Molecular-Level Devices and Machines

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1 Molecular-Level Devices and Machines Vincenzo Balzani, Alberto Credi, Margherita Venturi Concept: The concept of (macroscopic) device can be extended to the molecular level. A molecular-level device can be defined as an assembly of a discrete number of molecular components designed to achieve a Abstract: The concept of a macroscopic device can be extended to the molecular level by designing and synthesizing (supra)molecular species capable of performing specific functions. Molecularlevel devices operate via electronic and/or nuclear rearrangements and, like macroscopic devices, need energy to operate and signals to communicate with the operator. The energy needed to make the device work can be supplied as chemical energy, electrical energy, or light. Among the most useful techniques to monitor the operation of molecular-level devices are spectroscopy (particularly luminescence) and electrochemistry. A molecular-level electronic set for energy and specific function. A molecular-level machine is a particular type of molecularlevel device in which the component parts can display changes in their relative positions as a result of some external stimulus. electron transfer (wires, switches, antennas, plug/ socket, and extension systems) and various kinds of molecular-level machines (tweezers, pyston/cylinder systems, shuttles, systems based on catenanes, rotary motors) have already been synthesized and studied. The extension of the concept of a device to the molecular level is of interest, not only for basic research, but also for the growth of nanoscience and the development of nanotechnology. Molecular-level devices should find applications in information storage, display, and processing; in the long run, they are expected to lead to the construction of molecular-based (chemical) computers. Dipartimento di Chimica G. Ciamician, Università di Bologna, via Selmi 2, Bologna, Italy Prologue Phone: , Fax: , vbalzani ciam.unibo.it, Web: photochem.html In everyday life we make extensive use of macroscopic devices. A macroscopic device is an assembly of components designed to achieve a specific function. Each component of the device performs a simple act, whiletheentiredeviceperformsamorecomplex function, characteristic of the assembly. For example, the function performed by a hairdryer (production of hot wind) is the result of acts performedbyaswitch,aheater,andafan,suitablyconnected by electric wires and assembled in an appropriate framework. The concept of a device can be extended to the molecular level. [1 4] A molecular-level device can be defined as an assembly of a discrete number of molecular components (that is, a supramolecular structure) designed to achieve a specific function. Each molecular component performs a single act, while the entire assembly performs a more complex function, which results from the cooperation of the various molecular components. Molecular-level devices operate via electronic and/or nuclear rearrangements and, like macroscopic devices, are characterized by: (i) the kind of energy input supplied to make them work, (ii) the way in which their operation can be monitored, (iii) the possibility to repeat the operation at will (cyclic process), (iv) the time scale needed to complete a cycle, and (v) the performed function. As far as point (i) is concerned, a chemical reaction can be used, at least in principle, as an energy input. In such a case, however, if the device is to work cyclically [point (iii)], it will need addition of reactants at any step of the working cycle, and the accumulation of by-products resulting from the repeated addition of matter can compromise the operation of the device. On the basis of this consideration, the best energy inputs to make a molecular Keywords Molecular Devices Molecular Machines Molecular Wires Antenna Systems Molecular Switches Plug/socket Systems Pseudorotaxanes Rotaxanes Catenanes Supramolecular Chemistry Photochemistry Electrochemistry Luminescence

2 256 Molecular-Level Devices and Machines device work are photons and electrons (or holes). With appropriately chosen photochemically and electrochemically driven reactions, it is indeed possible to design very interesting molecular devices. In order to control and monitor the device operation [point (ii)], the electronic and/or nuclear rearrangements of the component parts should cause readable changes in some chemical or physical property of the system. In this regard, photochemical and electrochemical techniques are very useful since both photons [4] and electrons (or holes) [5] can play the dual role of writing (i.e., causing a change in the system) and reading (i.e., reporting the state of the system). The operation time scale of molecular devices [point (iv)] can range from less than picoseconds to seconds, depending on the type of rearrangement (electronic or nuclear) and the nature of the components involved. Finally, as far as point (v) is concerned, molecularlevel devices performing various kinds of functions can be imagined; some specific examples will be discussed below. The extension of the concept of