 In the past few years, the design of molecular machines has been an intriguing domain for chemists partially inspired by the goal to mimic some of the actions of biological motors. One of the most interesting subjects is to develop actuators or artificial muscles, which can contract and expand in a controllable fashion. Basically, the actuation mechanism is based on the fact that the actuator materials or assemblies of molecules are able to transduce optical, electrical or chemical stimulus into mechanical work through dimensional response. The advantages and limitations of the exploited artificial muscles have been generalized by Baughman and the strategies to improve the performance were suggested. A group of electro-active materials which attract most interests for actuation applications is composed of conducting polymers such as polypyrrole, polyaniline, and polythiophene and carbon nanotubes. One of the main operating principles of this class of redox-active materials is based on the intercalation of dopants and counterions in the polymer matrixes during the redox process, which induces volume change. The major disadvantage of this bulk mechanism is that the actuation frequency is diffusion limited by the counterion flux. In order to overcome this shortcoming, alternative mechanisms have been sought where the actuating mechanism is built on the intrinsic properties of the actuator materials. The intrinsic actuation mechanism of polyacetylene and carbon nanotubes that has been studied by our group is a quantum mechanical effect. It based on the principle that the frontier orbitals that are active during the redox process cause the elongation/shortening in the material. Current work extends these previous results and additionally explores new opportunities offered by conformationally flexible groups that can open and close upon redox processes. An alternative actuation mechanism involves multicenter pancake p-stacking interactions that can stabilize molecular aggregates. Selected references:
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