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...

No abstract

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.

No abstract

No abstract

Photoluminescence transients and time-resolved luminescence spectra are reported for the violet-blue emissions from epitaxial layers of Zn- and Cd-doped GaN. A typical decay time of 300 ns is reported for the blue GaN:Zn emission, peaking at about 2.89 eV. For GaN:Cd a somewhat longer decay time of τ≊1 μs dominates for the broad peak centered at ≊2.72 eV. In both cases it is concluded that the simple process involving a free-to-bound transition of an electron to a hole bound at the ZnGa, respectively, the CdGa acceptor is the dominating recombination mechanism corresponding to these characteristic decay rates.

No abstract

The red optical emission band in cubic GaN is examined for the first time by cathodoluminescence as well as steady-state and time-resolved photoluminescence. We establish a close analogy between the red emission band in cubic GaN and the yellow emission in wurtzite GaN, and assign it to a donor-acceptor process involving a shallow donor and a deep acceptor with similar ionization energies in both phases. We show a range of properties of the red emission which clearly indicate its donor-acceptor pair origin, namely the blueshift with excitation intensity, the power-law evolution at increasing excitation, and the blueshift with temperature. The line shape of the red band indicates that the deep acceptor is strongly coupled to the lattice. The photoluminescence decay curves show a fast decay at higher energies within the emission band, while at lower energies the decay is much slower, typical of donor-acceptor pair transitions. An analytical expression for the time-dependent emission intensity for donor-acceptor pair transitions in which one of the centers is strongly coupled to the lattice is derived, extending the theory of Thomas et al. ͓Phys. Rev. B 140, A202 ͑1995͔͒. The observed decay curves are found to be well described by the theory using the donor Bohr radius of 1.7 nm, while temperature quenching of the emission gives an activation energy of 15Ϯ5 meV. Evidence of Coulomb attraction between a shallow donor and a deep acceptor is presented. ͓S0163-1829͑99͒07631-6͔ PHYSICAL REVIEW B 15 AUGUST 1999-II VOLUME 60, NUMBER 8 PRB 60 0163-1829/99/60͑8͒/5464͑6͒/$15.00 5464