As summarized retrosynthetically in Scheme 10, the key steps for the synthesis of 47 and 48 are the preparation and photocyclization of the stilbene 54 containing the triptycene unit. Stilbene 54 would be formed by a Wittig reaction between aldehyde 55 and the ylide Photocyclization and cleavage of the methyl ether would give phenol 53 which should allow the incorporation of tethers of variable length to give 47 or Sestelo Scheme After Suzuki coupling of 60 with 61, a regioselective Diels-Alder addition of benzyne across the central ring of the anthracene afforded the desired aldehyde Wittig reaction Scheme 12 between the aldehyde 55 and the ylide 56, which was prepared from 7-methoxynaphthol , gave stilbene 54 as a Scheme Treatment of 63 with BBr3 afforded the desired phenol With the basic skeleton of 47 and 48 in hand, we proceeded to incorporate the tethers Scheme The three-carbon tether was attached to the helicene unit by reacting the phenol of 53 with 3-bromopropanol under basic conditions.
In an analogous way, the two-carbon tether was introduced by reaction of the phenol 53 with the methoxymethyl MOM ether of 2-bromoethanol. With the two molecular systems in hand, we then set out to separate and identify each of the three atropisomers of 47 and After extensive attempts, we found that by using semipreparative thin layer chromatography plates 0. The conformations of individual atropisomers were initially tentatively assigned by use of 1- and 2-D low temperature 1H NMR spectroscopy. After separating and determining the structures of atropisomers 47a, b and c Fig.
Accordingly, carbamate formation via isocyanate was chosen. In model studies with the simple triptyceneamine 67 and naphthol 68 Fig. Given the cleavage of 52 to 47b mentioned above, the unidirectional rotation of 47a to 47b had been achieved and a prototype of a molecular motor was in hand! Once the prototype molecular motor 47 had been shown to function as desired, we proceeded to study the rotation of the analogous molecular system 48, where the tether contains only two carbons.
Based on earlier considerations Fig. When the rotamer 48a was treated under the same reaction conditions [phosgene 1 equiv and Et3N 5 equiv ] as used with 47a, a mixture of isocyanate 69 and carbamate 70 Fig. Investigative studies showed that isocyanate 69 does not convert to carbamate 70 apparently due to geometric constrains and that formation of 70 occurs through the carbamoyl chloride 71 Scheme Accordingly, experimental conditions [phosgene mixed with 2,6-di tertbutyl pyridine and addition of triethylamine 5 equiv ] that favor the formation of the carbamoyl chloride 71 rather than isocyanate 69 were chosen.
Products from reaction of 48a with phosgene under original reaction conditions Scheme Furthermore, treatment of 48a and 2,6-di tertbutyl pyridine with phosgene and 14 equiv of triethylamine gave identical results but required only 15 min for reactions and rotation of 48a to 70 to be completed. The important conclusions from the study of 48a Fig. Much optimization remains to be done before we achieve a molecular system that rotates continuously and rivals the speed of its biological and mechanical counterparts. The next step, however, is now clear: To achieve repeated 50 T.
Efforts in that direction are now in progress. Meanwhile, the concepts underlying the present work should be of general application to the understanding of biological and other motors. Schematic representation of a continually operating molecular motor Rotary Motion in Single-Molecule Machines 51 8 Conclusions Over the last 10 years, efforts of several groups have led to the synthesis of a variety of molecular devices that exhibit controlled or coordinated rotation.
In our research group the ultimate achievement has been the synthesis of a molecular system that functions as a prototype of a motor. The utility of these systems in nanochemical engineering is still far in the future but their study has contributed to identifying some of the perils and promise of extrapolating macroscopic principles to the molecular level, and has provided conceptual insight into the operation of biological motors.
This work would not have been accomplished without the dedicated efforts of the coworkers named in the references. We thank them for their enormous contributions and the National Institutes of Health grant GM for partial support. In: Canton CR ed Bioorganic chemistry, 3rd edn. Reinhold Publishing Corp, New York, p Drexler KE Science and the future In: Encyclopedia Britannica, Chicago, p 4.
