Creation of helicene-based circularly polarized luminescence switching systems

Helicenes, composed of ortho-fused aromatic rings, have been recognized as excellent chiral dyes capable of showing circularly polarized luminescence (CPL) derived from inherent chirality. Although general carbohelicenes show CPL, weak fluorescence is often emitted in blue region. In contrast, incorporation of heteroatoms into the π-framework or periphery of helicenes modulates the electronic state, leading to intense fluorescence in the visible region. Furthermore, heteroatoms can respond to external stimuli such as acid/base, metal ions, and redox agents. Therefore, heteroatom-functionalized helicenes are good candidates for chiroptical switching systems with the changes in wavelength, intensity, on/off, and sign of CPL. Although examples of CPL switching systems were limited as compared to those of circular dichroism (CD) switching systems, they have been increasing gradually in the last decade. This review summarizes recent developments of helicene-based CPL switching systems, which are classified into pH-, coordination-, and redox-triggered switching systems.