Giulio Ragazzon

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 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.

The eternal youth of azobenzene: new photoactive molecular and supramolecular devices

Abstract The development of multicomponent chemical systems that can perform predetermined functions under external control – i.e., molecular devices – is a challenging task in chemistry and a fascinating objective in the frame of a bottom-up approach to nanostructures. Photochromic units undergo profound changes in their chemical and/or electronic structure upon light excitation, and are highly interesting for the construction of photocontrollable molecular devices, machines and materials. The E–Z photoisomerization of azobenzene – owing to its high efficiency, excellent reversibility and significant physico-chemical differences between the two forms – is a highly useful reaction in this regard. Azobenzene photoisomerization has been known for almost 80 years and has been exploited to implement light-induced functionalities with a large variety of compounds, biomolecules, nanosystems and materials. Here we present some of our recent investigations highlighting how this outstanding photochrome can be utilized to develop (supra)molecular systems with valuable light-induced functionalities.

An Artificial Molecular Transporter

Abstract The transport of substrates is one of the main tasks of biomolecular machines in living organisms. We report a synthetic small‐molecule system designed to catch, displace, and release molecular cargo in solution under external control. The system consists of a bistable rotaxane that behaves as an acid–base controlled molecular shuttle, whose ring component bears a tether ending with a nitrile group. The latter can be coordinated to a ruthenium complex that acts as the load, and dissociated upon irradiation with visible light. The cargo loading/unloading and ring transfer/return processes are reversible and can be controlled independently. The robust coordination bond ensures that the cargo remains attached to the device while the transport takes place.

Structural Changes of a Doubly Spin-Labeled Chemically Driven Molecular Shuttle Probed by PELDOR Spectroscopy

Gaining detailed information on the structural rearrangements associated with stimuli-induced molecular movements is of utmost importance for understanding the operation of molecular machines. Pulsed electron-electron double resonance (PELDOR) was employed to monitor the geometrical changes arising upon chemical switching of a [2]rotaxane that behaves as an acid-base-controlled molecular shuttle. To this aim, the rotaxane was endowed with stable nitroxide radical units in both the ring and axle components. The combination of PELDOR data and molecular dynamic calculations indicates that in the investigated rotaxane, the ring displacement along the axle, caused by the addition of a base, does not alter significantly the distance between the nitroxide labels, but it is accompanied by a profound change in the geometry adopted by the macrocycle.

Light-powered, artificial molecular pumps: a minimalistic approach

Summary The realization of artificial molecular motors capable of converting energy into mechanical work is a fascinating challenge of nanotechnology and requires reactive systems that can operate away from chemical equilibrium. This article describes the design and construction of a simple, supramolecular ensemble in which light irradiation causes the directional transit of a macrocycle along a nonsymmetric molecular axle, thus forming the basis for the development of artificial molecular pumps.

Synthesis by ring closing metathesis and properties of an electroactive calix[6]arene [2]catenane

Abstract Tris-(N-phenylureido)-calix[6]arenes are heteroditopic non-symmetric molecular wheels that, in apolar media, bind viologen-based molecular axles in a pseudorotaxane-type fashion. Because of the precise kinetic requirements associated with the threading process, in apolar solvents, the dicationic portion of the axle enters the calixarene annulus exclusively from the upper rim. With the general aim to develop new prototypes of molecular devices and machines whose functions could be governed through a wider set of control elements, we envisaged that the unique properties of these calixarene wheels could be transferred to the synthesis of new catenanes for the construction of unidirectional rotary motors. Herein, we describe the synthesis of a tris(N-phenylureido)calix[6]arene-based catenane by applying the intramolecular ring-closing metathesis reaction for the catenation step on a pre-formed pseudorotaxane.