a device to the molecular level is also of interest for the growth of nanoscience and the development of nanotechnology. Indeed, the miniaturization of components for the construction of useful devices, which is an essential feature of modern technology, is currently pursued by the large-downward (top-down) approach. This approach, however, which leads physicists and engineers to manipulate progressively smaller pieces of matter, has its intrinsic limitations. An alternative and promising strategy is offered by the small-upward (bottom-up) approach. Chemists, by the nature of their discipline, are already at the bottom, since they are able to manipulate molecules (i.e., the smallest entities with distinct shapes and properties) and are therefore in the ideal position to develop bottom-up strategies for the construction of nanoscale devices. In this chapter we will illustrate examples of three families of molecular-level devices: (i) devices for the transfer of electrons or electronic energy, (ii) devices capable of performing extensive nuclear motions, often called molecular-level machines, and(iii) devices whose function implies the occurrence of both electronic and nuclear rearrangements. Most of the examples that will be illustrated refer to devices studied in our laboratories. Devices Based on the Transfer of Electrons or Electronic Energy Apart from futuristic applications related, e.g., to the construction of a chemical computer, [6] the design and realization of a molecular-level electronic set (i.e., a set of molecular-level systems capable of playing functions that mimick those performed by macroscopic components in electronic devices) is of great scientific interest since it introduces new concepts into the field of chemistry and stimulates the ingenuity of research workers engaged in the emerging field of nanotechnology. In the last few years, many systems that could prove useful for information processing at the molecular level (e.g., wires, antennas, on/off switches, plug/socket devices, memories, logic gates) have been investigated. Wires and Related Systems An important function at the molecular level is photoinduced energy and electron transfer over long distances and/or along predetermined directions. This function can be obtained by linking donor and acceptor components by a rigid spacer, as illustrated in Figure 1a. An example [7] is given by the Ru(bpy) 3 2+ (ph) n Os(bpy) 3 2+ compounds (bpy = 2,2 bipyridine; ph = 1,4-phenylene; n =3,5,7)in which excitation of the Ru(bpy) 3 2+ moiety is followed by electronic energy transfer from the excited Ru(bpy) 3 2+ unit to the Os(bpy) 3 2+ one, as shown by the sensitized emission of the latter. For the compound with n =7(Figure1b),therateconstant for energy transfer over the 4.2 nm metal to - metal distance is s 1.IntheRu(bpy) 3 2+ (ph) n Os(bpy) 3 3+ compounds, obtained by chemical oxidation of the Os based moiety, photoexcitation of the Ru(bpy) 3 2+ unit causes the transfer of an electron to the Os-based one with a rate constant of s 1 for n=7 (Figure 1c). Unless the electron added to the Os(bpy) 3 3+ unit is rapidly removed, a back electron transfer reaction (rate constant s 1 for n=7) takes place from the Os(bpy) 3 2+ unit to the Ru(bpy) 3 3+ one. Spacers with energy levels or redox states in between those of the donor and acceptor may help energy or electron transfer (hopping mechanism). Spacers whose energy or redox levels can be manipulated by an external stimulus can play the role of switches for the energy- or electron-transfer processes. [4]

3 Devices Based on Nuclear Motions (Molecular-Level Machines) 257 Figure 1. Schematic representation of a molecularlevel wire (a) and examples of photoinduced energy (b) and electron (c) transfer processes. Antenna Systems In suitably designed dendrimers, electronic energy transfer can be channeled towards a specific position of the array. Compounds of this kind play the role of antennas for light harvesting. [4] Anumber of tree-like (dendritic) multicenter transitionmetal complexes based on Ru and Os as metals, 2,3- bis(2-pyridyl)pyrazine (2,3-dpp) and 2,5-bis(2-pyridyl) pyrazine (2,5-dpp) as bridging ligands, and 2,2 - bipyridine (bpy) and 2,2 -biquinoline (biq) as terminal ligands have been prepared with this aim. [8] The largest compounds contain 22 metal atoms, 21 bridging ligands (2,3-dpp), and 24 terminal ligands (bpy). They comprise 1090 atoms, with a molecular weight of Dalton (for the docosanuclear Ru complex), and an estimated size of 5 nm. Since the properties of the modular components are known and different modules can be located in the desired positions of the dendrimer array, synthetic control of the various properties can be obtained. It is therefore possible, as schematically showninfigure2,toconstructarrayswherethe electronic energy migration pattern can be predetermined, so as to channel the energy created by light absorption on the various components towards a selected module (antenna effect). Devices Based on Nuclear Motions (Molecular-Level Machines) A molecular-level machine is a particular type of molecular-level device in which the component parts can display changes in their