Computational Investigations of Novel Imines-Based Light-Driven Molecular Motors


Student thesis: Doctoral Thesis

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Award date22 Sep 2022


More and more powerful quantum computational approaches have been developed in recent years allowing one to deeply explore and illustrate the mysteries of the photo-induced molecular motors. These molecular motors are synthesized by using organic molecules as components; generally, a molecular motor consists of two parts, a rotor and a stator moiety, connected by a carbon-carbon double bond or a carbon-nitrogen double bond. Receiving an external stimulus, which could be in the form of light, temperature or an electric field, the motors are able to convert these stimuli into useful work through the unidirectional rotation of the rotor fragment, while the stator fragment is stationary. In this research, Density Functional Theory (DFT) and Time-Dependent Density Functional Theory (TDDFT) are employed to study the unidirectionality, the excited electron patterns in different states so as to investigate the effect of photoisomerization, and illustrate the intrinsic reaction coordinate connecting the reactant, transition state and the product, for understanding the thermal relaxation process, of several molecular systems.

We start with an overview of the first and the second-generation light-driven molecular motors with their potential applications. Afterward, we present the computational methods used in this work including advantages and principles of the DFT and the TDDFT, as well as a torque approach, which is developed by our group, that can be employed to predict the unidirectionality of any motor systems straightforwardly.

Then, we introduce the investigated system, a new type of rotary molecular motors having the similar photo/thermal isomerization processes based on the N-alkyl chiral imines which has been synthesized in 2006. It has a fascinating feature making it different from the overcrowded alkene motors. By modifying the composition of the stator part of the molecule, one can control the rotation of the rotor part to undergo a rotation of either 2 or 4 steps. In the 2 steps pathway, i.e., photoisomerization and nitrogen inversion, the target C=N rotor acts as a photoswitch, while in the 4 steps reaction, i.e., photoisomerization and ring inversion, it acts as a traditional molecular motor. The photoisomerization in either 2 or 4 steps shares the analogous mechanism, which is similar to the C=C motor, i.e., irradiating the molecules with electromagnetic radiation with suitable wavelength for some time will excite the molecule from its ground state to the excited state. While the thermal relaxation process is slightly different, a unique in-plane N inversion is occurred particularly when C=N rotor undergoes the 2 steps pathway, while in the 4 steps pathway, it relaxes like the overcrowded alkene-based molecular motor and complete the rotation unidirectionally. According to the computational results, this C=N rotor should be a good candidate of being a 2-step or 4-step photo-induced molecular motor when one can correctly control the stator part of it.

Finally, we demonstrate that, for an imine-based molecular switch, the existence of the benzene ring attached to the stator fragment alters the unidirectionality of the rotor fragment by means of illustrating the electron excitation patterns and demonstrate the thermal in-plane N inversion process by use of intrinsic reaction coordinate results. It holds a considerable implication for designing molecular motor. Meanwhile, for a 4-steps imine-based molecular motor, we also confirm all six configurations, four ground state structures plus two transition state structures in gas phase and in the solvent environments. We reveal its photo/thermal isomerization processes by plotting the photoisomerization paths and the intrinsic reaction coordinates, which actually shows the thermal ring inversion process. The findings in our works could offer new insights into the development of the molecular motors.

    Research areas

  • Molecular machines, Solvent effect, Torque, Excited state, Intrinsic reaction coordinate, Photoisomerization, Thermal isomerization