Introduction

The Faraday effect, a magneto-optical property, has received significant attention with regard to applications of magneto-optical materials. This effect causes the rotation of polarization by a material as the result of the application of an external magnetic field.1 The degree of rotation is linearly proportional to the component of the magnetic field in the direction of propagation. The Faraday effect originates from the Zeeman splitting of electronic states based on the angular momentum of electrons in the material. Inorganic lanthanide compounds such as Tb(III)-doped borosilicate glasses and Tb(III) garnet ceramics exhibit large Faraday effects because of the large angular momentum of the 4f electron, and such materials have been used to construct optical isolators employed in optical communication systems.2, 3, 4, 5

We recently described nonanuclear Tb(III) clusters coordinated with salicylate ligands that are a new type of lanthanide material exhibiting a large Faraday effect.6 These clusters are composed of 9 Tb(III) ions bridged by 10 μ-OHs and 16 salicylic acid esters as organic ligands. The Faraday rotation angles of the clusters in the visible region are much larger than those of Tb glasses. We expect the clusters to be applied to next-generation optical communication systems. We also identified a further unique property of these clusters: the rotation angles were different depending on the organic ligands. This finding has the possibility to lead to new molecular designs for Faraday effect materials based on the type of the organic ligands.

In the present study, we focused on introducing chiral ligands to the Tb(III) cluster to maximize the unique characteristics of these inorganic–organic hybrid complexes. Lanthanide complexes containing chiral ligands show significant circular dichroism (CD) and circularly polarized luminescence (CPL) resulting from the differing absorption or luminescent intensities between left- and right-handed circularly polarized light.7 These phenomena are derived from the chirality of the complexes; thus, their origin is markedly different from that of the Faraday effect.1, 8 The dissymmetry factors gCD and gCPL represent the magnitudes of the CD and CPL intensities, respectively, and the gCD and gCPL of the chiral lanthanide complexes are several hundred times larger than those of typical chiral organic molecules.8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22 These larger dissymmetry factors are attributed to the large transition magnetic dipole moment of the lanthanide ion that shows opposing vectors depending on chirality. This moment is based on the angular momentum of electrons accompanying 4f–4f transitions. In CD and CPL studies, Richardson9 established selection rules based on the S, L and J angular momentum quantum numbers of lanthanide 4f electron states, both experimentally and theoretically. Based on these rules, Muller and colleagues13 reported the largest gCPL value (>1.0) for a Eu(III) complex with chiral camphor-derivative ligands. As a general trend, Tb(III) complexes are also classified into a group having a large gCPL among the lanthanide complexes,22 expecting the large transition magnetic dipole moments of several transitions in the Tb(III) clusters.

For the first time, in this work, we introduced chiral ligands into Tb(III) clusters to investigate the effect of large transition magnetic dipole moments on the Faraday rotation. The chiral Tb(III) clusters [Tb9(sal-(R)-Bt)16(μ-OH)10]+[NO3]− (Tb-(R)-Bt: sal-(R)-Bt=(R)-2-butyl salicylate) and [Tb9(sal-(S)-Bt)16(μ-OH)10]+[NO3]− (Tb-(S)-Bt: sal-(S)-Bt=(S)-2-butyl salicylate) were synthesized by the complexation of Tb(NO3)·6H2O with chiral organic ligands in methanol (CH3OH) at 40 °C (Figure 1). The structures and chiroptical properties of these clusters were characterized using X-ray diffraction, CD and CPL measurements. We also performed Faraday effect measurements for these chiral Tb(III) clusters in poly(methyl methacrylate) (PMMA) films. The results showed that the Faraday rotation angles of these clusters were affected by their chirality. This study is the first report of a relationship between the Faraday effect and molecular chirality, providing new insights into the design of magneto-optical materials and also potentially opening up a novel field of chiral science.

Figure 1
figure 1

Synthetic scheme of a 2-butyl salicylate ligand and a nonanuclear Tb(III) cluster.

