[7]Helicene-Appended 𝑜-Carboranes: Sign-Changing Circularly Polarized Luminescence Based on Dual-Emissive Properties and Its Temperature Dependency

Circularly polarized luminescence (CPL) attracts tremendous attention owing to their applicability for the next-generation optoelectronics. Precise control over CPL sign as well as its stimuli-responsiveness is one of the next goals, especially for developing solid-state chiroptical materials. Herein, [7]helicene-appended 𝑜-carborane derivatives are synthesized and temperature-dependent CPL sign-switching behaviors are observed. Similar to the reported aryl-𝑜-carborane derivatives, the synthesized [7]helicene-appended derivatives show dual-emission properties between locally-excited (LE) and intramolecular charge transfer (ICT) emission, accompanied by excited-state deformation of 𝑜-carborane units. It should be noted that CPL sign inversion is observed upon emission species changing because of their different spatial orientation of the transition electronic and magnetic dipole moments. Owing to the spherical structure of the 𝑜-carborane scaffold, the dual-emission can be exhibited even in the condensed solid state, where structural relaxation is usually suppressed by the structural restriction caused by surrounding molecules. Finally, from temperature-dependent CPL measurements in the film state, stimuli-responsive CPL sign-switching properties are observed based on the change in LE/ICT dual-emission intensity ratio. The result is, to the best of the knowledge, the first example to offer stimuli-responsive materials that derive different optical properties by utilizing molecular motion in the excited state.