Theoretical Study On The Design,structure,and Nonlinear Optical Properties Of S₄ Symmetric Metal Complexes

Nonlinear optical（NLO）materials refer to materials that produce nonlinear optical effects under high intensity of light irradiation,such as optical frequency doubling,optical mixing,self focusing,etc.Due to their efficient frequency conversion and strong optical response,NLO materials have been widely used in fields of optical communication,laser technology,medicine,biotechnology,etc.On the other side,transition-metal complexes are promising NLO materials due to their unique d orbitals,high chemical stability,simple synthesis,wide absorption peak,etc.Particularly,the octapolar compounds with S₄symmetry have extremely excellent second-order NLO effect,which are ideal crystals to easily obtain non-central and non-dipole properties.In addition,the excellent transparency of octapolar compounds in the visible light region overcome the contradiction between NLO effects and transparency of materials.Therefore,the compounds with octapolar structures are expected to become high-performance NLO materials from the perspective of structural characteristics.However,due to the non-central and non-dipole structure requirements of the S₄symmetric octapolar molecules,designing and constructing this type of metal complexes NLO materials to promote the transition metal complex NLO material from theory to application remains a challenge.It is worth noting that by using the quantum chemistry methods to analyze the second-order NLO response of the transition metal complex system,and conducting theoretical calculations on the designed and constructed complex molecules,not only the NLO characteristics of the prepared complex materials can be predicted,but also the structure-activity relationship between the molecular structure and NLO response can be studied.This research provides theoretical calculation for the design of new NLO materials,and is of great significance for the development and design of NLO materials with novel structures.It is also a powerful tool for searching for new metal complexes.Therefore,in order to find more and better NLO materials,the purpose of this paper is to find meteal complexes with S₄symmetry through theoretical design,and then analyze their NLO properties through quantum chemical calculations.The main work of this paper is as follows:1.The fourth period transition metal ion and the organic ligand are combined to form a four-nucleated ring metal complex,aiming at finding and designing new metal complexes with symmetrical S4 octupole structure.It mainly uses quantum chemical calculation method to study the geometric structure,electronic structure,absorption spectrum and the first hyperpolarizability of the designed series of metal complexes.The results indicate that the coordination of central metal atoms of Ti,Cr,Fe,Ni,and Zn lead to the formation of S₄symmetric complexes,exhibiting excellent NLO properties.Particularly,the Zn-S₄complex has outstanding advantages in optical transparency and chemistry stability,indicating a good candidate for high performance of second-order NLO materials.2.On the basis of the molecular geometry structure of the S₄symmetric complex Zn-S₄,we further investigated the effect of substituent groups（-NO₂,-NH₂,-F,-Cl,-Br,-I）on the structure and properties of organic carboxylic acid ligands by quantum chemical calculation methods.The results show that the six designed metal complexes exhibit strong NLO response,and the introduction of substituents has an important effect on the geometric structure,electronic structure and linear and nonlinear optical properties of these complexes.Therefore,the complexes Zn-NO₂,Zn-NH₂,Zn-F,Zn-Cl,Zn-Br and Zn-I are all suitable candidates for nonlinear optical materials.3.Using density functional theory（DFT）and time-dependent density functional theory（TD-DFT）methods,the geometric structure,electronic structure and linear absorption spectral properties of six non-central and non-dipole S₄symmetric octupole cadmium complexes were discussed.Moreover,the second-order and third-order NLO properties were calculated at the CAM-BLTYP//6-31+G（d）/SDD calculation level.The research results show that these complexes have high stability,good transparency,and large NLO response coefficients,thus having potential application value in the field of NLO materials.At the same time,the fundamental effect of the substituents on the geometric structure,electronic structure,linear and NLO properties of these complexes are discussed.Therefore,the structure and NLO properties of these metal complexes can be adjusted through the modification of ligands,providing effective theoretical information and guidance for experimental synthesis of high-performance NLO materials.