Red to Near-Infrared Mechanochromism from Metal-free Polycrystals: Noncovalent Conformational Locks Facilitating Wide-Range Redshift.

Piezochromic organic materials that present a large difference in fluorescence wavelength in the near-infrared region have important potential applications; however, few such metal-free luminophores have been reported. In this study, we design and prepare π-conjugated electron acceptors whose planar conformation can be locked by the hydrogen-bonded interactions. The formation of a planar fused-ring geometry via noncovalent conformational locking can narrow the optical band gap, enhance the molecular stability and rigidity, as well as increase the radiative rate. As expected, the polymorphs Re-phase and Ni-phase emit the high-brightness fluorescence with wavelength maxima (λem,max) at 615 and 727 nm, respectively. Upon full grinding, the λem,max of Re-phase is bathochromically shifted to 775 nm. Detailed spectrophotometric and single-crystal X-ray analyses demonstrate that the ground powder of Re-phase becomes metastable as a consequence of noncovalent conformational locking and that the red to near-infrared (large colour difference) mechanochromism arises from the high degree of conformational coplanarity. This strategy is both conceptually and synthetically simple and offers a promising approach to the development of organic piezochromic materials with wide-range redshift and excellent penetrability.