Macrocyclic covalent encapsulation of diketopyrrolopyrroles: dual-state emission and mechanochromism

Abstract

The inherent tendency of conjugated π-systems towards π–π stacking aggregation in the solid state often compromises their intrinsic properties and limits their photophysical applications. To address this, we synthesized a series of macrocyclic diketopyrrolopyrrole (DPP) derivatives. These novel structures feature a DPP core covalently linked to double-sided straps at the lactam positions, forming an internally DPP-bridged macrocycle. Single-crystal X-ray diffraction analysis confirmed the encapsulated structures, revealing twisted conformations of the external macrocycles. These encapsulated systems displayed dual-state emission with remarkably high fluorescence quantum yields exceeding 94% in solution and over 12% in thin films, indicating effective suppression of intermolecular aggregation. Importantly, the external macrocycles are crucial in modulating stimuli-responsive luminescence. Specifically, Cycle2-DPP and Cycle2-DPP-OH, containing larger macrocycles, displayed pronounced mechanochromism resulting from crystalline-to-amorphous transitions, a behavior not observed in the more compact Cycle1-DPP and Cycle1-DPP-OH analogs. Furthermore, the mechanophore Cycle2-DPP-OH was integrated into linear polyurethane elastomers. The resulting polymers exhibited concentration-dependent aggregation and force-induced deaggregation, characterized by a hypsochromic shift in fluorescence. Therefore, macrocyclic covalent encapsulation represents a powerful strategy for designing stimuli-responsive luminescent materials.