Triazine Poly(carboxylic acid) Metal–Organic Frameworks and the Fluorescent Response with Lead Oxygen Clusters: [Pb7(COO)12X2] by Halogen Tuning (X = Cl, Br, or I)

A series of novel metal-organic frameworks were synthesized by using semirigid ligand 1,1',1″-(1,3,5-triazine-2,4,6-triyl)tripiperidine-4-carboxylic acid (H3TTPCA) and lead halide (Cl, Br, or I). The three complexes were characterized by elemental analysis, infrared spectroscopy, ultraviolet-visible spectroscopy, powder X-ray diffraction analysis, and thermogravimetric analysis. X-ray single-crystal diffraction analysis demonstrated that all three complexes were three-dimensional inorganic-organic framework structures with Pb-X2 (X = Cl, Br, or I). However, slight differences in the chemical environment were the focus of the coordinated halogen atoms and the different compositions of metal oxygen clusters: [Pb7(COO)12Cl2], [Pb7(COO)12Br2], and [Pb7(COO)12I2]. Because of the fluorescence of the organic ligand, the three complexes showed similar photoluminescence properties at room temperature, but the intensity of emissions decreased gradually with an increase in the atomic radius of coordinated halogen atoms. Interestingly, in the fluorescence response tests, complexes 1 and 2 displayed an optical signal of fluorescence "turn-on" while complex 3 showed an optical signal of fluorescence "turn-off". Here we aim to provide a possible mechanism to explain these unique and contradictory luminescence results.