Design of Thermochromic Luminescent Dyes Based on the Bis(ortho-carborane)-Substituted Benzobithiophene Structure

To obtain solid-state emissive materials having stimuli-responsive luminescent chromic properties without phase transition, benzobithiophenes modified with two o-carborane units having various substituents in the adjacent phenyl ring in o-carborane were designed and synthesized. Their emission colors were strongly affected not only by the substituents at the para-position of the phenyl ring but also by molecular distribution in the solid state. In particular, the emission colors were changed by heating without crystal phase transition. It was proposed that their thermochromic properties were correlated not with isomerization but with the molecular motion at the distorted benzobithiophene moiety.