From central chirality to helical chirality: two chiral coordination polymers based on a lactic acid derivative and their SHG responses

The chiral ligand ( R )-3-(1-carboxyethoxy)benzoic acid [( R )-H 2 cea], was synthesized by modifying an aromatic carboxylic acid with lactic acid. This ligand was used to react with Zn 2+ and Cd 2+ ions under hydrothermal conditions in the presence of N-containing heterocyclic auxiliary ligands [1,4-bis(2-methyl-1 H -imidazol-1-yl)benzene (1,4-bmib) and 1,1′-(2,5-dimethyl-1,4-phenylene)bis(1 H -imidazole) (2,5-dpb)], successfully constructing two chiral coordination polymers (CPs), namely, poly[[μ-1,4-bis(2-methyl-1 H -imidazol-1-yl)benzene-κ 2 N : N ′][μ-( R )-3-(1-carboxylatoethoxy)benzoato-κ 2 O : O ′]zinc(II)], [Zn(C 10 H 8 O 5 )(C 14 H 14 N 4 )] n or HCU40- R , and poly[[μ-1,4-bis(1 H -imidazol-1-yl)-2,5-dimethylbenzene-κ 2 N : N ′][μ-( R )-3-(1-carboxylatoethoxy)benzoato-κ 4 O , O ′: O ′′, O ′′′]cadmium(II)], [Cd(C 10 H 8 O 5 )(C 14 H 14 N 4 )] n or HCU41- R . Both CPs crystallize in the chiral space group P 2 1 , and the central chirality from the ( R )-cea 2− anion is transferred and amplified into supramolecular helical chirality. In HCU40- R , the Zn 2+ ions adopt a tetrahedral geometry, forming a one-dimensional right-handed helical chain that further extends into a two-dimensional helical layer. In HCU41- R , the Cd 2+ ions exhibit an octahedral geometry, displaying a double-helical chain and a twofold interpenetrated two-dimensional layered structure. Powder X-ray diffraction and elemental analysis confirmed the phase purity of the samples. Thermogravimetric analysis revealed thermal stability up to 350 °C for HCU40- R and 300 °C for HCU41- R . Fluorescence measurements showed emission peaks at 451 nm for HCU40- R and 491 nm for HCU41- R . Furthermore, both compounds exhibited second-order nonlinear optical (NLO) effects, with second harmonic generation (SHG) intensities of 0.38 and 0.35 times that of KDP, respectively.