We report the incrementally staged design, synthesis, characterization, and operation of a molecular machine that behaves like a nanoscale elevator. The operation of this device, which is made of a platformlike component interlocked with a trifurcated riglike component and is only 3.5 nanometers by 2.5 nanometers in size, relies on the integration of several structural and functional molecular subunits. This molecular elevator is considerably more complex and better organized than previously reported artificial molecular machines. It exhibits a clear-cut on-off reversible behavior, and it could develop forces up to around 200 piconewtons.

Directed motion at the nanoscale is a central attribute of life, and chemically driven motor proteins are nature’s choice to accomplish it. Motivated and inspired by such bionanodevices, in the past few decades chemists have developed artificial prototypes of molecular motors, namely, multicomponent synthetic species that exhibit directionally controlled, stimuli-induced movements of their parts. In this context, photonic and redox stimuli represent highly appealing modes of activation, particularly from a technological viewpoint. Here we describe the evolution of the field of photo- and redox-driven artificial molecular motors, and we provide a comprehensive review of the work published in the past 5 years. After an analysis of the general principles that govern controlled and directed movement at the molecular scale, we describe the fundamental photochemical and redox processes that can enable its realization. The main classes of light- and redox-driven molecular motors are illustrated, with a particular focus on recent designs, and a thorough description of the functions performed by these kinds of devices according to literature reports is presented. Limitations, challenges, and future perspectives of the field are critically discussed.

Biomolecular motors convert energy into directed motion and operate away from thermal equilibrium. The development of dynamic chemical systems that exploit dissipative (non-equilibrium) processes is a challenge in supramolecular chemistry and a premise for the realization of artificial nanoscale motors. Here, we report the relative unidirectional transit of a non-symmetric molecular axle through a macrocycle powered solely by light. The molecular machine rectifies Brownian fluctuations by energy and information ratchet mechanisms and can repeat its working cycle under photostationary conditions. The system epitomizes the conceptual and practical elements forming the basis of autonomous light-powered directed motion with a minimalist molecular design.

Semiconductor nanocrystal quantum dots (QDs), owing to their unique opto-electronic properties determined by quantum confinement effects, have been the subject of extensive investigations in different areas of science and technology in the past two decades. The electrochemical behaviour of QDs, particularly for CdSe and CdTe nanocrystals, has also been explored, although to a lesser extent compared to the optical properties. Voltammetric measurements can be used to probe the redox levels available for the nanocrystals, which is an invaluable piece of information if these systems are involved in electron transfer processes. Electrochemical data can also foster the interpretation of the spectroscopic properties of QDs, and give insightful information on their chemical composition, dimension, and surface properties. Hence, electrochemical methods constitute in principle an effective tool to probe the quality of QD samples in terms of purity, size dispersion, and surface defects. The scope of this critical review is to discuss the results of electrochemical studies carried out on CdSe and CdTe core and core-shell semiconductor nanocrystals of spherical shape. Examples of emerging or potential applications that exploit electroactive quantum dot-based systems will also be illustrated.

The development of solid materials that can be reversibly interconverted by light between forms with different physico-chemical properties is of great interest for separation, catalysis, optoelectronics, holography, mechanical actuation and solar energy conversion. Here, we describe a series of shape-persistent azobenzene tetramers that form porous molecular crystals in their E-configuration, the porosity of which can be tuned by changing the peripheral substituents on the molecule. Efficient E→Z photoisomerization of the azobenzene units takes place in the solid state and converts the crystals into a non-porous amorphous melt phase. Crystallinity and porosity are restored upon Z→E isomerization promoted by visible light irradiation or heating. We demonstrate that the photoisomerization enables reversible on/off switching of optical properties such as birefringence as well as the capture of CO2 from the gas phase. The linear design, structural versatility and synthetic accessibility make this new family of materials potentially interesting for technological applications.

A calix[6]arene wheel, whose cavity has been extended and rigidified by N-phenylureido groups on the upper rim, forms pseudorotaxane species with molecular axles containing the viologen (4,4'-bipyridinium) unit in CH(2)Cl(2) solution. In these conditions, the self-assembly process is very efficient, with associated DeltaG degrees values of around -8 kcal mol(-1). The counteranions of the bipyridinium-based threads play indeed an important role in the formation of the complex. The use of either tosylate or hexafluorophosphate salts of the guests affects both the stability of the complexes and the rate of the threading process. Such effects have been interpreted in terms of ion-pair recognition, suggesting that coordination of the counteranions of the viologen thread by the ureido groups of the calixarene wheel is crucial for the breaking of tight ion pairs prior to threading. The rate constants of the threading/dethreading reactions coupled with the redox processes of the viologen unit of the axle have been obtained by means of cyclic voltammetry. The pseudorotaxane species undergo fast dethreading (submicrosecond time scale) on electrochemical reduction of the guest. The heterogeneous electron-transfer kinetics for the reduction of the viologen unit is slowed upon encapsulation into the calixarene cavity.

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Inspired by the concept of multivalency in living systems, two mechanically interlocked molecules have been conceived that incorporate not once or twice but thrice the features of a pH-switchable [2]rotaxane with two orthogonal recognition sites for dibenzo[24]crown-8 (DB24C8), and 2,3-dinaphtho[24]crown-8 (DN24C8)-one a dialkylammonium ion (CH(2)NH(2)(+)CH(2)) and the other a bipyridinium dication (BIPY(2+)). Whereas at low pH, the CH(2)NH(2)(+)CH(2) sites bind the DB24C8/DN24C8 macrocycles preferentially, at high pH, deprotonation occurs with loss of hydrogen bonding and the macrocycles will move to the BIPY(2+) sites, where they can acquire some stabilizing [pi-pi] stacking interactions. Such mechanically interlocked molecules have been assembled from a trifurcated rig-like component wherein the dumbbell-like components of three [2]rotaxanes have one of their ends fused onto alternate positions (1,3,5) around a benzenoid core. The rig is mechanically interlocked by a platform based on a tritopic receptor, wherein either three benzo[24]crown-8 or three 2,3-naphtho[24]crown-8 macrocycles are fused onto a hexaoxatriphenylene core. The synthesis of these molecular elevators involves 1:1 complexation, followed by stoppering, i.e., feet are added to the rig. (1)H NMR spectroscopy and cyclic voltammetry, aided and abetted by absorption spectroscopy, have been employed to unravel the details of the mechanism by which the rig and platform components move on the alternate addition of base and acid. For each molecular elevator, the platform operates by taking three distinct steps associated with each of the three deprotonation/reprotonation processes. Thus, molecular elevators are more reminiscent of a legged animal than they are of passengers on freight elevators.