BODIHY based novel AIEgens and directive role of σ–spacers on their reversible mechanochromism

Synthesis of the novel hydrazone based ligands (L1–L3) and their borondifluoride (BF2) complexes (BODIHYs; B1–B3) have been described. The ligands and BODIHYs have been thoroughly characterized by spectroscopic studies (1H NMR, 13C NMR, 11B NMR, 19F NMR, ESI–MS, UV–vis and fluorescence) and structures of L1, L2 and B1–B3 determined by X–ray single crystal analyses. Emission behaviour of these compounds in solution and solid state have been fine tuned by varying the σ–spacers (–O, –S and –SO2). Vital role of various interactions in aggregation induced emission (AIE; B1 and B2), aggregation caused quenching (ACQ; B3) and reversible mechanochromism (B1–B3) have been rationalized by X-ray single crystal analyses. Dual emission band (~484 and 550 nm) for B3 have been attributed to the involvement of the intra-/and intermolecular charge transfer processes. The observed hypsochromic shift in these luminophores (upon grinding) successfully reverted back upon exposure to dichloromethane (CH2Cl2). An analysis of the powder X–ray diffraction (PXRD) pattern, before and after grinding advocated alteration of prevailing crystal packing by the external force as it rescinds the weak intra-/and intermolecular interactions of the molecule.