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- Herausgeber: Wiley-VCH GmbH
- Kategorie: Wissenschaft und neue Technologien
- Sprache: Englisch
- Veröffentlichungsjahr: 2024
Presenting comprehensively the emerging field of MIMs and MIMats - a must read for everyone who is working on this topic.
Das E-Book Mechanically Interlocked Materials wird angeboten von Wiley-VCH GmbH und wurde mit folgenden Begriffen kategorisiert:
Biopolymere, Biopolymers, Chemie, Chemistry, Electronic Materials, Elektronische Materialien, Materials Science, Materialwissenschaften, Polymer Science & Technology, Polymerwissenschaft u. -technologie, Sensor Materials, Sensoren, Supramolecular Chemistry, Supramolekulare Chemie
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Table of Contents
Cover
Table of Contents
Title Page
Copyright
Preface
References
1 Force-Promoted Transformations in Mechanically Linked Molecules
1.1 Introduction
1.2 SMFS in the Study of Non-covalent Interactions
1.3 Strength of Mechanical Bonds
1.4 Changes in Optical Properties – Reversible and Irreversible Changes of Optical Properties by Movement of Macrocycle in a Rotaxane
1.5 Conclusions
References
2 Colloidal Nanomaterials with Mechanically Interlocked Parts
2.1 Introduction
2.2 Installing and Actuating Mechanically Interlocked Molecular Architectures at Colloidal Nanoparticle Surfaces
2.3 Modulating Nanoparticle Physicochemical Properties Using Switchable Mechanically Interlocked Architectures
2.4 Interlocked Gates for Nanoparticle Pores: From Cargo Release to Nanoscale Communication
2.5 Mechanically Interlocked Molecular Links for Nanoparticle Assemblies
2.6 From Switches to Motors and Beyond: The Future of Colloidal Nanomaterials with Mechanically Interlocked Parts
References
3 Mechanically Interlocked Nanotubes
3.1 Introduction
3.2 Carbon Nanotubes
3.3 MINTs: Clipping Strategy
3.4 Other Strategies for the Preparation of MINTs
3.5 Application of MINTs
3.6 Conclusions
References
4 Concepts of Molecular Motors in Solution and on Surfaces
4.1 Light-driven Overcrowded Alkenes
4.2 Molecular Motors Based on Catenanes
4.3 Other Concepts of Molecular Motors
4.4 Computationally Designed Light-driven Molecular Motors
4.5 Molecular Motors on Surfaces
4.6 Conclusions and Outlook
Acknowledgments
References
Notes
5 MIMs in MOFs: Designing Mechanically Interlocked Molecules to Function Inside Metal–Organic Frameworks
5.1 Introduction
5.2 Coordination Polymers Using [2]Pseudorotaxanes as Linkers
5.3 Robust Dynamics
5.4 Techniques for Elucidating Dynamic Behavior in the Solid State
5.5 Rotational Motion of a MIM Wheel: UWDM-1, a Case Study
5.6 Translational Motion of a MIM Wheel: UWDM-4, a Case Study
5.7 MIM Linker Design Strategies
5.8 Controlling Dynamics and Switching of MIMs in MOFs
5.9 MIMs to Construct Poly-Threaded MOF Lattices
5.10 Applications and Future Perspectives
References
6 Mechanically Interlocked Proteins
6.1 Introduction
6.2 Classification of Mechanically Interlocked Proteins
6.3 Making Mechanically Interlocked Proteins
6.4 Biological Significance of Natural MIPs
6.5 Cultivating Mechanically Interlocked Proteins
6.6 Conclusion and Future Perspective
Acknowledgments
References
7 Recent Advances on Catenanes and Rotaxanes Made of DNA
7.1 Introduction
7.2 DNA Catenanes
7.3 DNA Rotaxanes
7.4 Conclusions and Outlook
References
8 Oligo- and Poly-catenanes from Molecular and Supramolecular Building Blocks
8.1 Introduction
8.2 [
n
]Molecular Necklaces by Cyclization of Polypseudorotaxanes
8.3 Main Chain Polycatenanes Composed of Covalent Macocyclic Building Block
8.4 Main Chain Nano-polycatenanes Composed of Non-covalent Building Block
8.5 Polycatenanes Composed of Metal–Organic Coordination Cages
8.6 Poly[2]catenane
8.7 Summary and Outlook
References
9 Synthesis, Properties, and Applications of Mechanically Interlocked Polymers
9.1 Introduction
9.2 Synthesis and Physical Properties of Polyrotaxanes
9.3 Applications
9.4 Conclusion and Outlook
References
Index
End User License Agreement
List of Tables
Chapter 4
Table 4.1 High-speed second-generation molecular motors.
List of Illustrations
Chapter 1
Figure 1.1 Response of catenanes and rotaxanes to external force.
Figure 1.2 Chemical structure and schematic representation of the AFM force ...
Figure 1.3 (a) Schematics of the single-molecule force spectroscopy experime...
Figure 1.4 (a) Schematics of the OT-based single-molecule force spectroscopy...
Figure 1.5 (a) Schematics of the single-molecule force spectroscopy experime...
Figure 1.6 Single-molecule force spectroscopy experiment of the deslipping b...
Figure 1.7 (a) Schematics of the single-molecule force spectroscopy experime...
Figure 1.8 Schematics of the AFM-based single-molecule force spectroscopy ex...
Figure 1.9 (a) Schematics of the AFM-based single-molecule force spectroscop...
Figure 1.10 Mechanical activation of Stoddart’s rotaxane in the center of a ...
Figure 1.11 Rotaxane as a force actuator in a Diels–Alder reaction. (a) Mech...
Figure 1.12 (a)Mechanical dissociation of rotaxane mechanophore
5
by an unst...
Figure 1.13 Sonication data for Craig’ s polymers. Limiting mass and length ...
Figure 1.14 Mechanical activation of catenane mechanophore
9
. (i) US (20 kHz...
Figure 1.15 (a) Schematic illustration of the activation of a rotaxane cross...
Figure 1.16 (a) Schematic illustration of reversible rotaxane mechanophores
Figure 1.17 (a) Schematic illustration of reversible and irreversible rotaxa...
Figure 1.18 (a) Activation of a mechanochromic hydrogen-bonded rotaxane
13
c...
Chapter 2
Figure 2.1 Schematic description of architectural categories for surface-imm...
Figure 2.2 Early examples of interlocked molecular structures constructed on...
Figure 2.3 End-tethered redox-responsive [2]pseudorotaxane and [2]rotaxane a...
