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

Abstract

Water-soluble Eu(III) and Tb(III) complexes with N,N'-bis(2-pyridylmethyl)-trans-1,2-diaminocyclohexane-N,N'-diacetic acid (Hbpcd) 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)(HO)] complexes (the closed-shell equivalents of [Eu(bpcd)(HO)] and [Tb(bpcd)(HO)]) indicate that the two trans-O,O and trans-N,N 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 (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 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.

Eu(iii) and Tb(iii) complexes of 6-fold coordinating ligands showing high affinity for the hydrogen carbonate ion: a spectroscopic and thermodynamic study

“…As the paramagnetic Eu(III) and Tb(III) complexes are rather difficult to model computationally, the analogues of the diamagnetic Y(III) ion were studied. It has been shown that Y(III) complexes may serve as suitable models for the Eu(III) analogues, [33][34][35][36] consistently with the fact that its ionic radius differs from that of Eu(III) ion by about 0.05 Å [and less for Tb(III)]. Geometry optimizations of the [Y(L)(H2O)n] complexes were carried out at DFT level in vacuum using the B3LYP exchangecorrelation functional.…”

“…39 This level of theory was previously demonstrated to provide correct geometries and thermochemical properties, maintaining the calculation feasible also with similar complex systems. 23,33 All final geometries were checked to be minima by vibrational analysis. Solvent effects were included by means of the PCM model.…”

Circularly polarized luminescence of Sm (III) and Eu (III) complexes with chiral ligand (R/S)‐BINAPO

“…[15][16][17] However, chiral complexes of luminescent lanthanide ions have been exploited for their circularly polarized luminescence properties for decades [18][19][20] and that continues with many recent studies. [21][22][23][24][25] Circularly polarized luminescence (CPL) is the differential emission of left vs right circularly polarized light. The observables in CPL spectroscopy are the difference in emission intensity of left vs right circularly polarized light (I L -I R ) and sum of the emission intensity of left vs right circularly polarized light (I L + I R ), which is the total luminescence.…”

“…27 Therefore, this mechanism depends on the type and arrangement of the ligands about the metal ion. 4,21,22,24,25,28,29 While Eu 3+ has been widely exploited for CPL applications, there are very few studies involving the CPL of Sm 3+ complexes. 18,27 Since 4f-4f transitions are magnetic dipole allowed (as long as ΔJ = 0,±1), the electric dipole transition mechanisms do not always dominate and can lead to large rotatory strengths compared with the dipole strength, resulting in a large g em (Equation 2).…”

“…26 The large g em and small number of Stark levels, leading to simpler to interpret CPL spectra structure, are key factors contributing to the large number of CPL studies involving chiral europium complexes (most with |g em | > 0.1). 4,21,22,24,25,28,29 While Eu 3+ has been widely exploited for CPL applications, there are very few studies involving the CPL of Sm 3+ complexes. [30][31][32][33][34][35][36] This is somewhat surprising given the fact that there are two magnetic-dipole allowed emissive transitions, 4 G 5/2 → 6 H 5/2 and 4 G 5/2 → 6 H 7/2 , within the 4f 5 configuration of Sm 3+ .…”