Aromatic guest inclusion by a tripodal ligand: Fluorescence and structural studies

The newly synthesized simple tripodal ligand tris-[2-(naphthalen-2-yloxy)-ethyl]-amine (L₁) act as a fluorescence signaling system for aromatic guest. It forms inclusion complexes with several electron deficient aromatic compounds. This inclusion phenomenon has been studied by steady-state fluorescence spectroscopy and solid-state structural analysis. Electron-rich L₁ shows dramatic color change and a concomitant quenching of luminescence in solution as well as solid phase when titrated with several other electron deficient aromatic guest molecules. Rather high selectivity towards the picric acid was observed. L₁ simultaneously forms inclusion complex and organic salt co-crystal with the composition [(L₁H⁺) (Pic⁻)]  PicH (PicH = picric acid) when crystallized in the presence of picric acid. In the solid state, it forms a strong π–π, C–Hπ and C–HO type interactions.     

Low-lying excited singlet states and S2→S0 luminescence of dipyrido[3,4-b: 2,3-d]phenazine

The absorption, fluorescence and excitation fluorescence spectra dipyrido[3,4-b:2,3-d]-phenazine (DPPZ1) have been measured in non-polar and polar matrices at room temperature, and were taken into account to explain the origin of the relatively weak emission of this molecule in both type of environment. The electronic structure of DPPZ1 was calculated using a modified INDO CI method. The geometry optimization has been performed using the MNDO method. According to the spectra and the results of calculations, the lowest excited singlet state S₁ of DPPZ1 molecule is of n,π^*-type and the next one, S₂ state, is of π,π^*-type. The energy gap ΔE_calc is equal 4770 cm⁻¹. The low efficiency of the emission observed in the hydroxylic solvent can be interpreted in terms of thermal quenching of the π,π^*-type fluorescence. However, experimental results obtained suggest that in nonpolar solvents the emission of the molecule examined is an anomalous S₂→S₀ fluorescence.     

S2 → S0 fluorescence in an aromatic thioketone, xanthione

In addition to the red phosphorescence (T₁(³ A₂n, π^*) → S₀) xanthione exhibits in solution an emission with a maximum at ≈ 23 000 cm⁻¹ and φ_f(298°) = 5 × 10⁻³. It is shown that this emission is fluorescence from the second excited singlet state (S₂ (¹A₁ π, π^*) → S₀).     

Enantioselective recognition of mandelic acid by a 3,6-dithiophen-2-yl-9H-carbazole-based chiral fluorescent bisboronic acid sensor

We have prepared chiral fluorescent bisboronic acid sensors with 3,6-dithiophen-2-yl-9H-carbazole as the fluorophore. The thiophene moiety was used to extend the π-conjugation framework of the fluorophore in order to red-shift the fluorescence emission and, at the same time, to enhance the novel process where the fluorophore serves as the electron donor of the photoinduced electron transfer process (d-PET) of the boronic acid sensors; i.e., the background fluorescence of the sensor 1 at acidic pH is weaker compared to that at neutral or basic pH, in stark contrast to the typical a-PET boronic acid sensors (where the fluorophore serves as the electron acceptor of the photoinduced electron transfer process). The benefit of the d-PET boronic acid sensors is that the recognition of the hydroxylic acids can be achieved at acidic pH. We found that the thiophene moiety is an efficient π-conjugation linker and electron donor; as a result, the d-PET contrast ratio of the sensors upon variation of the pH is improved 10-fold when compared to the previously reported d-PET sensors without the thiophene moiety. Enantioselective recognition of tartaric acid was achieved at acid pH, and the enantioselectivity (total response K(D)I(F)(D)/K(L)I(F)(L)) is 3.3. The fluorescence enhancement (I(F)(Sample)/I(F)(Blank)) of sensor 1 upon binding with tartaric acid is 3.5-fold at pH 3.0. With the fluorescent bisboronic acid sensor 1, enantioselective recognition of mandelic acid was achieved for the first time. To the best of our knowledge, this is the first time that the mandelic acid has been enantioselectively recognized using a chiral fluorescent boronic acid sensor. Chiral monoboronic acid sensor 2 and bisboronic acid sensor 3 without the thiophene moiety failed to enantioselectively recognize mandelic acid. Our findings with the thiophene-incorporated boronic acid sensors will be important for the design of d-PET fluorescent sensors for the enantioselective recognition of α-hydroxylic acids such as mandelic acid, given that it is currently a challenge to recognize these analytes with boronic acid fluorescent molecular sensors.     