Figure 2.4 Redox-responsive MIMs on metal NP cores. (a) Redox-switchable com...
Figure 2.5 Doubly tethered [2]rotaxanes and [3]rotaxanes on AuNP cores [22, ...
Figure 2.6 Modulation of hierarchical AuNP composite structure by mechanical...
Figure 2.7 End-tethered [2]pseudorotaxanes for spatiotemporal modulation of ...
Figure 2.8 Design strategies for stimuli-responsive control over MSNP pore a...
Figure 2.9 Redox-controlled cargo loading and release in MSNPs via co-confor...
Figure 2.10 pH-Triggered cargo release mechanisms for MSNP carriers compatib...
Figure 2.11 Cargo release by (a) light-triggered nanovalve opening [83], and...
Figure 2.12 MSNP nanocarriers optimized for live cell and
in vivo
pH-trigger...
Figure 2.13 Nanoscale structure (a), molecular mechanism (b), and cartoon re...
Figure 2.14 Size-selective pore access on MIM-gated MSNP nanocarriers. (a) S...
Figure 2.15 Nanoscale communication networks based on processing and exchang...
Figure 2.16 (a) Remoted-controlled NP aggregation–dispersion via photoswitch...
Figure 2.17 Nanoparticle assembly mediated by [3]pseudorotaxane complexes of...
Figure 2.18 Nanoparticle assemblies linked by redox-switched association of ...
Figure 2.19 Stepwise construction of AuNP dimers linked by [c2]daisy chain M...
Chapter 3
Figure 3.1 (a) Clipping strategy, based on the association of SWNTs by alken...
Figure 3.2 DCC strategy by von Delius. (a) Schematic representation of diffe...
Figure 3.3 (a) Scheme of “Ring-tossing” strategy, as described by Miki et al...
Figure 3.4 H-bonding strategy toward the encapsulation of SWNTs within self-...
Figure 3.5 (a) α-Helical coiled-coil peptide barrels (αHBs) encapsulate SWNT...
Figure 3.6 (a) Chemical structure of metallosquares used to try mechanical i...
Figure 3.7 (a) Chemical structures of the electron-donating (exTTF), electro...
Figure 3.8 (a) Cyclic voltammograms of AQ-MINT in 0.1 M NaOH under air (gree...
Figure 3.9 (a) Schematic representation of the three types of fillers invest...
Figure 3.10 (a) Chemical structure of the bisporphyrin U-shape precursor and...
Chapter 4
Figure 4.1 Schematic energy landscape of a Brownian motor. A particle is ini...
Figure 4.2 Schematic energy landscape of an energy ratchet. The particle (bl...
Figure 4.3 Schematic energy landscape of an information ratchet. The positio...
Figure 4.4 Ratchet-like system capable of unidirectional rotation. A chemi...
Figure 4.5 Second-generation molecular motor based on overcrowded alkenes. B...
Figure 4.6 Second generation of unidirectional molecular motors. In contrast...
Figure 4.7 Reversible molecular motor. A base-induced epimerization leads to...
Figure 4.8 Third generation of unidirectional molecular motors. Merging two ...
Figure 4.9 Photographs of a molecular motor doped liquid crystal. Depending ...
Figure 4.10 Photo-responsive nano-container. Depending on the state of the a...
Figure 4.11 Self-assembled artificial muscles made from molecular motors. (a...
Figure 4.12 Eight-shaped polymer system twisted by a molecular motor. Two st...
Figure 4.13 Contraction of a polymer gel by molecular motors. (a) UV-light a...
Figure 4.14 Nondirectional [2]-catenane system. By varying the thermodynamic...
Figure 4.15 Unidirectional rotation of two macrocycles in a [3]-catenane. Th...
Figure 4.16 Reversible molecular motor based on a [2]-catenane with two deta...
Figure 4.17 Chemical structure of the motor shown in Figure 4.16.
Figure 4.18 Symmetric [2]-catenane based molecular motor. The rate of remova...
Figure 4.19 Reversible, unidirectional motor driven by chemical fuel. The di...
Figure 4.20 Imine-based light-driven molecular motor. A light-induced isomer...
Figure 4.21 Visible light driven hemithioindigo chromophore-based molecular ...
Figure 4.22 Hemithioindigo chromophore-based motor undergoing an eight-shape...
Figure 4.23 Molecular motors grafted onto a thin gold film on a quartz subst...
Figure 4.24 (a) Azimuthal [73] and (b) altitudinal [75] molecular motors on ...
Figure 4.25 Tunable wettability of a gold on mica surface. (a) Attaching the...
Figure 4.26 Molecular motors on the Cu(111) surface. (a) Full rotary cycle...
Figure 4.27 BuSMe on Cu(111) as electric motor. (a) Due to rotation of the m...
Figure 4.28 Chemical structure and rotation of a platinum-porphyrin-based su...
Figure 4.29 Voltage pulse-induced rotation or deformation of supramolecular ...
Figure 4.30 Multicomponent molecular propeller on Au(111). (a, b) A three-bl...
Figure 4.31 Chemical structures of IoMes (a) and IRotate (b). Both the symme...
Figure 4.32 STM images (at 78 K) and energy landscape of IoMes (a–d) and IRo...
Figure 4.33 Directionality of the rotation of IRotate and IoMes on Au(111). ...
Figure 4.34 Achiral acetylene on the chiral PdGa() surface. The surface and...
Figure 4.35 Suggested model for the classical regime: only with an adequate ...
Chapter 5
Figure 5.1 Cartoon depictions of mechanically interlocked molecules (MIMs). ...
Figure 5.2 The judicious choice of linkers and nodes dictates whether the re...
Figure 5.3 Some linker systems used for the preparation of early examples of...
Figure 5.4 Two examples of three-periodic coordination polymers using [2]pse...
Figure 5.5 The case of
UWDM-1
. (a) The tetracarboxylic acid MIM linker combi...
Figure 5.6 The case of
UWDM-1
. (a) PXRD (25 °C) patterns for
UWDM-1
demonstr...
Figure 5.7 The case of
UWDM-1
. (a) VT
2
H SSNMR spectra, (b) the various moti...
Figure 5.8 The case of
UWDM-4
. (a) The MIM linker was used to create
UWDM-4
,...
Figure 5.9 The case of
UWDM-4
. (a) Partial VT CP MAS
13
C SSNMR showing the
1
...
Figure 5.10 The integral parts of [2]rotaxane linker. (a) A cartoon of a T-s...
Figure 5.11 Pillared MIMs in MOFs. (a) A cartoon depiction of a linear [2]ro...
