Chiral lanthanide lumino-glass for a circularly polarized light security device

Artificial light plays an essential role in information technologies such as optical telecommunications, data storage, security features, and the display of information. Here, we show a chiral lanthanide lumino-glass with extra-large circularly polarized luminescence (CPL) for advanced photonic security device applications. The chiral lanthanide glass is composed of a europium complex with the chiral (+)-3-(trifluoroacetyl)camphor ligand and the achiral glass promoter tris(2,6-dimethoxyphenyl)phosphine oxide ligand. The glass phase transition behavior of the Eu(III) complex is characterized using differential scanning calorimetry. The transparent amorphous glass shows CPL with extra-large dissymmetry factor of gCPL = 1.2. The brightness of the lumino-glass is one thousand times larger than that of Eu(III) luminophores embedded in polymer films of the same thickness at a Eu(III) concentration of 1 mM. The application of the chiral lanthanide lumino-glass in an advanced security paint is demonstrated.

A rtificial light plays an essential role in information technologies such as optical telecommunications, data storage, security features, and the display of information 1 . Organicbased luminophores that allow precise control of information have been extensively studied for the development of light-based information technologies 2,3 . This precise control originates from the molecular quantum design of organic luminophores [4][5][6] . The combination of the molecules with quantum design and flexible amorphous materials is expected to result in advancements in photo-science and -technology for industrial applications such as flexible organic light-emitting diodes (OLED) [7][8][9] .
In this work, we focus on an optical information technology based on chiral luminescent molecules. Recently, circularly polarized luminescence (CPL) from chiral organic molecules has attracted considerable attention because of its applications in sophisticated photo-information technologies, such as security tags, lasers, data storage, and organic electroluminescent (EL) devices for three-dimensional displays [10][11][12][13][14][15][16] . The CPL phenomena are characterized by the differential emission of right-and lefthanded circularly polarized light. The magnitude of CPL is given by the following equation 17,18 : where I L and I R are the emission intensities of the left-and righthanded CPL, respectively. In particular, lanthanide complexes containing chiral organic ligands exhibit dissymmetry factors (g CPL ) approximately a thousand times larger than those of chiral organic molecules [10][11][12][13][19][20][21][22] . Parker prepared anion sensors using the CPL signals of mononuclear Eu(III) and Tb(III) complexes with chiral ligands incorporating an azaxanthone sensitizer 23 . Muller and colleagues 24 observed an exceptionally large CPL in an Eu(III)-Cs(I) system with characteristic chiral β-diketonate ligands containing a camphor framework (tetrakis(3-heptafluorobutylryl-(+)-camphorato) (g CPL = −1.38) 24 , although the emission quantum yield was extremely low (Φ tot < 1.0%) 25 . Recently, we found that phosphine oxide ligands improve the quantum emission yield by controlling the energy quenching state for a chiral Eu(III) complex with camphor (Φ tot > 10%) 26 . The use of well-designed phosphine oxide ligands is a key strategy for the construction of chiral Eu(III) complexes with large g CPL and Φ tot values for advanced CPL luminophore applications.
Herein, we prepare a bulky phosphine oxide ligand with six methoxy groups, tmpo (tris(2,6-dimethoxyphenyl)phosphine oxide, Fig. 1a), for the construction of complexes with large g CPL and Φ tot values. The bulky ligand coordinates weakly to the Eu(III) ion; such weak coordination has been reported to be related to the enhancement of CPL 22 . The tmpo ligand is also expected to promote glass formation due to the multiple coordination sites provided by its phosphine oxide (P=O) 26 and methoxy groups (O-Me) 27 , which is beneficial for the fabrication of transparent materials. The ligand (±)-3-(trifluoroacetyl)camphor (±tfc) is selected to achieve a chiral ligand arrangement around the Eu center (Fig. 1b). The chiral Eu(III) complexes with phosphine oxide ligands are prepared by casting Eu(±tfc) 3 (H 2 O) 2 and tmpo ligands on a substrate. The glass phase transition behavior of the Eu(III) complex (Eu(±tfc)-tmpo) is characterized using differential scanning calorimetry. The resulting chiral Eu(III) lumino-glass exhibits extraordinary CPL intensity (g CPL = 1.2) and a relatively high emission quantum yield (Φ tot = 13 %). Chiral Eu(III) complexes with other phosphine oxide ligands (Fig. 1c, L1: Tris(2methoxyphenyl)phosphine oxide, L2: Tris(3-methoxyphenyl) phosphine oxide, L3: tris(4-methoxyphenyl)phosphine oxide, L4: tris(4-methylphenyl)phosphine oxide, L5: tris(2,4,6-trimethylphenyl)phosphine oxide, Supplementary Methods) are also prepared for comparison of their photophysical properties. Based on spectroscopic analyses, we determine that the extra-large g CPL of Eu (±tfc)-tmpo originates from its characteristic symmetric coordination structure with a strong crystal field. Using the chiral Eu(III) lumino-glass, the first application of CPL paint as a security ink (to display a sun or moon shape) are demonstrated. These glasstype chiral Eu(III) complexes are promising candidates for future devices with polarized light information.

