Artificial Molecular Rotors

Chem. Rev. 2005, 105, 1281−1376 1281 Artificial Molecular Rotors Gregg S. Kottas,† Laura I. Clarke,‡ Dominik Horinek,† and Josef Michl*,† Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309-0215, and Department of Physics, North Carolina State University, Raleigh, North Carolina 27695 Received September 8, 2004 Contents 5.4.2. Piano-Stool (Half-Sandwich) Transition 1330 Metal Complexes and Related 1. Introduction 1281 Compounds 2. Scope 1282 5.4.3. Complexes Bearing More Than One Cp 1331 3. Theoretical Issues 1285 Ring 3.1. Overview of Characteristic Energies 1285 5.4.4. Bisporphyrinato and Related Complexes 1331 3.1.1. Inertial Effects 1285 5.4.5. Metal Atoms as “Ball Bearings” 1334 3.1.2. Friction in Molecular Systems 1286 5.5. Rope-Skipping Rotors and Gyroscopes 1335 3.1.3. The Fluctuation−Dissipation Theorem 1286 5.6. Rotators in Inclusion (Supramolecular) 1338 3.1.4. Other Energies in the System 1287 Complexes 3.2. Rotor Behavior in Non-interacting Systems 1288 5.6.1. Rotation in Host−Guest Complexes 1338 3.2.1. Driven Motion 1288 5.6.2. Rotation in Self-Assembled Architectures 1340 3.2.2. Driving Fields 1291 5.6.3. Rotations in Molecular “Onion” 1342 3.2.3. Random Motion 1292 Complexes 3.2.4. Utilizing Rotor Systems in the Random 1297 5.7. Driven Unidirectional Molecular Rotors 1344 Motion Regime 5.7.1. Light-Driven Unidirectional Molecular 1345 3.3. Interacting Rotors 1298 Rotors 3.3.1. Steric Interactions 1298 5.7.2. Chemically Driven Rotors 1347 3.3.2. Electrostatic Interactions 1299 6. Rotors in Solids 1348 4. Experimental and Theoretical Methods 1300 6.1. Phenylene Group Rotations 1350 4.1. Dielectric Spectroscopy 1300 6.2. Geared Rotations in Solids 1352 4.2. Molecular Dynamics 1301 6.3. Solid-State Inclusion Complexes 1352 5. Rotors in Solution 1301 6.4. Rotations in Other Macromolecular Species 1353 5.1. Propellers, Gears, and Cogwheels 1301 6.5. Carbon Nanotubes and Fullerenes 1354 5.1.1. Nomenclature 1302 6.5.1. Carbon Nanotube Gears 1354 5.1.2. Historical Account of Molecular Propellers 1303 6.5.2. Fullerene Clusters 1355 and Gears 7. Rotors on Surfaces 1355 5.1.3. Rotation of Alkyls and Related Groups in 1304 7.1. Physisorbed Rotors 1355 Molecularly Geared Systems 7.2. Chemisorbed Rotors 1358 5.1.4. Biphenyls 1306 7.3. Wheels on Surfaces 1363 5.1.5. Arene Propellers and Gears 1307 8. Conclusions and Outlook 1365 5.1.6. Triptycenes 1311 9. References 1365 5.1.7. Aromatic Amides 1313 5.1.8. Gearing in Organometallic and Inorganic 1313 1. Introduction Systems 5.2. Rotation in Nonsandwich Porphyrins 1314 To many, periodic mechanical conformational mo- 5.2.1. Rotation of Phenyl Groups in 1315 tion of molecules is mesmerizing to a degree that is Phenylporphyrins (PPs) difficult to explain rationally: It is almost as hard 5.2.2. Rotations Involving Pyridylporphyrins 1319 to stop watching an image of internal rotation in a (PyPs) molecule as it is to stop watching the flames of a 5.2.3. Rotations Involving Nonsandwich 1320 campfire. Perhaps this is one of the reasons why Porphyrin Arrays chemists have been fascinated for decades with 5.3. Rotations about Triple Bonds 1326 molecular structures that permit internal mechanical 5.4. Rotations of Molecular Carousels (Sandwich 1328 motion at various degrees of complexity, from random Complexes) flipping to concerted geared motion and to motion 5.4.1. Metallocenes and Related Complexes 1328 intentionally driven in a unidirectional manner. The present review deals with man-made molecular ro- † University of Colorado. tors from the point of view of their potential utility ‡ North Carolina State University. for “molecular machinery”. 10.1021/cr0300993 CCC: $53.50 © 2005 American Chemical Society Published on Web 04/13/2005 1282 Chemical Reviews, 2005, Vol. 105, No. 4 Kottas et al. Gregg Kottas was born in 1974 in Buffalo, NY, and received his B.S. in Dominik Horinek was born in 1972 in Freiburg, Germany. He studied Chemistry from the University of Florida, performing undergraduate Chemistry at the University of Regensburg, Germany, where he got research under the direction of Merle A. Battiste. After an internship at interested in theoretical and computational Chemistry. In 2000, he finished the Eastman Chemical Company in Kingsport, TN, he moved on to his doctorate in the group of Prof. Bernhard Dick, Regensburg, where he graduate studies at the University of Colorado at Boulder under the worked on molecular dynamics simulations of matrix isolated molecules. direction of Prof. Josef Michl. Working on the synthesis and surface studies In 2001 he joined Prof. Josef Michl’s group at the University of Colorado of surface-mounted dipolar rotors, he received his Ph.D. in 2004. Currently, at Boulder as a Feodor Lynen Fellow of the Alexander von Humboldt he is a postdoctoral researcher at the University of Mu¨nster under the Foundation, where his interest is focused on