The widespread use of controlled molecular-level motion in key natural processes suggests that great rewards could come from bridging the gap between the present generation of synthetic molecular systems, which by and large rely upon electronic and chemical effects to carry out their functions, and the machines of the macroscopic world, which utilize the synchronized movements of smaller parts to perform specific tasks. This is a scientific area of great contemporary interest and extraordinary recent growth, yet the notion of molecular-level machines dates back to a time when the ideas surrounding the statistical nature of matter and the laws of thermodynamics were first being formulated. Here we outline the exciting successes in taming molecular-level movement thus far, the underlying principles that all experimental designs must follow, and the early progress made towards utilizing synthetic molecular structures to perform tasks using mechanical motion. We also highlight some of the issues and challenges that still need to be overcome.

Synthetic molecular motors and mechanical machines.

The miniaturization of components used in the construction of working devices is being pursued currently by the large-downward (top-down) fabrication. This approach, however, which obliges solid-state physicists and electronic engineers to manipulate progressively smaller and smaller pieces of matter, has its intrinsic limitations. An alternative approach is a small-upward (bottom-up) one, starting from the smallest compositions of matter that have distinct shapes and unique properties-namely molecules. In the context of this particular challenge, chemists have been extending the concept of a macroscopic machine to the molecular level. A molecular-level machine can be defined as an assembly of a distinct number of molecular components that are designed to perform machinelike movements (output) as a result of an appropriate external stimulation (input). In common with their macroscopic counterparts, a molecular machine is characterized by 1) the kind of energy input supplied to make it work, 2) the nature of the movements of its component parts, 3) the way in which its operation can be monitored and controlled, 4) the ability to make it repeat its operation in a cyclic fashion, 5) the timescale needed to complete a full cycle of movements, and 6) the purpose of its operation. Undoubtedly, the best energy inputs to make molecular machines work are photons or electrons. Indeed, with appropriately chosen photochemically and electrochemically driven reactions, it is possible to design and synthesize molecular machines that do work. Moreover, the dramatic increase in our fundamental understanding of self-assembly and self-organizational processes in chemical synthesis has aided and abetted the construction of artificial molecular machines through the development of new methods of noncovalent synthesis and the emergence of supramolecular assistance to covalent synthesis as a uniquely powerful synthetic tool. The aim of this review is to present a unified view of the field of molecular machines by focusing on past achievements, present limitations, and future perspectives. After analyzing a few important examples of natural molecular machines, the most significant developments in the field of artificial molecular machines are highlighted. The systems reviewed include 1) chemical rotors, 2) photochemically and electrochemically induced molecular (conformational) rearrangements, and 3) chemically, photochemically, and electrochemically controllable (co-conformational) motions in interlocked molecules (catenanes and rotaxanes), as well as in coordination and supramolecular complexes, including pseudorotaxanes. Artificial molecular machines based on biomolecules and interfacing artificial molecular machines with surfaces and solid supports are amongst some of the cutting-edge topics featured in this review. The extension of the concept of a machine to the molecular level is of interest not only for the sake of basic research, but also for the growth of nanoscience and the subsequent development of nanotechnology.

Artificial Molecular Machines.

Upconversion luminescent materials: advances and applications.

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

The review covers the knowledge on photoremovable protecting
groups and includes all relevant chromophores studied in the
time period of 2000–2012; the most relevant earlier works are
also discussed.

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Photoremovable Protecting Groups in Chemistry and Biology: Reaction Mechanisms and Efficacy

Photophysics of Organic Molecules in Solution Introduction Electronic States Radiative Transitions Nonradiative Transitions Excited State Kinetics Acknowledgments Bibliography Transition Metal Complexes Electronic Structure Types of Excited States and Electronic Transitions Absorption and Emission Bands Jablonski Diagram Photochemical Reactivity Electrochemical Behavior Polynuclear Metal Complexes Photophysical Properties of Organic Compounds Photophysics of Organic Molecules in Solution Triplet-State Energies: Ordered Flash Photolysis: Designing Experiments Low-Temperature Photophysics of Organic Molecules Absorption and Luminescence Spectra of Organic Compounds ESR and ODMR Parameters of the Triplet State Photophysical Properties of Transition Metal Complexes Photophysical Parameters Absorption and Emission Spectra Abbreviations Rate Constants of Excited-State Quenching Ionization Energies, Electron Affinities, and Reduction Potentials Ionization Energies and Electron Affinities Reduction Potentials Bond Dissociation Energies Solvent Properties Donor Number Luminescence Spectroscopy Measurements Correction of Luminescence Intensity Measurements in Solution Fluorescence Quantum Yield Standards Phosphorescence Quantum Yield Standards Luminescence Lifetime Standards Light Sources and Filters Spectral Distribution of Photochemical Sources Transmission Characteristics of Light Filters and Glasses Chemical Actinometry Ferrioxalate Actinometer Photochromic Actinometers Reinecke's Salt Actinometer Uranyl Oxalate Actinometer Other Actinometers Miscellaneous Spin-Orbit Coupling Hammett ? Constants Fundamental Constants and Conversion Factors Index *References included in each chapter

