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Previous studies have hypothesized that the principle of TL (transistor latency) in rare-earth complexes originates from direct excitation of rare earth elements, from antenna effects initiated by the excitation of organic molecules, or from the excitation of gases surrounding the sample (e.g., nitrogen). Whether or not the molecular assembly exhibits centrosymmetry has also been a subject of discussion. The chiral complex molecule used in this study, Chiral LnL^val (Ln = rare earth ion), is arranged in a very unusual space group "P"6? (or "P"6?), forming a lamellar structure. In this structure, aromatic rings of organic molecules are arranged in a sheet-like manner on one side, flanking the rare-earth layer, while nitrate ions are arranged on the other side. The next complex layer is deposited anisotropically while maintaining this orientation. In other words, it belongs to a system with broken centrosymmetry. For comparison, similar experiments were performed using racemic crystals of LnL^val and crystals in which the counter-anion was changed from nitrate ions to chloride ions, but neither exhibited TL.
When the TL of Chiral LnL^val was measured using DTS, the emission spectrum unique to rare earth ions was observed (Figure 2).
For example, a sharp band appears in the red wavelength region for Eu and in the green wavelength region for Tb. To understand the mechanism of this TL phenomenon, we conducted experiments using four further approaches.

(1) Dependence of TL intensity on stimulus strength (Figure 2): When the starting height of the drop of the stainless steel ball in the Chiral EuL ^val DTS is increased and the energy used for crushing is increased, the TL intensity in this system no longer changes from a height of 80 cm. This indicates that there is a limit to the mechanical energy required to induce TL.

(2) Environmental specificity of TL expression: To investigate whether the TL in this system is due to direct excitation of rare earth ions or via an antenna effect, the DTS itself was confined in an argon or nitrogen atmosphere and TL measurements were performed. The results showed that it was not dependent on the environment. Furthermore, by comparing the relative intensity of the TL spectrum using Chiral Tb/EuL^val, which is a mixture of Tb and Eu, with that of the PL spectrum, it was found that intermetallic energy transfer from Tb to Eu does not occur in the TL. In other words, it was found that the TL is not due to direct excitation of rare earth ions.

(3) Correlation between crystal grain size and TL expression (Figure 3): Crystals of the complex immediately after synthesis exhibit TL, but when the crystals are thoroughly crushed in a mortar, TL is no longer expressed. When these were measured by powder X-ray diffraction (PXRD) at the large synchrotron radiation facility SPring-8*? (BL02B2), the full width at half maximum of the XRD peak of the crushed sample increased, but the position of the diffraction peak did not change. Furthermore, upon recrystallization, the full width at half maximum of the XRD peak became sharp, similar to the case immediately after synthesis, and TL was observed. The effect of size was also observed from electron microscopy. In other words, during this crushing process, the crystal structure does not change significantly (no crystal phase transition occurs), but the crystal grain size decreases, indicating that a certain crystal grain size is necessary for TL expression.

(4) Confirmation of TL expression of organic molecules acting as optical antennas: Many conventional rare-earth complexes exhibiting TL have focused on the luminescence of the rare earth itself. Complexes with Gd attached are useful for understanding the luminescence properties of the organic compound. Chiral GdL^val was found to be a system that exhibits TL localized to the organic molecule. In other words, in conjunction with the considerations in (2), it was found that TL originating from rare-earth ions such as Eu is excited by mechanical stimulation of the ligand and proceeds via intramolecular energy transfer.

In summary, this study demonstrated that a system that converts mechanical energy into light emission using lamellar crystals of chiral rare-earth complexes containing amino acids is due to an antenna effect via the excited state (particularly the excited triplet state) of the ligand (Figure 4). Such a system has not yet been put into practical use, and elucidating its principle will provide a starting point for developing new light-emitting materials for the future.

In conducting this research, we received guidance and assistance from Professor Koichi Nozaki and Lecturer Munetaka Iwamura of Toyama University, and Dr. Kenta Goto of the Institute for Materials Chemistry and Engineering, Kyushu University. This research was carried out with support from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) Grants-in-Aid for Scientific Research, the Yamada Science Foundation, the Joint Research Center for Materials and Devices, and the MEXT Strategic Research Base Formation Support Program for Private Universities. Synchrotron radiation PXRD measurements were performed at the SPring-8 large synchrotron radiation facility as part of a project that was selected, and we received assistance from Hamamatsu Photonics K.K. and Teijin Limited in setting up the DTS. We would like to take this opportunity to express our gratitude.

*1: Soft crystals are a general term for molecular crystals that are highly ordered and flexible response systems. The conditions for formation of soft crystals and the phase transition phenomena have been dramatically advanced in the MEXT Grant-in-Aid for Scientific Research on Innovative Areas "Soft Crystals - Principles and Optical Functions of Highly Ordered and Flexible Response Systems," and they are developing as a new group of materials.
*2: Chirality refers to a relationship where molecules have the same skeletal structure but cannot overlap, like the relationship between a right hand and a left hand. For example, in the case of molecules, the aroma components of lemon and orange are chiral compounds, and because they have right-hand and left-hand molecular structures, they have different functions (in this case, aroma).
*3: Rare earth elements are a general term for scandium, yttrium, and lanthanides with atomic numbers 57 to 71. Rare earth complexes are compounds formed by bonding organic molecules to these elements. Here, we use complexes with samarium (Sm), europium (Eu), terbium (Tb), and dysprosium (Dy), which are known to exhibit luminescence, as well as gadolinium (Gd) for comparison.
*4: A RIKEN facility located in Harima Science Park City, Hyogo Prefecture, that produces the world's highest-performance synchrotron radiation. The Japan Synchrotron Radiation Research Institute (JASRI) provides user support. The name SPring-8 comes from Super Photon ring-8 GeV (gigaelectronvolts). At SPring-8, a wide range of research is conducted using this synchrotron radiation, from nanotechnology and biotechnology to industrial applications.

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