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

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