Herein, we report a new light-harvesting mixedligand Zr(IV)-based metal-organic framework (MOF),with underlying fcu topology, encompassing the [Zr6(μ3-O)4(μ3-OH)4(O2C−)12] cluster and an equimolar mixture of thiadiazole-and imidazole-functionalized ligands. The successful integration of ligands with similar structural features but with notable chemical distinction afforded the attainment of a highly efficient energy transfer. Notably, the very strong spectral overlap between the emission

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... m of benzimidazole (energy donor) and the absorption spectrum of thiadiazole (energy acceptor) provided an ideal platform to achieve very rapid (picosecond time scale) and highly efficient energy transfer (around 90% efficiency), as evidenced by timeresolved spectroscopy. Remarkably, the ultrafast timeresolved experiments quantified for the first time the anticipated close proximity of the two linkers with an average distance of 17 Å. This finding paves the way for the design and synthesis of periodic MOFs affording very efficient and fast energy transfer to mimic natural photosynthetic systems. Photosynthesis is one of the key processes in nature, and it is principally based on an energy transfer process that takes place between chlorophyll (an antenna molecule) and a carotenoid (a pigment molecule) present in floras. The energy transfer process in plants occurs by means of the absorption of a wide range of wavelengths of sunlight that can be transferred to the reaction center via energy transfer and is then converted into chemical energy.1-3 Several attempts have been made to design highly efficient natural energy transfer systems, such as assemblies of covalently bonded porphyrin arrays,4 dendrimers,5 chromogenic polymers,6 and selfassembled donor−acceptor supramolecular systems.7 This energy transfer process is most effective through assembly of an ordered network embedded within the same systems.8 Metal−organic frameworks (MOFs), an emerging class of porous crystalline materials, that are built up from multidentate organic building blocks and metal or metal clusters,9 have paved their way as bulk and thin films in diverse applications, including gas separation and storage,10 sensing,11 catalysis,12 and drug release.13 This versatility is a result of their permanent porosity, surface area, and structural and functional tunability. In particular, MOFs have been brought into the limelight as a suitable platform for exploration of directional energy transfer phenomena. This is mainly due to the distances and angles between linkers in MOF structures, which can be easily determined by different techniques such as single-crystal X-ray crystallography. The ability to design and fine-tune MOF structures can be approached methodically via the unlimited possibilities of altering and combining organic ligands and metals or metal clusters.14-15 In recent years, several research groups have made considerable progress in the design and development of MOFs for light-harvesting applications. In principle, energy transfer in MOFs can be introduced via various pathways, including metal-to-ligand, metal-to-metal, host-to-guest, and ligand-toligand scenarios.16-21 Several studies have reported stimulating strut-to-strut energy transfer pathways within MOFs encompassing porphyrin ligands.22-24 These early investigations encouraged us to explore novel structures containing two complementary ligands in a single MOF structure (mixed-ligand-based MOFs) to study ligand-toligand energy transfer. The mixed-ligand approach is the ideal design to co-assemble two ligands in a single MOF structure. Thus, we herein report a new zirconium-based mixed-ligand fcu MOF (Zr-ML-fcu-MOF), which was obtained by the solvothermal reaction between a zirconium salt and a equimolar mixture ratio of 4,4'-(1H-benzo[d]imidazole-4,7diyl)dibenzoic acid (BI) and 4,[1,2,5]thiadiazole-4,7-diyl)dibenzoic acid (TD)25-26 linkers (see Figure 1 ). The mixed-ligand strategy successfully yielded Zr-ML-fcu-MOF, because of the identical length, symmetry and connectivity of the two ligands (see Figure 1 ). The rapid and highly efficient ligand-to-ligand energy transfer from the benzimidazole linker to the thiadiazole linker in Zr-ML-fcu-MOF was confirmed by various techniques, such as steady-state and time-resolved luminescence measurements. The new Zr-ML-fcu-MOF was synthesized by a conventional method, as described in Figure 1 . ZrCl4 and TFA were mixed in DMF for 1 h, followed by the addition of BI and TD solutions. Heating for 24 h at 120 °C resulted in the formation of Zr-MLfcu-MOF as pale yellow fine crystals. The synthetic procedures are described in detail in the experimental section in the supporting information (SI). For comparison purposes, we have also synthesized single-ligand MOFs (Zr-BI-fcu-MOF and Zr-TD-fcu-MOF) of BI and TD by a conventional method, which also yielded white and yellow fine crystal products, respectively. Figure 2a shows the powder X-ray diffraction (PXRD) patterns of the three as-synthesized MOFs. The PXRD