Reversible piezochromic luminescence of coumarin hydrozone derivatives and the influence of substituents

Four benzoyl hydrazone coumarin derivatives with different substituents at the phenyl moiety, namely, BHC-OCH3, BHC-H, BHC-Br, and BHC-CF3, respectively, were synthesized and characterized via photoluminescence, wide-angle X-ray diffraction, differential scanning calorimetry, and other techniques. The results indicate that the compounds exhibit piezofluorochromic properties which are reversible upon fuming or heating. The fluorescence emission peaks show red shift upon grinding and the degrees of the red-shift are dependent on the electronic effect and steric size of the substituents. Based on the results, the roles of the intermolecular hydrogen bonds and the molecular charge transfer process were discussed in detail. This study will help researchers understand the piezofluorochromic mechanism and design new piezofluorochromic materials.