Mechanofluorochromic Material toward a Recoverable Microscale Force Sensor

in optical properties upon application of mechanical stimuli such as scratching or grinding, promises applications for force sensors. [20-36] Emission changes of coordi nation complex crystals, [24] polymeric micro crystals, [30] and organic fluorophores, [31-36] have been correlated to an applied force or pressure for new anticounterfeiting techno logies [23-28] and biological stress probes. [29] Force sensing via mechanofluoro chromic activity has been reported at the micro/nanoscales by direct measurement of emission changes upon in situ applica tion of varying amounts of mechanical stimulus. [24,30,31,33-36] Yet, one issue that remains to be explored is the recovery of the material. Emission changes, correlated with applied force, are induced by morpho logical changes in the material. [20-50] This means that subsequent sensing events, after the initial application of force, necessi tate recovering the original morphology-a process not so straightforward as this usu ally requires thermal annealing, [38] solvent fuming, [25,27] or recrystallization. [20-22,37] Selfrecovery, the spontaneous return to the initial state (of absorption, emission and morphology) of the scratched/ground material under ambient conditions, has been observed in derivatives based on Au(I) complexes, [28] pyrene, [39,40] anthracene, [41,42] tetraphenylethene, [43,44] indolylbenzothiadiazole, [45] triphenylamine, [46] boroncoordinated βdiketonate complexes, [47] and hexathiobenzene. [48] However, many MFCactive materials are left unexplored for multipleuse force sensing applications, not only due to the complexity of instrumentation required for such studies, [24,30,31,33-36] but also due to the lack of molecular design leading to reversibility [40] and a clear understanding of mechanism of selfrecovery. [30,45]