Investigation of the Photophysical and Chiroptical Properties of Eu(III), Tb(III), and Gd(III) Complexes with 4-(2-Methylbutyl) aminodipicolinic acid

We hypothesize that 4-(2-methylbutyl) aminodipicolinate (MEBADPA^2-), a derivative of 2,6-pyridine-dicarboxylate (DPA), will result in the formation of triple-helical complexes and that these compounds would lead to stable chiral emitting species on the luminescence time scale. We also hypothesize that if the intermolecular interactions between a chiral molecule and the Ln(III) complexes are stronger for one enantiomer than another, then there should be a measurable shift in the racemic equilibrium of the complexes using circularly polarized luminescence (CPL) spectroscopy, where the difference in the emission of left and right circularly polarized light is monitored. 

We resorted to direct excitation spectroscopy using an Argon-dye laser to study the speciation of the complexes. After direct excitation of the complexes at pH values of 3.3, 4.6, 7.3, and 12.1, we found that a 1:3 Ln(III):ligand complex requires an environment with pH = 12.1 or higher. In addition, steady-state and time-resolved luminescence measurements were conducted to determine the quantum yield of each lanthanide(III) complex. The quantum yield values are as follows: [Eu(MEBADPA)₃]^3- = 18.2%, [Tb(MEBADPA)₃]^3- = 15.6%, and [Gd(MEBADPA)₃]^3- = 1.59%.

These various measurements confirmed an efficient ligand-lanthanide(III) energy transfer, but a weak metal-centered luminescent quantum yield due to an incomplete intersystem-crossing-transfer. We have shown that the magnitude of this perturbation is larger with MEBADPA^2- as compared to the parent compound with DPA when L-proline derivatives are added. In conclusion, this work confirmed that our Ln(III) complexes, with the use of CPL, can be employed to determine the molecular structure of various chiral luminescent species.