Theoretical Research On The Braking Mechanism And Performance Of Iptycene Molecular Brake

Based on the theoretical calculation methods of quantum chemistry,in this paper,the density functional theory(DFT)and time-dependent density functional theory(TDDFT)were used to carry out the mechanism and performance of the iptycene molecular brake systems.The main research content includes the following three parts:First,the chemically controlled molecular brake system with triptycene(Tp)as the rotor and bipyridine as the brake shoe has been studied theoretically.In this study,the M06-2X hybrid functional combined with the 6-31G(d)basis set was used to optimize the Kelly-type molecular brake,and the relaxed potential energy surface scan processes of the rotation angle under the 6-311G(d,p)basis set was also performed,and then the analyses of the stagnation points on the minimum energy profiles(MEPs)was carried out.Under the def-TZVP basis set,structure optimization and transition state search(including intrinsic reaction coordinates,IRC)are performed on the minimum and maximum points on the MEPs respectively.Combined with the vibration analyses of thermodynamic corrections,the molecular brake rotation mechanism is obtained.The high-precision basis set of def2-TZVPP was applied to calculate the energy of each conformation.Therefore,the various thermodynamic parameters were analyzed,and the braking mechanism was pointed out.After that,the thermodynamics of the singlemolecule conformational conversion reaction was studied on the molecular brake system,and the dynamic performance was evaluated by the transition state theory(TST)analyses.Combined with the Restrained Electro Static Potential(RESP)charge,the molecular dynamics simulations were performed as well.Secondly,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 thermodynamic 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 intermediate of the isomerization reaction was searched and analyzed(combined with the IRC),thus,the braking mechanism of the light-driven molecular brake was proposed.Finally,four redox molecular brake systems with Tp as the rotor and tetrathiofulvalene(TTFV)as the deceleration pad were constructed,and their braking performance and redox performance were theoretically studied.In this study,the M06-2X hybrid functional and 6-31G(d)basis set combined density fitting basis set were used to optimize the geometric structure of three valence states(0,+1 and +2 valence).On the basis of optimized structure,the relaxed potential energy surface scanning calculations were performed respectively,thereby,the rotational energy barrier were calculated,and the mechanical properties of the four molecular brake systems were analyzed.Then,under the def-TZVP basis set,the thermodynamics quantities were calculated respectively.Combining the electrostatic potential(ESP)distributions and the RESP atomic charge distributions,the rotation mechanisms and braking mechanisms of the redox molecular brakes were analyzed and pointed out.According to vibration analyses,thermodynamic parameters were obtained.Under vacuum conditions,the computation processes of the frontier orbitals(HOMO and LUMO),spin densities and spin populations were performed.With reference to the energy level structures and the electronic structures,the analyses of the redox characteristics of such molecular brakes was figured out.