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 (H 3 BTC), a very common polyfunctional centric ligand, the Li + /Zn 2+ , Li + /Co 2+ , or Li + /Cd 2+ heterometallic combinations successfully led to five new noncentrosymmetric metal−organic frameworks, namely, {[Zn(HBTC)(), and {Li 2 [Zn 3 Li 5 (BTC) 4 (MTAZ)(H 2 O) 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 C 3 -symmery of BTC ligands. Clearly, our heterometalation approach greatly increases the possibility for induction of spontaneous resolution, which provides a fresh opportunity for the development of noncentrosymmetric functional MOF materials.

KC9H5O6(H2O): A Promising UV Nonlinear-Optical Material with Large Birefringence Based on a π-Conjugated (C9H5O6)−Group

“…Because the previously reported acentric compound {[Zn-(HBTC)(H 2 O)]} n possesses fundamental building units similar to those of KH 2 BTC, it is necessary to compare their structures. 34 {[Zn(HBTC)(H 2 O)]} n crystallizes in the chiral space group P6 1 . Unlike KH 2 BTC, the ZnO 4 tetrahedra in {[Zn(HBTC)(H 2 O)]} n are not directly connected to one another.…”

“…The BTC group owns a strong π-conjugation effect similar to that of (B 3 O 6 ) 3– and (C 3 N 3 O 3 ) 3– ; moreover, carboxyl groups outside the benzene ring are prone to coordination with metals to form new compounds with interesting structures. A metal–organic complex formed from trimesic acid as the ligand has wide applications in photocatalysis, electrocatalysis, CO oxidation, sensing of paraquat, etc. − However, there are rare applications in NLO or birefringent materials. − Such organic molecules usually have large dipole moment (μ) values, which cause them to align in the energy-favorable antiparallel pairing mode, generating a center symmetry . To break the center-symmetry trap, the “salt strategy” presented by Marder et al suggests that Coulombic interactions in salts can overcome such adverse dipole–dipole interaction, thereby promoting the formation of noncentrosymmetric (NCS) structures …”

Chiral metal–organic frameworks for photonics

“…This ‘heterometalation’ strategy was employed by Li et al , who synthesised several chiral MOFs from 1,3,5-benzenetricarboxylate and mixtures of Li + /Zn 2+ , Li + /Co 2+ , and Li + /Cd 2+ . 135 All were SHG-active and phase-matchable except the cobalt-containing framework, which likely suffered from total self-absorption of the second harmonic. The SHG efficiencies were moderate, ranging from 0.4–1.4 × KDP.…”

Cadmium‐Based Coordination Polymers from 1D to 3D: Synthesis, Structures, and Photoluminescent and Electrochemiluminescent Properties

“…In recent years, the rational design and construction of multifunctional coordination polymers (CPs) have attracted much attention, attributed to their intriguing structures and properties as well as great novel applications in gas sorption, heterogeneous catalysis, chemical sensing, nonlinear optics (NLO), and fluorescence, adsorbent, etc. Thus far, plenty of CPs assembled from inorganic vertices interconnected with organic ligands, have been synthesized by diversified approaches.…”