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Organic tribophosphorescence materials are rarely reported and the introduction of Br atoms may be a practical way to design such materials. Here four bromine-substituted fluorene-based derivatives are presented and BrFlu-CBr, having fluorescence-phosphorescence dual-emission induced not only by UV light but also by mechanical stimulus, manifests the highest phosphorescence efficiency of 4.56 % upon photoirradiation. During the grinding process, three different triboluminescent spectra were identified. Upon introduction of a mechanical stimulus, the triboluminescence emission is cyan, whereas after an extended period it changed to blue. After removing the mechanical stimulus, green-white phosphorescent emission was observed. Careful research on single-crystal structures and theoretical calculations demonstrate that strong Br⋅⋅⋅Br interactions are vital to facilitate spin-orbit coupling and promote intersystem crossing, thus generating the unique properties.
Wang et al. (Tue,) studied this question.