Elucidating the Excited State Behavior of Pyridyl Pyridinium Systems via Computational and Transient Absorption Studies of Tetrahedral Multichromophoric Arrays and their Model Compounds.

The tetrahedral shape-persistent molecule 14+ , containing four identical pyridyl pyridinium units connected via a sp3 hybridized carbon atom, has been investigated in detail by means of steady-state and time resolved spectroscopy. Remarkable photophysical properties are observed, particularly in comparison with protonated and methylated analogues (1H4 8+ , 1Me4 8+ ), which exhibit substantially shorter excited state lifetimes and lower emission quantum yields. Theoretical studies have rationalized the behavior of the tetrameric molecules relative to the monomers, with DFT and TD-DFT calculations corroborating steady-state (absorption and emission) and transient absorption spectra. The behavior of the monomeric compounds (each consisting in one of the four identical subunits of the tetramers, i. e., 2+ , 2H2+ and 2Me2+ ) considerably differs from that of the tetramers, indicating a strong electronic interaction between the subunits in the tetrameric species, likely promoted by the homoconjugation through the connecting sp3  C atom. 2+ is characterized by a peculiar S1 -S2 excited state inversion, whereas the short-lived emitting S1 state of 2H2+ and 2Me2+ exhibits a partial charge-transfer character, as substantiated by spectro-electrochemical studies. Among the six investigated systems, only 14+ is a sizeable luminophore (Φem =0.15), which is related to the peculiar features of its singlet state.

Kinetic and energetic insights into the dissipative non-equilibrium operation of an autonomous light-powered supramolecular pump.

Natural and artificial autonomous molecular machines operate by constantly dissipating energy coming from an external source to maintain a non-equilibrium state. Quantitative thermodynamic characterization of these dissipative states is highly challenging as they exist only as long as energy is provided. Here we report on the detailed physicochemical characterization of the dissipative operation of a supramolecular pump. The pump transduces light energy into chemical energy by bringing self-assembly reactions to non-equilibrium steady states. The composition of the system under light irradiation was followed in real time by 1H NMR for four different irradiation intensities. The experimental composition and photon flow were then fed into a theoretical model describing the non-equilibrium dissipation and the energy storage at the steady state. We quantitatively probed the relationship between the light energy input and the deviation of the dissipative state from thermodynamic equilibrium in this artificial system. Our results provide a testing ground for newly developed theoretical models for photoactivated artificial molecular machines operating away from thermodynamic equilibrium.

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.

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.

Direct synthetic routes to functionalised crown ethers.

Crown ethers are macrocyclic hosts that can complex a wide range of inorganic and organic cations as well as neutral guest species. Their widespread utilization in several areas of fundamental and applied chemistry strongly relies on strategies for their functionalisation, in order to obtain compounds that could carry out multiple functions and could be incorporated in sophisticated systems. Although functionalised crown ethers are normally synthesised by templated macrocyclisation using appropriately substituted starting materials, the direct addition of functional groups onto a pre-formed macrocyclic framework is a valuable yet underexplored alternative. Here we review the methodologies for the direct functionalisation of aliphatic and aromatic crown ethers sporadically reported in the literature over a period of four decades. The general approach for the introduction of moieties on aliphatic crown ethers involves a radical mediated cross dehydrogenative coupling initiated either by photochemical or thermal/chemical activation, while aromatic crown ethers are commonly derivatised via electrophilic aromatic substitution. Direct functionalization routes can reduce synthetic effort, allow the later modification of crown ether-based architectures, and disclose new ways to exploit these versatile macrocycles in contemporary supramolecular science and technology.

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

The mechanical bond has opened a new world for structural and dynamic stereochemistry, which is still largely underexplored and whose significance for various applications is becoming increasingly evident. We demonstrate that designed rearrangements involving both covalent and mechanical bonds can be integrated in [2]rotaxanes, leading to interesting consequences in terms of E/Z isomerization mechanisms. Two entirely distinct and concomitant stereomutations, pertaining to the same stereogenic element but involving different kinds of linkages within the molecule, are observed and are thoroughly characterized. The rate of the two processes is affected in opposite ways upon changing solvent polarity; such a phenomenon can be used to selectively modify the rate of each motion and adjust the relative contribution of the two mechanisms to the isomerization. Although the movements are not synchronized, an analysis of the intriguing fundamental implications for transition state theory, reaction pathway bifurcation, and microscopic reversibility was triggered by our experimental observations.

