Fluorescence quenching of water-soluble porphyrins by molecular oxygen

Quenching by molecular oxygen of excited states of water-soluble anionic 5,10,15,20-tetarkis-(4-sulfonatophenyl)-porphyrin (H₂TSPP) and cationic 5,10,15,20-tetrakis-(4-N-methylpyridyl)-porphyrin (H₂TMPyP) in aqueous ethanol was investigated. It was found that fluorescence quenching of dissociated (in water) and undissociated (in ethanol) forms of H₂TSPP was diffusion-controlled and occurred at distances close to contact ones (0.5–0.8 nm). Fluorescence of the dissociated form of H₂TSPP was quenched with rate constant k_S that was 1.7 times greater than that of the undissociated form. It was proposed that this was due to a decrease in the porphyrin molecule oxidation potential on going from the undissociated to the dissociated form. It was shown that the most probable reason for the dramatic increase in the rate constant of the fluorescence quenching of H₂TMPyP in water compared with that of H₂TSPP was the low-lying intramolecular charge-transfer state typical of H₂TMPyP. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 75, No. 2, pp. 170–176, March–April, 2008.     

Photophysical and photochemical properties of the water-soluble porphyrin complexes of metals of the platinum group

Consideration is given to phosphorescence and photochemical properties of the coordination compounds Pt(II), Pd(II), and Rh(III) with three water-soluble porphyrins, i.e., meso-tetrakis (4-N,N,N-trimethylaminophenyl) porphin (H₂TTMAP⁴⁺), meso-tetrakis (4-N-methylpyridyl) porphin (H₂TMPyP⁴⁺) and meso-tetrakis (4-sulfonatophenyl) porphin (H₂TSPP⁴⁻). It is shown that the process of photoreduction by an irreversibly oxidized electron donor (EDTA) proceeds only for the complexes with TMPyP, which is attributed to the strongest oxidative properties of these metal poprhyrins in the triplet excited state. The end products of the photoreduction reaction (metal chlorins and metallophlorin-anions) are established and its possible mechanism is suggested. To whom correspondence should be addressed. Reported at the VIIIth International Conference on Spectroscopy of Porphyrins and Their Analogs, Minsk, September 22–26, 1998. A. I. Gertsen Russian State Pedagogical University, 48, Moika Embankment, St. Petersburg, 191186, Russia. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 66, No. 4, pp. 528–531, July–August, 1999.     

Luminescence properties of calcium-iodide crystals

The effect of the conditions of preparation, temperature, and the action of x rays on the luminescence properties of calcium-iodide scintillation crystals is investigated. On the basis of the results of a study of the spectral characteristics of CaI₂ and CaI₂:H₂ crystals for optical and x-ray excitation in the temperature range 90–400 K, also taking into account the results of a study of the luminescence properties of CaI₂ crystals activated by Cl⁻, Br⁻, OH⁻, and Ca²⁺ impurities, it is suggested that the 236-nm band observed in the excitation spectra of crystals of calcium iodide may be caused by an uncontrollable hydrogen impurity. The luminescence of these crystals with maximum at 395 nm is ascribed to radiative recombination of excitons trapped at H⁻ ions. Zh. Tekh. Fiz. 69, 135–136 (January 1999)     

Doppler spectroscopy of a beam in the channel of a H− ion source

It is shown that high-accuracy contact-free measurements of the divergence and emittance of an accelerated H⁻ ion beam at the exit from the source can in principle be performed by passive Doppler spectroscopy of a beam of excited hydrogen atoms produced by neutralization of the ions with excitation on the residual gas in the source channel. The intensity of the H_α-line radiation detected by the Doppler system is calculated, taking into account the principal processes leading to the excitation and deexcitation of the 3s, 3p, and 3d levels of the hydrogen atoms in the beam, for residual gas densities of the order of 10⁻⁴–10⁻⁵ Torr in the source channel. The computed H_α-line intensity was confirmed experimentally, making it possible to perform photoelectronic detection of the spectral contour of the line in the current mode rather than the photon-counting mode. Zh. Tekh. Fiz. 68, 15–18 (June 1998)     

