Synthesis and Optical Performance of terbium complexes with octanoyl amino acids

Abstract

As multifunctional materials, amino acid rare-earth compounds exhibit unique physical and chemical properties. Considering their wide applications, we synthesized a new type of amino acid terbium complexes, which exhibit interesting optical properties. Three novel octanoyl amino acid terbium complexes were synthesized and investigated in this study: terbium octanoyl-alaninate (Tb(oct-ala)3·H2O), terbium (III) octanoyl-phenylalaninate (Tb(oct-phe)3·H2O), and terbium (III) octanyl-serinate (Tb(oct-ser)3·3H2O). The molecular structure and surface morphology of these three complexes were studied using various methods, including 1H (Nuclear Magnetic Resonance) NMR, Fourier-Transform Infrared Spectroscopy (FT-IR), powder X-Ray Diffraction (XRD), and Scanning Electron Microscope (SEM). The XRD results show that Tb(oct-phe)3·H2O and Tb(oct-ser)3·3H2O present long-range disorder and short-range order, while Tb(oct-ala)3·H2O shows long-range disorder and short-range disorder. SEM results revealed irregular shapes of the samples synthesized. Polarizing Microscopy (PM) observations show that Tb(oct-phe)3·H2O and Tb(oct-ser)3·3H2O samples are optically anisotropic at and above the room temperature. Tb(oct-ala)3·H2O exhibits optical homogeneity over the investigated temperature range. Also, the optical properties of complexes were examined using Ultraviolet–Visible Spectroscopy (UV–Vis), Emission Spectroscopy, Fluorescence Lifetime, and Quantum Yields (QY). The UV–Vis absorption spectra show that the complexes have excellent absorption properties in the UV region, and the absorption range of the complexes (200–400 nm) is larger than that of the ligands (200–270 nm). After irradiation with a 254 nm UV lamp, these three complexes are bright green in both solution and solid form. For the emission spectra, the luminescent intensity of the three complexes increases with an increase in concentration. Comparing the fluorescence spectra of the three powders, we observed that Tb(oct-phe)3·H2O has stronger luminescent intensity, longer fluorescence lifetime and larger quantum yield than the other two complexes. The structure of these terbium complexes was investigated using Density-Functional Theory (DFT) calculations and combining theory with experiment.

Keywords

• Octanoyl amino acid
• Terbium complexes
• UV–Vis absorption
• Luminescent properties
• Density-Functional Theory