Giant Shape-Persistent Tetrahedral Porphyrin System: Light-Induced Charge Separation.

Tetraphenylmethane appended with four pyridylpyridinium units works as a scaffold to self-assemble four ruthenium porphyrins in a tetrahedral shape-persistent giant architecture. The resulting supramolecular structure has been characterised in the solid state by X-ray single crystal analysis and in solution by various techniques. Multinuclear NMR spectroscopy confirms the 1 : 4 stoichiometry with the formation of a highly symmetric structure. The self-assembly process can be monitored by changes of the redox potentials, as well as by modifications in the visible absorption spectrum of the ruthenium porphyrin and by a complete quenching of both the bright fluorescence of the tetracationic scaffold and the weak phosphorescence of the ruthenium porphyrin. An ultrafast photoinduced electron transfer is responsible for this quenching process. The lifetime of the resulting charge separated state (800 ps) is about four times longer in the giant supramolecular structure compared to the model 1 : 1 complex formed by the ruthenium porphyrin and a single pyridylpyridinium unit. Electron delocalization over the tetrameric pyridinium structure is likely to be responsible for this effect.

Second-Generation Light-Fueled Supramolecular Pump.

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.

Artificial Supramolecular Pumps Powered by Light.

Invited for the cover of this issue is Alberto Credi and co-workers at the University of Bologna and National Research Council, Bologna, Italy. The image represents the photostationary non-equilibrium operation of supramolecular pumps as a hydraulic circuit in which water flows between reservoirs. Read the full text of the article at 10.1002/chem.202101163.

Artificial Supramolecular Pumps Powered by Light.

The development of artificial nanoscale motors that can use energy from a source to perform tasks requires systems capable of performing directionally controlled molecular movements and operating away from chemical equilibrium. Here, the design, synthesis and properties of pseudorotaxanes are described, in which a photon input triggers the unidirectional motion of a macrocyclic ring with respect to a non-symmetric molecular axle. The photoinduced energy ratcheting at the basis of the pumping mechanism is validated by measuring the relevant thermodynamic and kinetic parameters. Owing to the photochemical behavior of the azobenzene moiety embedded in the axle, the pump can repeat its operation cycle autonomously under continuous illumination. NMR spectroscopy was used to observe the dissipative non-equilibrium state generated in situ by light irradiation. We also show that fine changes in the axle structure lead to an improvement in the performance of the motor. Such results highlight the modularity and versatility of this minimalist pump design, which provides facile access to dynamic systems that operate under photoinduced non-equilibrium regimes.

Chemically Induced Mismatch of Rings and Stations in [3]Rotaxanes.

The mechanical interlocking of molecular components can lead to the appearance of novel and unconventional properties and processes, with potential relevance for applications in nanoscience, sensing, catalysis, and materials science. We describe a [3]rotaxane in which the number of recognition sites available on the axle component can be changed by acid-base inputs, encompassing cases in which this number is larger, equal to, or smaller than the number of interlocked macrocycles. These species exhibit very different properties and give rise to a unique network of acid-base reactions that leads to a fine pKa tuning of chemically equivalent acidic sites. The rotaxane where only one station is available for two rings exhibits a rich coconformational dynamics, unveiled by an integrated experimental and computational approach. In this compound, the two crown ethers compete for the sole recognition site, but can also come together to share it, driven by the need to minimize free energy without evident inter-ring interactions.

Light-Controlled Regioselective Synthesis of Fullerene Bis-Adducts.

Multi-functionalization and isomer-purity of fullerenes are crucial tasks for the development of their chemistry in various fields. In both current main approaches-tether-directed covalent functionalization and supramolecular masks-the control of regioselectivity requires multi-step synthetic procedures to prepare the desired tether or mask. Herein, we describe light-responsive tethers, containing an azobenzene photoswitch and two malonate groups, in the double cyclopropanation of [60]fullerene. The formation of the bis-adducts and their spectroscopic and photochemical properties, as well as the effect of azobenzene photoswitching on the regiochemistry of the bis-addition, have been studied. The behavior of the tethers depends on the geometry of the connection between the photoactive core and the malonate moieties. One tether lead to a strikingly different adduct distribution for the E and Z isomers, indicating that the covalent bis-functionalization of C60 can be controlled by light.