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Materials and methods

Materials

Salicylic acid (C6H4(OH)COOH), N,N-dimethyl-4-aminopyridine (DMAP, C7H10N2), PMMA ((C5H8O2)n, Lot PDF4279), CH3OH, and chloroform (CHCl3) were purchased from Wako Pure Chemical Industries, Ltd (Osaka, Japan). (R)-(–)-2-butanol (C4H9OH, >99.0%), (S)-(+)-2-butanol (C4H9OH, >98.0%) and 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide hydrochloride (EDC·HCl, C8H17N3·HCl) were purchased from Tokyo Chemical Industry Co., Ltd (Tokyo, Japan). Terbium(III) nitrate hexahydrate (Tb(NO3)3·6H2O) and triethylamine (Et3N, (C2H5)3N) were purchased from Kanto Chemical Co., Inc (Tokyo, Japan).

Apparatus

Proton nuclear magnetic resonance (1H-NMR) spectra were recorded in CDCl3 on an auto-NMR JEOL ECS 400 MHz spectrometer (JEOL, Tokyo, Japan); CHCl3 (δH=7.26 p.p.m.) was used as an internal reference. Fast atom bombardment–mass spectrometry was performed using a JEOL JMS-700TZ spectrometer. Elemental analyses were performed using a J-SCIENCE MICRO CORDER JM10 system (Kyoto, Japan). Infrared spectra were recorded on a JASCO FT/IR-350 spectrometer (Tokyo, Japan). X-ray diffraction data were obtained using a RIGAKU SmartLab X-ray diffractometer (Tokyo, Japan). Photoluminescence (PL) spectra were measured using a Horiba FluoroLog3 spectrofluorometer (Kyoto, Japan, excitation wavelength=380 nm). Luminescence quantum yields were measured using an JASCO FP-6300 spectrofluorometer with an integration sphere. The luminescence lifetimes were measured using the third harmonic (355 nm) of a Qswitched Nd:YAG laser. The electronic absorption, CD and CPL spectra were measured using a JASCO V-670 spectrophotometer, JASCO J-720 spectropolarimeter and JASCO CPL-200 spectrofluoropolarimeter (excitation wavelength=380 nm), respectively. The quantitative elemental analyses were performed using an inductively coupled plasma atomic emission spectroscopy (ICP-AES, SHIMADZU ICPE-9000, Kyoto, Japan) to determine the concentration of the Tb(III) ion in PMMA films for the Faraday rotation measurements. The Faraday effect measurements were performed using a JASCO Model K-250 spectrophotometer.

Synthesis of (R/S)-2-butyl salicylate [Sal-(R/S)-Bt]

Salicylic acid (2.0 g, 14.5 mmol), (R/S)-(−/+)-2-butanol (1.08 g, 14.5 mmol), EDC·HCl (2.78 g, 14.5 mmol) and DMAP (0.177 g, 1.45 mmol) were added to CHCl3 (40 ml). The mixture was stirred at room temperature for 5 h under Ar, washed with HCl aq. (2 × 30 ml) and NaHCO3 aq. (2 × 30 ml) and distilled H2O (2 × 30 ml). The organic layer was separated and dried with MgSO4, and the solvent was evaporated. The residue was chromatographed on silica gel eluting with ethyl acetate/hexane (10/90).

[Sal-(R)-Bt]. Yield: 10.0%. 1H-NMR (400 MHz, CDCl3): δ/p.p.m.=10.9 (s, 1H, -OH), 7.85 (d, 1H, J=8 Hz, Ar), 7.45 (t, 1H, J=8 Hz, Ar), 6.97 (d, 1H, J=8 Hz, Ar), 6.88 (t, 1H, J=8 Hz, Ar), 5.08–5.18 (m, 1H, -2CH), 1.64–1.84 (m, 2H, -3CH2), 1.36 (d, 3H, J=6 Hz, -1CH3), 0.98 (t, 3H, J=8 Hz, -4CH3). Elemental analysis: Calculated for C11H14O3: C, 68.02%, H, 7.27%. Measured: C, 68.13%, H, 7.52%.

[Sal-(S)-Bt]. Yield: 9.7%. 1H-NMR (400 MHz, CDCl3): δ/p.p.m.=10.9 (s, 1H, -OH), 7.85 (d, 1H, J=8 Hz, Ar), 7.45 (t, 1H, J=8 Hz, Ar), 6.97 (d, 1H, J=8 Hz, Ar), 6.88 (t, 1H, J=8 Hz, Ar), 5.08–5.18 (m, 1H, -2CH), 1.64–1.84 (m, 2H, -3CH2), 1.36 (d, 3H, J=6 Hz, -1CH3), 0.98 (t, 3H, J=8 Hz, -4CH3). Elemental analysis: Calculated for C11H14O3: C, 68.02%, H, 7.27%. Measured: C, 68.15%, H, 7.49%.

