Extraordinary Circularly Polarized Luminescence Activity Exhibited by Cesium Tetrakis(3-heptafluoro-butylryl-(+)-camphorato) Eu(III) Complexes in EtOH and CHCl₃ Solutions

Abstract

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...

Diastereoselective Self‐Assembly of a Homochiral Europium Triangle from a BipyoxazolineCarboxylate Ligand

“…Circularly polarised luminescence studies will be carried out in future work to investigate the influence of the concentration and of chirality on the photophysical properties. [23] In summary, we have reported a rare enantiopure trinuclear europium complex obtained in a novel concentrationdependent self-assembly process promoted by a new (S)-Phbipox À ligand. We have shown that selective homochiral recognition occurs, probably as a result of sterical interaction, during the self-assembly of the bis-ligand monomeric complexes to yield the final trinuclear species.…”

Circularly Polarized Luminescence from Simple Organic Molecules

“…Up to now, the highest levels of CPL have been mainly achieved from chiral lanthanide complexes, 2d–e,8d–f which typically exhibit | g lum | values within the 0.05–0.5 range (an exceptional value of 1.38 has been reported for an europium(III) complex). 14 However, the emission efficiencies of these complexes are usually small, due to the nature of the involved metal-centered electronic transitions. This fact makes difficult their use in certain CPL applications ( e.g.…”

Circularly Polarized Luminescence in Chiral Molecules and Supramolecular Assemblies

“…12 Among various chiral luminescent molecular systems investigated, Eu(III) complexes exhibit high g lum values, with ∼1.3 being the highest reported until date in cesium tetrakis(3-heptafluoro-butylryl-(+)-camphorato) Eu(III) complexes. 13 In contrast to inorganic complexes, the advantage with organic molecules is that their emission can be readily tuned by bringing about modulations in their excited state electronic levels. The modulations in the electronic states have been achieved by processes such as self-assembly, by chemical transformations of the substituents, and by changing the environment through external stimuli.…”