relative positions as a result of some external stimulus. [3,9 13] Although there are many chemical compounds whose structure and/or shape can be modified by an external stimulus (see, e.g., the photoinduced cis-trans isomerization processes), the term molecular-level machines is only used for systems showing large amplitude movements of molecular components. It is very important that such molecular-level motions are accompanied by changes of some chemical or physical property of the system, resulting in a readout signal that can be used to monitor the operation of the machine. The reversibility of the movement, i.e., the possibility to restore the initial situation by means of an opposite stimulus, is an essential feature of a molecular machine. Since such induced motions correspond to a binary logic, systems of this kind could also prove useful for information processing. The concept of machine at the molecular level is not new. Our body can be viewed as a very complex ensemble of molecular-level machines that power our motions, repair damage, and orchestrate our

4 258 Molecular-Level Devices and Machines Figure 2. (a) Pictorial representation of a homodecanuclear dendrimer (N N stands for bpy). (b) Schematic representation of the different energy-transfer patterns that can be obtained in such a structure on choosing different metals and ligands.the arrows indicate the exoergonic energytransfer steps; empty and full circles indicate Ru(II) and Os(II), respectively; in the peripheral position, circles and squares indicate M(bpy) 2 and M(biq) 2 components, respectively. The compounds shown have +20 electric charge. inner world of sense, emotion, and thought. [14] The problem of the construction of artificial molecularlevel machines, however, is quite new. Although this possibility was proposed by Richard P. Feynman, Nobel Laureate in Physics, in his famous address to the American Physical Society in 1959, [15] and clever examples of molecular-level machines based on the photoisomerization of azobenzene derivatives were reported in the early 1980s, [16] research on artificial molecular machines has only begun to develop in the last few years. [9 13] Since most of the recently designed molecular-level machines are based on pseudorotaxanes, rotaxanes, and catenanes, it is worthwhile recalling some relevant features of such compounds. Pseudorotaxanes, Rotaxanes, and Catenanes Rationale and efficient synthetic approaches for the preparation of complicated (supra)molecular systems like pseudorotaxanes, rotaxanes and catenanes have been devised only recently. [17] The strategies chosen by Stoddart and coworkers [18] are based on: (i) charge-transfer and C H...O hydrogenbonding interactions between an electron acceptor (e.g., 1,1 -dibenzyl-4,4 -bipyridinium dication) and an electron donor (e.g., 1,5-dinaphtho[38]crown-10, 1/5DN38C10), and/or (ii) hydrogen-bonding interactions between secondary ammonium functions (e.g., dibenzylammonium ion) and a suitable crown ether (dibenzo[24]crown-8, DB24C8) (Figure 3). Systems Featuring Charge-Transfer Interactions The charge-transfer interaction between electron-donor and electron-acceptor units has several important consequences from the spectroscopic and electrochemical viewpoints [19] and plays a fundamental role as far as the machine-like behaviour of these (supra)molecular species is concerned. The donor/acceptor interaction introduces low energy charge-transfer (CT) excited states which are responsible not only for the color of these (supra)molecular species (because of the presence of broad and weak absorption bands in the visible region), but also for the quenching of the potentially luminescent excited states localized on the molecular components (Figure 4). As far as the electrochemical behaviour is concerned, it should be

5 Devices Based on Nuclear Motions (Molecular-Level Machines) 259 Figure 3. Pictorial representation of the selfassembly of pseudorotaxanes based on (a) chargetransfer and C H O hydrogen bonding interactions between 1,1 -dibenzyl-4,4 bipyridinium dication and 1,5-dinaphtho[38] crown-10 (1/5DN38C10), and (b) hydrogen-bonding interactions between dibenzyl ammonium ion and dibenzo[24]crown-8 (DB24C8). A possible route towards the synthesis of rotaxanes and catenanes is also schematized. noted that, when engaged in CT interactions, the electron-donor and electron-acceptor units become more difficult to oxidize and to reduce, respectively. Furthermore, units which are topologically equivalent in an isolated component may not be so when the component is engaged in non-symmetric interactions with another component. Consider, for example, the macrocyclic component 1 shown in Figure 5. [20] Such a species exhibits a two-electron reduction process, which corresponds to the simultaneous first reduction of the two equivalent bipyridinium units and, at a more negative potential, another two-electron process, which corresponds to the second reduction of such Figure 4. Schematic energy level diagram for a catenane based on chargetransfer (CT) interactions and for its separated components. The wavy lines indicate nonradiative decay paths of the electronic excited states.

