A pair of ionic 1D Cu(ii) chain enantiomers simultaneously displaying large second- and third-harmonic generation responses

Abstract

By employing enantiomerically pure mono-bidentate N-donors (L_R/L_S) as chiral bridging ligands to react with Cu(ClO₄)₂(H₂O)₆ in CH₃CN–DMF mixed solvent, respectively, a pair of ionic one-dimensional (1D) Cu(II) chain enantiomers formulated as {[CuL_R(CH₃CN)(DMF)H₂O](ClO₄)₂}_n/{[CuL_S(CH₃CN)(DMF)H₂O](ClO₄)₂}_n (D-1/L-1) were isolated and structurally characterized, where L_R/L_S = (−)/(+)-4,5-pinenepyridyl-2-pyrazine. They crystallize in the noncentrosymmetric (NCS) P2₁2₁2₁ space group of an orthorhombic system due to the introduction of chiral L_R/L_S, and the ClO₄⁻ groups as counteranions reside in crystal lattices, thus leading to charge separation with large dipole moments in their molecular structures. Based on crystal samples, investigation on their nonlinear optical (NLO) behaviors showed that D-1 and L-1 display simultaneously much larger second- and third-harmonic generation (SHG and THG) responses than their analogues based on the same chiral N-donors (L_R/L_S) and Cu(NO₃)₂(H₂O)₃ with NO₃⁻ acting as the coordination group to bind Cu(II) ions. The SHG intensities of D-1/L-1 are 0.62/0.60 × KDP (KH₂PO₄), and THG intensities of D-1/L-1 are 238/228 × α-SiO₂. Our finding indicates that coordination polymers (CPs) with charge separation and NCS structures, i.e., ionic CPs with NCS arrangements are the ideal NLO crystalline materials for the simultaneous observation of large SHG and THG responses, thus providing a new approach to obtain NLO-active CP crystalline materials with high-performance SHG and THG responses.