Figure 5.12 Reticular chemistry is used to design different T-shaped MIM lin...
Figure 5.13 Reticular chemistry is used to design different T- or H-shaped M...
Figure 5.14 Coordinating groups are attached to the macrocyclic wheel rather...
Figure 5.15 Exchanging the MIM linker for a labile group in the large caviti...
Figure 5.16 Ring size, phase, and solvent effects on dynamics. (a) The linke...
Figure 5.17 The interaction of two different mobile components within a MOF....
Figure 5.18 Redox addressable MIMs in MOFs. (a) The charged macrocycle [
CBQT
Figure 5.19 Lattice linkers for poly-threaded MOFs. (a) Cartoon of an H-shap...
Chapter 6
Figure 6.1 Two categories of MIPs: knotted proteins and mechanoproteins.
Figure 6.2 Two pathways to make MIPs: one is by folding only, while the othe...
Figure 6.3 Functional benefits of MIPs include thermal stability, denaturant...
Figure 6.4 Cultivating artificial MIPs. (a) The homocatenane containing two ...
Chapter 7
Figure 7.1 Representation of a [2]catenane, [3]catenane, and [4]catenanes.
Figure 7.2 (a) Two methods to form [2]DNA catenanes and the topology of DNA ...
Figure 7.3 (a) Reversible switch of the rotor Ring α between three states (I...
Figure 7.4 (a) Reversible logic circuit based on the transformation of a dsD...
Figure 7.5 (a) Synthesis of a 6-helix tube with 24 single-stranded DNA stran...
Figure 7.6 (a) Model of a dsDNA (pseudo-) rotaxane. The hybridization betwee...
Figure 7.7 (a) Two strategies to control the switching between pseudo-rotaxa...
Figure 7.8 (a) Assembly of a single-stranded tile rotaxane. The SST stopper ...
Figure 7.9 (a) Switching of a DNA [3]rotaxane in response to input strands. ...
Chapter 8
Figure 8.1 Cartoon representations of (a) linear poly[
n
]catenane, (b) branch...
Figure 8.2 Sauvage’ s syntheses of (a) [2, 3]catenanes templated by copper(I...
Figure 8.3 (a) Schematic representation of the formation of a radial poly[
n
]...
Figure 8.4 Schematic illustration of the preparation of radial (a) [4, 5]cat...
Figure 8.5 Schematic illustration of synthesis of radial (a) [4] and (b) [7]...
Figure 8.6 (a) Chemical and (b) single-crystal X-ray crystal structures of a...
Figure 8.7 (a) Schematic illustration of Rowan’ s strategy for the synthesis...
Figure 8.8 (a) Molecular structure of DPN monomer and schematic representati...
Figure 8.9 (a–e) Upper panel displays protocols for separating toroids from ...
Figure 8.10 Fujita’ s triply interlocked cage complex is formed by the self-...
Figure 8.11 Synthesis of polycatenane based on metallocages reported by Lu a...
Figure 8.12 Schematic presentation of preparation of one-dimensional and thr...
Figure 8.13 Poly[2]catenanes reported by (a) Sauvage and (b) Greets.
Figure 8.14 Poly[2]catenanes reported by (a) and (b) Stoddart and (c) Craig....
Figure 8.15 (a) Schematic illustration of the preparation of a polymer gel b...
Figure 8.16 Cartoon representation of the formation of supramolecular main-c...
Figure 8.17 Schematic representation of the formation of a cross-linked poly...
Chapter 9
Figure 9.1 Graphical representation of mechanically interlocked molecules an...
Figure 9.2 Graphical representations of different subclasses of polyrotaxane...
Figure 9.3 Graphical representation of synthetic strategies towards main-cha...
Figure 9.4 Common rings for polyrotaxane synthesis. (a) Cyclodextrin. (b) Cr...
Figure 9.5 Schematic and molecular structure of α-cyclodextrin.
Figure 9.6 α-CD based PEG main-chain polyrotaxane.
Figure 9.7 Graphical representation of different architectures of slide-ring...
Figure 9.8 Typical stress–strain curves of polymers (R-shaped) and chemicall...
Figure 9.9 Schematic explanation of the mechanism of stress dissipation in s...
Figure 9.10 Polycaprolactone grafted polyrotaxanes as developed in [52].
Figure 9.11 Cartoon representation of (a) Insulated molecular wire and (b) p...
Figure 9.12 Schematic depiction of a stimuli responsive drug delivery system...
Figure 9.13 Polyrotaxane drug delivery system as described in Ref. [68].
Figure 9.14 Schematic showing the effect of the mobile rings on multivalent ...
Figure 9.15 Graphical and molecular representation of (a) mildly fluorescent...
Figure 9.16 Polyrotaxane designed for flexible electronics applications as i...
Figure 9.17 Molecular pump polyrotaxane based system as in [80].
Figure 9.18 Artificial muscle based on polyrotaxane as in [82].
Figure 9.19 α-CD/PEG based polyrotaxane crosslinked with polyacrylic acid fo...
Figure 9.20 Mechanically interlocked network for lithium metal batteries bas...
Guide
Cover
Table of Contents
Title Page
Copyright
Preface
Begin Reading
Index
End User License Agreement
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Mechanically Interlocked Materials
Polymers, Nanomaterials, MOFs, and more
Edited byEmilio M. Pérez
Editor
Prof. Emilio M. PérezIMDEA Nanosciencec/ Faraday 928049 MadridSpain
Cover Image: Courtesy of Patricia Bondia
All books published by WILEY-VCH are carefully produced. Nevertheless, authors, editors, and publisher do not warrant the information contained in these books, including this book, to be free of errors. Readers are advised to keep in mind that statements, data, illustrations, procedural details or other items may inadvertently be inaccurate.
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Print ISBN: 978-3-527-34793-3ePDF ISBN: 978-3-527-82895-1ePub ISBN: 978-3-527-82896-8oBook ISBN: 978-3-527-82897-5
Preface
Dear reader,
Welcome to the world of mechanically interlocked materials! You are about to discover the way that one of the most interesting tools of chemistry, the mechanical bond, is influencing the world of materials science.
