Lanthanide-2,3,5,6-Tetrabromoterephthalic Acid Metal-Organic Frameworks: Evolution of Halogen•••Halogen Interactions across the Lanthanide Series and Their Potential as Selective Bifunctional Sensors for the Detection of Fe3+, Cu2+, and Nitroaromatics

Two series of lanthanide metal-organic frameworks (MOFs) formulated as {Ln(2)(TBrTA)(3)(H2O)(8)center dot 2H(2)O}(n) [Ln = La (1), Ce (2), Pr (3), Nd (4), Sm (5), Eu (6), Gd (7), Tb (8), Dy (9)] series 1, {[Ln(3)(TBrTA)(6)][Ln(H2O)(8)]center dot 6H(2)O}(n) [Ln = Ho (10), Er (11), Tm (12), Yb (13), Lu (14)] series 2, respectively, were synthesized via slow evaporation with tetrabromoterephthalic acid (H(2)TBrTA) and structurally characterized by X-ray crystallography, infrared spectroscopy, and thermal analysis. Lanthanide contraction effects result in a change in the structure from a two-dimensional (2D) MOF with sql topology for La-Dy (series 1) to a three-dimensional (3D) MOF with pcu network topology for Ho-Lu (series 2). Supramolecular interactions also evolve from Br center dot center dot center dot Br, Br center dot center dot center dot O, pi-pi, and hydrogen bonding interactions in series 1 to Br center dot center dot center dot Br and hydrogen bonding interactions in series 2. The nature of Br center dot center dot center dot Br interactions transition from a combination of type I, type II, and quasi-type I/type II in series 1 to only type II in series 2. Furthermore, as the size of the Ln(III) ion decreases, the strength of the type II Br center dot center dot center dot Br interactions increases. The anionic frameworks in series 2 are charge balanced by [Ln(H2O)(8)](3+) cations in the pores that are encapsulated by an unprecedented three-dimensional (3D) "Star of David" dodecamer water cluster. Photophysical studies demonstrate that TBrTA is an efficient sensitizer of Eu3+(D-5(0)) and Tb3+(D-5(4)) luminescence as excitation into ligand bands leads to bright red and green emission, respectively, the spectral profiles of which display only metal centered emission bands. Emission from compound 6 is significantly quenched in the presence of Fe3+ (87%) and Cu2+ (75%) ions mainly due to competitive absorption of excitation energy, which leads to an inner-filter effect. In addition, the emission of 6 is more than 85% quenched by nitroaromatic compounds (NACs) such as 4-nitrophenol, dinitrophenol, and trinitrophenol (picric acid) with competitive absorption, photoinduced electron transfer, and electrostatic processes the main mechanisms of quenching. This study is the first to show that X center dot center dot center dot X interactions in extended structures such as MOFs can be tuned with the lanthanide contraction and also reveals that Ln-TBrTA MOFs can be used as bifunctional sensors for the detection of Fe3+ and Cu2+ as well as explosive nitroaromatics.