Handbook of Photochemistry

With the fabrication of molecular electronic devices (MEDs) and the construction of nanoelectromechanical systems (NEMSs) as incentives, two constitutionally isomeric, redox-controllable [2]rotaxanes have been synthesizedand characterized in solution. Therein, they both behave as near-perfect molecular switches, that is, to all intents and purposes, these two rotaxanes can be switched precisely by applying appropriate redox stimuli between two distinct chemomechanical states. Their dumbbell-shaped components are composed of polyether chains interrupted along their lengths by i) two π-electron rich recognition sites-a tetrathiafulvalene (TTF) unit and a 1,5-dioxynaphthalene (DNP) moiety-with ii) a rigid terphenylene spacer placed between the two recognition sites, and then terminated by iii) a hydrophobic tetraarylmethane stopper at one end and a hydrophilic dendritic stopper at the other end of the dumbbells, thus conferring amphiphilicity upon these molecules. A template-directed protocol produces a means to introduce the tetracationic cyclophane, cyclobis(paraquat-p-phenylene) (CBPQT 4 + ), which contains two π-electron accepting bipyridinium units, mechanically interlocked around the dumbbell-shaped components. Both the TTF unit and the DNP moiety are potential stations for CBPQT 4 + , since they can establish charge-transfer and hydrogen bonding interactions with the bipyridinium units of the cyclophane, thereby introducing bistability into the [2]rotaxanes. In both constitutional isomers, 'H NMR and absorption spectroscopies, together with electrochemical investigations, reveal that the CBPQT 4 + ring is predominantly located on the TTF unit, leading to the existence of a single translational isomer (co-conformation) in both cases. In addition, a model [2]rotaxane, incorporating hydrophobic tetraarylmethane stoppers at both ends of its dumbbell-shaped component, has also been synthesized as a point of reference. Molecular synthetic approaches were used to construct convergently the dumbbell-shaped compounds by assembling progressively smaller building blocks in the shape of the rigid spacer, the TTF unit and the DNP moiety, and the hydrophobic and hydrophilic stoppers. The two amphiphilic bistable [2]rotaxanes are constitutional isomers in the sense that, in one constitution, the TTF unit is adjacent to the hydrophobic stopper, whereas in the other, it is next to the hydrophilic stopper. All three bistable [2]rotaxanes have been isolated as green solids. Electrospray and fast atom bombardment mass spectra support the gross structural assignments given to all three of these mechanically interlocked compounds. Their photophysical and electrochemical properties have been investigated in acetonitrile. The results obtained from these investigations confirm that, in all three [2]rotaxanes, i) the CBPQT 4 + cyclophane encircles the TTF unit, ii) the CBPQT 4 + cyclophane shuttles between the TTF and DNP stations upon electrochemical or chemical oxidation/reduction of the TTF unit, and iii) folded conformations are present in which the CBPQT 4 + cyclophane, while encircling the TTF unit, interacts through its π-accepting bipyridinium exteriors with other π-donating components of the dumbbells, especially those located within the stoppers.

