Cruciform 9,10-distyryl-2,6-bis(p-dialkylamino-styryl)anthracene homologues exhibiting alkyl length-tunable piezochromic luminescence and heat-recovery temperature of ground states
A series of 2,6-bis(p-dialkylaminostyryl)-9,10-distyrylanthracene (FCn) cruciforms with N-alkyl chains of different lengths have been synthesized, and their aggregation-enhanced fluorescence and piezochromic luminescence (PFC) behaviours are investigated. These 9,10-distyrylanthracene-containing cruciforms exhibit relatively low fluorescence quantum yields (Φ) in THF solution (Φ ≈ 10%) and moderate aggregation-enhanced emission in aqueous media (Φ ≈ 25%), but strong and chain length-dependent solid-state fluorescence emission. Grinding and pressing experiments indicate that they are all effective PFC materials in terms of mechanical stress-induced spectral shifts (ΔλPFC = 23–54 nm), moreover, the longer alkyl-containing FCn shows a larger ΔλPFC. Powder X-ray diffraction and differential scanning calorimetry measurements reveal that the transformation between the crystalline and amorphous states upon external stimuli is responsible for the reversible PFC behaviour. It is found that increasing the N-alkyl length could effectively decrease the cold-crystallization temperature of the ground states to render the PFC states with a tunable heat-recovering temperature, and ground FC10 and FC12 solids can recover spontaneously to their original states at room temperature.