Efficient Energy Transfer (EnT) in Pyrene- and Porphyrin-Based Mixed-Ligand Metal-Organic Frameworks

Designing and synthesizing the ordered light-harvesting systems, possessing complementary absorption and energy-transfer process between the chromophores, are essential steps to accomplish successful mimicking of the natural photosynthetic systems. Metal-organic frameworks (MOFs) can be considered as an ideal system to achieve this due to their highly ordered structure, superior synthetic versatility, and tailorable functionality. Herein, we have synthesized the new light-harvesting mixed-ligand MOFs (MLMs, MLM-1-3) via solvothermal reactions between a Zr₆ cluster and a mixture of appropriate ratio of 1,3,6,8-tetrakis(p-benzoic acid)pyrene and [5,10,15,20-tetrakis(4-carboxy-phenyl)porphyrinato]-Zn(II) ligands. The identical symmetry and connectivity of the two ligands of the MLMs was the key parameter of successful synthesis as a single MOF form, and the ample overlap between the emission spectrum of pyrene and the absorption spectrum of porphyrin provided the ideal platform to design an efficient-energy transfer (EnT) process within the MLMs. We obtained the nanoscale maps of the fluorescence intensities and lifetimes of microsize MLM grains for unambiguous visualization of EnT phenomena occurring between two ligands in MLMs. Moreover, due to complementary absorption and energy transfer between the two ligands in the MLMs, our MLMs performed as superior photoinduced singlet-oxygen generators, verifying the enhanced light-harvesting properties of the pyrene- and porphyrin-based MLMs.