Helicene–Fluorescein Hybrids: A Reversible Base‐triggered (Chir)optical Switch with Sign Inversion of Circularly Polarized Luminescence

Novel circularly-polarized-luminescence (CPL) materials were designed and synthesized by integrating the intrinsic chirality of helicene with the acid/base-responsive properties of fluorescein. The synthesized helicene-fluorescein hybrids exhibit reversible switching between blue fluorescence under neutral conditions and red fluorescence under basic conditions. Furthermore, these hybrid compounds demonstrate unique chiroptical switching behavior with a rare base-triggered CPL sign inversion, where (P)-isomers show a negative CPL signal around 450 nm under neutral conditions and a positive signal around 620 nm under basic conditions, with three-fold enhanced dissymmetry factors (|glum|) under basic conditions. The structural basis for this switching was elucidated through TD-DFT calculations, revealing distinct angles between magnetic and electric transition dipole moments in closed versus open forms. Live-cell imaging experiments using HeLa cells revealed that the hybrid compounds exhibit intracellular red emission with minimal cytotoxicity, thus promising potential as chiral fluorescent probes for biological applications.