Calculation of electronic spectra of porphin and its derivatives by a modified INDO/S method

Quantum-chemical calculations of excited electronic states of porphin (H₂P) and Mg-porphin (MgP) have been carried out in the framework of the INDO/S method with varying off-diagonal matrix elements of the one-electron Hamiltonian {ie027-01} and electron-electron interaction integrals {ie027-02} as functions of the internuclear distance. It has been found that a simultaneous increase in the π-type overlap factor for {ie027-03} and decrease in the {ie027-04} integrals, as compared with {ie027-05} calculated by the Nishimoto-Mataga formula, make it possible to reproduce the positions of the Q-and B-transitions in the experimental absorption spectrum with a precision of ∼300 cm⁻¹. In this case, the N_x-transition intensity of the H₂P molecule is halved, which means that only two B-transitions should be related to the Soret band as in the four-orbital model. Using the proposed parameterization (INDO/Sm), electronic spectra have been calculated for a number of porphyrin compounds including chlorine, bacteriochlorin, tetrabenzoporphin, and tetraazaporphin. The results obtained agree with the experimental data to an accuracy of 300–700 cm⁻¹ whereas the accuracy of the standard INDO/S calculation is no better than 3000 cm⁻¹. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 75, No. 1, pp. 28–35, January–February, 2008.     

The infrared spectra of tutton salts 1. A comparative study of (NH4)2 M″(SO4)2·6H2O (M″=Ni,Co or Mg)

The infrared spectra of (NH₄)₂M″(SO₄)₂.6H₂O has been analysed in the region 4000–250 cm⁻¹. The dynamics of each crystal has been discussed in terms of 234 phonon modes, including 3 acoustical ones, using the unit cell approximation. The ambiguity in the assignments of the bands in the region 900–500 cm⁻¹ has been removed by assigning the bands in this region to the libratory modes of H₂O molecules. It has been concluded that the NH ₄ ⁺ and SO ₄ ²⁻ ions have a symmetry lower thanT _dand also the complex [M″(H₂O)₆]²⁺ has a symmetry lower than O _h . The hydrogen bonding is the strongest in the Ni-salt and the weakest in the Mg-salt.     

Kinetics of triplet-triplet absorption of anionic water-soluble porphyrins

It was established that in aqueous solutions of anionic porphyrins the processes of triplet-triplet (T-T) annihilation are absent (K₂<2.5·10⁵ M⁻¹ sec⁻¹), while a delayed annihilation fluorescence can be detected for these porphyrins in methanol (K₂≊3.5·10⁷ M⁻¹ sec⁻¹) and for their lipophilic analogs in organic solutions (K ₂≊2.5·10⁹ M⁻¹ sec⁻¹), and the kinetics of absorption of the latter porphyrins in triplet states is dependent on the intensity of the exciting radiation. The extremely small constant of T-T annihilation of anionic porphyrins can be due to the formation of a relatively dense solvate shell consisting of water molecules around the charged groups of the porphyrin molecules, which increases steric barriers and thus prevents an exchange-resonance transfer of energy between triplet excited molecules in aqueous solutions. It is also shown that porphyrin molecules can electrostatically repel one another when in their structure there are charged groups. To whom correspondence should be addressed. Deceased. Institute of Molecular and Atomic Physics, National Academy of Sciences of Belarus, 70 F. Skorina Ave., Minsk, 220072, Belarus; e-mail: llum@imaph.bas-net.by. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 66, No. 6, pp. 791–795, November–December, 1999.     

