Elastic properties of photoswitchable azobenzene polymers from molecular dynamics simulations.

Nanomechanical devices or molecular machines will, for a broad range of applications, most likely be powered by light or other kinds of electromagnetic radiation. The major reasons are ease of addressability, picosecond reaction times to external stimuli, and compatibility with a broad range of ambient substances, such as solvents, electrolytes, or gases. Azobenzene is a well-studied photoactive system, which can be photoswitched selectively from an extended trans to a more compact cis conformation by using light of wavelength 365 nm, whereas the reverse cis-to-trans isomerization is induced by light of wavelength 420 nm (Figure 1a). 6] Many processes, such as light-driven ion transport through biological membranes, can be steered by conformational switching of azobenzene chromophores. Azobenzene has been used frequently in synthetic photoresponsive systems for the regulation of the geometry and function of biomolecules. The isomerization of individual azobenzene molecules has also been studied by scanning tunneling microscopy. 21] Recently, a bistable polyazobenzene peptide was synthesized as a model system for a light-powered molecular machine, and its mechanical properties were characterized by means of single-molecule atomic force microscopy (AFM) experiments (Figure 1b, c). 23] Optical switching of the azobenzene polymers between their extended trans and compact cis conformations was demonstrated, and the corresponding change in the contour length of the polymer was detected. Thereby, in analogy to an Otto cycle, Gaub and co-workers established an optomechanical operating cycle, in which “optical” contraction against an external force delivered net mechanical work. Thus, they demonstrated that azobenzene polymers indeed hold great promise for future applications in nanotechnology, for example, as light-triggered molecular switches or cargo lifters. In a related experiment, Vancso and co-workers characterized a redox– mechanical cycle by using electrochemical AFM-based single-molecule force spectroscopy.