QUENCHING OF METHYLCYCLOHEXANE FLUORESCENCE BY METHANOL*

We measured the fluorescence emission spectrum and intensity decays of methylcyclohexane (MCH) when excited by simultaneous absorption of two photons at 298 nm. The steady-state intensities and lifetimes were both decreased by methanol, which was found to be an efficient quencher of MCH fluorescence. Methanol quenching of MCH is clearly dynamic, but the exact mechanism of quenching is unclear. Dynamic quenching of MCH was also observed by water and n-propanol. These results suggest that alkane fluorescence from biopolymers, if observable, will only occur from regions of the macromolecules that are not exposed to water.     

Amphiphilic Fluorescence Resonance Energy-Transfer Dyes: Synthesis,Fluorescence, and Aggregation Behavior in Water

Amphiphilic pyrene/perylene bis-chromophore dyes were synthesized from unsymmetrically substituted perylene bisimide dyes, which were obtained through three synthetic methods. The optical and aggregation behaviors of these functional dyes were studied by means of UV/Vis absorption and fluorescence spectroscopy, dynamic light scattering, and TEM. These dyes are highly fluorescent and cover the whole visible-light region. A donor/acceptor dye displays intramolecular fluorescence resonance energy transfer (FRET), with a high efficiency of up to 96.4 % from pyrene to perylene bisimide chromophores, which leads to a high fluorescence color sensitivity to environmental polarity. Under a λ=365 nm UV lamp, the light-emitting colors of the donor/acceptor dye change from green to yellow with increasing solvent polarity, which demonstrates application potential as a new class of FERT probes. The donor/acceptor dye in water was assembled into hollow vesicles with a narrow size distribution. The bilayer structure of the vesicular wall was directly observed by means of TEM. These vesicular aggregates in water are fluorescent at λ=650–850 nm within the near-infrared region.     

Thermal-cycling-induced spectral diffusion and thermal barriers in anisole-doped cyclohexane, an unusual multiphase host-guest system

The host-guest system of anisole incorporated into a cyclohexane matrix was investigated in a series of hole-burning experiments. This system is unusual in that cyclohexane can freeze into coexisting solid phases. The hole-burning experiments support the existence of two crystalline phases and one disordered phase. A second surprising characteristic of this system is that the quasi-line absorption features of the spectra appear inverted at low temperature because of unexpected dominance of fluorescence and phosphorescence.     

Facile fabrication of amphiphilic gold nanoparticles with V-shaped brushes from block copolymers with a trithiocarbonate group as the junction

Amphiphilic gold nanoparticles grafted with V-shaped brushes (Au-V-brushes) were prepared by grafting a polystyrene-b-poly(ethylene oxide) (PS-b-PEO) block copolymer with a trithiocarbonate group as the junction to the Au surface. The obtained Au-V-brushes were subjected to solubility test and UV-vis, FT-IR, TEM and DLS characterizations. It is found that the Au-V-brushes are soluble in both water and organic solvents. In the common solvent DMF, the size of the Au-V-brushes is about 17 nm, whereas in selective solvents (toluene and water) aggregates of 70-90 nm are formed. Phase transfer of the Au-V-brushes from the water phase into the toluene phase occurs upon addition of Na(2)SO(4) into water and the Au-V-brushes can also transfer from the toluene phase to the interface of toluene and water phases after addition of citric acid in the water phase.     

Isotropic-liquid crystalline phase diagram of a CdSe nanorod solution

We report the isotropic-liquid crystalline phase diagram of 3.0 nm x 60 nm CdSe nanorods dispersed in anhydrous cyclohexane. The coexistence concentrations of both phases are found to be lower and the biphasic region wider than the results predicted by the hard rod model, indicating that the attractive interaction between the nanorods may be important in the formation of the liquid crystalline phase in this system.     

