共查询到20条相似文献,搜索用时 31 毫秒
1.
Xiao Xia Zhang King Fai Li Kok Wai Cheah Xianju Zhou Peter A. Tanner 《Chemical physics letters》2004,400(4-6):331-335
A new 1.75 μm infrared emission transition of Y2O3:Er3+ is assigned to the 4S3/2 → 4I9/2 transition of Er3+ ions situated at the C2 sites of cubic RE2O3 (RE = Y, Gd, Lu). The intensities of features in the 1.54 μm 4I15/2–4I13/2 absorption transition due to Er3+ at S6 and C2 sites are consistent with the site occupation ratio and the relative magnetic dipole–electric dipole intensity contributions of Er3+ at the different sites. The 1.54 μm emission lines are predominantly from Er3+ ions at C2 sites. The different behaviours of the emission intensities 1.75 and 1.54 μm groups with change in Er3+ dopant ion concentration, preparation technique, Yb3+ co-doping, temperature change and different excitation line are rationalized. 相似文献
2.
Craig Pepmiller Eleanor Bedwell Rlobert R. Kuntz Camillo A. Ghiron† 《Photochemistry and photobiology》1983,38(3):273-280
Abstract— The conventional flash photolysis of 1-methylindole in aqueous media was studied at Λexcitation ≥290 nm. The transients observed 20 μs after excitation consisted mainly of the radical cation (R+ ). the hydrated electron (e- aq ) and the triplet state (T). Electron counting experiments indicate that photoionization is the only source of R+ with e- aq /R+ = 1.07±0.09 in neutral media. Quenching of the R+ yield with H+ indicates that the fluorescent state is the precursor to 80% of the photoionization events with the remainder probably arising from a prefluorescent state. The triplet decays with a lifetime of 29 μs in deaerated neutral media. This decay is unchanged by N2 O saturation, but T reacts with acrylamide with k ≥2.8 × 109 M -1 . In 2 M Br- , R+ and T yields are increased by factors of 2–3. Consideration of fluorescence quenching and T enhancement by Br-permits an estimate of φIsc between 0.33 and 0.49. The increased R+ yield at high Br-concentrations cannot be accounted for by induced photoionization or triplet state reactions. 相似文献
3.
Abstract— A single-sample method for estimating energy distribution and redistribution among the two photosystems using fluorescence lifetimes and transients at 77 K is presented. In this method,α(the fraction of photons absorbed by photosystem I, PSI) is F1(α) /(F1(α) + (τF 1(M) /τF 2(M) ).F2(M) ) where, F1(α) is the fluorescence intensity from PSI excited by photons initially absorbed by the latter, τF 1(M) and τF 2(M) are the maximum lifetimes of fluorescence from chlorophyll- a in PSI (1) and II (2), and, F2(M) is the maximum fluorescence intensity from PSII (P level). Analysis of the intensities and lifetimes of wavelength resolved fluorescence of thylakoids (pH 7.0), with and without cations, leads to the following conclusions: The addition of 10 m M Na+ to cation-depleted thylakoids (pH 7.0) increases α by ˜ 10%, while the subsequent addition of 10 m M Mg2+ leads to three principal concomitant changes (in the order of importance): a 50% decrease in PSII to PSI energy transfer, a 20% increase in other radiation-less losses, and a 10% decrease in α. 相似文献
4.
Abstract— The rate constant for quenching of 1 O2 by azide ion in water was determined to be (5.0 ± 0.4) × 108 M −1 s−1 using a variety of sensitizers (including humic acids) and 1 O2 acceptors. The apparent second-order rate constant decreases with pH below pH 5.5 in accordance with the protonation of azide ion to form hydrazoic acid (p K a = 4.6). Quenching by hydrazoic acid is at least 2 orders of magnitude slower than by azide ion. Greater than 99% of all interactions between 1 O2 and azide ion involve physical quenching rather than chemical reaction. Humic acid triplets are not significantly quenched by azide ion at concentrations less than 2 m M , allowing azide ion quenching to be used as a diagnostic test for the intermediacy of 1 O2 in photosensitized oxidations in natural surface waters. 相似文献
5.
