Alberto Credi

Artificial Molecular Machines.

The miniaturization of components used in the construction of working devices is being pursued currently by the large-downward (top-down) fabrication. This approach, however, which obliges solid-state physicists and electronic engineers to manipulate progressively smaller and smaller pieces of matter, has its intrinsic limitations. An alternative approach is a small-upward (bottom-up) one, starting from the smallest compositions of matter that have distinct shapes and unique properties-namely molecules. In the context of this particular challenge, chemists have been extending the concept of a macroscopic machine to the molecular level. A molecular-level machine can be defined as an assembly of a distinct number of molecular components that are designed to perform machinelike movements (output) as a result of an appropriate external stimulation (input). In common with their macroscopic counterparts, a molecular machine is characterized by 1) the kind of energy input supplied to make it work, 2) the nature of the movements of its component parts, 3) the way in which its operation can be monitored and controlled, 4) the ability to make it repeat its operation in a cyclic fashion, 5) the timescale needed to complete a full cycle of movements, and 6) the purpose of its operation. Undoubtedly, the best energy inputs to make molecular machines work are photons or electrons. Indeed, with appropriately chosen photochemically and electrochemically driven reactions, it is possible to design and synthesize molecular machines that do work. Moreover, the dramatic increase in our fundamental understanding of self-assembly and self-organizational processes in chemical synthesis has aided and abetted the construction of artificial molecular machines through the development of new methods of noncovalent synthesis and the emergence of supramolecular assistance to covalent synthesis as a uniquely powerful synthetic tool. The aim of this review is to present a unified view of the field of molecular machines by focusing on past achievements, present limitations, and future perspectives. After analyzing a few important examples of natural molecular machines, the most significant developments in the field of artificial molecular machines are highlighted. The systems reviewed include 1) chemical rotors, 2) photochemically and electrochemically induced molecular (conformational) rearrangements, and 3) chemically, photochemically, and electrochemically controllable (co-conformational) motions in interlocked molecules (catenanes and rotaxanes), as well as in coordination and supramolecular complexes, including pseudorotaxanes. Artificial molecular machines based on biomolecules and interfacing artificial molecular machines with surfaces and solid supports are amongst some of the cutting-edge topics featured in this review. The extension of the concept of a machine to the molecular level is of interest not only for the sake of basic research, but also for the growth of nanoscience and the subsequent development of nanotechnology.

Molecular Devices and Machines– A Journey into the Nano World

Preface.Reference.General Concepts.PART I: DEVICES FOR PROCESSING ELECTRONS AND ELECTRONIC ENERGY.Fundamental Principles of Electron and Energy Transfer.Wires and Related Systems.Switching Electron- and Energy-transfer Processes.Light-harvesting Antennae.Photoinduced Charge Separation and Solar Energy Conversion.PART II: MEMORIES, LOGIC GATES, AND RELATED SYSTEMS.Bistable Systems.Multistate-Multifunctional Systems.Logic Gates.PART III: MOLECULAR-SCALE MACHINES.Basic Principles of Molecular Machines.Spontaneous Mechanical-like Motions.Movements Related to Opening, Closing, and Translocation Functions.Rotary Movements.Threading-Dethreading Movements.Linear Movements.Motions in Catenanes.Appendix.Glossary.List of Abbreviations.Subject Index.

Künstliche molekulare Maschinen

Die zum Bau kleiner Maschinen notwendige Miniaturisierung von Komponenten erfolgt derzeit nach dem Verkleinerungsprinzip (top-down approach). Diesem Ansatz, der Festkorperphysiker und Elektronikingenieure zwingt, mit immer kleineren Materialbausteinen zu arbeiten, sind allerdings Grenzen gesetzt. Eine Alternative besteht im Vergroserungsprinzip (bottom-up approach), bei dem man von den kleinsten Teilen der Materie mit eindeutiger Form und definierten Eigenschaften, den Molekulen, ausgeht. Vor dem Hintergrund dieser Herausforderung haben Chemiker das Konzept der makroskopischen Maschine auf die molekulare Ebene ubertragen. Eine molekulare Maschine kann als eine Anordnung einer definierten Anzahl von molekularen Komponenten definiert werden, die so konzipiert wurden, dass sie als Reaktion auf geeignete externe Stimulation (input) maschinenahnliche Bewegungen ausfuhren (output). Genau wie ihr makroskopisches Gegenstuck ist eine molekulare Maschine durch folgende Merkmale charakterisiert: 1) die Art der Energie, die ihr zugefuhrt werden muss, damit sie funktioniert, 2) die Art der Bewegungen ihrer Komponenten, 3) die Methoden, durch die ihre Funktionen verfolgt und gesteuert werden konnen, 4) die Moglichkeit der cyclischen Wiederholung, 5) die Zeit, die fur die Durchfuhrung eines vollstandigen Arbeitscyclus benotigt wird, und 6) der Zweck ihrer Funktion. Zweifellos sind Photonen oder Elektronen die besten Energielieferanten fur molekulare Maschinen. So ist es moglich, mit sorgfaltig ausgewahlten photochemischen oder elektrochemischen Reaktionen, funktionierende molekulare Maschinen zu entwerfen und zu synthetisieren. Daruber hinaus hat unser rasch angewachsenes, fundamentales Verstandnis uber die Selbstorganisation und die ihr zugrunde liegenden Prozesse in der chemischen Synthese zum Aufbau kunstlicher molekularer Maschinen beigetragen. Dies geschah vor allem durch die Entwicklung neuer Methoden in der nichtkovalenten Synthese und das Aufkommen der supramolekular unterstutzten kovalenten Synthese als ausgesprochen leistungsfahiges Syntheseprinzip. Ziel dieses Ubersichtsartikels ist eine einheitliche Darstellung des Gebiets der molekularen Maschinen, wobei besonderes Augenmerk auf das in der Vergangenheit Erreichte, auf gegenwartig bestehende Grenzen und auf Zukunftsperspektiven gelegt werden soll. Nach der Beschreibung einiger naturlicher molekularer Maschinen werden die wichtigsten Entwicklungen auf dem Gebiet der kunstlichen molekularen Maschinen vorgestellt. Dabei wird auf folgende Systeme naher eingegangen: 1) chemische Rotoren, 2) photochemisch und elektrochemisch induzierte molekulare (konformative) Umlagerungen und 3) chemisch, photochemisch und elektrochemisch steuerbare (cokonformative) Bewegungen in ineinander greifenden (interlocked) Molekulen (Catenanen und Rotaxanen) sowie in Koordinationsverbindungen und supramolekularen Komplexen (darunter Pseudorotaxanen). Kunstliche, auf Biomolekulen basierende molekulare Maschinen und kunstliche molekulare Maschinen, die auf Oberflachen oder festen Tragern aufgebracht wurden, sind zwei der spannenden Entwicklungen, die besprochen werden. Die Erweiterung des Konzepts einer Maschine auf die molekulare Ebene ist nicht nur fur die Grundlagenforschung von Interesse, sondern auch fur die Weiterentwicklung der Nanowissenschaften und der daraus erwachsenden Nanotechnologie.

