Visible‐Light‐Driven Photoisomerization and Increased Rotation Speed of a Molecular Motor Acting as a Ligand in a Ruthenium(II) Complex

Photoresponsive Supramolecular Polymers: From Light-Controlled Small Molecules to Smart Materials

Photoresponsive supramolecular polymers are well-organized assemblies based on highly oriented and reversible noncovalent interactions containing photosensitive molecules as (co-)monomers. They have attracted increasing interest in smart materials and dynamic systems with precisely controllable functions, such as light-driven soft actuators, photoresponsive fluorescent anticounterfeiting and light-triggered electronic devices. The present review discusses light-activated molecules used in photoresponsive supramolecular polymers with their main photo-induced changes, e.g., geometry, dipole moment, and chirality. Based on these distinct changes, supramolecular polymers formed by light-activated molecules exhibit photoresponsive disassembly and reassembly. As a consequence, photo-induced supramolecular polymerization, "depolymerization," and regulation of the lengths and topologies are observed. Moreover, the light-controlled functions of supramolecular polymers, such as actuation, emission, and chirality transfer along length scales, are highlighted. Furthermore, a perspective on challenges and future opportunities is presented. Besides the challenge of moving from harmful UV light to visible/near IR light avoiding fatigue, and enabling biomedical applications, future opportunities include light-controlled supramolecular actuators with helical motion, light-modulated information transmission, optically recyclable materials, and multi-stimuli-responsive supramolecular systems.

Supramolecular porphyrin as an improved photocatalyst for chloroform decomposition

In this work, the outlying decoration of the free-base meso-(4-tetra) pyridyl porphyrin (H₂TPyP) with the RuCl(dppb)(5,5'-Me-bipy) ruthenium complex (here named Supra-H₂TPyP) is observed as an improved molecular photocatalyst for dye-mediated chloroform (CHCl₃) decomposition via one-photon absorption operating in the visible spectral range (532 nm and 645 nm). Supra-H₂TPyP offers a better option for CHCl₃ photodecomposition when compared to the same process mediated by pristine H₂TPyP, which requires either excited-state- or UV absorption. The chloroform photodecomposition rates for Supra-H₂TPyP as well as its excitation mechanisms are explored as a function of distinct laser irradiation conditions.

Coupled Rotary and Oscillatory Motion in a Second-Generation Molecular Motor Pd Complex

Molecular machines offer many opportunities for the development of responsive materials and introduce autonomous motion in molecular systems. While basic molecular switches and motors carry out one type of motion upon being exposed to an external stimulus, the development of molecular systems capable of performing coupled motions is essential for the development of more advanced molecular machinery. Overcrowded alkene-based rotary molecular motors are an ideal basis for the design of such systems as they undergo a controlled rotation initiated by light allowing for excellent spatio-temporal precision. Here, we present an example of a Pd complex of a second-generation rotary motor whose Pd center undergoes a coupled oscillatory motion relative to the motor core upon rotation of the motor. We have studied this phenomenon by UV-vis, NMR, and density functional theory calculations to support our conclusions. With this demonstration of a coupled rotation-oscillation motion powered by a light-driven molecular motor, we provide a solid basis for the development of more advanced molecular machines integrating different types of motion in their operation.

Surface-Mounted Dipolar Molecular Rotors Driven by External Electric Field, As Revealed by Torque Analyses

Driven by a high-speed rotating electric field (E-field), molecular motors with polar groups may perform a unidirectional, repetitive, and GHz frequency rotation and thus offer potential applications as nanostirrers. To drive the unidirectional rotation of molecular motors, it is crucial to consider factors of internal charge flow, thermal noise, molecular flexibility, and so forth before selecting an appropriate frequency of a rotating E-field. Herein, we studied two surface-mounted dipolar rotors of a "caltrop-like" molecule and a "sandwich" molecule by using quantum-mechanical computations in combination with torque analyses. We find that the rotational trend as indicated by the magnitude and the direction of torque vectors can sensitively change with the lag angle (α) between the dipolar arm and the E-field. The atomic charges timely flow within the molecule as the E-field rotates, so the lag angle α must be kept in particular intervals to maintain the rotor's unidirectional rotation. The thermal effect can substantially slow down the rotation of the dipolar rotor in the E-field. The flexible dipolar arm shows a more rigid geometry in the E-field with higher rotation speed. Our work would be useful for designing E-driven molecular rotors and for guiding their practical applications in future.

