Electronic chirality inversion of lanthanide complex induced by achiral molecules

A novel mechanism for chiroptical activity inversion based on the electronic structure of metal complexes without Λ- or Δ-type structure change was demonstrated spectroscopically and theoretically. To demonstrate the mechanism, a europium (Eu(III)) complex with chiral (+)-3-(trifluoroacetyl)camphor (+tfc) and achiral triphenylphosphine oxide (tppo) was prepared. The steric and electronic structures of the Eu(III) complex were adjusted by additional achiral tppo and coordinating acetone molecules, and were characterised by 1H NMR, photoluminescence, and emission lifetime measurements. The optical activity of the Eu(III) complex in solution was evaluated by circularly polarized luminescence (CPL) measurements. CPL sign inversion, which was independent of Λ- or Δ-type structure changes from the spectroscopic viewpoint, and a drastic CPL intensity enhancement were observed depending on the external achiral molecules around Eu(III) ion. These phenomena provide the first clarification of optical activity change associated with electronic structure rather than chiral coordination structure-type (Λ or Δ) under external environments.

exhibits a large dissymmetry factor on the CPL (g CPL ) by the introduction of chiral organic ligands; this is established by the effective contribution of the magnetic field component [27][28][29][30] . The optical activity signal of the lanthanide complex with chiral ligands has been evaluated by Λor Δ-type coordination structures previously 31,32 . Yuasa and Parker reported the chiroptical activity inversion of chiral Eu(III) complexes influenced by the different steric structures of achiral molecules 33,34 , which implied steric inversion between Λand Δ-type structures. Note that chiroptical activity is related to the electronic transition of the Eu(III) complex; therefore, the magnitude and sign could be also affected by the chiral electronic structure of the Eu(III) ion surrounded by external ligands (Fig. 1a).
In order to demonstrate the chiroptical activity based on the chiral electronic structure in external environments, we choose the chiral Eu(III) complex with bidentate (+)-3-(trifluoroacetyl)camphor (+tfc) and monodentate triphenylphosphine oxide (tppo) as the chiral and external achiral ligands, respectively. The Eu(III) complex [Eu(+tfc) 3 (tppo) 2 ] (Δ-type structure, as determined by X-ray single-crystal analysis) shows a large g CPL (−0.47) in acetone-d 6 35 . The chiral electronic structure of the Eu(III) complex in solution was adjusted by additional achiral tppo and acetone molecules (Fig. 1b). The coordination and electronic structures in liquid media were characterised using 1 H NMR, photoluminescence, and emission lifetime measurements. The chiroptical activities of the Eu(III) complex under external environments were evaluated using CPL measurements. In this study, we demonstrate a mechanism for the optical activity inversion based on the chiral electronic structure of the Eu(III) complex without Λor Δ-type structure change spectroscopically and theoretically, for the first time.

