Structural and luminescence studies of N-acyl amino acid-based praseodymium complexes

Rare-earth metal complexes have received wide attention for their excellent photo-physical properties and their light absorption capacity and energy transfer efficiency of organic ligands, which are the main factors affecting the photoluminescence of rare-earth metal co-mplexes. Therefore, in this study, four N -acyl amino acid praseodymium complexes, (Pr[CH 3 (CH 2 ) n CONHCH(CH 3 )COO] 3 , n = 4,6,8,10) with different carbon chain lengths were synthesized using H(hex-ala), H(oct-ala), H(dec-ala) and H(dod-ala) ligands and the Pr 3+ reaction method with the goal to obtain rare earth metal-organic complexes with good optical properties . The molecular structure of the praseodymium complexes was determined using CHN elemental analysis , 1 H NMR , and FT-IR spectroscopic analysis . Solubility tests in pure solvents have determined that Pr(oct-ala) 3 and Pr(dec-ala) 3 are highly soluble in polar solvents . However, Pr(dod-ala) 3 , which contains a long carbon chain and crystallizes easily, has a low solubility. The photophysical properties of the praseodymium complexes can be obtained by fluorescence spectroscopy in the visible region, which shows that the luminescence of the four praseodymium complexes occurs in the blue region, and the fluorescence lifetimes are about 0.32 μs. The effect of temperature change on the luminescence properties of the praseodymium complexes was analyzed by variable-temperature fluorescence spectroscopy . The fluorescence inte-nsity of Pr(dec-ala) 3 , which is more absorbent, is higher in the range of 25 °C–70 °C. The luminescence properties of praseodymium complexes in the near-infrared region were obtained by near-infrared emission spectroscopy , and three major emission intensities were found at 926 nm ( 1 D 2 → 3 F 2 ), 1001 nm ( 1 D 2 → 3 F 4 ), and 1376 nm ( 1 D 2 → 1 G 4 ), among which the intensity of the emission peaks of Pr(dod-ala) 3 was the strongest at 1001  nm, indicating a good match between the ligand-centered triplet state of Pr 3+ and the 1 D 2 emission state. In addition, in this study, the molecular structure of praseodymium complexes was geometrically optimized by density functional theory (DFT) method using Gaussian software, and the bond strengths and topological properties of the praseodymium complexes were obtained by Atoms in Molecules (AIM) theory analysis. The results show that there are closed-shell interactions between the ligand and Pr 3+ , in which Pr(oct-ala) 3 has the largest Pr–O bond strength among the complexes with different carbon chain lengths.