Spectroscopic characterization of phenazinium dye aggregates in water and acetonitrile media: effect of methyl substitution on the aggregation phenomenon

Absorption, fluorescence, and fluorescence excitation spectral studies of two planar, cationic phenazinium dyes, namely, phenosafranin (PSF) and safranin-T (ST), have been performed in protic and aprotic polar solvents. The studies reveal the formation of both J- and H-aggregates in concentrated solutions. The planarity of the phenazinium skeleton and the presence of a positive charge are attributed to be the driving force for this aggregation behavior. The aggregates are established to be dimers only. The positive inductive effect of the methyl substituents in safranin-T augments the aggregation process. The experiments reveal that for both dyes, the polar protic solvent favors the aggregation process more than the aprotic solvent.     

Photophysical behavior of lipophilic xanthene dyes without the involvement of photoinduced electron transfer mechanism

The correlation of dibutyl-ether-ester of xanthene dye structures with their photophysical properties is discussed with respect to their capability as fluorescent probes based on ultraviolet–visible absorption, fluorescence spectra and fluorescence lifetimes measured in different solvents. It was found that the dibutyl-ether-ester of fluorescein is very weakly emissive in aprotic solvents, but fairly strong fluorescent in alcohols. The dependence of fluorescence quantum yield (Φ_f) and lifetime (τ_f) on solvent polarity suggests non-involvement of the intra-molecular photoinduced electron transfer (PeT) mechanism, suggested previously to account for the emission efficiency of fluorescein derivatives. The xanthene dyes intend to self-assemble in aprotic solvents, less polar solvents facilitate the aggregation while hydrogen bonding disfavor it. The formation of non-emissive H-aggregates is proposed to be responsible for their fluorescent behavior. The esterification showed stronger influences on the photophysics than the etherification, i.e. the former caused larger reduction of Φ_f owing to the internal conversion. The halogenation decreases the fluorescence quantum yield and lifetime of the xanthene dyes, owing to the enhancement of inter-system crossing process.     

Synthesis and Characterization of Chitosan‐Based Polyelectrolyte Complexes Doped with Xanthene Dyes

Biocompatible chitosan‐based polyelectrolyte complexes (PECs) doped with xanthene dyes (fluorescein, eosin Y, erythrosin B, rhodamine 6G) were synthesized and characterized by scanning electron microscopy, dynamic light scattering, zeta potential measurements, and absorption and luminescence (including polarized, time‐resolved, and phosphorescence) spectroscopy. The results are discussed in terms of the mechanism and rigidity of dye–PEC binding, the heavy‐atom effect in dyes and PEC stability. Eosin Y is found to be the optimal dopant, providing both a high dye content in PECs and a high quantum yield of fluorescence.     

ISOLATION OF UNIQUE DIMER SPECTRA OF DYES FROM THE COMPOSITE SPECTRA OF AGGREGATED SOLUTIONS

Abstract— Aggregation of fluorescein, eosin and rose bengal in aqueous solutions has been studied. Mathematical expressions are derived to calculate the monomer mole fractions in moderately and highly concentrated solutions. The average values of the dimer dissociation constant, K_d , for fluorescein, eosin and rose bengal in aqueous solutions at pH 12 are 0.20, 9.0×10^-3 and 40 × 10^-3 moles/l respectively. A method is developed here to isolate the dimer spectrum from the composite spectrum. The uniqueness of this method is established by comparison with other known methods. The dimer spectra of all the three dyes are reported.     

