Use of Linear Polymers To Control the Preparation of Luminescent Organic Microcrystals

Abstract

Microcrystals of an organic fluorescent dye, 4-octylamino-7-nitrobenz-2-oxa-1,3-diazole, were generated using the reprecipitation method, which is a solvent exchange process. In the presence of polymers, namely, poly(acrylic acid), molecular weight 5100 g mol(-1) and 15 000 g mol(-1), and poly(acrylic acid-co-maleic acid), average molecular weight about 50 000 g mol(-1), used as their sodium salts, the reprecipitation process was strongly accelerated. The reprecipitation kinetics was monitored by UV/vis absorption spectroscopy and revealed a three-step mechanism, each step being influenced by the polymer. The size and shape of the microcrystals were analyzed by fluorescence microscopy. The microcrystals obtained in the presence of polymers were smaller and more regular than those prepared in water alone and were not agglomerated. When the polymer was placed in the reprecipitation medium before introducing the dye solution, the microcrystals displayed a rectangular shape. When the polymer was introduced 20 min after the beginning of the reprecipitation process, intricately structured flowerlike microcrystals were observed. Microanalysis revealed that the microcrystals contained noticeable amounts of polymer. The measurement of the surface electric zeta potential suggested that a proportion of the polymer was present at the microcrystal surface. This work gives a thorough insight into a field where trials have until now been performed in an empirical way. It opens new perspectives to produce low-cost organic microcrystals, potentially useful in optics or pharmaceutical sciences.

Stopped-flow analysis on the mechanism of perylene nanoparticle formation by the reprecipitation method

“…These nanocrystals have attracted great interest in recent years, owing to their unique optical and electronic properties [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17]. For example, several organic nanocrystals exhibit a blue shift in the absorption and emission peak wavelengths upon decreasing the crystal size below few hundred nanometers [3,5], a size range much larger compared to semiconductor quantum dots.…”

“…The process can be performed in mild conditions, as opposed to other techniques such as evaporation, enabling fabrication of nanoparticles from heatsensitive organic molecules. In spite of a wide variety of nanoparticles reported, there are only few reports on the direct observation of organic nanoparticle formation, and the kinetics of the process is not well understood [9,19]. Further insight into the reprecipitation process is essential for a better control over particle size and structure.…”

Nano‐ and Microparticles of Organic Fluorescent Dyes

“…The influence of the dendrimer size was also studied 29 . Then, we used linear polymers, such as the anionic sodium salt of poly(acrylic acid) or neutral poly(acrylamide), 30 and a natural polymer—calf thymus DNA 31 . All these macromolecules were placed in water before addition of the dye.…”

“…We also measured the zeta potential, which is the charge acquired by the particle when it is placed in solution. It appeared that the microcrystal charge varies with the additive used 30 . All this indicates that the microcrystals had incorporated a small amount of additive, and that a proportion of this additive was situated on their surface.…”

Predicting crystal structure and habit of organic micro-crystals by experimentally assisted molecular modelling (EAMM). The case of n-octylamino-NBD

“…Using a solvent-exchange process, specifically the reprecipitation method [26,27], compound 1 leads to microcrystals with a nanometric thickness. This dye is of particular interest because the size and shape of its micro-crystals can be drastically modified by complementing the re-precipitation medium with an additive, generally a macromolecule (dendrimer [28][29][30], polymer [31,32] and calf thymus DNA [33]). These fluorescent ultra-thin micro-crystals also display original spectroscopic properties such as light re-absorption and wave-guiding.…”