Redox-Controllable Amphiphilic [2]Rotaxanes

Two molecular shuttles/ switches-a slow one and a fast one- in the shape of amphiphilic, bistable [2]rotaxanes have been synthesized and characterized. Both [2]rotaxanes contain a hydrophobic, tetraarylmethane and a hydrophilic, dendritic stopper. They are comprised of two π-electron-rich stations-a monopyrrolotetrathiafulvalene unit and a 1,5-dioxynaphthalene moiety-which can act as recognition sites for the tetracationic cyclophane, cyclobis(paraquat-p-phenylene), to reside around. In addition, a model [2]rotaxane, incorporating only a monopyrrolotetrathiafulvalene unit in the rod section of the amphiphilic dumbbell component and cyclobis(paraquat-p-phenylene) as the ring component, has been investigated. The dumbbell-shaped components were constructed using conventional synthetic methodologies to assemble 1) the hydrophobic, tetraarylmethane stopper and 2) the hydrophilic, dendritic stopper. Next, 3) the hydrophobic stopper was fused to the 1,5-dioxynaphthalene moiety and/or the monopyrrolotetrathiafulvalene unit by appropriate alkylations, followed by 4) attachment of the hydrophilic stopper, once again by alkylation to give the dumbbell-shaped compounds. Finally, 5) the [2]rotaxanes were self-assembled by using the dumbbells as templates for the formation of the encircling cyclobis(paraquat-p-phenylene) tetracations. The two [2]rotaxanes differ in their arrangement of the π-electron-rich units, one in which the SMe group of the monopyrrolotetrathiafulvalene unit points toward the 1,5-dioxynaphthalene moiety (2 . 4PF 6 ) and another in which it points away from the 1,5-dioxynaphthalene moiety (3 . 4PF 6 ). This seemingly small difference in the orientation of the monopyrrolotetrathiafulvalene unit leads to profound changes in the physical properties of these rotaxanes. The bistable [2]rotaxanes were both isolated as brown solids. 1 H NMR and UV-visible spectroscopy, and electrochemical investigations, reveal the presence of both possible translational isomers at ambient temperature. As a consequence of the existence of both possible translational isomers in these bistable [2]rotaxanes, they exhibit a complex electrochemical behavior, which is further complicated by the presence of folded conformations wherein the monopyrrolotetrathiafulvalene unit is involved in an "alongside" interaction with the tetracationic cyclophane. In the molecular shuttle/switch 2 . 4PF 6 a "knob", in the shape of the SMe group, is situated between the monopyrrolotetrathiafulvalene and the 1,5-dioxynaphthalene recognition sites, making it possible to isolate both translational isomers (2 . 4 PF 6 . GREEN and 2 . 4 PF 6 . RED) and to investigate the kinetics of the shuttling of the cyclobis(paraquat-p-phenylene) tetracation between the two recognition sites. The shuttling processes, which are accompanied by clearly detectable color changes, can be followed by 'H NMR and UV-visible spectroscopy, allowing the rate constants and energies of activation for the translation of the cyclobis(paraquat-p-phenylene) tetracations between the two recognition sites to be determined. In the molecular shuttle/switch 3 . 4PF 6 , there is no "knob" situated between the 1,5-dioxynaphthalene and the monopyrrolotetrathiafulvalene recognition sites, resulting in a considerably faster shuttling of the cyclobis(paraquat-p-phenylene) tetracation between these two sites, making the separation of the two possible translational isomers of 3 . 4PF 6 impractical. However, the shuttling of the cyclobis(paraquat-p-phenylene) tetracation can be followed by dynamic 1 H NMR spectroscopy. At low temperatures, the major translational isomer is 3 . 4 PF 6 . RED, while 3 . 4 PF 6 . GREEN is the major isomer at higher temperature. In the bistable [2]rotaxanes shuttling of the cyclobis(paraquat-p-phenylene) tetracations can be driven by electrochemical oxidation of the monopyrrolotetrathiafulvalene unit. In complexes in which one of the two dumbbell stoppers is missing, electrochemical oxida

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Amphiphilic Bistable Rotaxanes

Selective Self‐Assembly and Acid–Base Controlled De‐/Rethreading of Pseudorotaxanes Constructed Using Multiple Recognition Motifs

Dynamic and static quenching of the luminescence of ruthenium(II) polypyridine complexes by heteropolytungstate anions. Direct measurements of intramolecular electron-transfer rate constants

M. Teresa Gandolfi

Papers

Handbook of Photochemistry

Redox-Controllable Amphiphilic [2]Rotaxanes

Amphiphilic Bistable Rotaxanes

Selective Self‐Assembly and Acid–Base Controlled De‐/Rethreading of Pseudorotaxanes Constructed Using Multiple Recognition Motifs

Dynamic and static quenching of the luminescence of ruthenium(II) polypyridine complexes by heteropolytungstate anions. Direct measurements of intramolecular electron-transfer rate constants