Synthesis of [Tb9(sal-(R/S)-Bt)16(μ-OH)10]+[NO3]− ([Tb-(R/S)-Bt])

(R/S)-2-butyl salicylate (0.20 g, 1.0 mmol) was dissolved in CH3OH, and Et3N (0.174 g, 1.72 mmol) was added to this solution with stirring at 40 °C. Tb(NO3)3·6H2O (0.26 g, 0.58 mmol) in CH3OH was added dropwise to this solution with further stirring for 20 min.6,23 A white powder, Tb-(R/S)-Bt, was obtained.

[Tb-(R)-Bt]. Selected IR (KBr, cm−1): 1680 (-C=O), 2930 (-CH2-), 2960 (-CH3). Elemental analysis: Calculated for C176H218NO61Tb9: C, 44.47%, H, 4.62%, N, 0.29%. Measured: C, 44.12%, H, 4.55%, N, <0.30%. Fast atom bombardment–mass spectrometry: m/z Calculated for C176H218O58Tb9: [M-NO3]+, 4691.75; Measured, 4691.79.

[Tb-(S)-Bt]. Selected IR (KBr, cm−1): 1680 (-C=O), 2930 (-CH2-), 2960 (-CH3). Elemental analysis: Calculated for C176H218NO61Tb9: C, 44.47%, H, 4.62%, N, 0.29%. Measured: C, 44.50%, H, 4.78%, N, 0.31%. Fast atom bombardment–mass spectrometry: m/z Calculated for C176H218O58Tb9: [M-NO3]+, 4691.75; Measured, 4691.76.

Spectral measurements

The electronic absorption and CD spectra of Tb-(R)-Bt (2.0 × 10−5 M) and Tb-(S)-Bt (2.0 × 10−5 M) were measured in CH3OH at room temperature. The photoluminescence, CPL spectra, luminescence quantum yields and emission lifetimes (1.0 × 10−4 M) were measured in CHCl3 at room temperature.

Faraday rotation measurements

The Tb(III) clusters (95 mg) obtained were added to CHCl3 (1 ml). Then, the solution (0.2 ml) was added to PMMA (2.8 g) dissolved in CHCl3 (10 ml).6 PMMA films were prepared on glass substrates using a casting method. The thickness of the PMMA film was ∼1.2 mm, and the transmittance was >90% in the 400–800 nm region. The external magnetic field was 15 kOe.

Results and discussion

Electronic states and chiroptical properties of the Tb(III) clusters

The Tb(III) clusters Tb-(R)-Bt and Tb-(S)-Bt were composed of 9 Tb(III) ions bridged by 10 μ-OHs and 16 chiral salicylate ligands. These structures were identified by infrared spectroscopy, fast atom bombardment–mass spectrometry and elemental analysis. X-ray diffraction patterns also demonstrated that the peaks generated by Tb-(R)-Bt were consistent with those produced by Tb-(S)-Bt (Supplementary Figure S1).

The absorption spectra of the Tb(III) cluster and the ligand are shown in Figure 2a. Intense absorption bands were observed in both Tb-(R)-Bt and Tb-(S)-Bt at ∼340 nm (ɛmax≈60 000 cm−1 M−1) because of the π–π* transition of the salicylate ligands (Figure 2a, red line). The spectrum of the Tb(III) clusters was red shifted and broadened compared with that of the ligands (Figure 2a, black line). This red shift is primarily attributed to destabilization of the highest occupied molecular orbital level as a result of the complexation.24 The 4f–4f transition of the Tb(III) ions (expected at 488 nm) was not observed because this transition is essentially forbidden by the Laporte rule (ɛmax<1 cm−1 M−1).25

Figure 2
figure 2

(a) Ultraviolet–visible (UV-vis) and (b) circular dichroism (CD) spectra of Sal-(R)-Bt (black line), Tb-(R)-Bt (red line) and Tb-(S)-Bt (blue line) in CH3OH (2.0 × 10−5 M).

