The HOF structures of nitrotetraphenylethene derivatives provide new insights into the nature of AIE and a way to design mechanoluminescent materials

This study probes the effect of intramolecular rotations on aggregation-induced emission (AIE) and leads to a kind of supramolecular mechanoluminescent material. Two hydrogen-bonded organic frameworks (HOFs), namely HOFTPE3N and HOFTPE4N, have been constructed from nitro-substituted tetraphenylethene (TPE) building blocks, namely tris(4-nitrophenyl)phenylethene (TPE3N) and tetrakis(4-nitrophenyl)ethene (TPE4N). Using single-crystal X-ray diffraction analysis, two types of pores are observed in the HOFTPE4N supramolecular structure. The pore sizes are 5.855 Å × 5.855 Å (α pores) and 7.218 Å × 7.218 Å (β pores). Powder X-ray diffraction and differential scanning calorimetry studies further reveal that the α pores, which contain nitrophenyl rings, quench the emission of HOFTPE4N. This emission can be turned on by breaking the α pores in the HOFs by grinding the sample. Temperature-dependent emission studies demonstrate that the emission quenching of HOFTPE4N is attributed to the intramolecular rotations of nitro-substituted phenyl units within the space of the α pores. These results clearly reveal AIE by controlling the intramolecular rotations, which can serve as a basis for developing mechanoluminescent materials.