Light-induced contraction and extension of single macromolecules on a modified graphite surface

Collective Molecular Machines: Multidimensionality and Reconfigurability

Molecular machines are key to cellular activity where they are involved in converting chemical and light energy into efficient mechanical work. During the last 60 years, designing molecular structures capable of generating unidirectional mechanical motion at the nanoscale has been the topic of intense research. Effective progress has been made, attributed to advances in various fields such as supramolecular chemistry, biology and nanotechnology, and informatics. However, individual molecular machines are only capable of producing nanometer work and generally have only a single functionality. In order to address these problems, collective behaviors realized by integrating several or more of these individual mechanical units in space and time have become a new paradigm. In this review, we comprehensively discuss recent developments in the collective behaviors of molecular machines. In particular, collective behavior is divided into two paradigms. One is the appropriate integration of molecular machines to efficiently amplify molecular motions and deformations to construct novel functional materials. The other is the construction of swarming modes at the supramolecular level to perform nanoscale or microscale operations. We discuss design strategies for both modes and focus on the modulation of features and properties. Subsequently, in order to address existing challenges, the idea of transferring experience gained in the field of micro/nano robotics is presented, offering prospects for future developments in the collective behavior of molecular machines.

Highly Cooperative Photoswitching in Dihydropyrene Dimers

We present a strategy to achieve highly cooperative photoswitching, where the initial switching event greatly facilitates subsequent switching of the neighboring unit. By linking donor/acceptor substituted dihydropyrenes via suitable π-conjugated bridges, the quantum yield of the second photochemical ring-opening process could be enhanced by more than two orders of magnitude as compared to the first ring-opening. As a result, the intermediate mixed switching state is not detected during photoisomerization although it is formed during the thermal back reaction. Comparing the switching behavior of various dimers, both experimentally and computationally, helped to unravel the crucial role of the bridging moiety connecting both photochromic units. The presented dihydropyrene dimer serves as model system for longer cooperative switching chains, which, in principle, should enable efficient and directional transfer of information along a molecularly defined path. Moreover, our concept allows to enhance the photosensitivity in oligomeric and polymeric systems and materials thereof.

Synthesis and Light-Induced Actuation of Photo-Labile 2-Pyridyl-1,2,3-Triazole Ru(bis-bipyridyl) Appended Ferrocene Rotors

To realise useful control over molecular motion in the future an extensive toolbox of both actionable molecules and stimuli-responsive units must be developed. Previously, our laboratory has reported 1,1'-disubstituted ferrocene (Fc) rotor units which assume a contracted/π-stacked conformation until complexation of cationic metal ions causes rotation about the Ferrocene (Fc) molecular 'ball-bearing'. Herein, we explore the potential of using the photochemical ejection of [Ru(2,2'-bipyridyl)₂]2+ units as a stimulus for the rotational contraction of new ferrocene rotor units. Fc rotors with both 'regular' and 'inverse' 2-pyridyl-1,2,3-triazole binding pockets and their corresponding [Ru(2,2'-bipyridyl)₂]2+ complexes were synthesised. The rotors and complexes were characterised using nuclear magnetic resonance (NMR) and ultraviolet (UV)-visible spectroscopies, Electro-Spray Ionisation Mass Spectrometry (ESI⁻MS), and electrochemistry. The 1,1'-disubstituted Fc ligands were shown to π-stack both in solution and solid state. Density Functional Theory (DFT) calculations (CAM-B3LYP/6-31G(d)) support the notion that complexation to [Ru(2,2'-bipyridyl)₂]2+ caused a rotation from the syn- to the anti-conformation. Upon photo-irradiation with UV light (254 nm), photo-ejection of the [Ru(2,2'-bipyridyl)₂(CH₃CN)₂]2+ units in acetonitrile was observed. The re-complexation of the [Ru(2,2'-bipyridyl)₂]2+ units could be achieved using acetone as the reaction solvent. However, the process was exceedingly slowly. Additionally, the Fc ligands slowly decomposed when exposed to UV irradiation meaning that only one extension and contraction cycle could be completed.

Orthogonal switching in four-state azobenzene mixed-dimers

Azobenzene multi-state switches whose isomerization can be orthogonally induced with photons and electrons are presented. Exposure to green, blue, or ultraviolet light allows toggling between three isomers, while the fourth one is formed selectively via electrocatalytic isomerization.

Diazonium-functionalized thin films from the spontaneous reaction of p-phenylenebis(diazonium) salts

Salts of the diazonium coupling agent p-phenylenebis(diazonium) form diazonium-terminated conjugated thin films on a variety of conductive and nonconductive surfaces by spontaneous reaction of the coupling agent with the surface. The resulting diazonium-bearing surface can be reacted with various organic and inorganic nucleophiles to form a functionalized surface. These surfaces have been characterized with voltammetry, XPS, infrared and Raman spectroscopy, and atomic force microscopy. Substrates that can be conveniently and quickly modified with this process include ordinary glass, gold, and an intact, fully assembled commercial screen-printed carbon electrode. The scope and convenience of this process make it promising for practical surface modification.

We present a surface functionalization procedure based on diazonium-functionalized thin films produced by spontaneous surface grafting of p-phenylene-bis(diazonium) cation.

