Thiazolothiazole-Based Luminescent Metal–Organic Frameworks with Ligand-to-Ligand Energy Transfer and Hg2+-Sensing Capabilities

Photoinduced electron and energy transfer through preorganized chromophore, donor, and acceptor arrays are key to light-harvesting capabilities of photosynthetic plants and bacteria. Mimicking the design principles of natural photosystems, we constructed a new luminescent pillared paddle wheel metal-organic framework (MOF), Zn2(NDC)2(DPTTZ), featuring naphthalene dicarboxylate (NDC) struts that served as antenna chromophores and energy donors and N,N'-di(4-pyridyl)thiazolo-[5,4-d]thiazole (DPTTZ) pillars as complementary energy acceptors and light emitters. Highly ordered arrangement and good overlap between the emission and absorption spectra of these two complementary energy donor and acceptor units enabled ligand-to-ligand Förster resonance energy transfer, allowing the MOF to display exclusively DPTTZ-centric blue emission (410 nm) regardless of the excitation of either chromophore at different wavelengths. In the presence of Hg2+, a toxic heavy metal ion, the photoluminescence (PL) of Zn2(NDC)2(DPTTZ) MOF underwent significant red-shift to 450 nm followed by quenching, whereas other transition metal ions (Mn2+, Fe2+, Co2+, Ni2+, Cu2+, and Cd2+) caused only fluorescence quenching but no shift. The free DPTTZ ligand also displayed similar, albeit less efficient, fluorescence changes, suggesting that the heavy atom effect and coordination of Hg2+ and other transition metal ions with the DPTTZ ligands were responsible for the fluorescence changes in the MOF. When exposed to a mixture of different metal ions, including Hg2+, the MOF still displayed the Hg2+-specific fluorescence signal, demonstrating that it could detect Hg2+ in the presence of other metal ions. The powder X-ray diffraction studies verified that the framework remained intact after being exposed to Hg2+ and other transition metal ions, and its original PL spectrum was restored upon washing. These studies demonstrated the light-harvesting and Hg2+ sensing capabilities of a new bichromophoric luminescent MOF featuring a seldom-used photoactive ligand, which will likely spark an explosion of TTZ-based MOFs for various optoelectronic applications in near future.