Metastable Molecular Cocktail of Crystalline and Liquid Difluoroboron β-Diketonate Complexes

The binary composites of liquid (<b>1</b>_<b>Liq</b>) and crystalline (<b>1</b>_<b>Cry</b>) difluoroboron β-diketonate (BF₂dbk) complexes exhibited a metastable nature arising from the intricate interplay between their liquid and crystalline components in bulk. Differential scanning calorimetry (DSC) measurements indicate nearly complete miscibility of <b>1</b>_<b>Cry</b> and <b>1</b>_<b>Liq</b> when the fractional volume of <b>1</b>_<b>Liq</b> occupied a substantial portion, corresponding to below 47 mol % of the <b>1</b>_<b>Cry</b> content. In contrast, polarized optical microscopic (POM) observations unveiled that the <b>1</b>_<b>Cry</b>/<b>1</b>_<b>Liq</b> composites between two glass slides crystallized regardless of the <b>1</b>_<b>Cry</b> content. This discrepancy indicated the occurrence of heterogeneous nucleation in the vicinity of the glass surface, highlighting the metastable nature of supercooled liquid and/or glassy phases of the <b>1</b>_<b>Cry</b>/<b>1</b>_<b>Liq</b> composites. Moreover, the <b>1</b>_<b>Cry</b>/<b>1</b>_<b>Liq</b> composites exhibited thermal hysteresis in the photophysical properties through thermally assisted excimer formation, which was observed on a scale distinct from DSC and POM observations.