Stepping Out of the Blue: From Visible to Near-IR Triggered Photoswitches

Light offers unique opportunities for controlling the activity of materials and biosystems with high spatiotemporal resolution. Molecular photoswitches are chromophores that undergo reversible isomerization between different states upon irradiation with light, allowing a convenient means to control their influence over the system of interest. However, a significant limitation of classical photoswitches is the requirement to initiate the switching in one or both directions using deleterious UV light with poor tissue penetration. Red-shifted photoswitches are hence in high demand and have attracted keen recent research interest. In this Review, we highlight recent progress towards the development of visible- and NIR-activated photoswitches characterized by distinct photochromic reaction mechanisms. We hope to inspire further endeavors in this field, allowing the full potential of these tools in biotechnology and materials chemistry applications to be realized.

Visible Light-Driven Molecular Switches and Motors: Recent Developments and Applications

Inspired by human vision, a diverse range of light-driven molecular switches and motors has been developed for fundamental understanding and application in material science and biology. Recently, the design and synthesis of visible light-driven molecular switches and motors have been actively pursued. This emerging trend is partly motivated to avoid the harmful effects of ultraviolet light, which was necessary to drive the classical molecular switches and motors at least in one direction, impeding their employment in biomedical and photopharmacology applications. Moreover, visible light-driven molecular switches and motors are demonstrated to enable benign optical materials for advanced photonic devices. Therefore, during the past several years, visible light-driven molecular switches based on azobenzene derivatives, diarylethenes, 1,2-dicyanodithienylethenes, hemithioindigo derivatives, iminothioindoxyls, donor-acceptor Stenhouse adducts, and overcrowded alkene based molecular motors have been judiciously designed, synthesized, and used in the development of functional materials and systems for a wide range of applications. In this Review, we present the recent developments toward the design of visible light-driven molecular switches and motors, with their applications in the fabrication of functional materials and systems in material science, bioscience, pharmacology, etc . The visible light-driven molecular switches and motors realized so far undoubtedly widen the scope of these interesting compounds for technological and biological applications. We hope this Review article could provide additional impetus and inspire further research interests for future exploration of visible light-driven advanced materials, systems, and devices.

Photochemical Energy Conversion with Artificial Molecular Machines

The exploitation of sunlight as a clean, renewable, and distributed energy source is key to facing the energetic demand of modern society in a sustainable and affordable fashion. In the past few decades, chemists have learned to make molecular machines, that is, synthetic chemical systems in which energy inputs cause controlled movements of molecular components that could be used to perform a task. A variety of artificial molecular machines operated by light have been constructed by implementing photochemical processes within appropriately designed (supra)molecular assemblies. These studies could open up new routes for the realization of nanostructured devices and materials capable to harness, convert, and store light energy.

Designing light-driven rotary molecular motors

The ability to induce and amplify motion at the molecular scale has seen tremendous progress ranging from simple molecular rotors to responsive materials. In the two decades since the discovery of light-driven rotary molecular motors, the development of these molecules has been extensive; moving from the realm of molecular chemistry to integration into dynamic molecular systems. They have been identified as actuators holding great potential to precisely control the dynamics of nanoscale devices, but integrating molecular motors effectively into evermore complex artificial molecular machinery is not trivial. Maximising efficiency without compromising function requires conscious and judicious selection of the structures used. In this perspective, we focus on the key aspects of motor design and discuss how to manipulate these properties without impeding motor integrity. Herein, we describe these principles in the context of molecular rotary motors featuring a central double bond axle and emphasise the strengths and weaknesses of each design, providing a comprehensive evaluation of all artificial light-driven rotary motor scaffolds currently present in the literature. Based on this discussion, we will explore the trajectory of research into the field of molecular motors in the coming years, including challenges to be addressed, potential applications, and future prospects.