A new boronic acid fluorescent reporter that changes emission intensities at three wavelengths upon sugar binding

The boronic acid moiety is a very useful functional group for the preparation of sugar sensors. Along this line, water-soluble boronic acids that change fluorescent properties upon sugar binding are especially useful as reporter units in fluorescent sensors for carbohydrates. Herein, we report the discovery of a new water-soluble boronic acid (1, dibenzofuran-4-boronic acid) that exhibits unique fluorescence changes at three wavelengths upon binding with sugars under near physiological conditions.     

Structure cristalline et conformation en solution aqueuse de la carnosine (β-alanyl-L-histidine)

Carnosine (β-alanyl-L-histidine) is a biologically active molecule involved in muscular metabolism. It crystallises in the C; space group with a = 24.725 Å b = 5,427 Å c = 8,004 Å β = 100,2° (Z = 4)

In the crystal, acid and basic groups are engaged in hydrogen bonds whose strength is evaluated through IR frequencies. Molecular conformation in the solid state is defined by τ₁ = /t-177° τ₂ = −38° φ = −96° ψ = +131° χ₁ = 181° χ₂₁ = 62°

NMR study of carnosine in aqueous solution indicates that rotation about CH₂-CH₂ is free and that the other angles take the following values: Ø −150° or −90° and X₁ = 165° or 315°. Infrared and Raman spectra suggest that τ₂ undergoes small changes when going from crystal to solution while ψ is close to +150°.     

Properties of two-component Langmuir monolayer of single chain perfluorinated carboxylic acids with dipalmitoylphosphatidylcholine (DPPC)

The surface pressure (π)– and the surface potential (ΔV)–area (A) isotherms were obtained for two-component monolayers of four different perfluorocarboxylic acids (FCns; perfluorododecanoic acid: FC12, perfluorotetradecanoic acid: FC14, perfluorohexadecanoic acid: FC16, perfluorooctadecanoic acid: FC18) with dipalmitoylphosphatidylcholine (DPPC) on substrate solution of 0.15 M NaCl (pH 2.0) at 298.2 K as a function of compositions in the mixtures by employing the Wilhelmy method, the ionizing electrode method, the fluorescence microscopy, and the atomic force microscopy. The data for the two-component monolayers on these systems were analyzed in terms of the additivity rule. Assuming a regular surface mixture, the Joos equation which allows one to describe the collapse pressure of a two-component monolayer with miscible components was used to declare the miscibility of the monolayer state, and an interaction parameter and an interaction energy were calculated. The new finding was that FCns and DPPC are miscible or immiscible depending on chain length increment of fluorocarbon part. That is, FC12/DPPC monolayer was perfectly miscible, and FC14/DPPC, and FC16/DPPC (0 ≤ X_FC16 ≤ 0.3) monolayers were partially miscible. While FC16/DPPC (0.3 < X_FC16 < 1) and FC18/DPPC systems are immiscible in the monolayer state. Furthermore, the mean molecular area, the surface dipole moment, and the phase diagrams enabled us to estimate the molecular orientation of four different perfluorocarboxylic acids/DPPC in the two-component monolayer state. One type of phase diagrams was obtained and classified into the positive azeotropic type. The miscibility of FCns and DPPC in the monolayer was also supported by fluorescence microscopy and atomic force microscopy. FC12/DPPC, FC14/DPPC and FC16/DPPC (0 ≤ X_FC16 ≤ 0.3) two-component monolayers on 0.15 M NaCl (pH 2) showed that FC12, FC14 and FC16 (0 ≤ X_FC16 ≤ 0.3) can dissolve or partially dissolve the ordered solid DPPC domains formed upon compression. This indicates that these fluorinated amphiphiles soften or harden the lipid depending on their chain length.     

