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Photoactive pseudorotaxanes and rotaxanes as artificial molecular machines
Synthetic Metals 139 (2003) 773–777
Photoactive pseudorotaxanes and rotaxanes as artificial molecular machines Miguel Clemente-León1 , Filippo Marchioni, Serena Silvi, Alberto Credi∗ Dipartimento di Chimica “G. Ciamician”, Università di Bologna, via Selmi 2, 40126 Bologna, Italy
Abstract A molecular machine is an assembly of a discrete number of molecular components (that is, a supramolecular structure) designed to perform mechanical-like movements as a consequence of appropriate external stimuli. The most convenient way to supply energy to an artificial molecular-level machine is through a photochemical reaction, that is, by using light. The two main types of photochemical reactions that can be employed to drive molecular machines are photoisomerization and photoinduced electron-transfer processes. By means of spectroscopic techniques, light can also be used to monitor the operation of the system. Two prototypes of photochemically driven molecular machines are described, namely, (i) a pseudorotaxane undergoing threading/dethreading motions, and (ii) a molecular shuttle based on a rotaxane. The extension of the concept of machine to the molecular-level is important not only for basic research, but also for the growth of nanoscience and the development of nanotechnology. © 2003 Elsevier Science B.V. All rights reserved. Keywords: Azobenzene; Electrochemistry; Electron-transfer; Luminescence; Ru complexes; Supramolecular chemistry
1. Introduction A molecular-level machine can be defined [1–4] as an assembly of a distinct number of molecular components that are designed to perform mechanical movements as a result of an appropriate external stimulation. Although there are many chemical compounds whose structure and/or shape can be modified by an external stimulus (see, e.g. photoisomerizable species), the term molecular machine is used only for systems showing large amplitude movements of the molecular components. Like macroscopic machines, molecular-level machines are characterized by (i) the kind of energy input supplied to make them work, (ii) the kind of movement performed by their components, (iii) the way in which their operation can be controlled and monitored, (iv) the possibility to repeat the operation at will and establish a cyclic process, (v) the time scale needed to complete a cycle of operation, and (vi) the function performed. Chemical, photochemical or electrochemical stimuli can be used to feed molecular-level machines [1–4]. Light stimulation, however, is the most convenient form of energy to make
∗
Corresponding author. Tel.: +39-051-209-9540; fax: +39-051-209-9456. E-mail address: [email protected] (A. Credi). 1 Present Address: Instituto de Ciencia Molecular, Universitat de Val`encia, Dr. Moliner 50, 46100 Burjassot, Spain.
molecular machines work [5], since it can be switched on and off easily and rapidly, it does not require “hard wiring”, and, by employing lasers, it provides the opportunity of working in small space regions and very short time domains. A further advantage offered by the use of photochemical techniques is that photons, besides supplying the energy needed to make a device work, can also be useful to read the state of the system (through spectroscopic methods) and thus to control and monitor its operation. Threaded and interlocked compounds such as pseudorotaxanes and rotaxanes [6], owing to their peculiar structure, are attractive candidates for the construction of artificial molecular machines. Fig. 1 shows pictorially two types of movements that can be imagined for these systems. Here we describe two examples of photochemically driven molecular motions recently studied in our laboratories, namely, the threading/dethreading of a pseudorotaxane, based on a cis/trans photoisomerization reaction, and the shuttling of the ring component in a rotaxane, which relies on a photoinduced electron-transfer process.
2. Light-driven threading/dethreading of a pseudorotaxane Photoisomerization reactions, particularly the well known [7] reversible cis/trans photoisomerization of the
0379-6779/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0379-6779(03)00328-X
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M. Clemente-Le´on et al. / Synthetic Metals 139 (2003) 773–777
Fig. 1. Pictorial representation of (a) threading/dethreading of the thread and ring components of a pseudorotaxane, and (b) shuttling of the macrocyclic component along the axle in a rotaxane.
