Expanding the Scope of Mechanochromic Luminescence Through Mechanical Atropisomerization in Biaxial Atropisomer Molecular Solids

Mechanoresponsive luminescent (MCL) materials hold promise for applications such as force sensing and lithography. This paper reports solid-state mechanical atropisomerization of biaxial atropisomers as a mechanism that enables mechanically activated photofluorochromic responses in molecular solids. A library of biaxial atropisomer small molecules are designed to undergo selective atropisomerization under mechanical stimulation and convert to their chemically inequivalent diastereomers. Mechanical activation converts a photoinactive solid of the parallel diarylethene atropisomer into an amorphous containing the photoswitchable antiparallel isomer, which subsequently undergo photocyclization under UV irradiation to exhibit photofluorochromism. Because this mechanical atropisomerization generates a distinct and thermally stable chemical species rather than a transient emissive state arising from lattice rearrangement or reversible bond scission, the resulting photofluorochromic response persists even after loss of the mechanically encoded supramolecular organization and is not erased by thermal reversion, distinguishing these mechanoactive biaxial atropisomers from conventional MCL solids. This persistence also enables a simple materials design strategy in which physical doping of small-molecule diarylethene atropisomers renders polymers mechanoresponsive, bypassing both covalent polymer modification required in conventional polymer mechanochemistry and the molecular-packing dependence of typical MCL solids. Importantly, solid-state mechanical atropisomerization was validated as a general mechanism within the tested subgroup of structurally diverse biaxial atropisomers and could guide the development of next-generation mechanoresponsive luminescent materials.