Lamellar-Cleavage-Induced Triboluminescence in Discrete Chiral Complexes with Lanthanides

Triboluminescence (TL), which is the light emission induced by mechanical stimuli, has attracted significant interest owing to its potential for energy conversion applications. Here, the TL mechanisms of chiral lanthanide complexes (LnL^val, Ln = Eu, Tb, Sm, Dy, and Gd) were elucidated by focusing on the role of lamellar cleavage of the crystal structure. The modified drop tower system (DTS) quantifies mechanical stimuli used to observe the TL spectra. Chiral EuL^val and TbL^val exhibit intense TL, in contrast to the absence of TL in their racemic counterparts. Structural analyses revealed lamella-like molecular stacking in the chiral crystals with anisotropic molecular orientations and flexibility along the c-axis. TL was reversibly observed by recrystallization when the crystal size was sufficiently large. Comparison of the TL and photoluminescence (PL) spectra, showed that TL of chiral LnL^val was enhanced through photo-antenna effects by the impact of lamellar cleavage. The TL values of the chiral Dy and Sm complexes were also evaluated. Furthermore, the absence of chemical reaction of the two nitrate ions with neighboring molecules in the lamella was confirmed. This system suggests the importance of the lamellar arrangement and provides potential design strategies for mechano-responsive materials for TL activation of Ln complexes.