Time-resolved fluorescence with an optical-fiber probe

Time-resolved fluorescence measurements are made with an optical-fiber probe approximately 16 m long. Fluorescence lifetimes for 1.00 μM solutions of rhodamine-B and rose bengal in ethanol were found to be 2.78 ± 0.04 ns and 0.77 ± 0.07 ns, respectively, similarin accuracy and precision to values obtained with conventional techniques. Calculations are used to investigate the limitations in remote determinations caused by temporal broadening. Results indicate that fiber lengths approaching 1 km can be used without significant loss in accuracy or precision.     

Lyman-α excitation spectra in the photodissociation of the doubly excited states of H2

A Lyman-α excitation spectrum has been observed using synchrotron radiation in the energy region corresponding to the double electron excitation of H₂. There exist in the spectrum three thresholds at 26.6±0.5 eV, 29.2±0.7 eV and 30.9±0.6 eV, and a dip at 34.1±0.5 eV. A Lyman-α excitation spectrum in the energy region corresponding to the single electron excitation has been also observed using a detection system which works as a band pass filter for detecting of Lyman-α fluorescence. The cross section of Lyman-α fluorescence in the photodissociation of the doubly excited states is very small, e.g., in the order of 10^?20 cm² at 30 eV, in comparison with that from the single electron excitation.     

Photophysical properties of poly-2-phenyl-5-(p-vinyl)-phenyloxazole

2-Phenyl-5-(p-vinyl)phenyloxazole (POS) has been synthesized. Copolymers of POS with styrene and the homopolymer poly–POS have been prepared. The polymers have been characterized through measurements of fluorescence decay times (using synchrotron radiation for excitation) and examination of excimer formation in the homopolymer. Fluorescence decay times of 0.6(±0.4) nsec and 9 (±0.2) nsec have been recorded at 298°K in toluene solution for monomer and excimer, respectively. Poly-POS exhibits excimer formation characterized by an activation energy of 5.9(±0.5) kJ/mole and a binding energy of 17.0(±1.0) kJ/mole.     

Rate constants for the gas phase reactions of OH with C5 through C7 aliphatic alcohols and ethers: Predicted and experimental values

Absolute rate constants were determined at 298 K for the gas phase reactions of hydroxyl radicals with several C₅ through C₇ aliphatic alcohols and ethers using the flash photolysis resonance fluorescence technique. The values obtained (in units of 10^?12 cm³ molecule^?1 s^?1) were: 3-methyl-2-butanol, (12.4 ± 0.7); 2-pentanol, (11.8 ± 0.8); 3-pentanol, (12.2 ± 0.7); cyclopentanol, (10.7 ± 0.7); 1-hexanol, (12.4 ± 0.7); 2-hexanol, (12.1 ± 0.7); 1-heptanol, (13.6 ± 1.3); methyl-n-butylether, (16.4 ± 0.6); ethyl-n-butylether, (22.8 ± 0.9); ethyl-t-butylether, (8.12 ± 0.32); and methyl-t-amylether, (7.91 ± 0.42). These results are discussed in terms of group reactivities in such molecules and are compared with values estimated from an additive structure-reactivity index.     

Lifetime of the B3Σ−u state of S2

Using the single-photon time correlation method, we have determined the lifetime of the S₂(B³Σ^?_u) state from the decay rate of the fluorescence at 370 nm. A lifetime of 45.0 ± 0.6 ns was measured, and the cross section for fluorescence quenching by S₂ as found to be 81.3 ± 4.7 A². A slight dependence of the lifetime on the wavelength of the excitation source over the range of 280 to 337 nm was observed.     

Phytochemical screening,antioxidant activity,total phenolic and total flavonoid contents of seven local varieties of Rosa indica L.

Rosa indica symbol of godness and beauty known for various healing power, has astringent, sedative, anti-inflammatory and antidepressant qualities. Standard methods were used for qualitative detection of phyto-compounds, and quantitative detection of antioxidants was done using DPPH radical scavenging assay, total phenolics and total flavonoids content were expressed in mg GAE/g dry weight and mg QE/g dry weight. Results revealed phyto-compounds presence in all varieties under study however maximum % inhibition was observed by R. indica var pink perfume (94 ± 0.6) with IC50 value 0.3376 ± 0.01 mg/mL. Highest phenolic and flavonoid content was observed in the leaves extract of R. indica var cardinal red, i.e. 3.3553 ± 0.11 (ethanol) mg of Gallic acid equivalents (GAE)/g dry weight and 3.736 ± 0.001(ethanol) mg of quercetin equivalents (QE)/g dry weight, respectively, at conc. 0.125 mg/mL. Our finding provides evidence that all varieties of rose contain medicinally important bioactive compounds and justifies their use for treatment of different diseases.     

