Peripheral Molecular Architecture Governs Reversible Solid-State Mechanochromism in AIE Luminogens

Luminescent materials exhibiting solid-state mechanochromism are attractive for coatings that visualize surface damage or record optical information. Because the emission of these materials is governed by the self-assembled structure of their luminogenic cores, the mechanochromic response is dictated by how readily this structure reorganizes under mechanical stress. To examine the role of molecular architecture in this process, we design two luminogens (CM1 and CM2) that share the same bis-cyanostilbene aggregation-induced emission (AIE) core but carry different self-assembling dendrons. The flexible alkyl taper dendron of CM1 has a low rearrangement barrier, so mechanical grinding drives a transition from a crystal to a metastable cubic phase of spherical supramolecular dendrimers, producing a large emission red-shift. In contrast, the rigid mesogenic dendron of CM2 resists the rearrangement, so grinding only breaks up its lamellar crystal and produces a minor spectral shift. Structural and optical analyses reveal that the magnitude of the mechanochromic response scales directly with the extent of the grinding-induced reorganization, and the behavior is reversibly switched by mechanical stimuli and thermal annealing. These results indicate that the mechanochromic contrast depends on the accessibility of a mechanically induced metastable state rather than on the emissive chromophore itself. The peripheral architecture controls this accessibility, offering a molecular-design handle for tuning the mechanochromic response of AIE luminogens without altering the emissive core.