Cent. Eur. J. Chem. • 6(3) • 2008 • 325–339 DOI: 10.2478/s11532-008-0033-4

Central European Journal of Chemistry

Molecular machines operated by light

Invited Review Alberto Credi*, Margherita Venturi

Dipartimento di Chimica “G. Ciamician”, Università di Bologna, Via Selmi 2 – 40126 Bologna, Italy

Received 11 February 2008; Accepted 22 Arpil 2008

Abstract: The bottom-up construction and operation of machines and motors of molecular size is a topic of great interest in nanoscience, and a fascinating challenge of nanotechnology. Researchers in this field are stimulated and inspired by the outstanding progress of mo- lecular biology that has begun to reveal the secrets of the natural nanomachines which constitute the material base of life. Like their macroscopic counterparts, nanoscale machines need energy to operate. Most molecular motors of the biological world are fueled by chemical reactions, but research in the last fifteen years has demonstrated that light energy can be used to power nanomachines by exploiting photochemical processes in appropriately designed artificial systems. As a matter of fact, light excitation exhibits several advantages with regard to the operation of the machine, and can also be used to monitor its state through spectroscopic methods. In this review we will illustrate the design principles at the basis of photochemically driven molecular machines, and we will describe a few examples based on rotaxane-type structures investigated in our laboratories. Keywords: Molecular device • Nanoscience • Photochemistry • Rotaxane • Supramolecular Chemistry

© Versita Warsaw and Springer-Verlag Berlin Heidelberg.

1. Introduction Richard Feynman stated in his famous talk in 1959 [4]. Research on supramolecular chemistry has shown that molecules are convenient nanometer-scale building The development of civilization has always been strictly blocks that can be used, in a bottom-up approach, related to the design and construction of devices – to construct ultraminiaturized devices and machines from wheel to jet engine – capable of facilitating man [5]. Chemists are in an ideal position to develop such movement and travelling. Nowadays the miniaturization a molecular approach to functional nanostructures race leads scientists to investigate the possibility of because they are able to design, synthesize, investigate designing and constructing machines and motors at the and operate with molecules – for instance, make them nanometer scale, that is, at the molecular level. Many react or get them together into larger assemblies. fields of technology, in particular information processing, Much of the inspiration to construct molecular have benefited from progressive miniaturization of the devices and machines comes from the outstanding components of devices in the last fifty years. A common progress of molecular biology that has begun to prediction is that further progress in miniaturization reveal the secrets of the natural nanomachines which will not only decrease the size and increase the power constitute the material base of life [6]. Surely, the of computers [1], but could also open the way to new supramolecular architectures of the biological world technologies in the fields of medicine, environment, are themselves the premier, proven examples of the energy and materials [2]. feasibility and utility of nanotechnology, and constitute a The top-down approach used so far for the sound rationale for attempting the realization of artificial construction of miniaturized devices is reaching molecular devices [7,8]. The bottom-up construction fundamental and practical limits, which include severe of machines as complex as those present in Nature cost limitations, for sizes below 50 nanometers [3]. is a prohibitive task. Therefore chemists have tried (i) Miniaturization, however, can be pushed further on to construct much simpler systems, without mimicking because ‘there is plenty of room at the bottom’, as