Fulvalene-Based Polycyclic Aromatic Hydrocarbon Ladder-Type Structures: Synthesis and Properties

Polycyclic aromatic hydrocarbons (PAHs) have found strong interest for their electronic properties and as model systems for graphene. While PAHs have been studied intensively as single units, here PAHs were constructed in ladder-type arrangements using cross-conjugated fulvalene and dithiafulvalene motifs as connecting units and moving forward a convenient synthetic approach for dimerizing (thio)ketones into olefins by the action of Lawesson's reagent. Some of the PAHs can also be regarded as "super-extended" tetrathiafulvalenes (TTFs) with some of the largest cores ever explored, being multi-redox systems that exhibit both reversible oxidations and reductions. Concomitant absorption redshifts were observed when expanding the ladder-type structures from one to two to three indenofluorene units, and optical and electrochemical HOMO-LUMO gaps were found to correlate linearly. Various conformations (and solid-state packing arrangements) were studied by X-ray crystallography and computations.

Visible-Light-Driven Rotation of Molecular Motors in Discrete Supramolecular Metallacycles

The organization of molecular motors in supramolecular assemblies to allow the amplification and transmission of motion and collective action is an important step toward future responsive systems. Metal-coordination-driven directional self-assembly into supramolecular metallacycles provides a powerful strategy to position several motor units in larger structures with well-defined geometries. Herein, we present a pyridyl-modified molecular motor ligand (MPY) which upon coordination with geometrically distinct di-Pt(II) acceptors assembles into discrete metallacycles of different sizes and shapes. This coordination leads to a red-shift of the absorption bands of molecular motors, making these motorized metallacycles responsive to visible light. Photochemical and thermal isomerization experiments demonstrated that the light-driven rotation of the motors in the metallacycles is similar to that in free MPY in solution. CD studies show that the helicity inversions associated with each isomerization step in the rotary cycle are preserved. To explore collective motion, the trimeric motor-containing metallacycle was aggregated with heparin through multiple electrostatic interactions, to construct a multi-component hierarchical system. SEM, TEM, and DLS measurements revealed that the photo- and thermal-responsive molecular motor units enabled selective manipulation of the secondary supramolecular aggregation process without dissociating the primary metallacycle structures. These visible-light-responsive metallacycles, with intrinsic multiple rotary motors, offer prospects for cooperative operations, dynamic hierarchical self-assembled systems, and adaptive materials.

Powering rotary molecular motors with low-intensity near-infrared light

Light-controlled artificial molecular machines hold tremendous potential to revolutionize molecular sciences as autonomous motion allows the design of smart materials and systems whose properties can respond, adapt, and be modified on command. One long-standing challenge toward future applicability has been the need to develop methods using low-energy, low-intensity, near-infrared light to power these nanomachines. Here, we describe a rotary molecular motor sensitized by a two-photon absorber, which efficiently operates under near-infrared light at intensities and wavelengths compatible with in vivo studies. Time-resolved spectroscopy was used to gain insight into the mechanism of energy transfer to the motor following initial two-photon excitation. Our results offer prospects toward in vitro and in vivo applications of artificial molecular motors.

