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International audience; High-pressure microfluidic systems exhibit favorable capacity to enhance mixing quality compared to conventional macroscale liquid systems. These mixing conditions are very interesting for the preparation of fluorescent organic nanoparticles by supercritical antisolvent process. In this work, fundamental mechanisms of microfluidic supercritical antisolvent process (µSAS) including thermodynamics, hydrodynamics, nucleation/growth phenomena, are investigated using a coupled experimental/simulation approach. Specifically, we determined experimentally the particle precipitation field in a CO2 / solvent medium using a fluorescent organic dye molecule which presents an enhancement of fluorescence intensity in its aggregated state (AIE effect). The results of the direct numerical simulation considering all the physical phenomena are compared with the experimental data for validation and deep understanding of the mechanisms. It is shown that despite ultra-short mixing time, the supersaturation field showed some fluctuation leading to variation of the nucleation times.

As a non-swelling clay, talc generally interacts very weakly with organic molecules. However, two new nanometric synthetic talcs that incorporate berberine chloride were successfully used as fluorescent fillers in clay polyurethane nanocomposites obtained by the blending method. A micrometric natural talc filler was also used for comparison. The clay polymer nanocomposites (CPN) were characterized by FTIR, molar mass analysis and XRD. All talc fillers were well dispersed into the polyurethane matrix even at high filler content of 5 wt%, as supported by TEM and SEM analyses. Morphological changes were confirmed by AFM. The optical properties of the fluorescent talcs, dye-doped polymer and CPN were compared. While the dye-doped polymer suffered from conventional fluorescence quenching due to dye aggregation, the emission quantum yield of the CPN was increased with increasing the filler content. The best emission efficiency was observed for the CPN that contains the smallest talc-berberine hybrid particles. The use of fluorescent synthetic talc resulted in materials with good thermal and mechanical properties, and can be considered as a new method to produce fluorescent CPN in view of multiple applications.

cited By 1; International audience; The optical properties of three bis-benzoxazole derivatives, namely 2,2’-diphenyl-6,6’-bibenzoxazole and its derivatives bearing methyl and tert-butyl groups at the para position, were investigated and compared to those of the corresponding subunits. As expected from the increase of the π-electron conjugated system, the absorption and fluorescence spectra were red-shifted when passing from mono-to bis-chromophores. The three bis-benzoxazoles showed superior fluorescence quantum yields in solution, but they were only weakly emissive in the solid state, the unsubstituted derivative even being virtually not photoluminescent. The solid-state behaviour was explained by the fact that bis-chromophoric molecules were locked in a non-planar conformation, as discussed on the basis of DFT calculations and X-ray data.

The fluorescence properties of four methoxy derivatives of 3-cyano-2-(N-ethoxycarbonyl)iminocoumarin, differing by the position and number of the methoxy substituents, were investigated. The 8-methoxy-iminocoumarin derivative was virtually non-fluorescent. The 7-methoxy-iminocoumarin derivative was only weakly fluorescent both in solution and in the solid state. The 6-methoxy-iminocoumarin derivative was poorly fluorescent in every solvent investigated, but it showed high photoluminescence in the solid state. To our knowledge, this is the first example of crystallization-enhanced emission (CEE) in the iminocoumarin series. The mechanism probably encompasses restricted rotation and favorable molecular packing. Moreover, this compound spontaneously crystallized as long microfibers. The 5,7-dimethoxy-iminocoumarin derivative exhibited good fluorescence in very polar solvents and was intensely luminescent in the crystalline state. The last two substitution patterns, 6-methoxy and 5,7-dimethoxy, could be of high interest for the design of new fluorescent materials.

3-Phenyl-5,7-dimethoxycoumarin and 3-(2-thienyl)-7-methoxycoumarin were strongly fluorescent in organic solution, with quantum yields reaching 0.84. In contrast, the iminocoumarin analogs of these dyes were poorly emissive. This shows that the behaviour of coumarins cannot be systematically transposed to their iminocoumarin analogs. The spectroscopic properties of iminocoumarin dimers were also examined. These dimers present little spectroscopic interest in comparison with the parent compounds. Above all, this study indicates the untapped potential of fluorescent 3-aryl-7-methoxycoumarins, which have scarcely been studied. In particular, 3-(2-thienyl)-7-methoxycoumarin exhibited good emission efficiency in a wide range of solvents, and could be very useful as a fluorescent label including in protic media.