Study of ion–solvent interactions and activation energy of LiBr in DMSO,H<Subscript>2</Subscript>O and DMSO–H<Subscript>2</Subscript>O mixtures at various temperatures

The Jones–Dole B coefficients of the electrolyte Lithium bromide (LiBr), reference salts tetra butyl ammonium tetra phenyl borate (BU₄NBPh₄), tetra butyl ammonium bromide (BU₄NBr), and potassium chloride (KCl) in dimethylsulfoxide (DMSO), water, and DMSO–water mixtures were obtained at different temperatures range from 25 to 45 °C For this, the relative viscosities were measured for Lithium bromide (LiBr) and reference salts in DMSO, water, and DMSO–water mixtures at above-mentioned temperatures. The B coefficients of these electrolytes were behaved as structure makers in DMSO, while in H₂O and DMSO–H₂O mixtures, the B-coefficient values were less positive showing the weak structure-making effect. Ionic viscosity B coefficients allow us to assess the behavior of ions in the solvent mixtures. In this study it was observed that all the values of ionic B coefficient of (Li⁺) were positive and small showing the weak structure-making effects. It was also observed that Br⁻ ions maintain negative B coefficient values in all DMSO–H₂O mixtures, except in 60% DMSO mole fraction. From this it can be concluded that Br⁻ ion behaved as a structure breaker in water and in all DMSO–H₂O mixtures except in 60% DMSO mole fraction mixtures. The low B _± values of alkali metal ions and Br⁻ ions in water are due to the breakdown of the tetrahedral structural of water and the formation of strongly structured solvated ion. It is also observed that the values of the energy of activation of the flow for LiBr are greater in DMSO–water mixtures and in pure water than in DMSO. This may be due the presence of a network of hydrogen bonds which cause the hindrance in the flow of the solution of LiBr in DMSO–water mixtures and in pure water than in DMSO.     

Variation of interface trap level charge density within the bandgap of 4H-SiC with varying oxide thickness

Interfacial characteristics of metal oxide-silicon carbide (MOSiC) structure with different thickness of SiO₂, thermally grown in steam ambient on Si-face of 4H-SiC (0 0 0 1) substrate were investigated. Variations in interface trapped level density (D _it) was systematically studied employing high-low (H-L) frequency C–V method. It was found that the distribution of D _it within the bandgap of 4H-SiC varied with oxide thickness. The calculated D _it value near the midgap of 4H-SiC remained almost stable for all oxide thicknesses in the range of 10⁹–10¹⁰ cm⁻² eV⁻¹. The D _it near the conduction band edge had been found to be of the order of 10¹¹ cm⁻² eV⁻¹ for thicker oxides and for thinner oxides D _it was found to be the range of 10¹⁰ cm⁻² eV⁻¹. The process had direct relevance in the fabrication of MOS-based device structures.     

Spectral components that form UV absorption spectrum of Ce3+ and Ce(IV) valence states in matrix of photothermorefractive glasses

We have measured the UV absorption spectra of photothermorefractive glasses of the system Na₂O-ZnO-Al₂O₃-NaF-SiO₂ doped by cerium oxide in the range of (2.8–5.0) × 10⁴ cm⁻¹ (360–200 nm). The spectra have been processed by the method of dispersion analysis based on the analytical convolution model for the complex dielectric function of glasses. We show that the absorption band centered at 3.3 × 10⁴ cm⁻¹ (∼303 nm) that is attributed to the transition 2F _5/2 → 5d in the Ce³⁺ ion, is an envelope of three spectral components. The broad absorption range (3.5–4.7) × 10⁴ cm⁻¹ (200–270 nm) that is commonly interpreted as a charge transfer band of the Ce(IV) valence state, is an envelope of at least three spectral components.     

Influence of heme periphery upon electronic structúre and Mössbauer spectra parameters of hemoglobin

Quantum-chemical analysis of electronic structure peculiarities and mossbauer spectra parameters was performed for penta-coordinated complex of ferro-protoporphyrin with imidazole (Fe(+2)PPIm). Peripheral substitutes (−CH₃, −C₂H₃, −C₂H₄COOH) introduced in the porphyrin macrocycle simulated real chemical structure of protoporphyrin (PP) in heme group of Hb. Calculations displayed that doubly occupied molecular orbitals (MO) of the peripheral substitutes (−CH=CH₂ and −CH₂−CH₂−COOH) always appeared near the occupat ion border. The orientation of vinyl fragment have the essential influence upon Fe₅ dorbital populations and quadrupole splitting_AE_Q for⁵B₁ and⁵B₂ terms. The values of isomer shift were insensitive to that modification of fragment orientation.     