* E-mail: [email protected] 325 Molecular machines operated by light

the complexity of the biological structures, (ii) to to apply at the nanoscale macroscopic engineering undestand the principles and processes at the basis of principles [48]. Biomolecular machines are made of their operation, and (iii) to investigate the challenging nanometer-size floppy molecules which operate at problems posed by interfacing artificial molecular constant temperature in the soft and chaotic environment machines with the macroscopic world, particularly as produced by the weak intermolecular forces and the far as energy supply and information exchange are ceaseless and random molecular movements. Gravity concerned. In the last few years the development of and inertia motions we are familiar with in our everyday powerful synthetic methodologies, combined with a experience are fully negligible at the molecular scale; device-driven ingenuity evolved from the attention viscous forces resulting from intermolecular interactions to functions and reactivity, have led to remarkable (including those with solvent water molecules) largely achievements in this field. Among the systems reported prevail and it is difficult to obtain directed motion. are molecular tweezers [9], propellers [10], rotors [11], This means that while we can describe the bottom-up turnstiles [12], gyroscopes [13,14], gears [15], brakes construction of a nanoscale device as an assembly of [16], a molecular pedal [17], ratchets [18], rotary motors suitable (molecular) components by analogy with what [19], shuttles [20], elevators [21], muscles [22], valves happens in the macroscopic world, we should not forget [23], processive artificial enzymes [24], walkers [25-27], that the design principles and the operating mechanisms vehicles [28], and catalytic self-propelled micro- and at the molecular level are different. nano-objects [29,30]. Several excellent reviews [31-46] For the above reasons, it is not easy to define and a monograph [47] dealing with artificial molecular the functions related to artificial molecular motions. A machines and motors are available. simple and immediate categorization is usually based In the first part of this review we will illustrate the on an iconic comparison with motions taking place in basic features of nanoscale machines and discuss their macroscopic systems (e.g. braking, locking, shuttling, implementation with molecular species. In the second rotating). Such a comparison presents the advantage of part we will describe a few examples, selected from an easy representation of molecular devices by cartoons our work, showing how photoinduced processes can that clearly explain their mechanical functions, but it be engineered within rotaxane-type structures with the also implies the danger of overlooking the substantial purpose of obtaining molecular machines driven by light. differences between the macroscopic and molecular A vast quotation of prominent work by other research worlds. groups will also be provided. Finally, we will critically In agreement with the recent literature [44,47], being analyze the limitations of the current systems and the aware that we are dealing with a difficult and potentially perspectives of this research field. controversial topic [37,44], we will comply with the minimum set of terms and definitions reported below: • Mechanical device: a particular type of device 2. Basic concepts designed to perform mechanical movements [49]. • Machine: a particular type of mechanical device 2.1. Molecular motions in artificial systems: designed to perform a specific mechanical terms and definitions movement under the action of a defined energy In the macroscopic world, devices and machines are input. assemblies of components designed to achieve a specific • Motor: a machine capable of using an energy input function. Each component of the assembly performs a to produce useful work. simple act, while the entire assembly performs a more Clearly, there is a hierarchy: a motor is also a complex, useful function, characteristic of that particular machine, and a machine is also a mechanical device, device or machine. In principle, the macroscopic but a mechanical device might not be a machine or a concepts of a device and a machine can be extended to motor and a machine might not be a motor. the molecular level [47]. It is also useful to discuss briefly the relation Nature shows, however, that nanoscale devices and between molecular switches, and molecular machines machines can hardly be considered as ‘shrunk’ versions and motors. A switch is a multi-state system whose of macroscopic counterparts because several intrinsic properties and effects on the environment are a function properties of molecular-level entities are quite different of its state [50,51]. Most often the interconversion from those of macroscopic objects. In fact, the design between two given states of a molecular switch can and construction of artificial molecular machines can take place by the same pathway that is travelled in take greater benefit from the knowledge of the working opposite directions (Figure 1, upper cycle). In this case, principles of natural ones rather than from sheer attempts any mechanical effect exerted on an external system

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surprisingly, the majority of the molecular motors of the biological world are powered by chemical reactions (e.g., ATP hydrolysis) [6-8]. Richard Feynman observed [4] that «an internal combustion engine of molecular size is impossible. Other chemical reactions, liberating energy when cold, can be used instead». This is exactly what happens in our body, where the chemical energy supplied by food is used in long series of slightly exergonic reactions to power the biological machinery that sustains life. If an artificial molecular machine has to work by inputs of chemical energy, it will need addition of fresh reactants (‘fuel’) at any step of its working cycle, with the concomitant formation of waste products. Accumulation of waste products, however, will compromise the operation of the device unless they are removed from the system, as it happens in our body as well as in macroscopic internal combustion engines. The need to remove waste products introduces noticeable limitations in the design and construction of artificial molecular Figure 1. Upper cycle: in a rotary switch the interconversion be- machines based on chemical fuel inputs. tween two states can take place by the same pathway Chemists have since long known that photochemical travelled in opposite directions. Lower cycle: in a rotary motor the forward and backward transitions between and electrochemical energy inputs can cause the two states follow different pathways. occurrence of endergonic and reversible reactions. In the last few years, the outstanding progress is cancelled out when the switch returns to its original made by supramolecular photochemistry [54] and state. Switches exist, however, in which the forward and electrochemistry [55] has thus led to the design and backward transitions between a pair of states follow construction of molecular machines powered by light or different pathways. A typical example is provided by a electrical energy which work without formation of waste rotary device undergoing a 360° unidirectional rotation products. through two directionally correlated half-rotations In the context of artificial nanomachines, light energy [19,52] (Figure 1, lower cycle). Switches of this kind stimulation possesses a number of further advantages can influence a system as a function of their switching compared to chemical or electrochemical stimulation. trajectory, and a physical task performed in a cycle is not First of all, the amount of energy conferred to a chemical inherently undone. This is a fundamental requirement system by using photons can be carefully controlled if a molecular motor has to be constructed. Therefore, by the wavelength and intensity of the exciting light, generally speaking, molecular machines are also in relation to the absorption spectrum of the targeted switches, whose states differ from one another for the species. Such an energy can be transmitted to molecules relative positioning of the various molecular components. without physically connecting them to the source (no However, in order to behave as motors, the above- ‘wiring’ is necessary), the only requirement being the described additional feature is required. Noteworthy, the transparency of the matrix at the excitation wavelength. vast majority of artificial molecular machines reported Other properties of light, such as polarization, can so far – including those described in section 3 – do not also be utilized. Lasers provide the opportunity of exhibit such a behavior and are therefore switches, but working in very small spaces and extremely short time not motors. A more thorough discussion on this topic domains, and near-field techniques allow excitation with can be found in reference [44]. nanometer resolution. On the other hand, the irradiation of large areas and volumes can be conveniently carried 2.2. Energy supply and monitoring signals out, thereby allowing the parallel (or even synchronous) As it happens in the macroscopic world, molecular- addressing of a very high number of individual level devices and machines need energy to operate nanodevices. and signals to communicate with the operator [53]. Because molecules are extremely small, the The most obvious way to supply energy to a chemical observation of motions at the molecular level, which system is through an exergonic chemical reaction. Not is crucial for monitoring the operation of a molecular