G. S. SINGHAL E. RABINOWITCH † J. HEVESI † V. SRINIVASAN ‡ 《Photochemistry and photobiology》1970,11(6):531-545
Abstract— The main absorption bands of thionine (Th+ ) and methylene blue (MB+ ) in aqueous solution lie at 598 nm and 664 nm, respectively. This position permits excitation energy transfer from Th+ to MB+ , but not vice versa. We describe here studies of such transfer between these molecules adsorbed on micelles of sodium lauryl sulfate (SLS), imitating, at least to some extent, the state of pigments in chloroplasts.
The SLS concentration was varied from 3.0 to 11 × 10-3 M. In the presence of dye, aggregation to micelles, each containing 70–100 detergent molecules, begins at about 3.0 × 10-3 M SLS. Practically all dye ions are adsorbed on these micelles as soon as their formation begins.
Energy transfer from adsorbed Th+ ions to adsorbed MB+ ions can be demonstrated by observing the quenching of the fluorescence of thionine and the sensitization of that of methylene blue.
At [Th+ ] = [MB+ ] = 1 × 10-5 M , the most efficient energy transfer (82 per cent efficiency, as derived from measurements of the quenching of Th+ fluorescence, or 90 per cent, as derived from sensitization of MB+ fluorescence) is observed at the lowest SLS-concentration (3.0 × 10-3 M ), when the only micelles present are those formed by aggregation of dye-carrying low molecular complexes of SLS with dye cations. Each micelle carries, under these conditions, 10–14 molecules of the two dyes, and the distance between two closest dye ions is about 16 A. Transfer becomes less efficient as the SLS-concentration increases, causing pigment molecules to distribute themselves among a greater number of micelles. 相似文献
The SLS concentration was varied from 3.0 to 11 × 10
Energy transfer from adsorbed Th
At [Th
6.
Abstract— The rapid bimolecular reaction SO + O3 = SO2 +O2 + 106 kcal/mole
yields electronically excited SO2 in the 3 B 1 and 1 B2 states with some vibrational excitation, as well as SO2 in its electronic ground state. It is shown that k1 = 1.5 x 1012 exp (-2100/ RT ) cm2 mole-1 s-1 and that the formation of electronically excited SO2 involves higher activation energies. 相似文献
yields electronically excited SO
7.
A series of novel photoactive hybrid materials with organic parts covalently linked to inorganic parts via the acylamino group have been assembled by sol–gel process. The organic parts as molecular bridge derive from α-hydroxypyridine (HP) functionalized by 3-(triethoxysilyl)-propyl isocyanate (TESPIC). Finally homogeneous, molecular-based hybrid materials with different microstructure (uniform spherical or clubbed) are obtained, in which no phase separation is observed. This may be ascribed as the different coordination behavior of metal ions (Eu3+ (Tb3+ ) or Zn2+ ). Red emission of Eu–HP–Si, green emission of Tb–HP–Si and violet-blue luminescence of Zn–HP–Si hybrids can be achieved within these molecular-based hybrid materials. Besides, both Eu(Tb) and Zn are introduced into the same hybrid systems (Eu(Zn)–HP–Si or Tb(Zn)–HP–Si) through the covalent Si–O bond, whose sphere particle size can be modified. Especially the photoluminescence behavior can be enhanced, suggesting that intramolecular energy transfer takes place between inert Zn2+ and Eu3+ (Tb3+ ) in the covalently bonded hybrid systems. 相似文献
8.
