Fluorescence Color Changes of Molecular Aggregation Processes of Difluoroboron β‐Diketonate Complexes With Heterocyclic Ring During Solvent Evaporation

Organic molecular crystals exhibit diverse packing motifs and polymorphic forms governed by weak intermolecular interactions. We have previously reported transient intermediates appearing during solvent‐evaporative crystallization by visualizing fluorescence spectral changes. In the present study, we investigate the crystallization process of heterocyclic β‐diketonate–boron difluoride (hetero–BF 2 ) complexes incorporating thiophene and furan rings by monitoring the evolution of fluorescence during solvent evaporation. Spectral analysis reveals that crystallization proceeds via dense liquid‐like cluster (LLC) intermediates. Unlike phenyl‐substituted BF 2 complexes that display a transient redshifted fluorescence band, the heteroaromatic derivatives exhibit broadened emission. Structural analysis and DFT calculations indicate that larger dipole moments in the heteroaromatic systems promote competitive formation of H‐ and J‐type aggregation motifs within the LLC states. This competitive formation broadened the emission profile and suppressed the formation of only a redshifted band. These findings demonstrate that heteroaromatic substitution in the present BF 2 systems reshapes the intermolecular interaction landscape of prenucleation clusters, thereby modulating aggregation pathways and crystallization behavior.