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Figure 2. Schematic representation of (a) ring shuttling and (b) ring rotation in rotaxanes, and (c) the threading-dethreading equilibrium involving the axle-type and ring components of a pseudorotaxane. machine, is not trivial. In general, the motion of the An important property of molecular machines, component parts should cause readable changes in related to energy supply and cyclic operation, is their some chemical or physical properties of the system. capability to exhibit an autonomous behaviour, that Photochemical methods are also useful in this regard. is, to keep operating, in a constant environment and As a matter of fact, photons can play with respect to without the intervention of an external operator, as chemical systems the dual role of writing (i.e., causing long as the energy source is available. Natural motors a change in the system) and reading (i.e., reporting the are autonomous, but most of the artificial systems state of the system) [54]. This is primarily true in nature, reported so far are not autonomous because, after the where sunlight photons are employed both as energy mechanical movement induced by a given input, they quanta in photosynthetic processes, and as information need another, opposite input to reset. Obviously, the elements in vision and other light-triggered processes. operation of a molecular machine is accompanied by For example, luminescence spectroscopy is a valuable partial degradation of free energy into heat, regardless method because it is easily accessible and offers good of the chemical, photochemical, and electrochemical sensitivity and selectivity, along with the possibility of nature of the energy input. time- and space-resolved studies [56]. In particular, flash Finally, the functions that can be performed by spectroscopic techniques with laser excitation allow the exploiting the movements of the component parts in study of extremely fast processes. molecular machines are various and, to a large extent, The use of light to power nanoscale devices is still unpredictable. In natural systems the molecular relevant for another important reason. If and when a motions are always aimed at obtaining specific nanotechnology-based industry will be developed, its functions, for example, catalysis, transport, gating. It is products will have to be powered by renewable energy worth noting that the changes in the physicochemical sources, because it has become clear that the problem properties related to the mechanical movements in of energy supply is a crucial one for human civilization molecular machines usually obey binary logic, and can for the years ahead [57]. In this frame, the construction thus be taken as a basis for information processing at of nanodevices, including natural-artificial hybrids [58] the molecular level. This point will be further discussed that harness solar energy in the form of visible or near- in the Conclusion section. UV light, is indeed an important possibility. 2.4. Rotaxane-type structures as nanoscale 2.3. Other features machines In addition to the kind of energy input supplied to make In principle, molecular machines can be designed starting them work and the way of monitoring their operation, from several kinds of molecular and supramolecular molecular machines are characterized by other features systems [31-47], including DNA [39,40]. However, for such as (i) the type of motion – for example, translation, the reasons mentioned below, most of the systems rotation, oscillation – performed by their components, constructed so far are based on interlocked molecular (ii) the possibility to repeat the operation in cycles, (iii) species such as rotaxanes and related species. The the time scale needed to complete a cycle, and (iv) the names of these compounds derive from the Latin function performed.

