Chain length-dependent piezofluorochromic behavior of 9,10-bis(p-alkoxystyryl)anthracenes

In the present work, 9,10-bis(p-alkoxystyryl)anthracenes (DSA-OCn) with various length of alkyl chains (n=1-10, 12, 14, 16, 18) are synthesized and the chain length-dependent piezofluorochromic (PFC) behaviors are systematically investigated. The results show that all the long alkoxy-containing DSA-OCn (n≥10) exhibit remarkable grinding-induced spectral shifts (ΔλPFC =46-53 nm) that are significantly larger than those of shorter alkyl chains (n≤9, ΔλPFC =14-20 nm). Powder wide angle X-ray diffraction indicates that ground solids are still crystalline but the packing modes have been affected. Differential scanning calorimetric curves indicate that almost all of the DSA-OCn exhibit two exothermic peaks and the lower-temperature peaks are affected by grinding. These findings imply that the existence of a mesophase transition peak and its change upon grinding are not related to the different PFC behaviors. IR spectra show that out-of-plane hydrogen bending modes of the phenyl rings have disappeared in the ground long alkoxy-containing DSA-OCn. This observation implicates a less twisted backbone conformation in the ground than unground states, which is responsible for the dichotomous PFC behavior.