Revealing the Origins of Mechanically Induced Fluorescence Changes in Organic Molecular Crystals

Mechanofluorochromic molecular materials display a change in fluorescence colour through mechanical stress. Complex structure-property relationships in both the crystalline and amorphous phases of these materials govern both the presence and strength of this behaviour, which is usually deemed the result of a mechanically-induced phase transition. However, the precise nature of the emitting species in each phase is often a matter of speculation, resulting from experimental data, which are difficult to interpret, and a lack of an acceptable theoretical model capable of capturing complex environmental effects. With a combined strategy using sophisticated experimental techniques and a new theoretical approach, here we show that the varied mechanofluorochromic behaviour of a series of difluoroboron diketonates is driven by the formation of low-energy exciton traps in the amorphous phase, with a limited number of traps giving rise to the full change in fluorescence colour. Our results highlight intrinsic structural links between crystalline and amorphous phases, and how these may be exploited for further development of powerful mechanofluorochromic assemblies.