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...