6 260 Molecular-Level Devices and Machines Figure 5. Correlations between the reduction potentials of two electron acceptor macrocycles and their catenanes with an electron donor crown ether. Black and white circles refer to reduction of bipyridinium and bis(pyridinium)ethylene units, respectively; processes marked with c involve two electrons. units. When macrocycle 1 is interlocked in the catenane 2 with 1/5DN38C10, which contains two dioxynaphthalene electron-donor units, its electrochemical behaviour changes drastically: (i) all the reduction processes take place at more negative potentials, as expected because the bipyridinium units are engaged in CT interactions with the electron-donor units of the crown ether, and (ii) the two electronacceptor units are no longer equivalent because the unit that resides inside the crown experiences a stronger CT interaction than the unit which resides alongside. Therefore, four distinct one-electron reduction processes are observed (Figure 5) which are assigned, starting from less negative potential values, to the first reduction of the alongside and inside bipyridinium units (first and second process), and to the second reduction of the alongside and inside units (third and fourth process). In macrocycle 3, [20] the two electron-acceptor units, a bipyridinium and a bis(pyridinium)ethylene, are different and therefore are reduced at different potentials, as expected on the basis of their electron-acceptor ability (Figure 5). When this macrocycle is interlocked with 1/5DN38C10 in the catenane 4, the bipyridinium unit occupies the inside position and therefore it becomes more difficult to reduce compared with the bis(pyridinium)ethylene one since it experiences a stronger CT interaction. As a consequence, the first reduction of the bis(pyridinium)ethylene unit becomes the first reduction process of the whole system and therefore is displaced toward less negative potentials with respect to same process in the free macrocycle 3, inwhich such a process follows the first reduction of the other unit. In this kind of pseudorotaxanes, rotaxanes, and catenanes, the stability of a specific (supra)molecular structure is a result, at least in part, of the CT interaction. In order to cause mechanical movements, such a CT interaction has to be destroyed. This requirement can be fulfilled by reduction of the electron-acceptor unit(s) or by oxidation of the electron-donor unit(s) by chemical, electrochemical, or photochemical redox processes. In most cases, the CT interaction can be restored by an opposite redox process, which thus promotes a reverse mechanical movement leading to the original structure. Systems Based on Hydrogen-Bonding Interactions Contrary to what happens in the case of CT interactions, hydrogen-bonding interactions between secondary ammonium centers and suitable crown ethers (Figure 3b) do not introduce low lying energy levels. [21] Therefore, even if the absorption bands of the molecular components of pseudorotaxanes, rotaxanes and catenanes based on this kind of interaction are often perturbed compared with the corresponding absorption bands of the isolated molecular components, no new band is present in the visible region. As far as luminescence is concerned, in the supramolecular architecture each component maintains its potentially luminescent levels, but intercomponent energy-transfer processes may often cause quenching and sensitization processes, as will be demonstrated better in the case of the plug/socket systems described in the next section. In principle, intercomponent electron transfer can also occur. The electrochemical properties of the separated components are more or less modified when the components are assembled. [21] In these compounds, mechanical movements can be caused by destroying the hydrogen bonding interaction which is responsible for assembly and spatial organization. This process can be easily caused by addition of a suitable base that is able to deprotonate the ammonium center. The movement can be reversed by addition of an acid that is able to reprotonate the amine function.