Production of chiral matrices with hybrid silver–thiocholesterol nanosystems and study of their properties

A new type of chiral matrix based on silver–thiocholesterol hybrid nanosystems adsorbed on silica gel has been proposed. The molar ratio of stabilized thiocholesterol (L) ligand and silver (Ag) was found to have little effect on the size of the resulting silver nanoparticles (SNPs). The average diameter of SNPs was 2.7 ± 0.4, 2.2 ± 0.4, and 2.1 ± 0.6 nm upon the ratios Ag: L = 1: 5, Ag: L = 1: 2, and Ag: L = 1: 0.5, respectively. The resulting chiral matrices possess enantioselectivity relative to the 1,1’-binaphthyl-2,2’-diamine (BNDA) and trifluoroanthranyl ethanol (TFAE) optical isomers. The TFAE optical isomers were successfully separated using thin layer chromatography (α = 1.56).     

Spectrofluorimetric determination of two β-agonist drugs in bulk and pharmaceutical dosage forms via derivatization with dansyl chloride

A sensitive, simple and rapid spectrofluorimetric method has been developed for the determination of both Fenoterol hydrobromide (FNT) and Ritodrine hydrochloride (RTH) in pure and dosage forms. The method is based on derivatization using dansyl chloride (DNS-Cl) as fluorogenic agent and measuring the fluorescence of the products at emission wavelengths of 517 and 515 nm after excitation at 348 and 343 nm for FNT and RTH, respectively. Different experimental parameters affecting the fluorescence intensities were carefully studied and optimized. The relation between fluorescence intensities and drug concentrations were rectilinear over the concentration range of 0.25–6.0 μg/mL for both drugs with a minimum detectability of 0.065 and 0.045 μg/mL for FNT and RTH, respectively. The percentage recoveries ±SD were 100.1 ± 0.9, 99.9 ± 0.6 for FNT and RTH respectively. The proposed method was successfully applied to the determination of both drugs in their commercial dosage forms. The obtained results were statistically validated and agreed well with those obtained with reference methods. A suggestion for the reaction pathway with DNS-Cl was postulated.     

Bulk and microscopic properties of copolymer networks in mixed aqueous solvents

Amphiphilic films and hydrogels have been prepared from ethanol/water solutions containing a hydrocarbon-grafted water-soluble cellulose ether. These materials are characterized by dispersed hydrophobic microdomains which form spontaneously in the solvent due to the inherent incompatibility of the side chains with water. At low applied shear stress, the microdomains behave as temporary linkages of finite lifetimes, imparting viscoelastic properties to the networks. The molecular weight between microdomains was found to be independent of the volume fraction of polymer in the gel, and the number of linkages per backbone ranges from 22.8 ± 1.3 to 26.2 ± 1.5 over the frequency range 30–50 rad/s. The behavior of the solutions and gels was characterized using fluorescence and dynamic rheological measurements. It was demonstrated that the microdomains are capable of sequestering water-insoluble solutes. © 1992 John Wiley & Sons, Inc.     

Rate coefficients for the reactions of O(3P) with selected biogenic Hydrocarbons

The rate coefficients for the reaction of O(³P) with the biogenic hydrocarbons Δ³-carene, α-pinene, and isoprene have been measured using a direct method for the first time. O(³P) was generated from the pulsed photolysis of NO₂ or O₃ at 308 nm, and measured by resonance fluorescence at 131 nm. Rate coefficients at room temperature for the biogenics are similar: (3.4 ± 0.6) × 10^?11, (3.7 ± 0.6) × 10^?11, and (3.5 ± 0.6) × 10^?11 cm³ molec^?1 s^?1, for Δ³-carene, α-pinene, and isoprene, respectively. The rate coefficients for the reaction of O(³P) with NO₂ and ethene were also measured with the same method, and these values are within 4% and 10% of the currently recommended values, respectively. The correlation between OH and O(³P)-alkene reaction rate coefficients is updated and discussed. © 1995 John Wiley & Sons, Inc.     

Calorimetric investigation of order–disorder transition in Cu0.6Pd0.4 and Cu0.85Pd0.15 alloys

Order–disorder phase transitions in Cu_0.6Pd_0.4 and Cu_0.85Pd_0.15 alloys have been investigated using differential scanning calorimetry and drop calorimetry. The differential scanning calorimetry measurements show that the transition in both these alloys are reversible in nature and the enthalpy increment measurements reveal that these transitions are first order in nature. The transition temperature of first-order phase transition in Cu_0.6Pd_0.4 and Cu_0.85Pd_0.15 alloys have been evaluated to be 884(±2) and 799(±2) K, respectively, from drop calorimetric measurements. The latent heat of first-order phase transition in Cu_0.6Pd_0.4 alloy were evaluated to be 31.2(±0.6) and 28.9(±0.5) J g^?1, by enthalpy increment and differential scanning calorimetry measurements, respectively. Similarly, the latent heat of first-order phase transition in Cu_0.85Pd_0.15 alloy were evaluated to be 23.1(±0.6) and 21.3(±0.5) J g^?1, by enthalpy increment and differential scanning calorimetry measurements, respectively. The solidus temperatures of Cu_0.6Pd_0.4 and Cu_0.85Pd_0.15 alloys were found to be 1,457(±2) and 1,360 K, respectively.     

