The DFT Study On Second-order Nonlinear Optical Property Of The Square-planar Tetrahedral Coordination VIII Metal (Ni, Rb) Complexes

Molecular materials with nonlinear optical （NLO） properties have attracted considerableattention due to their potential applications in novel optoelectronic devices fortelecommunications, information storage, optical switching and signal processing. Comparedwith inorganic and organic materials, metal complexes possess low-lying charge-transfer （CT）states that can be associated with large NLO responses. In experimental and theoreticalinvestigations, several effective methods have been developed to enhance the second-orderNLO responses of the metal complexes, such as appropriate ligand selection, donor-πbridge-acceptor （D-π-A） structure formation [15,16], and metal nd configuration and spinstate changes. A current challenge is the development of efficient strategies for the design ofswitchable NLO materials. Specific procedures that have been employed to switch NLOproperties include oxidation/reduction, protonation/deprotonation and photoisomerization.The NLO properties of the square-planar tetrahedral coordinationVⅡI metal （Ni, Rh）complexes have been studied by using quantum chemical calculations, the relationshipbetween molecular structure and NLO properties have been analyzed. The research resultssuggest:1. Quantum chemisty DFT B3LYP/6-31G（d）（SDD basis set for Rh ions） was employed tocalculate the second-order NLO properties and electronic spectra of Schiff base ligandscontaining ferrocene and their Ni（Ⅱ） complexes. The results are shown:（1） theα svalues donot change significantly from ligands to their Ni（Ⅱ） complexes, but enhanced by increasingthe conjugation bridge and adding withdrawing-electron substituent groups.（2） Ferrocenylgroups have different roles in ligands and complexes, acting as an accepter in ligands but adonor in complexes.（3） Compared with ligands, all complexes have a larger second-orderNLO coefficients. Theβ totvalues of the double bond bridge Schiff base ligands are muchlarger than the single bond bridge ones, but theβ totvalues of the double bond bridge Schiffbase complexes are close to the single bond bridge Schiff base complexes, metal Ni（Ⅱ） playsthe role of transporting electron in complexes.（4） By analyzing the composition of frontiermolecular orbitals and TD-HF calculations on electron spectrum: the results show that theligands and complexes were strongly assigned to the MLCT transitions.2. Quantum chemisty DFT PBE1PBE（UPBE1PBE）/6-31G（d）（SDD basis set for Rh ions） wasemployed to calculate the redox-switchable second-order NLO properties of a series of Rh（Ⅰ）complexes. The phdi ligand and metal Rh（Ⅰ） complexes are found to possess a significant potential for reversible switching and modulation of NLO properties by redox process. Theresults are shown:（1） the Rh ions are the oxidation centers for each of theone-electron-oxidized processes, while the Rh ion and the phdi ligand act as the reductioncenters. These results may alter the CT features of the redox species relative to their neutralspecies, and thus, the second-order NLO properties might be changed.（2） The substituentgroups （–CH₃,–NH₂,–CF₃and–NO₂） had some effects on the NLO responses of thesecomplexes, and the–NH2substituted3complex displayed the most desirable NLO properties.（3） For the complexes1,2,3and4, it was found that removing or adding one electron couldreduce the transition energy and change the CT feature, therefore, the static firsthyperpolarizabilities were enhanced.（4） With complexes2and3, theβ totvalues of theone-electron-reduced species2-and the one-electron-oxidized species3+were¹0.0and⁸.5times as large as those for their corresponding neutral complexes, respectively. Therefore, thecomplexes2and3were promising to become novel redox-switchable NLO molecularmaterials.（5） By analyzing the composition of frontier molecular orbitals and TD-HFcalculations on electron spectrum: the results show that the large NLO responses of theoxidized species were mainly related to the LLCT transitions combined with ILCT transitions,and the reduced species were strongly assigned to the MLCT transitions.