Detection of the structure of the wing of the Rayleigh line in ice, water, and heavy water using four-photon polarization spectroscopy

The spectral structure of the wing of the Rayleigh line in ice, ordinary water (H₂O), and heavy water (D₂O) is recorded in the frequency range 0–50 cm⁻¹ by means of four-photon polarization spectroscopy. It is shown that this structure can be explained by the collective rotational motion of molecules in cells determined by the structure of hexagonal ice. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 1, 12–14 (10 January 1999)     

Determination of Trace Mercury by Solid Substrate-Room Temperature Phosphorimetry Quenching Method Based on Catalytic Effect of Hg2+on Formation of the Ion Association Complex [Sn(XO)6]4+·[(Fin)4]

A new method for the determination of trace mercury by solid substrate-room temperature phosphorimetry (SS-RTP) quenching method has been established. In glycine-HCl buffer solution, xylenol orange (XO) can react with Sn⁴⁺ to form the complex [Sn(XO)₆]⁴⁺. [Sn(XO)₆]⁴⁺ can interact with Fin⁻ (fluorescein anion) to form the ion associate [Sn(XO)₆]⁴⁺·[(Fin)₄]⁻, which can emit strong and stable room temperature phosphorescence (RTP) on polyamide membrane (PAM). Hg²⁺ can catalyze H₂O₂ oxidizing the ion association complex [Sn(XO)₆]⁴⁺·[(Fin)₄]⁻, which causes the RTP to quench. The ΔIp value is directly proportional to the concentration of Hg²⁺ in the range of 0.016–1.6 fg spot⁻¹ (corresponding concentration: 0.040–4.0 pg ml⁻¹, 0.40 μl spot⁻¹), and the regression equation of working cure is ΔIp=10.03+83.15 m Hg²⁺ (fg spot⁻¹), (r=0.9987, n=6) and the detection limit (LD) is 3.6 ag spot⁻¹(corresponding concentration: 9.0×10^–15 g ml⁻¹, the sample volume: 0.4 μl). This simple, rapid, accurate method is of high selectivity and good repeatability, and it has been successfully applied to the determination of trace mercury in real samples. The reaction mechanism for catalyzing H₂O₂ oxidizing the ion association complex ([Sn(XO)₆]⁴⁺·[(Fin)₄]⁻) SS-RTP quenching method to determine trace mercury is also discussed.     

Catalytic and electrocatalytic activation of methane over manganese oxides deposited on YSZ

The catalytic and the electrocatalytic behavior of MnO_x oxides deposited on Yttria Stabilized Zirconia (YSZ) in the form of thin porous films, was studied during the reaction of methane activation at high methane to oxygen ratios. Experiments were carried out in a continuous flow well-mixed reactor (CSTR), at atmospheric total pressure and in a temperature range between 500–850 °C. It was found that the electrochemical pumping of oxygen anions (O²⁻) through the solid electrolyte (YSZ) affect drastically the rates of CO₂, C₂H₄ and C₂H₆ formation and consequently the C₂ selectivity. Paper presented at the 4th Euroconference on Solid State Ionics, Renvyle, Galway, Ireland, Sept. 13–19, 1997     

Europium-sensitized Chemiluminescence of System Tetracycline–H<Subscript>2</Subscript>O<Subscript>2</Subscript>–Fe(II)/(III) and Its Application to the Determination of Tetracycline

Chemiluminescence (CL) of the reaction system tetracycline–H₂O₂–Fe(II)/(III)–Eu(III) was used for the determination of tetracycline hydrochloride in water, pharmaceutical preparations, and honey. The CL spectrum registered for this system shows emission bands typical of Eu(III) ions, with a maximum at λ ∼ 600 nm, corresponding to the electronic transitions of ⁵D₀ → ⁷F₁ and ⁵D₀ → ⁷F₂. A strong chemiluminescence intensity characteristic of europium(III) ions in the system tetracycline–H₂O₂–Fe(II)/(III)–Eu(III), as contrasted to the emission of the system tetracycline–H₂O₂–Fe(II)/(III) without Eu(III), proves that the Eu(III) ion plays the role of a chemiluminescence sensitizer, accompanying tetracycline oxidation in the Fenton system (H₂O₂–Fe(II)/(III)). A linear dependence was observed for the integrated CL light intensity on the tetracycline concentration in the range of 2 × 10⁻⁷ to 3 × 10⁻⁵ mol l⁻¹ with the detection limit of 5 × 10⁻⁸ mol l⁻¹ in aqueous solution.     

