Modulating Excitation Energy of Luminescent Metal-Organic Frameworks for Detection of Cr(VI) in Water

Luminescent metal-organic frameworks (LMOFs) are newly emerged sensory materials, but rational design of LMOFs for specific analytes still faces many challenges for lack of sufficient knowledge about sensory mechanisms and effective approaches to match frameworks to analytes. In this paper, the highly fluorescent thiazolo[5,4-d]thiazole-extended viologen chromophore was for the first time incorporated to obtain a Zn(II) LMOF. The LMOF exhibits excellent stability in common solvents including boiling water and acidic to basic media. It can selectively sense CrO₄ ^2- and Cr₂O₇ ^2- in water with high sensitivity and good recyclability, the performance exceeding that of the free ligand and most previous LMOFs for sensing of Cr(VI). By plotting the quenching ratio against excitation energy, we obtained profiles that closely resemble the Cr(VI) absorption spectra. This illustrates a simple but useful experimental protocol to provide definite evidence for the sensing mechanism of competitive optical absorption. On the basis of density functional theory calculations in comparison with an isoreticular MOF, we demonstrate that the excitation energy can be modulated by varying the core motif of the extended viologen ligand to gain a better match between sensors and analytes.