The rotational process and conformation inversion of the second generation molecular motor (9‐(5‐methoxy‐2‐methyl‐2, 3‐dihydro‐1H‐cyclopentyl[a]naphthalene‐1‐subunit)‐9H‐fluorene) are calculated and analyzed by potential energy surface scanning at the level of density functional theory, the M06‐2X functional combined with the def‐TZVP basis set. The effects of four different donor–acceptor substituents on the molecular stability are mainly explored. The fully optimized geometries of molecular motors 1–5 are discussed at the M06‐2X/def‐TZVP theoretical level. The energy barrier analysis of molecular motors shows that the nature of the substituents will not have a significant impact on the thermal isomerization barrier of the motor. Theoretical analysis of frontier molecular orbital (FMO) shows that after replacing the methyl group with phenyl, methoxy, fluorine, and cyano, the stability of the molecule is reduced. It can be seen from the absorption spectrum that the substituent makes the molecular motor absorption peak significantly red shifted from the absorption spectrum. The study of nuclear magnetic resonance (NMR) shows that it may be affected by deshielding effects, and the substitution of the methyl substituent causes most of the chemical shift (δ) of the molecular motor to a downfield. Finally, it is verified that the conformation of the motors changed from a stable state to unstable state during the photoisomerization process. The calculated results have explained the rotation of molecular motor well and can be used in the design of molecular motor.

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The isomerization of the double bond plays an important role in the braking and de-braking of the light-driven molecular brake. Therefore, the Pp-type light-controlled molecular brake system containing the C=C double bond was theoretically studied. Combining the 6-31G(d) basis set, the ωB97XD functional with dispersion correction was applied to implement the (E)-configuration and (Z)-configuration initial optimization. Next, using the 6-311G(d,p) basis set, the relaxed potential energy surface scans of the rotation angle were operated, and then the optimization calculations of the transition states at the extremum high points. Analyzing the stagnation points and the rotational transition state on the MEPs, the rotation mechanism and basic energy parameters of the molecular brake were obtained. Then the DFT computations at ground states and the TD-DFT computations of vertical excitation energy was put into practice at the accuracy of the def-TZVP basis set for the two configurations, and using the natural transition orbital (NTOs) analyses combining the excitation energies and absorption spectrums of the molecules, the electronic transition characteristics and electron transfer properties of light-driven molecular brake were studied. Afterwards, in order to investigate the photo-induced isomerization reaction, the C=C double bond was scanned on the relaxed potential energy surface, and the intermediates of the isomerization reaction was searched and analyzed, thus, the braking mechanism of the light-driven molecular brake was proposed.

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