Computational Studies of Molecular Rotors in Solvents and External Stimulus

溶劑和外部刺激下的分子馬達計算研究

Student thesis: Doctoral Thesis

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Award date3 Sep 2021

Abstract

The enormous progress in computational approaches in recent years has developed new ways to investigate artificial molecular systems. The techniques are considered as promising tools parallel to the experiments, which can provide deep understandings of the potential applications of molecular rotors. The design of new rotors using organic molecules as components can turn the external stimulus into useful work. The external energy could be in the form of light, heat, or an electric field. In this research, quantum mechanical calculations combined with molecular dynamics simulations are adopted generally to study several molecular rotors and their capability of unidirectional rotation.

Herein, we begin with a comprehensive overview of molecular motors in different generations and some potential applications. Then, in a later chapter, we introduce the computational methods used in our work, such as static and dynamic features of density functional theory (DFT) and density functional tight-binding (DFTB) methods. The torque approach is also discussed, which is effectively used to probe the rotation patterns in any molecular system.

Next, we introduced a model of interlocking molecular rotors named as gears. The gears are based on the organic hexaethynlyl carbon rings capable of transferring the rotation in gas phases. The response of molecular gears in solvent environments is investigated when the gears are physisorbed on the graphene surface. The influence of several kinds of solvents on the rotation transfer reveals a pattern dependent on the viscosity of solvents. The torque approach is highly effective here in understanding better the experimental situation of molecular rotors working in liquid media. Interestingly, a counter-intuitive behavior is observed where the solvent with higher viscosity exerts less influence on hindering the rotation transfer between the gears. The results provide a design of a reasonable molecular motor for pumping fluid at the nanoscale.

Subsequently, it is shown that the widely studied light-activated fluorene-based molecular rotary motor can be used to design a huge molecular nanocar. The first nanocar synthesized and examined in B.L. Feringa’s lab can be activated electrically and mounted on metal surfaces. However, our computational study provides the mechanism of the photoisomerization process in the nanocar that can control the speed and direction of motion. We also employed the charged system of anions and cations to compare energy conversion efficiency and overcome the barrier. The results show that nanocar can not be activated photochemically to walk in a straight direction. In contrast to light, the electric charge can drive the molecule in a controlled manner.

Finally, we investigated a system of light-driven molecular motors derived from imines. The design is made up of two kinds of cis-trans photoisomerization processes followed by in-plane nitrogen inversion that imines can go through. Next to the imine, the placement of the stereogenic center leads to the preferred rotation, and two stereoisomers are formed. The in-plane inversion of the rotor part of the molecule results in thermal helix inversion. The electronic ground and excited state potential energy curves help understand the preferential direction of rotation in chiral imines. The findings in the above chapters are considered together to conclude that fine-tuning in the design of molecular rotors can provide a controlled motion at the molecular level. Based on the findings of the above investigations, it is inferred that molecular motors can demonstrate controlled movement with an optimal machine design.

    Research areas

  • Molecular machines, Nanocar, Nanogears, Solvent effects, Torque, Excited state, Molecular dynamics, Photoisomerization, Potential energy surface