Luminescent properties of carbon-rich starburst gold(I) acetylide complexes. Crystal structure of [TEE][Au(PCy3)]4 ([TEE]H4=tetraethynylethene)

Two carbon-rich starburst gold(I) acetylide complexes [TEE][Au(PCy₃)]₄ (3, [TEE]H₄=tetraethynylethene) and [TEB][Au(PCy₃)]₃ (6, [TEB]H₃=1,3,5-triethynylbenzene) were prepared and their UV–vis absorption, emission and excitation spectra have been recorded. In fluid CH₂Cl₂ solutions, 3 exhibits prompt ¹(ππ*) fluorescence with λ_0–0 and λ_max at 413 and 428 nm, respectively, while 6 displays ³(ππ*) phosphorescence with λ_0–0 and λ_max at 446 and 479 nm, respectively. The crystal structure of 3·CH₂Cl₂ has been determined.     

Effect of extended conjugation with a phenylethynyl group on the fluorescence properties of water-soluble arylboronic acids

Boronic acids that change fluorescence properties upon sugar binding are very important reporter units for the development of small molecule lectin mimics (boronolectins). Aimed at developing long wavelength fluorescent boronic acid reporter compounds, we have designed and synthesized a series of boronic acid analogs 2a-d with an extended π conjugation. Such designs are based on earlier fluorescent boronic acids that change fluorescence properties upon sugar binding. Compared with the corresponding parent chromophores, these new compounds with extended conjugations show longer excitation and emission wavelengths as designed. The patterns of fluorescence changes for the new compounds are also different from that of the corresponding parent compounds.     

Water-soluble fluorescent boronic acid compounds for saccharide sensing: substituent effects on their fluorescence properties

Four new naphthalene-based boronic acid compounds (1-4) were synthesized. The effect of various carbohydrates on their fluorescence properties has been studied in aqueous phosphate buffer at pH 7.4. Different substitutions on the aniline group of the naphthalene ring resulted in significant differences in fluorescence properties for these four compounds. Compound 1 shows ratiometric fluorescence changes upon addition of a sugar. Compounds 2 and 3 do not show ratiometric fluorescence changes but show very large fluorescence intensity changes (about 70-fold fluorescence intensity increase). In addition to the quantifiable fluorescence property changes upon sugar addition, the fluorescence color changes of 1-3 are also visible to the naked eye. However, amidation of the aniline nitrogen atom significantly diminishes the fluorescence intensity of compound 4. The crystal structure of one boronic acid provided some insight into the structural features that are important for the fluorescence properties of these compounds.     

The triple fluorescence of benzanilide and the dielectric medium modulation of its competitive excitation

The fluorescence of the benzanilide molecule at 298 K is inferred to consist of three independent electronic transitions associated with the single ground-state molecular species. F₁ (λ_max340 nm), the normal fluorescence is observed weakly and is ascribed to an n,π*,-π,π* mixed state. F′₂ is ascribed to the proton-transfer imidol tautomer fluorescence (previously reported) with unresolved λ_max (inferred at ≈460 nm). F″₂ is ascribed to a charge-transfer state fluorescence to the ground state, and occurs as a resolved CT transition in tetrahydrofuran at λ_max 520 nm. Comparison of the spectra of N-methylbenzanilide exhibiting only F₁ and F″₂ (CT) permitted the analysis of the benzanilide spectra.     

A dual role of phenylboronic acid as a receptor for carbohydrates as well as a quencher for neighboring pyrene fluorophore

A simple amino acid based compound (1) containing a phenyl boronic group and pyrene fluorophore showed an enhanced fluorescence in aqueous solutions at physiological pH through suppression of the photoinduced electron transfer from pyrene to boronic acid on carbohydrate binding. The compound exhibited an interesting fluorescence change depending on pH with decreased emission intensity at acidic pH but enhanced emission intensity at basic pH unlike the fluorescent carbohydrate chemosensors using a PET process with amine and aryl-boronic acid. We have characterized a dual role of phenylboronic acid as a receptor for carbohydrates as well as a quencher for the fluorescence of pyrene fluorophore.     

The lipid-mediated hypothesis of fumonisin B1 toxicodynamics tested in model membranes

The disruption of lipidic metabolism was considered a good candidate to explain FB1 toxicity mechanism. In the present work we investigated molecular organizational changes induced by FB1–biomembrane interaction possibly involved in mycotoxic effects.