Thaddeus Prusik Tatiana Kolubayev Michael J. Morelli Henry C. Brenner 《Photochemistry and photobiology》1980,31(4):315-321
Abstract— The luminescence of acridine dyes intercalated in DNA was studied as a function of the concurrent binding of metal ions to DNA, in an effort to deduce specific site interactions of the dyes. Two dyes, proflavine (PF) and acridine orange (AO), and two metal ions, silver and mercuric, were used. Both ions quench the fluorescence of the dyes in aqueous solution at room temperature. The metal ions have a different effect on the fluorescence of these dyes when they are intercalated between the base pairs of DNA. The fluorescence of AO is decreased when silver is bound, while the fluorescence of PF is enhanced. Since Ag+ initially binds to GC sites in DNA, which quench the PF fluorescence, it ostensibly 'turns off' the quenching by DNA at these sites, and this effect is greater than the quenching effect of the silver ion itself. Hg2+ ion initially binds to AT sites in DNA. Since both dyes fluoresce from AT sites, Hg2 + is expected to quench their fluorescence. This behavior is observed at low r (metal ion/base). At higher r values, however, where Hg2 + is expected to begin binding to GC sites, the fluorescence of PF is enhanced. These quenching turn-off effects are tentatively interpreted in terms of a change in the structure of the dye/DNA complex which occurs when a metal ion binds at the intercalation site. At 77 K. no fluorescence enhancement is observed when metal ions bind; Ag+ quenches the fluorescence and enhances the phosphorescence of both dyes. Qualitatively similar results are obtained with Hg2 + . 相似文献
9.
Abstract— Laser flash photolysis of subtilisin BPN'at 265 nm has shown that photoionization of tryptophanyl (Trp) and tyrosinyl (Tyr) residues are the principal initial photochemical reactions. The initial products are the corresponding oxidized radicals. Trp and Tyr, and hydrated electrons (eaq ) which react with the enzyme at: k (eaq + subt. BPN') = 2.1 × 1010 M−1 s−1 . The photoionization quantum yield was 0.032 ± 0.005 at 265 nm, which was enhanced 3.5-fold by simultaneous excitation at 265 and 530 nm. The photoionization yields were unchanged by 3 M bromide ion and 8 M urea. which did affect the enzyme fluorescence excited at 265 and 295 nm. A similar lack of correlation between the effects of perturbants on the photionization yields and fluorescence yields was found for subtilisin Carlsherg. The results indicate that the monophotonic and biphotonic ionization of the Trp residues does not involve the thermally-equilibrated. lowest excited singlet state and that singlet energy transfer from Tyr to Trp does not contribute to Trp photoionization. The photoinactivation quantum yield was 0.014 for 265 nm laser excitation. which was not changed by simultaneous 530 nm excitation. The corresponding quantum yield was 0.009 for low intensity 254 nm radiation, indicative of a biphotonic contribution to photoinactivation. The results are explained by postulating that photolysis of Trp-113 leads to disruption of hydrogen bonding to Asn-117 and a shift in the primary chain sequence associated with the aromatic substrate binding sites. The photoionization quantum yields in subtilisin BPN'and subtilisin Carlsberg agree with a model based on the assumption that exposed Trp and Tyr residues contribute independently at intrinsic photoionization efficiencies characteristic of the chromophores. 相似文献
10.
Abstract— The spectra and molar absorbances of the HO2 and O2 - free radicals have been redetermined in aqueous formate solutions by pulse and stopped-flow radiolysis as well as by 60 Co gamma-ray studies. The extinction coefficients at the corresponding maxima and 23°C are 225 = 1400 ± 80 M -1 cm-1 and 225 = 2350 ± 120 M -1 cm-1 respectively. Reevaluation of earlier published rate data in terms of the new extinction coefficients yielded the following rate constants for the spontaneous decay of HO2 and O2 - : K Ho2+HO2 = (8.60 ± 0.62) × 105 M -1 s-1 ; K Ho2+O2- = (1.02 ± 0.49) × 108 M -1 s-1 ; K Ho2+O2- < 0.35 M -1 s-1 . For the equilibrium HO2 → O2 - + H+ the dissociation constant is K Ho2 = (2.05 ± 0.39) × 10-5 M or p K HO2 = 4.69 ± 0.08. G (O2 - ) has been evaluated as a function of formate concentration. 相似文献
11.
