university of groningen basic molecular devices ter wiel

University of Groningen

Basic molecular devicester Wiel, Matthijs

IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.

Document VersionPublisher's PDF, also known as Version of record

Publication date:2004

Link to publication in University of Groningen/UMCG research database

Citation for published version (APA):ter Wiel, M. (2004). Basic molecular devices. s.n.

CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).

The publication may also be distributed here under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license.More information can be found on the University of Groningen website: https://www.rug.nl/library/open-access/self-archiving-pure/taverne-amendment.

Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.

Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.

Download date: 16-04-2022

https://research.rug.nl/en/publications/2ae88514-7344-4bc0-8306-8114e7a22c78

Hoofdstuk 1 - voor pdf.doc

Chapter 1 Nanotechnology and Molecular Devices

In the first chapter of this thesis an overview is given of the efforts to control the movement at the molecular level. Inspired by the machinery of nature, in the past years numerous artificial molecular devices, motors and machines have been reported in the literature. Despite many claims, there are only a small number of papers which really deal with controlled molecular motion. The design, synthesis and application of sterically overcrowded alkenes to achieve the creation of a working molecular motor will be discussed in more detail. Finally, at the end of the chapter an outline of this thesis will be given.

2


Basic Molecular Devices

1.1 Introduction In modern daily life, everybody makes use of innumerable devices and machines. Especially the development of microelectronics has greatly influenced the way we live today. This has mainly been made possible by application of silicon based chip technology. However, since the construction of silicon based electronical circuits is reaching its limits, scientist are seeking new paradigms for advanced devices. This research area, most commonly referred to as nanoscience, has evolved in the last decade to a whole new multidisciplinary branch of science.1,2

In physics, the emphasis has been on miniaturization of existing devices an approach referred to as “top-down”. Chemistry deals with matter at the molecular level. Here, devices are to be constructed from molecular building blocks or molecular assemblies. The approach of using these small building blocks to construct larger functional devices, is commonly referred upon as “bottom-up”.

The review presented in this first chapter will focus on the development of new synthetic molecular machinery in recent years. Also some examples of striking molecular machinery present in nature will be discussed briefly. A more detailed overview will be given on the design, synthesis and application of sterically overcrowded alkenes for the construction of molecular devices, which is the main subject of this thesis.

1.2 Machinery Evolved in Nature The molecular machinery of nature, developed during the millions of years of the evolutionary process, are among the most fascinating structures on this planet. Although these extremely complicated molecules are unlikely to be reproduced in the laboratory in the near future, they serve as valuable sources of inspiration for artificial systems. An excellent introduction into biomolecular motors can be found in a book edited by Schliwa.3 The intention of this review is not to cover the area of these biological motors, but a few pertinent examples will be discussed briefly. A particularly relevant article is that of Hess et al., 4 in which the possibilities of using biomolecular motors to power nanodevices are discussed.

1.2.1 F0F1-ATP Synthase One of the most fascinating structures is that of the F0F1-ATPase molecular motor. For their combined efforts in cell functioning, J. C. Skou, P. Boyer5 and J. Walker6 shared the Nobel prize in chemistry in 1997. The contribution of the latter two was chiefly in the area of the functioning of this ATPase protein assembly. A readable introduction into the subject is given in the Nobel lectures published in Angewandte Chemie.7

3


Nanotechnology and Molecular Devices

Figure 1.1 The F0F1-ATP Synthase.

The motor protein, which is found in mitochondria, bacteria and chloroplasts, is used to convert the energy stored in a transmembrane proton gradient to chemical energy (ATP). The protein assembly consists of two domains, the F0-domain, embedded in the bilayer, and a more soluble F1-domain, sticking out of the membrane. The two domains are connected by a central axis. By allowing protons to pass the membrane, a unidirectional rotation of the F0-domain takes place. The rotary motion of the F0-domain is used for the ATP synthesis in the F1-domain. Direct evidence for the 360° rotary movement was obtained by Yoshida et al. 8 by mounting a labelled motor molecule to a surface. Amazingly, in the whole process chemical energy is converted into mechanical energy with nearly 100% efficiency.9

1.2.2 Bacteria Flagellar Motors Most bacteria that actively seek improved living conditions use for their propulsion a flagellar motor.10 The passive, rigid helical structure of the flagella is driven by a rotary motor embedded in the cell membrane. Like the ATPase molecular motor described above, these motors are driven by a flux of either protons or sodium ions across the inner membrane of the bacterium. It has been estimated that in swimming cells, the speed of rotation of these filaments can be up to thousand rotations per second. The structure of the bacterium flaggellar motor is much more complicated than that of the ATPase molecular motor and is actually one of the most complex known in cells. The total flagellar motor consists in total of hundreds of different proteins. By means of a flux of ions, a torque is generated by 8 to 16 torque generators attached to the inside of the outer membrane. By interaction with proteins at the end of the filament, this torque can be translated to rotation of the fagella. The exact mechanism is, due to the complexity of the entire stucture, not known.

4



1.2.3 Myosin, Kinesin and Dynein Molecular Motors Myosin, dynein and kinesin, which form a family, are translational molecular motors that are able to convert the energy released by ATP hydrolysis into mechanical work along a linear substrate.11 Although myosins are mainly associated with the work they perform during muscle contraction, they perform a wide variety of functions.12 Myosin consist of two heads connected through a thin filament. In muscle movement, many aligned myosins combine and move in one direction along the actine filaments present in muscles. The myosin is, however, not capable of continuous movement along an actine filament since it is only attached to the actin in the brief period of time that the motion is induced. The stepwise movement of myosin along an actin filament has been observed directly by attachment of a fluorescent label. The enzyme was shown to walk in regular steps of 5.3 nm along the actin filament.13 The main function of kinesin motors is the transport of mRNA and proteins in mitosis and meiosis and in membrane transport. Like myosin, it moves along linear molecules, in this case along microtubuli, driven by the hydrolysis of ATP to ADP. Dyneins are assemblies of several proteins that cooperate to translocate attached cargo to the end of microtubuli and are involved in a number of cellular processes including mitosis and transport.14

1.2.4 The Ribosome: A Molecular Machine Although a large number of biomolecules can be considered molecular motors and their movement is somehow controlled, they cannot be considered as molecular machines when compared to the macroscopic machines we know. One exception is the intriguing molecular machine, the ribosome, present in every cell for the synthesis of proteins. The ribosome consists of two main components, the 30S and 50S particles. Each of these subunits comprises an assembly of both RNA and proteins.15 The smaller of the two subunits (30S) is used to recognize mRNA and tRNA whereas the large subunit (50S) is used for the synthesis of the protein. The ribosome can construct from raw materials, present in the form of tRNA, and instructions, present as mRNA, any desired protein or enzym and can therefore be considered as a true molecular machine.

1.3 Artificial Molecular Machinery Inspired by the molecular machinery found in nature, much attention has been paid in recent years to the development of molecules that can fulfil a certain function. The “bottom-up” approach towards functionalized nanostructure is a field of research that has emerged only in the last few years. A number of groups have created functional systems based on molecules or molecular assemblies, but so far no new paradigm for “bottom-up” construction of molecular devices has emerged. This area of research has been intensively reviewed by Stoddart,16 Balzani,17 Kelly,18 Sauvage19, Feringa20 and others21. The review of Balzani, Credi, Raymo and Stoddart that appeared in Angewandte Chemie is of special interest, as it gives a complete coverage of the literature up to 2000.22

5



1.3.1 Sterically Controlled Movement The early examples of controlled molecular movement involved sterically hindered molecules.23 The basic idea behind these molecules is that movement in one part of the molecule is restricted by the presence of the other part of the molecule. In this way, their movements are coupled. Prominent examples are the geared propellor structures such as 1.1 in figure 1.2 studied by Mislow.24 There is, however, no control over the direction or speed of rotation in these examples.

B

1.1 Figure 1.2 Example of a molecular propellor of Mislow.24

1.3.2 Chemical Rotors Based on the earlier research of Mislow, Akkerman and Iwamura on geared systems, Bedard and Moore published molecular turnstiles (figure 1.3).25 In a framework consisting of acetylenes and benzenes, a rotor is able to rotate around a bis-acetylene axis.

R

R

O

OMe

OMe

1.4 R=

OMe1.3 R=1.2 R= H

tBu

tBu

tBu

tBu

Figure 1.3 Bedard and Moore’s molecular turnstile.

The speed of rotation around the central axis of the molecule is governed by the size of the substituents attached to the rotor designated as R. When the R is small (R= H, 1.2) the dynamic process of rotation around the acetylene bond cannot be measured. Increasing the size of the substituent (R= CH2OMe, 1.3) allows the rotary process to be followed by 1H

6



NMR at low temperatures. The bulky aromatic substituent in 1.4 does not allow rotation around the axis, even not at elevated temperatures. Although the speed of the rotation can be influenced to some extent, the direction of the rotation is random.

Comparable to the previous example, the molecular machinery of Kelly et al.28 makes use of steric hindrance to control rotation in molecules. Unlike in previous examples, additional functionalities are included into the molecules enabling them to respond to external stimuli. Based on these principles, a molecular brake26, ratchet27 and ultimately a motor28 were synthesized. Although, in the first two examples, no control over the directional was achieved, the molecular motor was able to perform a controlled unidirectional 120° rotation.29 The rotation is performed in five consecutive steps and needs two external stimuli as depicted in scheme 1.1. The first step is the conversion of the amine to an isocyanate (a). After a slight rotation around the central axis (b) the alcohol moiety reacts to form a cyclic urethane (c). This conformation is dynamically unstable and a unidirectional rotation takes place to convert the molecule to another atropoisomer (d). In the final step (e), the urethane moiety is hydrolyzed resulting in the original functionalities of the molecule and completing the 120° rotation around the central axis of the molecule.