Synthetic chemists started wondering about the possibility of connecting molecular fragments mechanically, like the links on a chain, very early on. The first monograph on the subject was written by Schill more than 50 years ago [1] and mentions that the earliest known discussion of such compounds, attributed to Willstätter, dates back somewhere between 1900 and 1912. A [2]catenane (two interlocked rings) was first intentionally made by Wasserman using statistical approaches in 1960 [2, 3], followed seven years later by the synthesis of a rotaxane by Harrison and Harrison [4]. In the meantime, Schill had started exploring a covalent template approach and succeeded in making catenanes in 1964 [5, 6]. The advent of noncovalent templated synthetic methods, where weak intermolecular interactions are used to place functional groups in the right positions, allowed chemists to make interlocked molecules in meaningful yields. This approach was pioneered in the early 1980s by Jean-Pierre Sauvage, with the use of metal–ligand interactions [7, 8] and later expanded to π–π and charge–transfer interactions by Sir J. Fraser Stoddart [9] and to H-bonding by David Leigh [10, 11]. At approximately the same time, Fujita used dynamic metal–ligand interactions to promote the formation of a [2]catenane from two separate macrocycles [12]. These are some of the most prominent milestones and groups that enabled the synthesis of mechanically interlocked molecules (MIMs), and, therefore, the study of the unique features of the mechanical bond. We now know that the mechanical bond is strong (it cannot be taken apart without breaking covalent bonds within the components), but it is also respectful of the structural integrity of each part of the ensemble, combining the best of covalent and supramolecular chemistries [13]. But the star of the show is perhaps the dynamic nature of the mechanical bond: the components of MIMs can move with respect to each other. This has allowed the construction of fascinating synthetic molecular machines, which account for two-thirds of the 2016 Nobel Prize in Chemistry awarded to Stoddart [14] and Sauvage [15] with the work of Ben Feringa on molecular machines that move around covalent linkages claiming the remaining third [16].
While the chemistry of MIMs is a relatively recent endeavor, materials science is one of the oldest and most decisive of human interests: materials have literally defined ages and civilizations. This is so because making tools and shaping objects to perform a specific function is perhaps the key defining characteristic of humans [17] – although several other animals can use tools, their manufacturing abilities are typically much more limited [18]. Of course, the function of the tool is directly related to the material it is made of, which makes materials the cornerstone of civilization [19]. For example, the three-age system that divides human prehistory into Stone Age, Bronze Age, and Iron Age is attributed to Christian Jürgensen Thomsen c. 1836 and is still widely accepted [20]. The history of steel is a most fascinating one, including its serendipitous discovery (most likely iron was left for too long in coal ovens), the famous Damascus sabres that are often cited as one of the earliest examples of nanotechnology [21], and its key role in the Second Industrial Revolution, after mass production of steel was made possible by the Bessemer and the Siemens-Martins processes [22]. The invention of modern (Portland) cement by Joseph Aspdin [23] re-shaped the world to such an extent that its production has been traditionally used as one of the most reliable indicators of economic development [24]. It is such a central material that its eco-friendly use is currently a focal point in the battle against climate change [25]. Introduced by Alexander Parkes at the London International Exhibition in 1862, Parkesine is the first known example of a man-made plastic [26]. However, it is Baekeland’s bakelite that inaugurated the field of synthetic polymers [27], that would be officially born with Staudinger’s famous 1920 article “Über polymerisation” [28]. The influence that synthetic polymers have had on modern life cannot be overstated. On the positive side, they have increased our quality of life tremendously. Their role as enablers of other technologies is unmatched by any other family of materials. Hi-tech or low-tech, everything and anything is made of (or contains a bit of) plastic: the keyboard I am writing on, the screen you are looking at, the helmet I wear when I ride my bike, the plane that you flew in for your last holiday, the clothes you are wearing, or the wrapping of the frozen pizza you plan to have for dinner… just look around! [29]. This super-intensive use of plastics also has its dark side, of course, because synthetic polymers are typically not biodegradable, and pollution by (micro)plastics has become a major concern, that will require an entirely new approach to the way we make and use polymers [30]. And we still have to talk about semiconductors and the way that electronics have changed our way of living! The invention of the transistor by Shockley, Bardeen and Brattain [31] was recognized with the Nobel Prize in Physics in 1956. The development of electronics has indeed been mostly a physics endeavor, but one where the purity of silicon (and other semiconductors) has played a major role [32]. Once more, materials enable technology.
As the chemistry of the mechanical bond progressed, it was soon apparent that its characteristics made it a very interesting tool for materials chemistry. For example, strength and adaptability are often sought after together, but far from easy to achieve within the same material. In principle, the dynamic nature of the mechanical bond makes it a good candidate for adaptable materials, while its strength is typically comparable to that of a covalent bond. But there are many other opportunities, as you will see!
Each chapter of the book is completely self-standing. Our contributors have done an excellent job of putting their subject into perspective, summarizing the main advances, and providing a far-reaching outlook for their specific field. So, if you are only (or particularly) interested in a certain subject, by all means, dive into the corresponding chapter directly. If you prefer to get a global view of the subject, we have tried to help you by organizing the book in the following way:
The first section of the book is dedicated to understanding at the fundamental level how the dynamic character of MIMs can contribute to the dynamic and mechanical properties of materials. To understand, we need to measure in detail how individual MIMs respond under mechanical stress. We will begin with a chapter on single-molecule force spectroscopy of MIMs by James Ormson, Anne-Sophie Duwez, and Guillaume De Bo. Next, we start moving from the purely molecular to the materials world step by step. We will first see how MIMs behave on very small and essentially 0D materials. Euan Kay will explain how to interface MIMs and nanoparticles and the opportunities that arise from the combination of these two very hot research topics. The following contribution is by Alejandro López-Moreno and myself, and will take us to 1D materials. It is focused on the making and studying of mechanically interlocked derivatives of single-walled carbon nanotubes. We will progress one step further to the second dimension by seeing how molecular motors behave when they are supported on surfaces in a chapter prepared by Monika Schied. Finally, we complete this first stage of our journey with the third dimension, in the chapter prepared by Benjamin Wilson and Steve Loeb. They write about how to organized MIMs and study their motion within 3D porous crystalline solids.
The next block is dedicated to biopolymers featuring mechanical bonds. Proteins are the paradigmatic functional biomolecule. We will start with a chapter on mechanically interlocked proteins by Yu-Xiang Wang, Wen-Hao Wu, and Wen-Bin Zhang. The authors will take you through both natural and synthetic peptides and proteins featuring mechanical bonds. You might be surprised to learn that up to 6% of the structures deposited in the Protein Data Bank contain some form of interlocking! The following chapter is by Yinzhou Ma, Ze Yu, and Julián Valero, who will give us an overview of DNA-based mechanically interlocked structures. Besides being the information material of the cell, DNA is one of the most versatile building blocks for nanotechnology, so get ready to be amazed with intricate structures and examples of advanced DNA-based molecular machines.