For an overview, see Travis J Science Stryer L Molecular motors. In: Biochemistry, 4th edn. Wiley, New York Block SM Nature 52 T. Sestelo Rotary Motion in Single-Molecule Machines For a bibliography of applications, see www. However, the number of synthetic molecular ensembles whose dynamic behavior is reminiscent of biological motors is presently very limited. If a ring is threaded onto a rod, it can either rotate around the axle or undergo a translation movement.
Similarly, in catenanes, a ring can glide at will within another ring. Several examples of such compounds have been elaborated and studied in recent years, using threaded and interlocked molecules either based on acceptor-donor and hydrogen-bonded complexes or on transition metal complexes. These systems are multicomponent assemblies undergoing large amplitude geometrical changes or leading to the locomotion of one of the components, Structure and Bonding, Vol.
Sauvage either under the action of an external stimulus pH change, redox process, light pulse, etc. The number of synthetic molecular ensembles whose dynamic behavior is reminiscent of biological motors is presently very limited. Similarly, in catenanes i. Recently, a new dimension has been added by moving components of these Fig. In fact, compounds containing interlocking rings or rings threaded onto an acyclic fragment are the ideal precursors to molecular machines, i.
As schematically represented in Fig. However, molecular machines tend to refer to large amplitude motions leading to real translocation of some parts of the compound, reversibility being of course an essential feature of the system. Several examples of such compounds have been elaborated and studied in recent years, using threaded and interlocked molecules either based on acceptor-donor and hydrogen-bonded complexes  or on transition metal complexes [18, 19].
In the purely organic systems, interesting processes have been evidenced, such as dethreading of an acyclic component from a ring under the action of light or translation of a ring between two distinct positions of a thread, induced by a redox signal. The group of Stoddart has recently created a vast and new family of interlocking and threaded compounds constructed on acceptor-donor aromatic complexes. These authors have demonstrated that molecular movements can be induced in such systems, either by irradiating the compounds with visible light in the presence of other additional reagents acting as an electron donor or acceptor, or by using electrochemical reactions Fig.
For instance, a sulfoxide is Fig. Interlocking rings and threaded systems can be considered as elemental working parts of future molecular machines. Sauvage Fig. A switchable rotaxane based on acceptor-donor complexes . Before oxidation, the electron acceptor ring interacts preferentially with the benzidine nucleus donor.
After electrochemical oxidation of the latter, the ring is shifted towards the biphenol group. Another pertinent example of redox-induced movement is based on a multifunctional system incorporating sets of ligands forming two distinct coordination sites, adapted to either Fe II or Fe III [37, 38]. By changing the iron oxidation state, translocation of the metal is observed Fig. Hopping of an iron center between two coordination sites.
In the stable form of the Fe II complex, the metal is bound to three bipy ligands. After oxidation to Fe III , the metal center moves to the anionic site  the string to thread through the ring. This threading step is generally quantitative provided that the stoichiometry of the reaction is carefully respected, due to the selective formation of very stable tetrahedral copper I complexes Fig. It can be extended to strings containing two identical coordination sites, thus permitting the threading of two identical rings Fig. It can also be generalized to molecular strings containing two different sites, such as bidentate and terdentate coordinating units Fig.
In this case, again because of the very strong preference of copper I for 4-coordinate complexes tetrahedral or distorted tetrahedral , the threading process will be very selective and lead to a situation in which the ring is exclusively associated with the bidentate chelate fragment of the string, in its coordination to copper I , as indicated in Fig. It is of course this latter compound which will be prompted to undergo motions by changing the redox state. This characteristic will provide the driving force for setting our systems in motion.
Whereas a coordination number CN of 4, usually with a roughly tetrahedral arrangement of ligands, corresponds to stable monovalent systems, copper II requires higher coordination numbers. Photochemical and thermal isomerization processes in a biphenanthrylene derivative  trigonal bipyramidal geometries or 6 octahedral arrangement, with JahnTeller distortion. Thus, by switching alternatively from copper I to copper II , one should be able to induce changes in the molecule so as to afford a coordination situation favorable to the corresponding oxidation state.
