Syntheses and structures of two novel fluorescent metal-organic frameworks generated from a tridentate donor-acceptor motif ligand.

The tridentate organic ligand 4,4',4''-(4,4,8,8,12,12-hexamethyl-8,12-dihydro-4H-benzo[9,1]quinolizino[3,4,5,6,7-defg]acridine-2,6,10-triyl)tribenzoic acid (HL) has been synthesized (as the methanol 1.25-solvate, CHNO·1.25CHOH). As a donor-acceptor motif molecule, HL possess strong intramolecular charge transfer (ICT) fluorescence. Through hydrogen bonds, HL molecules construct a two-dimensional (2D) network, which pack together into three-dimensional (3D) networks with an ABC stacking pattern in the crystalline state. Based on HL and M(NO) salts (M = Cd and Zn) under solvothermal conditions, two metal-organic frameworks (MOFs), namely, catena-poly[[triaquacadmium(II)]-μ-10-(4-carboxyphenyl)-4,4'-(4,4,8,8,12,12-hexamethyl-8,12-dihydro-4H-benzo[9,1]quinolizino[3,4,5,6,7-defg]acridine-2,6-diyl)dibenzoato], [Cd(CHNO)(HO)], I, and poly[[μ-4,4',4''-(4,4,8,8,12,12-hexamethyl-8,12-dihydro-4H-benzo[9,1]quinolizino[3,4,5,6,7-defg]acridine-2,6,10-triyl)tribenzoato](μ-hydroxido)zinc(II)], [Zn(CHNO)(OH)], II, were synthesized. Single-crystal analysis revealed that both MOFs adopt a 3D structure. In I, partly deprotonated HL behaves as a bidentate ligand to link a Cd ion to form a one-dimensional chain. In the solid state of I, the existence of weak interactions, such as O-H...O hydrogen bonds and π-π interactions, plays an essential role in aligning 2D nets and 3D networks with AB packing patterns for I. The deprotonated ligand L in II is utilized as a tridentate building block to bind Zn ions to construct 3D networks, where unusual ZnO clusters act as connection nodes. As a donor-acceptor molecule, HL exhibits fluorescence with a photoluminescence quantum yield (PLQY) of 70% in the solid state. In comparison, the PL of both MOFs is red-shifted with even higher PLQYs of 79 and 85% for I and II, respectively.