Strongly Circularly Polarized Emission from Water-Soluble Eu(III)- and Tb(III)-Based Complexes: A Structural and Spectroscopic Study

Water-soluble Eu­(III) and Tb­(III) complexes with N,N′-bis­(2-pyridylmethyl)-trans-1,2-diaminocyclohexane-N,N′-diacetic acid (H2bpcd) have been synthesized and characterized in their racemic and enantiopure forms. The ligand has been designed to bind Ln­(III) ions, providing a dissymmetric environment able to solicit strong chiroptical features while at the same time leaving a few coordination sites available for engaging further ancillary ligands. Potentiometric studies show that Ln­(III) complexes have a relatively good stability and that at pH 7 the [Ln­(bpcd)]+ species is largely dominant. DFT calculations carried out on the (S,S)-[Y­(bpcd)­(H2O)5]+ complexes (the closed-shell equivalents of [Eu­(bpcd)­(H2O)5]+ and [Tb­(bpcd)­(H2O)5]+) indicate that the two trans-O,O and trans-Npy,Npy configurations are equally stable in solution and present two coordinated water molecules. This is in agreement with the hydration number ∼2.6 determined by luminescence lifetime measurements on Tb­(III) and Eu­(III) complexes. A detailed optical and chiroptical spectroscopic characterization has been carried out and reveals that the complexes display an efficient luminescence in the visible spectral range accompanied by a strong CPL activity. A value for g lum (around 0.1 on the top of the 546 nm band) for the Tb-based complex has been found. This is one of the highest g lum values measured up to now for chiral Tb complexes. These results suggest that in principle Tb­(bpcd)Cl is suitable to be employed as a CPL bioprobe for relevant analytes in aqueous media.