O

HO

NH2

O

HO

NCO

O

HO

NCO

O

N

OOOO

HO

NH2 N

OO

O

a) b)

c)

d)e)

Scheme 1.1 The chemically driven molecular motor of Kelly et al.28 capable of a controlled unidirectional 120° rotation around its central axis.

Although the design of the motor molecule is very elegant and it is the only example of a controlled unidirectional movement driven by a chemical reaction, the system has some drawbacks. Most importantly, the motion is not repetitive. After the 120° rotation the

7



system has no possibilities to rotate further. Second, for one 120° rotation, two different stimuli are required (reaction with phosgene and hydrolysis). Finally, poisonous phosgene is used as a fuel to drive the rotation. Nevertheless, the molecule is an elegant example which demonstrates that unidirectional rotation can be performed by a proper choice of molecular geometry and chemical transformations.

In a twin rotor system based on a radialene frame, the rate of rotation around the butatriene axis can be controlled by variation of the substituents.30 The fluorene substituted 1.5 gives the slowest rotation, whereas the thioxanthene 1.6 and dihydroanthracene 1.7 substituted rotors give much faster rotation. In this system there is, however, no control over in the speed of rotation other than by exchanging the bridging atoms and no control is achieved either over the direction of rotation.

CC

CC

X

X

Ph

Ph

Ph

Ph

1.5 X= none1.6 X= C(CH3)21.7 X= S

Figure 1.4 The twin rotor based on a radialene frame.

Molecular rotors designed in order to study the behavior on a surface by external control via an STM-tip have been reported by Viala et al. 31 The molecules consist of a polyaromatic board with a central rotor which should be kept several Å above a metal surface by appropriate spacers. Manipulation of the rotor in the middle of the main board by a STM-tip could lead to induced switching effects.

1.3.3 Electrically or Electrochemically Induced Motion The idea using the rotation around an aryl-acetylene bond by Bedard and Moore25 was also explored in the group of Garcia-Garibay,32-37 who reported in a number of papers molecules designed to function as molecular compasses or gyroscopes. In these molecules a central aryl rotor is held in a framework consisting of two triptycenes or two tritylmoieties by two acetylenes. In the solid state, the framework should be held static in the crystal lattice, shielding the rotor part from interactions allowing it to orient under the influence of electric or magnetic fields analogously to its macroscopic counterpart, the compass. Functionalization of the rotor part with donor-acceptor moieties should allow the the rotor to respond to the external electrical, magnetical or optical stimuli and allow the molecules to reorient.

8



The measurement performed on a model compound 1.8, which lacks substituents on the rotor part, showed that in the solid state the activation barrier for the rotation was comparable to that of the molecular turnstile of Bedard and Moore (see above).32

NO2

H2N

1.8 1.101.9 Figure 1.5 Two examples of a molecular gyroscope or compass investigated in the group of Garcia-Garibay.

Although in the gas and liquid phase rotation occurs freely, for the model compounds having a triptycene moiety such as 1.9 for shielding, the rotation in the solid state was severely hindered due to crystal packing effects.33 In any case, the rotation is one of the fastest observed for phenylenes in the solid state. The functionalization of the central rotor moieties with either donating, accepting or both donating and accepting moieties (see 1.10), has been described together with the behavior of these molecular compasses in the solid state.34 A problem is that during crystallization of these molecule, solvent molecules are incorporated into the crystal lattice. Despite the introduction of more bulky substituents onto the framework, cocrystallization still occurred.35 More detailed investigations of the first model compound revealed that whether a solvated benzene molecule was present in the crystal lattice or not, the barrier was only raised slightly in the solid state.36 In one example, the phenylene groups experienced motion in the gigahertz regime, but accurate thermodynamic parameters could not be obtained.37 However, despite the main targets of the research, no measurement were performed to determine the dipolar motion in the solid state.

1.3.4 Photochemically Induced Motion A molecular machine driven by light was reported by the group of Gaub.38 A polypeptide containing azobenzene moieties was covalently bound between a glass surface and an STM tip. It was shown that the polymer could be lengthened and contracted by switching the azobenzene moieties between their cis and trans isomeric forms with different wavelenghts

9



of light. The polymer was able to contract against an externally applied force by the STM tip, thus delivering mechanical work.

The chiral molecular scissors 1.11 developed by the group of Aida are another molecular gadget which uses photo-induced cis-trans isomerizations.39 A pair of scissors consists of three essential parts, i.e. handles, pivot and blades. In the molecule, these three components are mimicked by respectively an azobenzene, a ferrocene and two arene moieties respectively. By reversible isomerization of the azobenzene between a cis- and trans-isomer, the conformational changes in this moiety are translated into movement of the two arene moieties. The ferrocene part acts as a pivot since the two cyclopentadiene moieties rotate freely. Although the selectivity of the switching process is not so efficient, the molecule can be switched between a cis- and trans-geometry by alternating the wavelength of irradiation between UV-Vis and visible light.

NN

Fe

N N

Fe

h ν

trans-1.11 cis-1.11

back and forth

movement of arenes

Scheme 1.2 The “molecular scissors” 1.11 of Aida.39

1.4 Rotaxanes: Molecular Shuttles A rotaxane is a molecular assembly that is composed of a macrocyclic and a dumbbell-shaped component that are not covalently linked.40 The macrocycle is trapped onto the linear part of the dumbbell by two bulky stoppers. Although no covalent bond is present, the two components cannot dissociate from each other due to steric bulk. Onto the linear part of the dumbbell two identical or two different recognition stations can be present. In the first case, the result will be that the macrocycle has no preference for either site on the thread, which leads to two degenerate situations. If two different sites are present in the thread of the dumbbell, the rotaxane can exist as a mixture of two different conformations. The population of the macrocycle on each site on the thread reflects directly the strength of the interactions between macrocycle and station on the thread. If the properties of these sites can be addressed by external stimuli (chemical, electrochemical or photochemical), it is possible to change the relative binding energies of the macrocycle to both sites and create a back and forth (translational or linear) movement between the two stations on the thread.

10



Application of external stimuli will therefore enable rotaxanes to behave as molecular shuttles. Rotaxanes have already proven to be versatile components for the contruction of molecular devices, both in solution and in the solid state, since they permit large amplitudinal motions of their components.

1.4.1 Chemical Induced Motion in Rotaxanes Pioneering work to use rotaxanes for nanotechnological applications has been performed in the groups of Stoddart and Balzani and has been subject of a number of reviews.16,17 More recent work has been performed in the group of Leigh, who use a templated synthesis to effectively prepare rotaxane 1.12 as shown in figure 1.6.41

OO

N

O

NH H

NO

NH

N

O O

NH HH

4 OO

HN

Ph

Ph

HO

1.12

Figure 1.6 The chemically controlled rotaxane of Leigh.

The system consists of a peptide macrocycle that is hydrogen bonded to the succinimide site on the thread of the dumbbell. The other site, a phenol group, is only weakly hydrogen bonding, both as an acceptor and donor, but its phenolate anion is a powerful hydrogen bond acceptor. The shuttling process proved to be highly dependent on the solvent and could only successfully be induced in polar hydrogen bond accepting solvents. The nature of the base and competing anions did not affect the movement of the macrocycle.

A particularly elegant example of a supramolecular device mimicking the action of processive enzymes along a strand of DNA, has been designed in the group of Nolte.42 The catalytic group present in these systems consists of a manganese porphyrin complex that is attached onto a glycoluril clip molecule (figure 1.7). The two units together form a cavity. Since the t-butylpyridine needed for activation of the metal complex can only coordinate on the outside of the cavity, the oxidation reactions catalyzed by the manganese-porphyrin complex can only be performed on the inside of the cavity. When the entire complex is threaded onto a polybutadiene polymer strand, the catalyst is able to epoxidize the double bonds of the polymer to yield a polyepoxide in quantitative yield using iodosylbenzene or sodium hypochlorite as the oxidants. The system therefore acts as a simple analogue of an enzyme. It remained, however, unclear whether the catalytic process moves in steps along the polymer and sequentially oxidizes the double bonds or gives random oxidization of the alkene moieties.

11



Figure 1.7 The molecular machine developed in the group of Nolte (reproduced with permission of Nature).

A switching system based on degenerate rotaxanes, in other words two identical recognition sites on the thread, has been devised by Otera et al.43 The shuttling of the macrocycle can be reversibly switched on and off by complexation of a site in between the two station with a copper ion (see structures 1.13 in figure 1.8).

O

O

O

O

OOO

O

Stopper

N N

O

O

N N

Stopper

NN

O

O

O

O

O OO

O

Stopper

NN

O

O

NN

Stopper

N N

CuI

1.13

Figure 1.8 Switching system of Otera et al.43

In the group of Sauvage copper complexes are used for the construction of molecular devices. Based on previously developed chemistry for the synthesis of a doubly threaded rotaxane, a multicomponent device was constructed that was capable of undergoing reversible contraction or stretching.44 By the analogy with real muscles, in which the myosin filament moves along the actin polymer, this complex was called a molecular muscle. The molecular muscle consists of two units, each containing a ring and a thread. On the thread two different coordination sites, a phenanthroline and a terpyridine, are situated and into the ring a single phenanthroline has been incorporated.

