Design, synthesis, photophysical properties and intrinsic mechanism of two difluoroboron β-diketonate complexes with TPE and N-alkyl pyrrole units

It is a hot topic to furnish aggregation-caused quenching luminogens with aggregation-induced emission (AIE) active units and explore their optical properties and inherent mechanism. In this paper, two newly designed and synthesized difluoroboron β-diketonate complexes (TPE-BF2-2AP and TPE-BF2-2MP) with tetra (phenyl) ethylene (TPE) and N-alkyl pyrrole unit exhibit solvatochromism, AIE and mechanochromic properties. Although the alkyl substituents with slight difference, TPE-BF2-2AP relative to TPE-BF2-2MP show distinct increased stokes shift and red shifted fluorescence emission in organic solvents. More interestingly, single crystal analysis reveals that TPE-BF2-2MP forms head-to-head anti-parallel stacking, while TPE-BF2-2AP adopts a scarce head-to-head cross non-parallel stacking, leading to enhanced quantum efficiency from 0.24 to 0.34 before/after grinding, and blue shifted fluorescence emission compared with TPE-BF2-2MP in aggregated states and crystalline states. In addition, the two luminogens exhibit multi-state emission properties by grinding, solvent fumigation and heating due to incomplete transformation between amorphous and crystalline state.