The widespread use of molecular machines in biology has long suggested that great rewards could come from bridging the gap between synthetic molecular systems and the machines of the macroscopic world. In the last two decades, it has proved possible to design synthetic molecular systems with architectures where triggered large amplitude positional changes of submolecular components occur. Perhaps the best way to appreciate the technological potential of controlled molecular-level motion is to recognize that nanomotors and molecular-level machines lie at the heart of every significant biological process. Over billions of years of evolution, nature has not repeatedly chosen this solution for performing complex tasks without good reason. When mankind learns how to build artificial structures that can control and exploit molecular level motion and interface their effects directly with other molecular-level substructures and the outside world, it will potentially impact on every aspect of functional molecule and materials design. An improved understanding of physics and biology will surely follow.

The first steps on the long path to the invention of artificial molecular machines were arguably taken in 1827 when the Scottish botanist Robert Brown observed the haphazard motion of tiny particles under his microscope.1,2 The explanation for Brownian motion, that it is caused by bombardment of the particles by molecules as a consequence of the kinetic theory of matter, was later provided by Einstein, followed by experimental verification by Perrin.3,4 The random thermal motion of molecules and its implications for the laws of thermodynamics in turn inspired Gedankenexperiments (“thought experiments”) that explored the interplay (and apparent paradoxes) of Brownian motion and the Second Law of Thermodynamics. Richard Feynman’s famous 1959 lecture “There’s plenty of room at the bottom” outlined some of the promise that manmade molecular machines might hold.5,6 However, Feynman’s talk came at a time before chemists had the necessary synthetic and analytical tools to make molecular machines. While interest among synthetic chemists began to grow in the 1970s and 1980s, progress accelerated in the 1990s, particularly with the invention of methods to make mechanically interlocked molecular systems (catenanes and rotaxanes) and control and switch the relative positions of their components.7−24

Here, we review triggered large-amplitude motions in molecular structures and the changes in properties these can produce. We concentrate on conformational and configurational changes in wholly covalently bonded molecules and on catenanes and rotaxanes in which switching is brought about by various stimuli (light, electrochemistry, pH, heat, solvent polarity, cation or anion binding, allosteric effects, temperature, reversible covalent bond formation, etc.). Finally, we discuss the latest generations of sophisticated synthetic molecular machine systems in which the controlled motion of subcomponents is used to perform complex tasks, paving the way to applications and the realization of a new era of “molecular nanotechnology”.

1.1. The Language Used To Describe Molecular Machines
Terminology needs to be properly and appropriately defined and these meanings used consistently to effectively convey scientific concepts. Nowhere is the need for accurate scientific language more apparent than in the field of molecular machines. Much of the terminology used to describe molecular-level machines has its origins in observations made by biologists and physicists, and their findings and descriptions have often been misinterpreted and misunderstood by chemists. In 2007 we formalized definitions of some common terms used in the field (e.g., “machine”, “switch”, “motor”, “ratchet”, etc.) so that chemists could use them in a manner consistent with the meanings understood by biologists and physicists who study molecular-level machines.14

The word “machine” implies a mechanical movement that accomplishes a useful task. This Review concentrates on systems where a stimulus triggers the controlled, relatively large amplitude (or directional) motion of one molecular or submolecular component relative to another that can potentially result in a net task being performed. Molecular machines can be further categorized into various classes such as “motors” and “switches” whose behavior differs significantly.14 For example, in a rotaxane-based “switch”, the change in position of a macrocycle on the thread of the rotaxane influences the system only as a function of state. Returning the components of a molecular switch to their original position undoes any work done, and so a switch cannot be used repetitively and progressively to do work. A “motor”, on the other hand, influences a system as a function of trajectory, meaning that when the components of a molecular motor return to their original positions, for example, after a 360° directional rotation, any work that has been done is not undone unless the motor is subsequently rotated by 360° in the reverse direction. This difference in behavior is significant; no “switch-based” molecular machine can be used to progressively perform work in the way that biological motors can, such as those from the kinesin, myosin, and dynein superfamilies, unless the switch is part of a larger ratchet mechanism.14

Artificial Molecular Machines

Supramolecular Chirality in Self-Assembled Systems

Recent Developments in the Preparation and Chemistry of Metallacycles and Metallacages via Coordination

Materials fabrication with nanoscale structural precision based on bottom-up-type self-assembly has become more important in various current disciplines in chemistry including materials chemistry, ...