Abstract. Photosynthetic reduction of nitrite to ammonia with type C chloroplasts from the heterocont alga Bumilleriopsis filiformis was investigated using 3,6-diaminodurene/ascorbate and 3,6-diaminodurene/dithioerythritol (DAD/DTE) as electron donor couple. Rates approach 6–10 μmol NO- 2 reduced/mg chlorophyll/h and are steady for up to 30 min. The presence of oxygen or NADP+ only slightly diminished the rates of nitrite reduction obtained with DAD/DTE. Illuminated chloroplasts reduce oxygen in the presence of DAD/DTE at 135 μmol/mg chlorophyll/h without acceptor supplied. Photosynthetic oxygen uptake by this system in the presence of ferredoxin and NO- 2 , however, is inhibited to 42% by nitrite reductase with concurrent nitrite reduction. NO- 3 and NO- 2 have no effect on photosystem I-mediated NADP+ reduction, NO- 2 (10 m M ) inhibits ferricyanide-mediated oxygen evolution to 72%. Also photosystem II reactions assayed e.g. with silicomolybdate are inhibited significantly by NO- 2 (1 m M ), but only slightly by NO- 3 . Nitrite reductase is inhibited by p -chloromercuribenzoate ( p CMB), and this inhibition is prevented by DTE. Results suggest that photosynthetic nitrite reduction can cope with low concentrations of either compound, provided relevant thiol groups are protected. 相似文献
12.
E. Ben-Hur M. Green A. Prager R. Kol I. Rosenthal 《Photochemistry and photobiology》1987,46(5):651-656
Abstract— The concentrations of Na+ , K+ , Ca2+ , Mg2+ and CI− ions in the cytoplasm of octopus photoreceptor cells were determined before and after illumination by electron probe X-ray microanalysis. The concentrations of these elements in the dark-adapted photoreceptor cells were: Na+ , 68.4; K+ , 111.4; Ca2+ , 4.0; Mg2+ , 16.4; and CI− , 102.9 m M /kg of cytoplasm. Illumination raised the concentration of Na+ by 58 m M and that of Cl− by 23 m M and reduced the K+ concentration by 47 m M /kg of cytoplasm. A trace increase of intracellular Ca2+ and a trace decrease of Mg2+ were observed. These results confirm the hypothesis that sodium ions flow in on illumination, and suggest the influx of chloride ions and the outflux of potassium ions during illumination. The intracellular concentrations of Na+ , K+ and Cl+ can give the basis for calculating the ion permeability of ion channels in octopus photoreceptor cell membranes, using values of the membrane potentials obtained by electrophysiological studies 相似文献
13.
QUENCHING OF CHLOROPHYLL FLUORESCENCE BY NITROBENZENE 总被引:1,自引:0,他引:1
G. S. Beddard Sheena Carlin L. Harris G. Porter C. J. Tredwell 《Photochemistry and photobiology》1978,27(4):433-438
Abstract—Nitrobenzene quenching of chlorophyll fluorescence in ethanol has been investigated. Steady state relative quantum yields have been measured and fluorescence decay rates were determined using both nanosecond photon counting and picosecond pulses from a mode-locked Nd3+ glass laser.
The fluorescence decay is described by
1( t )= I0 exp (- t/τ−At1/2 )
the form predicted for decay governed by the kinetics of the continuum model of diffusion controlled reactions. From the parameters of the fluorescence decay, the encounter distance is 5–7 A° the mutual diffusion coefficient is 0.62 × 10- -5 cm2s-1 ± 12%.
Some of the fluorescence quenching is also attributed to static quenching by a nitrobenzene-chlorophyll, ground-state complex. The equilibrium constant for formation of this ground-state complex was determined to be 4.1 M-1 . The combined dynamic and static quenching model allows calculation of quantum yields of fluorescence in good agreement with the experimentally determined quantum yields. 相似文献
The fluorescence decay is described by
1( t )= I
the form predicted for decay governed by the kinetics of the continuum model of diffusion controlled reactions. From the parameters of the fluorescence decay, the encounter distance is 5–7 A° the mutual diffusion coefficient is 0.62 × 10
Some of the fluorescence quenching is also attributed to static quenching by a nitrobenzene-chlorophyll, ground-state complex. The equilibrium constant for formation of this ground-state complex was determined to be 4.1 M
14.
