Introduction
CHAPTER MENU
- Technomimetics
- Examples of Technomimetic Molecules
- Manufacturing of Technomimetic Molecules
- Scope of the Book
1.1 Technomimetics
The term technomimetics was introduced about 20 years ago [1]. However, the origin of the idea to create individual molecules and molecular systems that can mimic conventional man-made devices in function, design, or mode of operations can be traced back to the late 1970s (e.g., molecular tweezers) [2]. Despite the early start, the major advances in this area were made only during the past two decades. Nowadays, this is one of the most prominent research areas, with hundreds of research papers published every year. The past decade also saw an increasing rate of practical applications, primarily in biomedical and material sciences fields, with a better recognition of technomimetic molecules as a distinct subclass of molecular devices [3]. However, the major practical impact of this technology is expected over several decades into the future. Construction and fine-tuning of complex functional technomimetic molecules remain the major unresolved problems.
Technomimetics represent a limited subclass of molecular devices and molecular machines. Molecular devices themselves constitute a more general family of individual molecules and molecular systems capable of providing valuable device-like functions. Many of them do have distinct conventional prototypes and therefore can be identified as technomimetic molecules. However, other molecular devices operate in a manner that is distinctively different or rare within the domain of conventional man-made devices and therefore do not fit within the technomimetics subclass [4].
The early set of technomimetic molecules includes simple molecular devices, such as container compounds (see also Chapter 2) [5, 6], gearing systems (see also Chapter 3) [7, 8], belts and tubes (see also Chapter 4) [9], and tweezers (see also Chapter 5) [10, 11]. Subsequent developments in the late 1990s and early 2000s added more complex technomimetic molecules, such as molecular brakes [12] and chemically driven motors [13]. The late 2000s and early 2010s saw an expansion of this set with advanced technomimetic molecules, such as molecular wheelbarrows [14], cars [15], and scissors [16]. Some examples of technomimetic molecules are presented in Section 1.2 for illustration purposes.
1.2 Examples of Technomimetic Molecules
Containers are perhaps the simplest conventional devices that can be easily constructed on a molecular scale. More importantly, molecular containers can closely mimic the functions of conventional containers by providing an isolated environment that can be filled with guest molecules or ions. Container compounds are attractive molecular carrier vehicles and already in high demand for biomedical applications. This area of research was active since the late 1980s and culminated with the synthesis of various carcerands and hemicarcerands prepared by simply joining two bowl-shaped cavitand units (Figure 1.1, see also Chapter 2) [5, 6].
Figure 1.1 The pyrogallol[4]arene-based molecular container compound and its conventional prototype (R?=?CH2CH2Ph, some hydrogen atoms and CH2Ph fragments were removed in the space-fill model for clarity) [17].
The inner space of these container compounds is described as "a novel reaction environment" [18] or "molecular reaction flask" [19] and can accommodate various molecular guests, including small organic molecules, reactive intermediates [20], and even fullerenes. Two examples of container compounds with benzaldehyde and fullerene C60 entrapped inside are presented in Figure 1.2.
Figure 1.2 X-ray structures of benzaldehyde (left) [21] and fullerene C60 (right) [22] guest molecules encapsulated in molecular containers (some hydrogen atoms and CH2CH2Ph fragments were removed for clarity).
Further progress in the container compound research field was achieved with the advent of endohedral fullerenes [23-26], characterized by the tightly meshed all-carbon network sidewalls. A major synthetic hurdle associated with the difficulty of filling up the empty internal cavities of fullerenes was eventually resolved with the implementation of modern ion implantation technology as well as "surgical" open-and-close synthetic methodology. An additional set of valuable endohedral metallofullerenes (Figure 1.3) was prepared by the optimization of the traditional arc discharge fullerene production process. The commercial importance of endohedral fullerenes is evident from the recent reports indicating that N@C60 is one of the most expensive (per gram) synthetic organic compounds ever made [27]. The high price is undoubtedly associated with a proposal to use this container compound in miniature atomic clock applications due to a very long electron phase coherence time of up to 250?µs at 170°K [28].
Figure 1.3 X-ray structures of endohedral metallofullerenes Sc2O@C80-C 2v (5) (left) [29], Sm3@C80-Ih (middle) [30], and Sc3NC@C80-Ih (right) [31].
Molecular gearing systems [1, 8] (Figure 1.4, see also Chapter 3) belong to an important class of technomimetic molecules, which clearly demonstrate the limits of conventional engineering within the molecular domain. Unlike conventional devices, molecular gearing systems are impossible to construct without gear slippage. However, the optimal molecular design allows for millions of correlated rotations between the gear slippage events. It is also possible to introduce various braking elements into molecular gearing systems, so the rotation can be controlled externally [32].
Figure 1.4 A simple bevel molecular gear, bis(triptycyl) ether, and its mechanical prototype [33].
Belts and tubes (see also Chapter 4) are perhaps the most abundant conventional construction elements and are widely used in modern technology, predominantly for the transportation of gases and liquids, and also as parts of various mechanical devices. Belt molecules were actually the earliest examples of technomimetics and were accidentally discovered more than a century ago [34], but the rational synthesis of molecular tubes with precise structures remains a challenge [9]. A recent example of a simple belt-like molecule, cyclo[4]fluorene, with distinctive green fluorescence, is presented in Figure 1.5 [35].
Figure 1.5 Cyclo[4]fluorene as a simple belt-like molecule (left), its X-ray structure (middle, n-C3H7 fragments were removed for clarity), and mechanical prototype (right).
Molecular tweezers [2] (see also Chapter 5) [10, 11] represent one of the most developed classes of technomimetics with potential applications ranging from advanced chemical sensors to novel biomedical agents [10, 11]. While simple molecular tweezers typically do not have precise pick-and-choose capabilities of conventional mechanical tweezers guided by human or artificial intelligence, the desired "intelligent" selectivity can be attained in more complex systems by the incorporation of multiple functional groups. A better application potential can be expected for dynamic molecular tweezers, which can be controlled allosterically by the presence of certain chemical species in the environment or responding to light, electrochemical, and mechanochemical stimuli. An example of molecular tweezers with porphyrin pincers is shown in Figure 1.6 [36].
Figure 1.6 Flexible molecular tweezers with porphyrin pincers (left) and X-ray structure of its complex with fullerene C60 (right, R and X fragments as well as hydrogen atoms were removed for clarity) [36].
Molecular scissors and pliers are elegant examples of externally controlled molecular devices, which undergo scissor-like transformation upon the application of certain stimuli, typically narrow-band electromagnetic radiation [16]. Unfortunately, the preponderance of current examples of molecular scissors can mimic their conventional mechanical prototypes only in the mode of action, not in the cutting function. It is, however, expected that these molecular devices can eventually be used to perform similar actions, such as chemical bond cutting. An early example of molecular scissors is presented in Figure 1.7 [37].
Figure 1.7 An early example of light-driven chiral molecular scissors.
Source: Adapted from Muraoka et al. 2003 [37]. Reproduced with permission of American Chemical Society.
Advanced versions of molecular pliers can mimic their conventional mechanical counterparts not only in the mode of action but also in the function. An example of molecular pliers that can be used to change the conformation of a heterocyclic guest molecule is presented in Figure 1.8...
