The ligand effect resulted in different fluorescence responses of two similar zinc-based MOFs to high-valence metal ions and amino acids

The strategy of ligand functionalization can be used to adjust and control the properties of Metal-Organic Frameworks (MOFs). Based on these considerations, we chose two similar ligands (bpta = 3,5-bis(4-pyridyl)-1,2,4-triazole, bpat = 3,5-bis (4-pyridyl)-4-amino-1,2,4-triazole) and synthesized two MOFs with similar structures, {Zn2(bpta)2(hpta)(H2O)2·6H2O}n (1), {Zn2(bpat)2(hpta)(H2O)4·4H2O}n (2), by introduction of Zn(NO3)2·6H2O and pyromellitic dianhydride (pmda) coligand. X-ray single-crystal diffraction reveals that two MOFs both crystallized in triclinic systems with NbO topology. We systematically studied their fluorescence sensing performance. The results reveal that they both show obvious fluorescence enhancement effect to high-valence metal ions (Fe3+, Al3+, Cr3+, In3+ and Zr4+) and some amino acids. Unlike compound 1, the change of fluorescence property is not only reflected in the intensity, but also in the emission peak position for compound 2 with amino group. In addition, 2-H2O has a high selectivity and sensitivity to tryptophan, and there is no shift of emission peak. We verified the mechanism for the difference in fluorescence response between compounds 1 and 2 by IR and X-ray photoelectron spectrum.