The principle is illustrated in Fig. Of course, if the acyclic molecular fragment which threads the ring does not bear blocking groups at its ends, dethreading may occur. The obvious improvement is to attach one or, better, two bulky groups at the extremities of the string in order to prevent unthreading . These new systems are represented in Fig. Molecular Machines and Motors 61 The semi-rotaxane was simply obtained by mixing stoichiometric amounts of 8, which is an improved version of 6 Fig.
It is real in the sense that demetallation furnishes a copper-free system 12 whose acyclic component will not dethread from the membered ring. The electrochemically induced molecular motions square scheme similar to those represented in Fig. From the CV measurements at different scan rates from 0.
As the two redox couples involved in these systems are separated by 0. In Fig. A reversible redox wave at 0. SCE attests to the tetrahedral environment around the copper I atom [18, 19, 45]. During the potential scan, for rates between 0. This fact evidences the high kinetic stability of the 4-coordinate copper II rotaxane generated at the electrode. During the electrolysis the red color of the solution changed to light green. According to the invariant shape of the signals with the scan rate of the copper II solution, it can be inferred that the rate constant for the reaction Cu I 5 to Cu I 4 62 L.
Sauvage Molecular Machines and Motors 63 b Fig. Copper I -induced threading of one or two rings on a molecular string. This threading process has been used in our group since the early s to make catenanes. A second electrolysis at 0. As all the Cu I 5 species formed electrochemically are quantitatively transformed into Cu I 4 species during the electrolysis, we can give a lower limit of 10 4 s 1for the rate constant of the chemical reaction. The residual signal at 0. The 6-coordinate complex evidenced by CV originates from the dethreading and coordination of the two linear fragments via their terpy units to a copper center.
Principle of the electrochemically induced molecular motions in a copper I complex pseudorotaxane. The stable four-coordinate monovalent complex is oxidized to an intermediate tetrahedral divalent species. Finally, the latter undergoes the reorganization process that regenerates the starting complex [the black circle represents Cu I and the white circle represents Cu II ] 64 L. Sauvage Molecular Machines and Motors 65 b Fig. The bis-terpy complex, which would be formed by decomplexation of the copper II center and recoordination to the terpy fragments of two different molecules of 12, is not detected.
This observation is important in relation with the general mechanism of the changeover step converting a 4-coordinate Cu II species [Cu II 4 ] into the corresponding stable 5-coordinate complex Fig. Conditions: MeCN 0. Sauvage [Cu II 5 ]. It tends to indicate that the conversion does not involve full demetallation of Cu II 4 followed by recomplexation but is rather an intramolecular reaction, probably consisting of several elemental dissociationassociation steps involving the phen and terpy fragments of the string as well as solvent molecules and, possibly, counterions.
The gliding motions, either for the divalent or the monovalent complex, are slow on the time scale of the voltammetry measurements lines 2 and 4. The driving force of this motion is again based on different geometrical preferences for Cu I and Cu II. But in this case, the wheel of the rotaxane is a bis-coordinating macrocycle and the axle incorporates only one bidentate moiety.
The advantage of this method is to limit dethreading of the macrocycle during the stoppering reaction. The membered macrocycle Mt33 [18, 19] contains two different coordinating sites: a terpyridine moiety and a 1, 10phenanthroline one. The molecular thread contains the phenanthroline bidentate unit dpp. The end-groups of the thread, i. These last species were selected as blocking groups since they are large enough to prevent dethreading of the ring. Molecular Machines and Motors 67 Fig. Principle of the electrochemically induced molecular motions in a copper complex rotaxane.
