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)-H2cea], was synthesized by modifying an aromatic carboxylic acid with lactic acid. This ligand was used to react with Zn2+ and Cd2+ ions under hydrothermal conditions in the presence of N-containing heterocyclic auxiliary ligands [1,4-bis(2-methyl-1H-imidazol-1-yl)benzene (1,4-bmib) and 1,1'-(2,5-dimethyl-1,4-phenylene)bis(1H-imidazole) (2,5-dpb)], successfully constructing two chiral coordination polymers (CPs), namely, poly[[μ-1,4-bis(2-methyl-1H-imidazol-1-yl)benzene-κ2N:N'][μ-(R)-3-(1-carboxylatoethoxy)benzoato-κ2O:O']zinc(II)], [Zn(C10H8O5)(C14H14N4)]n or HCU40-R, and poly[[μ-1,4-bis(1H-imidazol-1-yl)-2,5-dimethylbenzene-κ2N:N'][μ-(R)-3-(1-carboxylatoethoxy)benzoato-κ4O,O':O'',O''']cadmium(II)], [Cd(C10H8O5)(C14H14N4)]n or HCU41-R. Both CPs crystallize in the chiral space group P21, and the central chirality from the (R)-cea2- anion is transferred and amplified into supramolecular helical chirality. In HCU40-R, the Zn2+ 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 Cd2+ 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.