In the contracted situation the two copper(I) ions prefer four ligand coordination with the phenanthrolines (scheme 1.3). By removal of the copper(I) ions with KCN and subsequent

12



replacement with zinc(II) ions, the entire assembly can be contracted due to the preference of the zinc(II) ions for five ligand coordination. Hence, the contraction and stretching of this molecular muscle by as much as 18 Å could be performed by changing the metal ions in both rotaxanes. The most apparent weak point of this device is the long response time of the system and the use of external reagents. An improvement towards faster devices was made with a copper rotaxane that was less sterically hindered than its predecessors.45

Scheme 1.3 The “molecular muscle” developed in the group of Sauvage.44

1.4.2 Electrochemical Induced Motion in Rotaxanes The groups of Stoddart and Balzani have designed, synthesized and applied a large number of electrochemically controllable shuttles based on rotaxanes.16,17 Recent attention has focused on application of these shuttle systems onto surfaces and in organized assemblies. Some model compounds for these applications were reported recently in the literature.46

Similar rotaxanes have been used, for example, for the construction of a photochemically driven molecular abacus by the same groups.47 In this rotaxane, the macrocycle could effectively be switched between the two stations on the thread using both an electrochemical and a photochemical process. Other electrochemically driven rotaxanes have been reported by these groups. Two examples are a rotaxane that be moved effectively over 3.7 nm on the thread and a bistable rotaxane.48,49

The final paper discussed of the groups of Stoddart and Balzani in this section deals with the construction of a molecular elevator.50 This is a two component molecular device that consists of a trifurcated component containing in each of its three legs two different recognition sites. The platform that was designed to shuttle between these two stations is made up of loops in the form of three macrocycles fused to a central floor. The platform

13



could be moved by changing the binding affinities, either chemically or electrochemically, of the stations on the legs. The distance moved by the platform was estimated to be 0.7 nm and this movement generated approximately a force of 200pN.

Figure 1.9 The molecular elevator of Balzani and Stoddart (reproduced with permission of Science).

A rather peculiar molecular device based on a rotaxane was constructed from a cucurbituril macrocycle.51 In the oxidized form, a bisviologen compound is able to thread once through the macrocycle. Upon a two electron reduction, the bisviologen is able to thread twice through the ring. This (de)threading process was fully reversible. This same behavior can be controlled by irradiation with light in presence of a ruthenium complex allowing electron transfer.

1.4.3 Photoinduced Motion in Rotaxanes52 A pseudorotaxane based device that can be controlled by light was developed by Jeong et al.53 In this system an azobenzene moiety is incorporated into the thread. If the azobenzene has a trans-geometry, the pseudorotaxane cannot thread due to steric hindrance in the molecule. Irradiation switches the azobenzene to a cis-isomer, which bends away from the scaffold and allows assembly of the pseudorotaxane. Heating of the unstable cis-isomer of the azobenzenes results in the formation of the thermally stable trans-isomer which inhibits complexation. By addressing an azobenzene moiety, the assembly of a neighbouring pseudorotaxane is either prohibited or allowed due to steric reasons.

A pseudo-rotaxane devised by Balzani et al. 54 also uses the principle of cis-trans isomerization in azobenzenes. It was demonstrated that the complexation of an azobenzene derivative in an host could be switched by photoisomerization between the cis- and trans-isomers of the azobenzene moiety. Irradiation with 365 nm light led to the formation of nearly exclusively the cis-isomer and unthreading of the complex. The complex rethreads reversibly on irradiation with 436 nm light, which predominantly gives the trans-isomer.

14



The groups of Leigh and Brouwer have reported a hydrogen-bond assembled rotaxane 1.14 in which the translation of the macrocycle between the two stations on the thread of the dumbbell takes place after photo-exitation with a nanosecond laser pulse (figure 1.10).55

OO

N

O

NH H

NO

NH

N

O O

NH HH

N

O

O

tBu

tBu

1.14

Figure 1.10 The ultrafast molecular shuttle of Leigh and Brouwer.

Initially, the macrocycle is locked with four hydrogen bonds to a succinamide station on the thread. The other station, a naphthalimide, is only a poor hydrogen bond donor in its neutral state. However, upon photo- or electrochemical generation of the radical of this naphthalimide, the station will become a strong hydrogen bond acceptor and will provide a driving force for the macrocycle to shuttle along the thread. After excitation by a laser pulse, movement of the macrocycle from the succinimide station to the naphthalimide station takes about 1 µs. Charge recombination in approximately 100 µs will lead to shuttling of the macrocycle to its original position. The entire process occurs very fast at a rate of 1·104 s-1 and the movement of the macrocycle along the thread amounts approximately to 15Å. A quantum chemical account for the movement of this system was given by reducing the shuttling process to a single dimension.56 For a related rotaxane, lacking the naphthalimide station, a 3 Å movement of the macrocycle could be induced on a short (nanosecond) time scale by irradiation. 57 Originally, the system of Leigh and Brouwer discussed below, functioning by irradiation with laser pulses, was designed to function upon electrochemical stimuli. 58 Electrochemically the system is also effectively switched between the two stations of the rotaxane. This takes place in approximately 50 µs, 50 times slower as the shuttling process induced by irradiation.

That a rotaxane can be used as a chiroptical switch addressed by irradiation was also shown in the group of Leigh. Normally, in this rotaxane the macrocycle resides on a strongly bonding trans-fumaride station on the thread. Upon irradiation of this station, a cis-trans isomerization is induced and the macrocycle will shuttle to the glycine-leucine station located on the thread. As was anticipated, the macrocycle experiences the chiral environment of the leucine as was evidenced by the strong induced CD signal in the aromatic region. The photo-isomerizations are highly reproducible and the macrocycle is effectively shuttled using different wavelengths of light.59 A variation on this theme is a photochemically and thermally addressable rotaxane in which discrimination between the two stations on the thread occurs by the number of hydrogen bond that each site is able to

15



form with the macrocycle.60 The same rotaxane system could be used, albeit with low selectivity, as a three state system directed by different wavelengths of light and thermal energy.61 It was shown that the rotation of the macrocycle around the fumaramide site on the thread is significantly influenced by the conformation of this station. 62 When the fumaramide site has a trans-geometry, four hydrogen bonds can be formed, resulting in a slow rotation relative to the cis-geometry in which only two hydrogen bonds can be formed. The rotaxanes have been attached to a surface, but apart from the formation of ordered pattern, no internal movement of the rotaxane was described.63

A system involving a relatively short rotaxane was reported by Anderson et al.64 The movement of the macrocycle, a cyclodextrin, on the rotaxane thread containing a stilbene unit could effectively be controlled by irradiation. If the stilbene has a trans-geometry configuration, the cyclodextrin is free to move on the thread. Irradiation resulted in a cis-trans isomerization that locked the cyclodextrin selectively on one end of the thread.

A more advanced type of a cyclodextrin based rotaxane was reported by Tian et al.65 The motion of the cyclodextrin can be locked by complexation with an isophthalic acid unit. The disruption of the hydrogen bonds enables shuttling of the cyclodextrin on the rotaxane thread, a process which can be controlled by light. Read-out of the location of the cyclodextrin on the thread could be performed by fluorescence spectroscopy.

1.5 Catenanes: Movement of Interlocked Rings Catenanes are molecules that consist of two interlocked macrocycles. These are, just as is the case for the rotaxanes, not covalently linked but held together merely mechanically which prevents their dissociation.40 As in the rotaxanes, various recognition sites can be incorporated into the two rings. These sites can either be different or identical. When one of the two rings bears different sites, control over the internal movement can be achieved by application of external stimuli (chemical, electrochemical or photochemical). Like rotaxanes, catenanes have been incorporated successfully in a number of molecular devices, both in solution and in the solid state. Since most catenane chemistry has been reviewed intensively (see above) only new examples will be discussed.

A large number of interlocked devices based on catenanes have been reported by Stoddart and Balzani and their groups. A fairly recent example is that of a two catenanes containing in one of the rings no less than four recognition sites.66 On this ring one or two macrocycles were attached and it was demonstrated that these rings could effectively be moved using electrochemical oxidation and reduction.

Two molecular motors based on catenanes were recently described by Leigh et al.67 In the first of these motors, the macrocycle of a [2]catenane could effectively be switched between three stations on the catenane. Although it is the first example of selective shuttling between three stations on a [2]catenane, no control over the direction could be obtained. In a [3]catenane 1.15 in scheme 1.4, which consists of two small macrocycles and one 63-membered macrocycle, unidirectional rotation was possible by virtue of

16



simultaneous movement of the two smaller macrocycles along the sites on the thread of the large macrocycle (scheme 1.2).

Scheme 1.4 Rotory behavior of the catenane 1.15 of Leigh (reproduced with permission of Nature).

The four sites have different hydrogen bonding affinities for the smaller macrocycles and are a trans fumaramide (A), a tertiary amide trans fumaramide (B), a succinic amide ester (C) and an isolated amide group (D). Upon selective photochemical isomerization, the capabilities of the fumaramide and the tertiary fumaramide to form hydrogen bonds are altered due to cis-trans isomerization to give the respective cis fumaramide (A’) and cis tertiary fumaramide (B’). The affinities of these groups towards the smaller macrocycle are then decreased and the binding affinity of all stations is then A>B>C>D>A’>B’. In the thermodynamically most favorable situation the two rings will reside on stations A and B. Selective excitation of the A station (step 1 in scheme 1.3) gives the weak hydrogen bonding A’, and one ring will move from station A to C. Selective excitation of station B will cause conversion to B’ and a shift of the second ring from B’ to station D (step 2). Heating the system, causing the stations A’ and B’ to revert to A and B, will lead to shuttling of the rings back to these stations (step 3). At this state, the two smaller rings have only exchanged place, but repeating the same three step sequence will result in completion of the unidirectional rotation (steps 1,2 and 3). The entire process is selective since one

17



ring, when not moving, blocks one direction of passage for the other ring thus ensuring unidirectionality for the movement. Although it was shown that the [3]catenane is able to perform a unidirectional rotation, it is not a functional molecular device. No less than six separate external stimuli are required to perform a single 360° unidirectional rotation. The systems are also slow as the thermal steps require heating to 100°C and proceed at relatively low rates.

Another example of light driven motion involves a ruthenium catenane developed by the group of Sauvage.68 In this system, the rotation of the catenane is based on the formation of a dissociative excited state. Upon irradiation, a ligand is expulsed from the metal ion. Recoordination to the metal is effected by heating at elevated temperatures. Although the photochemical decoordination and thermal recomplexation are quantitative, the system does not rotate in a single direction, requires two different stimuli and suffers from a long response time in the thermal step.