computational studies of direction of Prof. Luisa De Cola, investigating the synthesis and surface-mounted molecular rotors. photophysics of inorganic and organometallic complexes with possible applications for new lighting sources. Photograph and biography for Josef Michl can be found on p 1199. quite complicated paths of all the nuclei to the overall rotation of the molecule in the laboratory frame.1 In gases, liquids, and certain solids, entire molecules can rotate as a whole around three indepen- dent axes, but we will not deal with this phenomenon presently, although molecules rotating in a solid could be referred to as rotors by the dictionary definition2 of the term (“a part that revolves in a stationary part”). Since internal rotation within molecules is nearly ubiquitous (for instance, all molecules with a methyl group qualify), we further restrict the scope of this review to studies of historical Laura Clarke is an Assistant Professor of Physics at North Carolina State importance, in which groundwork was laid to the University. She received her Master’s degree in Physics in 1996 and her current developments, and to studies in which such Ph.D. in Physics in December 1998 from the University of Oregon under rotation has been the focus of attention in the context the supervision of Martin N. Wybourne. Her postdoctoral work was conducted in the laboratory of John C. Price at the University of Colorado. of nanoscience and relevant to the ultimate construc- Her research focuses on electric, dielectric, and optical measurements of tion of molecular-size mechanical structures that organic materials. might perform useful functions. In many borderline instances, we were obliged to make subjective deci- 2. Scope sions concerning inclusion in the review, and we beg the reader for understanding where we may have In the context of nanoscience, it has been custom- erred and also in cases of inadvertent omissions. A ary to use the term molecular rotor for molecules that particularly difficult distinction has been the separa- consist of two parts that can easily rotate relative to tion of pendular motion, which we do not cover, and each other. The rotation is one-dimensional in that rotatory motion. We have taken the view that the it involves changes in a single angle. It is common function of a molecular rotor is continued rotation to view the part with the larger moment of inertia by 360° or more, and we have not treated exhaus- as stationary (the stator) and the part with the tively structures capable of rotating only part way, smaller moment of inertia as the rotator (the other- although some are mentioned in the context of wise more common term rotor has been preempted rotational barriers. as it refers to the whole molecule), but the distinction We only deal with artificial molecular rotors and is truly unambiguous only if the stationary part is pay no heed to naturally occurring protein-based fixed on or within a much more massive object, such rotors and motors, such as ATP synthase.3-8 These as a macroscopic one. In the absence of such mount- have been reviewed repeatedly in the recent past.9-11 ing, the rotator and the stator both turn around a Even among artificial rotors, we have arbitrarily common axis. In the absence of outside torque, a decided to deal only with compounds that have been simple computational procedure relates the possibly isolated as pure chemical species with a well defined Artificial Molecular Rotors Chemical Reviews, 2005, Vol. 105, No. 4 1283 structure, and not with mixtures. This eliminated from consideration almost all work on polymers. Their internal rotations represent a fascinating subject in its own right, and one only needs to think of crankshaft rotation12-20 or the work of Gaub and collaborators21,22 on a bistable nanoscopic machine in which a cis-trans isomerization of azobenzene units shortens a polymer chain and performs nanoscopic work. We have included both molecular rotors exhibiting thermally induced rotation and those designed so as to be capable of being driven by an external force, such as an electric field or a flow of a fluid. Our emphasis is on experimental results, but we have included computational and theoretical work, and Figure 1. Classification system for surface-bound RS section 3 provides a theoretical background for mo- molecular rotors. (See text and Table 1.) lecular rotors. It emphasizes the behavior of rotors mounted in a solid or on a surface, reflecting our the axis of a liquid crystal, controlling binding events belief that these situations are most relevant for or transport, etc.). nanotechnology. Little is said about methods (section Structurally, one can distinguish rotors in which 4), which are generally well known and not particu- the stator and the rotator are covalently attached to larly specific for the study of rotors.

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