Controlled disassembling of self-assembling systems: Toward artificial molecular-level devices and machines

Abstract

Investigations on self-assembling/induced-disassembling systems have led to the design of molecular-level devices capable of performing a variety of functions. Some of the work carried out in this field is illustrated.

Molecular self-assembly, a concept central to nature's forms and functions (1), is an important route toward the construction of artificial molecular-level devices and machines (2–14). The challenge for chemists engaged in this field resides in the “programming” of the system (15), i.e., in the design and synthesis of components that carry within their structures the pieces of information necessary not only for the construction of the desired supramolecular architecture but also for the performance of the required function. In the case of systems based on self-assembling, the function to be performed is often related to the occurrence of a reversible assembling/disassembling process. The system therefore has to be programmed to be able not only to self-assemble under thermodynamic control (Fig. 1a) but also to disassemble under a suitable energy input (Fig. 1b). Disassembling, of course, implies a chemical transformation of one of the assembled partners. As mentioned above, an important requirement is the possibility of repeating the self-assembling/induced-disassembling process at will (cyclic process), which implies the reset of the system (Fig. 1c) after induced disassembling has taken place.

Figure 1.

Schematic representation of a self-assembling/induced-disassembling/reset process that can be exploited to design molecular-level devices.

In this Perspective, we will illustrate some of the work we have done on self-assembling/induced-disassembling/reassembling processes with the specific purpose of designing systems capable of playing the role of molecular-level devices and machines.

Self-Assembling.

For the design of molecular-level devices, the system has to exist in well defined states. This means that the self-assembling equilibrium (Fig. 1a) has to be strongly displaced toward the assembled species. Therefore, the interaction driving the assembly must be relatively strong. The systems illustrated in this Perspective are based on charge–transfer and hydrogen-bonding interactions between molecular species that have been suitably designed to maximize such interactions. The designing principles include: (i) choice of molecular components that can give rise to an intimate supramolecular structure, usually in the form of a pseudorotaxane; (ii) presence of complementary strongly interacting (electron-donor/electron-acceptor, acid/base) units; (iii) multipoint interactions; and (iv) choice of a suitable weakly interacting solvent.

Induced Disassembling.

Disassembling of a thermodynamically stable supramolecular structure requires the destruction of the interaction responsible for the association process. This result can be achieved by an appropriate chemical reaction that transforms one of the assembled partners (Fig. 1b). For example, when the interaction responsible for complexation is donor/acceptor in nature, it can be destroyed by oxidation of the electron-donor unit or reduction of the electron-acceptor one. When the association is based on hydrogen bonding involving an ammonium center (N⁺—H···O), it can be easily destroyed by addition of a base. Once again, disassembling must be complete to avoid loss of definition of the system. The reaction needed to cause disassembling can be obtained by chemical, photochemical, or electrochemical stimulation (8, 13).

Reassembling (Reset).

An important requirement for any kind of device is the possibility of repeating the operation at will (cyclic process). When applied to the molecular-level systems described here, this concept implies reassembling, which can take place only after the occurrence of a chemical reaction opposite that responsible for disassembling (Fig. 1c). For example, if disassembling was obtained by reduction of an electron-acceptor unit, oxidation of such unit has to be performed to restore the donor/acceptor interaction and achieve reassembling.

Plug/Socket and Extension Devices

A macroscopic plug/socket system is characterized by, (i) the possibility of connecting/disconnecting the two components in a reversible way, and (ii) the occurrence of an energy flow from the socket to the plug when the two components are connected. Self-assembling supramolecular systems have recently been designed that can perform as molecular-level plug/socket devices. In the system illustrated in Fig. 2 (16), the plug-in function is related to the threading, driven by the formation of strong hydrogen bonds, of a (±)-binaphthocrown ether (1) by a (9-anthracenyl)benzylammonium ion (2H⁺), obtained by protonation of amine 2. In the plugged-in pseudorotaxane structure [1⋅2H]⁺, light excitation of the binaphthyl unit of the crown causes not binaphthyl fluorescence but the sensitized fluorescence of the anthracenyl unit of the thread, showing that electronic energy transfer takes place between the two chromophoric groups. The rate constant of the energy-transfer process is higher than 4 × 10⁹ s⁻¹. Addition of a stoichiometric amount of base, which deprotonates the ammonium ion, causes the reappearance of the binaphthyl fluorescence and the disappearance of the sensitized anthracenyl fluorescence, demonstrating that plug-out of the pseudorotaxane structure has occurred. The plug/socket molecular-level concept can be extended straightforwardly to the construction of systems where, (i) light excitation induces an electron flow instead of an energy flow, and (ii) the plug-in/plug-out function is stereoselective.

