The Power of Heterometalation through Lithium for Helix Chain-Based Noncentrosymmetric Metal–Organic Frameworks with Tunable Second-Harmonic Generation Effects

Abstract

We demonstrate herein a general strategy for developing acentric or chiral nonlinear optical MOFs from centrosymmetric ligands by shifting the asymmetrical roles to the metal centers through heterometalation. With 1,3,5-benzenetricarboxylic acid (H3BTC), a very common polyfunctional centric ligand, the Li+/Zn2+, Li+/Co2+, or Li+/Cd2+ heterometallic combinations successfully led to five new noncentrosymmetric metal–organic frameworks, namely, {[Zn(HBTC)(H2O)]}n (SNNU-71), {Li4[Cd2(HBTC)(BTC)2]}n (SNNU-72), {CoLi(BTC)(DMA)2}n (SNNU-73), {CdLi2(BTC)4/3(H2O)3}n (SNNU-74), and {Li2[Zn3Li5(BTC)4(MTAZ)(H2O)4]}n (MTAZ = 5-methyl tetrazole, SNNU-75). All these MOFs are acentric or chiral on the base of helical chain building blocks and four colorless members of them show remarkable and tunable SHG effects. Remarkably, more than 1000 metal-BTC frameworks are reported to date and most of them are centrosymmetric due to the C3-symmery of BTC ligands. Clearly, our heterometalation approach greatly increases the possib...