Luminescence Enhancement of Organic Nanoparticles Induced by Photocrosslinking

Abstract

Nanoparticles are interesting materials because they exhibit unique optical and chemical properties which differ from solvated molecules and the bulk solid state and can be finely tuned during synthesis. These properties make nanoparticles suitable for numerous applications such as catalysis, high-performance ceramic materials, microelectronic devices, sensing, high-density magnetic recording and photonic materials. Colloidal organic fluorescence nanoparticles (ONPs) can be described as a core–shell system in which the core built of fluorescent dye molecules is coated by a stabilizer layer (surfactants or polymers) and show very different fluorescent properties from those arising from the starting materials. Despite the ease of preparation (mild and sustainable conditions), and the range of starting materials, there is not an extensive literature on ONPs so far (in comparison with inorganic colloidal nanoparticles), most likely because their lack of stability, which limits their use in photonic applications. In fact, they are basically self-assembled nanostructures arranged through intermolecular interactions (such as hydrogen bonding, p–p stacking, and hydrophobic interactions) which can be weakened by medium changes (pH, polarity, hydrophobicity, dilution). Organic nanoparticles are usually prepared by mechanical milling of the raw material, by precipitation or condensation of the starting material. In general, in colloidal organic nanoparticles the fluorescent material is confined in a stabilizer shell ; thus additives such as polymers take over the functions to be a boundary layer and a colloidal stabilizer. Photoinduced polymerization (UV curing) of monomers or oligomers is one of the most efficient ways to prepare photocrosslinked polymer networks. In short times and selectively at the irradiated area, liquid resins can be transformed into solid polymers very resistant to heat and mechanical treatments. These effects made UV curing technology widely used in many industrial applications. Considering the ability of some polymers to undergo crosslinking processes, a robust protective shell for the prepared organic nanoparticles was created by UV curing. The caging of the ONPs in a photocrosslinked polymeric shell could result in a considerable improvement in the stability and hence in the emission properties of the colloidal nanomaterials. ONPs were first prepared by use of the reprecipitation method, following the literature procedures. Herein, (EE)-1phenyl-4-(1’-pyrenyl)-1,3-butadiene [(EE)-PyPhB, Scheme 1], synthesized as previously reported, was selected as organic dye to build up the particle cores.