Aggregation and Decomposition of a Pseudoisocyanine Dye in Dispersions of Layered Silicates

Swelling-Induced Chromotropism of Bionanocomposite Hydrogel Beads

Monodispersed gelatin hydrogel beads containing smectite with adsorbed cyanine dye exhibit chromotropic responses to compression and swelling/deswelling by solvent. Photoluminescence color of the beads changes by swelling in water (blue) and deswelling in ethanol (purple) reversibly. The forces generated by swelling/deswelling are thought to induce the transition between the J-aggregate and the monomer of cyanine dye adsorbed on smectite, giving the photoluminescent color changes.

Cyanine Dyes Containing Quinoline Moieties: History, Synthesis, Optical Properties, and Applications

Cyanine dyes carrying quinoline moieties are an important class of organic molecules that are of great interest for applications in many fields like medicine, pharmacology, and engineering. Despite their exceptional properties, such as stability, high molar extinction coefficients, and high pH-sensitivity, this class of dyes has been less analyzed and reviewed in the last few decades. Therefore, this review article focuses on discussing the history of quinoline compounds, various synthetic routes to prepare quinolinium salts and symmetrical and asymmetrical mono-, di-, tri-, penta- and heptamethine cyanine dyes, containing quinoline moieties, together with their optical properties and applications as photosensitizers in photodynamic therapy, probes in biomolecules for labeling of nucleic acids, as well as imaging agents.

Photophysical Behavior of Isocyanine/Clay Hybrids in the Solid State †

In the present study, we have attempted to investigate, for the first time, the photophysical behavior of 1,1'-diethyl-2,4'-cyanine (ICY)/clay mineral hybrids in the solid state. The effects promoted by ICY loading and clay type on the spectroscopic properties were studied by UV-vis diffuse reflectance spectroscopy (DR) and different fluorescence techniques. The hybrids were characterized by X-ray diffraction (XRD) and thermogravimetric analysis (TGA). UV-vis-DR revealed the formation of ICY H-aggregates in Wyoming montmorillonite (SWy-1) and Laponite (Lap); however, J-aggregates were predominant for ICY on Arizona (SAz-1) and Barasym (SYn-1) montmorillonites. The formation of J-aggregates was favored on clays with a high layer charge density (SAz-1 and SYn-1). Increasing ICY loading leads to an increase in H-aggregates, which become predominant in all of the samples. The fluorescence spectra of ICY-Lap and ICY-SYn-1 hybrids showed two emissive bands, and they were assigned to the monomeric and J-aggregate species. The fluorescence lifetime showed consistent and distinct values for the two species. The longer fluorescence lifetime can be assigned to the ICY monomers, while the second component has a short lifetime value and may be attributed to J-aggregate emission species. Moreover, confocal fluorescence micrographs showed two different fluorescent domains; monomers (greenish domain) and J-aggregates (orange domain) can be clearly distinguished. For ICY adsorbed on SWy-1 and SAz-1, the intensities of the fluorescence spectra were very low, and it was not possible to measure the fluorescence lifetimes due to high iron content in these clays, which acts as an efficient quencher of the excited singlet state of the dye molecules. XRD and TGA curves showed that the intercalation of ICY into the interlayer regions of SWy-1, SAz-1, and SYn-1 occurred for high dye concentration only. In the case of Laponite, ICY adsorbs on the external surface of the layer. Our studies indicate that the ICY-clays, in particular, ICY-SYn-1 and ICY-Lap, are promising hybrid materials with interesting optical and photophysical properties.

