Absorption, fluorescence, and acid-base equilibria of rhodamines in micellar media of sodium dodecyl sulfate.

Rhodamine dyes are widely used as molecular probes in different fields of science. The aim of this paper was to ascertain to what extent the structural peculiarities of the compounds influence their absorption, emission, and acid-base properties under unified conditions. The acid-base dissociation (HR(+)⇄R+H(+)) of a series of rhodamine dyes was studied in sodium n-dodecylsulfate micellar solutions. In this media, the form R exists as a zwitterion R(±). The indices of apparent ionization constants of fifteen rhodamine cations HR(+) with different substituents in the xanthene moiety vary within the range of pKa(app)=5.04 to 5.53. The distinct dependence of emission of rhodamines bound to micelles on pH of bulk water opens the possibility of using them as fluorescent interfacial acid-base indicators.

Fluorescence of aminofluoresceins as an indicative process allowing one to distinguish between micelles of cationic surfactants and micelle-like aggregates.

Among the vast set of fluorescein derivatives, the double charged R2- anions of aminofluoresceins are known to exhibit only low quantum yields of fluorescence, [Formula: see text]. The [Formula: see text] value becomes as high as that of the fluorescein dianion when the lone electron pair of the amino group is involved in a covalent bond. According to Munkholm et al (1990 J. Am. Chem. Soc. 112 2608-12), a much smaller increase in the emission intensity can be observed in the presence of surfactant micelles. However, all these observations refer to aqueous or alcoholic solvents. In this paper, we show that in the non-hydrogen bond donor (or 'aprotic') solvents DMSO and acetone, the quantum yields, φ, of the 4'- (or 5')-aminofluorescein R2- species amount to 61-67% and approach that of fluorescein (φ = 87%), whereas in water φ is only 0.6-0.8%. In glycerol, a solvent with an extremely high viscosity, the φ value is only 6-10%. We report on the enhancement of the fluorescence of the aminofluorescein dianions as an indicative process, which allows us to distinguish between the micelle-like aggregates of cationic dendrimers of low generation, common spherical surfactant micelles, and surfactant bilayers. Some of these colloidal aggregates partly restore the fluorescence of aminofluoresceins in aqueous media. By contrast, other positively charged micellar-like aggregates do not enhance the quantum yield of aminofluorescein R2- species. Results for several related systems, such as CTAB-coated SiO2 particles and reverse microemulsions, are briefly described, and the possible reasons for the observed phenomena are discussed.

Mercury ions complexation with a series of heterocyclic derivatives of 3-hydroxychromone: spectral effects and prospects for ultrasensitive Hg2+ probing.

Complexation of three 3-hydroxychromone derivatives bearing a nitrogen-containing heterocyclic moiety in the position 2 of the chromone bicycle - benzimidazole, quinoline, and 2,5-diphenyloxazole, with mercury(II) ions is reported. Formation of chelate complexes with the metal cations coordinated with the cavity formed by 3-OH and 4-C═O groups was shown, as well as the possibility of side moiety heteroatom participation in binding of metal ions. High sensitivity to mercury of 2,5-diphenyloxazole-substituted 3-hydroxychromone was elucidated, allowing to detect Hg(2+) below the maximum permissible concentration for drinking water. This makes the above-mentioned compound a prospective basis for development of sensors for ultralow mercury concentration detection in water. Unusual fluorescence ignition of 2-(quinolin-2-yl)-3-hydroxychromone at low Hg(2+) concentrations, rarely observed for heavy metals ions complexation with organic fluorescent ligands, was discussed.

Partitioning of 2,6-Bis(1H-Benzimidazol-2-yl)pyridine fluorophore into a phospholipid bilayer: complementary use of fluorescence quenching studies and molecular dynamics simulations.

Successful use of fluorescence sensing in elucidating the biophysical properties of lipid membranes requires knowledge of the distribution and location of an emitting molecule in the bilayer. We report here that 2,6-bis(1H-benzimidazol-2-yl)pyridine (BBP), which is almost non-fluorescent in aqueous solutions, reveals a strong emission enhancement in a hydrophobic environment of a phospholipid bilayer, making it interesting for fluorescence probing of water content in a lipid membrane. Comparing the fluorescence behavior of BBP in a wide variety of solvents with those in phospholipid vesicles, we suggest that the hydrogen bonding interactions between a BBP fluorophore and water molecules play a crucial role in the observed "light switch effect". Therefore, the loss of water-induced fluorescence quenching inside a membrane are thought to be due to deep penetration of BBP into the hydrophobic, water-free region of a bilayer. Characterized by strong quenching by transition metal ions in solution, BBP also demonstrated significant shielding from the action of the quencher in the presence of phospholipid vesicles. We used the increase in fluorescence intensity, measured upon titration of probe molecules with lipid vesicles, to estimate the partition constant and the Gibbs free energy (ΔG) of transfer of BBP from aqueous buffer into a membrane. Partitioning BBP revealed strongly favorable ΔG, which depends only slightly on the lipid composition of a bilayer, varying in a range from -6.5 to -7.0kcal/mol. To elucidate the binding interactions of the probe with a membrane on the molecular level, a distribution and favorable location of BBP in a POPC bilayer were modeled via atomistic molecular dynamics (MD) simulations using two different approaches: (i) free, diffusion-driven partitioning of the probe molecules into a bilayer and (ii) constrained umbrella sampling of a penetration profile of the dye molecule across a bilayer. Both of these MD approaches agreed with regard to the preferred location of a BBP fluorophore within the interfacial region of a bilayer, located between the hydrocarbon acyl tails and the initial portion of the lipid headgroups. MD simulations also revealed restricted permeability of water molecules into this region of a POPC bilayer, determining the strong fluorescence enhancement observed experimentally for the membrane-partitioned form of BBP.