The stable 4-coordinate monovalent complex [top left, the black circle represents Cu I ] is oxidized to an intermediate tetrahedral divalent species [top right, the white circle represents Cu II ]. This compound undergoes a complete reorganization process to afford the stable 5-coordinate Cu II complex [bottom right]. Upon reduction, the 5-coordinate monovalent state is formed as a transient [bottom left]. As will be discussed later, this structural analogy does not at all correspond to a dynamic analogy.
The electrochemical behavior of tetracoordinated Cu I complexes, i. Once again, this relatively high potential underlines the stability of the 4-coordinate Cu I complexes versus the corresponding Cu II ones. The redox potential of pentacoordinated copper complexes [18, 19, 44] is observed in a much more cathodic range. This potential 68 L. A reversible signal appears at 0. The weak signal at 0. Only one anodic peak at 0. The weak peak at 0.
The main cathodic peak at 0. The irreversible character of the wave at 0. Moreover, it was observed that the rearrangement rates from the less to the most stable geometries are drastically different for the two oxidation states of the metal. Indeed, the cyclic voltammetry response located at 0. Following the method of Nicholson and Shain , the rate constant value, k, of the chemical reaction, i. On the other hand, Fig. Current versus time was recorded and an exponential decrease of the intensity of the current was observed.
When the current was near to 0, a new cyclic voltammetry curve of the solution was measured, leading to a voltammogram similar to the one represented, Fig. In the analogous systems already studied [18, 19, 44], an important difference between the related rate constants had also been observed.
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Nevertheless, for the systems based on a copper catenate [18, 19] vide infra , where one of the macrocycles is monochelating and the other one hetero-bischelating, the two processes are much slower. It is thus clear that pirouetting of a macrocycle around its axle induced by changing the redox state of the central metal is a fast process as compared to the other related reactions. These different types of molecular motion: gliding, translating, and pirouetting are possible thanks to the kinetic lability of copper complexes.
Both rearrangements require a decoordination step of one of the chelates, followed by recomplexation by the other chelate. The activation barrier of this decoordination step is higher for the tetracoordinated Cu II to pentacoordinated Cu II process one than for the pentacoordinated Cu I to tetracoordinated Cu I process, due to the higher electronic requirements of Cu II.
Thus, the difference of molecular motion 72 L. Its principle is explained in Fig. The actual system and the full square-scheme are indicated in Fig. Electrochemically triggered rearrangement of a catenate containing two different rings. The principle is the same as the one described for the copper complex rotaxane Fig. Its redox potential is 0. SCE in acetonitrile, indicating a good stabilization of the copper II state. The same conversion process can also be monitored by EPR . If coordinating ions or solvent molecules are present in the medium, they could interact with the metal in this coordinatively unsaturated complex, in a transitory fashion, and thus lower the activation barrier of the rearrangement by stabilizing intermediate states.
Sauvage 3. Multistage systems seem to be uncommon, although they are particularly challenging and promising in relation to photo- and electrochemical devices aimed at important electronic functions and information storage. In particular, if molecules or molecular assemblies are to be utilized one day as information storage devices, it is obvious that the use of three-state systems will produce a great increase in information density as compared to bistable systems. The principle of the three-situation electromediated catenate is represented in Fig.
Although we will not discuss them in the present chapter, they afford very conclusive data and, in particular, they demonstrate unambiguously that the compound undergoes the rearrangement reactions schematically represented in Fig. The sequence of electron-transfer steps and ring-gliding motions corresponding to the cyclic process of Fig.
As expected, the cyclic voltammogram CV of this species is the same as for the starting complex, which is in accordance with the 4-coordinate situation for both oxidized and reduced forms. The ringgliding step will subsequently lead to a hexa-coordinated complex, the 5-coordinated compound being characterized as a transient species by electrochemistry only. Molecular Machines and Motors 75 Fig. Each ring of the catenate incorporates two different coordinating units. The pentacoordinated complexes were characterized as transient species only since the present system does not allow us to stop motions at this stage.
The 4-, 5-, and 6- coordinate copper complexes involved. The use of transition metals as templates and of their complexes as electroactive and mobile components turned out to be particularly useful in the construction of electromechanical molecular machines and machines based on coordination compounds. Nanoscopic motors and machines might lead to practical applications in the future as molecular information storage devices, or as nanoscale components in electronics, making the search for such molecules or molecular assemblies particularly important.