We will finish the book on a high note with two chapters on purely synthetic mechanically interlocked materials. First, we will learn about oligo- and polycatenanes in the chapter contributed by Sougata Datta, Atsushi Isobe, and Shiki Yagai. These were perhaps the structures that started it all, as the conceptual move from two interlocked rings to a full chain of them was immediate. Not so with the synthesis, though. Making and characterizing polycatenanes is a grand challenge, and you will see how different groups have tackled and succeeded at this problem. The final chapter of the book is on mechanically interlocked polymers and is authored by Leonie Braks and Ali Coskun. Among these structures are some of the most advanced mechanically interlocked materials that have already demonstrated significant improvements over traditional structures. Accordingly, it includes specific sections on potential applications in fields from composites to biomedicine to batteries.
I hope you find the complete journey instructive and that you will be amazed by some of the advances we have already achieved. I hope that you will learn plenty; I certainly have. But, more than anything, I hope you will find this book inspiring, and that reading what has been done will encourage you to set your gaze on all that is yet to come. And if you choose to set your sail towards that Ithaka, I hope that your road is a long one, full of adventure, full of discovery.
I want to conclude this preface by sincerely thanking all authors of the chapters of this book. It has been a long journey for us, too! Thank you very much for your contributions. I am sure you will forgive the many emails, the constantly moving deadlines, and the many delays… and that you will agree with me that the final result was worth the effort.
Madrid, SpainFebruary 2024
Emilio M. Pérez
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1Force-Promoted Transformations in Mechanically Linked Molecules
James Ormson1, Anne-Sophie Duwez2, and Guillaume De Bo1
1University of Manchester, Department of Chemistry, Oxford Road, Manchester M13 9PL, UK
2University of Liège, Molecular Systems, Department of Chemistry, Campus of Sart Tilman, Allee du Six aout, Liège 4000, Belgium
1.1 Introduction
Mechanical bonds present useful dynamic properties that have been instrumental in the development of molecular machines and smart materials, as their subcomponents can undergo large internal displacement (e.g. shuttling of a macrocycle along the axle of a rotaxane) [1]. When coupled with a directional elongation force, such a large amplitude movement can trigger a useful mechanochemical transformation [2]. As an external force is applied, molecules respond by bond rotation and bending before cleavage of covalent bonds occurs at high force. Mechanical bonds follow a similar two-stage response to external force. If the macrocycle of a mechanical bond is forced away from its equilibrium position, the molecule will go through a stage of deformation (Figure 1.1, dashed arrows) before the eventual cleavage of a covalent bond (Figure 1.1, plain arrows). An alternative non-covalent dissociation pathway is accessible if the macrocycle can pass over the stopper of a rotaxane (dethreading).
Different experimental techniques can be used to study the two force ranges involved in the application of force to mechanically interlocked molecules. Single-molecule force spectroscopy (SMFS) has been used to probe non-covalent (e.g. elongation, dethreading) and dynamic (e.g. shuttling) processes in interlocked molecules. These phenomena typically occur in the pN regime, and SMFS provides quantitative information on the force and elongation distance involved. Ultrasound activation generates forces in the nN regime where the scission of covalent bonds occurs. This technique has been used to activate mechanically interlocked molecules and elucidate their mechanism of dissociation under tension. This chapter will cover the recent advances in the study of mechanical bonds under external force moving from low force (SMFS, Section 1.2) to high force (ultrasonication in solution, Section 1.3) processes. The final section presents the recent developments in the use of interlocked molecules in mechanoresponsive materials (Section 1.4).
Figure 1.1 Response of catenanes and rotaxanes to external force.
1.2 SMFS in the Study of Non-covalent Interactions
SMFS consists of trapping and stretching a molecule between an atomic force microscopy (AFM) tip and a surface, or in a laser trap or between magnetic beads, to probe molecular processes in situ and real-time through the application of mechanical forces. For more than two decades, SMFS has proved efficacious in deciphering mechanistic information of individual biomolecules and in quantifying their force response to external stress. Such experiments have provided unprecedented insights into the structure and function of many biological systems, including DNA, proteins, enzymes, and biomolecular machines [3–15]. In addition to its widespread use with biological macromolecules, the technique has been adapted for the study of intra-molecular processes in synthetic macromolecules, including polymers [16], mechanophores [17–20], and interlocked systems [21, 22]. However, only a few investigations on intra-molecular processes and single-molecule mechanics have been realized on small molecules (1–5 nm), with successful examples including molecular recognition pairing [23], helical structures [24], and artificial molecular machines prototypes [25–30]. The rarity of such studies stems from the difficulty in developing proper tools and preparing appropriate molecules that can be interfaced with SMFS techniques, especially when one wants to probe sub-molecular motions.
1.2.1 Rotaxanes
Stoddart, Houk, Ho, and coworkers have pulled the macrocycle over the bulky end groups of a redox-active rotaxane with an AFM tip [31]. They compared the large forces required to de-thread oxidized and unoxidized molecules. They have used the switchable, bistable, [2]rotaxane R4+ shown in Figure 1.2, whose mechanism is based on the redox-controlled mechanical shuttling of the macrocycle along its axis. The molecule was specifically designed with a short thioctic acid linker tethered to the ring for tight attachment to a gold AFM tip and with a hydroxymethyl group on one of the stoppers for attachment to SiO2 substrates via covalently bound monolayers of isocyanatopropyl linkers. They used force spectroscopy to probe the steric and electrostatic interactions present in the ground and oxidized states of R4+ (Figure 1.2). Control measurements consisted of probing molecules to measure the repulsive steric interactions between the cyclobis(paraquat-p-phenylene) (CBPQT4+) ring and the diisopropylphenyl ether stopper. The probing of the R6+ molecules in an oxidizing solution was used to measure the repulsive force between the CBPQT4+ ring and an oxidized tetrathiafulvalene (TTF2+), which is responsible for molecular actuation. On the basis of the differences in the force required to pass the ring over the stopper in the presence and absence of the Fe(ClO4)3 oxidizing agent, they estimated the repulsive electrostatic barrier, which is ultimately responsible for the molecule’s actuation.
