Molecular Dynamics Simulations in Molecular Motors

Rotatory molecular machines have the potential to replace conventional electronic devices as their properties are tunable at molecular level through clean and fast light stimuli. When utilizing molecular motors, mechanical energies are obtained from light. However, the conversion rate is still quite low due to random rotations of the molecules. To improve the working efficiency, the requirement of unidirectional rotation and an in-depth understanding of the rotation mechanism is crucial. This proposal will investigate the yet unknown process of rotational processes in selected molecular motors in solution as well as crafted in metal surfaces using state-of-the-art techniques in theoretical chemistry. The results obtained are expected to offer some general guidelines to synthesize more efficient molecular motors.

In this project, we have carried out molecular dynamics simulations to investigate molecular motors, which have numerous potential applications in electronic devices due to their tunable optical properties in the transformation of light and heat into mechanical movements. The molecular motors could go through two processes, which require light and heat from the environment to make them rotate unidirectionally, however, it is still a challenge to increase their working efficiency due to the low photonic quantum yield and the slow thermal reaction. We have carried out different levels of theories to simulate these two steps for understanding the role of the solvent during the unidirectional rotation of molecular motors. Firstly, in the photo-isomerization process, the electron decays from its excited state to the ground state in about one picosecond, which is 12 orders of magnitude faster than a second. The reaction rate in this photo-reaction was obtained by theoretical simulations, which was similar to the observation in the experiment. We also simulated time-resolved spectra, which showed that in the first 200 fs the electron decayed fast, followed by a formation of a dark state, before the motor relaxed to the ground state in about 1 ps. From the analysis of the orientation of solvents, it was found that solvents did not change their distribution and orientation around the molecular motor, due to the low polarizability of the molecular motor and weak hydrogen bond between the molecular motor and solvents. Secondly, we investigated the thermal step during the rotation of the molecular motor by calculating the reaction barrier. A few molecular motors with different dipole moments were selected in simulations. Reaction barriers were calculated in the gas phase with a high-level method, while a moderate level of theory was carried out to simulate the thermal reaction path when solvents were included explicitly. Both theories indicated that more prominent solvation effected on those molecular motors with larger dipole moments. In summary, our investigations in both the photo and thermal steps cast further light on the functionalization of molecular motors. In particular, we aim at speeding up their rotation, which is promising in utilizing them in the electronic devices.