CHLOROPHYLL PHOTOCHEMISTRY IN CONDENSED MEDIA—II. TRIPLET STATE QUENCHING AND ELECTRON TRANSFER TO QUINONE IN LIPOSOMES*

The fate of excitation energy and electron transfer to quinones within Chl-a-containing phosphatidyl choline liposomes has been investigated. The bilayer membrane of the liposome stabilizes the Chl triplet state, as evidenced by a three-fold increase in the lifetime over that observed in ethanol solution. The relative triplet yield follows the relative fluorescence yield, indicative of quenching at the singlet level. Triplet state lifetimes are markedly shortened as the Chl concentration is increased, demonstrating that quenching occurs at the triplet level as well. This process is shown to be due to a collisional de-excitation. In the presence of quinones, the Chl triplet reduces the quinone resulting in production of long-lived electron transfer products. The percent conversion of Chl triplet to cation radical when benzoquinone is employed as acceptor is approximately 60 ± 10%, which is slightly less than in ethanol solution (70 ± 10%). The lifetime of the radical, however, can be as much as 1900 times longer. With respect to potentially useful photochemical energy conversion, the magnitude of this increased lifetime is far more significant than is the decreased radical yield.     

Reactions of CFBr studied by laser-induced fluorescence

Laser-induced fluorescence has been used to study reactions of CFBr radicals in a discharge-flow system. Arrhenius expressions of (1.9 ± 0.6) ξ 10^?12 exp(?762± 92/T) and (1.4 ± 0.3) ξ 10^?12 exp(?533 ± 62/T) cm³ molecule ^?1s^?1 for their reactions with Cl₂ and Br₂ respectively. Upper limits were obtained for the rate of reaction of CFBr with O₂ and F₂CCFBr.     

Temperature dependence of radiationless processes. Isoquinoline in solution

The temperature dependence of the fluorescence quantum yield φ_f, the fluorescence lifetime τ_f, and the oscillator strength f(S₀→S₁) of isoquinoline in solution has been measured between room temperature and 77 K. Following an Arrhenius type expression, φ_f in ethanol increases from 0.012±0.002 at 295 K to 0.61±0.03 at 77 K paralleled by an increase of τ_f from 0.25±0.10 ns to 9.0±0.2 ns. Over the same temperature range f(S₀→S₁) and the radiative fluorescence lifetime remain constant. By analyzing the temperature dependent data, it is shown that a spin-allowed internal conversion process with an activation energy of ～1500 cm^?1 is responsible for the observed temperature effect. A mechanism is proposed based upon a thermally activated depopulation of the S₁(ππ^*) state of isoquinoline via a slightly higher state, presumably the S₂(ππ^*) singlet state. An extremenly fast process involving the dissociation of the hydrogen bond deactivates this latter state, by possing S₁.     

THE PHOTOPHYSICS OF MEROCYANINE 540. A COMPARATIVE STUDY IN ETHANOL AND IN LIPOSOMES

Abstract— Absorption and fluorescence emission spectra, fluorescence lifetimes, fluorescence quantum yields, photoisomerization quantum yields and triplet quantum yields were measured for Merocyanine 540 (MC540) in ethanol and in large unilamellar dimyristoyl phosphatidylcholine vesicles. The major differences in the photophysics between the two media are the increase of the fluorescence quantum yield from 0.15 in ethanol to 0.6 in vesicles at 25° C, and the appearance of a second fluorescence decay with a lifetime of 1.87 ns in the latter medium. Upper and lower limits for the photoisomerization quantum yields were determined by combining the data from laser flash photolysis and optoacoustic spectroscopy. The decrease in photoisomerization quantum yield upon incorporation of the dye into the lipid bilayer by a factor 2 suggests that this process competes directly with fluorescence. The temperature dependence of the fluorescence and photoisomerization quantum yields in solution supports this model. In both media MC540 has a very low triplet quantum yield with values 0.002 > (> ø_T > 0.02 in ethanol and 0.01 > ø_T- > 0.09 in liposomes Our data are consistent with the model whereby the dye is incorporated into the lipid bilayer as a monomer with two different orientations and this model is adopted on the basis of the biexponential behaviour of the fluorescence and photoisomer decay.     