The concepts discussed in the present paper can certainly be generalized to organized assemblies of molecules liquid crystals or to molecular compo- Molecular Machines and Motors 77 nents attached to an electrode surface. Since a simple signal can change the shape and the volume of a compound and of its assemblies, fascinating features related to mechanics contraction or stretching could be imagined, reminiscent of biological systems such as muscles. Sauvage Urry DW Angew. Chem Int Ed Engl Pallavicini external stimulus in an aqueous medium: the variation of pH or the change of the redox potential.
In such circumstances, altering the binding tendencies of the two sites by the external input inverts the interaction priority, thus inducing the translocation of the mobile part to the site that has become energetically more favorable. The lability of the interaction ensures the occurrence of the reverse translocation process, following an input of the opposite nature e. In the present chapter, we will focus our attention on motions occurring in systems containing one or more transition metal centers: in particular, the controlled movement will result from the alteration of the labile metal-ligand bonds.
Examples will include the swinging of a pendant arm, which possesses a donor atom and is covalently linked to the ligand backbone of a macrocyclic complex; following a pH change or the variation of the redox potential, the pendant arm can be relocated either on the metal center or far away from it. Special attention will be devoted to systems able to sharply signal to the outside the occurrence of the movement through the drastic change of a given molecular property. An especially suitable property for signaling purposes is luminescence, which can be visually perceived and instrumentally detected with extreme sensitivity.
In this situation, Ni II is especially resistant to the demetallation due to the so-called kinetic macrocyclic effect . The Ni II AN bond itself with both amine and pyridine donor atoms is intrinsically labile: thus, inertness must derive from the steric constraints imposed by the cyclic ligand. In particular, it has been observed that demetallation in tetraaza macrocyclic complexes would require the simultaneous breaking of two metal-nitrogen bonds, an event which is mechanically impeded and poorly favored from a probability point of view.
Ni II likes six-coordination, according to an octahedral stereochemistry: thus, the dimethylamine group of the pendant arm goes to occupy one of the axial positions of the octahedron, the other axial position being occupied, in an aqueous solution, by a water molecule structure I in Fig. The interaction of Ni II with the axially bound amine nitrogen atom does not suffer from any steric constraint and is therefore labile.
Indeed, bringing pH from 7 to 5 causes the color of the solution to change from pale blue to yellow, while a relatively intense 82 V. Pallavicini Fig. The pH-controlled coordination of an amine pendant arm to a Ni II center encircled by a tetraaza macrocycle. When the pendant arm is apically bound, the Ni II complex has an elongated octahedral stereochemistry, the sixth coordination site being occupied by a water molecule I, high-spin state, blue-violet color.
On protonation, the pendant arm is detached, and a square-planar complex forms II, low-spin state, yellow color absorption band develops at nm. The latter band corresponds to a Ni II complex of square planar stereochemistry, in which the four nitrogen atoms of the macrocyclic ring remain coordinated to the metal and the ammonium group of the pendant arm stays far away from the coordination sphere structure II in Fig.
The change is quickly reversible and addition of standard base, enough to bring the pH back to 7, relocates the pendant arm on one of the axial positions of the octahedron, as indicated by the color change from yellow to pale blue. Notice that the pKA value associated to the dissociation of the ammonium group in the Ni II complex of 1 is 6. The substantial difference of the pKA values which corresponds to a free energy change of 4. Varying the nature of the donating group of the pendant arm is expected to alter the intensity of the Ni II AN interaction and, consequently, to change the pKA value.
For instance, in the functionalized macrocycle 2 the donor atom of the ethylamine pendant arm is a primary, rather than a tertiary, amine nitrogen atom, which gives rise to a stronger axial interaction with the metal. In this connection, it has to be noted that coordination by the cyclam-like tetramine ring of ligand 2 favors the access to the otherwise uncommon Ni III state.
