The density functional theory (DFT) B3LYP(UB3LYP) method based on the 6-31g* level was employed to investigate the polarizabilities and second-order nonlinear optical (NLO) properties of tetrathiafulvalene-fused Schiff base ligand and its Fe(III/II), Co(II), Ni(III/II), Cu(II) complexes. The electronic spectra of closed-shell complexes were analyzed by the time-dependent density functional theory (TD-DFT) method. The results show that tetrathiafulvalene-fused Schiff base ligand possess considerable large second-order NLO coefficient and divalent metal complexes are close to that of the ligand. While for trivalent metal complexes, the β values increase to 30 times as large as that of ligand. The β values in different spin multiplicity of metal complexes are differ little. Combined with the composition of frontier molecular orbitals, the second-order NLO coefficients of ligand and divalent metal complexes are mainly related to the intraligand charge transfer (ILCT) process. While the trivalent metal complexes are assigned to an ILCT transition and combined with an MLCT (metal-to-ligand charge transfer) transition, meanwhile, the frontier molecular orbitals energy gap of the trivalent metal complexes are significantly decreased, thus lead to enhance their second-order NLO coefficients.

Key words: tetrathiafulvalene-fused Schiff base, metal complexes, second-order NLO properties, DFT