Theoretical Study On The Performance And Movement Mechanism Of Three Classes Of Molecular Shuttle

We use quantum chemical methods to study the structure,properties and motion mechanism of three molecular shuttle systems.The three molecular shuttle systems are as follows:?1?a cation-controllable molecular shuttle of tetracationic cyclophane;?2?E/Z isomerization in response to photochemical or thermal stimuli;?3?triggering molecular shuttle by photochemical bond-cleavage strategyThe first,the macrocycle component tetracationic cyclophane(CBPQT⁴⁺)switches back and forth between[18]crown-6 derivative thread 1,containing a1,5-dioxynaphthalene residue and the 1,4-dioxybenzene derivative thread 2 under the action of alkali metal cations.The density functional theory?DFT?is used to study the motion mechanism of CBPQT⁴⁺motion in these two thread components.The binding ability of CBPQT⁴⁺to thread 1 and thread 2 is determined according to the binding strength.Then the electron localization function?ELF?and natural bond orbital?NBO?are used to judge whether the electrostatic repulsion of alkali metal cations and CBPQT⁴⁺is the direct cause of pseudorotaxane dissociation.The nuclear magnetic resonance?NMR?and electron density difference are employed to reveal the electron density transfer process before and after molecular shuttle interlocking.The theoretical calculation results are in agreement with the experiment and verify the conjecture of Stoddart et al..The binding ability between CBPQT⁴⁺and thread 1 is stronger than that of the thread component 2.Adding alkali metal cations,the binding ability of CBPQT⁴⁺to the thread 1 is greatly weakened,and dissociation occurs,and CBPQT⁴⁺binds to the thread 2.The second,the molecular shuttle can undergo E/Z isomerization in response to irradiation of UV light.Molecular shuttle configurations are optimized using different functional under the same basis set and compared with X-ray crystal.The most stable configuration under normal conditions is determined based on the energy parameters?binding energy,Gibbs free energy?.The time-dependent density functional theory?TD-DFT?is used to calculate the excited state energy to obtain the population ratio of E-type and Z-type molecular shuttle under ultraviolet irradiation.The natural bond orbital theory?NBO?analysis reveals the electrons transfer.The properties and strength of noncovalent interactions?NCI?in molecular shuttle are analyzed by using reduced density gradient?RDG?and independent gradient model?IGM?.The theoretical calculation results are consistent with the experiment.The pyridyl-acyl hydrazone group can serves as a brake latch,the molecular shuttle can switch between the Z-type and E-isomer states based on the external stimuli.The third,the methyl-6-nitroveratryloxycarbonyl group,known as a typical phototrigger,is covalently bonded into the Leigh-type hydrogen-bonding-assembled[2]-rotaxane which possessing two identical binding sites.The bulky amounts of responsive groups disappear under irradiation of UV light.The nucleophilic and electrophilic reaction sites in the molecular shuttle are analyzed by using condensed dual descriptor.The nuclear magnetic resonance analyzes the electrons transfer after chemical bond cleavage.According to a variety of theoretical methods on the noncovalent interactions in the molecular shuttle system,and the effects of bulky photolabile group on the molecular shuttle are obtained.The calculation results are consistent with the experiment.The macrocycle-thread interactions is relatively weak before the chemical bond breaks,and the macrocycle component can move back and forth between the two succinamide recognition sites;the chemical bond is bound to the molecular shuttle in the form of covalent bond,and the macrocycle–thread interactions is greatly enhanced,preventing the movement of the macrocycle.