Theoretical Research On The Light-driven Molecular Rotation Motor Of Polyene And Imine

The invention and development of machines run through the evolution of human society.Machines have released human from hard labour,inspired people to think and create,and promopted human being towards a higher civilization.Up to now,the developments of machine have acompolished automation and gianticism,and are on the way towards intelligentialize and digitization.Meanwhile,the advances of nanotechnology in recent decades have brings a new oppotinity for machines,that is,micromation.How small can machine be?The artificial molecular machines designed and synthesized by chemists,which are inspired by biological molecular machines and macro machines,have give an answer.The 2016 Nobel Prize in chemistry was awarded to three chemists,including J.-P.Sauvage,J.F.Stoddart and B.L.Feringa,not only for their outstanding contributions to the design and synthesis of artificial molecular machines,but also for the great potential of artificial molecular machines in various applications.As the core of molecular machines,molecular motors are so has been extraordinary concerned,especially the light-driven molecular rotary motors that exhibit repetitive unidirectional rotary motion by means of consecutive photochemical and thermal isomerization steps through the absorption of UV or visible light.The representative light-driven molecular rotary motors include the overcrowded-alkene based light-driven molecular rotary motors created by B.L.Feringa,and the imine-based ones synthesized by J.-M.Lehn.There are still many challenges to realize the application for the light-driven molecular rotary motor.One of the preconditions for designing and synthesizing the light-driven molecular rotary motors with excellent properties is the detailed understanding of their working mechanism.Therefore,in this thesis,we have chosen three molecular motors from the first and second generation overcrowded-alkene-based motor by Feringa,as well as an imine-based light-driven molecular rotary motor by Lehn,to study their working mechanisms with the high-level,multireference theoretical methodogies.Because the thermal steps of overcrowded-alkene-based light-driven molecular rotary motor have been deeply understood,the photoisomerization steps are mainly studied here.The comprehensive and comparative studies of the photoisomerization step’s reaction mechanisms of the first and second generation overcrowded-alkene-based motors,as well as the exploration of the overall working mechanism of the imine-based motors,provide mechansitic understanding the rotational properties of these different kinds of motors.The results not only are conducive to a comprehensive knowleage of the light-driven rotations around double bonds,but also provide theoretical reference for further optimization of molecular motors.This thesis mainly includes the following three contents:1.Taking a 9-(2,4,7-trimethyl-2,3-dihydro-1H-inden-1-ylidene)-9H-fluorene("the fluorene motor" in short)as research object,we used MS-CASPT2//SF-DFT and MS-CASPT2//CASSCF methods to study its photoisomerization mechanism.The directional rotations,in both the adiabatic excited-state decay and nonadiabatic transition,have been rationalized by characterizing the nature and topology of the potential energy surface as well as the Si/S0 MECIs.The SFDFT method has been verified,with respect to the CASSCF and MS-CASPT2//CASSCF references,to be an efficient and robust method in describing the near-degenerating regions between S0 and S1 states during the photo-induced C=C isomerization in fluorene motor,although the quality of the result at the FC region is more approximate(majorly due to the poor triplet reference near a singlet ground state minimum).The good performances in the extensive reaction path(including the most critical nearly degenerate situations where the nonadiabatic transition takes place)suggest the CASPT2//SFDFT combination as a reasonable alternative for medium and large molecular systems in which the high-level multireference methods are not applicable.2.Taking the 2,2’,7,7’-tetramethyl-1,1’-bis(indanylidene)("the stilbene motor" in short)as research object,we employed the SF-DFT method to research the photoisomerization process of the stilbene motor,in comparison with the(TD-)DFT result and the literaturally reported CASSCF results.Again,the SFDFT method has been verified to be an efficient and robust method in describing the photoisomerization process of the stilbene motor,in especial the nonradiative transition region.Moreover,comparing the difference between MECIs and S1-state minima in the energies and geometries of the fluorene and stilbene motors on the base of the SFDFT calculations have been addressed.A relationship between the geometric and electronic factors of MECIs and that of the S1-state potential energy surface was found.It well explains the reason why the fluorene and stilbene motors have different MECIs,which is expected to be useful in tuning the location and topology of S1/S0 MECI as well as the corresponding nonadiabatic transitions in the photoisomerization process.3.The working mechanism,including the mechanisms of the photoisomerization and the thermal isomerization steps,of the N-(3,3-dimethylbutan-2-yl)-5H-dibenzo[a,d][7]annulen-5-imine motor was calculated at the MS-CASPT2//CASSCF and MS-CASPT2//(TD-)DFT levels.The photoisomerization process of the imine-based motor molecule involves both the bright(π,π*)state and the dark(n,π*)state.The molecule is first excited to the S2 statewith a π→π*transition,then relaxes rapidly to the S1(n,π*)state and reaches the minimum of the energy profile of the S1 state through geometry relaxation.Finally,the molecule de-excites to the ground state through an MECI.Herein,the MECI with similar geometry and energy to the minimium of the S1 state keeps the directionality of rotation well,which confirms that the imine-based motor can be a good candidate for light-driven molecular rotary motors.For the thermal isomerization process of the imine-based motor,there are two possible reaction paths:one path with the in-plane N inversion mechanism,and another path with the mechanism of the ring inversion of the cycloheptatriene moiety,in addition the potential barriers of these two thermal isomerization paths are equal.The study and evaluation of the overall working mechanism of the imine-based motor can provide theory reference for the further optimization.