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 <i>ortho</i>-substituted 4-methoxyphenyl groups. A phenanthrene-fused scaffold was employed to balance conformational flexibility and redox stability, while an <i>ortho</i> 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 <i>ortho</i> substituents induces structural deformation that can lead to effective conjugation and enhanced switching ability. Thus, the <i>o</i>-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.