| Acylhydrazone molecular photoswitches have been widely used in important field s such as supramolecular self-assembly,materials science,and information storage.Especially the bipyridine acylhydrazone derivatives,because of their unique N-N=C fragment and the presence of multiple protons on this fragment,this kind of derivatives can be used as a good heteroditopic ion-pair receptor to capture or release ions under the irradiation of different wavelengths.However,there are few literatures about acylhydrazone photoswitches as heterotopic ion-pair receptor in previous reports.Because the absorption spectra of the E/Z isomers of such molecular switches cannot produce effective band separation,which limits the application prospects of acylhydrazone derivatives as ectopic ion-pair receptor.Therefore,this work designed a new acylhydrazone derivative molecule,(namely NBPD)from a theoretical point of view,which can’t only be isomerized under the irradiation of blue and green light,but also can absorb the different ions at different position.And this work had also proposed a new thermal isomerization mechanism through research the microscopic thermal isomerization process of novel acylhydrazone molecule precisely.Finally,this article fully explained the photoisomerization mechanism of acylhydrazone photoswitches by studying the transition states of excited states for the first time,and filled up the gap in the existing literature on the photoisomerization mechanism of acylhydrazone molecules.The main research work of this paper is as follows:1)Design of novel acylhydrazone moleculesThis work used the bipyridylhydrazone compound reported in the literature as the parent molecule.A total of 30 acylhydrazones molecular photoswitches had been obtained by modifying the structure of the parent molecule with different substituents.We studied the physical and chemical properties and substituent effects of these acylhydrazone molecular photoswitches,respectively.By comparing the absorption wavelength,oscillator strength,and electrostatic potential diagram of the E/Z isomers of different acylhydrazone derivatives,respectively,we finally screened out a novel acylhydrazone NBPD,which not only has excellent maximum absorption band(△λmax=λE-λz)separation,at the same time,its E isomer and Zisomer can be isomerized under blue light of 478.72 nm and green light of 525.34 nm,respectively.In addition,the NBPD molecule is also a good heteroditopic ion-pair receptor.2)Research on the mechanism of thermally induced isomerizationIn this paper,the quantum chemistry tool Gaussian 16 program with the method of PBE0 and 6-31+G(d)basis sets were used to study the thermal isomerization reaction mechanism of acylhydrazone molecular photoswitches in acetonitrile solution.In the process from the E-isomer to the Z-isomer,two transition states and an intermediate were obtained,and through frequency analysis and the computational of the intrinsic reaction coordination(IRC),the accuracy of the transition state and the intermediate were verified.At the same time,by accurately searching the potential energy surface diagram(PES)of the isomerization process from E to Zisomer,it was discovered that an independent shoulder peak exists on the PES.Unlike the transition state at the saddle point of the PES,the shoulder peak generated from the repulsive force when the carbonyl/furyl oxygen atom and two nitrogen atoms of the N-N=C fragment are in the same plane and close,so that the steric hindrance effect appears as a shoulder peak on the potential stationary surface.It divides the entire thermo-reaction mechanism into two processes including three stages:(ⅰ)E-isomer reaches the I0 of the shoulder peak through the first transition state TS1,and then,I0 changes along the potential energy surface to the first lowest energy point under the impetus of repulsion,the intermediate M1 can be generated;(ⅱ)The intermediate M1 passes through the TS2 and finally forms Z-isomer.In the entire thermal isomerization process,the first step reaction is the decision step of the reaction.The mechanism of photoisomerization of acylhydrazone has not yet been reported.This work proposed to explore the photoisomerization mechanism of molecular photoswitches by studying the transition states of excited states.This research was based on the novel type of acylhydrazone molecule as the research object,the vertical excited states were calculated used the TD-PBE0 method with 6-31+G(d)basis set in acetonitrile solution.Finally,the complete two transition states in the process of the photo-induced reaction were proposed,and there is one and only one imaginary frequency found for each excited-transition state by frequency analysis.The photoinduced reaction process is as follow:(ⅰ)The E-isomer is excited to the first excited state under the visible light of 478.72 nm to form ES-EiSO,and then passes through the first transition state ES-TS1 to form the intermediate ES-M,and finally forms the ground-state product of the Z-isomer;(ⅱ)and the Z-isomer is excited to the first excited state under visible light of 525.34 nm to form ES-ZiSO,and then passes through the second transition state ES-TS2 to form the ground-state product of the Eisomer finally. |
