Off–off–on chiroptical property switching of a pyrene luminophore by stepwise helicate formation

Abstract

Pyrene-based chiral ligands (L^R and L^S) consisting of two chiral imidazole sidearms at the 1,8-positions first form pyrene-bridged oligomers [(L^(R or S))₃(Zn²⁺)_n], which are chiroptically inactive, with a low concentration of zinc ions (Zn²⁺). Upon addition of more Zn²⁺, chiroptically active coordination helicates ([(L^(R or S))₃(Zn²⁺)₂]) are formed. The stepwise assembly process successfully facilitated an “off–off–on” chiroptical switching process of a pyrene luminophore controlled by the concentration of Zn²⁺. The resulting pyrene-bridged helicates exhibit high performance in terms of a dissymmetry factor of circular dichroism (|g_CD| = 0.006) and circularly polarized luminescence (|g_CPL| = 0.01).