Organic Nanoparticulate Photochromes

Abstract

Photochromic organic dyes can be widely used in materials for optically rewritable data storage, photonic switches, memories, sensors, or actuators. In recent years photochromic materials based on nanoparticles became particularly focused, since they can be dispersed in colloidal aqueous suspensions or incorporated in thin films, avoiding problems of light scattering or shallow light penetration in bulk materials. Spiropyrans, spirooxazines and diarylethenes were by far the most researched photochromes in nanoparticulate systems. Great effort was made to investigate photochromic dyes incorporated into organic nanoparticles via self-assembly strategies, covalent linkage or dispersion of the molecular species in polymers (doping). Nanoparticles composed of solely photochromic dyes were prepared by laser ablation and reprecipitation techniques. Photochromic dyes were microencapsulated by self-assembly, soap free-, emulsion/microemulsion/miniemulsion or free radical-(co)polymerization. Sol-gel processing from silane precursors to poly(organo)siloxane matrix is a common method to synthesize doped or core-shell photochromic organogels. Coloured forms of some photochromes display fluorescence; however, a more effective strategy for fluorescence modulation with photochromic molecules is integrating them, covalently or noncovalently, with a separate fluorophore in the same nanoparticles. These photoresponsive nanoparticles may find applications particularly in biological fields such as cell labelling and bioimaging. The purpose of this review is to summarize the preparation methods of organic nanoparticles containing photochromic dyes and to investigate their typical properties derived from their nanoparticulate character.Keywords: photochromic nanoparticle, laser ablation, reprecipitation, polymerization, doping, fluorescence INTRODUCTIONPhotochromism is a reversible structural and colour change of a species triggered in one or both directions by electromagnetic radiation [1]. The shift of optical absorption, due to change in molecular structure or conformation results in colourization, while the coloured product decolourizes after a structure rearrangement in the back (generally thermal) reaction. The reversible transformations of photochromic compounds are usually based on ring opening/closing steps, cis/trans isomerizations, proton transfer, electron transfer or cycloadditions [2]. Like other stimuli responsive materials (e.g. heat and pH-sensitive materials), photochromic devices are responsive to an external stimulus, in this case, light. Light is the most attractive stimulus due to the fact that its wavelength, intensity and focus can be tuned. Photochromic materials have been investigated extensively during the past few decades because of optical responses and nonlinear optical, electronic and magnetic properties [3]. Photochromic materials have a high potential for applications to ophthalmic lenses [4,5] [15], and also for microstructuring and patterning surfaces [16]. Therefore, photosensitive, nanoscal...

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Solubility, thermal and photoluminescence properties of triphenyl imidazole-containing polyimides

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