A new variation of the catenane theme was recently presented. In rotaxane 1.16 in figure 1.11, a large macrocycle contains two different metal complexes, each with a different affinity for a smaller ring depending on the oxidation state of the metal ions (Cu and Ni).69 By selective oxidation and reduction of the copper and nickel centers between 2+ and 3+, the position of the crown ether macrocycle can be controlled and reversibly moved from one metal to the other. The direction of rotation is, however, again random and not controlled.

N N

NN

HN

HN

N N

NN

NH

NH

O O

OO

O

OO

O

Ni2+Cu2+

Figure 1.11 Structure of the two metal containing catenane 1.16.

1.6 Rotary Motion in Metal Complexes Double-decker metal complexes have been used, especially by the group of Shinkai, for the construction of supramolecular scaffolds based on allosteric effects.70 A series of cerium bisporphyrinate double-deckers has been synthesized, which were able to bind up to three silver ions via a positive homotropic allosteric mechanism. 71 Surprisingly, upon complexation of silver ions in the π-clefts of the porphyrins, the rate of rotation of the porphyrin ligands is dramatically increased. However, control over the direction of the rotation was not achieved.

18



Using as somewhat different approach, Aida et al. 72 employed similar bisporphyrinate double-decker complexes as redox-responsive rotating modules. The internal rotation of these enantiomerically pure cerium and zirconium bisporphyrinate complexes was studied in relation to their oxidation state. In case of the cerium bisporphyrinate complex control over the rate of racemization of the was obtained by switching between a Ce3+ and a Ce4+ metal center. For the the zirconium complex control was achieved by oxidation of the porphyrin ligands. Again, the direction of the rotation cannot be controlled.

Another approach using a half metallocene ruthenium complex has been followed by Carella et al.73 Two ligands rotate freely around a metal center (Ru2+) and the direction of rotation is random. Even ferrocene could be considered a molecular device employing these criteria. It was stated by the authors that by mounting the lower ligand to a surface, the complex can be used as a nanomechanical device.

The concept of sandwiching metal ions has been extended by the group of Shinoya.74 Three silver ions were coordinated between the two disk-type ligands 1.16 and 1.17 (scheme 1.5). However, no control was achieved over the rate of rotation of these complexes. Although no formal goals for a nanotechnological device were proposed by the authors, the complex might be able to function as a molecular ball-bearing.

NS

N

SN

S

NS

N

SN

S

N

SN

S

N S

3 Ag(O3SCH3)

1.16

1.17

Scheme 1.5 The molecular ball-bearings of Shinoya.74

A sophisticated example, which features a charge transfer induced molecular movement, is the controlled rotation of a functionalized metallacarborane shown in scheme 1.6.75 The nickel complex has, depending on the oxidation state of the nickel, either a cisoid (Ni4+) or a transoid (Ni3+) sandwich structure. Unlike previously reported systems, the rotation of the

19



ligands relative to each other cannot only be controlled by subsequent oxidation and reduction, but also by a photochemical stimulus. Irradiation of the cisoid (Ni4+) complex leads to the formation of an excited complex, which preferentially has its ligands oriented in a transoid (Ni4+*) way. Although this process is not unidirectional, the authors suggest that placement of additional substituents (R1-R4) onto the ligand might allow control over the direction of the rotation. The efficiency of this device was calculated to be 4.2% which was associated with an average torque of 8 x 103 N·m·mol-1.

Scheme 1.6 Controlled back and forth rotation in a carborane nickel complex (reproduced with permission of Science).

1.7 Molecular Devices Based on Overcrowded Alkenes Overcrowded biphenanthrylidenes were initially studied for their interesting photochromic and stereochemical properties.76,77 Due to the steric hindrance around their central double bond, these extended stilbene type molecules adopt an helical shape and are therefore chiral. Noteworthy are the intense Cotton effects in the CD spectra of both trans- and cis-isomers of 1.18, which reflect their twisted π-systems (figure 1.12).

1.18 1.18 Figure 1.12 Biphenanthrylidenes trans-1.18 and cis-1.18.

At the time, the absolute configuration of these compounds was not know. More recent investigations revealed that the absolute configuration of these molecules could quite

20



accurately be predicted by making use of the excition coupling method.78 Furthermore, detailed studies on the behavior of optically active trans- and cis-1.18 revealed that the apparently much more hindered cis-1.18 racemizes much more easily than the trans-1.18.79 An explanation for this peculiar behavior was provided by theoretical studies.80 In order to prove unequivocally the absolute configuration of these helical molecules, a center of known configuration was introduced next to the sterically hindered double bond (figure 1.13).81 Analysis revealed that in the trans- and cis-isomer of 1.19, the additional methyl substituent had little or no effect on the UV-Vis and CD spectra, but allowed the unequivocal determination of the absolute configuration. Only the trans-isomer 1.19 could be obtained from synthesis. The cis-isomer 1.19 was prepared independently by irradiation. During this irradiation, another trans-isomer of 1.19 was obtained, which was demonstrated to be thermally unstable.82

1.19 1.19 1.19 Figure 1.13 Methyl substituted overcrowded alkene 1.19.

Detailed investigations on the photochemistry of the bisphenanthrylidene revealed that this system was capable of a unidirectional rotary motion around the central double bond, as is depicted in scheme 1.7.83,29 This rotation around the central double bond is achieved by combining two energetically uphill, photochemical steps and two energetically downhill, thermal steps. Starting from (3’R,3’R)-(P,P)-trans-1.19, in which both methyl substituents adopt an energetically favored axial orientation, a trans-cis isomerization occurs upon irradiation with light forming (3’R,3’R)-(M,M)-cis-1.19. Upon formation of (3’R,3’R)-(M,M)-cis-1.19, not only the helicity of the molecule changes from (P,P) to (M,M), but also the orientation of the methyl substituents changes from the favorable axial orientation to an unfavorable diequatorial orientation. In order to release the steric strain associated with this diequatorial orientation, (3’R,3’R)-(M,M)-cis-1.19 undergoes at room temperature a thermally induced helix inversion to form (3’R, 3’R)-(P,P)-cis-1.19, which has its methyl substituent again in the favored axial orientation. Another photochemical step then leads to the unstable (3’R,3’R)-(M,M)-trans-1.19 with the methyl substituents in an equatorial conformation. A full unidirectional 360° rotation around the central double bond is then completed by a thermal helix inversion from (3’R,3’R)-(M,M)-trans-1.19 to the initial stage of the rotary cycle, (3’R,3’R)-(P,P)-trans-1.19. The direction of the rotation is solely controlled by the absolute configuration at the stereogenic centers. Continuous irradiation and heating of the molecule will result in a repetitive rotation around the central double bond. This motor molecule was the first example of an artificial system capable of a controlled, unidirectional, 360° rotation. Since it is capable of a repetitive motion and

21



conversion of one source of energy to another, the molecule can be consided as a primitive sort of molecular motor. It must be noted, however, that although the molecule is reasonably simple, this motor is by far the most efficient system reported so far. Moreover, the molecule not only functions as a molecular motor on the molecular level, but is also able to exert macroscopically observable effects. By doping the motor in a liquid crystalline host, the molecule could be used to control the size of the pitch of a cholesteric phase.84 The magnitude of the helical twisting power of the stable trans-1.19 relative to the stable cis-1.19 and unstable trans-1.19 allowed a direct color read-out of the system ranging througout the entire visible spectrum and direct visualization of the state of the rotary motor system.

stable (3R,3'R)-(P,P)-cis-1.19diaxial methyl groups

unstable (3R,3'R)-(M,M)-trans-1.19diequatorial methyl groups

stable (3R,3'R)-(P,P)-trans-1.19diaxial methyl groups

unstable (3R,3'R)-(M,M)-cis-1.19diequatorial methyl groups

20°C60°C step 2step 4

>280 nm

>280 nm

step 1

step 3

Scheme 1.7 The first molecular motor capable of controlled 360° rotation.

1.7.1 Overcrowded Alkenes as Chiroptical Molecular Switches Chiroptical molecular switches based on sterically overcrowded alkenes have been the subject of reviews published in 199385 and 200086 and have been intensively investigated in our group. 87 Initially, these helical molecules were designed to function as molecular

22



switches to perform a preferably unlimited number of back and forth turns of nearly 180° with the central double bond as axis. These molecules consist of a tetrahydrophenanthrene or 2,3-dihydronaphtho(thio)pyran upper part connected by a double bond to a tricyclic lower part (e.g. xanthene, thioxanthene or anthracene). Due to the steric crowding, the molecules adopt helical conformations to avoid unfavorable interactions. Despite the absence of a stereogenic center, these molecules are chiral due to their helical shape and adopt conformations with a positive (P) or negative (M) helix. Under the influence of light, a cis-trans-isomerization occurs and the upper half of the molecule turns 180° with respect to the lower half of the molecule. Using light of different wavelengths, this photochemical process can be directed to give either excess of the (P)-pseudoenantiomer or the (M)-pseudoenantiomer (see scheme 1.8) for the donor-acceptor molecular switch 1.20.88 The photochemical switching mechanism was investigated by some theoretical methods to account for the observed selectivity.89

S

S

Me2N NO2

(M)-cis-1.20

S

S

NO2Me2N

(P)-trans-1.20

365 nm

435 nm

30 : 7090 : 10 Scheme 1.8 Stereoselective photoisomerization of the donor-acceptor substituted molecular switch 1.20.

An advantage of these helical molecules is that they have intense Cotton effects facilitating read-out by CD spectroscopy. The stability of the molecule is ensured by incorporation of large sulfur atoms in the upper and lower halves of the molecule. Racemization of the molecules can occur by thermal isomerization involving helix inversion when the upper half of the molecule slips over the lower half. By increasing the steric hindrance, the barrier for this thermal interconversion is increased. In the example in scheme 1.8 both these atoms are sulfur atoms. The effect of these large sulfur atoms is that they push the upper and lower halves of the molecules towards each other and therefore increase the steric hindrance and the racemization barrier.