FB1 was self-aggregated with a critical micellar concentration of 1.97 mM. FB1 (0–81.4 μM), decreased in a dose-dependent manner, the fluorescence anisotropy of TMA-DPH (from 0.349 ± 0.003 to 0.1720 ± 0.0035) in dpPC bilayers, whilst no differences were registered with DPH. At 5.6 μM in the subphase, FB1 increased the lateral surface pressure (π) of a Langmuir film to an extent that depended on the monolayer composition (Δπ_{dpPC:DOTAP 3:1} > Δπ_dpPC:dpPA3:1 > Δπ_dpPC), the molecular packing (Δπ decreased linearly as a function of the initial π) and the subphase pH (Δπ_{pH 2.6} > Δπ_{pH 7.4} and maximal π allowing the drug penetration π_cut-off was 34.3 and 27.7 mN/m at pH 2.63 and 7.4, respectively). FB1 increased the surface potential of dpPC and dpPC:DOTAP monolayers and decreased that of dpPC:dpPA. This suggested that FB1 acquired different orientations and/or foldings depending on the surface electrostatics and the toxin charge state. Moreover, FB1–lipid interactions were transduced into long-range effects at the mesoscopic level affecting the lipidic self-separated lateral domains shape and density.     

Fluororeactands for the Detection of Saccharides Based on Hemicyanine Dyes with a Boronic Acid Receptor

Three hemicyanine dyes with boronic acid receptor functions have been synthesized in a two step procedure. These dyes are capable of forming a covalent bond between their boronic acid moiety and the diol moiety of saccharides which causes fluorescence to change. In detail, the indicator dyes exhibit absorbance maxima at around 460 nm and emission at around 600 nm, show increases in fluorescence upon exposure to saccharides and can be used in aqueous solution at physiological pH.     

Effect of solvent on the subnanosecond decay of the second excited singlet state of tetramethylindanethione

The S₂ → S₀ fluorescence spectra and quantum yields and the S₂ lifetimes of 2,2,3,3-tetramethylindanethione (TMIT) have been measured in several solvents using a synchronously pumped picosecond dye laser excitation system. The S₂ nonradiative decay rate is markedly solvent dependent. In inert perfluoroalkane solvents remarkably large S₂-S₀ fluorescence quantum yields (θ_f = 0.14) and long S₂ lifetimes (τ = 880 ps) are measured. Hydrocarbons are efficient excited-state quenchers.     

Pyrene-Based Fluorescent Porous Organic Polymers for Recognition and Detection of Pesticides

Eating vegetables with pesticide residues over a long period of time causes serious adverse effects on the human body, such as acute poisoning, chronic poisoning, and endocrine system interference. To achieve the goal of a healthy society, it is an urgent issue to find a simple and effective method to detect organic pesticides. In this work, two fluorescent porous organic polymers, LNU-45 and LNU-47 (abbreviation for Liaoning University), were prepared using π-conjugated dibromopyrene monomer and boronic acid compounds as building units through a Suzuki coupling reaction. Due to the large π-electron delocalization effect, the resulting polymers revealed enhanced fluorescence performance. Significantly, in sharp contrast with the planar π-conjugated polymer framework (LNU-47), the distorted conjugated structure (LNU-45) shows a higher specific surface area and provides a broad interface for analyte interaction, which is helpful to achieve rapid response and detection sensitivity. LNU-45 exhibits strong fluorescence emission at 469 nm after excitation at 365 nm in THF solution, providing strong evidence for its suitability as a luminescent chemosensor for organic pesticides. The fluorescence quenching coefficients of LNU-45 for trifluralin and dicloran were 5710 and 12,000 (LNU-47 sample by ca. 1.98 and 3.38 times), respectively. Therefore, LNU-45 serves as an effective “real-time” sensor for the detection of trifluralin and dicloran with high sensitivity and selectivity.     

Synthesis and Single Crystal X-ray Studies of 3-（3,5-Bis（trifluoromethyl）phenyl） Quinoline and 3-（4-Fluoro-3-methyiphenyl） Quinoline