On carrying out an exhaustive electrolysis experiment on a solution of the Ni II complex of 2, which is also 10 3 M in HClO4 yellow, pendant arm protonated by setting the potential of the platinum gauze acting as a working electrode at 0. There is evidence from the ESR spectrum splitting of the g feature into three lines of equal intensity that the pendant arm is axially bound to the metal. Thus, the Ni II -to-Ni III oxidation induces the deprotonation of the ammonium group of the pendant arm and its simultaneous relocation on the metal center; at the same time, a water molecule goes to occupy the other axial position to complete the octahedral coordination see the b to c equilibrium in Fig.
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In particular, in a 0. Pallavicini occupied by water molecules structure d in Fig. The multi-step equilibrium in Fig. Nickel scorpionate complexes are convenient systems for carrying out controlled molecular motions driven by either a pH change or a variation of the redox potential. We were recently interested in associating the pH-controlled motion of the pendant arm in nickel II scorpionate complexes to the generation of a luminescent signal.
In this perspective, an anthracene fragment An was covalently linked to the nitrogen donor atom of the ethylamine side-chain of scorpionand 2, through a -CH2-spacer, to give 3, which will be indicated in the following as L . It is linker, Ni II -C9, as calculated through molecular modeling, is 7. In this connection, it has to be noted Fig. The molecular movement of the pendant arm from I to II is signaled by a partial quenching of the light-emission [due to an energy transfer process involving the Ni II center]. Complete quenching step from II to III is associated to the deprotonation of the water molecule axially bound to the metal [which promotes the occurrence of an electron transfer process from Ni II to the excited anthracene fragment] 86 V.
Deprotonation of the metal-bound water molecule structure III in Fig. There is evidence for an electron transfer nature of this process. This effect is not related to any drastic nuclear rearrangement, but results from the moderate electron reorganization on the metal center following OH binding. The Ni II -3 system is different in that it controls three levels of illumination: high-low-off, as switches operating car headlights do in the macroscopic world.
Other metal ions, e.
Molecular Machines: Principles and Mechanisms
When the side-chain is protonated, the Cu II complex exhibits a square stereochemistry. Following pendant arm deprotonation and metal binding, the complex adopts a square-pyramidal coordination geometry. The stereochemical rearrangement induces a redviolet to blue-violet color change, which corresponds to the shift of the Cu II d-d broad absorption band towards the higher wavelengths.
Color signaling of pH-induced pendant arm relocation is less eye-catching for Cu II than for Ni II , where axial binding induces a drastic change of the electronic structure of the d8 cation from low-spin to high-spin , a unique feature, which cannot be experienced by a d9 center like Cu II. Figure 5 displays the distribution diagram of the species present at the equilibrium in the case of the Cu II scorpionate complex. Also, the IF vs. However, the stereochemical change promoting the latter quenching process cannot be the same as hypothesized Fig.
It is therefore suggested that the OH anion replaces in the apical site of the square pyramid the amine group of the pendant arm, which is removed from the coordination sphere. The controlled motion of an arm covalently linked to a ring, a further scorpionand-like system, has been designed also with purely organic systems, in the absence of metal ions.
System 4 undergoes two twoelectron reduction steps, at 0. Molecular Movements and Translocations 89 Fig. The process takes place in an MeCN solution in the course of a controlled potential electrolysis experiment Thus, the p-p interaction vanishes and the naphthalene containing pendant arm dethreads.
In these circumstances, one would expect that, on dethreading, any quenching mechanism ceases and the intrinsic emission of the 1, 5-dioxynaphthalene fragment is restored. However, when carrying out an exhaustive electrolysis experiment with the potential of the working electrode set at 0. SCE i. However, occurrence of the redox 90 V. Pallavicini induced dethreading process in system 4 is demonstrated by the disappearance of the CT absorption band at nm and by the development of the band of the reduced bipyridinium units.
