Theoretical studies on the spectroscopic properties of a series of halide Zinc (II) complexes with pyridinylimine and pyridinylmethylamine derivatives

The ground structures of pyridinylimine derivatives (L1-a = N-(pyridin-2-ylmethylene)aniline and L1-b = N1,N1-(dimethyl-N4-pyridin-2-ylmethylene)benzene-1,4-diamine) and pyridinylmethylamine derivatives (L2-a = N-(pyridin-2-ylmethyl)aniline and L2-b = N1,N1-(dimethyl-N4-pyridin-2-ylmethy)benzene-1,4-diamine) and their corresponding Zn (II) complexes ([Zn(L1-a)Cl2] (A), [Zn(L1-b)Cl2] (B), [Zn(L2-a)Cl2] (C), and [Zn(L2-b)Cl2] (D)) are optimized at three DFT levels, i.e., B3LYP, B3PW91, and M06. The spectroscopic properties are calculated using time-dependent density functional theory (TD-DFT) in gas and in solution. The occupied orbitals involved in the transitions have mixed character of the Cl atom p orbital and ligand-based π orbital, while the lowest unoccupied molecular orbital (LUMO) presents π* orbital character. Two UV–Vis absorption peaks located at 252 and 353 nm are assigned to (p, π) → π* transition with mixed intraligand charge-transfer (ILCT)/ligand–ligand charge-transfer (LLCT) character and π → π* transition with ILCT character for complex A, respectively. With respect to complex B, the absorption bands show red shift with two peaks at 281 and 470 nm with the appearance of an electron-releasing group (–N(CH3)2) because the energy gap (ΔE|HOMO–LUMO|) is decreased. Red shift phenomenon is also observed between other two complexes C and D. Comparison between theoretical and experimental results for the structures and spectra is discussed in detail.