An iminocoumarin derivative bearing a fatty chain, specifically N-palmitoyl-3-cyano-7-diethylamino-2-iminocoumarin, was synthesized and used to prepare particles owing to a solvent exchange process (reprecipitation method). The composition of the reprecipitation medium was allowed to vary. In all cases, very thin particles that emitted yellow–orange light were obtained. In water alone, they looked like nanofibers or nanoribbons. Addition of surfactants at concentrations lower than the CMC favored the formation of bladed microcrystals. In the presence of acetonitrile, elongated microcrystals with good wave-guiding properties were obtained. The fluorescence properties were discussed on the basis of the crystal packing mode. It appears that the flexibility of the imino function leads to an original crystal structure, which is compatible with solid-state fluorescence. This property, together with the ease of synthesis, makes this family of compounds interesting for subsequent use as optically-active nanomaterials.

The unexpected formation of nanocrystals in dilute dye solutions was revealed using the hydrophobic dye, 7-methoxycoumarin, the spectroscopic behavior of which has been frequently studied in water; comparison was made with the water-soluble, sulfonated analogue. A solution of the dye at 10 −5 M concentration range was found to be a suspension of nanocrystals; at concentrations of 1 × 10 −4 M, > 70% dye was in the form of nanocrystals. Surprisingly, the presence of these nanocrystals was difficult to detect by routine absorbance and fluorescence spectroscopy and was mainly noticeable by measurement of fluorescence quantum yield, as the nanocrystals were poorly emissive compared to dissolved dye molecules; electron microscopy and dynamic light scattering gave clear evidence for the presence of nanocrystals. This article illustrates the fact that the formation of nanocrystals, although abundant, may easily go unnoticed.

The condensation of 3-cyano-7-diethylamino-iminocoumarin with various isocyanates and diisocyanates as C-electrophiles gave a new series of N-substituted iminocoumarins and bis-iminocoumarins. The ten compounds obtained were characterized using IR, 1 H NMR, 13 C NMR, elemental analysis and mass spectrometry. Their optical properties, studied in dichloromethane by UV/vis absorption and fluorescence spectroscopy, were found to depend strongly upon the nature of the substituent borne by the imino group. For the sake of comparison, the optical properties of selected compounds were also analyzed in ethanol. Of the iminocoumarins studied, the alkylurea derivatives displayed the most interesting spectroscopic characteristics and may be of potential use as novel fluorescent probes in media of various polarity.

Various chiral N-palmitoyl amino acid surfactants (AAS) derived from methionine, proline, leucine, threonine, phenylalanine and phenylglycine were prepared and converted to their sodium salt. The properties of the aggregates formed in aqueous solution were studied for both the optically-active compounds and their racemic mixture. Characterization was made by surface tensiometry, fluorimetry, dynamic light scattering, circular dichroism (CD) and transmission electron microscopy. It appeared that most of the AAS studied in this work spontaneously formed different types of aggregates, including micrometer-sized aggregates. No significant difference could be found between the critical aggregation concentration (cac) value of pure enantiomers and that of the racemic forms. CD spectra did not reveal any aggregation-induced chirality.

A liquid/liquid interfacial technique was used to prepare biscoumarin dyes by condensing 7-hydroxy-4-methylcoumarin with various diacyl chlorides in the presence of a phase-transfer catalyst such as triethylbenzylammonium chloride. The reaction conditions were optimized. Six new biscoumarin derivatives were thus prepared in high yield. Their optical properties were studied in chloroform by UV/vis absorption and fluorescence spectroscopies. It appeared that the shape of the absorption spectra varies with the nature of the tether between both chromophores. In contrast, the excitation and emission spectra were almost unchanged, although the compounds exhibited drastic differences in their fluorescence efficiency.

Six new iminocoumarin dyes, bearing a cyano group at the 3-position and a diethylamino group at the 7-position, were synthesized. These compounds differ by the nature of the acyl substituent borne by the imino group. They were studied in CH 2 Cl 2 and ethanol by UV/vis absorption and fluorescence spectroscopy. This work shows that the optical properties strongly depend on the nature of the substituent borne by the imino group, and indicates which type of substituent is favourable for a given application. It confirms that iminocoumarin derivatives could lead to a new generation of fluorescent probes, prone to easy modification of their chemical structure.