Fluorescence of proteins induced by two-photo absorption

At room temperature, the UV fluorescence of trypsin, thyroglobulin, hemoglobin and ablumin are observed; these molecules undergo two-photon absorption when they are excited with the radiation of a frequency-duplicated Q-switched Nd:YAG laser. It is shown that the fluorescence of these proteins comes from the tryptophan residues and that it has a red shift of about 20 nm relative to the one-photon fluorescence. We suggest that the weak emission from tyrosine arises from the forbidden two-photon transition. The influence of concentration on the fluorescence of hemoglobin and tryptophan is discussed. The two-photon absorption cross-sections are estimated.     

Study on the subgel-phase formation using an asymmetric phospholipid bilayer membrane by high-pressure fluorometry

The myristoylpalmitoylphosphatidylcholine (MPPC) bilayer membrane shows a complicated temperature-pressure phase diagram. The large portion of the lamellar gel (L(β)'), ripple gel (P(β)'), and pressure-induced gel (L(β)I) phases exist as metastable phases due to the extremely stable subgel (L(c)) phase. The stable L(c) phase enables us to examine the properties of the L(c) phase. The phases of the MPPC bilayers under atmospheric and high pressures were studied by small-angle neutron scattering (SANS) and fluorescence spectroscopy using a polarity-sensitive fluorescent probe Prodan. The SANS measurements clearly demonstrated the existence of the metastable L(β)I phase with the smallest lamellar repeat distance. From a second-derivative analysis of the fluorescence data, the line shape for the L(c) phase under high pressure was characterized by a broad peak with a minimum of ca. 460 nm. The line shapes and the minimum intensity wavelength (λ″(min)) values changed with pressure, indicating that the L(c) phase has highly pressure-sensible structure. The λ″(min) values of the L(c) phase spectra were split into ca. 430 and 500 nm in the L(β)I phase region, which corresponds to the formation of a interdigitated subgel L(c) (L(c)I) phase. Moreover, the phase transitions related to the L(c) phase were reversible transitions under high pressure. Taking into account the fluorescence behavior of Prodan for the L(c) phase, we concluded that the structure of the L(c) phase is highly probably a staggered structure, which can transform into the L(c)I phase easily.     

Homodimeric monomethine cyanine dyes as fluorescent probes of biopolymers

The fluorescence properties of newly synthesized homodimeric monomethine cyanine dyes in the presence of biopolymers are investigated. They do not fluoresce in TE buffer and bidistilled water but become strongly fluorescent (Q(F)=0.3-0.9) in the region 530-650 nm when bound to dsDNA and ssDNA. The detection limit of dsDNA is about 1.7 ng/ml. Some of dyes studied are able to distinguish between dsDNA and ssDNA, RNA, BSA in solution and gel electrophoresis. The influence of different factors (temperature, pH and viscosity of the medium, presence of histone) on the formation of the dye-biopolymer complexes is investigated. The results of steady-state and dynamic fluorescence measurements concerning the different types of binding between dyes and biopolymers show that the new dyes are applicable in molecular biology as highly sensitive fluorescence labels.     

Fine tuning of gadolinium doped ceria electrolyte nanoparticles via reverse microemulsion process

Gadolinium doped ceria (Gd–CeO₂) nanoparticles have been synthesized by an reverse microemulsion system using cyclohexane as the oil phase, a non-ionic surfactant Igepal CO 520 and their mixed aqueous solutions of gadolinium III nitrate hexahydrate and cerium III nitrate hexahydrate as the water phase. The control of particle size was achieved by varying the water to surfactant molar ratio. The synthesized and calcined powders were characterized by thermogravimetry-differential thermal analysis (TGA-DTA), X-ray diffraction analysis (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS), Fourier transform infrared spectroscopy (FTIR) and Raman spectroscopy. The XRD results show that all the samples calcined at 700 °C were single phase cubic fluorite structure. The average size of the particle was found to increase with increase in water to surfactant molar ratio (R). The mean diameter of the particle for various value of R varies between 8–15 nm (SEM) and 7.5–11 nm (TEM), respectively. EDS confirm the presence of gadolinia and ceria phase in the nanopowder calcined at 700 °C. FTIR analysis was carried to monitor the elimination of residual oil and surfactant phases from the microemulsion-derived precursor and calcined powder. Raman spectroscopy and DTA evidenced the formation of a solid solution of gadolinium doped ceria at room temperature.     

