Theoretical Design Of Conjugated Polyene-Based Light-Driven Molecular Motors Based On Push-Pull Electronic Structure

For the past few recent years,various synthetic molecular motors continually create.Among them,the light-driven molecular motors designed by Feringa team has attracted much attention due to its renewable driving mode and vast development prospect.First of all,light energy has the merits of high cleanliness,easy control and good selectivity.Secondly,there are two chiral elements in the structure of the overcrowded-alkene,the stereocenter and the helicity of the molecule.The coordinated interaction between the two chiral elements can ensure the continuous unidirectional rotation of the motors,which can be applied in many aspects.Generally,unidirectional rotation is achieved through continuous photoisomerization and thermal isomerization steps(light step?thermal step?light step?thermal step).Based on the classical model of light driven molecular motor(fluorene motor),how to design the light-driven molecular motors with different rotation speed and make the motors more widely used are the problems to be considered.According to the structure of fluorene motor,three motor models are designed,which are DMA-indene fluorene motor with electron donating dimethylamino group in rotor,lactone fluorene-like motor with electron withdrawing oxygen group in stator and DMA-indene lactone fluorene-like motor with push-pull electron group in rotor and stator.(TD-)DFT and SF-DFT methods are used in this paper.The study aim of this paper is to clarify the design reasons and photochemical reaction mechanism of the overcrowded-alkene-based motor.The main contents include the following two parts:In the first part,We designed the DMA-indene fluorene motor with electron donating dimethylamine group in the rotor and the lactone fluorine-like motor with electron withdrawing oxygen group in the stator,and used(TD-)BHHLYP/6-31G(d)methods to research its photoisomerization mechanism.The results show that on the S1 potential energy curve,the DMA-indene fluorene motor relaxes from the FC point to the central rotation angle of 90° region,and its energy difference(26.5 kcal/mol)is greater than that of the fluorene motor in this process(25.4 kcal/mol),indicating that the DMA-indene fluorene motor has a large driving force and a fast structural relaxation process.The energy difference between the vertical structure(?=90°)of S1 state potential energy curve and the vertical structure(?=90°)of S0 state potential energy curve of DMA-indene fluorene based motor(21.8 kcal/mol)is smaller than that of fluorene motor(25.8 kcal/mol).The non-adiabatic coupling is strengthened,which is conducive to the non-radiative transition process.However,the UV absorption spectra showed that the DMA-indene fluorene motor could not rotate under visible light.For the lactone fluorine-like motor,on the S1 potential energy curve,the energy difference(10.8 kcal/mol)of the lactone fluorene motor is smaller than that(25.4 kcal/mol)of the fluorene motor from the FC point to the central rotation angle of 90°region,indicating that the driving force of the lactone fluorene motor is smaller and the structural relaxation process is slower.The energy difference between the vertical structure(?=90°)of the S1 energy curve and the vertical structure(?=90°)of the S0 energy curve of the DMA-indene fluorene motor(16.7 kcal/mol)is significantly smaller than that of the fluorene motor(25.8 kcal/mol),which greatly improves the non-radiation transition process.The UV absorption spectra showed that the lactone fluorine-like motor could rotate under visible light.By comparing and analyzing the two motors,it can be found that the acceleration of the thermal step will affect the non-radiative transition process of the light process,and the two restrict each other.Adding electron-donating and electron-withdrawing substituents to the rotor and stator of the fluorene motor,the nature of the motor are changed,and its affect on light and thermal process is different,each has its advantages and disadvantages.Therefore,fluorene motors can be further modified.In the second part,a dimethylamine lactone fluorine-like motor model with electron donor dimethylamino and electron acceptor oxygen in the rotor and stator was designed,and used(TD-)BHHLYP/6-31 G(d)methods to research its photoisomerization mechanism.The consequence indicated that in the photoisomerization process,the energy difference between the vertical structure(?=90°)of the S1 trend energy curve of the dimethylamine lactone fluorine-like motor and the vertical structure(?=90°)of the So trend energy curve(12.3 kcal/mol)was smaller than that of the fluorene motor(25.8 kcal/mol).Compared with the fluorene-based motor,the non-adiabatic coupling of the dimethylamine lactone fluorine-like motor was strengthened,and the key point CIs in the non-radiation process optimized by the SF-BHHLYP/6-31G(d)method had little difference with the structure and energy of the excited stable intermediate,which was beneficial to the non-radiation transition process.In addition,it can be observed from the absorption spectrum that the dimethylamine lactone fluorine-like motor can rotate under visible light.At the same time,the energy of the two transition states TS2 and TS4 in the step2 and step4 thermal helix inversion process is lower than that of the thermal cis-trans isomerization transition states(TS1 and TS3)in the photoisomerization process,which can ensure the continuous unidirectional rotation of the motor.Structurally,the central C=C bond of dimethylamine lactone fluorine-like motor is longer than that of fluorene motor,which greatly reduces the potential barrier of thermal isomerization process and increases the motor speed.This study not only proved that the designed dimethylamine lactone fluorine-like motor model can be used as a light-driven molecular motor,but also introduced the push-pull electronic group to effectively regulate the photoisomerization and thermal isomerization process,which provided a theoretical basis for the design of new light-driven molecular motors.