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words rota and axis for wheel and axle, respectively. evidenced by the examples in the following sections, Rotaxanes [59] are minimally composed (Figure 2ab) this feature for molecular machines is most commonly of a dumbbell-shaped molecule surrounded by a fulfilled by utilizing molecular components that possess macrocyclic compound (the ‘ring’) and terminated by rigid subunits in their structures. bulky groups (‘stoppers’) that prevent disassembly. If Interestingly, the dumbbell component of a molecular the stoppers are not present, the assembled species shuttle exerts on the ring motion the same type of is denoted as a pseudorotaxane and in solution it directional restriction imposed by the protein track for equilibrates with the separated axle-type and ring linear biomolecular motors (an actin filament for myosin components (Figure 2c). Important features of these and a microtubule for kinesin and dynein) [6]. It should systems derive from noncovalent interactions between also be noted that interlocked molecular architectures the components that contain complementary recognition are largely present in natural systems – for instance, sites. Such interactions are responsible for the self- DNA catenanes and rotaxanes are known [59]. Many assembly of pseudorotaxanes and efficient template- processive enzymes, that is, enzymes that remain directed syntheses of rotaxanes, and include: electron attached to their biopolymer substrates (DNA, RNA or donor-acceptor ability, hydrogen bonding, hydrophobic- proteins) and perform multiple rounds of catalysis before hydrophylic character, π-π stacking, electrostatic forces dissociating, are thought to exhibit a rotaxane structure, and, on the side of the strong interaction limit, metal- as confirmed for example by the observation of the ligand bonding. crystal structure of DNA λ-exonuclease [61]. Clearly, Rotaxanes are appealing systems for the construction the unique aspect of the rotaxane architecture, that is, of molecular machines because (i) the mechanical the mechanical binding of the catalyst with the substrate bond enables a large variety of mutual arrangements which leaves the former free to displace itself along the of the molecular components while conferring stability latter without losing the system’s integrity, is utilized by to the system, (ii) the interlocked architecture limits the Nature to enhance the activity of processive enzymes. amplitude of the intercomponent motion in the three directions, (iii) the stability of a specific arrangement (co-conformation) is determined by the strength 3. Examples of the intercomponent interactions, and (iv) such interactions can be modulated by external stimulation. 3.1. Pseudorotaxane threading-dethreading Two interesting molecular motions can be envisaged based on cis-trans photoisomerization in rotaxanes, namely translation, i.e., shuttling of the Cis-trans photoisomerization reactions involving ring along the axle (Figure 2a), and rotation of the ring –N=N–, –C=N– or –C=C– double bonds are well known around the axle (Figure 2b). Hence, rotaxanes are good processes [62]. In general, they are extremely clean prototypes for the construction of both linear and rotary and reversible reactions, the prototypical case being molecular machines. Systems of the first type, termed the cis-trans isomerization of azobenzene [63]. These molecular shuttles, constitute indeed the most common are ideal processes to obtain light-driven operation of implementation of the molecular machine concept with molecular machines because they bring about evident rotaxanes. structural changes that can be exploited to cause large The assembly-disassembly of the axle-type and amplitude motions in suitably designed molecular and ring components of a pseudorotaxane (Figure 2c) is supramolecular systems. As a matter of fact, molecular reminiscent of the threading-dethreading of a needle tweezers based on the azobenzene unit were the first and can be controlled by external stimulation [47,60]. examples of light-driven molecular machines reported Studies on switchable pseudorotaxanes are important in the literature [9,64]. The photoisomerization about a for the development of less trivial unimolecular –C=C– bond is at the basis of a very interesting class machines based on rotaxanes and related interlocked of molecular rotary motors based on sterically hindered compounds. alkenes [19,65,66]. Molecular shuttles relying on the The cartoons shown in Figure 2, while providing a photoisomerization of azobenzene or stilbene units simple structural and topological representation, are have also been reported [67-70]. somewhat misleading because they give the impression An example of a pseudorotaxane whose that rotaxanes are made of rigid molecular components, threading-dethreading can be controlled by means which is not the case for the vast majority of the systems of a photoisomerization process is shown in Figure 3 reported so far. However, in order to obtain clear-cut [71]. The thread-like species trans-1, which contains a mechanical movements the molecular components π-electron rich azobiphenoxy unit, and the π-electron should exhibit at least some stiffness. As it will be