Molecular Stark effect spectroscopy of diflavonol and inhomogeneous broadening of its electronic spectra in erythrocyte membranes

Using spectroscopy of the molecular Stark effect and fluorescence spectroscopy, we study the characteristics of diflavonol 3,7-dihydroxy-2,8-di(4-dimethylaminophenyl)-4H,6H-pyrano[3,2-g]chromene-4,6-dione (DFME), which demonstrates intramolecular charge and proton phototransfer. In the ground state, this dye has only one form and, in the excited state, it has two forms, i.e., normal and phototautomeric. We found that, for the normal form of DFME, the transition dipole moment that is responsible for the absorption (m _a ), the dipole moment in the equilibrium ground state (μ _g ), and the change of the dipole moment upon transition of the molecule in the excited Franck-Condon state (Δ ^a μ) are parallel. In the ground equilibrium state, the dipole moments in 1,4-dioxane and cyclohexane are equal to μg = 12.2 × 10⁻³⁰ C m and μ _g = 11.0 × 10⁻³⁰ C m, respectively. Upon excitation, they increase by Δ ^a μ = 61 × 10⁻³⁰ C m and Δ ^a μ = 50.2 × 10⁻³⁰ C m in these solvents. We study the spectral characteristics of DFME in organic solvents and erythrocyte membranes. A spectral inhomogeneity of DFME in erythrocyte ghosts is found. The inhomogeneous broadening of fluorescence spectra is manifested as a long-wavelength shift of the band of the normal form of DFME by 1640 cm⁻¹ upon excitation at the red edge of the absorption spectrum.     

Electrochemical double layer capacitor based on cellulose-PVA hybrid electrolyte

A polyvinyl alcohol (PVA) cellulose electrolyte was prepared by casting a solution of PVA and H₃PO₄ on both sides of a cellulose membrane (filter paper). The ionic species H⁺ are caused by the H₃PO₄ acid which is entrapped inside the PVA and in the pores of the filter paper. The electrolyte was sandwiched between two carbon electrodes to form an electric double layer capacitor (EDLC). The EDLC exhibits a good charge and discharge characteristics with a capacitance value of 30 Fg⁻¹. Paper presented at the International Conference on Functional Materials and Devices 2005, Kuala Lumpur, Malaysia, June 6 – 8, 2005.     

A theoretical study of hydrogen adsorption on Li, Be, Na, and Mg atoms attached to aromatic hydrocarbons

The binding energy of a hydrogen molecule on metal atoms (Li, Be, Na, and Mg) attached to aromatic hydrocarbon molecules (benzene and anthracene) was calculated using an ab initio molecular orbital method at the MP2(FC)/cc-pVTZ level with basis set superposition error (BSSE) correction. The energy tended to become more negative as the metal atom had a more positive charge and a smaller radius. The energies of Li₂C₆H₆-H₂, Li₂C₁₄H₁₀-H₂, Na₂C₁₄H₁₀-H₂, and MgC₁₄H₁₀-H₂ were −2.7 to −2.2, −4.0 to −3.1, −2.8 to −0.3, and −1.3 kcal/mol, respectively. Most of these energies were more negative than those on the hydrocarbons without metal atoms (ca. −1 kcal/mol). Analyzing the Lennard–Jones type potential with the parameters determined by the MP2 calculations, it was found that these energies mainly consisted of the induction force caused by the positive charge of the metal atom and the dispersion force from the nearest C₆-ring. The energy of BeC₁₄H₁₀-H₂ was more negative (−8.6 kcal/mol) than of the other complexes. The hydrogen molecule in this complex had a comparatively longer H–H distance and a more positive H₂ charge than the others. These data suggest that the hydrogen adsorption on this complex involves a charge transfer process in addition to physisorption interactions. The hydrogen binding energies in some Li₂C₁₄H₁₀-H₂ systems (∼−4.0 kcal/mol) and BeC₁₄H₁₀-H₂ are promising to operate hydrogen storage/release at ambient temperature with moderate pressure.     

