Supramolecular switching

Figure 11. Schematic representation of the concept of a Figure 10 A, B, and C are suitably chosen chromophoric three-pole supramolecular switching of energy transfer. The groups. For more details, see the text, cartoons correspond to the molecular components shown in...

Scheme 15.2 Cartoon displaying the electrochemically reversible three-pole supramolecular switch based on the three-component mixture CBPQT4 + -1/5DN38C10-TTF. (See the color version of this figure in Color Plates section.)...

In summary, phthalocyanines modified with crown ethers are interesting synthetic targets as they are prone to form columnar phases. Their electron conductivity and complexation properties make them interesting candidates for the design of sensor materials or supramolecular switches. 

Fig. 18. A chemically driven supramolecular switch. In acetonitrile solution, the cyclophane CBPQT4+ resides preferentially around the better electron-donating thread 12. Addition of an excess of potassium ions to the solution results in a strong complex being formed between these cations and the crown ether portion of thread 12. Electrostatic repulsion between the cyclophane and the thread leads to the cyclophane residing upon the poorer tr-electron donating - yet electrically neutral -thread 13...

The next incremental step up the ladder of complexity of switching was the development [38] of a three-pole supramolecular switch. Such a switch is one that can be adjusted to any one of three different settings. Bolstered by the success and rich electrochemistry of the dual mode switch, a three-pole switch was designed without delay and investigated. 

Fig. 12. A multicomponent supramolecular switching system based on competition between the crown ether 28 and hexylamine (30) for the binding of 29. The binding of 30 depends on its protonation state. The absorbance spectra are adapted from [55] with permission...

Ferrocene (Fc) possesses a rich synthetic chemistry, a three-dimensional structure which allows the preparation of many derivatives, high thermal stability, and good solubility in common organic solvents. These characteristics allow the synthesis of a great variety of liquid-crystalline materials. Furthermore, its unique electrochemical properties (fast and reversible one-electron transfer process) make Fc a valuable building block for the elaboration of redox-active supramolecular switches. In 30 years (1976-2006), liquid-crystalline ferrocenes have been established as a versatile class of metallomesogens. The aim of this chapter is to highlight the main results obtained for... 

Klajn R, Stoddart JF, Grzybowski BA. Nanoparticles functionalised with reversible molecular and supramolecular switches. Chem Soc Rev 2010 39(6) 2203—37. 

Figure 13.75. Supramolecular switches for photoinduced electron transfer (PeT).

Example 3 Supramolecular switching guest reduction drives decomplexation... 

The shape of the CV tidation reflects the mechanism of supramolecular switching. In the CV, the cathodic reduction peak at —0.86 V is shifted 230 mV compared to where it started and it simply decreases in peak intensity. By contrast, the anodic oxidation peak displays a shifting in its position from —0.57 to —0.67 V (only 100 mV). Neither of these new peak positions corresponds to the value of —1.1 V that was employed in the simulation. This situation... 

Figure 9 Supramolecular switching under thermodynamic control. (a) Square scheme for the reduction of G in the presence of host H. The selected thermodynamic parameters correspond to a strong decrease in complex stability upon reduction of the guest, (b) Simulated CV titration corresponding to increasing concentrations of host, from 0 equivalent (red trace) to 5 equivalent (blue trace), (c) Simulated CVs showing the dependence of peak potentials on scan rate 0.01 V s (magenta) and 10 V s (green).

Figure 11 Supramolecular switching under kinetic control, (a) Simulated CV titrations of host (0,0.25,0.5,0.75,1, 2,4, 5 equivalents) into redox-active guest at 0.1 V s . (b) Diffeent simulated CVs showing the scan-rate dependence of the relative peak intensities. The two traces shown correspond to 0.01 Vs (magenta) and 10 Vs (green).

Now that it is possible to measure (or estimate with simulations) the host-guest binding constants of the redox center in two oxidation states, one has the basis for supramolecular switches and molecular machines. In such situations, the CV is able to both stimulate the switching by oxidation or reduction and to analyze the outcome. 

A switch can be as simple as a two state system for example, H G + e H G . However, it is the ensuing chemical step, that is, H-G H + G , which allows the stimulated state (H G ) to potentially do some work (AG). What is unique to systems held together by noncovalent interactions is the ability to achieve motion during this chemical step. A supramolecular switch that leads to the destabilization of the host-guest complex pushes the position of equilibrium from one side of an association reaction to the other, in which the CVs simulated above in Figures 9 and 11 are exemplary. 

All that is required to make a redox-stimulated supramolecular switch is (i) confirmation that the redox change weakened (or strengthened) the complex by an appreciable amount such that (ii) conditions can be selected (see below) to harness the change in stability. A typical switch effects at least a 90% population inversion, that is, from 10 1 to 1 10. For host-guest complexes, this requirement usually translates into a problem of selecting a concentration where the population of the starting state, for example, H G, is 10-fold different from the switched state, for example, H + G . While a ratio of the equilibrium constants of 10 might seem appropriate, recall that... 

TTF c CBPQT Figure 13 A three-pole supramolecular switch. 

MIMs, such as catenanes (see Self-Assembled Links Catenanes, Self-Processes) and rotaxanes (see Rotax-anes—Self-Assembled Links, Self-Processes), share aU the good attributes of supramolecular switches while dispensing with the concentration dependence. The cost involved in this functional simplicity is paid for by more lengthy syntheses. The benefits of the added conhol provided by the molecular format are greater than the sum... 

Kobuke and colleagues described a complementary zinc porphyrin-magnesium phthalocyanine dimer that can be considered as a supramolecular switch (Figure 24). This switch is based on the variation of fluorescence induced by a change of ligands. More precisely, an imidazole-appended porphyrinatozinc-phthalocyaninatomagnesium complex was shown to spontaneously dimerize into a supramolecular short dimer (state I) in which the imidazole units are bound to the central magnesium atom of the Pcs. On addition of 2 equivalents (with respect to the dimer) of dimethyl sulfoxide (DMSO) or 1-methylimidazole to the dimer, a new extended dimer (state II) is formed in which... 

Fulgide derivatives have been widely used as photocontrol-lable units for the design of supramolecular switches. The photochromism of these components occurs between one of the colorless open form and the photocyclized colored form. Fulgides are not luminescent, at room temperature, unless... 

Figure 10 Chemical formula and simplifled electronic states diagram of a three-component supramolecular switch containing a fulgimide interposed between an anthracene, the energy donor, and an amino coumarin ester moiety as the luminescent group, the energy acceptor. (Reprinted from Ref. 22, Copyright (1993), with permission from Elsevier.)...

Tetrathiafulvalene (TTF) is an excellent candidate for redox-driven molecular switch because it can be easily oxidized into TTF" and TTF + ions. In 1999, Stoddart et al. reported a three-pole supramolecular switch (Figure 38), in which the mechanical action was mediated by the electrochemical adjustment of the TTF oxidation... 

Chen H, Yang H, Xu W, Tan Y. A supramolecular switch based on three binding states of a pyrene derivate a reversible three-state switch with only two stimuli. RSC Advances 2013 3 13311-7. 

Another interesting application of water-soluble dendrimers has been described by Stoddart and co-workers and utilises ferrocene containing carbohydrate dendrimers [102]. A ferrocene core substituted with glucopyranosyl residues was found to form complexes with 3-cyclodextrin and has formed the basis for a supramolecular switch. 

MOFs Gated by Pillar[n]arene-based Supramolecular Switches... 

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