Light powered molecular machines.

The bottom-up construction and operation of mechanical machines of molecular size is a topic of high interest for nanoscience, and a fascinating challenge of nanotechnology. Like their macroscopic counterparts, nanoscale machines need energy to operate. Although most molecular motors of the biological world are fueled by chemical reactions, light is a very good choice to power artificial molecular machines because it can also be used to monitor the state of the machine, and makes it possible to obtain systems that show autonomous operation and do not generate waste products. By adopting an incrementally staged design strategy, photoinduced processes can be engineered within multicomponent (supramolecular) species with the purpose of obtaining light-powered molecular machines. Such an approach is illustrated in this tutorial review by describing some examples based on rotaxanes investigated in our laboratories.

Autonomous artificial nanomotor powered by sunlight

Light excitation powers the reversible shuttling movement of the ring component of a rotaxane between two stations located at a 1.3-nm distance on its dumbbell-shaped component. The photoinduced shuttling movement, which occurs in solution, is based on a “four-stroke” synchronized sequence of electronic and nuclear processes. At room temperature the deactivation time of the high-energy charge-transfer state obtained by light excitation is ≈10 μs, and the time period required for the ring-displacement process is on the order of 100 μs. The rotaxane behaves as an autonomous linear motor and operates with a quantum efficiency up to ≈12%. The investigated system is a unique example of an artificial linear nanomotor because it gathers together the following features: (i) it is powered by visible light (e.g., sunlight); (ii) it exhibits autonomous behavior, like motor proteins; (iii) it does not generate waste products; (iv) its operation can rely only on intramolecular processes, allowing in principle operation at the single-molecule level; (v) it can be driven at a frequency of 1 kHz; (vi) it works in mild environmental conditions (i.e., fluid solution at ambient temperature); and (vii) it is stable for at least 103 cycles.

Artificial nanomachines based on interlocked molecular species: recent advances

The bottom-up construction and operation of nanoscale machines and motors, that is, supramolecular systems wherein the molecular components can be set in motion in a controlled manner for ultimately accomplishing a function, is a topic of great interest in nanoscience and a fascinating challenge of nanotechnology. The field of artificial molecular machines and motors is growing at an astonishing rate and is attracting a great deal of interest. Research in the last decade has shown that species made of interlocked molecular components like rotaxanes, catenanes and related systems are most attractive candidates. In recent times, the evolution of the structural and functional design of such systems has led to the construction and operation of complex molecular machines that, in some cases, are able to do specific tasks. This tutorial review is intended to discuss the design principles for nanomachines based on interlocked molecules, and to provide a timely overview on representative prototype systems.

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.

A Simple Unimolecular Multiplexer/Demultiplexer

Here we show that the reversible acid/base switching of the absorption and photoluminescence properties of a fluorophore as simple as 8-methoxyquinoline in solution can form the basis for molecular 2:1 multiplexing and 1:2 demultiplexing with a clear-cut digital response.

The bottom-up approach to molecular-level devices and machines

The macroscopic concepts of a device and a machine can be extended to the molecular level. Molecular-level devices and machines are constructed by a bottom-up approach. The atom-by-atom bottom-up approach is unrealistic from the chemical viewpoint. The bottom-up approach molecule-by-molecule following the guidelines of supramolecular (multicomponent) chemistry has proved to be successful. The extension of the concepts of a device and a machine to the molecular level is of interest not only for basic research, but also for the growth of nanoscience and the development of nanotechnology.

From observed to corrected luminescence intensity of solution systems: an easy-to-apply correction method for standard spectrofluorimeters

Abstract Although spectrofluorimetry is a very sensitive analytical technique, its use is limited by the non linear relationship between the concentration of the analyte of interest and the generated electric signal. Here we discuss a very easy correction method which takes into account all the instrumental factors and transforms the observed intensity value in a quantity, the corrected luminescence intensity, that is directly proportional to the concentration of the observed luminophore. At the end of the discussion, a general formula is presented together with two examples illustrating the method and its application.