Shape-Shifting Azo Dye Polymers: Towards Sunlight-Driven Molecular Devices

The development of stimuli-responsive polymers is among the key goals of modern materials science. The structure and properties of such switchable materials can be designed to be controlled via various stimuli, among which light is frequently the most powerful trigger. Light is a gentle energy source that can target materials remotely, and with extremely high spatial and temporal resolution easily and cheaply. Reversible light-control over molecular mechanical properties in particular has in recent years attracted great interest due to potential applications as optical-to-mechanical conversion actuators and 'devices', enabling 'molecular robotic machines'. In this review, some recent examples and emerging trends in this exciting field of research are highlighted, covering a wide variety of polymer hosts that contain azobenzene photo-reversible switches. It is hoped that this review will help stimulate more interest towards the development of light-reversible materials for energy harvesting and conversion, and their successful incorporation into a wide variety of current and future high-tech applications in devices.

Solid surface vs. liquid surface: nanoarchitectonics, molecular machines, and DNA origami

Comparisons of science and technology between these solid and liquid surfaces would be a good navigation for current-to-future developments.

Photoswitchable NIR-Emitting Gold Nanoparticles

Photo-switching of the NIR emission of gold nanoparticles (GNP) upon photo-isomerization of azobenzene ligands, bound to the surface, is demonstrated. Photophysical results confirm the occurrence of an excitation energy transfer process from the ligands to the GNP that produces sensitized NIR emission. Because of this process, the excitation efficiency of the gold core, upon excitation of the ligands, is much higher for the trans form than for the cis one, and t→c photo-isomerization causes a relevant decrease of the GNP NIR emission. As a consequence, photo-isomerization can be monitored by ratiometric detection of the NIR emission upon dual excitation. The photo-isomerization process was followed in real-time through the simultaneous detection of absorbance and luminescence changes using a dedicated setup. Surprisingly, the photo-isomerization rate of the ligands, bound to the GNP surface, was the same as measured for the chromophores in solution. This outcome demonstrated that excitation energy transfer to gold assists photo-isomerization, rather than competing with it. These results pave the road to the development of new, NIR-emitting, stimuli-responsive nanomaterials for theranostics.

Cooperative Switching in Nanofibers of Azobenzene Oligomers

Next-generation molecular devices and machines demand the integration of molecular switches into hierarchical assemblies to amplify the response of the system from the molecular level to the meso- or macro-scale. Here, we demonstrate that multi-azobenzene oligomers can assemble to form robust supramolecular nanofibers in which they can be switched repeatedly between the E- and Z-configuration. While in isolated oligomers the azobenzene units undergo reversible photoisomerization independently, in the nanofibers they are coupled via intermolecular interactions and switch cooperatively as evidenced by unusual thermal and kinetic behavior. We find that the photoisomerization rate from the Z-isomer to the E-isomer depends on the fraction of Z-azobenzene in the nanofibers, and is increased by more than a factor of 4 in Z-rich fibers when compared to E-rich fibers. This demonstrates the great potential of coupling individual photochromic units for increasing their quantum efficiency in the solid state with potential relevance for actuation and sensing.

Wholly Synthetic Molecular Machines

The past quarter of a century has witnessed an increasing engagement on the part of physicists and chemists in the design and synthesis of molecular machines de novo. This minireview traces the development of artificial molecular machines from their prototypes in the form of shuttles and switches to their emergence as motors and pumps where supplies of energy in the form of chemical fuel, electrochemical potential and light activation become a minimum requirement for them to function away from equilibrium. The challenge facing this rapidly growing community of scientists and engineers today is one of putting wholly synthetic molecules to work, both individually and as collections. Here, we highlight some of the recent conceptual and practical advances relating to the operation of wholly synthetic rotary and linear motors.

Controlling Motion at the Nanoscale: Rise of the Molecular Machines

As our understanding and control of intra- and intermolecular interactions evolve, ever more complex molecular systems are synthesized and assembled that are capable of performing work or completing sophisticated tasks at the molecular scale. Commonly referred to as molecular machines, these dynamic systems comprise an astonishingly diverse class of motifs and are designed to respond to a plethora of actuation stimuli. In this Review, we outline the conditions that distinguish simple switches and rotors from machines and draw from a variety of fields to highlight some of the most exciting recent examples of opportunities for driven molecular mechanics. Emphasis is placed on the need for controllable and hierarchical assembly of these molecular components to display measurable effects at the micro-, meso-, and macroscales. As in Nature, this strategy will lead to dramatic amplification of the work performed via the collective action of many machines organized in linear chains, on functionalized surfaces, or in three-dimensional assemblies.

Watching single molecules move in response to light

Nature has long inspired scientists with its seemingly unlimited ability to harness solar energy and to utilize it to drive various physiological processes. With the help of man-made molecular photoswitches, we now have the potential to outperform natural systems in many ways, with the ultimate goal of fabricating multifunctional materials that operate at different light wavelengths. An important challenge in developing light-controlled artificial molecular machines lies in attaining a detailed understanding of the photoisomerization-coupled conformational changes that occur in macromolecules and molecular assemblies. In this issue of ACS Nano, Bléger, Rabe, and co-workers use force microscopy to provide interesting insights into the behavior of individual photoresponsive molecules and to identify contraction, extension, and crawling events accompanying light-induced isomerization.