Photophysical and Chiroptical Properties of the Enantiomers of N,N'-Bis(1-phenylpropyl)-2,6-pyridinecarboxamide and their Chiral 9-Coordinate Ln(3+) Complexes.

The R,R and S,S enantiomers of N,N'-bis(1-phenylpropyl)-2,6-pyridinedicarboxamide, L(Et), react with Ln(3+) ions (Ln = La, Eu, Gd, and Tb) to give stable [Ln((R,R)- and (S,S)-L(Et))(3)](3+) in anhydrous acetonitrile solution, as evidenced by various spectroscopic measurements, including NMR and luminescence titrations. In addition to the characteristic Eu(3+) and Tb(3+) luminescence bands, the steady-state and time-resolved luminescence spectra of the aforementioned complexes show the residual ligand-centered emission of the (1)ππ* to (3)ππ* states, indicating an incomplete intersystem crossing (ISC) transfer from the (1)ππ* to (3)ππ* and ligand-to-Ln(3+) energy transfer, respectively. The high circularly polarized luminescence (CPL) activity of [Eu(L(Et))(3)](3+) confirms that using a single enantiomer of L(Et) induces the preferential formation of one chiral [Eu(L(Et))(3)](3+) complex, consistent with the [EuL (3)](3+) complexes formed with other ligands derived from a 2,6-pyridine dicarboxamide moiety. Furthermore, the CPL sign patterns of complexes with (R,R) or (S,S) enantiomer of L(Et) are consistent with the CPL sign pattern of related [LnL (3)](3+) complexes with the (R,R) or (S,S) enantiomer of the respective ligands in this family.