329 Molecular machines operated by light

Figure 3. Threading-dethreading of 1 and 24+ as a consequence of the cis-trans photoisomerization of the azobenzene-type unit contained in the thread-like component 1. deficient macrocycle 24+ self-assemble very efficiently to give a pseudorotaxane, stabilized by electron donor- acceptor interactions. From fluorescence titrations we × 5 –1 obtained an association constant Ka = 1.5 10 M in MeCN at room temperature. In the pseudorotaxane structure, the intense fluorescence characteristic of free 4+ λ 2 ( max = 434 nm, Figure 4) is completely quenched by the donor-acceptor interaction. Irradiation of a MeCN solution containing 1.0 × 10‑4 M trans-1 and 24+ (ca. 80% complexed species) with 365-nm light – almost exclusively absorbed by the trans- azobiphenoxy unit – causes strong absorption spectral changes, as expected for the well known trans → cis photoisomerization of the azobenzene-type moiety. Figure 4. Fluorescence spectrum of a 1:1 mixture (1.0×10–4 M) Such spectral changes are accompanied by a parallel of trans-1 and 24+ in MeCN at room temperature (full line), and fluorescence spectrum of the same mixture increase in the intensity of the fluorescence band after irradiation at 365 nm until a photostationary state characteristic of free 24+ (Figure 4). This behavior shows is reached (dashed line). Inset: changes in intensity of the fluorescence of the free macrocyclic ring 24+ upon that photoisomerization is accompanied by dethreading consecutive trans → cis (irradiation at 365 nm, grey ar- (Figure 3), a result which is confirmed by the finding that cis → trans eas) and (irradiation at 436 nm, white areas) the association constant of 24+ with cis-1 is 15 times photoisomerization cycles involving component 1. Exci- tation is performed in an isosbestic point at 411 nm. smaller than that with trans-1. On irradiation at 436 nm or by warming the solution in the dark the trans isomer of 1 can be regenerated. This process is accompanied

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Figure 5. Structure formula of rotaxane 36+ and schematic representation of its operation as an autonomous molecular shuttle driven by visible light. by a parallel decrease in the fluorescence intensity 3.2. Molecular shuttles based on photoinduced λ at max = 434 nm, indicating that the trans-1 species 4+ electron transfer rethreads through the macrocycle 2 . Although this Photoinduced electron-transfer reactions are of primary system is a rudimental attempt towards the making of importance both in natural photosynthetic devices light-driven molecular machines, it should be noted that it and in artificial systems 72[ ]. The first attempts aimed exhibits a number of valuable features. Firstly, threading- at exploiting photoinduced electron transfer to bring dethreading is controlled exclusively by light energy, about large-amplitude controllable molecular motions without generation of waste products. Furthermore, involved the light-driven threading-dethreading of owing to the reversibility of the photoisomerization pseudorotaxane-type complexes [73-78]. process, the light-driven dethreading-rethreading On the basis of the experience gained with some cycle can be repeated at will (Figure 4, inset). Another of these model systems [76], the bistable rotaxane 36+ relevant feature of this system is that it exhibits profound (Figure 5) was specifically designed [79] to achieve changes of a strong fluorescence signal. photoinduced ring shuttling in solution. This compound

331 Molecular machines operated by light

has a modular structure; its ring component R is a absorption experiments allowed us to determine [79,80] π-electron-donating bis-p-phenylene-34-crown-10, the time constants of processes 4 and 5, which are 47 whereas its dumbbell component is made of several and 6.7 µs, respectively, at 303 K. Hence, the efficiency