azobenzene group, have long been used to exert photochemical control on chemical systems (For early examples, see [8]). We have recently studied [9] a pseudorotaxane, illustrated in Fig. 2, in which dethreading/rethreading is based on such a principle and is powered exclusively by light energy. The thread-like species trans-1, which contains an electron-rich azobiphenoxy unit, and the electron-acceptor macrocycle 24+ self-assemble very efficiently in acetonitrile solution to give a pseudorotaxane, stabilized mainly by -electron-donor/acceptor interactions. In the pseudorotaxane structure, the intense fluorescence band characteristic of free 24+ is completely quenched by the donor/acceptor interaction. Irradiation with 365 nm light of a solution containing trans-1 and 24+ , in which the majority of the
species are assembled to give the pseudorotaxane, causes trans → cis photoisomerization of 1. Since the affinity of the macrocycle for cis-1 is much lower than that for trans-1, photoexcitation causes a dethreading process (Fig. 2), as indicated by a substantial increase in the fluorescence intensity of free 24+ (Fig. 3). On irradiation at 436 nm or by warming the solution in the dark the trans isomer of 1 can be reformed and, as a result, it threads inside the macrocycle once again. Owing to the full reversibility of the photoisomerization process, the light-driven dethreading/rethreading cycle can be repeated many times on the same solution (Fig. 3, inset). Another relevant feature of this system is that it exhibits profound changes of a strong fluorescence signal.
Fig. 2. Dethreading/rethreading of pseudorotaxane [1·2]4+ as a consequence of the cis/trans photoisomerization of the azobenzene-type unit contained in the thread-like component 1.
M. Clemente-Le´on et al. / Synthetic Metals 139 (2003) 773–777
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3. A photocontrollable molecular shuttle
Fig. 3. Fluorescence spectra (λexc = 411 nm) of an equimolar mixture (1×10−4 mol/l, CH3 CN solution, r.t.) of trans-1 and 24+ (full line) and of the same mixture after irradiation (20 min) at 365 nm (dashed line). The inset shows the changes in intensity of the fluorescence associated with the free macrocyclic ring 24+ upon consecutive trans → cis (20 min of irradiation at 365 nm, dark areas) and cis → trans (10 min of irradiation at 436 nm, light areas) photoisomerisation cycles.
In rotaxanes containing two different recognition sites (‘stations’) in the axle component it is possible to make the macrocycle ‘shuttle’ between the two stations by an external stimulus [1–4]. Elegant examples of molecular shuttles operated by light have been reported in the last few years [10]. Taking advantage of our past investigations on photocontrolled dethreading of pseudorotaxanes [11], we have studied rotaxane 36+ [12], specifically designed to achieve photoinduced shuttling of its ring component. This compound is made (Fig. 4, top) of the electron-donor macrocycle R, and an axle component which contains (i) [Ru(bpy)3 ]2+ (P) as one of its stoppers, (ii) a 4,4 -bipyridinium unit (A1 ) and a 3,3 -dimethyl-4,4 -bipyridinium unit (A2 ) as electron-accepting stations, (iii) a p-terphenyl-type ring system as a rigid spacer (S), and (iv) a tetraarylmethane group as the second stopper (T). The stable translational isomer of rotaxane 36+ is the one in which the R component encircles the A1 unit, in keeping with the fact that this station is a better electron acceptor than the other one. The strategy devised in order to obtain the photoinduced abacus-like movement of the R macrocycle between the two stations A1 and A2 relies on intramolecular processes, i.e. on processes involving only the rotaxane components. As illustrated in
Fig. 4. (a) Rotaxane 36+ and (b) schematic representation of the mechanism for the photoinduced shuttling movement of macrocycle R between the two stations A1 and A2 located on the axle component.