Figure 1.2 Chemical structure and schematic representation of the AFM force spectroscopy experiment of the bistable [2]rotaxane R4+, in which an electron-poor cyclobis(paraquat-p-phenylene) (CBPQT4+) ring is confined to an axis containing two electron-rich recognition sites, tetrathiafulvalene (TTF) and 1,5-dioxynaphthalene (DNP), by the presence of bulky 2,6-diisopropylphenyl ether stoppers at each end. The stopper close to the DNP site carries a hydroxymethyl group for subsequent attachment to silicon wafers. (a) The CBPQT4+ ring, which carries a very short tether terminated by a thioctic acid ester for attachment to a gold-coated AFM tip, displays a stronger interaction with TTF than with DNP and thus resides selectively on the former. (b) Chemical oxidation of TTF to TTF2+ results in a strong charge–charge repulsion between the CBPQT4+ ring and TTF2+, a situation that causes the CBPQT4+ ring to shuttle to DNP in the oxidized [2]rotaxane R6+.
Source: Brough et al. [31] / National Academy of Science.
By using the measured force spectroscopy values of 66 pN for the dethreading barrier under nonoxidative conditions and 145 pN for the oxidant-induced electrostatic repulsion barrier, along with the distance of 8.6 Å of the energy barrier produced from molecular simulations for the dethreading of the CBPQT4+ ring over the bulky stopper, and 13.0 Å as the distance necessary for the CBPQT4+ ring to travel from the 1,5-dioxynaphthalene (DNP) recognition site to the TTF2+ dication, the difference in interaction energies can be calculated as 19 kcal mol−1. By combining the 19 kcal mol−1 difference in oxidative and nonoxidative interaction energies with the theoretically determined value of 46 kcal mol−1 for the energy associated with ground-state dethreading, a final value of 65 kcal mol−1 was obtained, representing the amount of repulsive actuation energy between CBPQT4+ and TTF2+ produced by the oxidation of R4+. As estimated by this approach, 65 kcal mol−1 represents the upper limit of the total actuation energy.
Unfortunately, the irreversible rupture of molecules due to the dethreading required by this experiment prohibits the realization of a relaxing step to complete the cycle required to directly probe the mechanical work generated against the external load and to verify the estimation of 65 kcal mol−1.
In 2011, Duwez, Leigh, and coworkers succeeded in detecting sub-molecular movements in a hydrogen-bonded rotaxane and in directly measuring the work generated by this molecule against a load [25]. They designed a hydrogen-bonded rotaxane with a poly(ethylene oxide) (PEO) tether attached to the ring to track its motion by an AFM cantilever (Figure 1.3). The [2]rotaxane consists of a ring mechanically locked onto a thread by two bulky groups situated at either end of the axle. The thread bears a fumaramide and a succinic amide-ester site, each of which can bind to the macrocycle through four intercomponent hydrogen bonds [32]. The ring predominantly resides over the fumaramide site (green in Figure 1.3), the occupancy ratio being higher than 95 : 5, compared to the succinimide ester (orange in Figure 1.3). A chain, suitable for binding to an AFM tip, was attached to the macrocycle. Close to the fumaramide binding site, a 1,2-dithiolane ring was introduced to enable the grafting of the rotaxane onto a gold surface.
Figure 1.3 (a) Schematics of the single-molecule force spectroscopy experiment of the rotaxane. The rotaxane is grafted onto gold, and the PEO tether is caught by the AFM tip. (b) Chemical structure of the rotaxane molecule. The rotaxane consists of a benzylic amide molecular ring (in blue) mechanically locked onto an axle by bulky diphenylethyl ester groups situated at either end. The axle bears a fumaramide group (in green) and a succinic amide-ester group (in orange), either of which can act as a binding site for the ring through up to four intercomponent hydrogen bonds. The affinity of the ring for the fumaramide site is much higher than for the succinic amide-ester site so the fumaramide: succinic amide-ester occupancy ratio is higher than 95 : 5. Next to the fumaramide binding site, a disulfide group (in red) was introduced to enable the grafting of the molecule onto gold substrates. A PEO tether (in blue) is attached to the ring in order to link the molecule to the AFM probe and track the motion of the ring along the axle.
Source: Lussis et al. [25] / Springer Nature.
They used the cantilever of an AFM microscope to catch the tether, then applied a mechanical load to the ring of the rotaxane and followed its movement. The caught molecules were stretched in a controlled manner by moving the tip away from the substrate at a fixed pulling rate, and the force–extension profiles were measured. Thanks to the long tether, the detachment of the macrocycle from its station is directly observable in the stretching profile. Indeed, the progressive stretching of the PEO tether gives the characteristic parabolic profile of a random coil, which loses entropy on stretching, inducing a restoring force. Once the force exerted on the tether exceeds the strength of the hydrogen bonds between the macrocycle and the fumaramide station, the hydrogen bonds break, which results in the appearance of a small peak in the main force profile. This peak appears at about 27 pN in dimethylformamide and 45 pN in tetrachloroethane (TCE), reflecting the relative strengths of solute–solute hydrogen bonding in the two solvents. They fitted the pulling profiles with the worm-like chain model (an entropic elasticity model that predicts the relationship between the extension of a linear polymer and the entropic restoring force). The fits of the profile before and after the rupture peak give the increase in length (ΔLc) of the molecule after the rupture of the inter-component hydrogen bonds of about 4 nm, which is consistent with the theoretical length of the fully extended thread between the two binding sites.
Pulling–relaxing cycles were also recorded, and they observed a peak, which is the signature of force acting on the tether. This shows that the macrocycle has traveled back from the succinic amide-ester site to the fumaramide site and that it is able to generate force when rebinding against the external load exerted by the cantilever. It means that if we trap the ring and then slightly decrease the force on the tether while maintaining the pulling, the ring is capable of pulling more strongly to recover its preferred position. In TCE, the macrocycle is able to travel back against an external load of 30 pN. The mechanical work produced by this sub-molecular motion is about 6 kcal mol−1.
They also used the Crooks fluctuation theorem to estimate the free energy driving the macrocycle to bind to the fumaramide binding site at zero force. The work done on/by the molecules during pulling/relaxing is given by the areas below the corresponding force–extension curves. The rupture and rebinding work distributions overlap over a large range of work values and cross at a value of the work ΔG of 9.3 ± 2.3 kBT = 5.5 ± 1.3 kcal mol−1. The work value for a loading rate of 500 pN s−1 is close to this binding-energy difference between the two hydrogen-bonding motifs, indicating that under these conditions, the rotaxane is able to make use of almost all the energy available from hydrogen bonding to perform work along the direction of the applied load.
This example was the first quantitative measurement of the work done by a single synthetic molecular machine. It shows that the biased Brownian motions caused by thermal energy can be harnessed within a single synthetic small molecule to generate significant directional forces of similar magnitude to those generated by natural molecular machines, which are a lot bigger.