Layer-by-layer Nanoarchitectonics: Invention, Innovation, and Evolution

There is great need for stand-alone luminescence-based chemosensors that exemplify selectivity, sensitivity, and applicability and that overcome the challenges that arise from complex, real-world media. Discussed herein are recent developments toward these goals in the field of supramolecular luminescent chemosensors, including macrocycles, polymers, and nanomaterials. Specific focus is placed on the development of new macrocycle hosts since 2010, coupled with considerations of the underlying principles of supramolecular chemistry as well as analytes of interest and common luminophores. State-of-the-art developments in the fields of polymer and nanomaterial sensors are also examined, and some remaining unsolved challenges in the area of chemosensors are discussed.

/pdf/supramolecular-luminescent-sensors-1rs45r4um4.pdf

Supramolecular Luminescent Sensors

In the 25 years since its Nobel Prize in chemistry, supramolecular chemistry based on molecular recognition has been paid much attention in scientific and technological fields. Nanotechnology and the related areas seek breakthrough methods of nanofabrication based on rational organization through assembly of constituent molecules. Advanced biochemistry, medical applications, and environmental and energy technologies also depend on the importance of specific interactions between molecules. In those current fields, molecular recognition is now being re-evaluated. In this review, we re-examine current trends in molecular recognition from the viewpoint of the surrounding media, that is (i) the solution phase for development of basic science and molecular design advances; (ii) at nano/materials interfaces for emerging technologies and applications. The first section of this review includes molecular recognition frontiers, receptor design based on combinatorial approaches, organic capsule receptors, metallo-capsule receptors, helical receptors, dendrimer receptors, and the future design of receptor architectures. The following section summarizes topics related to molecular recognition at interfaces including fundamentals of molecular recognition, sensing and detection, structure formation, molecular machines, molecular recognition involving polymers and related materials, and molecular recognition processes in nanostructured materials.

Molecular recognition: from solution science to nano/materials technology.

Chirality sensing and size recognition of N-Boc-amino acids by cage-type dimeric lanthanide complexes: chirality detection of N-Boc-aspartate anions via luminescence colour change.

A series of quadruple-stranded Na(+) and Ca(2+) complexes with octadentate cyclen ligands was synthesized to produce complexes that contained four different side-arm combinations (one triazole-coumarin group and three pyridine groups (1), four pyridine groups (2), one triazole-coumarin group and three quinoline groups (3), and four quinoline groups (4)). X-ray crystallographic analysis revealed that no significant changes occurred in the stereostructure of these complexes upon replacing one pyridine group with a triazole-coumarin moiety, or by replacing Na(+) ions with Ca(2+) ions, although the coordination number of the complexes in the solid state decreased when pyridine groups were replaced by quinoline groups. In solution, all of the side arms were arranged in a propeller-like pattern to yield an enantiomer pair of Δ and Λ forms in each metal complex. The addition of a tert-butoxycarbonyl (Boc)-protected amino acid anion, that is, a coordinative chiral carboxylate anion, to the cyclen-Ca(2+) complex induced circular dichroism (CD) signals in the aromatic region by forming a 1:1 mixture of diastereomeric ternary complexes with opposite complex chirality, whilst the corresponding Na(+) complexes rarely showed any response. In complexes 1-Ca(2+) and 3-Ca(2+), this chirality-transfer process was efficiently followed by considering the induction of the CD signals at two different wavelengths, that is, the coumarin-chromophore region and the aza-aromatic region. The sign and intensity of the CD signal were significantly dependent on both the nature of the aza-aromatic moiety and the enantiomeric purity of the external anion. These Ca(2+) complexes worked as effective probes for the determination of the enantiomeric excess of the chiral anion. The cyclen-Ca(2+) complexes also interacted with the non-coordinative Δ-TRISPHAT anion through an ion-pairing mechanism to achieve chirality transfer from the anion to the metal complex; both complexes 1-Ca(2+) and 3-Ca(2+) clearly showed induced CD signals in the coumarin-chromophore region, owing to ion-paring interactions with the Δ-TRISPHAT anion. Thus, the proper combination of an octadentate cyclen ligand and a metal center demonstrated effective chirality transfer.

Chirality Transfer in Propeller-Shaped Cyclen ? Calcium(II) Complexes: Metal-Coordinating and Ion-Pairing Anion Procedures

A chirality rewriting cycle mediated by a dynamic cyclen–calcium complex

Two macrotricyclic ligands composed of two face-to-face octadentate metal chelates were synthesized. These cage-shaped disodium complexes had special recognition ability for various counter anions. Specific chiral dicarboxylates bound to the complexes within the cavity and exhibited chirality induction properties. For instance, N-Boc-Asp dianion strongly induced circular dichroism (CD) signals, but N-Boc-Glu dianion, which is one carbon longer, did not.

https://chemistry-europe.onlinelibrary.wiley.com/doi/pdf/10.1002/open.201402049

Chirality Sensing and Size Discrimination of Anions by Macrotricyclic Cyclen–Disodium Complexes

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