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the bottom part of Fig. 4, this mechanism is based on the following four operations [12]: (a) Destabilization of the stable translational isomer: Light excitation of the photoactive unit P (process 1) is followed by the transfer of an electron from the excited state to the A1 station, which is encircled by the ring R (process 2), with the consequent ‘deactivation’ of this station; such a photoinduced electron-transfer process competes with the intrinsic decay of ∗ P (process 3). (b) Ring displacement: The ring moves from the reduced A1 station to A2 (process 4), a step that has to compete with the back electron-transfer process from A1− (still encircled by R) to the oxidized photoactive unit, P+ (process 5). (c) Electronic reset: A back electron-transfer process from the ‘free’ A1− station to P+ (process 6) restores the electron-acceptor power to the A1 station. (d) Nuclear reset: As a consequence of the electronic reset, back movement of the ring from A2 to A1 takes place (process 7). Electrochemical investigations (CH3 CN solution, 25 ◦ C) on the reduction of the A1 and A2 units in the rotaxane and in its axle component have revealed [12] that in 36+ the macrocycle moves from A1 to A2 upon injection of one electron into the A1 station. Moreover, steady-state and time-resolved absorption and luminescence studies, including laser flash photolysis, have demonstrated [12] that light excitation of P, which can be performed in a selective manner by using visible light, causes an electron-transfer process to the A1 station. The back electron-transfer that regenerates the ground state occurs with a rate constant of 2.4 × 105 s−1 , which means that the A1 station remains reduced for some microseconds before giving the electron back to the oxidized Ru complex. It is clear that a very precise organization in the dimensions of space, energy and time is mandatory in order to obtain a supramolecular species capable of performing a specific function. A key point for the operation mechanism reported in Fig. 4 is the favorable competition of ring displacement (process 4) with back electron-transfer (process 5). While the back electron-transfer kinetics have been measured by laser flash photolysis (vide supra), the determination of the rate constant for the displacement of the ring towards A2 upon one-electron reduction of A1 is not straightforward. Voltammetric methods can provide a means to study the kinetics of the structural rearrangement associated with redox processes. The reversibility of the reduction processes of the A1 and A2 units of 36+ , observed in conventional voltammetric experiments, indicates that at room temperature the rate of the electrochemically induced ring shuttling is fast on the time scale accessible with potential sweep rates ≤1 V/s. Very fast sweep rates could not be used because of severe adsorption of 36+ on both Pt and graphite ultramicroelectrodes. An alternative possibility [13] is to perform voltammetric experiments with conventional sweep
Fig. 5. Cyclic voltammogram (0.5 mM, DMF with TEAP 0.05 M, −60 ◦ C, glassy carbon as working electrode, sweep rate 0.5 V/s) showing the first reduction process for rotaxane 36+ (full line) and its axle component (dashed line).
rates, but lowering the temperature in order to slow down the ring shuttling motion. Fig. 5 shows the cyclic voltammetric waves, recorded in N,N-dimethylformamide at −60 ◦ C, for the first reduction process of rotaxane 36+ and its axle component, which takes place on the A1 unit in both cases. The change of the cyclic voltammetric behavior of 36+ with sweep rate, observed at different temperatures in the range −10 to −60 ◦ C, is consistent with a chemical reaction—that is, ring shuttling—being coupled with the reduction process. Digital simulations of the voltammetric curves are underway with the purpose of evaluating the rate constants for the motion of the macrocycle. Photochemical ring shuttling has indeed been obtained with 36+ by using a less demanding mechanism based on the use of external sacrificial reactants besides light irradiation [12]. 4. Conclusion The progresses made in the fields of supramolecular chemistry and related disciplines have led in recent years to design and construct artificial molecular-level machines. For several reasons, light stimulation is particularly convenient for operating molecular machines. Photochemical reactions such as photoisomerization and photoinduced electron-transfer processes have been successfully used for this purpose. The research on artificial molecular-level machines is of interest not only for the development of nanotechnology, but also for 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 which introduces novel concepts into chemistry as a scientific discipline. Acknowledgements We are indebted to Professors V. Balzani and M. Venturi for their continuous help and inspiration, and to Professor
M. Clemente-Le´on et al. / Synthetic Metals 139 (2003) 773–777
J.F. Stoddart and his group for a long lasting and profitable collaboration. Financial support from the EU (project HPRN-CT-2000-00029), the University of Bologna and MIUR is gratefully acknowledged.
