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

The binary composites of liquid (1_Liq) and crystalline (1_Cry) 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 1_Cry and 1_Liq when the fractional volume of 1_Liq occupied a substantial portion, corresponding to below 47 mol % of the 1_Cry content. In contrast, polarized optical microscopic (POM) observations unveiled that the 1_Cry/1_Liq composites between two glass slides crystallized regardless of the 1_Cry 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 1_Cry/1_Liq composites. Moreover, the 1_Cry/1_Liq 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.