutes the premise to the construction of molecular machines and motors, which in a not-too-distant future could find applications in fields such as materials science, information technology, energy conversion, diagnostics, and medicine. In the past 25 years the development of supramolecular chemistry has enabled the construction of an interesting variety of artificial molecular machines. These devices operate via electronic and molecular rearrangements and, like the macroscopic counterparts, they need energy to work as well as signals to communicate with the operator. Here we outline the design principles at the basis of redox switching of molecular motion in artificial nanodevices. Redox processes, chemically, electrically, or photochemically induced, can indeed supply the energy to bring about molecular motions. Moreover, in the case of electrically and photochemically induced processes, electrochemical and photochemical techniques can be used to read the state of the system, and thus to control and monitor the operation of the device. Some selected examples are also reported to describe the most representative achievements in this research area.

Photoresponsive host-guest systems based on a new azobenzene-containing cryptand

A new azobenzene-bridged cryptand was synthesized; it can be well controlled between cis and trans isomers by irradiation with different wavelengths or by being heated. It was found that this cryptand exhibits an ON-OFF binding ability with 2,7-diazapyrenium (DAP) derivatives. It binds DAP derivatives as the cis isomer only, as demonstrated by various methods. The fluorescence property of DAP derivatives endows these host-guest systems with detectable fluorescent output signals, which makes monitoring these photoresponsive systems convenient.

Changing stations in single bistable rotaxane molecules under electrochemical control

We have directly observed electrochemically driven single-molecule station changes within bistable rotaxane molecules anchored laterally on gold surfaces. These observations were achieved by employing molecular designs that significantly reduced the mobility and enhanced the assembly of molecules in orientations conducive to direct measurement using scanning tunneling microscopy. The results reveal molecular-level details of the station changes of surface-bound bistable rotaxane molecules, correlated with their different redox states. The mechanical motions within these mechanically interlocked molecules are influenced by their interactions with the surface and with neighboring molecules, as well as by the conformations of the dumbbell component.

Beta-substituted terthiophene [2]rotaxanes

Two kinds of beta-substituted terthiophene [2]rotaxanes were synthesized using the host-guest pairs of the electron-deficient cyclophane cyclobis(paraquat-p-phenylene) (CBPQT(4+)) and the electron-rich terthiophenes with diethyleneglycol chains at the beta-position. One is made from the alpha-position non-substituted terthiophene (3 T-beta-Rx) and the other is made from the alpha-dibromo-substituted terthiophene (3 TBr-beta-Rx). The binding constants of the beta-substituted terthiophene threads were confirmed to be smaller than that of the alpha-substituted terthiophene analogue. By UV/Vis absorption measurements, we confirmed the charge-transfer (CT) band in the visible region with an extinction coefficient of approximately 10(2) (M(-1) cm(-1)). Strong, but not quantitative, quenching of the terthiophene fluorescence was confirmed for the [2]rotaxanes. Although the beta-substituted terthiophene thread was electrochemically polymerizable, the [2]rotaxane 3 T-beta-Rx was not polymerizable. This result indicates that the interlocked CBPQT(4+) macrocycle effectively suppresses the electrochemical polymerization of the terthiophene unit because electrostatic repulsive and steric effects of CBPQT(4+) hinder the dimerization of the terthiophene radical cations. In the electrochemical measurement, we confirmed the shift of the first reduction peak towards less negative potential compared to free CBPQT(4+) and the splitting of the second reduction peak. These electrochemical behaviors are similar to those observed for the highly-constrained [2]rotaxanes. The beta-substituted terthiophene [2]rotaxanes reported herein are important key compounds to prepare polythiophene polyrotaxanes.

A mechanical actuator driven electrochemically by artificial molecular muscles

A microcantilever, coated with a monolayer of redox-controllable, bistable [3]rotaxane molecules (artificial molecular muscles), undergoes reversible deflections when subjected to alternating oxidizing and reducing electrochemical potentials. The microcantilever devices were prepared by precoating one surface with a gold film and allowing the palindromic [3]rotaxane molecules to adsorb selectively onto one side of the microcantilevers, utilizing thiol-gold chemistry. An electrochemical cell was employed in the experiments, and deflections were monitored both as a function of (i) the scan rate (< or =20 mV s(-1)) and (ii) the time for potential step experiments at oxidizing (>+0.4 V) and reducing (<+0.2 V) potentials. The different directions and magnitudes of the deflections for the microcantilevers, which were coated with artificial molecular muscles, were compared with (i) data from nominally bare microcantilevers precoated with gold and (ii) those coated with two types of control compounds, namely, dumbbell molecules to simulate the redox activity of the palindromic bistable [3]rotaxane molecules and inactive 1-dodecanethiol molecules. The comparisons demonstrate that the artificial molecular muscles are responsible for the deflections, which can be repeated over many cycles. The microcantilevers deflect in one direction following oxidation and in the opposite direction upon reduction. The approximately 550 nm deflections were calculated to be commensurate with forces per molecule of approximately 650 pN. The thermal relaxation that characterizes the device's deflection is consistent with the double bistability associated with the palindromic [3]rotaxane and reflects a metastable contracted state. The use of the cooperative forces generated by these self-assembled, nanometer-scale artificial molecular muscles that are electrically wired to an external power supply constitutes a seminal step toward molecular-machine-based nanoelectromechanical systems (NEMS).

A bistable poly[2]catenane forms nanosuperstructures

Side-chain poly[2]catenanes at the click of a switch! A bistable side-chain poly[2]catenane has been synthesized and found to form hierarchical self-assembled hollow superstructures of nanoscale dimensions in solution. Molecular electromechanical switching (see picture) of the material is demonstrated, and the ground-state equilibrium thermodynamics and switching kinetics are examined as the initial steps towards processible molecular-based electronic devices and nanoelectromechanical systems.