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These clusters exhibited bisignate CD bands because of the π–π* transitions of the salicylate ligands (Figure 2b), the signs of which were dependent on the asymmetric centers of the organic ligands. These bisignate spectra were consistent with the absorption maximum peak (335 nm) and shoulder peak (365 nm), respectively, that was attributed to an exciton coupling.8, 26, 27, 28, 29 These results indicate that intramolecular interactions might occur between neighboring chiral ligands in the Tb(III) clusters. The band of Tb(III) clusters is not related to CT transition because the Tb(III) ion shows high reduction potential, and its ligand-to-metal charge transfer transition state is located at a higher energy (>60 000 cm−1). Because the electronic state of the Tb(III) ion depends on the coordination geometry formed by the ligands,25 the intramolecular interactions in the Tb(III) clusters affect the photophysical properties based on the transitions of the Tb(III) ions.

To assess the differences between the electronic states of Tb-(R)-Bt and Tb-(S)-Bt, we carried out high-resolution photoluminescence measurements. The photoluminescence spectra of the Tb(III) clusters in CHCl3 showed four sharp emission bands in the region of 450–650 nm that were attributed to the 5D4→7FJ (J=6, 5, 4, 3) transitions of the Tb(III) ions (Figure 3 and Supplementary Figure S2).25 In contrast to the absorption spectra, emission bands resulting from the π–π* transitions of the ligands were not observed. This effect was attributed to the intersystem crossing of the ligand from the excited singlet state to the excited triplet state promoted by the spin orbit coupling of the Tb(III) ions. The crystal field splitting (Stark splitting) of Tb-(R)-Bt was consistent with that of Tb-(S)-Bt (Figure 3, inset). In time-resolved spectroscopy, the luminescence decays (5D4→7F5 transition) of the Tb(III) clusters were single exponential (Supplementary Figure S3). These results demonstrate that Tb-(R)-Bt and Tb-(S)-Bt form mirror image stereoisomers in solution, and these clusters are thus expected to show identical photophysical properties, with the exception of their chiroptical characteristics.

Figure 3
figure 3

High-resolution photoluminescence (PL) spectra of Tb-(R)-Bt (red line) and Tb-(S)-Bt (blue line) in CHCl3 (1.0 × 10−4 M).

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We also performed CPL measurements for these clusters (Figure 4a and b), and the resulting signals were observed to correspond to the transitions of the Tb(III) ions. The observed photophysical properties are summarized in Table 1. Within the experimental error, the clusters had the same luminescence quantum yield, Φπ−π*, and luminescence lifetime, τobs, although their gCPL signs were inverted. The largest gCPL values were observed at the 5D4→7F5 transitions (gCPL=±0.04) that were on the same order of magnitude as previously reported for Tb(III) complexes.12, 16, 22, 24 The large gCPL consequently leads to a large gCD,9 and this is expected to give large transition magnetic dipole moments around the Faraday-active wavelength based on the 4f–4f absorption.

Figure 4
figure 4

(a) Photoluminescence (PL) and (b) circularly polarized luminescence (CPL) spectra of Tb-(R)-Bt (red line) and Tb-(S)-Bt (blue line) in CHCl3 (1.0 × 10−4 M).

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Table 1 Luminescence quantum yields (Φπ−π*), luminescence lifetimes (τobs) and dissymmetry factors gCPL of Tb(III) clusters
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The Faraday effect of the chiral Tb(III) clusters

The wavelength dependence of the Faraday effect was measured using PMMA films containing Tb-(R)-Bt or Tb-(S)-Bt. The Faraday effect was determined based on the Verdet constant, V, as defined in the following equation:1

where θ, H and l represent the Faraday rotation angle, the external magnetic field and the thickness of the film, respectively. In these experimental trials, we determined the concentration C of the Tb(III) ions in the film using inductively coupled plasma atomic emission spectroscopy. Based on previous work,6 we subsequently calculated the Verdet constant, Vc, normalized by the concentration of Tb(III) ions in the film, as follows:

The measured rotation angle consists of a magnetic rotation, θF, due to the Faraday effect and a natural rotation, θCD, due to chirality. To extract θCD from the total rotational angle, two rotational angles, θ+ and θ−, were measured by applying an external magnetic field parallel (+) and antiparallel (−) to the direction of light propagation, respectively (Supplementary Figure S4). These parameters are represented as follows:

The Faraday rotation angle was obtained from Equations (3a and b).