More recently, they detected the real-time fluctuations in the force experiments on these rotaxanes and revealed an intermediate weak binding state during shuttling[30]. They mechanically triggered the translocation of the ring between the two principal binding sites on the axle. Equilibrium fluctuations reveal another interacting site involving the two oxygen atoms in the middle of the thread. They characterized the ring occupancy distribution over time, which confirms the intermediate in both shuttling directions. The study provides evidence of weak hydrogen bonds that are difficult to detect using other methods and shows how the composition of the thread can significantly influence the shuttling dynamics by slowing down the ring motion between the principal binding sites. More generally, the study illustrates the utility that single-molecule experiments, such as force spectroscopy, can offer for elucidating the structure and dynamics of synthetic molecular machines.
Ibarra and coworkers studied the dynamics of a related rotaxane, modified to contain a very long thread so that the molecule could be investigated using optical tweezers in an aqueous environment (Figure 1.4) [27]. Force-clamp experiments under low external forces were made possible due to the high stability of the trapping, which allowed them to measure the dynamics of the rotaxane in an aqueous medium. They observed a lower rupture force of 8.5 pN for the breaking of hydrogen bonds between the macrocycle and the fumaramide station, in agreement with the relative strengths of hydrogen bonding in water and organic solvents, intercomponent hydrogen bonds being stronger in less polar solvents. Hopping events were also evidenced between the fumaramide and succinic amide-ester stations, but no intermediate states were observed. Since these experiments were performed in aqueous conditions, we suggest that the formation of weak hydrogen bonds between the macrocycle and the polyether thread would be much less favorable, and so, under those experimental conditions, the intermediate states may not be significant. Furthermore, the detection of such close intermediate states is difficult, given the spatial resolution of optical tweezers.
Their experimental set-up allowed very stable pulling–relaxing experiments to probe real-time shuttling events. Under these conditions, hundreds of shuttling events of the macrocycle between the two stations were monitored in real-time during minutes. From these data, they obtained crucial information about the dynamics of shuttling and a detailed picture of the energy landscape at different forces.
In 2018, Duwez, Sluysmans, Stoddart, and coworkers investigated the mechanochemical properties of donor–acceptor oligorotaxane foldamers, in which interactions between the mechanically interlocked component parts dictate the single-molecule assembly into a folded secondary structure (Figure 1.5) [28].
They used AFM-based SMFS to mechanically unfold molecules made of oligomeric dumbbells incorporating DNP units encircled by CBPQT4+ rings. Real-time capture of fluctuations between unfolded and folded states revealed that the molecules exert forces of up to 50 pN against a mechanical load of up to 150 pN and displayed transition times of less than 10 μs. They showed that the folding is at least as fast as that observed in proteins and remarkably more robust, thanks to the mechanically interlocked structure.
The mechanical breaking of the donor–acceptor interactions responsible for the folded structure was measured over a broad range of loading rates [29]. The results showed a high constant rupture force over a leading rate covering three orders of magnitude. In comparison with dynamic force spectroscopy performed during the past 20 years on various (bio)molecules, the near-equilibrium regime of oligorotaxanes persists at much higher loading rates, at which biomolecules have reached their kinetic regime, illustrating the high-speed dynamics and remarkable rebinding capabilities of the intramolecular donor–acceptor interactions. The authors investigated single interactions at a time in pulling–relaxing cycles and evidenced the near-equilibrium stochastic behavior. Indeed, even if the pulling experiments are performed near thermodynamic equilibrium, small molecules are always being submitted to random thermal fluctuations that constantly modify their unfolding and refolding paths. Using the Crooks fluctuation theorem, they measured the mechanical work produced during the breaking and rebinding to determine a free-energy difference, ΔG, of 6 kcal mol−1 between the two local conformations around a single bond.
Figure 1.4 (a) Schematics of the OT-based single-molecule force spectroscopy experiment of the rotaxane. The rotaxane is attached to two polystyrene beads (diameter ∼3 μm) through two dsDNA molecules: the 2686 bp dsDNA molecule connects the macrocycle to the bead via digoxigenin–antidigoxigenin (Dig–αDig) connections. The 830 bp dsDNA molecule connects the shuttle to a bead held by suction on a micropipette, via biotin–streptavidin connections at one end and Dig–αDig at the other end. Load is applied to the system by moving the optical trap away from the micropipette. At constant load, the shuttling dynamics of the macrocycle are inferred by measuring the motions of the bead. (b) Chemical structure of the rotaxane molecule modified to be trapped in a laser in an aqueous medium. The axle contains fumaramide (green) and succinic amide-ester (orange) stations separated by a very long oligoethyleneglycol spacer. A DNA oligonucleotide was covalently attached to the macrocycle to allow the manipulation in the optical trap.
Figure 1.5 (a) Schematics of the single-molecule force spectroscopy experiment of the oligorotaxane. The theoretical variation of length after breaking one interaction between a free DNP and the contiguous ring is about 1.2 nm, on the basis of crystallographic data. (b) Chemical structure of the oligorotaxane molecule made of oligomeric dumbbells incorporating 1,5-dioxynaphthalene units encircled by cyclobis(paraquat-p-phenylene) rings.
1.2.2 Poly-(pseudo)rotaxanes
In contrast to rotaxanes, pseudorotaxanes are kinetically labile as their macrocycle is able to dethread spontaneously (i.e. the stoppers are too small to retain the macrocycle). This ability has been studied experimentally [33–40] and computationally [33, 35, 41], and subsequent mechanochemical studies have provided information of the forces needed to overcome steric groups and distinguish rotaxanes from pseudrotaxanes. Cyclodextrins (CD) spontaneously form pseudorotaxanes by threading onto polyethyleneglycol (PEG) chains [42], and the dethreading behaviour of these systems has been studied by Round and co-workers in 2008 [43]. Polyrotaxanes of varied lengths were probed by AFM (Figure 1.6). Pulling experiments at loading rate in the range of 7–12 nN s−1 indicate a rupture force around 114 pN with a rupture length corresponding to length of the PEG axle and the length of the tip tether. This observation, and the fact that the detachment of the of the polymer from the tip occurs at a higher force, suggests that the rotaxane disassembles with the dethreading of CD macrocycle over the bulky tricarboxylic acid benzoyl stopper.