covalently linked units. They are a Ru(II) polypyridine of ring displacement from the photo-reduced A1 station complex (P2+), a p-terphenyl-type rigid spacer (S), amounts to 0.12; because all the successive processes 2+ a 4,4’-bipyridinium (A1 ) and a 3,3’-dimethyl-4,4’- have no competitors, the overall shuttling quantum yield 2+ π bipyridinium (A2 ) -electron-accepting stations, and a is simply 0.16×0.12=0.02. This somewhat disappointing tetraarylmethane group as the terminal stopper (T). The figure is compensated by the fact that the investigated Ru-based unit plays the dual role of a light-fueled power system gathers together the following features: (i) it is station and a stopper, whereas the mechanical switch powered by visible light (in other words, sunlight); (ii) it consists of the two electron-accepting stations and the exhibits autonomous behavior, like motor proteins; (iii) - electron-donating macrocycle. Six PF6 ions are present it does not generate waste products; (iv) its operation as the counteranions of the positively charged rotaxane. can rely only on intramolecular processes, allowing in The stable translational isomer of rotaxane 36+ is the principle operation at the single-molecule level; (v) it can 2+ one in which the R component encircles the A1 unit, in be driven at a frequency of about 1 kHz; (vi) it works keeping with the fact that this station is a better electron in mild environmental conditions (i.e., fluid solution at acceptor than the other one. ambient temperature); and (vii) it is stable for at least The strategy devised in order to obtain the 103 cycles. photoinduced shuttling movement of R between the A thorough computational investigation on 36+ has 2+ 2+ two stations A1 and A2 is based on a ‘four stroke’ revealed [82] that the rate limiting step for the shuttling synchronized sequence of electron-transfer and motion (process 4 in Figure 5) could be related to the – molecular rearrangement processes, as illustrated in detachment of the PF6 counteranions from the station 2+ Figure 5 [79,80]. Light excitation of the photoactive unit that has to receive the ring (A2 ). If such a station were P2+ (process 1) is followed by the transfer of an electron not hindered by anions, the shuttling motion would be 2+ from this unit to A1 (process 2) which competes with almost barrierless and occur with a time constant as the intrinsic decay of the P2+ excited state (process fast as 20 ns at 300 K. Hence, the shuttling quantum 2+ 3). After the reduction of A1 , with the consequent yield could be substantially improved by adopting ‘deactivation’ of this station, the ring moves (process 4) weakly coordinating counteranions for 36+, or by 2+ by 1.3 nm to encircle A2 , a step that is in competition changing the solvent. The latter choice, however, would + with the back electron-transfer from A1 (still encircled also affect the energetics and kinetics of the electron- by R) to the oxidized unit P3+ (process 5). Eventually, a transfer processes. Unfortunately, experiments in these + back electron-transfer from the ‘free’ reduced station A1 directions are not easy because of difficulties related to to the oxidized unit P3+ (process 6) restores the electron counteranion exchange for 36+, and solubility issues. acceptor power to this radical cationic station. As a The molecular shuttle 36+ can also be operated, consequence of the electronic reset, thermally activated with a higher quantum yield, by a sacrificial mechanism 2+ 2+ back movement of the ring from A2 to A1 takes place [79] based on the participation of external reducing (process 7). (triethanolamine) and oxidizing (dioxygen) species, and By means of steady-state and time-resolved by an intermolecular mechanism [80] involving the kinetic spectroscopic experiments complemented by assistance of an external electron relay (phenothiazine), electrochemical measurements in acetonitrile solution, which is not consumed. However, operation by the we showed [80] that the absorption of a visible photon sacrificial mechanism does not afford an autonomous by 36+ can cause the occurrence of a forward and behavior and leads to consumption of chemical fuels back ring movement, that is, a full mechanical cycle and formation of waste products. On the other hand, according to the mechanism illustrated in Figure 5 [81]. the assistance by an electron relay affords autonomous The key issues of this mechanism are the competition operation in which only photons are consumed, but the between processes 2 (photoinduced electron transfer) mechanism is no longer based solely on intra-rotaxane and 3 (intrinsic excited-state deactivation), and between processes. processes 4 (ring displacement from the reduced We have also investigated systems related to 36+ in + 2+ 2+ A1 station) and 5 (back electron transfer). From which either the position of the A1 and A2 stations is luminescence measurements we found [79] that the exchanged with respect to the P unit [83], a different Ru- time constants of processes 2 and 3 at 303 K are 4.0 based moiety is employed [84], or the photosensitizer and 0.9 µs, respectively, corresponding to a quantum is connected noncovalently to an electron-accepting yield of 0.16 for the electron-transfer process. Transient station [85,86].

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Figure 6. Structure formula of rotaxane 44+ and schematic representation of its operation as an autonomous molecular shuttle driven by visible light. Recently, a second-generation molecular shuttle was electron-accepting ring, which is confined in the region designed and constructed [87]. The formula of rotaxane of the dumbbell delimited by the two stoppers T1 and 4+ 4 and its operation scheme, which is similar to that T2, encircles the better electron-donor TTF station. In of 36+ (Figure 5), are shown in Figure 6. The system solution, excitation of the porphyrin unit with visible light is composed of a cyclobis(paraquat-p-phenylene) (process 1 in Figure 6) should cause an electron transfer 4+ electron-accepting ring (R ) and a multicomponent to C60 (process 2); then, an electron shift from TTF to the dumbbell comprising a light-harvesting porphyrin (P) oxidized porphyrin (process 4) should destabilize the which acts as an electron donor in the excited state, original structure, causing the displacement of R4+ from + a C60 electron acceptor (A), and a tetrathiafulvalene D1 to D2 (process 5). Subsequent back electron transfer

(TTF, D1) and a dioxynaphthalene (DON, D2) electron- (process 7) and macrocycle replacement (process 8) donating stations. In the stable translational isomer the regenerate the starting isomer.