The optical properties of 10 new iminocoumarin dyes, bearing a cyano group at the 3-position, were studied in CH2Cl2 by UV/vis absorption and fluorescence spectroscopy. Five of the dyes bear a free imino group, and differ by the nature and position of their electron-donor group. Among them, the dye bearing a diethylamino group at the 7-position displayed the most interesting optical properties. All the compounds were compared to analogues containing an ethoxycarbonyl group on the imino function. The presence of the ethoxycarbonyl group systematically induced a shift of the absorption and emission spectra towards long wavelengths. For the three dyes where charge transfer is weak, substitution reduced fluorescence efficiency. In contrast, the fluorescence quantum yield and lifetime were slightly increased for the compounds with strong charge transfer, and especially for that bearing a diethylamino group at the 7-position. This shows that substitution on the imino group can be a convenient way to obtain good fluorescent probes designed for various purposes.

The optical properties of two new iminocoumarin dyes, bearing a cyano group at the 3-position and an N-diethylamino group at the 7-position, were studied by UV–vis absorption and fluorescence spectroscopy in a large number of solvents. Dye 1 bears a free imino group, while dye 2 is substituted by an ethoxycarbonyl group on the imino function. The properties of these dyes were compared with those of 3-cyano-7-diethylaminocoumarin (3). The study shows that the three compounds display close spectroscopic behaviour in a large range of solvents, but the iminocoumarins exhibit much better fluorescence efficiency than the coumarin in polar and moderately protic solvents. This shows that iminocoumarin derivatives can advantageously replace coumarins in this type of solvents, and confirms that substitution on the imino group is a convenient way to obtain good fluorescent probes designed for various purposes. To cite this article: H. Turki et al., C. R. Chimie 9 (2006).

This paper focuses on various chromophores that have been covalently linked to two azacrown or benzocrown ether units. These compounds can interact with near-UV or visible light. By comparison with monocrown dyes, they display particular behaviour in the presence of cations. The crowns may have the possibility to act in concert, thus forming sandwich complexes around the guest. This often leads to increased selectivity towards large cations. This is the case for photochromic crown ethers, which allow the photochemical control of cation binding, as well as for some optical sensors, designed for cation recognition. Biscrown dyes can also be rigid structures in which the two spatially separated crowns cannot co-operate intramolecularly. However, intermolecular sandwich complexes can be detected. They range from discrete face-to-face arrangements to extended supramolecular architectures, which can generate nanosized aggregates and materials. The variety of possible combinations with cations bestows a particular interest on biscrown dyes.

The optical properties of six new 3,3′-phenylenebiscoumarin dyes, differing by the substitution pattern of the central phenyl group and by the presence of methoxy substituents, were investigated in CH 2 Cl 2 by UV/vis absorption and fluorescence spectroscopy. The spectroscopic behaviour of one of the dyes was also analysed in five other solvents. By comparison with monocoumarins, the absorption and emission spectra of the biscoumarin derivatives were shifted to high wavelengths. Most of them displayed high fluorescence quantum yields (0.7–0.8) and lifetimes in the nanosecond range, which can make these dyes very useful as new fluorescent probes.

International audience; The fluorescence quenching of 1,5-bis[4-(diethylamino)phenyl]penta-1,4-dien-3-one (1), 1-[4-(diethylamino)phenyl]-5-phenylpenta-1,4-dien-3-one (2) and chalcone 3 (1-[4-(diethylamino)phenyl]but-1-en-3-one) was studied in acetonitrile using ferrocene and three commercially available derivatives, 1,1′-diacetylferrocene, acetylferrocene and 1,1′-dimethylferrocene. The spectroscopic and electrochemical characteristics of all the compounds were investigated. The feasibility of the quenching process was calculated, showing in particular that electron transfer was thermodynamically possible in every case. The steady-state fluorescence of the three organic dyes was then measured in the presence and in the absence of the ferrocenyl derivatives and the data were corrected for inner-filter effects. 1,1′-Diacetylferrocene and acetylferrocene proved to be the most efficient quenchers, and among the three dyes investigated, chalcone 3 was the most sensitive to the presence of the ferrocenyl derivatives. However, the fluorescence decay times of the ketocyanines were not affected by the presence of the ferrocene derivatives, indicating that quenching was not diffusion-controlled but rather the consequence of the formation of a ground state complex.

