ABSTRACT
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 arti„cial biomimics, is the rhodopsin/retinal protein system that enables
4.1 Introduction .......................................................................................................................... 107 4.2 Photo-Mechanical Azo Polymers ......................................................................................... 108
4.2.1 Azobenzene Chromophores ...................................................................................... 110 4.2.2 Azobenzene Photochemistry .................................................................................... 112 4.2.3 Classes of Azobenzene Systems ............................................................................... 114
4.3 Photo-Induced Motions and Movements .............................................................................. 117 4.3.1 Molecular Motion ..................................................................................................... 117 4.3.2 Macroscopic Motion ................................................................................................. 118 4.3.3 Phototransport across Surfaces ................................................................................. 119
4.3.3.1 Photopatterning and Photomorphing ......................................................... 121 4.3.3.2 Dependence of Phototransport on Material Properties ............................. 121 4.3.3.3 Photo-Pressure Mechanism of Azo Photomotion ...................................... 122
4.4 Nano-Fabrication Applications of Azo Photomotion ...........................................................124 4.5 Photo-Induced Mechanical Response and Actuation ........................................................... 125
4.5.1 Photoactuation in Monolayers and Interfacial Films ................................................ 126 4.5.2 Photoactuation in Amorphous Thin Films ............................................................... 127 4.5.3 Photoactuation in Liquid Crystalline Azo Polymers ................................................ 129 4.5.4 Photoactuation in Azobenzene Crystals ................................................................... 135
4.6 Applications in Robotics ....................................................................................................... 136 4.7 Conclusions and Outlook ...................................................................................................... 142 References ...................................................................................................................................... 143
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 ampli„ed 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 ampli„ed 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 dif„cult. Perhaps the best arti„cial 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 ampli„cation of the simple geometric change.
