Polymorphic crystals and their luminescence switching of triphenylacrylonitrile derivatives upon solvent vapour, mechanical, and thermal stimuli

Abstract

For piezo-, vapo-, and thermochromic materials, it remains a challenge to figure out the underlying reason for fluorescence color changes upon external stimulation and determine why only some fluorophores reveal emission switching. A novel triphenylacrylonitrile derivative (TPAN-MeO) with remarkably twisted conformations has been carefully prepared via the Suzuki coupling reaction. The fluorescence of TPAN-MeO in the aggregate state depends on the polymorphic forms: three crystalline forms BCrys, SCrys and YCrys exhibit bright blue, sky-blue and yellow emission, respectively; meanwhile the amorphous powders are also strongly fluorescent with green emission. The crystals BCrys and SCrys exhibit mechano- and piezochromism in that grinding and high pressure could alter the emission colour, respectively. In addition, the amorphous film exhibits vapo- and thermochromic behaviour in that organic vapour and heating could change the green colour into sky-blue. Interestingly, the solvent vapour and heating stimuli can trigger a crystal-to-crystal transformation between SCrys form and YCrys form.