Ultrafast excited-state dynamics of eosin B: a potential probe of the hydrogen-bonding properties of the environment

The photophysics of two dyes from the xanthene family, eosin B (EB), and eosin Y (EY) has been investigated in various solvents by femtosecond transient absorption spectroscopy, first, to clarify the huge disparity of the EB fluorescence lifetimes reported in literature, and, second, to understand the mechanism responsible for the ultrafast excited-state deactivation of EB in water. The excited-state lifetime of EB was found to be much shorter in water and in other protic solvents, due to the occurrence of hydrogen-bond assisted nonradiative deactivation. This mechanism is associated with the hydrogen bonds between the solvent molecules and the nitro groups of EB, which become stronger upon optical excitation due to the charge-transfer character of the excited-state. This process is not operative with EY, where the nitro groups are replaced by bromine atoms. Therefore, the excited-state lifetime of EB in solution is directly related to the strength of the solvent as a hydrogen-bond donor, offering the possibility to build a corresponding scale based on the fluorescence quantum yield or lifetime of EB. This scale of hydrogen-bonding strength could be especially useful for studies of liquid interfaces by time-resolved surface second harmonic generation.     

The effect of dyes on photo-induced chemiluminescence emission from polymers

Weak photo-induced chemiluminescence (PICL) emission is observed when polymers are exposed to UVA or visible light. The presence of dyes can either increase PICL intensity via Type I photosensitisation which generates polymer free radicals, or reduce it via photo-protection. PICL studies on the eight Blue Wool Standards (BWSs) that are used commercially as lightfastness standards show higher PICL intensity from the least photostable BWSs that use triphenylmethane dyes and lower intensity from more photostable BWSs using UVA and visible wavelengths. The relative PICL intensities do not correlate in a stepwise manner with lightfastness ratings of the BWSs. However dye/polymer combinations that emit high levels of PICL relative to the undyed material are unlikely to have acceptable lightfastness. The xanthene dyes fluorescein and eosin Y are more strongly photosensitising than triphenylmethane dyes on wool and both produce higher PICL emission than undyed wool when irradiated with visible light.     

Solvent and substituent effects on aggregation constants of perylene bisimide π-stacks--a linear free energy relationship analysis

A series of six perylene bisimides (PBIs) with hydrophilic and hydrophobic side chains at the imide nitrogens were applied for a comparative study of the solvent and structural effects on the aggregation behaviour of this class of dyes. A comparison of the binding constants in tetrachloromethane at room temperature revealed the highest binding constant of about 10(5) M(-1) for a PBI bearing 3,4,5-tridodecyloxyphenyl substituents at the imide nitrogens, followed by 3,4,5-tridodecylphenyl and alkyl-substituted PBIs, whereas no aggregation could be observed in the accessible concentration range for PBIs equipped with bulky 2,6-diisopropylphenyl substituents at the imide nitrogens. The aggregation behaviour of three properly soluble compounds was investigated in 17 different solvents covering a broad polarity range from nonpolar n-hexane to highly polar DMSO and water. Linear free energy relationships (LFER) revealed a biphasic behaviour between Gibbs free energies of aggregation and common empirical solvent polarity scales indicating particularly strong π-π stacking interactions in nonpolar aliphatic and polar alcoholic solvents whilst the weakest binding is observed in dichloromethane and chloroform. Accordingly, PBI aggregation is dominated by electrostatic interactions in nonpolar solvents and by solvophobic interactions in protic solvents. In water, the aggregation constant is increased far beyond LFER expectations pointing at a pronounced hydrophobic effect.     

Synthesis and Optical Properties of Water‐Soluble Polyglycerol‐Dendronized Rylene Bisimide Dyes

Four new water‐soluble polyglycerol‐dendronized perylene, terrylene, and quaterrylene bisimides have been synthesized and characterized with respect to their optical properties in polar organic solvents and water by using UV/Vis and fluorescence spectroscopy. All of these dyes were highly soluble in water, but the size of the chosen polyglycerol dendron was only sufficient to completely suppress dye aggregation for the core‐unsubstituted perylene derivative. Their high solubility in water and their absorption and emission wavelengths up to the NIR region make the core‐unsubstituted perylene and terrylene bisimides ideal candidates for applications in bioimaging, whilst the lack of fluorescence for quaterrylene bisimide in all polar solvents does not warrant further investigation of this chromophore in fluorescence and imaging applications. Likewise, tuning of the emission of rylene bisimides towards longer wavelengths by employing electron‐donating bay substituents is not a promising strategy, owing to the lower fluorescence quantum yields in polar solvents and, in particular, in water.     