Visible-Light-Driven Rotation of Molecular Motors in a Dual-Function Metal-Organic Framework Enabled by Energy Transfer

The visible-light-driven rotation of an overcrowded alkene-based molecular motor strut in a dual-function metal-organic framework (MOF) is reported. Two types of functional linkers, a palladium-porphyrin photosensitizer and a bispyridine-derived molecular motor, were used to construct the framework capable of harvesting low-energy green light to power the rotary motion. The molecular motor was introduced in the framework using the postsynthetic solvent-assisted linker exchange (SALE) method, and the structure of the material was confirmed by powder (PXRD) and single-crystal X-ray (SC-XRD) diffraction. The large decrease in the phosphorescence lifetime and intensity of the porphyrin in the MOFs upon introduction of the molecular motor pillars confirms efficient triplet-to-triplet energy transfer between the porphyrin linkers and the molecular motor. Near-infrared Raman spectroscopy revealed that the visible light-driven rotation of the molecular motor proceeds in the solid state at rates similar to those observed in solution.

Photoswitchable catalysis using organometallic complexes

Photoswitchable catalysis using organometallic complexes: a ligand design perspective.

Light-Activated Organic Molecular Motors and Their Applications

Molecular motors are at the heart of cellular machinery, and they are involved in converting chemical and light energy inputs into efficient mechanical work. From a synthetic perspective, the most advanced molecular motors are rotators that are activated by light wherein a molecular subcomponent rotates unidirectionally around an axis. The mechanical work produced by arrays of molecular motors can be used to induce a macroscopic effect. Light activation offers advantages over biological chemically activated molecular motors because one can direct precise spatiotemporal inputs while conducting reactions in the gas phase, in solution and in vacuum, while generating no chemical byproducts or waste. In this review, we describe the origins of the first light-activated rotary motors and their modes of function, the structural modifications that led to newer motor designs with optimized rotary properties at variable activation wavelengths. Presented are molecular motor attachments to surfaces, their insertion into supramolecular structures and photomodulating materials, their use in catalysis, and their action in biological environments to produce exciting new prospects for biomedicine.

1,2-Dithienyldicyanoethene-Based, Visible-Light-Driven, Chiral Fluorescent Molecular Switch: Rewritable Multimodal Photonic Devices

Reported here is the first example of a 1,2-dithienyldicyanoethene-based visible-light-driven chiral fluorescent molecular switch that exhibits reversible trans to cis photoisomerization. The trans form in solution almost completely transforms into the cis form, accompanied by a 10-fold decrease in its fluorescence intensity within 60 seconds when exposed to green light (520 nm). The reverse isomerization proceeds upon irradiation with blue light (405 nm). When doped into commercially available achiral liquid crystal hosts, this molecular switch efficiently induces luminescent helical superstructures, that is, a cholesteric phase. The intensity of the circularly polarized fluorescence as well as the selective reflection wavelength of the induced cholesteric phases can be reversibly tuned using visible light of two different wavelengths. Optically rewritable photonic devices using cholesteric films containing this molecular switch are described.

Photoefficient 2nd generation molecular motors responsive to visible light

A new series of visible light-driven artificial rotary molecular motors is presented including the most red-shifted example to date.

Molecular motors that operate with high efficiency using visible light are attractive for numerous applications. Here the synthesis and characterisation of three novel visible light switchable 2^nd generation molecular motors is presented. Two of them are based on push–pull systems with the third one possessing an extended π-system. With a maximum effective excitation wavelength of 530 nm we designed the most red-shifted artificial rotary motor known to date. All three motors benefit from efficient switching to the metastable isomer, high quantum yields and excellent photostability setting them apart from visible light switchable motors reported previously. The activation barriers of the rate-determining thermal helix inversion could be accurately predicted using DFT calculations and differences between the motors can be explained by distinct transition state structures. Enantiomers of push–pull motors were successfully separated and their helical twisting power in E7 liquid crystals was determined.

Visible-Light-Driven Tunable Molecular Motors Based on Oxindole

Molecular rotary motors based on oxindole which can be driven by visible light are presented. This novel class of motors can be easily synthesized via a Knoevenagel condensation, and the choice of different upper halves allows for the facile tuning of their rotational speed. The four-step rotational cycle was explored using DFT calculations, and the expected photochemical and thermal isomerization behavior was confirmed by NMR, UV/vis, and CD spectroscopy. These oxindole motors offer attractive prospects for functional materials responsive to light.

