Circularly polarized blue fluorescence based on chiral heteroleptic six-coordinate bis-pyrazolonate-Zn2+ complexes

Abstract

Applying molecular design to chiral organo-Zn²⁺ complexes, a new pair of chiral heteroleptic bis-pyrazolonate-Zn²⁺ enantiomers [Zn(PMBP)₂(1R,2R-Chxn)] (R,R-Zn²⁺; HPMBP = 1-phenyl-3-methyl-4-benzoyl-5-pyrazolone and 1R,2R-Chxn = (1R,2R)-cyclohexane-1,2-diamine) and [Zn(PMBP)₂(1S,2S-Chxn)] (S,S-Zn²⁺; 1S,2S-Chxn = (1S,2S)-cyclohexane-1,2-diamine) have been synthesized and characterized in terms of photophysical and thermodynamic properties. In addition to a small Flack parameter (0.05(3)) associated with the solid-state elucidation of S,S-Zn²⁺, the circular dichroism (CD) and circularly polarized light (CPL) spectra for the chiral Zn²⁺ enantiomers show perfect mirror symmetry, establishing that the enantiopure 1,2-diamines successfully induce the optical isomerism of R,R-Zn²⁺ and S,S-Zn²⁺. As a result of the combined strong chiral induction capability of chiral 1,2-diamines and excellent photophysical properties of the pyrazolone ligand (PMBP)⁻, the two Zn²⁺ enantiomers exhibit high-quality pure blue fluorescence (Φ_PL = 9–10%) and significant CPL activity (|g_lum| = 0.0065–0.0068). The heteroleptic strategy adopted in this study offers a new route to develop high-performance chiroptical luminophores.