A high contrast mechanochromic luminescent diacetylene-linked bis-benzothiadiazole derivative

Abstract

Two new rod-shaped molecules based on a directly-linked or a diacetylene-linked bis-benzothiadiazole core flanked by two p-nonylphenyl groups have been synthesized and their potential stimuli-responsive behaviour was investigated. The high molecular anisotropy and the presence of flexible chains induce these molecules to organize into lamellar self-assemblies, which can be influenced by the action of external stimuli. The two compounds present a polymorphic transformation upon heating which can be reversed upon shearing, but it is only in the diacetylene linked derivative that this transformation is accompanied by a change of its emission colour. In fact, in this material, the phase interconversion involves a high contrast luminescence alteration which can be rationalized considering the high conformational flexibility of the bridging unit which offers a great opportunity to change the surrounding of the molecules in the solid upon minor stimuli. Conversely, in the directly connected bis-benzothiadiazole, the molecules are closely packed in the layers, through cooperative chalcogen and aromatic interactions, probably preventing the displacement of the molecules within the layers and therefore changes in the environment of their aromatic parts responsible for the colour and emissive properties of the solids. The results of this study point to the flexible diacetylene moiety as an attractive design element in the construction of stimuli responsible light emitting materials. We hope that this observation will facilitate the design of new mechanochromic materials in the near future.