Mechanochromic luminescence
Mechanochromic luminescence references to intensity and/or color changes of 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.Mechanochromic Fluorescent Materials, Editors: Jiarui Xu, Zhenguo Chi, Royal Society of Chemistry, Cambridge 2014, https://pubs.rsc.org/en/content/ebook/978-1-78262-322-9 The most common cause of ML is related to changes of intermolecular interactions of dyes and pigments, which gives rise to various strong and/or weak 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.