Aromatic Difluoroboron β-Ketoiminate Complexes: Effect of Substituents on Mechanofluorochromism and Polymorphic Behavior.

Aromatic difluoroboron β-ketoiminate complexes (ketimBF₂) are structural nitrogen-containing analogues of aromatic difluoroboron β-diketonates (diketBF₂). Aggregation-induced emission (AIE) and polymorphic behavior allow ketimBF₂ to exhibit mechanofluorochromic (MFC) properties. A detailed comparative study of the luminescence of a wide range of ketimBF₂ with H- and CH₃-substituents of nitrogen atom and diketBF₂ with various substituents of the chelate ring (methyl, phenyl, toluoyl, anisoyl, biphenyl, naphthyl, anthracyl) in the solid state was carried out. As a result, regularities of the influence of substituents on the luminescent and MFC properties for 21 dyes have been established. Replacing one oxygen atom in diketBF₂ with nitrogen atom in the chelate ring (H-substituted ketimBF₂) changes the nature of the molecular stacking in crystals, which is manifested in different MFC properties. The introduction of a methyl group into the chelate ring (CH₃-substituted ketimBF₂) induces steric hindrance, which prevents the efficient formation of supramolecular structures and, consequently, leads to the shortest-wavelength monomeric emission in the solid state in comparison with oxygen and H-substituted nitrogen analogues. Methoxy derivative of H-substituted ketimBF₂ exhibits non-reversible fluorochromic switching after annealing and can be used as temperature indicator to control unauthorized heating of temperature-sensitive substances during transportation and storage.