Efficient and reversible acidofluorochromic features on a solid platform for reusable security writing: A structure-property relationship study on anthracenyl π-conjugates

Reversible solid-state acidochromic (SSAC) behavior offers facile and handy fluorescence (FL)-based pH detection and secured ink or rewritable platform for anti-counterfeiting applications. The acidochromism based detection primarily depends on the pyridine-based linkers in the solution state but remains ill-defined in the solid-state. This work describes the degree of emission responses, primarily in the solid-state if the N-atom of pyridyl ring alters the position in toluene/thiophene-connected structurally twisted anthracenyl π-conjugates. The role of N-atom at the 4th position is more sensitive against acid fumes, especially in the thiophene-linked analog that displays 108 nm redshift, compared to the toluene analog (85 nm). Changing N-atom's position from 4th to 3rd in the ring, only tolyl-linked analog is significantly responsive with 48 nm redshift. Single-crystal structures of protonated molecules have been judiciously determined and found more planar compared to the originals. The crystal structure and molecular packing analyses dictate that the rotors are significantly twisted with multiple noncovalent interactions, crucially responsible for the admirable solid-state emission. The gained planarity of the molecules upon protonation can elucidate the redshifted emission with quenching. Further, stronger H-bonding and excited state stabilization for a few pyridinium salts may be accountable for reducing the emission intensity and redshifted emission, respectively. High-contrast emission color variation on protonation-deprotonation has been employed as a handy polymer-blended emitting platform for convenient and reusable security writings. [Display omitted] •Comparative solid-state emission studies between (positional) isomeric toluene/thiophene-connected structurally twisted anthracenyl π-pyridyl conjugates.•Efficient solid-state acidofluorochromism with reversible emission switching.•Detailed analysis of crystal packing and noncovalent interactions.•Intense and reusable security writing on solid polymer matrix.