Liquid-liquid phase-transfer of magnetic nanoparticles in organic solvents

We report a novel route for the preparation of well-defined colloidal dispersions of magnetic nanoparticles stabilized by steric repulsion in organic solvents. The usual methods standardly lead to the surfaction of multiparticle aggregates, incompatible with our long-term aim of studying and modeling the influence of magnetic dipolar interactions in colloidal dispersions which are free of aggregates, all other interactions being perfectly defined. A new and reproducible method based on a surfactant-mediated liquid-liquid phase transfer of individually dispersed gamma-Fe(2)O(3) nanoparticles from an aqueous colloidal dispersion to an organic phase is developed. The choice of the reagent and the preparation techniques is discussed. Among several solvent/surfactant pairs, the cyclohexane/dimethyldidodecylammonium bromide (DDAB) system is found to fulfill the colloidal stability criterion: aggregation does not appear, even upon aging. A complete transfer of isolated particles is observed above a threshold in DDAB concentration. The nanoparticle surface is then fully covered with adsorbed DDAB molecules, each surfactant head occupying a surface of 0.57+/-0.05 nm(2). The volume fraction of the cyclohexane-based organosols is easily tunable up to a volume fraction of 12% by modifying the volume ratio of the organic and of the aqueous phases during the liquid-liquid phase transfer.     

Electronic spectroscopy and photochromic mechanism of bis-Schiff bases, N, N''''-bis(2-hydroxy-1-naphthylidene)-1,4-phenyldiamine

The steady state and transient state absorption spectra and fluorescence spectra of N,N'-bis(2-hy-droxy-1-napbthylidcne)-1,4-phenyldiamme ( BNP ) in cyclohexane and acetonitrile were determined.The pho-tochromic mechanism was discussed In nonpolar solvents,BNP exists mainly in the enol form and has the absorption maximum in the UV region In polar solvents,however,both the enol and proton transfer tautomer are formed,but the farmer is the main one Fluorescence emissions result from the excited state of proton transfer product.     

1,2‐Dithienyldicyanoethene‐Based,Visible‐Light‐Driven,Chiral Fluorescent Molecular Switch: Rewritable Multimodal Photonic Devices

Reported here is the first example of a 1,2‐dithienyldicyanoethene‐based visible‐light‐driven chiral fluorescent molecular switch that exhibits reversible trans to cis photoisomerization. The trans form in solution almost completely transforms into the cis form, accompanied by a 10‐fold decrease in its fluorescence intensity within 60 seconds when exposed to green light (520 nm). The reverse isomerization proceeds upon irradiation with blue light (405 nm). When doped into commercially available achiral liquid crystal hosts, this molecular switch efficiently induces luminescent helical superstructures, that is, a cholesteric phase. The intensity of the circularly polarized fluorescence as well as the selective reflection wavelength of the induced cholesteric phases can be reversibly tuned using visible light of two different wavelengths. Optically rewritable photonic devices using cholesteric films containing this molecular switch are described.     

双连续微乳体系中PbS纳米管的控制合成(英文)

PbS nanotubes were synthesized by the tiny water channels in bicontinuous microemulsions consisted of p-octyl polyethylene glycol phenylether (OP)/n-amyl alcohol/cyclohexane/water. The possible formation mechanism of PbS nanotubes is also discussed based on their shape evolutions. The results indicate that the formation of PbS nanotubes is probably via the process of the nucleation, growth, assemblage and crystallization.     