7 Devices Based on Nuclear Motions (Molecular-Level Machines) 261 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. We have shown that, in suitably designed systems, the movement of such a rudimentary molecular machine can be driven by chemical energy or electrical energy and, most importantly, by light. The first attempt at designing [22] aphotochemically driven molecular-level machine of a pseudorotaxane type was based (Figure 6a) on the use of an rethreading. The threading, dethreading, and rethreading processes can be easily monitored by absorption and fluorescence spectroscopy. Second generation photochemically driven machines were subsequently designed where the piston/ cylinder pseudorotaxane superstructure incorporates the light-fueled motor (i.e., the photosensitizer) in the wire (Figure 6b) [23] or in the macrocyclic ring (Figure 6c). [24] In both cases, excitation of the photosensitiser with visible light in the presence of a sacrificial donor causes reduction of the electron-acceptor unit and, as a consequence, dethreading. Rethreading can be obtained by allowing oxygen to enter the solution. Figure 6. Light-driven dethreading of: (a) a pseudorotaxane by excitation of an external photosensitizer, (b) a pseudorotaxane incorporating a photosensitizer as a stopper in the wire-type component, (c) a pseudorotaxane incorporating a photosensitizer in the macrocyclic component. external electron-transfer photosensitizer. As a result of CT interactions, the electron-acceptor ring 1 and a dioxynaphthalene-based electron-donor wire selfassemble in aqueous solution. Irradiation with visible light of an external electron-transfer photosensitiser [e.g., Ru(bpy) 3 2+ ] causes reduction of one of the bipyridinium units of the ring (the back electron-transfer reaction is prevented by the presence of a sacrificial reductant like triethanolamine). Once the ring has received an electron, the interaction responsible for self-assembly is partly destroyed and therefore the wire dethreads from the reduced ring. If oxygen is allowed to enter the solution, oxidation of the reduced bipyridinium unit restores the interaction and causes Chemically induced dethreading/rethreading in similar pseudorotaxane systems has been shown to behave according to the XOR logic function. [25] A Molecular Abacus Rotaxanes are made of dumbbell-shaped and ring components which exhibit some kind of interaction originating from complementary chemical properties. In rotaxanes containing two different recognition sites in the dumbbell-shaped component, it is possible to switch the position of the ring between the two stations by an external stimu-

8 262 Molecular-Level Devices and Machines Figure 7. An example of molecular abacus: the ring can be switched between the two stations of the dumbbellshaped component by base/acid inputs. lus. [21] A system which behaves as a molecular abacus is shown in Figure 7. It is made of a DB24C8 ring and a dumbbell-shaped component containing a dialkylammonium center and a 4,4 -bipyridinium unit. An anthracene moiety is used as a stopper because its absorption, luminescence, and redox properties are useful to monitor the state of the system. The DB24C8 ring exhibits 100 % selectivity for the ammonium recognition site and therefore the rotaxane exists as only one of the two possible translational isomers, as evidenced by X-ray crystallography. Deprotonation of the ammonium center, however, causes 100 % displacement of the ring component to the bipyridinium unit. Reprotonation directs the crown ring back onto the ammonium center. Such a switching process has been investigated by 1 H NMR spectroscopy and by electrochemical and photophysical measurements. Futhermore, in the deprotonated rotaxane, it is possible to displace the crown ring from the bipyridinium station by destroying the charge-transfer interaction through: (i) electrochemical reduction of the bipyridinium station or (ii) electrochemical oxidation of the crown ring. In a catenane, structural changes caused by circumrotation of one ring with respect to the other can be clearly evidenced when one of the two rings contains two non-equivalent units. In the catenane shown in Figure 8, [26] the ring containing the electron-acceptor units is symmetric, whereas the other ring is non-symmetric since it contains two different electron-donor units, namely, a tetrathiafulvalene (TTF) and a 1,5-dioxynaphthalene (DMN) unit. In a catenane structure, the inside electron donor experiences the effect of two electronacceptor units, whereas the alongside electron donor experiences the effect of only one electron acceptor. Therefore, in the catenane shown in Figure 8, the better electron donor (i.e., TTF) enters the ring and the less good one (i.e., DMN) remains alongside, as shown by a variety of techniques, including X-ray crystallography. On electrochemical oxidation, the first unit that undergoes oxidation is TTF, which thus loses its electron-donating properties. The disruption of the CT interaction and the electrostatic repulsion between TTF + and the tetracat- Electrochemically Driven Motion of a Ring in Catenanes Figure 8. Electrochemically controlled movements of the ring components upon one-electron oxidation/reduction in a catenane containing a non-symmetric ring.

