Controlling circularly polarized luminescence of a pyrene modified chiral Zn(II) complex based on a temperature-dependent diastereomer equilibrium and solid-state excimer formation

A pyrene modified chiral Schiff-base Zn(II) complex was synthesized and its circularly polarized luminescence (CPL) properties were examined. Single crystal X-ray diffraction analysis of the (S,S)-isomer revealed its distorted tetrahedral coordination geometry with Λ coordination chirality. An equilibrium of the two diastereomers with different coordination chiralities, Λ-(S,S) and Δ-(S,S), was observed in solution, and the ratio of the isomers was found to change with temperature. The complex showed negligible CPL in CHCl₃ at room temperature, whereas a clear mirror-image signal was obtained when the solution was cooled to 253 K. Interestingly, excimer-derived long wavelength emission was also observed in the solid state, resulting in CPL sign inversion compared to the solution state. The relationship between their photophysical properties and coordination geometries were theoretically investigated by density functional theory (DFT) calculations.