1,2‐Dithienyldicyanoethene‐Based,Visible‐Light‐Driven,Chiral Fluorescent Molecular Switch: Rewritable Multimodal Photonic Devices

Reported here is the first example of a 1,2‐dithienyldicyanoethene‐based visible‐light‐driven chiral fluorescent molecular switch that exhibits reversible trans to cis photoisomerization. The trans form in solution almost completely transforms into the cis form, accompanied by a 10‐fold decrease in its fluorescence intensity within 60 seconds when exposed to green light (520 nm). The reverse isomerization proceeds upon irradiation with blue light (405 nm). When doped into commercially available achiral liquid crystal hosts, this molecular switch efficiently induces luminescent helical superstructures, that is, a cholesteric phase. The intensity of the circularly polarized fluorescence as well as the selective reflection wavelength of the induced cholesteric phases can be reversibly tuned using visible light of two different wavelengths. Optically rewritable photonic devices using cholesteric films containing this molecular switch are described.     

Synthesis and Photophysical Properties of Dimethoxybis(3,3,3‐trifluoropropen‐1‐yl)benzenes: Compact Chromophores Exhibiting Violet Fluorescence in the Solid State

Dimethoxybis(3,3,3‐trifluo‐ropropen‐1‐yl)benzenes were prepared through palladium‐catalyzed double cross‐coupling reactions of diiododimethoxybenzenes with CF₃C≡CZnCl, followed by reduction of CF₃C≡C groups with LiAlH₄ or H₂ in the presence of the Lindlar catalyst. The edges of the absorption spectra of 1,2‐(MeO)₂‐4,5‐(CF₃CHC=CH)₂benzenes 1 and 1,3‐(MeO)₂‐4,6‐(CF₃CH=CH)₂benzenes 2 in cyclohexane ranged from 348 to 360 nm, whereas the absorption spectra of 1,4‐(MeO)₂‐2,5‐[(E)‐CF₃CH=CH]₂ benzene ((E)‐ 3 ) ended at 406 nm. These findings indicate that the effective conjugation length of (E)‐ 3 was significantly larger than those of 1 and 2 . Consistently, 1 and 2 in cyclohexane exhibited fluorescence with emission maxima in the UV region, whereas (E)‐ 3 in cyclohexane emitted violet light with an emission maximum at 407 nm. All the fluorescence spectra of 1 – 3 in various solvents redshifted as the solvent polarity increased. The photoluminescence of 1 , E‐1 , Z‐1 , 2 , E‐2 , E‐2H , Z‐2 , E‐3 , E‐3H , Z‐3 in the solid states was also observed with emission maxima in the violet region. It is important to note that the quantum yields of (E)‐ 3 in a neat thin film and in a doped polymer film were 0.37 and 0.49, respectively. Density functional theory calculations suggested that the fluorine atoms contribute to a slight extension of both the HOMOs and the LUMOs, as well as narrowing of the HOMO–LUMO gaps when compared with the corresponding fluorine‐free analogues. In the case of (E)‐ 3 , it is suggested that the HOMO–LUMO transition includes charge transfer from the ethereal oxygen atoms to the C(sp²) CF₃ moieties.     

Electronic spectroscopy and photochromic mechanism of bis-Schiff bases, N, N′-bis(2-hydroxy-l-naphthylidene)-l, 4-phenyldiamine

The steady state and transient state absorption spectra and fluorescence spectra of N, Ń-bis(2-hydroxy-1-naphthylidene)-1, 4-phenyldiamine (BNP) in cyclohexane and acetonitrile were determined. The photochromic mechanism was discussed. In nonpolar solvents, BNP exists mainly in the enol form and has the absorption maximum in the UV region. In polar solvents, however, both the enol and proton transfer tautomer are formed, but the former is the main one. Fluorescence emissions result from the excited state of proton transfer product. Project supported by the National Natural Science Foundation of China and the Foundation of Chinese Academy of Sciences.     

