Dual-State Mechano- and Electrochromic Responses Enabled by Sterically Strained Diazaanthraquinodimethanes

Stimulus-responsive materials with reversible optical properties are of significant interest for the development of functional systems. Herein, we present the design and synthesis of diazaanthraquinodimethane (N₂AQD) derivatives incorporating ortho-substituted 4-methoxyphenyl groups. A phenanthrene-fused scaffold was employed to balance conformational flexibility and redox stability, while an ortho substitution strategy was used to control the molecular conformation and near-infrared (NIR) absorption. Whereas neutral donors and their corresponding dications exhibit folded and twisted conformations, respectively, greater steric strain upon introduction of ortho substituents induces structural deformation that can lead to effective conjugation and enhanced switching ability. Thus, the o-dimethyl-substituted derivative showed a red-shifted absorption in both neutral and dicationic states. Mechanochromic behavior was observed in a solid state, involving both ground-state (color) and excited-state (emission) changes, which is due to a crystal-to-amorphous phase transition. Additionally, reversible electrochromism was demonstrated in solution. These results highlight N₂AQDs as promising platforms for multifunctional chromic materials that respond to mechanical and electrical stimuli.