REVIEWS Artificial Molecular Machines Vincenzo Balzani,* Alberto Credi, Françisco M. Raymo, and J. Fraser Stoddart* The miniaturization of components which its operation can be monitored ular machines, the most significant used in the construction of working and controlled, 4) the ability to make it developments in the field of artificial devices is being pursued currently by repeat its operation in a cyclic fashion, molecular machines are highlighted. the large-downward (top-down) fabri- 5) the timescale needed to complete a The systems reviewed include 1) chem- cation. This approach, however, which full cycle of movements, and 6) the ical rotors, 2) photochemically and obliges solid-state physicists and elec- purpose of its operation. Undoubtedly, electrochemically induced molecular tronic engineers to manipulate pro- the best energy inputs to make molec- (conformational) rearrangements, and gressively smaller and smaller pieces of ular machines work are photons or 3) chemically, photochemically, and matter, has its intrinsic limitations. An electrons. Indeed, with appropriately electrochemically controllable (co- alternative approach is a small-upward chosen photochemically and electro- conformational) motions in inter- (bottom-up) one, starting from the chemically driven reactions, it is possi- locked molecules (catenanes and ro- smallest compositions of matter that ble to design and synthesize molecular taxanes), as well as in coordination and have distinct shapes and unique prop- machines that do work. Moreover, the supramolecular complexes, including ertiesÐnamely molecules. In the con- dramatic increase in our fundamental pseudorotaxanes. Artificial molecular text of this particular challenge, chem- understanding of self-assembly and machines based on biomolecules and ists have been extending the concept of self-organizational processes in chem- interfacing artificial molecular ma- a macroscopic machine to the molec- ical synthesis has aided and abetted the chines with surfaces and solid supports ular level. A molecular-level machine construction of artificial molecular are amongst some of the cutting-edge can be defined as an assembly of a machines through the development of topics featured in this review. The distinct number of molecular compo- new methods of noncovalent synthesis extension of the concept of a machine nents that are designed to perform and the emergence of supramolecular to the molecular level is of interest not machinelike movements (output) as a assistance to covalent synthesis as a only for the sake of basic research, but result of an appropriate external stim- uniquely powerful synthetic tool. The also for the growth of nanoscience and ulation (input). In common with their aim of this review is to present a the subsequent development of nano- macroscopic counterparts, a molecular unified view of the field of molecular technology. machine is characterized by 1) the kind machines by focusing on past achieve- of energy input supplied to make it ments, present limitations, and future Keywords: catenanes ´ molecular ma- work, 2) the nature of the movements perspectives. After analyzing a few chines ´ nanodevices ´ photochemistry of its component parts, 3) the way in important examples of natural molec- ´ rotaxanes ´ supramolecular chemistry [*] Prof. V. Balzani, Dr. A. Credi Dipartimento di Chimica ªG. Ciamicianº 1. Molecular-Scale Machines UniversitaÁ di Bologna What would be the utility of such machines? Who knows? I cannot via Selmi 2, 40126 Bologna (Italy) see exactly what would happen, but I can hardly doubt that when we Fax : (39)051-2099456 have some control of the arrangement of things on a molecular scale E-mail: [email protected] we will get an enormously greater range of possible properties that Prof. J. F. Stoddart substances can have, and of the different things we can do. Department of Chemistry and Biochemistry [1] University of California, Los Angeles Richard P. Feynman (1959) 405 Hilgard Avenue, Los Angeles, CA 90095-1569 (USA) Fax : (1)310-206-1843 E-mail: [email protected] 1.1. The Concept of a Machine at the Molecular Level Prof. F. M. Raymo Center for Supramolecular Science In everyday life we make extensive use of (macroscopic) Department of Chemistry, University of Miami [2] 1301 Memorial Drive, Coral Gables, FL 33146-0431 (USA) machines. A machine is ªan apparatus for applying mechan- E-mail: [email protected] ical power, having several parts each with a definite functionº. Angew. Chem. Int. Ed. 2000, 39, 3348 ± 3391 WILEY-VCH Verlag GmbH, D-69451 Weinheim, 2000 1433-7851/00/3919-3349 $ 17.50+.50/0 3349 REVIEWS V. Balzani, J. F. Stoddart et al. V. Balzani A. Credi F. M. Raymo J. F. Stoddart Vincenzo Balzani was born in Forlimpopoli (Italy) in 1936. He