Circularly Polarized Luminescence from New Heteroleptic Eu(III) and Tb(III) Complexes

The complexes [Eu-(bpcd)-(tta)],[Eu-(bpcd)-(Coum)], and [Tb-(bpcd)-(Coum)][tta = 2-thenoyltrifluoroacetyl-acetonate, Coum = 3-acetyl-4-hydroxy-coumarin,and bpcd = N,N '-bis-(2-pyridylmethyl)-trans-1,2-diaminocyclohexane-N,N '-diacetate] have been synthesized and characterizedfrom photophysical and thermodynamic points of view. The optical andchiroptical properties of these complexes, such as the total luminescence,decay curves of the Ln-(III) luminescence, electronic circular dichroism,and circularly polarized luminescence, have been investigated. Interestingly,the number of coordinated solvent (methanol) molecules is sensitiveto the nature of the metal ion. This number, estimated by spectroscopy,is >1 for Eu-(III)-based complexes and <1 for Tb-(III)-based complexes.A possible explanation for this behavior is provided via the studyof the minimum energy structure obtained by density functional theory(DFT) calculations on the model complexes of the diamagnetic Y-(III)and La-(III) counterparts [Y-(bpcd)-(tta)], [Y-(bpcd)-(Coum)], and [La-(bpcd)-(Coum)].By time-dependent DFT calculations, estimation of donor-acceptor(D-A) distances and of the energy position of the S-1 and T-1 ligand excited states involved in the antenna effect was possible. These data are useful for rationalizingthe different sensitization efficiencies (eta(sens))of the antennae toward Eu-(III) and Tb-(III). The ttaligand is an optimal antenna for sensitizing Eu-(III)luminescence, while the Coum ligand sensitizes better Tb-(III) luminescence{phi(ovl) = 55%; eta(sens) >= 55%for the [Tb-(bpcd)-(Coum)] complex}. Finally, for the [Eu-(bpcd)-(tta)]complex, a sizable value of g (lum) (0.26)and a good quantum yield (26%) were measured.Chiral, DACH-based Eu-(III) and Tb-(III) heteroleptic complexesexhibit good metal-centered circularly polarized luminescence activityupon excitation of the antenna ligands. The tta andCoum ligands are optimal antennae for Eu-(III) andTb-(III) luminescence, respectively. The Tb-(III)-based complex showsa very good overall quantum yield of similar to 55%.