Mechanoluminescence (ML) is the nonthermal luminescence generated in the process of force-to-light conversion, which has broad prospects in stress sensing, wearable devices, biomechanics, and multiple information anticounterfeiting. Multivalence emitter ions utilize their own self-reduction process to realize multiband ML without introducing another dopant, such as Eu3+/Eu2+, Sm3+/Sm2+, and Mn4+/Mn2+. However, self-reduction-induced ML in bismuth-activated materials has rarely been reported so far. In this work, a novel visible-to-near-infrared (vis–NIR) ML induced by the self-reduction of Bi3+ to Bi2+ in the spinel-type compound (MgGa2O4) is reported. The photoluminescence (PL) spectra, PL excitation (PLE) spectra, and PL lifetime curves demonstrate that Bi3+/Bi2+ ions are the main luminescence centers. Notably, the possible self-reduction model is proposed, where a magnesium vacancy (VMg″) is considered as the driving force for the self-reduction of Bi3+ to Bi2+. Furthermore, an oxygen vacancy (VO••) is confirmed by electron paramagnetic resonance (EPR) spectroscopy. Combined with thermoluminescence (TL) glow curves and ML spectra, a plausible trap-controlled ML mechanism is illustrated, where electron–hole (VO••/VMg″) pairs play a significant role in capturing electrons and holes. It is worth noting that the proof-of-concept dual-mode electronic signature application is implemented based on the flexible ML film, which improves the capabilities of signature anticounterfeiting for high-level security applications. Besides, multistimulus-responsive luminescence behaviors of the ML film are realized under the excitation of a 254 nm UV lamp, thermal disturbance, 980 nm laser, and mechanical stimuli. In general, this study provides new insights into designing vis–NIR ML materials toward wider application possibilities.
The development of organic phosphors with excitation-dependent luminescence color is tremendously challenging for organic room temperature phosphorescent (ORTP) systems. Herein, we designed and synthesized a series of novel ORTP materials based on pyranone. Under the excitation of 254 nm, the materials exhibited the rare phenomenon of emitting almost pure phosphorescence, whereas strong cyan photoluminescence and weak green phosphorescence were observed upon exposure to 360 nm excitation light. Experiment analysis and theoretical calculations demonstrate that the regulation of luminescent color is attributed to the different transition process of excitons from singlet excited states (S 1 and S 2 ) to triplet excited states (T 1 ) under various excitation wavelengths. The materials still can maintain the crystalline state under various extreme conditions (ground, recrystallization, high pressure), thereby maintaining the ORTP properties. In addition, the ORTP properties of the materials are very stable to water/heat/light/air. Taking advantage of the multiple luminescence colors, excellent crystallinity and stability, these phosphors can be developed as an advanced anti-counterfeiting ink for printing and hand-painting on a wide range of media, showing an extremely ideal application prospect.
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