Reversible Photocontrol of Thrombin Activity by Replacing Loops of Thrombin Binding Aptamer using Azobenzene Derivatives

The photoisomerization of azobenzenes provides a general means for the photocontrol of many important biomolecular structures and organismal functions. For temporal and spatial control activity of thrombin binding aptamer (TBA) by light, azobenzene derivatives were carefully selected as light-triggered molecular switches to replace TT loops and the TGT loop of TBA to reversibly control enzyme activity. These molecules interconverted between the trans and cis states under alternate UV and visible light irradiation, which consequently triggered reversible formation of G-quadruplex morphology. In addition, we investigated the impact of three azobenzene derivatives on stability, thrombin binding ability, and anticoagulant properties. The result showed that 4,4'-bis(hydroxymethyl)azobenzene at the TGT loop position significantly photoregulated affinity to thrombin and blood clotting in human plasma, which provided a successful strategy to control blood clotting in human plasma and a further evidence for design of TBA analogues with pivotal positions of modifications.

Toward Fast and Efficient Visible-Light-Driven Molecular Motors: A Minimal Design

A key goal in the development of light-driven rotary molecular motors is to facilitate their usage in biology and medicine by shifting the required irradiation wavelengths from the UV regime to the nondestructive visible regime. Although some progress has been made toward this goal, most available visible-light-driven motors either have relatively low quantum yields or require that thermal steps follow the photoisomerizations that underlie the rotary motion. Here, a minimal design for visible-light-driven motors without these drawbacks is presented and evaluated on the basis of state-of-the-art quantum chemical calculations and molecular dynamics simulations. The design, featuring dihydropyridinium and cyclohexenylidene motifs and comprising only five conjugated double bonds, is found to produce a full 360° rotation through fast photoisomerizations (excited-state lifetimes of ≈170-250 fs) powered by photons with energies well below 3 eV.

Self-Propelled Micro/Nanomotors for Sensing and Environmental Remediation

Self-propelled micro/nanomotors have gained attention for successful application in cargo delivery, therapeutic treatments, sensing, and environmental remediation. Unique characteristics such as high speed, motion control, selectivity, and functionability promote the application of micro/nanomotors in analytical sciences. Here, the recent advancements and main challenges regarding the application of self-propelled micro/nanomotors in sensing and environmental remediation are discussed. The current state of micro/nanomotors is reviewed, emphasizing the period of the last five years, then their developments into the future applications for enhanced sensing and efficient purification of water resources are extrapolated.

Cation-Modulated Rotary Speed in a Light-Driven Crown Ether Functionalized Molecular Motor

The design and synthesis of an overcrowded-alkene based molecular motor featuring a crown ether integrated in its stator structure has been accomplished. The photostationary state ratios and rotational speed of this motor can be modulated by cation coordination to the crown ether moiety, which can be reversed upon the addition of a competing chelating agent, thus achieving a dynamic control over the rotational behavior of the motor.