References [1] V. Balzani, A. Credi, M. Venturi, Molecular Devices and Machines— A Journey into the Nano World, Wiley, Weinheim, 2003. [2] V. Balzani, A. Credi, F.M. Raymo, J.F. Stoddart, Angew. Chem. Int. Ed. 39 (2000) 3348. [3] J.F. Stoddart (Guest Ed.), Acc. Chem. Res. 34 (6) (2001) (Special Issue on Molecular Machines). [4] J.-P. Sauvage (Guest Ed.), Struct. Bond. 99 (2001) (Volume on Molecular Machines and Motors).
777
[5] R. Ballardini, et al., Acc. Chem. Res. 34 (2001) 445. [6] J.-P. Sauvage, C.O. Dietrich-Buchecker (Eds.), Molecular Catenanes, Rotaxanes and Knots, Wiley, Weinheim, 1999. [7] H. Rau, in: H. Dürr, H. Bouas-Laurent (Eds.), Photochromism: Molecules and Systems, Elsevier, Amsterdam, 1990 (Chapter 4). [8] (a) A. Ueno, et al., J. Am. Chem. Soc. 101 (1979) 2779; (b) A. Ueno, et al., J. Chem. Soc., Chem. Commun. (1980) 837; (c) S. Shinkai, et al., J. Am. Chem. Soc. 103 (1981) 111. [9] V. Balzani, A. Credi, F. Marchioni, J.F. Stoddart, Chem. Commun. (2001) 1861. [10] (a) H. Murakami, et al., J. Am. Chem. Soc. 119 (1997) 7605; (b) N. Armaroli, et al., J. Am. Chem. Soc. 121 (1999) 4397; (c) A.M. Brouwer, et al., Science 291 (2001) 2124. [11] (a) R. Ballardini, et al., Angew. Chem. Int. Ed. Engl. 32 (1993) 1301; (b) P.R. Ashton, et al., Chem. Eur. J. 4 (1998) 2411; (c) P.R. Ashton, et al., J. Am. Chem. Soc. 120 (1998) 11190. [12] P.R. Ashton, et al., Chem. Eur. J. 6 (2000) 3558. [13] V. Balzani, A. Credi, M. Venturi, et al., in preparation.
Photoactive pseudorotaxanes and rotaxanes as artificial molecular machines Miguel Clemente-León1 , Filippo Marchioni, Serena Silvi, Alberto Credi∗ Dipartimento di Chimica “G. Ciamician”, Università di Bologna, via Selmi 2, 40126 Bologna, Italy
Abstract A molecular machine is an assembly of a discrete number of molecular components (that is, a supramolecular structure) designed to perform mechanical-like movements as a consequence of appropriate external stimuli. The most convenient way to supply energy to an artificial molecular-level machine is through a photochemical reaction, that is, by using light. The two main types of photochemical reactions that can be employed to drive molecular machines are photoisomerization and photoinduced electron-transfer processes. By means of spectroscopic techniques, light can also be used to monitor the operation of the system. Two prototypes of photochemically driven molecular machines are described, namely, (i) a pseudorotaxane undergoing threading/dethreading motions, and (ii) a molecular shuttle based on a rotaxane. The extension of the concept of machine to the molecular-level is important not only for basic research, but also for the growth of nanoscience and the development of nanotechnology. © 2003 Elsevier Science B.V. All rights reserved. Keywords: Azobenzene; Electrochemistry; Electron-transfer; Luminescence; Ru complexes; Supramolecular chemistry
1. Introduction A molecular-level machine can be defined [1–4] as an assembly of a distinct number of molecular components that are designed to perform mechanical movements as a result of an appropriate external stimulation. Although there are many chemical compounds whose structure and/or shape can be modified by an external stimulus (see, e.g. photoisomerizable species), the term molecular machine is used only for systems showing large amplitude movements of the molecular components. Like macroscopic machines, molecular-level machines are characterized by (i) the kind of energy input supplied to make them work, (ii) the kind of movement performed by their components, (iii) the way in which their operation can be controlled and monitored, (iv) the possibility to repeat the operation at will and establish a cyclic process, (v) the time scale needed to complete a cycle of operation, and (vi) the function performed. Chemical, photochemical or electrochemical stimuli can be used to feed molecular-level machines [1–4]. Light stimulation, however, is the most convenient form of energy to make
∗
Corresponding author. Tel.: +39-051-209-9540; fax: +39-051-209-9456. E-mail address: [email protected] (A. Credi). 1 Present Address: Instituto de Ciencia Molecular, Universitat de Val`encia, Dr. Moliner 50, 46100 Burjassot, Spain.