Figure 2.

Acid/base controlled plug-in/plug-out of the (9-anthracenyl)benzylammonium ion 2H⁺ with the (±)-binaphthocrown ether 1. The occurrence of photoinduced energy transfer in the plug-in state is schematized.

The above concept has more recently been used to design and construct a self-assembling supramolecular system that mimics, at the molecular level, the function played by a macroscopic extension.

Electrochemically Controlled Switches

In self-assembling systems based on electron donor/acceptor interactions, induced disassembling/self-reassembling can be controlled by redox processes, which make possible the design of electrochemical switching devices (17). An example of a supramolecular system that can be switched reversibly in three different states through electrochemical control of the properties of one component is illustrated in Fig. 3 (18). Tetrathiafulvalene (TTF) is stable in three different oxidation states, TTF(0), TTF⁺, and TTF²⁺. On oxidation, the electron-donor power of tetrathiafulvalene decreases with a concomitant increase in the electron-acceptor properties. Whereas TTF(0) plays the role of electron donor and gives a 1:1 charge–transfer complex with the electron-acceptor macrocycle 3⁴⁺, TTF²⁺ plays the role of an electron acceptor and gives rise to a charge–transfer complex with the electron-donor macrocycle 4. TTF⁺ does not show any electron-donor/electron-acceptor character. The system illustrated in Fig. 3 consists of an MeCN solution containing TTF and the two macrocycles 3⁴⁺ and 4. Electrochemical experiments show that, depending on the potential value, TTF can be (i) free in the TTF⁺ state, (ii) complexed with the electron-acceptor 3⁴⁺ host in the TTF(0) state, or (iii) complexed with the electron-donor host 4 in the TTF²⁺ state. In such a three-state system, switching (“writing”) can be performed electrochemically, and the state of the system can be monitored (“reading”) by spectroscopic and electrochemical techniques. An interesting aspect of this system, from the applicative viewpoint, is that the three different states (Fig. 3) exhibit different colors (electrochromic behavior).

Figure 3.

Components of a three-pole supramolecular switch and schematic representation of the ranges of electrochemical stability of the three states available to the system.

Photochemically Driven Piston/Cylinder Systems

Dethreading/rethreading of the wire and ring components of a pseudorotaxane reminds the movement of a piston in a cylinder. To control this function by light excitation, a “light-fueled” motor (19) (i.e., a photosensitizer) has been incorporated in the wire (Fig. 4a) (20) or in the macrocyclic ring (Fig. 4b) (21) of pseudorotaxanes based on donor/acceptor interactions. In both cases, in deaerated solution, excitation of the photosensitizer with visible light in the presence of a sacrificial electron donor (e.g., triethanolamine) causes reduction of an electron-acceptor unit and, as a consequence, dethreading takes place. Rethreading can be obtained by allowing oxygen to enter the solution. Photochemical systems that rely on such a sensitizer–scavenger strategy, however, produce waste species from the decomposition of the reductant scavenger and from the consumption of dioxygen.

Figure 4.

Light-driven dethreading of pseudorotaxanes by excitation of a photosensitizer (a) contained as a stopper in the wire-type component, and (b) incorporated in the macrocyclic ring.

A system in which dethreading/rethreading is exclusively governed by light energy without generation of any waste product is illustrated in Fig. 5 (22). The thread-like species trans-5, which contains a π-electron-rich azobiphenoxy unit, and the electron-accepting host 6⁴⁺ self-assemble very efficiently in MeCN solution, as shown by the complete quenching in the pseudorotaxane structure of the characteristic fluorescence of 6⁴⁺. Irradiation with 365-nm light of a solution containing trans-5 and 6⁴⁺ (80% complexed species) causes photoisomerization of trans-5 to cis-5. Because the interaction of 6⁴⁺ with cis-5 is much weaker than that with trans-5, photoexcitation causes a dethreading process (Fig. 5), as indicated by the strong increase in the fluorescence intensity of 6⁴⁺. On irradiation at 436 nm or by warming the solution in the dark, trans-5 is reformed and rethreads inside 6⁴⁺, restoring the initial structure.

Figure 5.