Indigoid Photoswitches: Visible Light Responsive Molecular Tools

Indigoid photoswitches comprise a class of chromophores that are derived from the parent and well-known indigo dye. Different from most photoswitches their core structures absorb in the visible region of the spectrum in both isomeric states even without substitutions, which makes them especially interesting for applications not tolerant of high-energy UV light. Also different from most current photoswitching systems, they provide highly rigid structures that undergo large yet precisely controllable geometry changes upon photoisomerization. The favorable combination of pronounced photochromism, fast and efficient photoreactions, and high thermal bistability have led to a strongly increased interest in indigoid photoswitches over the last years. As a result, intriguing applications of these chromophores as reversible triggering units in supramolecular and biological chemistry, the field of molecular machines, or smart molecules have been put forward. In this Account current developments in the synthesis, mechanistic understanding of light responsiveness, advantageous properties as phototools, and new applications of indigoid photoswitches are summarized with the focus on hemithioindigo, hemiindigo, and indigo as key examples. Many methods for the synthesis of hemithioindigos are known, but derivatives with a fourth substituent at the double bond could not easily be prepared because of the resulting increased steric hindrance in the products. Recent efforts in our laboratory have provided two different methods to prepare these highly promising photoswitches in very efficient ways. One method is especially designed for the introduction of sterically hindered ketones while the second one allows rapid structural diversification in only three high-yielding synthetic steps. Given the lesser prominence of indigoid photoswitches, mechanistic understanding of their excited state behavior and therefore rational design opportunities for photophysical properties are also much less developed compared to, for example, azobenzenes or stilbenes. By testing different substitution patterns, we were able to produce strongly beneficial property combinations in hemithioindigo, hemiindigo, or indigo photoswitches, for example, red-light responsiveness together with very high thermal bistability of the switching states. This is of particular importance for photopharmacological and biological applications of these switches to reduce the damage from high-energy light and to enable deep penetration of the light into tissues. An additional ground state twisting in hemithioindigo allowed us to control the type of light-induced bond rotation simply by the polarity of the solvent. With the aid of time-resolved spectroscopy and quantum yield measurements, we could show that in apolar cyclohexane exclusive double bond rotation takes place while in polar DMSO sole single bond rotation is observed. Such precise control over geometrical changes is of great interest for the construction of future sophisticated molecular machinery. In this field, we have introduced hemithioindigo photoswitches as novel core structure for molecular motors providing very fast directional motions upon irradiation with visible light. The mechanism of the directional rotation adheres to a four-step process, which could directly be observed in situ with a slower second-generation motor. Further applications of indigoid photoswitches were made in our laboratory in the realms of photocontrolled folding and host-guest chemistry as well as in molecular digital information processing showcasing the great versatility and enormous future promise of indigoid photoswitches.

Molecular rotary motors: Unidirectional motion around double bonds

The field of synthetic molecular machines has quickly evolved in recent years, growing from a fundamental curiosity to a highly active field of chemistry. Many different applications are being explored in areas such as catalysis, self-assembled and nanostructured materials, and molecular electronics. Rotary molecular motors hold great promise for achieving dynamic control of molecular functions as well as for powering nanoscale devices. However, for these motors to reach their full potential, many challenges still need to be addressed. In this paper we focus on the design principles of rotary motors featuring a double-bond axle and discuss the major challenges that are ahead of us. Although great progress has been made, further design improvements, for example in terms of efficiency, energy input, and environmental adaptability, will be crucial to fully exploit the opportunities that these rotary motors offer.

Light-driven micro/nanomotors: from fundamentals to applications

Light, as an external stimulus, is capable of driving the motion of micro/nanomotors (MNMs) with the advantages of reversible, wireless and remote manoeuvre on demand with excellent spatial and temporal resolution. This review focuses on the state-of-the-art light-driven MNMs, which are able to move in liquids or on a substrate surface by converting light energy into mechanical work. The general design strategies for constructing asymmetric fields around light-driven MNMs to propel themselves are introduced as well as the photoactive materials for light-driven MNMs, including photocatalytic materials, photothermal materials and photochromic materials. Then, the propulsion mechanisms and motion behaviors of the so far developed light-driven MNMs are illustrated in detail involving light-induced phoretic propulsion, bubble recoil and interfacial tension gradient, followed by recent progress in the light-driven movement of liquid crystalline elastomers based on light-induced deformation. An outlook is further presented on the future development of light-driven MNMs towards overcoming key challenges after summarizing the potential applications in biomedical, environmental and micro/nanoengineering fields.

