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 (A) over circle x 5.855 (A) over circle (alpha pores) and 7.218 (A) over circle X 7.218 (A) over circle (beta pores). Powder X-ray diffraction and differential scanning calorimetry studies further reveal that the alpha pores, which contain nitrophenyl rings, quench the emission of HOFTPE4N. This emission can be turned on by breaking the alpha 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 alpha pores. These results clearly reveal AIE by controlling the intramolecular rotations, which can serve as a basis for developing mechanoluminescent materials.