Fluorescence spectroscopy is widely used as a tool for elucidating the structure and dynamics of the micellar medium. A prerequisite commonly encountered for quantitative approaches is that the fluorophore resides exclusively in the micellar phase. Providing the fluorophore with a long alkyl chain may appear advantageous with regard to fixing the probe into the micelle. The present work was aimed at determining which are the consequences of this process from a spectroscopic viewpoint. The nitrobenzoxadiazolyl (NBD) moiety, which leads to well known fluorescent probes, was directly grafted on three fatty amines, the chain length of which varied from 8 to 18 carbon atoms. The spectroscopic properties of these NBD derivatives were investigated in three different micellar media: SDS, CTAB and TX100. The dyes were incorporated into micelles, where they were located in the interfacial region, whatever the chain length. When the dyes were previously dissolved in ethanol, and subsequently placed in the presence of the surfactant solution, complete solubilization was obtained. However, when the surfactant solution was used to dissolve directly a thin film of dye, a certain amount of dye remained non-incorporated and formed microcrystals, whose quantity and size increased with chain length. These microcrystals were mainly detected by UV/Vis-absorption and fluorescence microscopy. They induced drastic errors in the determination of fluorescence quantum yields, although they hardly interfered with other steady-state measurements and with dynamical fluorescence measurements. In conclusion, it appeared that for a small, non-ionic fluorophore such as NBD, the presence of a long alkyl chain is not an advantage. It slows down the incorporation process, unless some alcohol is introduced in the medium. Short-chain probes are therefore best suited for the study of the micellar medium.

International audience; Owing to their fairly high stability under visible irradiation, ferrocene and ferrocenyl derivatives are widely used in luminescent systems. They are classical quenchers of excited states. Both energy and electron transfer may be involved, depending on the nature of the excited species. Inter- or intramolecular quenching are encountered. Applications span from the study of reaction mechanisms to that of organized or biological media. Recently, dyads and polyads designed for their ability to mimic photosynthetic centers or for their photodiode properties have also been obtained. Finally, the incorporation of a ferrocenyl derivative in a luminescent system does not necessarily lead to luminescence quenching. New applications are emerging, in which advantage is taken of the presence of ferrocene acting as a redox center: this gives optically and electrochemically active sensors. The present review encompasses the literature up to November 1999.

4-Hydroxycoumarin (1a) and a polysubstituted derivative (2a) were treated with chlorodiphenylphosphate to give the corresponding diphenylphosphatidylcoumarins, lc and 2c, respectively. For the sake of comparison, the hydroxycoumarins 1a and 2a were also esterified by acetic acid to yield 1b and 2b, respectively. These compounds were studied by UV/vis-absorption and fluorescence spectroscopy. In the 4-hydroxycou-marin series, fluorescence was quenched after substitution by the phosphate group, which is reminiscent of the behaviour of 4-methyl-umbelliferyl-phosphate (4-MUP). In contrast, for the polysubstituted compounds, the fluorescence quantum yield of the phosphate derivative was much higher than that of the corresponding hydroxycoumarin. For both series, the phosphate-bearing derivatives were markedly less emissive than the acetyl ester analogues (1b and 2b).

The spectroscopic and photophysical properties of two 7-nitrobenz-2-oxa-1,3-diazol-4-yl (NBD) derivatives with different substituents on the nitrogen group are reported in 18 solvents. The solvatochromic shifts were analysed by correlating with polarity scales. The results, together with the help of modified neglect of diatomic overlap (MNDO) calculations, enable the polarity of the ground and first singlet excited states to be determined. Experiments based on variations in temperature and viscosity establish that the two probes undergo different de-excitation pathways. The possibilities of internal rotation leading to a twisted intramolecular charge transfer (TICT) state in the case of diethylamino-NBD are discussed. A study in binary solvent mixtures outlines specific solvent—solute interactions. Appropriate restrictions are emphasized on the utilization of NBD probes in biological fields.