9 Devices Based on Electronic and Nuclear Motions 263 ionic macrocycle cause circumrotation of one ring to yield the translational isomer with the DMN moiety positioned inside the acceptor (Figure 8). Upon reduction of TTF +, the switching is fully reversible. The oxidation/reduction cycle, which is accompanied by a clearly detectable color change, can be monitored by 1 H NMR spectroscopy, UV/Vis spectroscopy, and cyclic voltammetry. [26] Devices Based on Electronic and Nuclear Motions In the two preceding sections, we have illustrated examples of molecular-level devices working on the basis of either electron or nuclear movements. In other devices, which will be described in this section, the function they perform is based on both electronic and nuclear rearrangements, that take place in distinct steps. Plug/Socket and Related Systems A macroscopic plug/socket system is characterized by the following two features: (i) the possibility to connect/disconnect the two components in a reversible way, and (ii) the occurrence of electron flow from the socket to the plug when the two components are connected. Supramolecular systems have recently been designed that may be considered as molecular-level plug/socket devices. Plug in/plug out is reversibly controlled by acid/base reactions, and the photoinduced flow of electronic energy (or electrons) takes place in the plug-in state. The plug-in function can be based (see Figure 9) [27] on the threading of a ( e )-binaphthocrown ether by a (9-anthracenyl)benzylammonium ion. The association process can be reversed quantitatively (plug out) by addition of a suitable base like tributylamine. In the plug-in state (pseudorotaxane), the quenching of the binaphthyl-type fluorescence is accompanied by the sensitization of the fluorescence of the anthracenyl unit of the ammonium ion. The rate constant for electronic energy transfer from the binaphthyl unit of the crown to the anthracenyl unit of the wire is higher than s 1. Addition of a stoichiometric amount of base to the pseudorotaxane structure causes the revival of the binaphthyl fluorescence and the disappearance of the anthracenyl fluorescence upon excitation in the binaphthyl bands, demonstrating that plug out has happened. The plug/socket molecular-level concept can be extended straightforwardly to the construction of molecular-scale extensions and to the design 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 (the enantiomeric recognition of chiral ammonium ions by chiral crown ethers is well known). Thus, the plug/ socket molecular-level systems are devices whose function is based on both electronic and nuclear movements, caused by two distinct external input(s). Electrochemically Controlled Switches In host-guest systems based on electron donor/ acceptor interactions, association/dissociation can be driven by redox processes so that it is possible to design electrochemical switches than can be used to control energy- and electron-transfer processes. Figure 9. Acid/base controlled plug in/plug out of (9- anthracenyl)benzylammonium ion with a (±)-binaphthocrown ether. The occurrence of photoinduced energy transfer in the plug in state is schematized.

10 264 Molecular-Level Devices and Machines An example of a system that can be switched reversibly in three different states through electrochemical control of the guest properties of one component is illustrated in Figure 10. [28] Tetrathiafulvalene is stable in three different oxidation states, TTF(0), TTF +,andttf 2+. On oxidation, the electrondonor power of tetrathiafulvalene decreases with a The mechanical movements taking place in this supramolecular system, in which a free molecule can be driven electrochemically to associate with either of two different receptors, opens the way to futuristic applications in the field of molecular-level signal processors. [6] One can conceive second generation systems wherein the electrochemically driven Figure 10. Components of a three-state system and schematic representation of the ranges of electrochemical stability of the three states available to the system. 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 cyclobis(paraquat-p-phenylene) electron-acceptor macrocycle 1, TTF 2+ plays the role of an electron acceptor and gives rise to a charge-transfer complex with the 1/5DN38C10 electron-donor macrocycle. TTF + does not show any electron donor/acceptor character. The system illustrated in Figure 10 consists of acetonitrile solutions of TTF and the two macrocycles 1 and 1/5DN38C10. Electrochemical experiments carried out on such a system show that, depending on the potential range, TTF can be: (i) free in the TTF + state, (ii) complexed with the electron-acceptor host in the TTF(0) state, or (iii) complexed with the electron-donor host in the TTF 2+ state. The reversibility of the electrochemical processes shows that complexation/decomplexation and, as a consequence, the exchange of the guest between the two hosts, are fast processes compared to the time scale of the electrochemical experiments. In such a three-state system, switching ( writing ) can be performed electrochemically and the state of the system can be monitored ( reading ) by absorption, emission, and NMR spectroscopies, in addition to electrochemical techniques. movements can control the selection of the partner in energy- or electron-transfer processes. Consider, e.g., a system (Figure 11) where a chromophoric group A is appended to the potential guest and chromophoric groups B and C, whose lowest excited state is lower than that of A, are appended to the potential hosts 1 and 1/5DN38C10, respectively. In such a system, light excitation of A will lead to no energy transfer (OFF), energy transfer to B (ON 1), or energy transfer to C (ON 2), depending on the potential value selected by the operator. Admittedly, the donor/acceptor-based molecular-level connection posesseverelimitationsastothechoiceofthechromophoric groups to be used if energy transfer has to proceed through the charge-transfer connections. However, it does not seem unlikely that systems can be designed where molecular association: (i) relies on a different kind of interaction (e.g., hydrogenbonding), or (ii) simply plays the role of bringing the two chromophoric groups to a suitable distance for through-space energy transfer. Similar switching of electron-transfer processes could also be performed. More complex energy- and/or electron-transfer patterns are conceivable. The strategy described in this section could also be used, in principle, to catalyze chemical reactions.

