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Mechanochromic luminescence (ML) references to intensity and/or color changes of (solid-state) luminescent materials induced by mechanical forces, such as rubbing, crushing, pressing, shearing, or smearing. Unlike " triboluminescence" which does not require additional excitation source other than force itself, ML is often manifested by external photoexcitation such as a UV lamp. ^[1] The most common cause of ML is related to changes of intermolecular interactions of dyes and pigments, which gives rise to various strong ( exciton splitting) and/or weak ( Forster) excited state interactions. For example, a certain boron complex of sunscreen compound avobenzone exhibits reversible ML. A recent detailed study ^[2] suggests that ML from the boron complex consists of two critical coupled steps: 1) generation of low energy exciton trap via mechanical perturbation; and 2) exciton migration from regions where photoexcitation results in a higher excited state. Since solid-state energy transfer can be very efficient, only a small fraction of the low-energy exciton traps is required when mechanical force is applied. As a result, for crystalline ML materials, XRD measurement may not able to detect changes before and after mechanical stimuli while its photoluminescence can be quite different.

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