Spectroscopic properties of N1,N5-bis[pyridine-2-methylene]-thiocarbohydrazone and its corresponding zinc (II) and nickel (II) metal complexes: A DFT and TD-DFT study

To elucidate the structure information and spectroscopic properties, the ground structures of N1,N5-bis[pyridine-2-methylene]-thiocarbohydrazone ligand (L) and their corresponding Zn (II) complex ((H2L)ZnCl2) (A) and Ni (II) complex ((H2L)NiCl2) (B) are optimized at the M06 method. Two conformers, i.e., cis- or trans-, are considered for every species. The ligand presents almost planar structure; while two metal complexes show distorted tetrahedral structure. Subsequently, the spectroscopic properties are calculated using time-dependent density functional theory (TD-DFT) with the polarized continuum model (PCM). For the ligand L, all UV–vis absorption peaks are originated from (p, π) → π* with a character of intraligand charge-transfer (ILCT). With respect to complex A ((H2L)ZnCl2), the lowest-energy absorption peak is correlated with HOMO → LUMO transition, which is ascribed to (p, π) → π* with a mixed ligand–ligand charge-transfer and intraligand charge-transfer (LL/ILCT) character. Compared with complex A, the maximum absorption wavelength of complex B ((H2L)NiCl2) exhibits red shift.