Cent. Eur. J. Chem. • 6(3) • 2008 • 325–339 DOI: 10.2478/s11532-008-0033-4 Central European Journal of Chemistry Molecular machines operated by light Invited Review Alberto Credi*, Margherita Venturi Dipartimento di Chimica “G. Ciamician”, Università di Bologna, Via Selmi 2 – 40126 Bologna, Italy Received 11 February 2008; Accepted 22 Arpil 2008 Abstract: The bottom-up construction and operation of machines and motors of molecular size is a topic of great interest in nanoscience, and a fascinating challenge of nanotechnology. Researchers in this field are stimulated and inspired by the outstanding progress of mo- lecular biology that has begun to reveal the secrets of the natural nanomachines which constitute the material base of life. Like their macroscopic counterparts, nanoscale machines need energy to operate. Most molecular motors of the biological world are fueled by chemical reactions, but research in the last fifteen years has demonstrated that light energy can be used to power nanomachines by exploiting photochemical processes in appropriately designed artificial systems. As a matter of fact, light excitation exhibits several advantages with regard to the operation of the machine, and can also be used to monitor its state through spectroscopic methods. In this review we will illustrate the design principles at the basis of photochemically driven molecular machines, and we will describe a few examples based on rotaxane-type structures investigated in our laboratories. Keywords: Molecular device • Nanoscience • Photochemistry • Rotaxane • Supramolecular Chemistry © Versita Warsaw and Springer-Verlag Berlin Heidelberg. 1. Introduction Richard Feynman stated in his famous talk in 1959 [4]. Research on supramolecular chemistry has shown that molecules are convenient nanometer-scale building The development of civilization has always been strictly blocks that can be used, in a bottom-up approach, related to the design and construction of devices – to construct ultraminiaturized devices and machines from wheel to jet engine – capable of facilitating man [5]. Chemists are in an ideal position to develop such movement and travelling. Nowadays the miniaturization a molecular approach to functional nanostructures race leads scientists to investigate the possibility of because they are able to design, synthesize, investigate designing and constructing machines and motors at the and operate with molecules – for instance, make them nanometer scale, that is, at the molecular level. Many react or get them together into larger assemblies. fields of technology, in particular information processing, Much of the inspiration to construct molecular have benefited from progressive miniaturization of the devices and machines comes from the outstanding components of devices in the last fifty years. A common progress of molecular biology that has begun to prediction is that further progress in miniaturization reveal the secrets of the natural nanomachines which will not only decrease the size and increase the power constitute the material base of life [6]. Surely, the of computers [1], but could also open the way to new supramolecular architectures of the biological world technologies in the fields of medicine, environment, are themselves the premier, proven examples of the energy and materials [2]. feasibility and utility of nanotechnology, and constitute a The top-down approach used so far for the sound rationale for attempting the realization of artificial construction of miniaturized devices is reaching molecular devices [7,8]. The bottom-up construction fundamental and practical limits, which include severe of machines as complex as those present in Nature cost limitations, for sizes below 50 nanometers [3]. is a prohibitive task. Therefore chemists have tried (i) Miniaturization, however, can be pushed further on to construct much simpler systems, without mimicking because ‘there is plenty of room at the bottom’, as * E-mail: [email protected] 325 Molecular machines operated by light the complexity of the biological structures, (ii) to to apply at the nanoscale macroscopic engineering undestand the principles and processes at the basis of principles [48]. Biomolecular machines are made of their operation, and (iii) to investigate the challenging nanometer-size floppy molecules which operate at problems posed by interfacing artificial molecular constant temperature in the soft and chaotic environment machines with the macroscopic world, particularly as produced by the weak intermolecular forces and the far as energy supply and information exchange are ceaseless and random molecular movements. Gravity concerned. In the last few years the development of and inertia motions we are familiar with in our everyday powerful synthetic methodologies, combined with a experience are fully negligible at the molecular scale; device-driven ingenuity evolved from the attention viscous forces resulting from intermolecular interactions to functions and reactivity, have led to remarkable (including those with solvent water molecules) largely achievements in this field. Among the systems reported prevail and it is difficult to obtain directed motion. are molecular tweezers [9], propellers [10], rotors [11], This means that while we can describe the bottom-up turnstiles [12], gyroscopes [13,14], gears [15], brakes construction of a nanoscale device as an assembly of [16], a molecular pedal [17], ratchets [18], rotary motors suitable (molecular) components by analogy with what [19], shuttles [20], elevators [21], muscles [22], valves happens in the macroscopic world, we should not forget [23], processive artificial enzymes [24], walkers [25-27], that the design principles and the operating mechanisms vehicles [28], and catalytic self-propelled micro- and at the molecular level are different. nano-objects [29,30]. Several excellent reviews [31-46] For the above reasons, it is not easy to define and a monograph [47] dealing with artificial molecular the functions related to artificial molecular motions. A machines and motors are available. simple and immediate categorization is usually based In the first part of this review we will illustrate the on an iconic comparison with motions taking place in basic features of nanoscale machines and discuss their macroscopic systems (e.g. braking, locking, shuttling, implementation with molecular species. In the second rotating). Such a comparison presents the advantage of part we will describe a few examples, selected from an easy representation of molecular devices by cartoons our work, showing how photoinduced processes can that clearly explain their mechanical functions, but it be engineered within rotaxane-type structures with the also implies the danger of overlooking the substantial purpose of obtaining molecular machines driven by light. differences between the macroscopic and molecular A vast quotation of prominent work by other research worlds. groups will also be provided. Finally, we will critically In agreement with the recent literature [44,47], being analyze the limitations of the current systems and the aware that we are dealing with a difficult and potentially perspectives of this research field. controversial topic [37,44], we will comply with the minimum set of terms and definitions reported below: • Mechanical device: a particular type of device 2. Basic concepts designed to perform mechanical movements [49]. • Machine: a particular type of mechanical device 2.1. Molecular motions in artificial systems: designed to perform a specific mechanical terms and definitions movement under the action of a defined energy In the macroscopic world, devices and machines are input. assemblies of components designed to achieve a specific • Motor: a machine capable of using an energy input function. Each component of the assembly performs a to produce useful work. simple act, while the entire assembly performs a more Clearly, there is a hierarchy: a motor is also a complex, useful function, characteristic of that particular machine, and a machine is also a mechanical device, device or machine. In principle, the macroscopic but a mechanical device might not be a machine or a concepts of a device and a machine can be extended to motor and a machine might not be a motor. the molecular level [47]. It is also useful to discuss briefly the relation Nature shows, however, that nanoscale devices and between molecular switches, and molecular machines machines can hardly be considered as ‘shrunk’ versions and motors. A switch is a multi-state system whose of macroscopic counterparts because several intrinsic properties and effects on the environment are a function properties of molecular-level entities are quite different of its state [50,51]. Most often the interconversion from those of macroscopic objects. In fact, the design between two given states of a molecular switch can and construction of artificial molecular machines can take place by the same pathway that is travelled in take greater benefit from the knowledge of the working opposite directions (Figure 1, upper cycle). In this case, principles of natural ones rather than from sheer attempts any mechanical effect exerted on an external system 326 A. Credi, M. Venturi surprisingly, the majority of the molecular motors of the biological world are powered by chemical reactions (e.g., ATP hydrolysis) [6-8]. Richard Feynman observed [4] that «an internal combustion engine of molecular size is impossible. Other chemical reactions, liberating energy when cold, can