Vincenzo Balzani

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.

Electron transfer in chemistry

Volume 1: Principles, theories, methods and techniques- principles and theories/ methods and techniques. Volume 2: Organic, organometallic, and inorganic molecules- organic molecules/ organometallic and inorganic molecules. Volume 3: Natural and artificial supramolecular systems- Biological processes and artificial model systems/ artificial supramolecular systems. Volume 4: Polymers, heterogeneous systems, solid state systems, gas phase systems- polymers/ heterogeneous systems/ gas phase systems. Volume 5: Applications- redox catalysis/ molecular-level electronics/ other applications.

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.

Photochemistry and Photophysics of Coordination Compounds: Ruthenium

Ruthenium compounds, particularly Ru(II) polypyridine complexes, are the class of transition metal complexes which has been most deeply investigated from a photochemical viewpoint. The reason for such great interest stems from a unique combination of chemical stability, redox properties, excited-state reactivity, luminescence emission, and excited-state lifetime. Ruthenium polypyridine complexes are indeed good visible light absorbers, feature relatively intense and long-lived luminescence, and can undergo reversible redox processes in both the ground and excited states. This chapter presents some general concepts on the photochemical properties of Ru(II) polypyridine complexes and gives an overview of various research topics involving ruthenium photochemistry which have emerged in the last 15 years. In particular, aspects connected to supramolecular photochemistry and photophysics are discussed, such as multicomponent systems for light harvesting and photoinduced charge separation, systems for photoinduced multielectron/hole storage, and photocatalytic processes based on supramolecular Ru(II) polypyridine species. Interaction with biological systems and dye-sensitized photoelectrochemical cells are also briefly discussed.

The hydrogen issue.

Hydrogen is often proposed as the fuel of the future, but the transformation from the present fossil fuel economy to a hydrogen economy will need the solution of numerous complex scientific and technological issues, which will require several decades to be accomplished. Hydrogen is not an alternative fuel, but an energy carrier that has to be produced by using energy, starting from hydrogen-rich compounds. Production from gasoline or natural gas does not offer any advantage over the direct use of such fuels. Production from coal by gasification techniques with capture and sequestration of CO₂ could be an interim solution. Water splitting by artificial photosynthesis, photobiological methods based on algae, and high temperatures obtained by nuclear or concentrated solar power plants are promising approaches, but still far from practical applications. In the next decades, the development of the hydrogen economy will most likely rely on water electrolysis by using enormous amounts of electric power, which in its turn has to be generated. Producing electricity by burning fossil fuels, of course, cannot be a rational solution. Hydroelectric power can give but a very modest contribution. Therefore, it will be necessary to generate large amounts of electric power by nuclear energy of by renewable energies. A hydrogen economy based on nuclear electricity would imply the construction of thousands of fission reactors, thereby magnifying all the problems related to the use of nuclear energy (e.g., safe disposal of radioactive waste, nuclear proliferation, plant decommissioning, uranium shortage). In principle, wind, photovoltaic, and concentrated solar power have the potential to produce enormous amounts of electric power, but, except for wind, such technologies are too underdeveloped and expensive to tackle such a big task in a short period of time. A full development of a hydrogen economy needs also improvement in hydrogen storage, transportation and distribution. Hydrogen and electricity can be easily interconverted by electrolysis and fuel cells, and which of these two energy carriers will prevail, particularly in the crucial field of road vehicle powering, will depend on the solutions found for their peculiar drawbacks, namely storage for electricity and transportation and distribution for hydrogen. There is little doubt that power production by renewable energies, energy storage by hydrogen, and electric power transportation and distribution by smart electric grids will play an essential role in phasing out fossil fuels.

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.

Photochemistry and photophysics of Ru(II)polypyridine complexes in the Bologna group. From early studies to recent developments

Abstract The investigations carried out in the Bologna group on Ru(bpy)32+ (bpy=2,2′-bipyridine) and related systems are reviewed. The following topics are discussed: (i) bimolecular energy and electron-transfer processes (including the measure of the rate of self-exchange energy-transfer); (ii) tuning the excited state properties by changing the ligands (including a caged version of Ru(bpy)32+); (iii) chemi- and electrochemiluminescent processes (including the description of an artificial firefly); (iv) photochemistry without light; (v) molecular-level wires for energy and electron transfer; (vi) dendrimers for light harvesting; (vi) light-powered molecular machines.

ANTENNA EFFECT IN LUMINESCENT LANTHANIDE CRYPTATES: A PHOTOPHYSICAL STUDY

Excited state emission and absorption decay measurements have been made on the cage‐type cryptate complexes [M bpy.bpy.bpy]n+, where Mn+= Na+, La3+, Eu3+, Gd3+ or Tb3+ and [bpy.bpy.bpy] is a tris‐bipyridine macrobicyclic cryptand. Excitation has been performed in the high intensity 1π‐π* cryptand band with maximum at about 300 nm. Experiments have been carried out in H2O or D2O solutions and at 300 and 77 K to evaluate the rate constants of radiative and nonradiative decay processes. For Mn+= Na+, La3+ and Gd3+ the lowest excited state of the cryptate is a 3ππ* level of the cryptand which decays in the microsecond time scale at room temperature in H2O solution and in the second‐millisecond time scale at 77 K in MeOH‐EtOH. For Mn+= Eu3+, the lowest excited state is the luminescent 5D0 Eu3+ level which in H2O solution is populated with 10% efficiency and decays to the ground state with rate constants 2.9 × 103 s_1 at room temperature and 1.2 × 103 s−′ at 77 K. The relatively low efficiency of 5D0 population upon 1ππ* excitation is attributed to the presence of a ligand‐to‐metal charge transfer level through which 1ππ* decays directly to the ground state. For Mn+= Tb3+ the lowest excited state is the luminescent 5D4 Tb3+ level. The process of 5D4 population upon 1ππ* excitation is ˜100% efficient, but at room temperature it is followed by a high‐efficiency, activated back energy transfer from the 5D4 Tb3+ level to the 3ππ* ligand level because of the relatively small energy gap between the two levels (1200 cm_1) and the intrinsically long lifetime of 5D4. At 77 K back energy transfer cannot take place and the 5D4 Tb3* level deactivates to the ground state with rate constant 5.9 × 102 s‐′ (H2O solution). The relevance of these results toward the optimization of Eu3+ and Tb3+ cryptates as luminescent probes is discussed.