The title compounds 3-（3,5-bis（trifluoromethyl）phenyl）quinoline （1） and 3-（4- fluoro-3-methylphenyl）quinoline （2） were synthesized through Suzuki-Miyaura Cross coupling reaction of 3-bromoquinoloine with aryl boronic acids. The title compounds were characterized by single-crystal X-ray diffraction, 1H NMR, 13C NMR, El-MS, elemental analysis and IR. The crystals of 3-（3,5-bis（trifluoromethyl）phenyl）quinoline （C17H9F6N, Mr = 341.25） belongs to the monoclinic system, space group P21n, a = 12.3072（13）, b = 4.9378（6）, c = 24.493（2） A, V= 1473.1（3） A3, Z = 4, Dc = 1.539 Mg m-3, 2 - 0.71073A, μ = 0.144 mm^-1, F（000） = 688, the final R = 0.0715 and wR = 0.1873 for 1875 obserwed reflections with I 〉 2σ（I） and the crystal of 3-（4-fluoro-3- methylphenyl）quinoline （C16H12FN, Mr= 237.27） belongs to the orthorhombic system, space group Pca21, a = 23.794（2）, b = 3.9094（3）, c = 25.669（2） A, V = 2387.7（4） A3, Z = 8, D, = 1.320 Mg m-3, 2 = 0.71073 A, μ = 0.088 mm-1, F（000） = 992, the final R = 0.0534 and wR = 0.1188 for 2270 observed reflections with I 〉 2σ（I）.     

Measurement of atomic L shell fluorescence (ω1, ω2, ω3) and Auger (a1, a2 and a3) yields for some elements in the atomic number range 59≤Z≤85

A comprehensive set of theoretical Coster–Kronig and fluorescence yields are presented for atomic numbers 18≤Z≤100. These quantities are based on ab initio relativistic calculations. Agreement with experimental values is fair for ω₁ and generally good for ω₂, ω₃ (Z≥54) [1]. Therefore, atomic L shell fluorescence (ω₁, ω₂, ω₃) and Auger yields (a₁, a₂ and a₃) for some elements in the atomic number range 59≤Z≤85 were determined. These selected measured semi-empirical values were also fitted by least squares to polynomials in the Z of the form ∑_na_nZⁿ and compared with theoretical and with earlier fitted values.     

Synthesis and evaluation of thermal, photophysical and magnetic properties of novel starlike fullerene–organosilane macromolecules

Thermal, photophysical and magnetic properties of some novel fullerenol–silane adducts are described. Excellent improvement of thermal stability and high char yield due to the presence of silicon is the key feature of these adducts. Highest luminescence quenching due to maximum π–π electronic interactions between phenyl ring and fullerene are observed in the aromatic-silane adducts and the quenching ability of the aromatic ring reduces with further delocalization of the π-electrons as in naphthyl silane. The alkyl vinyl silane, on the other hand, records better fluorescence intensity owing to increase population of the electron density (+I effect) and non-effective charge transfer complex formation between isolated vinylic double bond and fullerene. Emission peak positions of these adducts are comparable to fullerenol because of control derivatization of fullerene ring causing less perturbation of the symmetric π-electronic system. These adducts are paramagnetic in nature with peaks around 3515 G and higher g-values (2.005–2.009) compared to fullerenol (1.985). The fullerenol–silane adducts are synthesized using fullerenol as substrate and different chloro and alkoxy silanes as silylating agents adopting simple nucleophilic displacement and transesterification reactions. All the fullerenol–silane adducts are characterized spectroscopically.     

The cis—trans isomerization of nitrostilbenes XV: mixed singlet and triplet mechanism for trans → cis photoisomerization of 4-nitro-4′-dialkylaminostilbenes in non-polar solvents

The quantum yields of direct cis trans photoisomerization (φ_c → _t and φ_{t → c}) and of fluorescence of the trans isomers (φ_f) of three 4-nitro-4′-R-stilbenes (R amino (1), dimethylamino (2) and diethylamino (3)) were measured in several saturated hydrocarbons. Formation and decay of the lowest triplet state was observed by nanosecond laser flash photolysis. The triplet yield (φ_T), the triplet lifetime (τ_T), φ_{t → c} and φ_f were measured as a function of temperature and of the concentration of the quenchers ferrocene, azulene (Q) and oxygen. Twisting in the triplet, involving a ³t* ³p* equilibrium, analogous to that in other 4-nitrostilbenes, is suggested on the basis of the effects of temperature and quenchers on φ_T and τ_T. The trans → cis photoisomerization of 1 follows the triplet route almost completely. The existence of a singlet pathway (20% – 30% contribution) for 2 and 3 in non-polar solvents at room temperature is concluded from the non-linear dependence of the φ⁰_{t → c}/φ_{t → c} ratio on the concentration of Q. For these two nitrostilbenes a mixed singlet—triplet mechanism for the trans → cis photoisomerization is suggested.