Controllable detreading/rethreading of a pseudorotaxane based on a trans-cis photoisomerization reaction.

Logic Gates

Computers are based on semiconductor logic gates that perform binary operations. Logic gates are switches whose output state (0 or 1) depends on the input conditions (0 or 1). Molecular systems that can perform simple YES and NOT logic operations are very common, and fluorescence is a particularly useful signal to monitor such operations (5). To perform more complex logic operations, however, carefully designed supramolecular systems are needed. Fig. 6 shows schematically the changes that have to occur in a chemical system to perform the AND, OR, and XOR fundamental logic operations under the action of two chemical inputs (X and Y). For illustration purposes, the equivalent (from a logic viewpoint) electric circuits are also shown. In the equivalent circuit of the AND gate, the switches are connected in series (Fig. 6a), whereas in the case of the OR gate, the switches are connected in parallel (Fig. 6b). Interesting examples of supramolecular systems that behave as AND or OR gates have been reported (23, 24).

Figure 6.

Schematic representation of a chemical system (P) which performs the AND (a), OR (b), and XOR (c) logic operations under the action of two chemical inputs (X and Y). The truth tables of such operations are also shown, along with their representations based on electric circuit schemes.

An XOR (eXclusive OR) gate is a much more complex device, as one can understand from the fact that its equivalent circuit contains two bipolar switches (Fig. 6c). The supramolecular system illustrated in Fig. 7 performs according to the XOR logic (25). The pseudorotaxane [7⋅8]²⁺ results from self-assembly, in CH₂Cl₂/MeCN 9:1 solution, of the electron-accepting 2,7-dibenzyldiazapyrenium dication 7²⁺ with the crown ether 8, which contains two 2,3-dioxynaphthalene electron-donating units. In the pseudorotaxane structure, the electron-deficient diazapyrenium unit is sandwiched between the electron-rich 2,3-dioxynaphthalene units of 8. Because of the electron-donor/electron-acceptor interaction, a low energy charge–transfer excited state is formed that is responsible, inter alia, for the disappearance of the strong fluorescence exhibited by 8 (λ_max = 343 nm). On addition of tributylamine (B, Fig. 7), a 1:2 adduct [7⋅B₂]²⁺ is formed between the 2,7-dibenzyldiazapyrenium dication and the amine, with the consequent dethreading of the pseudorotaxane structure (process I in Fig. 7). This process causes strong spectral changes, including the recovery of the fluorescence of 8. Subsequent addition of a stoichiometric amount of trifluoromethanesulfonic acid unlocks 7²⁺ from the [7⋅B₂]²⁺ adduct and allows rethreading between 7²⁺ and 8 to give back the pseudorotaxane [7⋅8]²⁺ (process II in Fig. 7). This process, of course, is accompanied by spectral changes opposite those observed on addition of amine. Processes I and II can be repeated on the same solution by repeating the addition of amine and acid. As shown in Fig. 7, the dethreading/rethreading cycle can also be performed by reverting the order of the two inputs. Processes I and III both cause a strong increase of emission intensity at 343 nm, which is cancelled by processes II and IV, respectively. Therefore, the described supramolecular system shows the input/output relationships indicated by the truth table of the XOR logic gate (Fig. 6c): the strong fluorescent signal at 343 nm is present (output: 1) only when either amine or H⁺ (inputs X and Y in the truth table) are added (i.e., X: 1 and Y: 0, or vice versa); conversely, the fluorescent signal is absent (output: 0) when none or both of the inputs are present (i.e., X = Y: 0 or X = Y: 1).

Figure 7.

Schematic representation of the threading/dethreading pattern of pseudorotaxane [7⋅8]²⁺, which corresponds to an XOR logic function.

Conclusion

In the last few years, investigations of self-assembling/induced-disassembling processes have led to the design of molecular-level devices capable of performing a variety of functions. It should be noted, of course, that the described systems work in solution, i.e., incoherently. For most kinds of applications, they need to be interfaced with the macroscopic world by ordering them in some way, for example at an interface or on a surface, so they can behave coherently either in parallel or in series. Research on this topic is developing at a growing rate (11, 12).

The extension of the concept of a device to the molecular level is of interest not only for the development of nanotechnology but also for the growth of basic research. Looking at supramolecular chemistry from the viewpoint of functions with references to devices of the macroscopic world is indeed a very interesting exercise that introduces novel concepts into chemistry as a scientific discipline.