Doping of molecular switches in liquid crystalline hosts has been an important research area in our group.87b,c,f An approach towards more practical applications was performed by Van Delden et al. in our group. 90 Doping a chiroptical molecular switch in a liquid crystalline hosts, the pitch of the liquid crystal and hence the reflection color can be controlled. This chiral information can be subsequently stored by photopolymerization of the liquid crystalline host.

A very similar chiroptical switch has been reported by Chen and Chou (scheme 1.9).91 As for the second generation molecular motors discussed below, their system makes use of the bistability of two isomers of an overcrowded chiral alkene. By irradiation at λ= 280 nm, the

23



stable form 1.21 can be selectively switched to its unstable isomer 1.21 with a diastereomeric excess of 99.2%. Complete reversal to the stable 1.21 takes place by heating at 130°. Photochemical reversal of the process is, however, less efficient, giving at λ= 280 nm the stable and unstable isomers in a 67:33 ratio. A minor detail is that the experimentally observed results are not in agreement with the calculation performed for the stable and unstable forms of 1.21 in the article. Although the selectivity of the switching process could not be determined in a liquid crystalline host, both isomers of switch 1.21 are compatible with a liquid crystalline matrix like K-15.

hν = 280 nm

hν = 254 nmor ∆T= 130°C

stable-1.21 unstable-1.21 Scheme 1.9 The chiroptical molecular switch of Chen and Chou.

1.7.2 Second Generation Molecular Motors In the previous section, the extensive research on the overcrowded biphenanthrylidines focussed on unidirectional rotary motors and chiroptical molecular switches based upon chiral overcrowded alkenes has been outlined. Combining these two ideas, it would be interesting to see how a chiroptical switch would behave if a single stereocenter was introduced in the upper half. Therefore, a series of molecules were studied in order to examine their rotary behavior. The first molecule of these series, 1.22, containing a sulfur atom in both lower and upper half, has the shape of a chiroptical molecular switch, but contains additionally a stereogenic carbon center in the upper half and a methoxy substituent in the lower half.92 As was established by X-ray crystallography and supported by calculations on the AM1 theoretical level, the substituent has in the energetically most favorable conformation of the molecule an axial orientation. The equatorial orientation of the methyl substituent of the molecule is thermally unstable and will revert to the stable conformation upon heating at elevated temperatures. The unidirectional rotation can be probed by the methoxy substituent in the lower half of the molecule, giving rise to distinct cis- and trans- isomers. Using optically active material, the rotary cycle for the second generation molecular motors was followed by CD spectroscopy and the entire unidirectional rotation is depicted in scheme 1.10 below.

24



S

S

MeO

S

S

MeO

S

S

MeO

S

S

MeO

(2'R)-(M)-trans-1.22

(2'R)-(P)-trans-1.22

(2'R)-(P)-cis-1.22

(2'R)-(M)-cis-1.22

T = 60°C

h ν

365 nm14:86

h ν

365 nm89:11

T = 60°C

Scheme 1.10 Unidirectional rotation of the second generation molecular motor 1.22.

Upon irradiation of the optically active (2’R)-(M)-trans-1.22, with the methyl substituent in an axial orientation, with light of λ= 365 nm in n-hexane, a photochemically induced cis-trans isomerization takes place, leading to the unstable (2’R)-(P)-cis-1.22, with the methyl substituent in an equatorial orientation. Upon heating at 60°C, the unstable (2’R)-(P)-cis-1.22 performs an irreversible thermal helix inversion to give the stable (2’R)-(M)-cis-1.22. Irradiation of the stable (2’R)-(M)-cis-1.22 with light of λ= 365 nm leads to formation of the unstable (2’R)-(P)-trans-1.22, which thermally reverts to the stable (2’R)-(M)-trans-1.22. In total, the upper half of the molecule is able to perform a unidirectional rotation around the central double bond with respect to the lower half. The entire rotary process was followed by CD spectroscopy, indicating the reversal of helicity in each step, going from an overall (P)-helix to an overall (M)-helix or vice versa. Like the first-generation molecular motor, the whole process is driven by light energy, but unidirectionality is maintained by the one-way thermal helix inversions governed by the orientation of the methyl substituent in the upper half.

The disadvantage of the molecular motor 1.22 is that the barrier for the thermally induced helix inversions are relatively high and have to be performed at elevated temperatures. If the motor is ever to be used in a device, the motor needs to be able to function at room temperature or even lower since the speed of rotation around the central double bond should not be limited by the thermal step. Therefore, in order to tune the helix inversion process, the (hetero-)atoms in upper and lower halves were altered and their influence on the rotary process was studied. Based on these considerations it is anticipated that, in contrast to the chiroptical molecular switches (vide supra), the barriers of the process should be lowered as much as possible and the atomic radii of the bridging atoms X and Y in scheme 1.11 should be as small as possible.

25



X

Y

R

X

Y

R

X

Y

R

stable forms unstable formsunstable forms

h ν T = 60°C

R R R

1.23: X= S, Y= S, R= H1.24: X= S, Y= S, R= OMe1.25: X= S, Y= O, R= H1.26: X= S, Y= C(CH3)2, R= H1.27: X= CH2, Y= S, R= H1.28: X= CH2, Y= C(CH3)2, R=H1.29: X= CH2, Y= CHCH, R=H

Scheme 1.11 Determination of the barrier of thermal helix conversion of the second-generation molecular motors by varying the bridging atoms X and Y.

Systematic variation of the X and Y atoms in the second-generation molecular motors showed that there was indeed a large dependence of the size of the bridging atoms X and Y on the thermal isomerization barrier.93 Changing the substituent X=S and Y=C(CH3)2 in 1.26 to X=CH2 and Y=S in 1.27 leads to an approximate 300 fold increase in the speed of the thermal helix inversion. Although from a steric point of view Y=S is larger than Y=C(CH3)2, the barrier for the thermal helix inversion is lower. Extra substituents (OMe) in 1.24 attached to the lower half of the motor in 1.24 do not appear to influence the barrier of the thermal helix inversion to a large extent.

In order to decrease the thermal barrier further and to create a faster motor, the steric hindrance in the molecule was diminished by replacing the tetrahydrophenanthrene moiety in the upper half by a tetrahydronaphthalene.94 Although of a series of molecules with different lower halves only 1.30 molecule was able to perform the photochemical and thermal interconversion between the instable and stable forms induced by the stereogenic center, the system showed peculiar behavior. Instead of having a more labile unstable form due to the expected lowered steric hindrance, the unstable form was more stable than all previous examples and reverted only slowly to the stable form by heating (scheme 1.12)

h ν ∆

stable-1.30 unstable-1.30 stable-1.30 Scheme 1.12 Increased thermal barrier for sterically less-hindered alkene 1.30.

26



Another example of these second-generation molecular motors is the donor-acceptor substituted motor 1.31.95 The lower part of this molecule with dimethylamino donor and nitro acceptor moieties was also used for the highly selective chiroptical molecular switch mentioned above. Interestingly, the motor 1.31 in figure 1.14 can perform its rotary cycle using visible light (λ= 435 nm). The use of visible light as a driving force is prefered over using UV-Vis light since light of higher wavelengths corresponds to lower energy consumption and hence an enhanced efficiency.

S

S

Me2N NO2

(2'R)-(M)-cis-1.31 Figure 1.14 Molecular motor1.31 driven by visible light.

Concluding, the molecular motors based on sterically overcrowded alkenes are versatile molecules, which are able to perform selectively and unidirectional rotation around the central double bond. The main objectives for further developments are decrease of the thermal isomerization barrier and the functionalization of the molecule to be able to use derivatives in nanotechnological devices.

1.8 Molecular Devices on Surfaces For the realization of the construction of nanotechnological devices, it is necessary to integrate the molecules shown above into highly functional systems. Most of the molecules presented have been studied in solution. In recent years, research is steadily shifting its emphasis from systems controlled in solution to those in confined environments such as surfaces or Langmuir-Blodgett films.16c A review on earlier applications of electronically induced translation motion on surfaces was published by Shipway and Willner.96 A next logical step in the development of machinery to be used for nanotechnological devices is to organize these molecular components on surfaces or to form aggregates by self-assembly. On a surface, or other organized assemblies, the nanotechnological devices have to be operated in unison in order to use the combined potential of all molecules.

The route chosen to attach catenanes to a surface by the group of Sauvage involved the use of a disulfide moiety in one of the catenanes to link this molecule onto the surface.97 After absorption to the gold surface, a catenate is formed in which gold atoms are elements of one of the rings. In a more advanced paper, the systems were investigated by STM, ATM, IR and electrochemistry, but no motion of the catenane rings were observed.98

In the group of Michl, surface mounted rotor 1.32 (figure 1.15) has been fabricated in which the axis of rotation is parallel to a gold surface.99 Binding takes place through the

27



sulfur containing side chains attached on the cyclopentadiene rings of the rotor. This dipolar rotor should respond to external electrical stimuli. Some evidence was provided that the rotor rapidly rotates at room temperature and that this rotation could be influenced to some extent by the tip of an STM. Although no experimental proof was provided, the rotation of the device in an alternating electrical field was predicted to be unidirectional by computer similations.

Ph

PhCo

Ph

CoPh

R

R

R

R

R

R

R

RR

RPh Ph

R=HgS(CH2)2SMe

FFFF

1.32

Figure 1.15 The surface bound rotor of Michl.

A surface bound system designed by Tour et al. 100 consists of caltrop-shaped molecules equipped with an electrically addressable arm 1.33 (figure 1.16). Although bonding to a gold surface was shown to occur by the tripod via three thiol moieties, actual switching of the system was not shown to be feasible. The idea is that the donor-acceptor moieties in the arm of the system should allow alignment and manipulation of the arm in an electric field.

Si

N NMe2O2N

S

SS

Gold Surface

1.33

Figure 1.16 The surface bound caltrop-shaped rotor 1.33 of Tour.

