Solid-state fluorescence properties and reversible piezochromic luminescence of aggregation-induced emission-active 9,10-bis[(9,9-dialkylfluorene-2-yl)vinyl]anthracenes

In this work, we have synthesized 9,10-bis[(9,9-dialkylfluorene-2-yl)vinyl]anthracene derivatives (FLA-Cn) with propyl, pentyl, and dodecyl side chains to investigate their fluorescence properties. The results show that FLA-Cn exhibit not only an aggregation-induced emission effect but also reversible piezofluorochromic (PFC) behaviour. Interestingly, the fluorescence emission and grinding-induced spectral shifts (Delta lambda(PFC)) of FLA-Cn solids are alkyl length-dependent: the longer alkyl-containing FLA-Cn solids show more blue-shifted emission and larger Delta lambda(PFC). Moreover, the fluorescence emission of ground FLA-C12 solid can recover spontaneously at room temperature, Powder wide-angle X-ray diffraction and differential scanning calorimetry experiments reveal that the transformation between crystalline and amorphous states under various external stimuli is responsible for the PFC behaviour, and the spontaneous recovering emission of amorphized FLA-C12 solid is ascribed to its low cold-crystallization temperature. This work demonstrates once again the accessibility of tuning the solid-state optical properties of organic fluorophores by combining the simple alternation of molecular chemical structure and the physical change of aggregate morphology under external stimuli.