Leveraging intermolecular contacts of polymorphs for tuning solid-state emission

Two crystalline polymorphs of a conjugated organic fluorophore were obtained through separate synthetic protocols despite crystallizing under identical conditions. The unique polymorphs were confirmed by single-crystal X-ray diffraction. The two polymorphs had markedly different supramolecular packing. Thus, polymorph A was stabilized by directional C–H···N/S interactions with limited π overlap, while polymorph B assembled into extended π–π stacks and a denser network with short contacts. These structural differences resulted in distinct photophysics. Indeed, the emission quantum yield (Φfl) of Polymorph B was 4-fold lower along with multiexponential excited state kinetics compared to Polymorph A in addition to a 38 nm blue-shift in the emission. The metastable polymorph B could be converted to the thermodynamically stable Polymorph A by grinding the pristine crystal. Both intra- and supramolecular contacts could be leveraged to guide the crystal packing for modulating the emissive properties of the intrinsic fluorophores.