Influence of Hydroxycoumarin Substituents on the Photophysical\nProperties of Chiroptical Tb(III) and Eu(III) Complexes

In\nthis article, the synthesis, density functional theory (DFT)\nstructural characterization, and spectroscopic investigation of chiral\nand heteroleptic Tb(III) and Eu(III) complexes are presented. These\nmolecules are characterized by two different ligands: the enantiopure <i>N,N</i>′-bis(2-pyridylmethyl)-<i>trans</i>-1,2-diaminocyclohexane-<i>N,N</i>′-diacetic acid (H<sub>2</sub>bpcd) and a hydroxycoumarin-based\nligand bearing different substituents in C(3) position (<i>i.e.</i>, acetyl group in Coum, ethyl ester in CoumA, secondary and tertiary\namides in CoumB and CoumC, respectively). The coumarin ligands exhibited\ndifferent luminescence sensitization efficiency toward Tb(III) and\nEu(III) ions in the related complexes of chemical formula [Ln(bpcd)(Coum)],\n[Ln(bpcd)(CoumA)], [Ln(bpcd)(CoumB)], [Ln(bpcd)(CoumC)]. Through theoretical\ncalculations of intramolecular energy transfer (IET) processes (ligand-to-metal)\nin Eu(III) and Tb(III) complexes, along with quantum yield calculations,\nwe provide a reasonable explanation for the observed differences in\ntheir luminescence properties. The nature of the coumarin ligand also\naffects the chiroptical properties of the Tb(III) complexes [i.e.,\ncircularly polarized luminescence (CPL) and electronic circular dichroism\n(ECD)].