Theoretical studies on the spectroscopic properties of a series of palladium (II) complexes with 1-allyl-3-(2-pyridyl)thiourea

To explore the spectroscopic properties, a series of palladium complexes are investigated theoretically through density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations. Full geometry optimizations of 1-allyl-3-(2-pyridyl)thiourea (APTU) and its corresponding Pd (II) complexes [Pd(APTU)Cl2] (A), trans-[Pd(APTU)2]2+ (B), cis-[Pd(APTU)2]2+ (C), together with two thiolic tautomeric APTU-based Pd (II) complex cis-[Pd(APTU)2] (D) and trans-[Pd(APTU)2] (E) are carried out at the B3LYP, B3PW91, and M06 levels. All of them present slightly distorted square-planar geometries. On the basis of the optimized geometries, the absorption spectra in ethanol are evaluated by using three TD-DFT methods with the PCM model. Experimental absorption spectra are well reproduced by our time-dependent density functional theory calculations. Both UV–vis absorption peaks at 219.64 and 254.80 nm for complex A are mainly attributed to (p,π,d) → π* with a mixed character of intraligand charge-transfer (ILCT)/ligand–ligand charge-transfer (LLCT)/metal-ligand charge-transfer (MLCT) transitions. Another weak absorption band of complex A is located at 313.17 nm largely originated from HOMO−6 to LUMO transition. There are two isomers with small energy difference for [Pd(APTU)2]2+, i.e., trans-[Pd(APTU)2]2+ (B) and cis-[Pd(APTU)2]2+ (C). Only when both isomers are considered, four absorption peaks can be obtained, which is in line with the experimental results. The absorption peaks of complexes D and E present obvious red shift that is associated with their lower HOMO–LUMO gap (ΔE|HOMO–LUMO|) among five complexes. Differences and similarities in molecular orbitals and electronic transition characters for five complexes are discussed in detail.