Deeply colored anionic polymethine dyes derived from bis(2,2,3,3,4,4,5,5-octafluoropentyl) 9 H-fluorene-2,7-disulfonate

Anionic symmetric polymethine dyes have been synthesized based on bis(2,2,3,3,4,4,5,5-octafluoropentyl) 9H-fluorene-2,7-disulfonate, which exhibit intensive absorption in the near IR region of the spectrum: pentamethine with an open polymethine chain (⋉_max = 790 nm) and nonamethine with six- and five-membered saturated rings at δ-positions of the chain (@#@ ⋉_max = 1044 and 1086 nm, respectively). To make comparisons, anionic dyes with analogous structure of the polymethine chain, viz., 1,3-dimethylbarbituric and 1,3-diethylthiobarbituric acid derivatives, have been also synthesized. Quantum chemical analysis of the electron structure of the dyes has been performed. Their solvatochromism in polar solvents depends weakly on electrophilicity and nucleophilicity of the medium and is primarily determined by universal interactions. This is the reason that they, in contrast to cationic dyes, retain a high selectivity of absorption (sharp absorption bands) and universal contour of electronic bands in strong polar solvents even with the long polymethine chain. Special solvatochromic effects of the anionic dyes in weakly polar media have been revealed and interpreted. The role of a counterion in these effects has been analyzed.     

荧光素染料与富马酸酮替芬的光谱研究及应用

在弱酸性缓冲介质中,富马酸酮替芬和某些卤代荧光素类染料(曙红Y、荧光桃红和曙红B)借助静电引力和疏水作用力形成离子缔合物,引起共振光散射光谱、吸收光谱及荧光光谱的变化。实验表明,曙红Y体系灵敏度最高。对曙红Y、荧光桃红和曙红B体系,线性范围分别为0.12~8.4μg/mL、0.24~8.4μg/mL和0.18~6.0μg/mL,检测限分别为12.72 ng/mL、12.52μg/mL和18.21μg/mL。方法已用于分析富马酸酮替芬片剂、血清及尿样。     

Xanthene dye functionalized norbornenes for the use in ring opening metathesis polymerization

A convenient synthetic route towards polymerizable fluorescein, dicholorofluorescein, and eosin dyes is presented. Polymerizability was provided by linking 2‐norbornene carboxylic acid, 11‐bromo‐undecyl ester to the dye's carboxylate functionality. Although the monomers bearing dichlorofluorescein and eosin were obtained in high yield, the related fluorescein bearing monomer could only be obtained in low yield. In the latter case, concurring etherification and esterification led to a product mixture of the desired carboxy modified monomer and a double substituted by‐product. The dye‐monomers were used successfully for the preparation of statistical copolymers with endo,exo‐2,3‐norbornene dicarboxylic acid dimethylester by ROMP. Absorption and luminescence characteristics and, in particular, the acid/base sensitive behavior of the parent dyes were preserved in the monomers and copolymers. The absorption and emission maxima in THF solution and in the solid state were red shifted in comparison to the aqueous samples of the parent dyes. Dye‐copolymers exhibited good film forming properties. Solid state luminescence studies of the copolymers revealed an increasing sensitivity towards NEt₃ vapor in the order fluorescein < dichlorofluorescein < eosin bearing copolymer. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 1336–1348, 2007     

Bis(merocyanine) Homo-Folda-Dimers: Evaluation of Electronic and Spectral Changes in Well-Defined Dye Aggregate Geometries

A series of three bis(merocyanine) dyes comprising chromophores of different conjugation lengths has been synthesized and the intramolecular aggregation process was investigated by UV/Vis absorption spectroscopy. The spectral changes observed upon variation of the solvent polarity reveal a folding process resulting in a cofacial π-stack of two chromophores with a decrease of the aggregation tendency with increasing chromophore length and solvent polarity. Solvent-dependent UV/Vis studies of the monomeric reference dyes show a significant increase of the polyene-like character for dyes with longer polymethine chains in nonpolar solvents, which is reversed upon aggregation due to the polarizability effect of the adjacent chromophore within the dye stack. The pronounced hypsochromic shift of the absorption band observed upon aggregation indicates strong coupling of the dyes’ transition dipole moments, which was confirmed by quantum-chemical analysis.     

