Bis(triarylmethylium)-type Macrocyclic Dications: Mechanochromic Emission Extending to the Red Region

Macrocyclic dications 2²⁺ composed of two triarylmethylium units were designed and synthesized. In contrast to the reference monocations 1⁺ , macrocyclic dications 2²⁺ exhibited mechanochromic emission extending to the red region (-900 nm), since the luminescence color in a solid state can reversibly change due to their constrained structures granted by alkylene linkers and the choice of a proper counterion. X-ray diffraction and spectroscopic analyses revealed that such mechanochromic behavior was induced by the crystal-to-amorphous transition. A change in the intermolecular interaction of macrocyclic dications 2²⁺ would be the key to realizing a change in the emission pattern, since the color of the molecules did not change by applying mechanical stimuli. These findings may suggest a design strategy for creating a variety of stimuli-responsive materials, especially for carbocation-based fluorescent materials.