A Benzothiophene‐Extended Rhodamine Mechanophore Enabling Mechanochromic Near‐Infrared Polymers

Comprehensive Summary Mechanochromic mechanophores that operate in the near‐infrared (NIR) region are highly desirable for applications requiring deep tissue penetration, high signal‐to‐noise ratio, and low background interference. However, most rhodamine‐based mechanophores exhibit optical responses confined to the visible region. Herein, we report the design and synthesis of a benzothiophene‐conjugation‐extended NIR rhodamine mechanophore, termed 2XR‐Rh . Theoretical calculations (constrained geometry simulates external force) and small‐molecule model studies confirm that 2XR‐Rh undergoes selective C–N bond cleavage under mechanical force, with the ring‐opened form displaying a significantly red‐shifted absorption maximum at 727 nm and emission at 777 nm, compared to conventional rhodamine mechanophores. When incorporated into linear polymethacrylates via single‐electron transfer living radical polymerization (SET‐LRP), 2XR‐Rh exhibits force‐dependent, efficient ultrasound‐induced ring‐opening. More importantly, in double‐network elastomers containing polar monomers (2‐hydroxyethyl acrylate/2‐methoxyethyl acrylate), the mechanophore shows pronounced mechanochromic and mechano‐fluorogenic responses under both tensile and compressive loading, enabling visual and fluorescence‐based stress sensing in the solid state. This work demonstrates that benzothiophene‐based π‐extension is an effective strategy to push rhodamine mechanophores into the NIR region while retaining excellent force‐responsiveness and thermal reversibility, offering a promising platform for advanced stress‐sensing materials.