Results and discussion
Luminescence intensity of Eu(III) lumino-glass. Emission properties of Eu(III) lumino-glass ( Fig. 2) were evaluated using emission spectra and time-resolved emission measurement. The emission spectra of the Eu(+tfc)-tmpo(n) films (n = 1, 2, 3, n: number of equivalents relative to Eu(+tfc) 3     spectra were normalized using the integrated intensity of the magnetic dipole transition (MD: 5 D 0 → 7 F 1 ). Two peaks were observed at 584 and 594 nm for the MD transition (Fig. 3, inset). The strong emission bands near 612 nm were attributed to the electric dipole transition (ED: 5 D 0 → 7 F 2 ). The emission band of Eu(+tfc)-tmpo(1) was different from those of Eu(+tfc)-tmpo (2) and Eu(+tfc)-tmpo(3). The luminescence intensity ratio of the ED/MD transition (A ED /A MD ) decreased with increasing n (i.e., increasing number of tmpo molecules), indicating that the Eu(III) complex with tmpo ligands forms a symmetric coordination geometry 28 . The crystal field splitting in the MD transition was found to be 274 cm −1 . Small A ED /A MD and large crystal field splitting values were also observed in a comparative study with other Eu(III) complexes (Supplementary Note 2-3, Supplementary Figs. 4-7, and Supplementary Table 1). These results indicated that a symmetric coordination structure with a strong crystal field was formed by the tmpo ligands, which was attributed to the large steric hindrance and multiple interaction of tmpo ligands.
To obtain information regarding the coordination structure around the luminescent Eu(III) center, the time-resolved emission profiles of the complexes were measured. The emission lifetimes were estimated using triple exponential decays (Table 1), and were consistent with the existence of several metastable structures. The relative lifetime of the τ 3 component increased with the addition of more equivalents of tmpo. The emission decay phenomenon of Eu(+tfc)-tmpo(2) was similar to that of Eu (+tfc)-tmpo(3). The emission quantum efficiencies excited by the tfc ligands (Φ tot ) of the films are also shown in Table 1. The Φ tot values also increased with the number of equivalents of tmpo added. The emission quantum yield of Eu(+tfc)-tmpo(2) (Φ tot = 13 %) was much larger than that of previously reported Eu(III)-Cs(I) systems (Φ tot < 1%) with a large g CPL value (−1.38) in solution 24,25 .  Table 3, and Supplementary Data 1-3). The g CPL values at 594 nm (film: g CPL = −1.2, solution: g CPL = −1.0) were as large as that previously reported (g CPL = −1.38) for Eu(III)-Cs(I) systems 24,25 . These values were different from the small g CPL value of a mononuclear Eu (III) complex with tightly coordinated phosphine oxide ligands (Eu(+tfc) 3 (tppo) 2 , tppo: triphenylphosphine oxide, g CPL = 0.09) and a polynuclear Eu(III) complex ([Eu(+tfc) 3 dpbp] n , dpbp: 4,4-bis (diphenylphosphoryl)biphenyl, g CPL = 0.17) 26 . The g CPL value of Eu (+tfc)-tmpo(2) was also higher than those of the other Eu(III) lumino-glasses in a comparative study ( Supplementary Fig. 7 and Supplementary Table 1). In the comparative compounds, the Eu (+tfc)-L1(2) showed a relatively large g CPL signal. We attributed the large g CPL value of the Eu(III) lumino-glass to the large J-mixing (J: total angular momentum) 22 induced by the symmetric coordination geometry with a strong crystal field. In addition, the methoxy group in the ortho-position in the phosphine oxide ligands play an important role in the formation of the characteristic coordination field.
Demonstration of a CPL application. We also demonstrated a CPL paint application using the amorphous Eu(III) lumino-glass. The prepared samples are shown in Fig. 5a. Transparent glasses of Eu(+tfc)-tmpo(2) and its enantiomer (Eu(−tfc)-tmpo(2)) were prepared on a substrate using a casting method (Eu(+tfc)-tmpo (2): sun symbol; Eu(−tfc)-tmpo(2): moon symbol). The experimental setup for visualizing the CPL image is depicted in Fig. 5b. In the setup, the emission of the lumino-glasses is detected using a camera with a linear polarizer, a rotatable λ/4 plate, and a bandpass filter (594 nm). The left-or right-handed polarized light is detected by rotating the λ/4 plate 90°clockwise or anticlockwise using an angle controller. The photographs of the emission of the samples using total light (without the λ/4 plate), left-handed, and right-handed circularly polarized light are shown in Fig. 5c, d, and e, respectively. The total emission from the Eu(+) and Eu(−) lumino-glass is observed in photograph (Fig. 5c). Under clockwise and anticlockwise rotation, the sun and moon shapes of the Eu(+) and Eu(−) lumino-glasses were observed, respectively (photographs Fig. 5d, e). This is the first demonstration of nakedeye detection of a security paint based on the CPL phenomenon using amorphous Eu(III) lumino-glasses.
To the best of our knowledge, the first lanthanide lumino-glass with an extra-large CPL and a relatively high emission quantum yield was reported. We revealed that symmetric coordination with a strong crystal field is a key factor for the enhancement of  g CPL . We also successfully demonstrated a security paint based on the CPL phenomenon using Eu(III) lumino-glass for the first time. Our results provide new insights into the design of Eu(III) complexes with excellent CPL performance and high emission intensity, which should lead to new applications of CPL materials as security inks.
Apparatus. Electrospray ionization mass spectra (ESI-MS) were measured by using a Thermo Scientific Exactive instrument. Elemental analyses were performed on an Exeter Analytical CE440. Emission spectra and emission lifetimes were measured using a Horiba/Jobin-Yvon FluoroLog-3 spectrofluorometer. CPL spectra were measured using a JASCO CPL-300 spectrofluoropolarimeter. DSC measurements were recorded on a SII DSC 7020 heat flux meter. Proton nuclear magnetic resonance ( 1 H NMR) spectra were recorded on an auto-NMR JEOL ECS 400 MHz.