On the other hand, following two-electron oxidation, the tetracationic cyclophane is restored and the side-chain threads again. Oxidation can be carried out either electrochemically or chemically for instance, by bubbling dioxygen into the MeCN solution. A unique example of light-driven motion of a side-chain covalently linked to a ring was described by Shinkai in the s and is illustrated in Fig.
In system 5, the side-chain ends with an ammonium group, which can interact electrostatically with the ethereal oxygen atoms of an crown-6 subunit. The pendant arm contains also an azobenzene fragment, whose lightinduced isomerization controls the electrostatic interaction with the crown.
In the cis isomer, the ammonium group of the pendant arm can interact electrostatically with the polyether ring. It has to be noted that when the pendant arm ends with an amine group rather than with an ammonium group , which excludes any Fig. A light-driven movement of an pendant arm which bears an ammonium group and is covalently linked to a crown ether.
UV irradiation induces the trans-to-cis rearrangement of the azobenzene fragment. The self-complexing process is favored by the establishing of hydrogen bonding interactions between the ammonium group and the oxygen atoms of the crown. However, there exists another way to produce mechanical work at the molecular level: i. In the following, we will consider examples of translocation of metal ions driven by either a pH or redox potential change and of inorganic anions induced by a variation of the redox potential.
An important consequence of deprotonation is that binding tendencies towards M are substantially enhanced.
The model for the pH driven translocation of a metal ion between the two compartments of a heteroditopic ligand. In these circumstances, when compartment A exists in its protonated form, AHn, the metal prefers to reside in B. Then, on subsequent addition of standard acid and protonation of An , the metal leaves the no longer appealing compartment AHn and moves back to B. The reversible pHcontrolled translocation of M between compartments A and B in a two-box ditopic receptor is pictorially illustrated in Fig. In view of the very poor coordinating tendencies of the amide Molecular Movements and Translocations 93 nitrogen atoms, the N4 donor set of the AH2 compartment is expected to form poorly stable complexes with any kind of metal.
Thus, the doubly deprotonated A2 donor moiety is expected to form particularly stable metal complexes. The donor set of compartment B consists of two amine groups and two pyridine nitrogen atoms. Coordinating tendencies of B are expected to be much higher than those of AH2, but distinctly lower than those of A2. In particular, the acidity controlled motion of the Ni II ion between compartments A and B of the ditopic ligand 6 has been recently described.
Pallavicini determined through non-linear least-squares treatment of pH-metric titration data. This argument is mainly based on the spectral features of the two complex species. These band are typically observed with a high-spin Ni II ion in an octahedral stereochemical environment the two unpaired electrons occupy the x2 y2 and z2 d orbitals. This coordinative environment can be provided by the two amine nitrogen atoms, the two quinoline nitrogen atoms and the oxygen atoms of two water molecules. Such a band is typically observed with Ni II complexes of square stereochemistry, which form with quadridentate ligands exerting strong in-plane interactions.
This is the case of the tetraaza donor set consisting of two deprotonated amide groups and two amine groups. Thus, a pH variation from 7. Moreover, the occurrence of the translocation can be distinctly perceived both visually through a sharp color change, from pale blue to yellow and instrumentally through the development of the absorption band of the [Ni II L ] complex, centered at nm. The family of spectra recorded in the course of the experiment is shown in Fig. The development of the band at nm indicates that the Ni II ion has moved to the A2 compartment.
In this connection, one must consider that the preliminary step of each translocation process, either direct or inverse, should involve the dissociation of the coordinative bonds. Interest in associating molecular motion to the generation of a luminescent signal prompted us to equip the previously described ditopic system with a light-emitting subunit. In particular, an anthracene fragment was linked through a -CH2- spacer to the middle carbon atoms of the aliphatic chain joining the amide groups of compartment A, to give system 7 .
In 96 V. Thus, also with system 7, the Ni II ion can be moved back and forth between the two compartments by a moderate pH change from 7 to 9, and vice versa , i. However, the rates of both direct and reverse processes are remarkably slower for 7 than for 6. This is probably due to the presence of the bulky anthracene substituent, which slows down the conformational rearrangement to which the ligating backbone is Fig.