More successful attemps to use functional devices on a surface have been performed by the combined groups of Stoddart, Zink, Heath and Balzani. Very recently a review on these surface bound systems has appeared by Stoddart et al.16c Initially, redox-switchable catenanes were incorporated into Langmuir-Blodgett films and then transferred to a gold or

28



mica surface.101 Although these catenanes effectively can be switched in solution, nothing was reported on their switching behavior on the surfaces. The first attempts for the construction of a nanomechanical device focused on the use of a nanoporous matrix and a silica surface, which yielded both primitive functioning devices.102 The pseudorotaxanes were shown to thread and dethread efficiently onto the functionalized glass surface and in the matrix. Similarly, redox active rotaxanes were used for the construction of logic gates. For this purpose a monolayer of the rotaxane was sandwiched between metal electrodes.103 It was demonstrated that the device could effectively be switched from an initial state to a final state in an irreversible manner. An improved device using similar technology was constructed from a catenane sandwiched between an n-type silicon bottom electrode and a metallic top electrode. 104 The switch can be opened and closed by applying external electrical stimuli and was able to perform this opening/closing cycle multiple times. Some more of these devices were reported somewhat later by the same groups using slightly altered rotaxanes and a catenane. 105 Only the catenane based device exhibited stable operation over multiple cycles, whilst functioning of the rotaxane based devices was either complicated by formation of molecular domains in the Langmuir-Blodgett film or poor switching behavior.

In a more recent paper, pseudorotaxanes have been used for the construction of a supramolecular nanovalve.106 In mesoporous silica, luminescent molecules are trapped by capping pseudorotaxanes attached on the surface of the silica. The luminescent molecules are released from the silica pore in solution by disassembling the pseudorotaxane. The devices can, however, operate only one time. Once the luminescent molecules are released, they cannot be filled anymore.

1.9 Conclusions and Contents of this Thesis The number and variety of molecular systems that are of potential use in nanotechnological devices have exploded over the last decade. For “bottom-up” construction of these devices, it has become very important to control molecular movement and molecular conformation for future applications. The chiroptical molecular switches and molecular motors have been shown to be prominent members in this vastly expanding family of molecular devices. With these systems, almost full control can be achieved over the internal movement of the molecules. In the case of the chiroptical switches, this is the near 180° cis-trans isomerization over the double bond and in the case of the molecular motor this is a full 360° rotation around the central double bond. In order to improve the properties of these molecules, further investigations have been performed and these are described in this thesis. The emphasis will be on the synthesis of new molecular motors and examination of their behavior. Furthermore, initial studies are presented that might lead the way towards application in real nanotechnological devices.

The original motor molecule 1.19, as shown above, is the subject of investigation of chapter 2. By structural variation of the substituents adjacent to the central double bond, it was expected to gain more insight into the peculiar behavior of this molecule. Most importantly, the question was addressed what the influence is of the substituents on the 360° rotary

29



cycle. In order to study these molecules, CD and UV-Vis spectroscopy and X-ray crystallography have been used intensively. Since for these measurements optically pure material is required, an asymmetric synthesis route towards these molecules has been explored. Furthermore, the possibility has been investigated whether other substituents could be attached onto the naphthalene moiety of the motor molecule to see whether such a functionalized molecular motor could be used as a scaffold to attach a variety of functional groups.

In chapter 3, the basic concept of the first molecular motor capable of a unidirectional 360° has been extended. The original molecule consists of two six-membered rings connected by a double bond. As has been shown, an interplay between the steric hindrance and conformation of the molecule makes it able to perform a unidirectional rotation around the central double bond. It would be interesting to see whether this design of a molecular motor can be improved, especially since the rotation is still relatively slow at room temperature. The main focus in chapter 3 was on the synthesis of a molecular motor in which the two six-membered ring have been replaced by two five-membered rings. Although the difference between these two motors is small, the use of two five-membered rings affected the interplay between steric hindrance and rotational behavior to a large extent. The smaller five-membered ring decreased the steric crowding in the molecule and hence allowed faster rotation processes and easier synthesis. Furthermore, the scope of the design has been investigated by the introduction of a larger substituents onto the five-membered ring.

The use of a five-membered ring has been extended in chapter 4. In the previous chapter, two five-membered rings were employed to induce a unidirectional rotary movement around the central double bond. In each of these two rings, a methyl substituent was introduced onto a stereogenic carbon atom to control the direction of the rotation. From the original motor molecule with two six-membered rings, a second-generation molecular motor was developed with one six-membered ring with a single stereocenter. This strategy has been also employed for the molecular motor with five-membered rings. Hence, a molecular motor has been developed that consists of an upper part with a five-membered ring containing a single stereocenter and a lower part as has been employed in previous examples.

In the chapter 5, a structural variation of the second-generation molecular motors is discussed. In these motor molecules a single methyl substituent governs the direction of rotation around the central double bond. This methyl substituent is attached to the carbon atom next to the central double bond. The rate of rotation for these molecules is, however, still too low for application in nanotechnological devices. Therefore, the idea was to lower the steric hindrance in the “fjord-region” by moving the methyl substituent to the carbon atom adjacent to the sulfur atom, one carbon atom further away from the double bond. It was anticipated that this would result in an increased speed of rotation while preserving the unidirectionality of the rotary process.

To function as a motor in a real nanotechnological device, the motor molecules have to be attached to a surface or be incorporated into an assembly in order to be able to generate a torque and to perform work. An approach to a molecular motor that can be used as such has

30



been presented in chapter 6. Sites of attachment had to be introduced to the molecular motors. These functionalized motor molecules first had to be synthesized and then attached to gold or glass surfaces. Moreover, it has been investigated whether these molecules were still able to perform a unidirectional rotation. Since direct observation of rotation of a single molecule on a surface was hard to investigate, gold colloids were chosen to function as a model of a gold surface.

In the last chapter, chapter 7, the previously introduced concept of controlled speed of rotation of an o-xylyl-rotor by the upper part of an overcrowded alkene has been extended to the molecular motors. It has been investigated whether the methyl substituted upper part would allow control of the speed of rotation of the xylyl-moiety and whether the position of the upper part could be controlled by light and thermal isomerizations. Furthermore a more detailed investigation has been performed to determine which parameters are of influence for the original molecular rotor previously investigated in our group.

1.10 References 1) Special Issue of Scientific American: Nanotech, The Science of the Small Gets Down to Business,

September 2001. 2) Encyclopedia of Nanoscience and Nanotechnology; H. Singh Nalwa (Ed.); American Scientific

Publishers, Stevenson Ranch CA, 2004. 3) Molecular Motors; M. Schliwa (Ed.); Wiley-VCH, Weinheim, 2003. 4) H. Hess, G. D. Bachand, V. Vogel, Chem. Eur. J. 2004, 10, 2110-2116. 5) P. D. Boyer, Annu. Rev. Biochem. 1997, 66, 717-749. 6) J. A. Abrahams, A. G. W. Leslie, R. Lutter, J. E. Walker, Nature, 1994, 370, 621-628. 7a) P. D. Boyer, Angew. Chem., Int. Ed. 1998, 37, 2296-2307; b) J. E. Walker, Angew. Chem., Int. Ed.

1998, 37, 2308-2319. 8) H. Noji, R. Yasuda, M. Yoshida, K. Kinosita Jr., Naure, 1997, 386, 299-302. 9) H. Wang, G. Oster, Nature, 1998, 396, 279-282. 10) For a review see: R. Berry; Molecular Motors: Chapter 4, The Bacterial Flagellar Motor; M.

Schliwa (Ed.); Wiley-VCH, Weinheim, 2003, 112-140. 11) For a review on myosin and kinesin motors see: a) R. D. Vale, R. A. Milligan, Science, 2000, 288,

88-94. 12 ) For a review on myosins see: J. A. Spudich; Molecular Motors: Chapter 11, Quantative

Measurements of Myosin Movement In Vitro; M. Schliwa (Ed.), Wiley-VCH, Weinheim, 2003, 271-303.

13) K. Kitamura, M. Tokunaga, A. H. Iwane, T. Yanagida, Nature, 1999, 397, 129-134. 14) For a review see: S. M. King; Molecular Motors: Chapter 2, Dynein Motors: Structure ,

Mechanochemistry and Regulation; M. Schliwa (Ed.); Wiley-VCH, Weinheim, 2003, 45-78. 15) E. Westhof, N. Leontis, Angew. Chem., Int. Ed. 2000, 39, 1587-1591. 16a) A. R. Pease, J. O. Jeppesen, J. F. Stoddart, Y. Luo, C. P. Collier, J. R. Heath, Acc. Chem. Res.

2001, 34, 433-444; b) A. R. Pease, J. F. Stoddart, Struct. Bond. 2001, 99, 189-236; c) A. H. Flood, R. J. A. Ramirez, W. Q. Deng, R. P. Muller, W. A. Goddard, J. F. Stoddart, Aust. J. Chem. 2004, 57, 301-322.

31


17a) V. Balzani, A. Credi, The Chemical Record, 2001, 1, 422-435; b) R. Ballardini, V. Balzani, A.

Credi, M. T. Gandolfi, M. Venturi, Struct. Bond. 2001, 99, 163-188; c) V. Balzani, A. Credi, M. Venturi, Pure Appl. Chem. 2003, 75, 541-547; d) R. Ballardini, V. Balzani, A. Credi, M. T. Gandolfi, M. Venturi, Acc. Chem. Res. 2001, 34, 445-455; e) V. Balzani, M. Venturi, A. Credi; Molecular Devices and Machines: A Journey into the Nanoworld; Wiley-VCH, Weinheim, 2004.

18) For reviews see: a) T. R. Kelly, Acc. Chem. Res. 2001, 34, 514-522; b) T. R. Kelly, J. P. Sestelo, Struct. Bond. 2001, 99, 19-53.