The Faraday rotation spectra and the differential spectrum, ΔVC, are shown in Figure 5a and b. In Figure 5a, negative Faraday rotation angles are observed at ∼450 nm. The Verdet constant of the Tb-(R)-Bt (VC,max(446 nm)=−5.2 × 10−4 deg Oe−1 cm−1 M−1) was larger than that of the Tb-(S)-Bt (VC, max(446 nm)=−3.4 × 10−4 deg Oe−1 cm−1 M−1). This result represents the first observation of a Faraday effect that is dependent on the chirality of the clusters.

Figure 5
figure 5

(a) The Faraday rotation spectra of Tb-(R)-Bt (red line) and Tb-(S)-Bt (blue line) and (b) the differential spectrum between Tb-(R)-Bt and Tb-(S)-Bt in poly(methyl methacrylate) (PMMA) films.

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Generally, the Faraday rotation angle can be expressed as the sum of three terms resulting from three electronic conditions, as follows:1, 30, 31

Here, f1 and f2 are functions that provide an explicit description of the shape of the Faraday effect. The Faraday A and C terms primarily reflect the degeneracy of the excited and the ground states, respectively, whereas the Faraday B term is related to the mixing of the ground and two or more excited states by the magnetic field. In general, the Faraday C term is larger than the Faraday A and B terms.31 The wavelengths observed in the Faraday effect measurements are associated with several 4f-4f transitions, such as 7F6→5D4 (488 nm), 7F6→5D3 (380 nm)32 and the broad 4f–5d (below 300 nm)33 transitions of the Tb(III) ions and the S0→T1 transitions (470 nm) of the ligands.34 The degenerate ground state is not formed by diamagnetic organic molecules such as the ligands, but rather by the paramagnetic Tb(III) ions depending on the total angular momentum, J. In this case, the 7F6 (J=6) state of the Tb(III) ions can degenerate into 13 states (2J+1). For this reason, the observed signals were primarily due to the transitions of the Tb(III) ions.

The photophysical properties of the Tb(III) ions depend on their coordination geometry. We have previously reported that the Faraday rotation angles of nonanuclear Tb(III) cluster derivatives were changed by altering a substituent in the ligands.6 This effect results not from changing the electronic state of the ligands, but rather from varying the geometry around the Tb(III) ions of the cluster by using different ester substituents in the ligands.35 We also demonstrated that the shape of the Stark splitting of Tb-(R)-Bt was consistent with that of Tb-(S)-Bt based on their geometries (Figure 3 and Supplementary Figure S2), suggesting that difference in the Faraday rotation angle is not derived from configuration interactions between the 4f–4f and 4f–5d transitions. These aspects of the photophysical behavior indicate that the difference between the Faraday rotation angles observed in our experimental work might be attributable to the large transition magnetic dipole moments of Tb-(R)-Bt and Tb-(S)-Bt. The sign of the transition magnetic dipole moments in the Faraday B term inverts depending on the chirality,1 indicating that this term might be affected by interactions between the 4f–4f (7F6→5D4) transition and the other 4f–4f transitions such as 7F6→5D3 or the 4f–5d transition.1 The chirality-induced magneto-optical effect observed in our work is a different phenomenon from magneto-chiral dichroism (that is, the absorption dependence of a chiral molecule on the direction of a magnetic field).36, 37, 38 Although it is difficult to determine the exact origin of the phenomenon at this stage, this is the first report that Faraday rotation can be affected by chiroptical properties. This result could be a key factor in the design of new materials exhibiting the Faraday effect and may also broaden the field of chiral science.

Conclusion

In the present study, we assessed the chiroptical and magneto-optical properties of novel chiral nonanuclear Tb(III) clusters that are composed of 9 Tb(III) ions and 16 chiral 2-butyl salicylate ligands. Faraday effect measurements indicated that the Faraday rotation angles depended on the chirality of the ligands in the Tb(III) clusters. These results provide significant new insight regarding the interrelation between chirality and the Faraday effect. To clarify this mechanism and develop novel magneto-optical materials exhibiting pronounced Faraday rotations, the study of the Faraday effect associated with chiral clusters with large gCD values is now in progress.