Theoretical Study Of Artificial Molecular Motors - Azobenzene Photoisomerization Mechanism And Load-bearing Capacity,

Biomotors such as kinesin and dynein show us that robust track-walking is possible down to molecular scale. Our group designed a laser-powered molecular locomotive that is able to do that on an easily constructed track. The core of the machine is its work cycle that periodically converts optical energy into mechanical work, which is further rectified into processive, directional motion. Azobenzene molecule is a major candidate for the backbone of the molecular locomotive. It is also widely used in other mechanical nanodevices. This thesis investigates theoretically the optomechanical conversion capacity of azobenzene molecule.Nonadiabatic dynamical simulations were carried out to study cis-to-trans isomerization of azobenzene under laser irradiation and/or external mechanical loads. We used a semiclassical electron-radiation-ion dynamics method that is able to describe the coevolution of the structural dynamics and the underlying electronic dynamics in a real-time manner. It is found that azobenzene photoisomerization occurs predominantly by an out-of-plane rotation mechanism even under a nontrivial resisting force of several tens of piconewtons. We have repeated the simulations systematically for a broad range of parameters for laser pulses, but could not find any photoisomerization event by a previously suggested in-plane inversion mechanism.The simulations found that the photoisomerization process can be held back by an external resisting force of 90-200 pN depending on the frequency and intensity of the lasers. This study also found that a pure mechanical isomerization is possible from the cis-to-trans state if the azobenzene molecule is stretched by an external force of about 1250-1650 pN. Remarkably, the mechanical isomerization first proceeds through a mechanically activated inversion, and then is diverted to an ultrafast downhill rotation that accomplishes the isomerization. Implications of these findings to azobenzene-based nanomotors and nanodevices are discussed.