19a) J.-P. Collin, C. Dietrich-Buchecker, P. Gaviña, M. C. Jimenez-Molero, J.-P. Sauvage, Acc. Chem. Res. 2001, 34, 477-487; b) M. C. Jimenez-Molero, C. Dietrich-Buchecker, J.-P. Sauvage, Chem. Commun. 2003, 1613-1616; c) L. Raehm, J.-P. Sauvage, Struct. Bond. 2001, 99, 55-78; d) B. X. Colasson, C. Dietrich-Buchecker, M. C. Jimenez-Molero, J.-P. Sauvage, J. Phys. Org. Chem. 2002, 15, 476-483; e) C. Dietrich-Buchecker, M. C. Jimenez-Molero, V. Sartor, J.-P. Sauvage, Pure Appl. Chem. 2003, 75, 1383-1393.

20a) B. L. Feringa, N. Koumura, R. A. van Delden, M. K. J. ter Wiel, Appl. Phys. A. 2002, 75, 301-308; b) B. L. Feringa, R. A. van Delden, M. K. J. ter Wiel, Pure Appl. Chem. 2003, 75, 563-575; c) B. L. Feringa, Acc. Chem. Res. 2001, 34, 504-513.

21a) C. P. Mandl, B. König, Angew. Chem Int. Ed. 2004, 43, 1622-1624; b) C. Joachim, J. K. Gimzewski, Struct. Bond. 2001, 99, 1-18; c) V. Amendola, L. Fabrizzi, C. Mangano, P. Pallavicini, Struct. Bond. 2001, 99, 79-116; d) V. Amendola, L. Fabrizzi, C. Mangano, P. Pallavicini, Acc. Chem. Res. 2001, 34, 488-493; e) J. Liu, M. Gómez-Kaifer, A. E. Kaifer, Struct. Bond. 2001, 99, 141-162; f) C. A. Schalley, K. Beizai, F. Vögtle, Acc. Chem. Res. 2001, 43, 465-476; g) C. A. Schalley, Angew. Chem., Int. Ed. 2002, 41, 1513-1515; h) R. M. Raymo, Adv. Mat. 2002, 14, 401-414.

22) V. Balzani, A. Credi, F. M. Raymo, J. F. Stoddart, Angew. Chem., Int. Ed. 2000, 39, 3348-3391. 23a) O. S. Akkerman, Recl. Trav. Chim. Pays-Bas 1970, 89, 673-679; b) K. Mislow, Acc. Chem. Res.

1976, 9, 26-33; c) R. Glaser, J. F. Blount, K. Mislow, Acc. Chem. Res. 1980, 102, 2777-2786; d) F. Cozzi, A. Guenzi, C. A. Johnson, K. Mislow, W. D. Hounshell, J. F. Blount, J. Am. Chem. Soc. 1981, 103, 957-958; e) U. Berg, T. Liljefors, C. Roussel, J. Sandström, Acc. Chem. Res. 1985, 18, 80-86; f) H. Iwamura, K. Mislow, Acc. Chem. Res. 1988, 21, 175-182.

24) J. P. Hummel, D. Gust, K. Mislow, J. Am. Chem. Soc. 1974, 96, 3679-3681. 25) T. C. Bedard, J. S. Moore, J. Am. Chem. Soc. 1995, 117, 10662-10671. 26a) T. R. Kelly, M. C. Bowyer, K. V. Bhaskar, D. Bebbington, A. Garcia, F. Lang, M. H. Kim, M. P.

Jette, J. Am. Chem. Soc. 1994, 116, 3657-3658; b) A. P. Davis, Angew. Chem., Int. Ed. 1998, 37, 909-910.

27) T. R. Kelly, J. P. Sestelo, I. Tellitu, J. Org. Chem. 1998, 63, 3655-3665. 28a) T. R. Kelly, H. De Silva, R. A. Silva, Nature, 1999, 401, 150-152; b) T. R. Kelly, R. A. Silva, H.

De Silva, S. Jasmin, Y. Zhao, J. Am. Chem. Soc. 2000, 122, 6934-6949. 29) For a discussion see: A. P. Davis, Nature 1999, 401, 120-121 30) Y. Kuwatani, G. Yamamoto, M. Iyoda, Org. Lett. 2003, 5, 3371-3374. 31) C. Viala, A. Secchi, A. Gourdon, Eur. J. Org. Chem. 2002, 4185-4189. 32) Z. Dominguez, H. Dang, M. J. Strouse, M. A. Garcia-Garibay, J. Am. Chem. Soc. 2002, 124,

2398-2399. 33) C. E. Godlinez, G. Zepeda, M. A. Garcia-Garibay, J. Am. Chem. Soc. 2002, 124, 4701-4707. 34) Z. Dominguez, T.-A. V. Khuong, H. Dang, C. A. Sanrame, J. E. Nuñez, M. A. Garcia-Garibay, J.

Am. Chem. Soc. 2003, 125, 8827-8837. 35) C. E. Godlinez, G. Zapeda, C. J. Mortko, H. Dang, M. A. Garcia-Garibay, J. Org. Chem. 2004,

69, 1652-1662.

32



36) Z. Dominguez, H. Dang, M. J. Strouse, M. A. Garcia-Garibay, J. Am. Chem. Soc. 2002, 124,

7719-7727. 37) T.-A. V. Khuong, G. Zepeda, R. Ruiz, S. I. Khan, M. A. Garcia-Garibay, Crystal Growth &

Design 2004, 4, 15-18. 38) T. Hugel, N. B. Holland, A. Cattani, L. Moroder, M. Seitz, H. E. Gaub, Science, 2002, 296, 1103-

1106. 39) T. Muraoka, K. Kinbara, Y. Kobayashi, T. Aida, J. Am. Chem. Soc. 2003, 125, 5612-5613. 40) Molecular Catenanes, Rotaxanes and Knots; J.-P. Sauvage, C. Dietrich-Buchecker (Eds.); 1999,

Wiley-VCH, Weinheim. 41) C. M. Keaveney, D. A. Leigh, Angew. Chem., Int. Ed. 2004, 43, 1222-1224. 42a) P. Thordarson, E. J. A. Bijsterveld, A. E. Rowan, R. J. M. Nolte, Nature, 2003, 424, 915-918;

see for a discussion: b) P. R. Carlier, Angew. Chem., Int. Ed. 2004, 43, 2602-2605. 43) L. Jiang, J. Okano, A. Orita, J. Otera, Angew. Chem., Int. Ed. 2004, 43, 2121-2124. 44) M. C. Jiménez, C. Dietrich-Buchecker, J.-P. Sauvage, Angew. Chem., Int. Ed. 2000, 39, 3284-

3287. 45) I. Poleschak, J.-M. Kern, J.-P. Sauvage, Chem. Commun. 2004, 474-476. 46a) S. Kang, S. A. Vignon, H.-R. Tseng, J. F. Stoddart, Chem. Eur. J. 2004, 10, 2555-2564; b) H.-R.

Tseng, S. A. Vignon, P. C. Celeste, J. Perkins, J. O. Jeppesen, A. Di Fabio, R. Ballardini, M. T. Gandolfi, M. Venturi, V. Balzani, J. F. Stoddart, Chem. Eur. J. 2004, 10, 155-172.

47) P. R. Ashton, R. Ballardini, V. Balzani, A. Credi, K. R. Dress, E. Ishow, C. J. Kleverlaan, O. Kocian, J. A. Preece, N. Spencer, J. F. Stoddart, M. Venturi, S. Wenger, Chem. Eur. J. 2000, 6, 3558-3574.

48) H.-R. Tseng, S. A. Vignon, J. F. Stoddart, Angew. Chem., Int. Ed. 2003, 42, 1491-1495. 49) J. O. Jeppesen, J. Becher, J. F. Stoddart, Org. Lett. 2002, 4, 557-560. 50a) J. D. Badjic, V. Balzani, A. Credi, S. Silvi, J. F. Stoddart, Science, 2004, 303, 1845-1849; b) V.

Balzani, M. Clemente-León, A. Credi, J. N. Lowe, J. D. Badjic, J. F. Stoddart, D. J. Williams, Chem. Eur. J. 2003, 9, 5348-5360; c) J. D. Badjic, V. Balzani, A. Credi, J. N. Lowe, S. Silvi, J. F. Stoddart, Chem. Eur. J. 2004, 10, 1926-1935; d) M. C. T. Fyfe, J. N. Lowe, J. F. Stoddart, D. J. Williams, Org. Lett. 2000, 2, 1221-1224.

51) W. S. Jeon, A. Y. Ziganshina, J. W. Lee, Y. H. Ko, J.-K. Kang, C. Lee, K. Kim, Angew. Chem., Int. Ed. 2003, 42, 4097-4100.

52) For reviews concerning rotaxanes see: a) A. C. Benniston, Chem. Soc. Rev. 1996, 25, 427-435; b) M.-J. Blanco, M. C. Jiménez, J. C. Chambron, V. Heitz, M. Linke, J.-P. Sauvage, Chem. Soc. Rev. 1999, 28, 293-305.

53) K.-S. Jeong, K.-J. Chang, Y.-J. An, Chem Commun, 2003, 1450-1451. 54) V. Balzani, A. Credi, F. Marchioni, J. F. Stoddart, Chem. Commun. 2001, 1860-1861. 55) A. M. Brouwer, C. Frochot, F. G. Gatti, D. A. Leigh, L. Mottier, F. Paolucci, S. Roffia, G. W. H.

Wurpel, Science, 2001, 291, 2124-2128. 56) D. A. Leigh, A. Trosi, F. Zerbetto, Angew. Chem., Int. Ed. 2000, 39, 350-353. 57) G. W. H. Wurpel, A. M. Brouwer, I. H. M. van Stokkum, A. Farran, D. A. Leigh, J. Am. Chem.

Soc. 2001, 123, 11327-11328. 58) A. Altieri, F. G. Gatti, E. R. Kay, D. A. Leigh, D. Martel, F. Paolucci, A. M. Z. Slawin, J. K. Y.

