Theoretical Study On Second-Order Nonlinear Optical Properties Of Tetracoordinate D~8 Transition Metal Pt(?)/Pd(?) Complexes

Due to its peculiar d~8 electron configurations and tetra-coordinated bonding mode of complexes,abundant photophysical properties,excellent luminescent properties and synthesis of the characteristics of easy operation,transition metal Pt(?)/Pd(?)complexes has become one of the hot topics in the study of materials chemistry,which show a broad application prospect in environmental science,biology,medicine,chemical sensors and supramolecular self-assembly,etc.Transition metal Pt(?)/Pd(?)complexes with unique plane structure and d-?conjugate formed by metal ion d orbitals and organic ligand is beneficial to improve the degree of the intramolecular charge transfer.In addition,the complexes have many advantages such as high chemical stability,easy functionalized modifications and wide absorption peaks in the visible light range.They have great potential to become a new metal complexes nonlinear optical(NLO)materials.Up to now,in order to meet the increasing demand for material functionalization,it has been the focus of people to seek and design new metal NLO materials with high efficiency and stability,and with good light transmittance and high NLO performance.However,it is of great guiding significance to analyze the second-order NLO response mechanism of the system at the micro level,and reveal the structure-NLO properties for the development and design of functional NLO materials with novel structures and excellent luminescence properties.In this paper,the density functional theory(DFT)combined with analytical derivative and sum-over-states methods are used to show a systematic theoretical study of a series of d~8 transition metal Pt(?)/Pd(?)complexes.The effects of substituent modification,photoisomerization,molecular configuration and size effect on NLO properties of the metal complexes were investigated.The geometric structure,electronic structure,electronic transition form,ultraviolet-visible absorption spectra and NLO properties of the system were studied and analyzed,and the microscopic mechanism of NLO response was further clarified.In addition,with the reversible redox reaction of the complex as a reference,significant second-order redox NLO switches were obtained,which provided a theoretical basis for the design of excellent functional NLO materials.The main conclusions observed are as follows:(1)The second-order NLO properties of a series of Pt(?)dithienylcyclopentene(DTE)based complexes were comparatively studied via DFT and TD-DFT method with the aim of investigating the influence of substituent modification and photoisomerization on their first hyperpolarizabilities.The calculations demonstrate that the introduction of quinolone into the complexes can lead to the larger second-order NLO response.Compared with monometallic Pt(?)complexes,bimetallic complexes exhibit stronger charge transfer ability.The first hyperpolarizabilities of closed-ring complexes are larger than those of open-ring complexes due to the better?-conjugated delocalization and smaller excitation energies.(2)The electronic structures and NLO response of these intriguing isomeric Pt(?)acetylacetone complexes with donor-acceptor structure were systematically studied.The effects of the combination of different donor and acceptor on the photoelectric and NLO properties of the studied isomeric systems were investigated by virtue of adjusting the atom coordination position of metal complexes.Complex 2 with the stronger electron donating capacity of TPA and electron-withdrawing ability py T unit has various charge transfer form,thus it produces the highest?tot value.In addition,a noteworthy finding was that the first hyperpolarizability of the oxidation state of the complex was increased obviously by redox reaction,which show the good redox NLO switching effect.(3)The circular Pd(?)-nanohoop complexes with 2,2'-bipyridine embedded CPPs ligand and curved complexes decorated with alkyl tether accompanied by linear counterparts were systematically designed and comparatively analyzed.We have investigated the effects of size effect and polarization environment on the electronic structure,absorption spectrum and NLO properties of these Pd(?)complexes.A noteworthily interesting finding is that the maximum?tot values are observed in alkyl chain modified curved complexes,which are positively correlated with the alkyl tether length.In comparison to conventional linear species,nanohoop ligand coordinated with metal Pd(?)can achieve remarkably improved NLO response,and it effectively overcomes the trade-off between good transparency and high second-order NLO response.Therefore,Pd(?)nanohoop complexes is a kind of NLO material with practical application.(4)On the basis of the circular Pd(?)nanohoop complexes,a series of“Russian doll”complexes consisting of two nested Pd(?)nanorings complex were studied.This work is done to explore the effects of the relative size of host and guest on the electronic structure,the stability and interaction,as well as second order NLO property of the nested complexes.A new structure-nonlinear optical property correlation regarding embedded Pd(?)nanorings complexes was proposed.The results show that the strong interactions occur in those complexes with small size difference of two rings by five phenyl units.There is the weak?-?noncovalent interaction between the host and guest ring.The relatively large first hyperpolarizabilities are observed in complexes where two nested nanohoops possess small size difference,and the microscopic mechanism of NLO response of such embedded Pd(?)nanorings complexes was further clarified.