Photo- and Redox-Driven Artificial Molecular Motors

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

Reversible Photoswitching and Isomer-Dependent Diffusion of Single Azobenzene Tetramers on a Metal Surface

Azobenzene is a prototypical molecular switch that can be reversibly photoisomerized between the nearly planar and apolar trans form, and the distorted, polar cis form. Thus far, most studies related to azobenzene derivatives have focused on planar adsorbed molecules. We present here the study of a three-dimensional shape-persistent molecular architecture consisting of four tetrahedrally arranged azobenzene units and adsorbed on a Ag(111) surface. While the azobenzenes of the tripod in contact with the surface lose their switching ability, different isomers of the upright standing arm of the tetramer are obtained reversibly and efficiently by illumination at different wavelengths, revealing time constants of only a few minutes. Diffusion on the surface turns out to be dependent on the isomeric state - trans or cis - of the upright oriented azobenzene group. Hence, molecular mobility can be modulated via their isomeric state, which suggests that for instance molecular growth processes could be controlled by external stimuli

Light-powered autonomous and directional molecular motion of a dissipative self-assembling system

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 nonsymmetric 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

Stereodynamics of E/Z isomerization in rotaxanes through mechanical shuttling and covalent bond rotation

We report on a set of rotaxanes with symmetrical axles equipped with a central amide group that installs E/Z stereoisomerism owing to the ring position along the axle. Isomerization by concomitant rotation about the amide bond and ring shuttling along the axle was thoroughly characterized in different solvents. The results trigger a discussion on core concepts, such as microscopic reversibility and transition state theory, and provide insights for designing molecules capable to transform and transmit motion between subcomponents

Supramolecular assemblies of semiconductor quantum dots and a bis(bipyridinium) derivative : luminescence quenching and aggregation phenomena

We have synthesized CdSe and CdSe–ZnS core–shell luminescent nanocrystal quantum dots and studied their interaction with a ditopic bis(bipyridinium) compound in solution. The latter strongly quenches the luminescence of the quantum dots by a static mechanism, indicating that the nanocrystal and molecular components undergo association in the ground state. Photoexcitation of these inorganic–organic hybrids causes an electron-transfer process from the conduction band of the nanocrystal to the LUMO of the molecule. The ability of the bipyridinium-type species to trigger association of the quantum dots is evidenced by spectrofluorimetric titrations and DLS measurements in solution, and confirmed by TEM experiments on surfaces. The quantum dot–molecule complexes can be disassembled in solution by addition of a calixarene host capable of encapsulating the bipyridinium units of the molecular connector. Our results demonstrate that supramolecular chemistry offers convenient ways to control the aggregation of semiconductor nanocrystals, a crucial task for the generation of nanostructured arrays with well defined properties

Second-Generation Light-Fueled Supramolecular Pump

open7noFinancial support from the European Research Council (H2020 Advanced Grant 692981 to A.C.)We describe the modular design of a pseudorotaxane-based supramolecular pump and its photochemically driven autonomous nonequilibrium operation in a dissipative regime. These properties derive from careful engineering of the energy maxima and minima along the threading coordinate and their light-triggered modulation. Unlike its precursor, this second-generation system is amenable to functionalization for integration into more complex devices.openCanton, Martina; Groppi, Jessica; Casimiro, Lorenzo; Corra, Stefano; Baroncini, Massimo; Silvi, Serena; Credi, AlbertoCanton, Martina; Groppi, Jessica; Casimiro, Lorenzo; Corra, Stefano; Baroncini, Massimo; Silvi, Serena; Credi, Albert

Supramolecular assemblies of semiconductor quantum dots and a bis(bipyridinium) derivative: luminescence quenching and aggregation phenomena

We have synthesized CdSe and CdSe\u2013ZnS core\u2013shell luminescent nanocrystal quantum dots and studied their interaction with a ditopic bis(bipyridinium) compound in solution. The latter strongly quenches the luminescence of the quantum dots by a static mechanism, indicating that the nanocrystal and molecular components undergo association in the ground state. Photoexcitation of these inorganic\u2013organic hybrids causes an electron-transfer process from the conduction band of the nanocrystal to the LUMO of the molecule. The ability of the bipyridinium-type species to trigger association of the quantum dots is evidenced by spectrofluorimetric titrations and DLS measurements in solution, and confirmed by TEM experiments on surfaces. The quantum dot\u2013molecule complexes can be disassembled in solution by addition of a calixarene host capable of encapsulating the bipyridinium units of the molecular connector. Our results demonstrate that supramolecular chemistry offers convenient ways to control the aggregation of semiconductor nanocrystals, a crucial task for the generation of nanostructured arrays with well defined properties

Photoinduced Autonomous Nonequilibrium Operation of a Molecular Shuttle by Combined Isomerization and Proton Transfer Through a Catalytic Pathway

We describe a [2]rotaxane whose recognition sites for the ring are a dibenzylammonium moiety, endowed with acidic and H-bonding donor properties, and an imidazolium center bearing a photoactive phenylazo substituent. Light irradiation of this compound triggers a network of E/Z isomerization and proton transfer reactions that enable autonomous and reversible ring shuttling away from equilibrium

Solution and Solid-State Emission Toggling of a Photochromic Hydrazone

The proliferation of light-activated switches in recent years has enabled their use in a broad range of applications encompassing an array of research fields and disciplines. All current systems, however, have limitations (e.g., from complicated synthesis to incompatibility in biologically relevant media and lack of switching in the solid-state) that can stifle their real-life application. Here we report on a system that packs most, if not all, the desired, targeted and sought-after traits from photochromic compounds (bistability, switching in various media ranging from serum to solid-state, while exhibiting ON/OFF fluorescence emission switching, and two-photon assisted near-infrared light toggling) in an easily accessible structure

Chemical On/Off Switching of Mechanically Planar Chirality and Chiral Anion Recognition in a [2]Rotaxane Molecular Shuttle

We exploit a reversible acid-base triggered molecular shuttling process to switch an appropriately designed rotaxane between prochiral and mechanically planar chiral forms. The mechanically planar enantiomers and their interconversion, arising from ring shuttling, have been characterized by NMR spectroscopy. We also show that the supramolecular interaction of the positively charged rotaxane with optically active anions causes an imbalance in the population of the two enantiomeric co-conformations. This result represents an unprecedented example of chiral molecular recognition and can disclose innovative approaches to enantioselective sensing and catalysis