Results
Coordination structures in solution. X-ray crystallography measurements indicated that the coordination structure of the Eu(III) complex in the solid is an eight-coordinated Δ-type structure composed of three chiral +tfc and two tppo ligands 35 . To evaluate the conformation of [Eu(+tfc) 3 (tppo) 2 ] (Eu(+)) in acetone, 1 H NMR spectra with additional n equivalents (n = 0, 8, 28, 48, and 98 relative to Eu(+)) of tppo molecules, namely Eu(+)-Exn, were acquired ( Fig. 2 and Supplementary Table S1). In the low-magnetic-field side, protons of tppo molecules in Eu(+)-Ex0 show broad peaks in Fig. 2 (black; A). The line-broadening and chemical shifts originate from the exchange reaction and paramagnetic effect on the metal complex 36,37 . The paramagnetic effect of the Eu(III) ion generally induces little broadening (bandwidth; nearly 10 Hz) 37 . Therefore, the large broadening of tppo signals in our experiment (Fig. 2, black; A, bandwidth; nearly 300 Hz) is mainly caused by their exchange reaction in acetone-d 6 . The lower-magnetic-field shift in Eu(+)-Ex0 (Fig. 2, black; A) is influenced by the direct coordination of tppo ligands with the Eu(III) ion.
NMR peaks of the chiral +tfc ligands in the Eu(III) complex were observed in the high-magnetic-field side (Fig. 2, signals B -I). Eu(+)-Ex8 (green) provided effective chemical shifts of +tfc ligands compared with those of Eu(+)-Ex0 (black). We also observed gradual shifts at the B, E, G, and I peaks of Eu(+)-Ex28 (purple), -Ex48 (red), and -Ex98 (blue). The effective shifts of the tppo and +tfc signals indicate that the Eu(III) complex with tppo molecules is rearranged by additional tppo molecules, resulting in the formation of several equilibrium states in acetone-d 6 .
Luminescence properties. Photophysical properties of Eu(III) complexes are affected by the coordination geometry 38 . The emission spectra of Eu(+)-Ex0 and -Ex498 in acetone (1 × 10 −3 M) are shown in Fig. 3 (black; a, and red; b). The Eu(III) complexes show sharp emission peaks in the region of 570-630 nm, which are attributed to the 5 D 0 → 7 F J (J = 0, 1, and 2) transitions of Eu(III) ions. The spectra were normalised with respect to the integrated intensities of the magnetic dipole transition ( 5 D 0 → 7 F 1 ). Their spectral shapes in liquid media were different from that of [Eu(+tfc) 3 (tppo) 2 ] in the solid state ( Supplementary Fig. S1). The emission spectra for the 5 D 0 → 7 F 1 and 5 D 0 → 7 F 2 transitions were also changed in response to the concentration of the tppo molecules in solution. In particular, the 5 D 0 → 7 F 1 transition band is composed of three Stark sublevels under the electric field (crystal field). Eu(+)-Ex0 showed three peaks at 584.5, 588, and 593.5 nm in the 5 D 0 → 7 F 1 transition ( Fig. 3 inset, black; a), whereas Eu(+)-Ex498 showed two peaks at 588 and 593 nm ( Fig. 3 inset, red; b). The 5 D 0 → 7 F 1 transition of Eu(+)-Ex0 at a lower concentration (1 × 10 −5 M, Fig. 3, blue; c) also showed three peaks at 584, 587.5, and 593.5 nm, which are similar to that of Eu(+)-Ex0 at a higher concentration. The small peak at 587.5 nm can be attributed to the 5 D 1 → 7 F 3 transition, which is sometimes observed in the same energy region as the 5 D 0 → 7 F 1 transition 38 . The emission bands at around 612 nm are attributed to hypersensitive electric dipole transitions ( 5 D 0 → 7 F 2 ), which are strongly dependent on the local symmetry of the Eu(III) ion. The change of spectral shape is influenced by the rearrangement of coordination geometries of the Eu(III) complex depending on additional tppo molecules. In case of the non-coordinating toluene solution, the emission spectra of Eu(+)-Ex0 and -Ex48 were similar in shape to that of Eu(+)-Ex498 in acetone, irrespective of the amount of additional tppo molecules ( Supplementary Fig. S2). We propose that the inner coordination structure of Eu(+)-Ex498 in acetone is composed of one Eu(III) ion, three +tfc ligands, and two tppo ligands. The time-resolved emission profiles of Eu(+)-Ex0 and -Ex498 in acetone (1 × 10 −3 M) were measured to clarify their coordination structures. The emission lifetimes were estimated using triple (for Eu(+)-Ex0) or double (for Eu(+)-Ex498) exponential functions to analyse several conformations in solution (Fig. 4a,b). The estimated   Fig. 4c). We consider that the τ 1 component is the Eu(III) complex with coordinating acetone molecules (Fig. 4a,d). The τ 1 and τ 2 component ratios decreased and increased, respectively, with increasing amount of tppo molecules. The main τ 2 value of Eu(+)-Ex498 was found to be 0.12 ms (97%). The lifetime τ 2 in acetone was similar to the single-lifetime component in toluene (Supplementary Table S2), indicating that the Eu(III) ion is attached with three +tfc and two tppo ligands (Fig. 4b,d). We revealed that the two types of steric structures with τ 1 and τ 2 components were reorganized in response to the external tppo and acetone molecules.
Chiroptical properties. The CPL spectra and dissymmetry factors of Eu(+)-Ex0 and -Ex498 are shown in Fig. 5 and Table 1, respectively. The CPL signals for the 5 D 0 → 7 F 1 transition were composed of two peaks at    Fig. S4b) 39 .
Considering the presence of τ 1 and τ 2 components in the emission lifetime measurements, the observed CPL spectra of Eu(+)-Ex0 and -Ex498 in acetone (1 × 10 −3 M) were attributed to several equilibrium states of the Eu(III) complex in acetone. The large negative g CPL of Eu(+)-Ex0 is dominated by the τ 1 component related to coordinating acetone molecules ( Supplementary Figs S5-S7). The small positive g CPL of Eu(+)-Ex498 is related to the τ 2 component of the eight-coordinated Eu(III) complex with two inner tppo ligands; this was supported by the similar positive g CPL (+0.036) in toluene ( Supplementary Fig. S8).