Competitive Self and Induced Aggregation of Calix[4]arene Ethers and Their Interaction with Pinacyanol Chloride and Methylene Blue in Nonaqueous Media

Long chain calix[4]arene ethers have been examined for aggregation in nonaqueous solvents by using UV-vis molecular absorbance spectroscopy. It has been observed that tetraalkylated (alkyl = hexadecyl and octadecyl, respectively) calix[4]arene ethers tend to aggregate in chloroform and tetrahydrofuran, possibly via π–π stacking interactions of the phenyl moieties, and the aggregation process appears to be facilitated by the alkyl chains. The analogous dialkylated compounds do not show any self-aggregation, plausibly due to strong hydrogen bonding between the –OH and the –O– of calix aryl ether which seems to disrupt the aggregation process. Addition of the anionic surfactant sodium dodecylsulfate (SDS) appears to hinder the aggregation process in nonpolar chloroform but the same surfactant facilitates aggregation in the polar tetrahydrofuran. The cationic surfactant (cetyltrimethyl ammonium bromide) and the nonionic surfactant (Brij-35) have no effect on this aggregation process. Unexpectedly, SDS induces aggregation of dialkylated calix[4]arene ethers in chloroform. It has been observed that the aggregated form of the tetraalkylated calix[4]arene ethers tend to increase the dimerization efficiency of cationic dyes (pinacyanol chloride and methylene blue) in chloroform.     

The absorption and structure of fluorescein and its ethyl derivatives in various solutions

Ethyl fluorescein and diethyl fluorescein were synthesized. Their absorption spectra in different pH solutions and in organic solvents with different polarity were measured. These were systematically compared with those of fluorescein, and the forms of these dyes in various media were reasonably assigned.     

Adsorption Characteristics of Activated Carbon for the Reclamation of Eosin Y and Indigo Carmine Colored Effluents and New Isotherm Model

The adsorption response of eosin Y and indigo carmine acid dyes on activated carbon as a function of system temperature for a fixed concentration was investigated at various temperatures via adsorption isotherms and their thermodynamic quantities such as enthalpy, entropy, and Gibbs free energy changes. The adsorption data were exploited to develop a new adsorption isotherm. The new isotherm was developed with the spirit of solid–liquid phase equilibrium and regular solution theory. The proposed model has four adjustable constants and correlates adsorption isotherm in terms of the system temperature and melting temperature of the dye. The effect of pH on the removal of acid dyes was reported. The pH variation was observed to affect the adsorption efficiency. The removal of eosin Y and indigo carmine decreased from 99.4% to 82.6% and 92.38% to 79.48%, respectively, when the pH of the solution varied from 2 to 12. The thermodynamic analysis of the process reveals that the process of the removal of acid dyes is exothermic and spontaneous. Moreover, the kinetics parameters of the batch process are reported.     

Adsorption of xanthene dyes by lysozyme crystals

Adsorption characteristics of cross-linked lysozyme crystals of different morphologies (tetragonal, orthorhombic, triclinic and monoclinic) were examined using four anionic dyes (fluorescein, eosin, erythrosin, and rose bengal), one zwitterionic dye (rhodamine B), and one cationic dye (rhodamine 6G). The adsorption isotherms were of the Langmuir type for all examined systems with the exception of rhodamine B adsorption by monoclinic crystals. The weakest adsorption was observed for the cationic dye, rhodamine B, whereas dianionic dyes, eosin, rose bengal, and erythrosin were strongly adsorbed on the protein surface. The adsorption capacities of the crystals for the dyes were found to depend on both charge and hydrophobicity of the dye, reflecting the heterogeneous character of the lysozyme pore surface. The adsorption affinity of the crystals for the dyes was a function of the dyes' hydrophobicity. Furthermore, the crystal morphology was identified as an additional factor determining capacity and affinity for dye adsorption. Differences between crystals prepared in the presence of the same precipitant were lower than between morphologies prepared with different precipitants.     