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Figure 8. (a) The acid-base controlled threading-dethreading of pseudorotaxane [5⊃6-H]2+. (b) The acid-base and light controlled interconversion between spiropyran 7 and + Figure 7. Cyclic voltammetry of 44+ in N,N-dimethylformamide protonated merocyanine 8-H . (1.0x10–3 M, 0.1 M tetrabutylammonium hexafluorophos- phate, glassy carbon electrode) recorded at 200 mV TTF oxidation by an electric potential generated s–1 with assignment of each of the peaks to a particular internally through intramolecular photoinduced electron electroactive unit of the rotaxane. The much lower than transfer is currently under investigation. In general expected current intensity for TTF oxidation is attributed to the existence of folded conformations in which the terms, these results indicate that, as the structural TTF unit, being buried inside a complex molecular struc- complexity increases, the overall properties of the ture, is not accessible for electron transfer processes with the electrode. system cannot be easily rationalized solely on the basis of the type and sequence of the functional units Rotaxane 44+ is expected to exhibit a better incorporated in the molecular framework – that is, its performance as a light-driven autonomous molecular ‘primary’ structure. Higher-level conformational effects, shuttle compared with the first-generation compound which are reminiscent of those related to the secondary 36+ for the following three reasons. First, by using a triad and tertiary structure of biomolecules [7], have to be approach [88,89], a relatively long lived charge-separated taken into consideration. The comprehension of these state should be obtained with a high efficiency. Second, effects constitutes a stimulating scientific problem, the electrostatic repulsion between the photo-oxidized and a necessary step for the design of novel artificial D + station and the R4+ ring is expected to speed up the 1 molecular devices and machines. displacement of the latter (process 5 in Figure 6). Third, Finally, it should be recalled that molecular shuttles the hampering effect of the counteranions discussed like those described in this section could not perform for 36+ is not expected to be dramatic in the case of 44+ a net mechanical work in a full cycle of operation [90] because the stations are originally uncharged. because – as for any reversible molecular shuttle – the UV-Vis-NIR absorption spectroscopic investigations work done in the ‘forward’ stroke would be cancelled by and voltammetric experiments (Figure 7) revealed the ‘backward’ stroke (section 2.1). To reach this goal, [87] remarkable electronic interactions between the more advanced molecular machines and/or a better various units of 44+, pointing to the existence of folded engineering of their operating environment (e.g., a conformations in solution. Interestingly, the TTF unit can surface or a membrane) are required. be electrochemically oxidized only in a limited fraction of the rotaxane molecules (Figure 7); in these species, removal of one electron from the TTF unit causes the 3.3. Threading-dethreading motions controlled shuttling of the R4+ ring away from this station. Most likely, by photoinduced proton transfer rotaxane 44+ occurs as conformations in which the TTF As shown by the examples described in the previous unit is buried inside a complex molecular structure and section, a modular construction of light-driven molecular is therefore protected against oxidation performed by machines pursued by integrating photoactive and an electric potential applied externally. Such a behavior mechanically switchable systems in a molecular limits the efficiency for the operation of 44+ as a redox- assembly is a quite demanding task. In fact, the majority driven molecular shuttle. The possibility of achieving of artificial molecular machines reported so 31 far[ - 36,38-40,43-47] are powered by chemical energy,