Remote electrochemical modulation of pKa in a rotaxane by co-conformational allostery

Significance Rotaxanes are species in which a macrocyclic molecule—the ring—is interlocked with a dumbbell-shaped component—the axle. The translational motion of the ring along the axle provides the basis for constructing molecular machinery. In this paper we show that such a dynamic process enables the transfer of chemical information between two distant sites; as a result, the acidity of one site can be reversibly modulated by redox switching at the other site. Possibilities emerge not only for the rational design of species with tailor-made acid–base properties but also for the development of model systems to understand some of Nature’s most effective regulatory mechanisms—namely, allostery and proton-coupled electron transfer. Allosteric control, one of Nature’s most effective ways to regulate functions in biomolecular machinery, involves the transfer of information between distant sites. The mechanistic details of such a transfer are still an object of intensive investigation and debate, and the idea that intramolecular communication could be enabled by dynamic processes is gaining attention as a complement to traditional explanations. Mechanically interlocked molecules, owing to the particular kind of connection between their components and the resulting dynamic behavior, are attractive systems to investigate allosteric mechanisms and exploit them to develop functionalities with artificial species. We show that the pKa of an ammonium site located on the axle component of a [2]rotaxane can be reversibly modulated by changing the affinity of a remote recognition site for the interlocked crown ether ring through electrochemical stimulation. The use of a reversible ternary redox switch enables us to set the pKa to three different values, encompassing more than seven units. Our results demonstrate that in the axle the two sites do not communicate, and that in the rotaxane the transfer of information between them is made possible by the shuttling of the ring, that is, by a dynamic intramolecular process. The investigated coupling of electron- and proton-transfer reactions is reminiscent of the operation of the protein complex I of the respiratory chain.

Synthesis of Densely Packaged, Ultrasmall Pt02 Clusters within a Thioether‐Functionalized MOF: Catalytic Activity in Industrial Reactions at Low Temperature

The gram-scale synthesis, stabilization, and characterization of well-defined ultrasmall subnanometric catalytic clusters on solids is a challenge. The chemical synthesis and X-ray snapshots of Pt02 clusters, homogenously distributed and densely packaged within the channels of a metal-organic framework, is presented. This hybrid material catalyzes efficiently, and even more importantly from an economic and environmental viewpoint, at low temperature (25 to 140 °C), energetically costly industrial reactions in the gas phase such as HCN production, CO2 methanation, and alkene hydrogenations. These results open the way for the design of precisely defined catalytically active ultrasmall metal clusters in solids for technically easier, cheaper, and dramatically less-dangerous industrial reactions.

Redox-Switchable Calix[6]arene-Based Isomeric Rotaxanes

Operating molecular machines are based on switchable systems whose components can be set in motion in a controllable fashion. The presence of nonsymmetrical elements is a mandatory requirement to obtain and demonstrate the unidirectionality of motion. Calixarene-based macrocycles have proved to be very efficient hosts in the design of oriented rotaxanes and of pseudorotaxanes with strict control over the direction of complexation. A series of two-station rotaxanes based on bipyridinium-ammonium axles was synthesized and characterized. A recently reported supramolecularly assisted strategy for the synthesis of different orientational isomers was exploited, and the ammonium unit was identified as a proper secondary station for the calixarene. Displacement of the macrocycle was triggered by electrochemical reduction of the bipyridinium primary station, and it was shown that the shuttling is influenced both by the length of the chain of the axle component and by the position of the secondary station with respect to the calixarene rims.

Electrochemically Controlled Supramolecular Switches and Machines

The device-driven ingenuity of chemists led to the development of artificial molecular devices and machines. Nowadays they represent an established class of systems that mostly rely on supramolecular interactions. In analogy with their macroscopic counterparts, molecular devices and machines need an energy supply to operate. Electrochemical inputs offer desirable features, indeed they are ideal to interface the macroscopic world with the molecular entities—through the electrodes—and can be used both to operate and monitor the state of the system. In this article, selected examples of self-assembling and interlocked systems that can be redox switched are illustrated. Moreover, examples of the emerging class of electroactive systems that operate away from equilibrium are discussed. In the initial sections, the general concepts at the basis of the electrochemical operation and analysis of supramolecular systems are also covered, in order to facilitate the readers who are approaching the topic from different backgrounds.