The DFT Study On Second-order Nonlinear Optical Property Of The Open-shell Schiff Base Ligands And Complexes

Materials with optimized nonlinear optical (NLO) properties are required for various applications, including processing of optical signals, optical data storage, nanophotonics, and biophotonics. A current challenge is the development of efficient strategies for the design of switchable nonlinear optical (NLO) materials. To achieve a pronounced switching effect, the molecule must be stable in two (or more) states that exhibit very different NLO responses. Complete reversibility and high switching speed are also highly desirable for practical applications. Specific procedures that have been employed to switch NLO properties include protonation/deprotonation, oxidation/reduction, and photoisomerization. Open-shell metal complexes offer a larger variety of structures, low-lying charge transfer electronic states, high environmental stability, and a diversity of electronic/redox properties tunable by virtue of the coordinated metal center. Thus Open-shell metal complexes may become an important matertials on switching NLO responses.The NLO properties of Open-shell metal complexes have been studied by using quantum chemical calculations, the relationship between molecular structure and NLO properties have been analyzed. The research results suggest:1. Quantum chemistry DFT B3LYP (UB3LYP)/6-31g(d) and the time-dependent Hartree-Fock (TD-HF) method were employed to calculate the second-order nonlinear optical properties of Schiff base ligands, open-shell Fe(III) metal complexes and closed-shell Ni(II) metal complexes. The results are shown: (1) The large second-order NLO response of complexes are created by a greaterÏ€charge delocalization over the chelate ring and the existence of new excited states. (2) We obtained the structure involving the strong electron donating NH₂ group on the R1 and the strong electron accepting NO2 group on the R2 exhibits the strongest enhancement of its first hyperpolarizability, and"switches on"the second-order NLO response by exchange donor/acceptor. (3) While for the substitution metal complex, we note a strong increase in on going from Fe to Ni, it confirms that closed-shell Schiff base complexes can have larger hyperpolariz-ability compared with the corresponding open-shell ones. (4) Theβ_tot of two spin states Fe(III) complexes could be affected by the donor/acceptor, which indicate that the second-order NLO could be adjusted through introducing the substituents to the systems with different spin multiplicity. (5) By analyzing the composition of frontier molecular orbitals and TD-HF calculations on electron spectrum, the results show that the large NLO response is strongly related to the intraligand charge transfer transitions (ILCT).2. Quantum chemistry DFT B3LYP (UB3LYP)/6-31g(d) method were employed to clculate the second-order nonlinear optical properties of the TEMPO-bound dithiolate ligands and metal (Pt, Pd) complexes in redox process. The TEMPO-bound dithiolate ligands and metal (Pt, Pd) complexes are found to possess a significant potential for reversible switching and modulation of NLO properties by redox process. The results show that for ligands and complexes, they have the different redox center. Ligands can exist in four stable redox states, and exhibit three forms of nonlinearity"off"in the L·, LH and ³L^- species and one form of nonlinearity"on"in the ¹L⁺ species. The DFT-FF calculations indicate that the redox processes significantly affect the TEMPO moiety as donor/acceptor ability and change the charge transfer feature. Thus, theβ_tot value of one-electron-oxidized species, ¹L⁺, increases to 461×10^-30 esu, 303 times as large as that of TEMPO·radical ligand. For complexes, theβ_tot values of PtL·and PtLH are large and very close in magnitude of 222×10^-30 esu, 246×10^-30 esu respectively, and this attribute to their specific structure. The specific structure combining of a dithiolate electron-donor ligand which is more easily oxidized and a diimine electron-acceptor ligand which is more easily reduced, at the same time, the two ligands coordinated to a M(II) center lead to a large optical charge transfer. The one-electron-reduced and -oxidized of PtL·cause a decrease in theβ_tot values. The two forms of nonlinearity"off"in the 3PtL- and ³PtL⁺ species and two forms of nonlinearity"on"in the PtL·and PtLH species could become the principal permit switching between four redox states. By analyzing TDDFT calculations on electron spectrum, the results show that large NLO response is strongly related to the ligand-to-ligand charge transfer (LLCT) transitions. Our investigation has not only turned out the origin of larger NLO values in TEMPO-bound dithiolate ligands and metal (Pt, Pd) complexes but also predicted the possible significant potential of the present studies system as a new type of"On/Off"switchable NLO molecular material.