PULSE RADIOLYSIS STUDIES IN MODEL LIPID SYSTEMS: FORMATION AND BEHAVIOR OF PEROXY RADICALS IN FATTY ACIDS 总被引:1,自引:0,他引:1
Abstract— Radiolytic formation and peroxidation of fatty acid radicals have been investigated by pulse radiolysis techniques in oleate, linoleate, linolenate and arachidonate systems. A strong absorption band at 280 nm associated with conjugated radicals, Rconj , formed in polyunsaturated fatty acid moieties has been used as a probe for kinetic processes occurring at doubly allylic sites in the hydrocarbon chain. Formation of Rconj by O- has been found to be more efficient than the less selective OH radical. Peroxidation of Rconj is shown to be somewhat slower, ( k R + O2 ˜ 3 × 108 M -1 s-1 ), than O2 reactions with radicals in oleate ( k R + O2 = 1 × 109 M -1 s-1 ). Peroxy radicals generated in these reactions disappear slowly by essentially second order processes (2 k RO1 ˜ 107 M -1 s-1 ). The superoxide radical, O- 2 , shows little if any reactivity towards 0.01 M linolenate or arachidonate over periods of 20 s. 相似文献
15.
Abstract— The influence of chloride ion on the rate of decay of triplet methylene blue in 0.01 M acid in the absence and presence of ferrous ions was investigated by means of laser flash-photolysis monitored by kinetic spectrophotometry. Chloride weakly accelerates decay of 3 MBH2± in aqueous solution in the absence of Fe(II). Quenching of 3 MBH2+ by Fe(II) is more strongly catalyzed by Cl- in both water and 50 v/v% aq. CH3 CN. The uncatalyzed quenching constant, k 5 , is of the order of 1 × 106 M -1 s-1 while in 4.8 M aqueous chloride ( μ – 7.2 M ) k 5 = (37.2 ± 1.8) × 106 M -1 s-1 . A possible role of chloride is as a bridging species in quenching via electron transfer between 3 MBH2+ and Fe(II). 相似文献
16.
在以往的研究工作中,人们经常关注给定基质具有多个格位所对应多发光中心的光致发射。 取决于所掺杂的基质,多个格位的晶体化合物应该可以提供相应的多发光中心,但某些化合物有时只会表现一个发光中心,由此产生“争议性”发光中心。 据以往研究,以稀土离子掺杂而为人所知的经典镁黄长石(Ca2MgSi2O7)长余辉化合物具有“争议性”Ca格位。 为此,在本文中以非稀土离子(Bi3+)特有的发光特性从侧面来研究Ca2MgSi2O7化合物中的“争议性”Ca格位问题。 结果表明,在250和276 nm激发波长激发下,所有的Bi3+掺杂Ca2MgSi2O7样品均有峰位位于582和350 nm的两个Bi3+特征发射带。结合晶体结构分析,光谱结果表明,Ca2MgSi2O7:Bi3+荧光粉具有对应于六配位和八配位的两个不同Ca2+格位的两个Bi3+发光中心,即,Bi3+(Ⅰ)(~582 nm)和Bi3+(Ⅱ)(~350 nm)发光中心。 此外,光谱分析进一步表明,Bi3+(Ⅰ)与Bi3+(Ⅱ)具有单向的能量传递,而且,发现这两个发光中心所对应的相对发射强度是依赖于激发波长和Bi3+掺杂浓度。 实验证明了Ca2MgSi2O7晶体化合物具有两个Ca格位,而不是有些工作中所讨论的一个Ca格位。 本文工作可以为已知或未知的具有“争议性”格位的晶体化合物的验证提供新的借鉴。 相似文献
17.
