Tandem OLEDs Deliver High Luminous Efficiency
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to those of the template. The MIPs become capable of selectively absorbing the template species. Because of the stability, predesigned selectivity, and easy preparation of MIPs, they have been used for affinity chromatographic stationary phases, artificial antibodies, synthesis mediators, sensor components, and adsorbents for solid-phase extraction. The common forms of MIPs are bulk materials and membranes. N. Minoura of the National Institute of Advanced Industrial Science and Technology in Japan, K. Idei of Japan Science and Technology Corporation, A. Rachkov of Nihon University, and colleagues have prepared and characterized molecularly imprinted polymer membranes with a photoregulated ability to interact reversibly with a predetermined compound. The researchers report their work in the December 16, 2003 issue of Chemistry of Materials. In their synthesis, p-phenylazoacrylanilide (PhAAAn), a derivative of azobenzene with photoresponsive properties, was selected as the functional monomer. After it was polymerized, the polymer’s chemical properties changed reversibly by isomerization of this photoresponsive chromophore. Dansylamide (DA) was chosen as the template in the MIP synthesis because DA has the ability to decrease the rate of photoisomerization in solutions of PhAAAn. To improve the membrane’s flexibility, the cross-linker consisted of a mixture of ethylene glycol dimethacrylate (EGDMA) and tetraethylene glycol diacrylate (TEGDA). After DA, PhAAAn, EGDMA, and TEGDA were dissolved in acetonitrile, the freeradical polymerization was initiated by 2,2-azobis (4-methoxy-2,4-dimethylvaleronitrile) between two glass plates at 35°C. The formed MIP membrane was about 80 µm thick. Another control membrane was prepared using the same procedure without adding the template DA. The capacity and selectivity of the membranes were tested by incubating the membranes in the DA acetonitrile solutions at a concentration of 10 µM. The MIP membrane absorbed more DA than the control membrane, and its capacity is about 1.2 nmol of DA/cm2, or 0.15 nmol/mm3. The affinity of the binding sites can be controlled by illuminating the samples with the light of different wavelengths. Upon illumination with UV light, the absorbed DA from the MIP membrane
was released. Following a second illumination with visible light, the concentration of DA in solution and in the MIP membrane returned to levels close to those seen before the application of UV radiation. The control membrane did not show this property. Because the MIP membrane was tailored by imprinting DA, it also showed the selectivity and recognition to DA rather than to other molecules. Such MIP membranes showed a higher absorption level of DA than other reported levels of photoinduced effects in azobenzene-containing materials. The researchers said that molecular imprinting technology can be used to design photonic devices such as a photoswitches and photosensors. YUE HU
Tandem OLEDs Deliver High Luminous Efficiency Organic light-emitting diodes (OLEDs) are being developed for their
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