334 A. Credi, M. Venturi

in terms of the mechanism shown in Figure 9. Starting from a mixture of the complex [5⊃6-H]2+ and 7 in a 1:1 ratio, a thermal proton transfer occurs from 6-H2+ to the photochrome, yielding 8-H+ and the deprotonated guest 6+, which escapes from the cavity of 5. Subsequent light irradiation of 8-H+ in the visible region causes an opposite proton transfer, converting 6+ into 6-H2+; the latter species then rethreads into the calixarene macrocycle. Thermal equilibration-light irradiation cycles were performed on the same solution without loss of signal, showing that the overall switching process is reversible. As the reset of the system occurs thermally, its operation under continuous light irradiation can give rise to autonomous behavior. In practice, because of the large difference in the time scale of the dark and light parts of the cycle, the photostationary state is strongly displaced towards the [5⊃6-H]2+/7 mixture, unless irradiation is carried out with Figure 9. Scheme of threading-dethreading processes in pseudor- very low intensity and/or the temperature is increased. otaxane [5⊃6-H]2+ controlled by means of photoinduced proton exchange with the photochromic system 7/8-H+. Nevertheless, such a behavior could be employed to implement a memory effect in the system [95]. This most typically supplied by acid-base reactions. It would system, although far from applications, is interesting therefore be interesting to identify viable strategies for because it provides a general principle for the operation using light to operate ‘stand alone’ pH-driven molecular of photoinactive acid-base controllable molecular machines. We showed recently [91] that the acid-base machines with light. controlled threading-dethreading of a pseudorotaxane in solution can be operated by light-induced intermolecular proton transfer with a photochromic switch. The tris(N-phenylureido)calix[6]arene 5 (Figure 8a) 4. Conclusion forms fairly stable pseudorotaxane complexes with 4,4’-bipyridinium derivatives in apolar solvents [92]. The results described show that compounds capable of performing large amplitude, non-trivial and controlled Therefore, we envisaged that compound 6-H2+, obtained by protonation of the pyridyl nitrogen of the 4,4’- mechanical movements upon light stimulation can be obtained by utilizing careful incremental design pyridylpyridinium thread-like 6+ (Figure 8a), could thread strategies, the tools of modern synthetic chemistry, and into the cavity of 5 as well. In fact, spectrophotometric titrations and voltammetric experiments showed that a the paradigms of supramolecular chemistry, together very stable pseudorotaxane complex is formed between with some inspiration from natural systems. Such achievements enable to devise future 5 and 6-H2+ in CH Cl . Deprotonation of 6-H2+ with a 2 2 developments which are under investigation in our base (e.g., tributylamine) in CH2Cl2 leads to dethreading of the pseudorotaxane (Figure 8a). and other laboratories, namely: (i) the design and In order to trigger the self-assembly and disassembly construction of more sophisticated artificial molecular of this pseudorotaxane by light, we identified a species motors and machines, showing complex motions (Figure 8b) that occurs in two forms, interconvertible and better performances in terms of stability, speed, into one another by light irradiation, exhibiting smaller switching, and so forth; (ii) the use of such systems to do molecular-level tasks such as uptake-release, and larger pK than that of 6-H2+, respectively [93,94]. a transportation, catalysis, and mechanical gating of In the presence of an acid, the colorless spiropyran 7 is converted into the yellow protonated merocyanine molecular channels; and (iii) the possibility of exploiting their logic behavior for information processing at the form 8-H+ [93]. Upon irradiation with visible light, 8-H+ molecular level and, in the long run, for the construction releases a proton, isomerizing back to 7 (Figure 8b) [94]. of chemical computers. We characterized this system by performing repeated The majority of the artificial molecular machines photochemical and thermal equilibration experiments developed so far operate in solution, that is, in an and using UV-vis absorption spectroscopy to monitor incoherent fashion and without control of spatial its state [91]. The observed behavior can be interpreted positioning. The studies in solution of complicated chemical systems such as molecular machines and

335 Molecular machines operated by light

motors are indeed of fundamental importance to Apart from more or less futuristic applications, the understand their operation mechanisms; moreover, for study of motion at the molecular level and the extension some use (e.g., drug delivery) molecular machines will of the concept of motor and machine to the nanoscale have to work in liquid solution. In this regard, it should are fascinating topics for basic research. Looking be recalled that motor proteins operate in – or at least in at molecular and supramolecular species from the contact with – an aqueous solution. However, it seems viewpoint of functions with references to devices of the reasonable that, before artificial molecular machines macroscopic world is indeed a very interesting exercise can find applications in many fields of technology, they which introduces novel concepts into Chemistry as a have to be interfaced with the macroscopic world by scientific discipline. ordering them in some way so that they can behave coherently and can be addressed in space [96]. New generations of molecular machines and motors Acknowledgments organized at interfaces [97], deposited on surfaces [98-102], embedded into liquid crystals [103-105] and We would like to thank prof. Vincenzo Balzani for his polymers [106], or immobilized into membranes [107] support, and prof. J. Fraser Stoddart and his group or porous materials [23,108], have started to appear. for a long lasting and fruitful cooperation. We are On the basis of recent experiments [109,110] showing also grateful to all our collaborators and coworkers that the collective operation of machine-molecules in whose names appear in the reference list. Financial carefully engineered surface-deposited monolayers support from Ministero dell’Università e della Ricerca can indeed develop mechanical work at a larger scale, (PRIN 2006034123_003), Ministero degli Affari Esteri one can optimistically hope that useful devices based (DGPCC), Regione Emilia-Romagna (NANOFABER) on artificial nanomachines will see the light in a not too and Università di Bologna (Progetto strategico distant future. CompReNDe) is gratefully acknowledged.

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