Study of spectral behaviour and optical properties of cis/trans-bis(N-methylpyridinium-4-yl)diphenyl porphyrin adsorbed on layered silicates

The spectral properties of two dicationic porphyrins, cis- and trans-bis(N-methylpyridinium-4-yl)diphenyl porphyrins, upon adsorption on clay mineral templates were investigated. A series of reduced charge Nanocor montmorillonites was used as host templates. The main impact was given on the influence of the layer charge on the absorption and fluorescence spectra. Adsorption of the porphyrins led to significant changes in their spectra but preserved the photoactivity of the dyes. The changes can be attributed to two phenomena: structural changes (flattening) of the porphyrin molecules and molecular aggregation. Latter one was more influenced by the molecular structure of the dyes and influenced the properties of mainly trans-isomer. The formation of the assemblies was significantly influenced by the layer charge of montmorillonite template. This study presents the example how one is able to influence the spectral properties and possibly also the functionality of adsorbed photoactive molecules via a strategy of appropriate combination of a photoactive component with specific inorganic templates.

Interactions between C.I. Basic Blue 41 and aluminosilicate sorbents

Four aluminosilicate sorbents (montmorillonite, bentonite, raw perlite, and expanded perlite) were employed for retention of the cationic dye C.I. Basic Blue 41. Interactions between the clay and the dyestuff were investigated at several temperatures and clay:dye ratios. The mechanism behind the adsorption involves the formation of H-aggregates of the dye on both clays, followed by dye migration into the interlayer in the case of montmorillonite. Time-dependent absorbance spectra revealed the presence of various dye species in montmorillonite. Introduction of the dye molecules into the interlamellar space occurs more rapidly in bentonite than in montmorillonite. The dye molecules inserted between the clay leaves adopt different orientations and, eventually, stack in layers at increased dye loadings for both montmorillonite and bentonite. Higher dye aggregates are then present as suggested by diffuse reflectance spectroscopy. Dye sorption on both raw and expanded perlite proceeds via H-aggregate formation as well.

Molecular Orientation of Rhodamine Dyes on Surfaces of Layered Silicates

Films of the layered silicates fluorohectorite (FH) and saponite (Sap) with various rhodamine dyes were prepared. The dyes with acidic as well as large hydrophobic groups in their molecule were not adsorbed on the surface of FH, which was interpreted in terms of high charge density on the surface of this silicate. All adsorbed dyes formed similar forms, such as isolated cations and H-type molecular aggregates, which were characterized by different spectral properties. Polarized ultraviolet-visible (UV-vis) spectroscopy was used for the characterization of the molecular orientation of dye chromophores on the silicate surface. The isolated dye cations and species, which absorbed light at the low energy part of the spectra, were only slightly tilted with respect to the plane of the silicate surface. The cations forming H-aggregates and absorbing light at low wavelengths were oriented in a nearly perpendicular fashion. The nearly perpendicular orientation was observed as a strong increase of dichroic ratio with film tilting. The orientation of the cations in H-aggregates depends partially on the structure of the dye molecule, namely, on the type of amino group (primary, secondary, or tertiary) in the dye molecule. The type of amino groups probably plays a role in the suitable orientation of dye cations for effective electrostatic interaction between the cations and the negatively charged siloxane surface. X-ray powder diffraction could not distinguish dye phases of dye monomers and molecular aggregates.

l-Tartaric acid assisted binary organogel system: strongly enhanced fluorescence induced by supramolecular assembly

A new series of binary organogels, which showed good gelated capability and strongly enhanced fluorescence emission, were designed and prepared by simply mixing two components of L-tartaric acid and alkoxyl substituted stilbazoles. It was found that L-tartaric acid could not only introduce stilbazole with fluorescent property into the gel system, but also provided a main motif for the formation of the gel-phase via multiple hydrogen bonds. Meanwhile, pi-pi interactions between the complexes also played a key role in the gel formation. The decreased nonradiative process, suggested by the longer fluorescent lifetime of aggregates than that of monomers and the presence of the J-aggregation for the aggregated state, favored the enhanced fluorescence emission.