The same authors have used a CD-based pseudorotaxane to measure the strength of a calcium-mediated eggbox junction (a cross-link between sequences of oligoguluronic acids [oligoGs]) [44]. After some equilibration time, an AFM tip was used to pick an α-CD in order to unzip the eggbox junction. The authors determined how much force is required to slide along a single strand of oligoGs. Slipping over one guluronic acid requires about 47 pN, while unzipping requires 58, 112, and 141 pN (at 6–20 nN s−1) for oligoGs containing 2, 4, and 8 Ca2+, respectively. The strength of the first cross-link depends on the number of subsequent cross-links, and inspection of the force–extension curves of the different oligomers show that the CD does not break one complex at a time but rather disrupts a sequence of four Ca2+ at once. The same group used a similar design measure the strength of a calcium-mediated eggbox junction (a cross-link between sequences of oligoguluronic acids [oligoGs]) [44]. After some equilibration time, an AFM tip was used to pick an α-CD to unzip the eggbox junction. They found that this macrocycle requires ∼47 pN to slip over one guluronic acid, while unzipping requires 58, 112, and 141 pN (at 6–20 nN s−1) for oligoGs containing 2, 4, and 8 Ca2+, respectively. Inspection of the force-extension curves showed that the first cross-link is influenced by the subsequent cross links causing 4 Ca2+ units to break in a single event.
Figure 1.6 Single-molecule force spectroscopy experiment of the deslipping behavior of α-CD over a tricarboxylic acid benzoyl stopper on a PEG track. The red arrow indicates the direction of force.
They also exploited the sliding and deslipping behavior of α-CD to map the position of specific groups along a polymer chain and called their approach “sliding contact force spectroscopy” [45].
1.2.3 Catenanes
In 2014, Duwez, Fustin, Leigh, and coworkers probed the mobility of the rings in a [2]catenane of the benzylic amide family [26]. They selected two catenanes that differ only by methylation of the amide groups and the corresponding ability to form inter-component hydrogen bonds (Figure 1.7). Catenane CatNH has secondary amide groups and can thus form hydrogen bonds between the two macrocycles, restricting their mobility. In catenane CatNMe, the amide groups are methylated, and the rings are relatively free to rotate. PEO chains were grafted at each side of the catenane for pulling with the AFM tip. The experiment consisted of performing approach-retraction cycles with the AFM tip on the molecules adsorbed on the substrate in both TCE and DMF. The persistence length of each molecule was extracted by fitting the force–extension curves with a worm-like chain model.
Figure 1.7 (a) Schematics of the single-molecule force spectroscopy experiment of the catenane. (b) Chemical structure of the [2]catenane of the benzylic amide family.
Catenane CatNMe, which is unable to form any internal hydrogen bonds, showed the same persistence length (Lp) in both solvents (0.45 nm). In contrast, catenane CatNH showed a large increase in Lp from DMF (0.5 nm) to TCE (1 nm), due to strong hydrogen bond interactions between the two macrocycles that transform the catenane in a long rigid segment. This result showed that SMFS is able to address the mobility of the rings of a single catenane unit, and demonstrates the impact that a single mechanical bond can have on the mechanical properties of polymers.
In 2019, Zhang, Huang, and coworkers investigated a poly[2]catenane made of the same catenane units of the benzylic amide family with inter-component hydrogen bonds (Figure 1.8) [22]. The catenane units are separated by short alkane tethers. When pulling on these polymers, they observed force–extension curves with a regular sawtooth pattern containing evenly spaced peaks. The contour length increments, 1.68 ± 0.12 nm, were in good agreement with the length change expected (1.5 nm) after the rupture of the inter-component hydrogen bonds from the theoretical modeling of the structures. They performed the pulling experiments at eight different pulling rates and determined the characteristic force rupture for the inter-component hydrogen bonds. They could observe both the equilibrium (below ∼1500 pN s−1) and kinetic regimes (above ∼1500 pN s−1). Kinetic and thermodynamic parameters of the dissociation were extracted from the data with the Bell–Evans model [46]. The spontaneous unbinding rate in the absence of external force was estimated at about 0.07 s−1, and the distance between the bound state and the transition state was estimated at about 2.82 Å. Since the near-equilibrium regime is probed, it would be interesting to re-analyze the data with the Friddle–Noy–De Yoreo model that consists of a nonlinear fit including both the near-equilibrium (loading rate-independent) and the kinetic regimes where the force is proportional to the logarithm of the loading rate [47]. Cycles of one pulling and one relaxing experiment were also realized and showed that the hydrogen bonds can reform between the macrocycles of the catenanes during the relaxing step. The rebinding force was determined at the different loading rates. The spontaneous rebinding rate was estimated at 3.6 × 105 s−1, and the distance between the unbinding state and the transition state was estimated at 3.51 Å, using the Bell–Evans model.
Figure 1.8 Schematics of the AFM-based single-molecule force spectroscopy experiment of a poly[2]catenane.
Janshoff, Marszalek, and coworkers designed an oligomer system made of interlocked calix[4]arenes dimers [21]. The dimers are held together by 16 hydrogen bridges, and the interlocking limits how far the dimers can be separated after the rupture of the H bonds (Figure 1.9). The design prevents the bond rupture from being irreversible and thus enables the study of the rebinding of the calix[4]arenes dimers. Oligomers were prepared by tethering several of these dimers together. The force–extension curves obtained by pulling the molecule displayed a regular sawtooth pattern indicative of the successive rupture of the calixarene dimers. The assignment of the peaks was unambiguous in light of their equidistance, which corresponds to the theoretical distance for the dimer separation. The strength and reversibility of the hydrogen bonds were measured at various loading rates, covering both the equilibrium regime and out-of-equilibrium regime, corresponding to pulling rates close to the spontaneous bond dissociation rate or much higher than this rate, respectively. The results, together with stochastic modeling using a three-well potential under external load, allowed the reconstruction of the energy landscape of the system.
Figure 1.9 (a) Schematics of the AFM-based single-molecule force spectroscopy experiment of interlocked calix[4]arenes dimeric capsules. (b) Chemical structure of the capsules bound together by 16 hydrogen bonds.
1.3 Strength of Mechanical Bonds
In polymer mechanochemistry, polymer chains are used to activate mechanophores (force-sensitive molecules) in a way reminiscent of a tug of war. This can be achieved in solution with ultrasound-induced cavitation, where acoustic waves cause the nucleation, growth, and collapse of microbubbles. As bubbles collapse, a high gradient elongational flow is created in the surroundings, causing a nearby macromolecule to be stretched until covalent bond scission occurs in the central region of the chain [48]. This technique has been used to activate a variety of mechanophores [49], sometimes along unusual reaction pathways [50–52], prepare functional materials [53], carry out in situ catalysis [54, 55], or induce the release of small molecules [56]