Three-phase equilibrium in solutions of polystyrene homologues in cyclohexane

The equilibrium of three liquid phases in solutions of two polystyrene homologues in cyclohexane has been studied experimentally for different values of the average molecular weights of the two polymer species. The compositions of the phases have been determined in some cases. The variation in the size of the three-phase region with change in the molecular weights is discussed, particularly in connection with the occurrence of a tricritical point. Some qualitative observations of the phase behavior in mixed solvent systems are mentioned.     

Separations of tocopherols and methylated tocols on cyclodextrin-bonded silica

-, β-, γ-, and δ-tocopherols (methyl-substituted tocols) and 5,7-dimethyltocol were separated by normal-phase HPLC on β- or γ-cyclodextrin-bonded silica (CDS) with fluorescence detection. The HPLC behavior of the tocol components was studied under various mobile phase conditions. Hexane or cyclohexane was used in combination with oxygen-containing solvents (alcohol, ether, and esters) in binary and ternary mobile phases. Capacity factors (k′) and separation factors () for adjacent tocol components were determined. Incorporation of non-polar ethers in the hydrocarbon (hexane or cyclohexane) mobile phases favored the separation of β- and γ-tocopherols with improved values, which enabled trace analysis of the β-isomer present in soybean oil. Analyte solutes tended to be more strongly adsorbed in mobile phases containing branched-chain alcohols and ethers than in those containing the corresponding straight-chain solvents. Generally, the k′ values obtained with hexane mobile phases or with the β-CDS phase were greater than those observed in HPLC with cyclohexane mobile phases or with the γ-CDS phase.     

FLUORESCENCE SPECTROSCOPY OF A P700-CHLOROPHYLL-PROTEIN COMPLEX*

Abstract. Chlorophyll-protein complexes enriched in the Photosystem I reaction center chlorophyll (P700) exhibit a fluorescence emission maximum at 696 nm at - 196°C The height of this 696 nm emission relative to the emission at 683 nm from antenna chlorophyll a increases proportionally with the P700 concentration while the total fluorescence yield of the complex decreases. The 696 nm emission could possibly be from an absorbing form of antenna chlorophyll a that may be somewhat enriched along with P700 in Photosystem I fractions. However, evidence resulting from glycerol treatment which appears to decrease the rate of resonance energy transfer between antenna chlorophyll and P700 favors the hypothesis that the emission comes from a photooxidized P700 dimer (Chl⁺-Chl) absorbing near 690 nm. In turn, this fluorescence evidence provides additional support for the model of a P700 dimer involving exciton interaction. Absorption in the wavelength region of 450 nm specifically excites emission at 696 nm from the P700-chlorophyll complex.     

Spectroscopic and stability characteristics of Ga-tetraphenyl porphyrin- and Ge- and Sn-tetra (p-methyl-phenyl) porphyrin chlorides

The electronic absorption, emission and excitation spectra of the titled porphyrin complexes have been studied both in cyclohexane and in microemulsion media. The spectral data indicate minimal aggregation of the complexes in microemulsions. The fluorescence quantum yield (φ_f) values are generally higher in microemulsion media compared with cyclohexane. Solutions of the complexes are generally unstable upon prolonged photoirradiation giving open macrorings. A photochemical electron transfer from benzidine to the porphyrin complexes is induced by photoirradiation (λ_ex. = 420 nm) giving the characteristic blue benzidine monoradical cation. This represents a simple photochromic reaction based on inter-molecular photoelectron transfer. The porphyrin complexes undergo fluorescence quenching by benzidine. The fluorescence quenching of the porphyrin complexes using benzidine as a quencher does not give linear Stern-Volmer plots indicating a static-type quenching mechanism. The porphyrin complexes show instability with respect to organic oxidants. The oxidative decomposition of the Ga-porphyrin complex induced by o-chlorodibenzoyl peroxide follows a second-order kinetic law with an activation energy E_a = 64.6 kJ mol⁻¹.