Wong, J. Am. Chem. Soc. 2003, 125, 8644-8654. 59) G. Bottari, D. A. Leigh, E. M. Pérez, J. Am. Chem. Soc. 2003, 125, 13360-13361. 60) A. Altieri, G. Bottari, F. Dehez, D. A. Leigh, J. K. Y. Wong, F. Zerbetto, Angew. Chem., Int. Ed.

2003, 42, 2296-2300. 61) G. Bottari, F. Dehez, D. A. Leigh, P. J. Nash, E. M. Pérez, J. K. Y. Wong, F. Zerbetto, Angew.

Chem., Int. Ed. 2003, 42, 5886-5889.

33


62) F. G. Gatti, S. León, J. K. Y. Wong, G. Bottari, A. Altieri, M. A. Farran Morales, S. J. Teat, C.

Frochot, D. A. Leigh, A. M. Brouwer, F. Zerbetto, Proc. Nat. Acad. Sci. 2003, 100, 10-14. 63) M. Cavallini, F. Biscarini, S. León, F. Zerbetto, G. Bottari, D. A. Leigh, Science, 2003, 299, 531. 64) C. A. Stanier, S. J. Alderman, T. D. W. Claridge, H. L. Anderson, Angew. Chem., Int. Ed. 2002,

41, 1769-1772. 65) Q.-C. Wang, D.-H. Qu, J. Ren, K. Chen, H. Tian, Angew. Chem., Int. Ed. 2004, 43, 2661-2665. 66) P. R. Ashton, V. Baldoni, V. Balzani, A. Credi, H. D. A. Hoffmann, M.-V. Martínez-Díaz, F. M.

Raymo, J. F. Stoddart, M. Venturi, Chem. Eur. J. 2001, 7, 3482-3493. 67) D. A. Leigh, J. K. Y. Wong, F. Dehez, F. Zerbetto, Nature, 2003, 424, 174-179. 68) a) F. Arico, P. Mobian, J.-M. Kern, J.-P. Sauvage, Org. Lett. 2000, 5, 1887-1890; b) P. Mobian,

J.-M. Kern, J.-P. Sauvage, Angew. Chem., Int. Ed. 2004, 43, 2392-2395. 69) B. Korybut-Daszkiewicz, A. Wieckowska, R. Bilewicz, S. Domagała, K. Wozniak, Angew.

Chem., Int. Ed. 2004, 43, 1668-1672. 70a) S. Shinkai, M. Ikeda, A. Sugasaki, M. Takeuchi, Acc. Chem. Res. 2001, 34, 494-503; b) M.

Takeuchi, M. Ikeda, A. Sugasaki, S. Shinkai, Acc. Chem. Res. 2001, 34, 865-873. 71a) M. Ikeda, T. Tanida, M. Takeuchi, S. Shinkai, Org. Lett. 2000, 2, 1803-1805; b) M. Ikeda, M.

Takeuchi, S. Shinkai, Y. Naruta, S. Sakamoto, K. Yamaguchi, Chem. Eur. J. 2002, 8, 5542-5550. 72) K. Tashiro, K. Konishi, T. Aida, J. Am. Chem. Soc. 2000, 122, 7921-7926. 73) A. Carella, J. Jaud, G. Rapenne, J.-P. Launay, Chem. Commun. 2003, 2434-2435. 74a) S. Hiraoka, T. Yi, M. Shiro, M. Shionoya, J. Am. Chem. Soc. 2002, 124, 14510-14511; b) S.

Hiraoka, K. Harano, T. Tanaka, M. Shiro, M. Shionoya, Angew. Chem. Int. 2003, 42, 5182-5185; c) S. Hiraoka, M. Shiro, M. Shionoya, J. Am. Chem. Soc. 2004, 126, 1214-1218.

75) M. F. Hawthorne, J. I. Zink, J. M. Skelton, M. J. Bayer, C. Liu, E. Livshits, R. Baer, D. Neuhauser, Science, 2004, 303, 1849-1851.

76) B. Feringa, H. Wynberg, J. Am. Chem. Soc. 1977, 99, 602-603. 77) B. Feringa, H. Wynberg, A. J. Duisenberg, A. L. Spek, Recl. Trav. Chim. Pays-Bas 1979, 98, 1-2. 78) N. Harada, A. Saito, N. Koumura, H. Uda, B. de Lange, W. F. Jager, H. Wynberg, B. L. Feringa,

J. Am. Chem. Soc. 1997, 119, 7241-7248. 79) N. Harada, A. Saito, N. Koumura, D. C. Roe, W. F. Jager, R. W. J. Zijstra, B. de Lange, B. L.

Feringa, J. Am. Chem. Soc. 1997, 119, 7249-7255. 80) R. W. J. Zijlstra, W. F. Jager, B. de Lange, P. Th. Van Duijnen, B. L. Feringa, H. Goto, A. Saito,

N. Koumura, N. Harada, J. Org. Chem. 1999, 64, 1667-1674. 81) N. Harada, N. Koumura, B. L. Feringa, J. Am. Chem. Soc. 1997, 119, 7256-7264. 82) N. Koumura, N. Harada, Chem. Lett. 1998, 1151-1152. 83) N. Koumura, R. W. J. Zijstra, R. A. van Delden, N. Harada, B. L. Feringa, Nature 1999, 401,

152-155. 84) R. A. van Delden, N. Koumura, N. Harada, B. L. Feringa, Proc. Nat. Acad. Sci. 2002, 99, 4945-

4949. 85) B. L. Feringa, W. F. Jager, B. de Lange, Tetrahedron 1993, 49, 8267-8310. 86) B. L. Feringa, R. A. van Delden, N. Koumura, E. M. Geertsema, Chem. Rev. 2000, 100,1789-

1816. 87a) B. de Lange, Chiroptical Molecular Switches, Ph. D. Thesis, 1993, Rijksuniversiteit Groningen;

b) W. F. Jager, Chiroptical Molecular Switches, Ph. D. Thesis, 1994; c) N. M. P. Huck, Chiroptical Molecular Switches, Ph. D. Thesis, 1997, Rijksuniversiteit Groningen d) A. M. Schoevaars, Chiroptical Molecular Switches, Ph. D. Thesis, 1998, Rijksuniversiteit Groningen; e) E. M. Geertsema, From Overcrowded Alkenes towards Molecular Motors, Ph. D. Thesis, 2003,

34



Rijksuniversiteit Groningen; f) R. A. van Delden, Controlling Molecular Chirality and Motion, Ph. D. Thesis, 2002, Rijksuniversiteit Groningen.

88) W. F. Jager, J. C. de Jong, B. de Lange, N. M. P. Huck, A. Meetsma, B. L. Feringa, Angew. Chem., Int. Ed. Engl. Ed. 1995, 34, 348-350.

89) S. F. Braga, D. S. Galvão, P. M. V. B. Barone, S. O. Dantas, J. Phys. Chem. B. 2001, 105, 8334-8338.

90) R. A. van Delden, M. B. van Gelder, N. M. P. Huck, B. L. Feringa, Adv. Funct. Mat. 2003, 13, 319-324.

91) C.-T. Chen, Y.C. Chou, J. Am. Chem. Soc. 2000, 122, 7662-7672. 92) N. Koumura, E. M. Geertsema, A. Meetsma, B. L. Feringa, J. Am.Chem. Soc. 2000, 122, 12005-

12006. 93) N. Koumura, E. M. Geertsema, M. B. van Gelder, A. Meetsma, B. L. Feringa, J. Am. Chem. Soc.

2002, 124, 5037-5051. 94) E. M. Geertsema, N. Koumura, M. K. J. ter Wiel, A. Meetsma, B. L. Feringa, Chem. Commun.

2002, 2962-2963. 95) R. A. van Delden, N. Koumura, A. M. Schoevaars, A. Meetsma, B. L. Feringa, Org. Biomol.

Chem. 2003, 1, 33-35. 96) A. N. Shipway, I. Willner, Acc. Chem. Res. 2001, 34, 412-432. 97) L. Raehm, C. Hamann, J.-M. Kern, J.P. Sauvage, Org. Lett. 2000, 2, 1991-1994. 98) L. Raehm, J.-M. Kern, J.-P. Sauvage, C. Hamann, S. Palacin, J.-P. Bourgoin, Chem. Eur. J. 2002,

8, 2153-2162. 99) X. Zheng, M. E. Mulcahy, D. Horinek, F. Galeotti, T. F. Magnera, J. Michl, J. Am. Chem. Soc.

2004, 126, 4540-4542. 100) H. Jian, J. M. Tour, J. Org. Chem. 2003, 68, 5091-5103. 101) M. Asakawa, M. Higuchi, G. Mattersteig, T. Nakamura, A. R. Pease, F. M. Raymo, T. Shimizu,

J. F. Stoddart, Adv. Mater. 2000, 12, 1099-1102. 102) S. Chia, J. Cao, J. F. Stoddart, J. I. Zink, Angew. Chem., Int. Ed. 2001, 40, 2447-2451. 103) C. P. Collier, E. W. Wong, M. Belohradsk ý, F. M. Raymo, J. F. Stoddart, P. J. Kuekes, R. S.

Williams, J. R. Heath, Science, 1999, 285, 391-394. 104) C. P. Collier, G. Mattersteig, E. W. Wong, Y. Luo, J. Sampaio, F. M. Raymo, J. F. Stoddart, J.

R. Heath, Science, 2000, 289, 1172-1175. 105) C. P. Collier, J. O. Jeppesen, Y. Luo, E. W. Wong, J. R. Heath, J. F. Stoddart, J. Am. Chem. Soc.

2001, 123, 12632-12641. 106) R. Hernandez, H.-R. Tseng, J. W. Wong, J. F. Stoddart, J. I. Zink, J. Am. Chem. Soc. 2004, 126,

3370-3371.

university of groningen basic molecular devices ter wiel

Documents