Discussion
The dissymmetry factor g CPL is expressed in terms of the transition electric dipole moment µ → and transition magnetic dipole moment → m as follows 28 , where θ is the angle between µ → and m → . When µ θ → = → =°m ( 0 ), equation (1) provides the largest g CPL value (=2) mathematically (Fig. 6a). In the region m / 1 µ → → < (Fig. 6a,b, orange regions), the Eu(III) complex with a large µ → provides a large g CPL value. In general, the → m value in the 5 D 0 → 7 F 1 transition is larger than the µ → value 40 . The intensity of µ → in the 5 D 0 → 7 F 1 transition depends on the crystal field around the Eu(III) ion 41,42 . The 7 F 1 energy level of the Eu(III) ion in a typical eight-coordinate structure (C 4v or D 2d ) splits into two Stark sublevels (Fig. 6b) 38 . The two bands at 583 and 594 nm in the CPL spectra are assigned to the A 1 → A 2 and A 1 → E transitions 42 , respectively, in Fig. 6b,c. The observed CPL signal in the A 1 → E transition was inverted from minus to plus, while that in the A 1 → A 2 transition retained the minus sign. In C 4v or D 2d symmetry, the direct product A 2 (=A 1 × A 2 ) is expressed in terms of the electric dipole (ED) forbidden and magnetic dipole (MD) allowed transitions (R z ) on the character table in group theory (Fig. 6c, Supplementary Tables S3 and S4). On the other hand, the direct product E (=A 1 × E) produces ED and MD allowed transitions ((x, y); (R x , R y ), Fig. 6c, Supplementary Tables S3 and S4). The CPL sign at 583 nm (ED forbidden A 1 → A 2 transition) reflects the intrinsic Λor Δ-type structure, because of insensitive electronic state mixing. Considering the same CPL sign at 583 nm In contrast, the CPL sign in the ED and MD allowed A 1 → E transition is sensitive to electronic state mixing even for the same chiral structure-type (Λ or Δ). The effective sign inversion and drastic intensity change of the CPL signal in the A 1 → E transition should be caused by the change of µ → based on electronic state mixing.
In the 5 D 0 → 7 F 1 transition, µ → is mainly altered by the J-mixing of 7 F 2 or 7 F 3 sublevels into 7 F 1 38,43 . In the photoluminescence spectra, the Stark splitting energy of the τ 1 component (270 cm −1 , Fig. 3 inset, blue; c) was larger than that of the τ 2 component (160 cm −1 , Fig. 3 inset, red; b). The large Stark splitting energy suggests the large J-mixing in the A 1 → E transition of the τ 1 component 43 . The J-mixing increases the µ → value at the ED allowed A 1 → E transition relative to that at the ED forbidden A 1 → A 2 transition, which is consistent with relatively large emission intensity at the A 1 → E transition (593.5 nm) of the τ 1 component (Fig. 3 inset, blue; c). The µ → increase leads to the large g CPL value in equation (1). The angle θ between µ → and → m of the τ 1 component is larger than 90°, whereas that of the τ 2 component is smaller than 90°, suggesting that the angle is established by µ → vector change due to J-mixing. The extra-large enhancement of g CPL from +0.013 to −1.0 also indicates that J-mixing promotes the direction of µ → and → m to antiparallel, leading to the large g CPL . We demonstrated that the CPL sign and intensity are strongly influenced by the chiral electronic structure depending on the µ → under J-mixing in the same chiral structure-type.

Conclusion
We successfully observed a CPL sign inversion with a drastic g CPL change from +0.013 to −1.0 for the Eu(III) complex with the same chiral coordination structure-type. The CPL phenomena were attributed to the chiral electronic structure depending on the µ → under J-mixing. We also achieved an extra-large g CPL (−1.3) of the Eu(III) complex with chiral +tfc ligands in DMSO, suggesting that the g CPL of the Eu(III) complex could be enhanced by J-mixing with a small 4f-5d mixing character (Supplementary Figs S12 and S13, Table S5). The results provide a novel aspect for the optical activity of metal complexes and molecular design of chiral lanthanide complex by maximising the g CPL value.
Apparatus. Elemental analyses were performed on an Exeter Analytical CE440. Proton nuclear magnetic resonance ( 1 H NMR) spectra were recorded in acetone-d 6 on an auto-NMR JEOL ECS 400 MHz; Acetone (δ H = 2.05 ppm) was used as an internal reference. Emission spectra and emission lifetimes were measured using a Horiba/Jobin-Yvon FluoroLog-3 spectrofluorometer. CPL spectra were measured using a JASCO CPL-200 spectrofluoropolarimeter.