molecular machines work [5], since it can be switched on and off easily and rapidly, it does not require “hard wiring”, and, by employing lasers, it provides the opportunity of working in small space regions and very short time domains. A further advantage offered by the use of photochemical techniques is that photons, besides supplying the energy needed to make a device work, can also be useful to read the state of the system (through spectroscopic methods) and thus to control and monitor its operation. Threaded and interlocked compounds such as pseudorotaxanes and rotaxanes [6], owing to their peculiar structure, are attractive candidates for the construction of artificial molecular machines. Fig. 1 shows pictorially two types of movements that can be imagined for these systems. Here we describe two examples of photochemically driven molecular motions recently studied in our laboratories, namely, the threading/dethreading of a pseudorotaxane, based on a cis/trans photoisomerization reaction, and the shuttling of the ring component in a rotaxane, which relies on a photoinduced electron-transfer process.
2. Light-driven threading/dethreading of a pseudorotaxane Photoisomerization reactions, particularly the well known [7] reversible cis/trans photoisomerization of the
0379-6779/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0379-6779(03)00328-X
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M. Clemente-Le´on et al. / Synthetic Metals 139 (2003) 773–777
Fig. 1. Pictorial representation of (a) threading/dethreading of the thread and ring components of a pseudorotaxane, and (b) shuttling of the macrocyclic component along the axle in a rotaxane.
azobenzene group, have long been used to exert photochemical control on chemical systems (For early examples, see [8]). We have recently studied [9] a pseudorotaxane, illustrated in Fig. 2, in which dethreading/rethreading is based on such a principle and is powered exclusively by light energy. The thread-like species trans-1, which contains an electron-rich azobiphenoxy unit, and the electron-acceptor macrocycle 24+ self-assemble very efficiently in acetonitrile solution to give a pseudorotaxane, stabilized mainly by -electron-donor/acceptor interactions. In the pseudorotaxane structure, the intense fluorescence band characteristic of free 24+ is completely quenched by the donor/acceptor interaction. Irradiation with 365 nm light of a solution containing trans-1 and 24+ , in which the majority of the
species are assembled to give the pseudorotaxane, causes trans → cis photoisomerization of 1. Since the affinity of the macrocycle for cis-1 is much lower than that for trans-1, photoexcitation causes a dethreading process (Fig. 2), as indicated by a substantial increase in the fluorescence intensity of free 24+ (Fig. 3). On irradiation at 436 nm or by warming the solution in the dark the trans isomer of 1 can be reformed and, as a result, it threads inside the macrocycle once again. Owing to the full reversibility of the photoisomerization process, the light-driven dethreading/rethreading cycle can be repeated many times on the same solution (Fig. 3, inset). Another relevant feature of this system is that it exhibits profound changes of a strong fluorescence signal.
Fig. 2. Dethreading/rethreading of pseudorotaxane [1·2]4+ as a consequence of the cis/trans photoisomerization of the azobenzene-type unit contained in the thread-like component 1.
M. Clemente-Le´on et al. / Synthetic Metals 139 (2003) 773–777
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3. A photocontrollable molecular shuttle
Fig. 3. Fluorescence spectra (λexc = 411 nm) of an equimolar mixture (1×10−4 mol/l, CH3 CN solution, r.t.) of trans-1 and 24+ (full line) and of the same mixture after irradiation (20 min) at 365 nm (dashed line). The inset shows the changes in intensity of the fluorescence associated with the free macrocyclic ring 24+ upon consecutive trans → cis (20 min of irradiation at 365 nm, dark areas) and cis → trans (10 min of irradiation at 436 nm, light areas) photoisomerisation cycles.
