Construction Design And Theoretical Study Of Novel Molecular Tweezers

This thesis aims to carry out structural design and theoretical research on molecular tweezers.From the perspective of quantum computational chemistry,the recognition mechanism of the two recognition sites?IS?based on the spacer group?S?to the substrate and the physical properties of such molecular tweezers are described.The main contents are as follows:Firstly,density functional theory?DFT?was used to illustrate a novel molecular tweezers?L₁?based on porphyrin-azobenzene-crown ether triads receptor.In order to study the effect of porphyrin metallization,the metal ion Zn²⁺was introduced into complex L₁and formed the complex L₁/Zn²⁺?L₂?,and the noncovalent interaction between complex L₂and the substrate sodium cyanide,namely the 1:1 sandwich complex L₂·Na CN,was studied.The full optimized geometric structures were calculated at PBE0/6-311G?d,p?level.Natural bond orbital?NBO?analysis was used to study the interaction of L₂and L₂·Na CN molecules.The absorption spectra of L₁,L₂and L₂·Na CN were investigated by CAM-B3LYP and time-dependent density functional theory method?TD-DFT?.DFT calculations show that the geometric structure of compound L₂binding sodium cyanide has undergone greatly changes,forming of a“sandwich”configuration complex.Second,¹H and ¹³C NMR spectra indicated that the compound L₂·Na CN was formed in the ditopic fashion.Because the crown ether/Zinc-porphyrin complex?L₂?recognizes sodium cyanide,the proton signal shifts significantly toward upfield.It demonstrates that L₂is suitable ditopic receptor to recognize sodium cyanide salt,which the crown ether group recognizes the metal sodium ion and the Zn?II?porphyrin partially recognizes the cyanide ion.Secondly,a molecular tweezers?L₃?with Zn?II?-Schiff base unit and benzo-15-crown-5 unit connected by a spacer azo and complex L₄with the alkaline metal cation Na⁺were theoretically studied.The ditopic recognition of hydrophobic tryptophan was described.Geometrical optimization was carried out using?B97XD functional and def2-SVP basis set,and the absorption spectra and excited-states were calculated using TD-DFT//?B97XD/def2-TZVP level.The absorption spectra data show some significantly shifts in the absorption band due to the present of Na⁺or tryptophan.In addition,interfragment interactions between receptor L and tryptophan were analyzed in detail by independent gradient model?IGM?and topological properties of atoms in molecules theory?AIM?,which is found to contribute to forming the metal-ligand bonds,intermolecular H-bond and van der Waals interaction in 1:1 sandwich complex.The above results demonstrate that the complex L₄is a ditopic receptor to be utilized to recognize amphiphilic molecule-tryptophan.Thirdly,two switchable tweezers based on bis-Schiff base?Zn-Salphen?complex were designed,which are tweezers 1 based on terpyridine ligand and tweezers 2 based on 4'-bromophenyl-terpyridine ligand,respectively.The theoretical calculation was carried out using B3LYP-D3 functional and def2-SVP basis set.Free tweezers 1 and 2can be converted from a“W”open form to a“U”closed form upon Ru?III?coordination.The switching performances were characterized by ¹H NMR and absorption spectra.¹H NMR spectra showed that terpyridine protons had an obvious upfield shift during complexation with Ru Cl₃.The absorption spectrum was observed in the closed tweezers with a clear red shift and a decreased oscillator strength.In addition,the tweezers were reopened by introducing aromatic molecule pyrazine in the“U”-shaped conformation to form a host-guest system.The recognition ability of two Zn-Salphen binding sites was studied by geometrical optimization and absorption spectra.Molecular tweezers 1 and 2 have the physical characteristics of mechanical switches at the molecular scale,and that“U”-shaped tweezers can recognize guest pyrazine.