Stabilization of Optical Molecules Using Dendritic Boxes against Singlet Oxygen in Photobleaching
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Stabilization of Optical Molecules Using Dendritic Boxes against Singlet Oxygen in Photobleaching Sonoko Otomo, Akira Otomo and Shinro Mashiko Kansai Advanced Research Center, Communications Research Laboratory, Kobe, 651-2492, Japan ABSTRACT The stabilizing effect of dendritic boxes (DBoxes) as a means to protect against photodegradation of encapsulated dye molecules was experimentally investigated. We focused on photoinduced oxidation and studied the ability of DBoxes to protect dyes inside them from reactive singlet oxygen. Rubrene was captured in a DBox, and then the box was closed at the surface by other stable molecules. A singlet oxygen generator was added to a Rubrene solution to exaggerate photo-oxidation in the complicated total-photodegradation process, and then the solution was exposed to a laser beam to generate singlet oxygen. Bleaching of the encapsulated Rubrene in a DBox was 50 times slower than that of Rubrene alone. We also compared the dependence of surface molecules of a DBox with other kinds of molecules that have different efficiencies of quenching singlet oxygen. Further improved stability was observed by attaching singlet-oxygen-quencher molecules on the DBox surface. INTRODUCTION Organic π-conjugated molecules possess excellent linear and nonlinear optical properties, such as high emission efficiency and large nonlinearity [1,2]. They are expected to enable production of high-speed, low-energy-consumption optoelectronic devices. However, photoinduced degradation (photobleaching) stands in the way of applying organic optical molecules for a wide range of photonic devices [3-5]. In general, photobleaching mainly consists of two processes, photo-induced oxidation and photothermal decomposition. Although the photoinduced oxidation process is not simple, it is certain that singlet oxygen self-generated by an excited triplet species of optoelectronic molecules plays an important role in photooxidation. Sealing packages of optoelectronic devices is a simple solution and has been applied to electroluminescent devices. However, an intrinsically stable molecular structure is needed to simplify device processing and expand the range of application field. Dendrimers are attracting attention as a novel framework for optical molecules. These molecules are useful for photonic applications because their step-wise synthesis process makes it possible to hybridize many different optoelectronic functions on a molecular scale with precisely controlled structures [6-8]. Their intrinsic hyperbranched architecture leads to a characteristic shell-like structure that has lower molecular density inside than outside. This structure should prevent the molecules inside from reacting chemically with active species outside the shell, without changing the optoelectronic properties of the molecules inside. A polypropyleneimide dendrimer was reported by Jansen et al. as a molecular container that easily capture small molecules and keep them inside by attaching bulky molecules on the dendrimer surface [9,10]. We examine the s
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