In rotaxanes containing two different recognition sites (‘stations’) in the axle component it is possible to make the macrocycle ‘shuttle’ between the two stations by an external stimulus [1–4]. Elegant examples of molecular shuttles operated by light have been reported in the last few years [10]. Taking advantage of our past investigations on photocontrolled dethreading of pseudorotaxanes [11], we have studied rotaxane 36+ [12], specifically designed to achieve photoinduced shuttling of its ring component. This compound is made (Fig. 4, top) of the electron-donor macrocycle R, and an axle component which contains (i) [Ru(bpy)3 ]2+ (P) as one of its stoppers, (ii) a 4,4 -bipyridinium unit (A1 ) and a 3,3 -dimethyl-4,4 -bipyridinium unit (A2 ) as electron-accepting stations, (iii) a p-terphenyl-type ring system as a rigid spacer (S), and (iv) a tetraarylmethane group as the second stopper (T). The stable translational isomer of rotaxane 36+ is the one in which the R component encircles the A1 unit, in keeping with the fact that this station is a better electron acceptor than the other one. The strategy devised in order to obtain the photoinduced abacus-like movement of the R macrocycle between the two stations A1 and A2 relies on intramolecular processes, i.e. on processes involving only the rotaxane components. As illustrated in
Fig. 4. (a) Rotaxane 36+ and (b) schematic representation of the mechanism for the photoinduced shuttling movement of macrocycle R between the two stations A1 and A2 located on the axle component.
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the bottom part of Fig. 4, this mechanism is based on the following four operations [12]: (a) Destabilization of the stable translational isomer: Light excitation of the photoactive unit P (process 1) is followed by the transfer of an electron from the excited state to the A1 station, which is encircled by the ring R (process 2), with the consequent ‘deactivation’ of this station; such a photoinduced electron-transfer process competes with the intrinsic decay of ∗ P (process 3). (b) Ring displacement: The ring moves from the reduced A1 station to A2 (process 4), a step that has to compete with the back electron-transfer process from A1− (still encircled by R) to the oxidized photoactive unit, P+ (process 5). (c) Electronic reset: A back electron-transfer process from the ‘free’ A1− station to P+ (process 6) restores the electron-acceptor power to the A1 station. (d) Nuclear reset: As a consequence of the electronic reset, back movement of the ring from A2 to A1 takes place (process 7). Electrochemical investigations (CH3 CN solution, 25 ◦ C) on the reduction of the A1 and A2 units in the rotaxane and in its axle component have revealed [12] that in 36+ the macrocycle moves from A1 to A2 upon injection of one electron into the A1 station. Moreover, steady-state and time-resolved absorption and luminescence studies, including laser flash photolysis, have demonstrated [12] that light excitation of P, which can be performed in a selective manner by using visible light, causes an electron-transfer process to the A1 station. The back electron-transfer that regenerates the ground state occurs with a rate constant of 2.4 × 105 s−1 , which means that the A1 station remains reduced for some microseconds before giving the electron back to the oxidized Ru complex. It is clear that a very precise organization in the dimensions of space, energy and time is mandatory in order to obtain a supramolecular species capable of performing a specific function. A key point for the operation mechanism reported in Fig. 4 is the favorable competition of ring displacement (process 4) with back electron-transfer (process 5). While the back electron-transfer kinetics have been measured by laser flash photolysis (vide supra), the determination of the rate constant for the displacement of the ring towards A2 upon one-electron reduction of A1 is not straightforward. Voltammetric methods can provide a means to study the kinetics of the structural rearrangement associated with redox processes. The reversibility of the reduction processes of the A1 and A2 units of 36+ , observed in conventional voltammetric experiments, indicates that at room temperature the rate of the electrochemically induced ring shuttling is fast on the time scale accessible with potential sweep rates ≤1 V/s. Very fast sweep rates could not be used because of severe adsorption of 36+ on both Pt and graphite ultramicroelectrodes. An alternative possibility [13] is to perform voltammetric experiments with conventional sweep
Fig. 5. Cyclic voltammogram (0.5 mM, DMF with TEAP 0.05 M, −60 ◦ C, glassy carbon as working electrode, sweep rate 0.5 V/s) showing the first reduction process for rotaxane 36+ (full line) and its axle component (dashed line).