Light-Powered Micro/Nanomotors

Designed micro/nanomotors are micro/nanoscale machines capable of autonomous motion in fluids, which have been emerging in recent decades owing to their great potential for biomedical and environmental applications. Among them, light-powered micro/nanomotors, in which motion is driven by light, exhibit various advantages in their precise motion manipulation and thereby a superior scope for application. This review summarizes recent advances in the design, manufacture and motion manipulation of different types of light-powered micro/nanomotors. Their structural features and motion performance are reviewed and compared. The challenges and opportunities of light-powered micro/nanomotors are also discussed. With rapidly increasing innovation, advanced, intelligent and multifunctional light-powered micro/nanomachines will certainly bring profound impacts and changes for human life in the future.

The Art of Building Small: From Molecular Switches to Motors (Nobel Lecture)

A journey into the nano-world: The ability to design, use and control motor-like functions at the molecular level sets the stage for numerous dynamic molecular systems. In his Nobel Lecture, B. L. Feringa describes the evolution of the field of molecular motors and explains how to program and control molecules by incorporating responsive and adaptive properties.

Visible-Light Excitation of a Molecular Motor with an Extended Aromatic Core

Exploring routes to visible-light-driven rotary motors, the possibility of red-shifting the excitation wavelength of molecular motors by extension of the aromatic core is studied. Introducing a dibenzofluorenyl moiety in a standard molecular motor resulted in red-shifting of the absorption spectrum. UV/vis and 1H NMR spectroscopy showed that these motors could be isomerized with light of wavelengths up to 490 nm and that the structural modification did not impair the anticipated rotary behavior. Extension of the aromatic core is therefore a suitable strategy to apply in pursuit of visible-light-driven molecular motors.

Artificial molecular motors

Artificial molecular motors take inspiration from motor proteins, nature's solution for achieving directional molecular level motion. An overview is given of the principal designs of artificial molecular motors and their modes of operation. We identify some key challenges remaining in the field.

Allosteric Regulation of the Rotational Speed in a Light-Driven Molecular Motor

The rotational speed of an overcrowded alkene-based molecular rotary motor, having an integrated 4,5-diazafluorenyl coordination motif, can be regulated allosterically via the binding of metal ions. DFT calculations have been used to predict the relative speed of rotation of three different (i.e., zinc, palladium, and platinum) metal dichloride complexes. The photochemical and thermal isomerization behavior of these complexes has been studied in detail using UV–vis and ¹H NMR spectroscopy. Our results confirm that metal coordination induces a contraction of the diazafluorenyl lower half, resulting in a reduction of the steric hindrance in the “fjord” region of the molecule, which causes an increase of the rotational speed. Importantly, metal complexation can be accomplished in situ and is found to be reversible upon the addition of a competing ligand. Consequently, the rotational behavior of these molecular motors can be dynamically controlled with chemical additives.

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.

OFF-ON-OFF Dual Emission at Visible and UV Wavelengths from Carbazole Functionalized β-Diketonate Europium(III) Complex

This work demonstrates dual emission "OFF-ON-OFF" switching at visible and UV wavelengths of a carbazole functionalized β-diketone (LH) by a simple change of a europium(III) ion (Eu(3+)) concentration in the submicromolar concentration range. In the presence of 0.25 equiv of Eu(3+) (5 μM), LH forms a luminescent 4:1 complex ([Eu(3+)(L(-))4](-)) exhibiting dual emission at 357 and 613 nm resulting from the local excitation of the carbazole ring and ligand-sensitized luminescence from the Eu(3+)-β-diketonate unit, respectively. The 4:1 complex begins to convert into a 2:1 complex ([Eu(3+)(L(-))2](+)) via a 3:1 complex [Eu(3+)(L(-))3] above a molar ratio ([Eu(3+)]/[LH]) of 0.25, which provides the opportunity for binding of solvent methanol molecules to the vacant site of the Eu(3+) ion in the complex ([Eu(3+)(L(-))2(MeOH)n](+)). The OH oscillators of coordinated methanol molecules facilitate the nonradiative pathway of the Eu(3+) emission; hence the emission at 613 nm almost disappears above the 0.50 equivalent of Eu(3+) (11 μM), while the UV emission at 357 nm remains mostly constant over the whole concentration range.