A. J. W. G. Visser G. J. van Ommen G. van Ark F. Müller J. D. W. van Voorst 《Photochemistry and photobiology》1974,20(3):227-232
Abstract— Using high-intensity actinic light, the chlorophyll a fluorescence transient from HCO- 3 -depleted chloroplasts shows a rapid initial rise (O → I) followed by a slow phase (I → P). In the presence of HCO- 3 , the O → I rise is delayed but the I → P phase is much more rapid. Using low-intensity actinic light, the chlorophyll a fluorescence transient from 3-(3,4-dichlorophenyl)-1,1 dimethylurea (DCMU)-treated chloroplasts is delayed in the presence of HCO- 3 . Bicarbonate increases the amount of delayed light emission from chloroplasts given 10 s illumination with weak blue light (0·4 W/m2 ). DCMU greatly increases the amount of delayed light seen in the presence of HCO- 3 under these conditions but decreases the amount seen in the absence of HCO- 3 . It is suggested that HCO- 3 may somehow form or stabilize, in the dark, a number of reaction centers corresponding to the S1 state in the model of B. Forbush, B. Kok and M. McGloin ( Photochem. Photobiol. 14, 307–321, 1971). 相似文献
18.
Mannam Krishnamurthy Ashok K. Mishra Sneh K. Dogra 《Photochemistry and photobiology》1987,45(3):359-364
Abstract— The absorption and fluorescence spectra of indole-4-carboxylic acid in various solvents have indicated that the -COOH group is more planar with respect to the indole ring in the first excited singlet state (S1 ) than in the ground (S0 ) state. Relatively large Stokes' shifts indicate that polarisability and dipole moment of the molecule are increased predominantly upon excitation. Prototropic reactions in the S0 and S1 states are the same. The -COO- and -COOH+2 groups are not coplanar in the S0 , but coplanar in the S1 state. pH-dependent fluorescence spectra have revealed that both protonation and deprotonation of the -COOH group increase the basicity of the molecule upon excitation. 相似文献
19.
Abstract— Laser flash photolysis of trans -[Rh(dppe)2 X2 ][PF6 ] (X=Br and I; dppe=bis(diphenylphosphino)ethane) in CH2 Cl2 or CH3 CN produces the d7 Rh(II) radicals, [Rh(dppe)2 X]+ , and halogen atoms. The kinetics of the disappearance of [Rh(dppe)2 X]+ radicals in CH2 Cl2 or CH3 CN were mixed order: H-atom abstraction from solvent to produce the rhodium hydrides, [RhH(dppe)2 X][PF6 ], and Rh/X recombination. In the poor H-atom donor solvent, benzonitrile, Rh/Br recombination was observed to be uncomplicated by competing H-atom abstraction. The hydride complexes [RhH(dppe)2 X][PF6 ], formed by H-atom abstraction were completely characterized by 31 P{1 H}-NMR, 1 H-NMR, and mass specrometry. Cyclohexene was used as an effective trap for photogenerated Br atoms and yielded bromocyclohexane and 3-bromocyclohexene in a relative yield, 1:9. The photochemical mechanism is discussed in light of the transient absorbance and trapping studies. 相似文献
20.
Abstract— Chlorophyll a in aqueous hexane solutions has two fluorescence peaks, two fluorescence excitation peaks in the blue, and two fluorescence excitation peaks in the red. One of these set of peaks (excitation 2.34μ -1 and 1.52μ -1, fluorescence 1–50μ -1 ) is identical with those found in methanol solutions of chlorophyll a and is assigned to an unaggregated species, while the other set of peaks (excitation 2.44μ -1 and 1.475μ -l , fluorescence 1.48μ -l ) is assigned to a species aggregated in the presence of water. The spectral shifts occurring in the aggregate follow the selection rules obtained by McRae and Kasha for a linear stack of parallel dye molecules. The sharply increased overlap of absorption and fluorescence leads to resonance fluorescence in the aggregate. The resonance fluorescence is demonstrated experimentally. 相似文献