Reductive Fluorescence Quenching of the Photoexcited Free Base <Emphasis Type="Italic">meso</Emphasis>-Tetrakis (Pentafluorophenyl) Porphyrin by Amines

Steady state and time resolved fluorescence quenching behaviors of meso-Tetrakis (pentafluorophenyl) porphyrin (H₂F₂₀TPP) in presence of different aliphatic and aromatic amines have been executed in homogeneous dichloromethane (DCM) solution. At room temperature in DCM, free base (H₂F₂₀TPP) shows fluorescence with two distinct peaks at 640 and 711 nm and natural lifetime τ _f = 9.8 ns which are very similar to that of meso-tetraphenyl porphyrin (TPP). Unlike TPP, addition of both aliphatic and aromatic amines to a solution containing H₂F₂₀TPP results in an efficient decrease in fluorescence intensity without altering the shape and peak position of fluorescence emission. Upon addition of amines there was no change in optical absorption spectra of H₂F₂₀TPP. The fluorescence quenching rate constants ranged from 1 × 10⁹ to 4 × 10⁹ s⁻¹, which are one order below to the diffusion control limit, and temperature dependent quenching rate constants yield the activation energies which are found to be order of 0.1 eV. Femto second transient absorption studies reveal the existence of amine cation radical and porphyrin anion radicals with very short decay time (15 ps). The fluorescence quenching reaction follows Stern–Volmer kinetics. Steady state and time-resolved data are interpreted within general kinetic scheme of Marcus semi-classical model which attributes bimolecular electron transfer process between amines and the lowest excited singlet state of H₂F₂₀TPP. Calculated internal reorganization energies are found to be in between 0.04 and 0.22 ev. Variation of electron transfer rate as function of free energy change (∆G⁰) points the ET reactions in the present systems are in Marcus normal region. This is the first example of reductive fluorescence quenching of free base neutral porphyrins in homogeneous organic solvent ever known.     

Study of nonplanar distortions of Ni(II)-mesomethylporphyrins using spectroscopy of resonance Raman scattering

The role of mesomethyl substitution and of steric interaction of peripheral substituents in the formation of nonplanar distortions of the porphyrin macrocycle is studied for a series of Ni(II)-porphyrins. Resonance Raman spectra are recorded for Ni(II)-5-methyl-2,3,7,8,12,13,17,18-octaethylporphyrin (1); Ni(II)-2,5,8,12,18-pentamethyl-3,7,13,17-tetraethylporphyrin (2); and Ni(II)-3,5,7,12,17-pentamethyl-2,8,12,17-tetraethylporphyrin (3). The obtained spectra are compared with spectra of Ni-octaethylporphyrin (Ni-OEP) and Ni-etioporphyrin II. It is found that introducing one methyl group into a mesoposition leads to significant low-frequency distortions of the structure-sensitive lines of resonance Raman scattering. Specifically, frequenciesν ₃ ,ν ₁₁ ,ν ₁₉ ,ν ₂ , andν ₁₀ decrease by 10–44 cm⁻¹ while frequencyν ₄ remains practically unchanged. It is shown that the observed spectral variations are caused by nonplanar distortions of the structure of Ni-porphyrins. The same tendency of the line displacement for tetragonal Ni-OEP and for compounds 1, 2, and 3 indicates that CH₃-mesosubstituted Ni-porphyrins have a “corrugated” structure. The degree of nonplanarity is modulated by alkyl groups on pyrrole rings and increases in the porphyrin series 3→2→1. All mesomethyl-substituted Ni-porphyrins have spectral indications that the considered compounds are structurally heterogeneous in solution due to the presence of conformers with different degrees of planarity. Institute of Molecular and Atomic Physics, National Academy of Sciences of Belarus, 70, F. Skorina Ave., Minsk, 220072, Belarus. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 66, No. 2, pp. 170–176, March–April, 1999.