Artificial light-harvesting antenna materials have rapidly gained growing interest in recent years because of their applications in the design of sensors, [1] light-emitting diodes, [2] and solar cells. [3] The long-range ordered organization of donors and acceptors on the nano-to micrometer scale is crucial for efficient Förster resonance energy transfer (FRET) processes in these materials. [4,5] Various elegant strategies have been developed to achieve organized multichromophoric systems, such as organogels [6a] and hybrid hydrogels, [6b] vesicles, [7] and biomolecule-based assemblies. [8,9] Recently, novel approaches to host-guest light-harvesting systems were achieved by loading dye molecules into a single crystal zeolite [10] or periodic mesoporous organosilica.[11]These organizations of dye molecules into long-range ordered solids have proven to be very promising for attaining the desired macroscopic properties. To date, however, the use of nanocrystalline metal-organic frameworks (MOFs) as lightharvesting materials is less explored. MOFs, also known as coordination polymers that are assembled from organic ligands and metal ions, are a very promising type of material with a wide range of potential properties and applications including gas sorption, catalysis, magnetism, fluorescence, and nonlinear optics. [12] Recently, increasing interest has been paid to the miniaturization of MOFs to the nanometer scale; these miniaturized coordination polymers can overcome, to some extent, the limited solution-based behavior of their corresponding bulk materials.[13] The so-called nanoscale coordination polymers [13] have potential applications such as ion exchange, [14] multimodal bioimaging, [15] drug delivery, and sensing.[16] Recent studies showed that some fluorescent molecules confined in coordination polymer nanoparticles by novel adaptive self-assembly or host-guest strategies exhibit remarkably enhanced fluorescence and/or efficient FRET. [17,18] Herein, we envisaged the use of nanoscale metalorganic frameworks (NMOFs) as light harvesting antenna materials because chromophores densely embedded within the frameworks can increase the light absorption crosssection while solution-based behavior of nanocrystals provides potential for further applications.In light-harvesting systems, the energy-transfer efficiency and optical properties always depend on the donor/acceptor ratios. Compared with encapsulation by weak noncovalent interactions, self-assembly by stronger metal-ligand complexation can stabilize different components in the frameworks and decrease the possibility of leakage, which is especially important for sensors beased on FRET.[1] However, the arrangement of different components into long-range ordered frameworks is challenging. [13] Owing to different intermolecular forces in the precursor solution, such as counter ionic interactions, hydrogen bonding, and p-p interactions, coordination polymers easily aggregate to form amorphous particles. Recently, crystalline NMOFs have been prepared by surfactan...

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We report the self-assembly of stable nanoscale coordination polymers (NCPs), which exhibit temperature-controlled guest encapsulation and release, as well as an efficient light-harvesting property. NCPs are obtained by coordination-directed organization of pi-conjugated dicarboxylate (L1) and lanthanide metal ions Gd(III), Eu(III), and Yb(III) in a DMF system. Guest molecules trans-4-styryl-1-methylpyridiniumiodide (D1) and methylene blue (D2) can be encapsulated into NCPs, and the loading amounts can be controlled by changing reaction temperatures. Small angle X-ray diffraction (SAXRD) results reveal that the self-assembled discus-like NCPs exhibit long-range ordered structures, which remain unchanged after guest encapsulations. Experimental results reveal that the negatively charged local environment around the metal connector is the driving force for the encapsulation of cationic guests. The D1 molecules encapsulated in NCPs at 140 degrees C can be released gradually at room temperature in DMF. Guest-loaded NCPs exhibit efficient light harvesting with energy transfer from the framework to the guest D1 molecule, which is studied by photoluminescence and fluorescence lifetime decays. This coordination-directed encapsulation approach is general and should be extended to the fabrication of a wide range of multifunctional nanomaterials.

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