Helicene-Based Ligands Enable Strong Magneto-Chiral Dichroism in a Chiral Ytterbium Complex

Abstract

Here we report the first experimental observation of magneto-chiral dichroism (MChD) detected through light absorption in an enantiopure lanthanide complex. The P and M enantiomers of [YbIII((X)-L)­(hfac)3] (X = P, M; L = 3-(2-pyridyl)-4-aza[6]-helicene; hfac = 1,1,1,5,5,5-hexafluoro­acetyl­acetonate), where the chirality is held by the helicene-based ligand, were studied in the near-infrared spectral window. When irradiated with unpolarized light in a magnetic field, these chiral complexes exhibit a strong MChD signal (g MChD ca. 0.12 T–1) associated with the 2F5/2 ← 2F7/2 electronic transition of YbIII. The low temperature absorption and MChD spectra reveal a fine structure associated with crystal field splitting and vibronic coupling. The temperature dependence of the main dichroic signal detected up to 150 K allowed, for the first time, the disentanglement of the two main microscopic contributions to the dichroic signal predicted by the MChD theory. These findings pave the way toward probing MChD in chiral lanthanide-based single-molecule magnets.

The Underexplored Field of Lanthanide Complexes with Helicene Ligands: Towards Chiral Lanthanide Single Molecule Magnets

“…Finally, the recent magneto-chiral dichroism (MChD) measurements for an optically pure lanthanide-helicene complex [Yb(hfac)3(L1)] [73] confirm the theory that the MChD signal is coupled with the magnetization of the metal center. This constitutes an important breakthrough that validates the concept of the optical detection of magnetization in molecular magnets using unpolarized light, as introduced by Train et al in 2008 [52], and extends it to lanthanide single molecule magnets (Ln-SMMs).…”

“…Of note, there are only two examples of Ln-helicene complexes; 1 [68] and 8 [73] were obtained in optically pure forms. Preparation of optically pure Ln-helicene compounds is a fundamental requirement for the observation of the NCD and MChD phenomena in the bulk material, as both effects cancel out in racemic mixtures.…”

“…Section 2 reveals that the coupling between the chirality of the helicene ligand and the magnetism of the lanthanide central ion can only be revealed for pure enantiomeric forms. Recently, the racemic compound [Ln(hfac)3(L1)] (8), discussed in the previous section, was obtained in the form of pure enantiomers ([Yb(hfac)3((P)-L1)] 8P and [Yb(hfac)3((M)-L1)] 8M [73]. These compounds are isostructural with the pure enantiomers 1P and 1M of the Dy(III) analog (also discussed in the beginning of the previous section and reported in 2016 [68]).…”

Chiral or Luminescent Lanthanide Single-Molecule Magnets Involving Bridging Redox Active Triad Ligand

“…These so-called "hot bands" usually disappear upon the lowering of the temperature that depopulates upper excited states [28,[67][68][69]. Consequently, these "hot bands" as well as other contributions of weak intensities that appeared in the emission spectrum may be also attributed to the vibronic coupling between electronic transitions and low energy vibrational modes as recently suggested for such Yb III ion in molecular systems [38,70]. Similar light excitation into the ILCT at 22,220 cm −1 (Figure S4) for 2 cannot give the visible Dy III centered emission because of the too low-energy absorption band of L.…”

“…In a general manner, the addition of one or more physical properties to the SMM behavior paves the way to the design of multi-properties SMM as well as the study of a possible synergy between the different properties. Moreover, when chirality is added to the (luminescent) SMM behavior [21][22][23][24][25][26][27][28], chiral SMM [29], ferroelectric SMM [30] as well as chiral luminescent SMM [31][32][33][34][35][36] and magneto-chiral SMM [37,38] can be obtained. To these three properties, the introduction of redox-active ligands is of interest because it could permit to switch both optical and magnetic properties depending on the oxidation state of the ligand [39][40][41][42][43][44][45][46].…”