Precision Molecular Threading/Dethreading.

The general principles guiding the design of molecular machines based on interlocked structures are well known. Nonetheless, the identification of suitable molecular components for a precise tuning of the energetic parameters that determine the mechanical link is still challenging. Indeed, what are the reasons of the "all-or-nothing" effect, which turns a molecular "speed-bump" into a stopper in pseudorotaxane-based architectures? Here we investigate the threading and dethreading processes for a representative class of molecular components, based on symmetric dibenzylammonium axles and dibenzo[24]crown-8 ether, with a joint experimental-computational strategy. From the analysis of quantitative data and an atomistic insight, we derive simple rules correlating the kinetic behaviour with the substitution pattern, and provide rational guidelines for the design of modules to be integrated in molecular switches and motors with sophisticated dynamic features.

Photoactivated Artificial Molecular Machines that Can Perform Tasks.

Research on artificial photoactivated molecular machines has moved in recent years from a basic scientific endeavor toward a more applicative effort. Nowadays, the prospect of reproducing the operation of natural nanomachines with artificial counterparts is no longer a dream but a concrete possibility. The progress toward the construction of molecular-machine-based devices and materials in which light irradiation results in the execution of a task as a result of nanoscale movements is illustrated here. After a brief description of a few basic types of photoactivated molecular machines, significant examples of their exploitation to perform predetermined functions are presented. These include switchable catalysts, nanoactuators that interact with cellular membranes, transporters of small molecular cargos, and active joints capable of mechanically coupling molecular-scale movements. Investigations aimed at harnessing the collective operation of a multitude of molecular machines organized in arrays to perform tasks at the microscale and macroscale in hard and soft materials are also reviewed. Surfaces, gels, liquid crystals, polymers, and self-assembled nanostructures are described wherein the nanoscale movement of embedded molecular machines is amplified, allowing the realization of muscle-like actuators, microfluidic devices, and polymeric materials for light energy transduction and storage.

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.

Design of photo-activated molecular machines: highlights from the past ten years.

Molecular machines powered by light are appropriately designed multicomponent molecular structures that exploit photochemical processes to perform controlled movements. We discuss the advances in the design and investigation of light-driven molecular machines by presenting a selection of examples from the past decade.

Photocontrolled self-assembly of azobenzene nanocontainers in water: light-triggered uptake and release of lipophilic molecules.

An easy to synthesize azobenzene based amphiphile spontaneously self-assembles into monodisperse nanoaggregates in water. The large difference in the critical aggregation concentration between the E and Z stereoisomeric forms enables photocontrol of its aggregation state over a wide concentration range: light-triggered release and uptake of lipophilic molecules is achieved in aqueous solution.

Threading-gated photochromism in [2]pseudorotaxanes.

Rigid, Y-shaped imidazole compounds containing the bis(thienyl)ethene moiety were designed and synthesized. The 4,5-bis(benzothienyl)-2-phenylimidazolium cations were then used as axles for [2]pseudorotaxane formation with 24-membered crown ether wheels. It was demonstrated using 1H NMR spectroscopy, UV-Vis absorption and emission spectroscopies that this host-guest interaction results in significant changes in the photochromic properties of the imidazolium axles. This is a rare example of gated photochromism, which exploits the recognition event of an interpenetrated molecular system to tune the photochromic properties in one of the components.

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 coconformations. This result represents an unprecedented example of chiral molecular recognition and can disclose innovative approaches to enantioselective sensing and catalysis.

Solution and solid state photochromism in a family of shape persistent azobenzene tetramers functionalized with alkyloxy substituents.

Shape-persistent azobenzene tetramers functionalized at the periphery with alkyloxy substituents of different lengths have been synthesized and their photochemical behaviour has been investigated. Efficient E→Z photoisomerization of the azobenzene units takes place both in solution and in the solid state, a highly desirable yet uncommon property for azobenzene-type photochromic compounds. The solid state E→Z photoisomerization is accompanied by an isothermal crystal-amorphous phase transformation; successively, anisotropic crystals can be grown upon promoting the Z→E isomerization by thermal annealing of the irradiated samples. These results validate the strategy of engineering multiphotochromic architectures with a rigid star-shaped geometry to preserve the solution-based photoreactivity also in the solid state. The observed unexpected photoinduced alignment makes these materials potentially attractive for the development of photo-patternable and photo-responsive surfaces.