Two
series of lanthanide metal–organic frameworks (MOFs)
formulated as {Ln2(TBrTA)3(H2O)8·2H2O}
n
[Ln = La (1), Ce (2), Pr (3), Nd (4), Sm (5), Eu (6), Gd (7), Tb (8), Dy (9)]
series 1, {[Ln3(TBrTA)6][Ln(H2O)8]·6H2O}
n
[Ln = Ho (10), Er (11), Tm (12), Yb (13), Lu (14)] series 2, respectively, were synthesized via slow evaporation with
tetrabromoterephthalic acid (H2TBrTA) 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···Br, Br···O, π–π, and
hydrogen bonding interactions in series 1 to Br···Br and hydrogen bonding interactions in
series 2. The nature of Br···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···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+(5D0) and Tb3+(5D4) 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···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.
