Fluorescence mutation and structural evolution of a π-conjugated molecular crystal during phase transition

Abstract

Two thermodynamically stable crystalline phases (B- and G-phases) were found for a twistable π-conjugated molecule, CN-DSB, condensed from its solution. We investigated the structural evolution at the molecular and supramolecular levels as the crystalline phase transforms from the B-phase to G-phase under varied temperature or pressure. The intermolecular interactions were undermined before phase transition as the B-phase crystal was stimulated with an external energy. Heating the B-phase crystal up to 175 °C or applying stress up to its critical pressure (0.75 GPa) initially resulted in mixture phases or disordered state. At this stage, the molecules slightly adjust from a twisting configuration to a planar configuration, corresponding to the gradual red shift of the fluorescence spectra. Above the phase transition point, the initial intermolecular interaction of the B-phase is broken down, and the CN-DSB molecules re-assemble to the new phase—a new thermodynamic equilibrium state—corresponding to the sudden change of the emission color. Furthermore, the property of thermal-induced phase transition can be used to fabricate patterns on the CN-DSB crystal surface, and a uniform raster has been prepared by femtosecond laser direct writing (FsLDW) on the B-phase. The investigations provide new insight and understanding for the crystal phase transition and may contribute to process innovation in optical devices.