A pictorial view of the Ni II translocation within the heteroditopic ligand 7. It is suggested that the metal translocation process of the type illustrated in Fig. In this situation, it is expected that a bulky substituent e. Translocation should be associated with the occasional folding of the ditopic receptor 6 or 7 around the ideal line passing through the two amine nitrogen atoms, which acts as a hinge.
This situation is pictorially illustrated in Fig. Thus, the transition state for the translocation process corresponds to a situation of maximum folding, in which the two halves of the receptor the two pages of the book are brought one to face the other at the closest possible distance, an event which precedes the metal transfer.
The bulky anthracene substituent raises the energy of such a transition state, thus reducing the rate of both direct and reverse translocation processes. Intramolecular coordination rearrangement related to the acidic behavior of the amide group has been observed also in the platinum II complex of macrocycles 8 and 9, which contain two amide groups in one side, and two and three thioether sulfur atoms, respectively, in the opposite side .
The systems proved to be particularly efficient as trigger elements to control the organisation of liquid crystalline LC phases in a fully reversible manner; an important findings in the context of novel display materials in cooperation with Philips research. The first stereoselective switching process based on circularly polarised light CPL was demonstrated and a chiral amplification mechanism based on molecular motion, from CPL via molecular chirality to supramolecular chiral assemblies, established in Science.
Figure 1: Large array electronic device based on self-assembled monolayers of photoswitchable molecular wires. The diarylethene photo-switches have been developed by the Feringa and Browne groups to allow for electronic communication and dynamics to be controlled in a reversible manner. This class of molecule has also been applied to single molecule photoswitching of molecular conductance using the break-junction technique, switchable molecular wires , large array electronic devices Adv. Furthermore, using chiral diarylethene switches in conjunction with hydrogen bonding functionalities, light-induced reversible transcription of supramolecular chirality into molecular chirality in self-assembled fibres could be demonstrated Science, By designing functional overcrowded alkene type molecular switches, reversible three-state switching of luminescence, control of dynamic surfaces and surface patterning, dynamic assembly and switchable magnetic properties were realised.
Molecular switches have been incorporated successfully in membrane channel proteins allowing for the realisation of a nanovalve to light and pH triggered opening of nanopores that are major components in cell membranes en route to smart drug delivery devices Science, Figure 2. Furthermore a bio-hybrid translational motor system was designed and the light induced control of the Protein translocation by the SecY lateral gate in a the membrane was recently achieved.
Figure 2: Photomechanical nanovalve device based on hybrid protein channel with optical switch embedded in membrane of vesicle allowing controlled delivery. A recent example of the level of complexity that can be achieved with molecular switching functions by the research team is demonstrated in our recent report of the design and realization of a dynamic, complex self-organized system that is not only responsive to external signals but also multicomponent in nature Figure 3. These self-assembled, vesicle capped nanotubes are selective disassembled by light Nature Nanotech.
The walls of the nanotubes are 3-nm- thick bilayers and are made from amphiphilic molecules with two hydrophobic legs that interdigitate when the molecules self-assemble into bilayers. In the presence of phospholipids, a phase separation between the phospholipids and the amphiphilic molecules creates uniform nanotubes, which are end-capped by vesicles that can be chemically altered or removed and reattached without affecting the nanotubes. The presence of an intrinsic photoswitchable and fluorescent core in the amphiphilic molecules allows fast and highly controlled disassembly of the nanotubes upon irradiation, and distinct disassembly processes can be observed in real time using fluorescence microscopy.
Figure 3: Self- assembled vesicle capped nanotubes comprising two distinct amphiphiles that can be selectively de-capped or disassembled by light. Due to the presence of intrinsic optical switch and fluorescent core the disassembly process can be controlled by an external signal and observed in real-time. Our program on Molecular Motors has focused on control of rotary molecular and more recently also on translational motion culminating recently in the coupling of rotary and translational motion at the nanoscale.
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