Molecular , Supra molecular , and Macromolecular Motors and Artificial Muscles

Abstract

Recent developments in chemical synthesis, nanoscale assembly, and molecularscale measurements enable the extension of the concept of macroscopic machines to the molecular and supramolecular levels. Molecular machines are capable of performing mechanical movements in response to external stimuli. They offer the potential to couple electrical or other forms of energy to mechanical action at the nanoand molecular scales. Working hierarchically and in concert, they can form actuators referred to as artificial muscles, in analogy to biological systems. We describe the principles behind driven motion and assembly at the molecular scale and recent advances in the field of molecular-level electromechanical machines, molecular motors, and artificial muscles. We discuss the challenges and successes in making these assemblies work cooperatively to function at larger scales. Molecular, Supra molecular, and Macromolecular Motors and Artificial Muscles Dongbo Li, Walter F. Paxton, Ray H. Baughman, Tony Jun Huang, J. Fraser Stoddart, and Paul S. Weiss Introduction The rapid scaling of electronic devices and other structures to smaller dimensions has driven intense study of new techniques for the assembly and construction of devices and machines with functional components based on nanometerscale building blocks. In the last few decades, remarkable advances in chemical synthesis, nanoscale assembly, and molecular measurements have enabled the development of such molecular-scale machines. A variety of nanoscopic species, including single molecules,1–3 supramolecular assemblies (a discrete number of molecules connected through noncovalent bonds),4–6 nanoparticles,7,8 nanowires,9–11 and nanotubes,12–16 have been used as components, each offering unique mechanical properties. Ultimately, success at miniaturization will extend the macroscopic concept of machines to the molecular level. In principle, a molecular machine consists of a discrete number of molecules or other nanoscale components, each capable of performing mechanical movements in response to external stimuli.17 While such machines can be as simple as single molecules undergoing conformational changes, they also may function hierarchically and in concert, performing complex functions in analogy to biological systems. One of the most promising examples of this complexity to date, artificial muscle, is discussed in this overview article. Like their macroscopic counterparts, molecular machines must be powered by electrical,1,3 chemical,18 electrochemical,19 or optical20–23 means. Although certain key examples of such mechanical coupling are already understood, new strategies for energetic coupling at the nanoscale will enable new applications. Thus, understanding the stimulus-response behavior of molecules and assemblies at the nanometer scale is critical for the practical implementation and optimization of molecular machines. We survey recent progress toward the control and understanding of nano mechanical motion in prototypical molecular systems. Although there are a variety of artificial molecular machines, such as molecular logic gates24 and molecular tweezers of DNA25, we focus primarily on three classes of artificial molecular machines that have proven especially promising in this rapidly developing field: electric field–driven single-molecule switches, chemically or electrochemically actuated bistable rotaxane molecular motors, and electrochemically powered carbon nanotube-based artificial muscles. In each area, we discuss the fundamental principles of molecular assembly, nanomechanical performance characteristics of the singleor supramolecular species, and the physical mechanism underlying stimulated molecular motion. We also analyze the technological challenges and perspectives associated with improving the performance of each molecular machine. Electric Field–Driven SingleMolecule Conductance Switching Attempts to miniaturize microelectronic devices to the nanometer scale have led to tremendous research efforts to exploit synthetic and biological molecules as active components of devices. In order to gain insight into how molecules and assemblies function as self-contained nanoelectronic and nanomechanical devices, understanding the behavior of individual molecules and supramolecular assemblies is of critical importance.3,26–28 The key challenges for this research include not only fabricating precise assemblies of molecules in controlled environments but also accessing their properties and functions at the molecular and nanometer scales. Recently, selfand directed assembly (methods to organize molecules and to control their placement based on chemical and noncovalent interactions for self-assembly; also, processing conditions and processing order for directed assembly) in combination with scanning probe microscopy techniques have led to significant progress in electric field–driven single-molecule switching. Here, we discuss detailed experimental observations of the switching of individual and bundled molecules and the mechanism behind driven motion. MRS BULLETIN • VOLUME 34 • SEPTEMBER 2009 • www.mrs.org/bulletin 671 Current challenges and future perspectives on utilizing single molecules as switches and developing them into nanometer electromechanical devices are summarized. Self-Assembly and Measurements In order to study and to control the environment and thus the stability of single-molecule switches and motors, these molecules are inserted into twodimensional self-assembled monolayer (SAM) matrices.3,26,27,29–31 Alkanethiolate molecules (CnH2n+1S ) in the matrix have low electrical conductivity and low chemical reactivity, and when assembled into monolayers, their dynamics are understood sufficiently to allow straightforward control and manipulation. In addition, specific types of interactions can be designed and positioned into the functional molecules and their surrounding matrices.3,27,32,33 All of these favorable features make alkanethiolate monolayers well suited for serving as host matrices for insertion of target molecules.34–36 Figure 1a schematically shows the process of inserting guest molecules into an n-alkanethiolate SAM on Au{111}, which serves as the substrate surface for the formation of SAM, the physical support and connection, and one of the electronic contacts. Figure 1b shows a molecularresolution scanning tunneling microscopy (STM) image of a typical alkanethiolate SAM, in which a number of different types of defects, including domain boundaries, step edges, gold substrate vacancy islands, and disordered SAM regions, are observed.3 These defects and their densities can be controlled by processing the matrix.37,38 After exposing the alkanethiolate SAMs to guest molecules either in solution, vapor, or by contact (e.g., with a patterned polymer stamp), single molecules can be selectively inserted at the defect sites.3,23,26,27,30,31,39–45 Due to physical constraints from the surrounding matrix and (optional) designed intermolecular interactions, the inserted molecules are limited in their motion and stabilized in specific conformations. The resulting surface structure, as shown in Figure 1c, consists of isolated molecules protruding from host SAM matrices, enabling measurements of driven molecular switching at the single-molecule level by STM.39 In this measurement configuration, a sharp metallic STM tip addresses the isolated molecule and creates a circuit with the molecule and substrate, in which the tip serves as top electrode and the gold substrate as the bottom electrode.31,39,46,47 Although STM images convolve the probe tip and surface structures and therefore 672 MRS BULLETIN • VOLUME 34 • SEPTEMBER 2009 • www.mrs.org/bulletin Molecular, Supra molecular, and Macromolecular Motors and Artificial Muscles

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Cite this paper

@inproceedings{Li2009MolecularS, title={Molecular , Supra molecular , and Macromolecular Motors and Artificial Muscles}, author={Dongbo Li and Walter F. Paxton and Ray H Baughman and Tony Jun Huang and Paul S. Weiss}, year={2009} }