fulltext37 - Nano- and Microparticles of Organic...

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Nano- and Microparticles of Organic Fluorescent Dyes Self-organization and Optical Properties S UZANNE F ERY -F ORGUES ,M OUHAMMAD A BYAN , AND J EAN -F RANC ¸ OIS L AMERE Universit´e Paul Sabatier, Laboratoire des Interactions Mol´eculaires R´eactivit´e Chimique et Photochimique (UMR CNRS 5623), Toulouse, France Organic nanostructured materials are of increasing interest for applications in the fields of bio- analysis, photocatalysis, photonics, and organic light-emitting diodes. However, their preparation is still difficult to control and their optical properties are inadequately known. A solvent-exchange process called the “reprecipitation method” was used here to prepare nano- and microcrystals from fluorescent dyes belonging, for example, to the coumarin and nitrobenzoxadiazole (NBD) se- ries. Typically, the dyes were dissolved in a hydrophilic organic solvent and then suddenly placed in an aqueous environment, where they spontaneously produce molecular assemblies. According to the self-association properties of the dyes and to the experimental conditions used, the nano- and microcrystals obtained exhibited different sizes and shapes, as observed by fluorescence and electron microscopy. In some cases, the crystal habit was controlled by adding some additives to the reprecipitation medium. The overall optical properties of the free-standing particles in sus- pension were generally quite close to those of the dissolved dyes. However, strong distortions of the absorption and emission spectra were observed for crystals grown in the presence of ionic ad- ditives. Under the fluorescence microscope, individual microcrystals may show peculiar emission characteristics, displaying bright and dark zones, or behaving like tiny optical fibers. Key words: nanocrystal; microcrystal; nanofiber; NBD; coumarin; reprecipitation; fluorescence Introduction Organic nano- and microcrystals belong to the in- termediate states that fill the gap between bulk ma- terials and single molecules. These very small objects measure, respectively, less than 100 nanometers and up to some tens of micrometers. Those of interest in this paper are made of molecules that exhibit partic- ular photophysical or photochemical properties. They have been relatively little studied by spectroscopists and photochemists for a long time, and their real develop- ment began only some 15 years ago. The attention that is paid to them at the moment is justified by their numerous potential applications as photoactive ma- terials. Actually, they offer distinct advantages in this area. With their high density of molecules, dye nano- and microcrystals absorb strongly in the ultraviolet and the visible. They are also particularly stable and show little degradation upon light irradiation, unlike dis- solved molecules. These characteristics, together with Address for correspondence: Suzanne Fery-Forgues, Laboratoire IMRCP, Universit´e Paul Sabatier, 31062 Toulouse cedex 9, France. Voice: + 335 61 55 68 05; fax:
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fulltext37 - Nano- and Microparticles of Organic...

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