Advances in Pure Organic Mechanoluminescence Materials

Versatile Method of Generating Triboluminescence in Polymer Films Blended with Common Luminophores

“…No correlation was observed between TL properties and the crystal space group, with both noncentrosymmetric TPE ( P 2 1 ) and RiTB ( P 1) and centrosymmetric Ir­(ppy) 3 ( P 3̅) and Eu­(acac) 3 phen ( P 2 1 /c) crystalline samples showing TL in the crystalline state. This is also consistent with our previous results and many recent examples showing TL in centrosymmetric crystals. ,, …”

“…1,2,5,6,9,23,24 While many earlier studies reported that this phenomenon is most commonly observed in noncentrosymmetric (piezoelectric) crystals, 22 several other studies showed that TL may also be present in centrosymmetric crystals (as well as achiral polymers and metals), and no full correlation is present between the lack of centrosymmetry and TL. 10,11 To establish whether noncentrosymmetric crystals were required to observe TL among our selected compounds, we (re)examined their crystal structures by single-crystal X-ray diffraction (SC-XRD) (Table S1) and powder X-ray diffraction (PXRD), which confirmed the absence of polymorphs in the studied samples and that the PXRD patterns were consistent with patterns simulated based on the SC XRD structures (Figures S10 and 11). No correlation was observed between TL properties and the crystal space group, with both noncentrosymmetric TPE (P2 1 ) and RiTB (P1) and centrosymmetric Ir(ppy) 3 (P3̅ ) and Eu(acac) 3 phen (P2 1 /c) crystalline samples showing TL in the crystalline state.…”

“…This is also consistent with our previous results 18 and many recent examples showing TL in centrosymmetric crystals. 6,9,10 Next, we prepared luminophore-blended PMMA films by dissolving the luminophores and the polymers in dichloromethane solution, followed by casting on glass and drying under vacuum. Transparent films were obtained, with a film thickness of ca.…”

“…In contrast to photoluminescence (PL), TL does not require light irradiation to produce the excited state (Figure ), which offers great advantages for the development of mechanoresponsive materials capable of visualizing applied mechanical force or as components for self-powered light-emitting devices. A variety of materials have been discovered to show TL, including photoluminescent as well as nonphotoluminescent compounds such as sugars, inorganic salts, rare-earth doped ceramics, and semiconducting particles. − The most widely studied TL materials are crystalline solids that show emission upon their fracture. In practical terms, the utilization of crystalline materials to observe TL imposes serious limitations to reliable preparation and repeatable use, as the observed effect is usually a function of crystal size and tends to diminish after mechanical damage. , …”

“…Although currently there is no unifying theory to explain the origin of TL in solids, it has often been attributed to the formation of oppositely charged planes upon crystal fracture, generating a sufficiently strong electric field to cause discharge through the surrounding gas, which is manifested by the presence of gas-specific sharp emission lines in the TL spectrum of nonphotoluminescent materials. If the materials are tribophotoluminescent (emit PL when fractured), this may result in excitation of the luminophore via absorption of the UV emission from the excited gas or directly by electron bombardment. ,,,,,, While many earlier studies reported that this phenomenon is most commonly observed in noncentrosymmetric (piezoelectric) crystals, several other studies showed that TL may also be present in centrosymmetric crystals (as well as achiral polymers and metals), and no full correlation is present between the lack of centrosymmetry and TL. , To establish whether noncentrosymmetric crystals were required to observe TL among our selected compounds, we (re)­examined their crystal structures by single-crystal X-ray diffraction (SC-XRD) (Table S1) and powder X-ray diffraction (PXRD), which confirmed the absence of polymorphs in the studied samples and that the PXRD patterns were consistent with patterns simulated based on the SC XRD structures (Figures S10 and 11). No correlation was observed between TL properties and the crystal space group, with both noncentrosymmetric TPE ( P 2 1 ) and RiTB ( P 1) and centrosymmetric Ir­(ppy) 3 ( P 3̅) and Eu­(acac) 3 phen ( P 2 1 /c) crystalline samples showing TL in the crystalline state.…”