5-(4-Fluoro-phen-yl)-2-[2-(5-phenyl-1,3-oxazol-2-yl)phen-yl]-1,3-oxazole.

In the title compound, C(24)H(15)FN(2)O(2), the dihedral angles between the central benzene ring and the oxazole rings are 10.7 (6) and 64.1 (5)°. The dihedral angles between the oxazole rings and their pendant rings are 2.0 (3) and 24.3 (2)°. The F atoms are disordered over two sites with occupancies of 0.627 (3) and 0.373 (3) in the phenyl-ene-oxazol-yl-phenyl and in oxazol-yl-phenyl fragments, respectively. In the crystal structure, mol-ecules are linked through a network of C-H⋯F and weak π-π stacking inter-actions.

Heterogeneous wide range pH-sensing materials allowing ratiometric fluorescence detection based on structurally rigid analogs of 2,6-distyrylpyridine.

The new sensing materials based on the microsized silica gel powder with non-covalently immobilized structurally rigid analogs of 2,6-distyrylpyrydine ((3E,5E)-3,5-dibenzylidene-8-phenyl-1,2,3,5,6,7-hexahydrodicyclopenta[b,e]pyridines) were developed and tested. Most of the investigated compositions demonstrate linear ratiometric fluorescence response on pH in the physiologically important interval (pH 6-9). The compound with the greatest number of protolytic centers within the studied series demonstrated the widest pH sensitivity range, however in this case the analytical signal was the lowest. The prospects for the practical application of the investigated materials in the fiber optics sensing devices were outlined.

Tautomerism of acridin-9-amines substituted at the exocyclic nitrogen atom: spectroscopic investigations and theoretical studies.

In the ground electronic state, the first two (1 and 2) of the compounds investigated--9-(methoxyamino)acridine (1), 9-hydrazinoacridine (2), N-(2-chloroethyl)acridin-9-amine (3) and N-(5-methylpyridin-2-yl)acridin-9-amine (4)--exist principally in the imino tautomeric form, while the other two (3 and 4) can coexist as amino and imino tautomers in solvents of various polarities and abilities to enter into specific interactions. These features of the molecules are reflected in the experimental absorption spectra and the predicted thermodynamic and spectral data. The predicted thermodynamic characteristics suggest that in the S1 state, the imino tautomers of 1, 2 and 4 and the amino tautomer of 3 are more stable than their tautomeric counterparts. However, the predicted rates of intersystem crossing suggest that the imino tautomers of 1-3 and the amino tautomer of 4 lose excitation energy very rapidly, so that only their counterpart tautomers in fact emit radiation. This explains why 1 and 2 do not fluoresce and why the amino form of 3 and the imino form of 4 are the emitters. 3 and 4 thus represent acridin-9-amines whose imino forms are preferred in the ground state, but whose respective amino and imino forms are preferred in the excited state. Because 3 and 4 are capable of tautomeric transformations in the ground and excited states, and also enter into specific interactions with solvents, these compounds could be potent spectral indicators or probes of environmental properties.

Synthesis of water-soluble, ring-substituted squaraine dyes and their evaluation as fluorescent probes and labels.

A series of ring-substituted squaraines absorbing and emitting in the red and NIR spectral region was synthesized and their spectral and photophysical properties (quantum yields, fluorescence lifetimes) and photostabilities were measured and compared to Cy5, a commonly used fluorescent label. The absorption maxima in aqueous media were found to be between 628 and 667 nm and the emission maxima are between 642 and 685 nm. Squaraine dyes exhibit high extinction coefficients (163,000-265,000 M(-1) cm(-1)) and lower quantum yields (2-7%) in aqueous buffer but high quantum yields (up to 45%) and long fluorescence lifetimes (up to 3.3 ns) in presence of BSA. Dicyanomethylene- and thio-substituted squaraines exhibit an additional absorption around 400 nm with extinction coefficients between 21,500 and 44,500 M(-1) cm(-1). These dyes are excitable not only with red but also with blue diode lasers or light emitting diodes. Due to the favourable spectral and photophysical properties these dyes can be used as fluorescent probes and labels for intensity- and fluorescence lifetime-based biomedical applications.

Radiationless deactivation of the excited phototautomer form and molecular structure of ESIPT-compounds.

Correlation between the character of electronic density redistribution in the excited normal and phototautomer forms, which are involved in the excited state intramolecular proton transfer (ESIPT) reaction, and the rates of primary photoprocesses--proton phototransfer and radiationless deactivation of the phototautomer form--was analysed on the qualitative level. It was revealed that noticeable improvement of fluorescence properties of ESIPT molecules could be achieved by directed modification of their chemical structure, most importantly by the introduction of electron withdrawing substituents into their proton donor moieties. Several facts are presented in favour of the last statement.