Intermolecular hydrogen bonding in chlorine dioxide photochemistry: A time-resolved resonance Raman study

The geminate-recombination and vibrational-relaxation dynamics of chlorine dioxide (OClO) dissolved in ethanol and 2,2,2-trifluoroethanol (TFE) are investigated using time-resolved resonance Raman spectroscopy. Stokes spectra are measured as a function of time following photoexcitation using degenerate pump and probe wavelengths of 398 nm. For OClO dissolved in ethanol, subpicosecond geminate recombination occurs resulting in the reformation of ground-state OClO with a quantum yield of 0.5±0.1. Following recombination, intermolecular-vibrational relaxation of OClO occurs with a time constant of 31±10 ps. For OClO dissolved in TFE, recombination occurs with a time constant of 1.8±0.8 ps and a quantum yield of only 0.3±0.1. The intermolecular-vibrational-relaxation time constant of OClO in TFE is 79±27 ps. The reduced geminate-recombination quantum yield, delayed recombination, and slower vibrational relaxation for OClO in TFE is interpreted in terms of greater self-association of the solvent. Degenerate pump–probe experiments are also presented that demonstrate decay of the Cl-solvent charge-transfer complex on the ∼1-ns time scale in ethanol and TFE. This time is significantly longer than the abstraction times observed for other systems demonstrating that Cl hydrogen abstraction from alcohols occurs in the presence of a significant energy barrier.     

Thermodynamics of vinyl ethers—XXI: Evaluation of some cis interaction energies

Thermodynamics of isomerization reactions on some substituted vinyl methyl ethers have been studied for evaluating the magnitudes of the interaction energies S[R¹... R²] between the substituents R¹ and R², juxtaposed in a cis position across the CC bond of vinyl ethers. The results obtained are (values given in kJ mol^-1): S[Me... t-Bu]=18.2±1.0, S[Ph...i-Pr]=11±2, S[i-Pr...Et] =6.1±0.6, S[i-Pr... i-Pr] = 6.0 ± 0.6, S[0...t-Bu]=2.9±0.5, S[O...i-Pr]= -0.7±0.5, S[O...Et] = ?1.5±0.5, S[O...Ph]= ?2.1+-0.6, and S[O... Me] =?2.9 ±0.2 (the symbol O stands for the ethereal oxygen atom of vinyl ethers). The negative interaction energy values reveal that the cis interaction between the ethereal oxygen and the alkyl (aryl) group concerned is stabilizing.     

Photochemically induced fluorimetric determination of the antimalarial mefloquine

Mefloquine (MQ) was determined quantitatively in pharmaceutical tablets by irradiating the MQ solution for 2–7 min with a high-pressure UV lamp, measuring simultaneously the increase in fluorescence signal at room temperature (298 K) and comparing the signal with that of a standard. The effects of several solvents (acetone, dioxane, ethanol, methanol, propan-2-ol, tetrahydrofuran, water) and “pH” on the fluorescence properties of MQ and its photochemical decomposition product were studied. The limit of detection of MQ was 50 ng ml^?1. The mean overall recovery from pharmaceutical tablets was 102.3 ± 4%.     

Fluorescence properties of fluoranthene as a function of temperature and environment

The fluoranthene fluorescence properties were investigated in a variety of environments, i.e. in different solvents and temperatures, in the solid state, and in the vapor phase. The emission maximum was found to be independent of environment. The absorption spectrum in different solvents exhibits only minor changes. In solution, the fluorescence lifetime shows a slight inverse relationship to the solvent dielectric constant. With water/methanol mixtures of varying composition as the solvent, the lifetime decreases linearly with increasing mole fraction of water. At 77K, the fluoranthene fluorescence lifetime in frozen polar and nonpolar solvents are the same within experimental error. In hexane the fluorescence lifetime is independent of temperature (77±3 and 82±7 ns, at room temperature and 77 K, respectively). In methanol the lifetime is 64±3 ns at room temperature and increases linearly to 80±4 ns at 77 K. In the vapor phase the lifetime is 32±1 ns. No fluorescence quantum yield change was observed for either S₁ or S₂ manifold excitation.     

Reactions of the fluorobromocarbene radical studied by laser-induced fluorescence

CFBr radicals produced by the reaction of atomic oxygen with F₂CCFBr were monitored in a discharge flow system by fluorescence excited at 424 nm. The rate coefficients for reactions of the CFBr radicals were measured between 298 and 358 K, and the following values were obtained in units of cm³/molec·s: O₂ < 2 × 10^?16 at 353 K; NO < 10^?14 at 298 K; F₂CCFBr < 10^?15 at 298 K; Cl₂ (1.9 ± 0.6) × 10^?12 exp(?762 ± 92/T) Br₂ (1.4 ± 0.3) × 10^?12 exp(?533 ± 62/T).