Influence of solvent and substituent on excited state characteristics of laser grade coumarin dyes

Absorption and fluorescence emission of 4 and 7 substituted coumarins viz. C 440, C 490, C 485 and C 311 have been studied in various polar and non-polar organic solvents. These coumarin dyes are substituted with alkyl, amine and fluorine groups at 4- and 7-positions. They give different absorption and emission spectra in different solvents. The study leads to a possible assignment of energy level scheme for such coumarins including the effect on ground state and excited state dipole moments due to substitutions. Excited state dipole moments of these dyes are calculated by solvetochromic data experimentally and theoretically these are calculated by PM 3 method. The dipole moments in excited state, for all molecules investigated here, are higher than the corresponding values in the ground state. The increase in dipole moment has been explained in terms of the nature of excited state and resonance structure.     

Metal ion doped polymers as media for optical memories

Polymer materials are making an impact on optical storage technology to develop high information density and fast access type memories with a high read-out efficiency. The principle and advantages of three-dimensional (3-D) data storage in the form of interference patterns (holograms) have been outlined. Three different information storage materials have been developed by doping metal ions such as Cr(VI) and Fe(III) in water-soluble polymers, namely poly(vinyl alcohol) (PVA) and poly(acrylic acid) (PAA), which need no further thermal or chemical treatment. Volume transmission holograms have been recorded in dichromated poly(vinyl alcohol) (DCPVA), ferric chloride doped poly(vinyl alcohol) (FePVA) and dichromated poly(acrylic acid) (DCPAA), with and without an electron donor (dimethyl formamide), and xanthene dyes (fluorescein, eosin Y and Rose Bengal). Different parameters influencing the holographic performance have been optimized to achieve a high real-time diffraction efficiency (～70%). An electron transfer process from the polymer matrix to Cr(VI)/Fe(III), leading to the photocrosslinking of the polymer in the form of an interference pattern, has been suggested as the mechanism of information storage (hologram recording) in these materials.     

Flourescence self-quenching of Halofluorescein dyes

The fluorescence quenching of halogenated derivatives (eosin and erythrosin) of fluorescein in aqueous solution was studied over a wide concentration range (5 × 10⁻⁶ – 10⁻¹ mol dm⁻³ where aggregates are formed. The rate constants of the quenching produced by the dimers and trimers of the dyes were determined at various temperatures and were compared with those of fluorescein quenching reported previously. The quenching mechanisms are also discussed.     

Effect of halogenated fluorescent compounds on bioluminescent reactions

The paper investigates an application of luminescent bioassays to monitor the toxicity of organic halides. Effects of xanthene dyes (fluorescein, eosin Y, and erythrosin B), used as model compounds, on bioluminescent reactions of firefly Luciola mingrelica, marine bacteria Photobacterium leiognathi, and hydroid polyp Obelia longissima were studied. Dependence of bioluminescence quenching constants on the atomic weight of halogen substituents in dye molecules was demonstrated. Bacterial bioluminescence was shown to be most sensitive to heavy halogen atoms involved in molecular structure; hence, it is suitable for construction of sensors to monitor toxicity of halogenated compounds. Mechanisms of bioluminescence quenching—energy transfer processes, collisional interactions, and enzyme–dye binding—were considered. Changes of bioluminescence (BL) spectra in the presence of the dyes were analyzed. Interactions of the dyes with enzymes were studied using fluorescence characteristics of the dyes in steady-state and time-resolved experiments. The dependences of fluorescence anisotropy of enzyme-bound dyes, the average fluorescence lifetime, and the number of exponential components in fluorescence decay on the atomic weight of halogen substituents were demonstrated. The results are discussed in terms of “dark effect of heavy halogen atom” in the process of enzyme–dye binding; hydrophobic interactions were assumed to be responsible for the effect.