received his ªLaureaº in Chemistry at the University of Bologna in 1960. After a few years as Assistant Professor at the University of Ferrara, he joined the Faculty of Science of the University of Bologna in 1969, where he has remained to this day. He was Director of the Photochemistry and Radiaton Chemistry Institute of the Italian National Research Council (1977 ± 1988) and Chairman of the European Photochemistry Association (1988 ± 1992). He has obtained several awards, including the Ziegler ± Natta Lectureship of the Gesellschaft Deutscher Chemiker (1994), the Italgas European Prize for Research and Innovation (1994), the Centenary Lectureship of the Royal Chemical Society (1996), and the Porter Medal for Photochemistry (2000). He is the author of two monographs and of more than 400 scientific papers. His research interests include photochemistry, electron-transfer reactions, supramolecular chemistry, dendrimer chemistry, and molecular-level devices and machines. Alberto Credi was born in Bologna (Italy) in 1970. After receiving a ªLaureaº in Chemistry from the University of Bologna (1994), he spent one more year in the same University as a research fellow (Ciba-Geigy), and six months as a visiting scientist (NATO) at the University of Virginia with Professor F. S. Richardson. In early 1999 he obtained his Ph.D in Chemical Sciences from the University of Bologna under the supervision of Professor V. Balzani. He was selected, as a graduate student, to attend the meeting Scientia Europña, and invited to illustrate his research at the University of Zurich, Switzerland. His dissertation, entitled ªMolecular-level Machines and Logic Gatesº, received the annual award of the Italian Chemical Society and, very recently, the 2000 IUPAC Prize for young Chemists. After a short period as a research assistant, he is presently researcher at ªG. Ciamicianº Department of Chemistry of the University of Bologna. His research interests, which include photophysics, photochemistry and electrochemistry of metal complexes, supramolecular systems, and dendrimers, are mainly focused on the development of molecular-level devices and machines. He is the author of about 60 publications in the field of molecular and supramolecular photochemistry and electrochemistry. Françisco M. Raymo was born (1969) and educated in Messina (Italy). He obtained a ªLaurea in Chimicaº (1992) from the University of Messina and a ªPh.D. in Chemistryº (1996) from the University of Birmingham. As a graduate student, he developed numerous approaches to the template-directed syntheses of catenanes and rotaxanes under the supervision of Professor J. F. Stoddart. Thereafter, he was a Postdoctoral Research Fellow at the University of Birmingham (1996 ± 1997) and, later, at UCLA (1997 ± 1999) working, in collaboration with Professors K. N. Houk and J. F. Stoddart, on the design, synthesis, and computational investigation of molecular machines incorporating interlocked components. Currently, he is an Assistant Professor in the Center for Supramolecular Science at the Department of Chemistry of the University of Miami. In 1998, he was selected as the ªMarie Curie Success Storyº for Chemistry by the European Commission and also received a ªChancellors Award for Postdoctoral Researchº at UCLA. His research interests range from the design and synthesis of artificial self-assembling systems, molecule-based materials, and mechanically interlocked molecules to crystal engineering, template-directed syntheses, and the computational investigation of recognition phenomena. He is the author of more than 90 publications in the areas of computational chemistry, chemical synthesis, materials science, and supramolecular chemistry. J. Fraser Stoddart was born in Edinburgh, Scotland in 1942. He received all of his degrees (B.Sc., 1964; Ph.D., 1966; D.Sc., 1980) from the University of Edinburgh. He carried out postdoctoral work at Queens University in Canada before joining the academic staff at the University of Sheffield in 1970. After a seven-year spell as the Professor of Organic Chemistry at the University of Birmingham, he moved, in 1997, to the Saul Winstein Chair of Organic Chemistry at UCLA. Much of his research effort over the last 15 years has been channeled into the design and construction of molecular machinery based on switchable catenanes and rotaxanes obtained by both noncovalent synthesis and supramolecular assistance to covalent synthesis. Present and future research goals include the interfacing of artificial molecular machines with the macroscopic world by applying self-assembly protocols to the fabrication of molecular devices. 3350 Angew. Chem. Int. Ed. 2000, 39, 3348 ± 3391 Artificial Molecular Machines REVIEWS When a machine is working, at least some of its