Mechanochromic and thermochromic fluorescent properties of cyanostilbene derivatives

• TPA-CNa presents higher fluorescence quantum efficiency and more extended planarity than TPA-CNb.
• TPA-CNa exhibits multi-stimuli-responsive fluorescence switching behaviours.
• The weak intermolecular interaction may induce mechanochromic fluorescence.

Two isomers (TPA-CNa and TPA-CNb) consisting of twisted triphenylamine and cyanostilbene were synthetized by Knoevenagel condensation and C–N coupling reaction. The fluorescent colours of two isomers in crystalline state were sky-blue for TPA-CNa and green for TPA-CNb with quantum yields (Φf) of 44.9% and 7.7%, respectively. The single crystal X-ray diffractometry revealed the fully different molecular conformations and packing modes of TPA-CNa (planar) and TPA-CNb (twisting), which might be the reason for different Φf. The reversible mechanochromic and thermochromic fluorescence switching was observed in crystalline TPA-CNa powders. The sky-blue crystals were changed into the green-emissive solids after grinding, and they recovered their original state upon heating at 60 °C over 2 min. The reversible conversion of fluorescence colour was also realized by a pure thermal process. Powder X-ray diffractometry clearly demonstrated that the mechanochromic and thermochromic behaviour could be attributed to the crystal-to-amorphisation phase transition.

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