rates, but lowering the temperature in order to slow down the ring shuttling motion. Fig. 5 shows the cyclic voltammetric waves, recorded in N,N-dimethylformamide at −60 ◦ C, for the first reduction process of rotaxane 36+ and its axle component, which takes place on the A1 unit in both cases. The change of the cyclic voltammetric behavior of 36+ with sweep rate, observed at different temperatures in the range −10 to −60 ◦ C, is consistent with a chemical reaction—that is, ring shuttling—being coupled with the reduction process. Digital simulations of the voltammetric curves are underway with the purpose of evaluating the rate constants for the motion of the macrocycle. Photochemical ring shuttling has indeed been obtained with 36+ by using a less demanding mechanism based on the use of external sacrificial reactants besides light irradiation [12]. 4. Conclusion The progresses made in the fields of supramolecular chemistry and related disciplines have led in recent years to design and construct artificial molecular-level machines. For several reasons, light stimulation is particularly convenient for operating molecular machines. Photochemical reactions such as photoisomerization and photoinduced electron-transfer processes have been successfully used for this purpose. The research on artificial molecular-level machines is of interest not only for the development of nanotechnology, but also for 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 which introduces novel concepts into chemistry as a scientific discipline. Acknowledgements We are indebted to Professors V. Balzani and M. Venturi for their continuous help and inspiration, and to Professor
M. Clemente-Le´on et al. / Synthetic Metals 139 (2003) 773–777
J.F. Stoddart and his group for a long lasting and profitable collaboration. Financial support from the EU (project HPRN-CT-2000-00029), the University of Bologna and MIUR is gratefully acknowledged.
References [1] V. Balzani, A. Credi, M. Venturi, Molecular Devices and Machines— A Journey into the Nano World, Wiley, Weinheim, 2003. [2] V. Balzani, A. Credi, F.M. Raymo, J.F. Stoddart, Angew. Chem. Int. Ed. 39 (2000) 3348. [3] J.F. Stoddart (Guest Ed.), Acc. Chem. Res. 34 (6) (2001) (Special Issue on Molecular Machines). [4] J.-P. Sauvage (Guest Ed.), Struct. Bond. 99 (2001) (Volume on Molecular Machines and Motors).
777
[5] R. Ballardini, et al., Acc. Chem. Res. 34 (2001) 445. [6] J.-P. Sauvage, C.O. Dietrich-Buchecker (Eds.), Molecular Catenanes, Rotaxanes and Knots, Wiley, Weinheim, 1999. [7] H. Rau, in: H. Dürr, H. Bouas-Laurent (Eds.), Photochromism: Molecules and Systems, Elsevier, Amsterdam, 1990 (Chapter 4). [8] (a) A. Ueno, et al., J. Am. Chem. Soc. 101 (1979) 2779; (b) A. Ueno, et al., J. Chem. Soc., Chem. Commun. (1980) 837; (c) S. Shinkai, et al., J. Am. Chem. Soc. 103 (1981) 111. [9] V. Balzani, A. Credi, F. Marchioni, J.F. Stoddart, Chem. Commun. (2001) 1861. [10] (a) H. Murakami, et al., J. Am. Chem. Soc. 119 (1997) 7605; (b) N. Armaroli, et al., J. Am. Chem. Soc. 121 (1999) 4397; (c) A.M. Brouwer, et al., Science 291 (2001) 2124. [11] (a) R. Ballardini, et al., Angew. Chem. Int. Ed. Engl. 32 (1993) 1301; (b) P.R. Ashton, et al., Chem. Eur. J. 4 (1998) 2411; (c) P.R. Ashton, et al., J. Am. Chem. Soc. 120 (1998) 11190. [12] P.R. Ashton, et al., Chem. Eur. J. 6 (2000) 3558. [13] V. Balzani, A. Credi, M. Venturi, et al., in preparation.