11 References and Notes 265 Figure 11. Schematic representation of the concept of a three-pole supramolecular switching of energy transfer. The cartoons correspond to the molecular components shown in Epilogue In his 1959 address to the American Physical Society, when discussing the possibility of constructing molecular-level machines, R. P. Feynman said: [15] An internal combustion engine of molecular size is impossible. Other chemical reactions, liberating energy when cold, can be used instead. Thedescribedexamplesofmolecular machines driven by redox or acid/base cold chemical reactions fulfil Feynman s prediction. Furthermore, the reported examples show that the primary energy source to drive such mechanical movements can be electricity or, even more interesting, light. In the same address, Feynman concluded his reflection on the construction of molecular-scale machines as follows: What would be the utility of such machines? Who knows? I cannot see exactly what would happen, but I can hardly doubt that when we have some control of the rearrangement of things on a molecular scale we will get an enormously greater range of possible properties that substances can have, and of different things we can do. We believe that these sentences are the most appropriate final comment to the work described in this chapter. Acknowledgements We would like to thank our colleagues and coworkers, whose names appear in the references quoted below, and particularly Prof. J. F. Stoddart and his group for a long lasting and most profitable collaboration. Financial support from EU (TMR grants FMRX-CT and FMRX-CT ), the University of Bologna (Funds for Selected Research Topics), and MURST (Supramolecular Devices Project) is gratefully acknowledged. Figure 10; A, B, and C are suitably chosen chromophoric groups. For more details, see the text. References and Notes 1. J.-M. Lehn, Supramolecular Chemistry, VCH, Weinheim, V. Balzani, F. Scandola, Supramolecular Photochemistry, Horwood, Chichester, V. Balzani, A. Credi, M. Venturi, Molecular-Level Devices, in Supramolecular Science: Where It Is and Where It Is Going (Eds. R. Ungaro, E. Dalcanale), Kluwer, Dordrecht, 1999, pp V. Balzani, F. Scandola, Photochemical and Photophysical Devices, in Comprehensive Supramolecular Chemistry (Eds. J. L. Atwood, J. E. D. Davies, D. D. MacNicol, F. Vögtle), Pergamon Press, Oxford, 1996, Vol. 10, pp P. L. Boulas, M. Gómez Kaifer, L. Echegoyen, Electrochemistry of Supramolecular Systems, Angew. Chem. Int. Ed. 1998, 37, D. Rouvray, Reckoning on Chemical Computers, Chem. Br. 1998, 34 (2), B. Schlicke, P. Belser, L. De Cola, E. Sabbioni, V. Balzani, Photonic Wires of Nanometric Dimensions. Electronic Energy Transfer in Rigid Rod like Ru(bpy) 3 2+ (ph) n Os(bpy) 3 2+ Compounds (ph=1,4 phenylene, n=3,5,7), J. Am. Chem. Soc. 1999, 121, , and unpublished results. 8. V. Balzani, S. Campagna, G. Denti, A. Juris, S. Serroni, M. Venturi, Designing Dendrimers Based on Transition- Metal Complexes. Light harvesting Properties and Predetermined Redox Patterns, Acc. Chem. Res. 1998, 31, V. Balzani, M. Gómez López, J. F. Stoddart, Molecular Machines, Acc. Chem. Res. 1998, 31, J.-P. Sauvage, Transition Metal-Containing Rotaxanes and Catenanes in Motions: Toward Molecular Machines and Motors, Acc. Chem. Res. 1998, 31, T. R. Kelly, H. De Silva, R. A. Silva, Unidirectional Rotary Motion in a Molecular System, Nature 1999, 401, N. Koumura, R. W. J. Zijlstra, R. A. van Delden, N. Harada, B. L. Feringa, Light-driven Monodirectional Molecular Rotor, Nature 1999, 401, V. Balzani, A. Credi, F. M. Raymo, J. F. Stoddart, Artificial Molecular Machines, Angew. Chem. Int. Ed., 2000, 39,

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