Circularly polarized luminescence (CPL) spectroscopy is the emission analogue to circular dichroism (CD) spectroscopy. 1 It is common to report the degree of CPL in terms of the luminescence dissymmetry factor, g lum (λ), which is defined as follows: g lum (λ) = 2ΔI/I = 2 (I L -I R )/(I L + I R ), where I L and I R refer respectively to the intensity of left and right circularly polarized emissions. A value of 0 for g lum corresponds to no circular polarization, while the absolute maximum value is 2. Although the development of useful correlation between CPL spectrum and chiral structure is still limited, 1 a study by Bruce et al. 2 conducted on welldefined DOTA-based macrocyclic Eu(III) complexes led to the following conclusion. The sign and magnitude of CPL are affected by the degree of helical twist of the complex, the nature of the ligand field, and the axial donor group solvation. Of special importance is that the CPL will reflect the time-averaged local helicity around the lanthanide(III) ion (i.e. the magnitude of g lum values increases with an increase in the degree of conformational rigidity of the complex).To date, the largest g lum value reported in the literature was for the commercially available NMR shift reagent tris(3-trifluoroacetyl-(+)-camphorato)europium(III), [Eu((+)-facam) 3 ], in dry DMSO (g lum value of -0.78 at 588.2 nm), 3,4 whereas lanthanidecontaining systems with chiral 2-hydroxyisophthalamide-, pyridyl diamide-,1-hydroxy-2-pyridinone-, or DOTA-based ligand derivatives exhibited g lum values as high as 0.5. 1,4-8 Recently, we reported on the isolation and structural characterization of sodium or cesium tetrakis(3-heptafluorobutylryl-(+)-camphorato) lanthanide(III) complexes, M I [Ln((+)-hfbc) 4 ], by X-ray analysis and/or CD spectroscopy. The solution structure is supposed to take a square antiprism eight coordination (SAPR-8) with Δ-configurational chirality on the basis of the exciton CD spectra. 9,10 In this communication, the CPL as well as CD of M I [Eu((+)-hfbc) 4 ] complexes (M I = Cs and Na) in CHCl 3 and EtOH were examined in order to reveal the detailed chiral configuration in solution. Of special interest is the importance of using CPL for selectively studying only luminescent chromophores present in the systems of interest, in contrast to CD, which is affected by most chromophores and/or equilibrium mixtures in an additive manner. The NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author ManuscriptThe CPL spectra of 2 mM solutions of M I [Eu((+)-hfbc) 4 ] (M I = Cs and Na) in CHCl 3 are plotted in Figure 1 in the spectral range of the 5 D 0 → 7 F 1 transition, which is particularly wellsuited for CPL measurements since it satisfies the magnetic-dipole selection rule, ΔJ = 0, ±1 (except 0↔0), respectively. As shown in Figure 1, the detection of a CPL signal confirmed the presence of stable chiral emitting species on the luminescence time scale. The g lum values of the M I [Eu((+)-hfbc) 4 ] (M I = Cs and Na) complexes amounted to +1.38 and +0.15 at 595 nm, respecti...

The luminescence and circularly polarized luminescence (CPL) spectra of MI[Eu((+)–hfbc)4] show a similar behavior to the exciton CD in the intraligand π–π* transitions when the alkali metal ions and solvents are manipulated. There is a difference in susceptibility in solvation toward the alkali metal ions but not toward the Eu(III) ion, as in the case of axially symmetric DOTA–type compounds. The remarkable CPL in the 4f–4f transitions provide much more information on stereospecific formation of chiral Eu(III) complexes, since CPL spectroscopy is limited to luminescent species and reflects selectively toward helicity of the local structural environment around the lanthanide(III). While in comparison, exciton CD reveals the chiral structural information from the helical arrangement of the four bladed chelates. Of special importance, the observation of the highest CPL activities measured to date for lanthanide(III)–containing compounds (i.e. Eu and Sm) in solution supports that the chirality of Lanthanide(III) in the excited state corresponds to that in the ground state, which was derived from the exciton CD.

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