Photomechanochromic vs. mechanochromic fluorescence of a unichromophoric bimodal molecular solid: multicolour fluorescence patterning

Mechanofluorochromism (MFC) of molecular solids generally results from the variation of intermolecular interactions induced by external mechanical forces. However, the use of internal photomechanical forces to perturb intermolecular interactions for multicolour fluorescence responses has yet to be demonstrated. Herein we report a unichromophoric anthracene–pentiptycene derivative (1) that displays both MFC and photomechanofluorochromism (PMFC), which lead to various fluorescence colours including red-green-blue (RGB) and near-pure white-light emission. Compound 1 crystallizes in two polymorphs, the yellow (Y) and green (G) emissive forms, in which the pairwise stacked anthracene groups undergo [4 + 4] photodimerization to form the UV (black) emissive photodimer 2 and meanwhile exert photomechanical stresses on the neighbouring molecules. While the photomechanical stresses cause an excimer-to-monomer switching that results in a blue fluorescent state for the Y form, a red-emissive “super dimer” is photomechanically produced for the G form. The recovery of the Y form demands heating, but the G form could be restored by selective photoexcitation of the super dimer. X-ray crystal structures of the Y and G forms and the photodimer 2 generated through single-crystal-to-single-crystal transformation provide a clue to the origins of polymorph-dependent PMFC. The corresponding MFC and mechano-activated vapofluorochromism (VFC) of 1 also shed light on the structure–property relationship. The ability to spatially and temporally control the fluorochromicity of 1 is demonstrated by a series of multicolour fluorescence patterning of “angelfishes”.