Photo-Mechanical Azo Polymers for Light-Powered Actuation and Artificial Muscles
Mahimwalla, Z.; Yager, K.G.; Mamiya, J.-i.; Shishido, A.; Barrett, C.J. "Photo-Mechanical Azo Polymers for Light-Powered Actuation and Artificial Muscles" in Optical Nano and Micro Actuator Technology, Knopf, G.K. and Otani, Y., eds.CRC Press 2012
, Chapter 4
We review photo-mechanical effects in azobenzene materials.
The change in shape inducible in some photo-reversible molecules using light can effect powerful changes to a variety of properties of a host material. This class of reversible light-switchable molecules includes photo-responsive molecules that photodimerize, such as coumarins and anthracenes; those that allow intramolecular photo-induced bond formation, such as fulgides, spiro-pyrans, and diarylethenes; and those that exhibit photoisomerization, such as stilbenes, crowded alkenes, and azobenzene. The most ubiquitous natural molecule for reversible shape change however, and perhaps the inspiration for all artificial biomimics, is the rhodopsin/retinal protein system that enables vision, which is perhaps the quintessential reversible photoswitch for performance and robustness. Here, the small retinal molecule embedded in a cage of rhodopsin helices isomerizes from a cis geometry to a trans geometry around a C=C double bond with the absorption of just a single photon. The modest shape change of just a few angstroms is quickly amplified however, and sets off a cascade of larger shape and chemical changes, eventually culminating in an electrical signal to the brain of a vision event, the energy of the input photon amplified many thousands of times in the process. Complicated biochemical pathways then revert the trans isomer back to cis and set the system back up for another cascade upon subsequent absorption. The reversibility is complete, and many subsequent cycles are possible. The reversion mechanism back to the initial cis state is complex and enzymatic however, so direct application of the retinal/rhodopsin photoswitch to engineering systems is difficult. Perhaps the best artificial mimic of this strong photo-switching effect however, for reversibility, speed, and simplicity of incorporation, is azobenzene. Trans and cis states can be switched in microseconds with low power light, reversibility of 105 and 106 cycles is routine before chemical fatigue, and a wide variety of molecular architectures is available to the synthetic materials chemist permitting facile anchoring and compatibility, as well as chemical and physical amplification of the simple geometric change.
This chapter focuses on the study and application of reversible changes in shape that can be induced with various material systems incorporating azobenzene to effect significant reversible mechanical actuation. This photo-mechanical effect can be defined as the reversible change in shape inducible in some molecules by the adsorption of light, which results in a significant macroscopic mechanical deformation of the host material. Thus, it does not include simple thermal expansion effects, nor does it include reversible but nonmechanical photoswitching or photochemistry, nor
any of the wide range of optical and electro-optical switching effects for which good reviews exist elsewhere. These azobenzenes are similarly of great interest for light energy harvesting applications across much of the solar spectrum, yet this emerging field is still in an early enough stage of research output as to not yet warrant review.