Characterization of supported solid thin films of Laponite clay. Intercalation of rhodamine 6G laser dye

The morphology of thin films of Laponite (Lap) clay elaborated by the evaporation method and spin-coating technique was analyzed by atomic force microscopy and scanning electron microscopy, indicating a better quality film for the latter procedure. Rhodamine 6G (R6G) laser dye was intercalated into these films by ion exchange mechanism, performed by immersing the clay film into adequate dye solutions in which the effect of the dye concentration, immersion time, and nature of the solvent on the adsorption process were checked. The adsorption of R6G at the interlayer space of Lap was analyzed by the X-ray diffraction technique, and the presence of several R6G species (monomers and aggregates) was characterized by absorption and fluorescence spectroscopies. Less viscous solvents lead to higher dye loadings, suggesting a diffusional process for the intercalation of the dye in the interlayer spaces of Lap, and polar solvents favor the swellability of the interlayer space giving rise to a more homogeneous distribution of R6G molecules through the film and decreasing the dye aggregation. With the aging of the samples, the dye molecules can migrate through the interlayer spaces, leading to a more expanded distribution of R6G molecules and to the dye deaggregation.

Isomerization of cationic azobenzene derivatives in dispersions and films of layered silicates

Photoisomerization reactions of cationic azobenzene dyes in solutions, dispersions, and films of layered silicates were studied by visible (Vis) spectroscopy. The dyes isomerized reversibly from thermodynamically more stable trans-isomers to cis-isomers when irradiated with ultraviolet (UV) light. Observed trends were compared with the optical changes of the dyes that occurred as a consequence of their adsorption at the silicate surface. Small fractions of the dyes are likely to have isomerized during the adsorption process, even without the UV-light irradiation. The aggregation of the dyes was another reaction taking place at the surface of the silicates. The extent of the UV-light-induced isomerization reactions was reduced for the adsorbed dye cations. The reaction proceeded readily for a dye with monovalent cations. However, the photoisomerization was practically negligible in both dispersions and films of layered silicates for a dye with bivalent cations, whereas the isomerization proceeded in solution. This phenomenon was interpreted in terms of the attractive electrostatic forces between the substrate and the dye cations, which hindered the isomerization reaction. The layer charge of silicates affected the orientation of the dye cations as observed by X-ray diffraction (XRD) measurements. However, the choice of silicate did not significantly affect the fundamental aspects and the described basic trends of the UV-light-induced isomerization reaction.

Enhanced emission and its switching in fluorescent organic nanoparticles

A new class of organic nanoparticles (CN-MBE nanoparticles) with a mean diameter of ca. 30-40 nm, which exhibit a strongly enhanced fluorescence emission, were prepared by a simple reprecipitation method. CN-MBE (1-cyano-trans-1,2-bis-(4'-methylbiphenyl)ethylene) is very weakly fluorescent in solution, but the intensity is increased by almost 700 times in the nanoparticles. Enhanced emission in CN-MBE nanoparticles is attributed to the synergetic effect of intramolecular planarization and J-type aggregate formation (restricted excimer formation) in nanopaticles. On/off fluorescence switching for organic vapor was demonstrated with CN-MBE nanoparticles.

Influence of the layer charge and clay particle size on the interactions between the cationic dye methylene blue and clays in an aqueous suspension

The spectroscopic behavior of the dye MB in suspensions of different clays have been used for evaluating layer charge density influence on the adsorption properties of the particles. The clays with higher charge density, like SAz-1 and SCa-3, promote a higher aggregation and do not show deaggregation at longer times, so that practically only the aggregate peak at approximately 570 nm is observed, without any change with time. This is due to, on one side, the larger particle size that decreases the surface area available for adsorption. Additionally, the clay layers will be held together more tightly, avoiding the migration of the dye to the interlamellar region. On the other hand, SWy-1, having a lower charge density, shows a completely different behavior. The dye molecules, initially adsorbed as aggregates on the outer surface of the clay, deaggregate to form monomers that migrate to the interlamellar spaces, giving rise to absorption bands at 670 and 760 nm. Experiments using Ca-exchanged SWy-1, variation of the ionic strength by addition of salt, and the use of different size fractions of the clays confirm the finding that the main factor ruling the adsorption behavior of the probe is the size of the clay particles.