Structures, photophysical properties and DFT studies of terbium complexes containing N-acyl amido alanine

In recent years, rare earth complexes containing amino acids have attracted much attention in numerous fields, such as light, electricity, catalysis, biological activity, drugs, etc. The rare earth element terbium is considered an important fluorescent metal due to its special electronic structure. It has excellent photophysical properties, such as high color purity, large Stokes shift, long excited state lifetime, and profuse energy levels. It has been used in the fields of fluorescent probes, medical radiation, and chemical sensors. Considering their wide applications, a series of acyl amino acid terbium complexes were synthesized using N-acyl amino acids as ligands, and their structures and photophysical properties were studied. The analysis of the theoretical and experimental results demonstrates that the energy transfer efficiency from ligands to the terbium ion emission levels increases with increasing alkyl carbon chain length, leading to differences in the luminescent intensity of the complexes. The energy transfer efficiency from ligands to rare earth ions was the result of a combination of the relative positions between the resonance energy levels of the central rare earth ion and the lowest triplet state energies of ligands as well as the intersystem crossing probability of the ligands. Moreover, the density functional theory (DFT) was used to determine the geometric structures and obtain 1 H NMR and FT-IR spectra of the terbium complexes. Their structures were further examined by comparing the calculated with the experimental spectra. The results show that the ligands coordinate to the central Tb 3+ through the bidentate chelation mode and that the theoretically determined structure was reliable.