Bright Lanthanide III Triboluminescence despite Low Photoluminescence, and Dual Triboluminescence and Mechano‐Responsive Photoluminescence

reactions in organic solvents. Mechanochromic luminescence (MCL) describes the deformation of molecular conformations alongside changed optical properties in response to mechanical stimuli. Crystalto-crystal/amorphous phase transitions produce shifts in the emission wavelength and/or changes in the emission intensities of organic and coordination compounds. [3] Despite the first description of blue-glowing sugar having been described in 1605, and the broad versatility of these mechanicallytriggered phenomena, triboluminescence (TL)-fracture-induced luminescence-is not yet understood in terms of a general molecular design, and the underlying photophysical processes remain unclear. TL is observed only in the moment in which force is applied and does not require a light source. Studying this instantaneous forceto-photon conversion is of critical not only from the perspective of fundamental science but also for industrial applications, which include irradiation-free force/stress sensors, security marking techniques, and health care devices. [4] Bright TL has been reported for various lanthanide III (Ln III) complexes with dibenzoylmethide (dbm), thenoyl-trifluoroacetylacetonate (tta), and hexafluoroacetylacetonate (hfa) ligands, In pursuit of a new family of mechanically responsive luminescent materials, it is aimed to differentiate triboluminescence (TL) from photoluminescence (PL). A β-diketonate ligand with tert-butyl groups (2,2,6,6-tetramethylheptane-3,5-dionate: tmh) is selected to quench Eu III-centered PL via ligand-to-metal charge transfer, whereas tmh provides efficient photosensitization of Tb III ions. Bright TL is observed from the Eu III and Tb III homodinuclear complexes despite the fact that their PL quantum yields differed by a factor of >50. Nanomechanical tests reveal the ductility of the crystals, suggesting they are ideal for accumulating deformation energy before breakage. Furthermore, a TL/PL color difference is observed for a Tb III /Eu III heterodinuclear complex, and grinding results in mechanochromic luminescence (MCL); this is the first example of a dual TL-and MCL-active lanthanide III coordination compound. The photophysical properties before, during, and after grinding are investigated and correlated with powder and single-crystal crystallographic data.