Beyond the Cr³⁺ Paradigm: Bi²⁺-Enabled Robust and Self-Recoverable Near-Infrared Mechanoluminescence for Mechanically...

Near-infrared (NIR) mechanoluminescence (ML) materials that directly convert mechanical stimuli into optical signals are highly desirable for mechanically adaptive optoelectronics. To date, however, this field has been overwhelmingly dominated by Cr³⁺-based phosphors, whose emission relies on piezoelectric-field-driven excitation. While effective in rigid inorganic hosts, such systems suffer from pronounced performance degradation when transferred into flexible matrices, resulting in poor repeatability, limited cyclic stability, and restricted applicability. Here, we move beyond the Cr³⁺ paradigm by introducing Bi²⁺ as an alternative and fundamentally distinct activator for flexible NIR ML. By incorporating a newly developed Sr₃(BO₃)₂:Bi²⁺ phosphor into a polydimethylsiloxane (PDMS) matrix, a chromium-free NIR ML elastomer is developed, leveraging a triboelectrification-induced interfacial charge transfer mechanism to achieve robust broadband emission peaking at 815 nm under diverse mechanical excitations. The composite demonstrates highly repeatable and cyclically stable NIR ML over 10,000 continuous stretching cycles, with an impressive initial power density (29.0 mW·m^-2) and self-recovery behavior (24.1%@1 min and 91.5%@24 h). Notably, its initial ML intensity exceeds that of state-of-the-art Cr³⁺-based counterparts (e.g., Ga₂O₃:Cr³⁺/PDMS) by more than 2.2-fold, while simultaneously exhibiting markedly enhanced cyclic stability. This work presents a chromium-free, self-powered, and self-recoverable NIR ML system, paving the way for mechanically adaptive optoelectronic devices.