首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献,搜索用时 562 毫秒
1.
Abstract— Although the vast majority of photochemical reactions in condensed phase lead directly to product molecules in the ground states it might be expected that in favorable cases a certain fraction of the reacting species would escape deactivation long enough to attain the product configuration adiabatically in an electronically excited state. In this communication, we report that di-9-methylanthracene, in addition to its normal diabatic course, also leads to small but finite amounts of both excited singlet monomer and excimer species. Oxygen quenching experiments seem to indicate that the monomer and excimer are derived from excited di-9-methylanthracene with little mutual interconversion. The fluorescence yields were determined for these processes in aerated solution at room temperature to be φ1= 0·00040 ± 0·00005 and φ2= 0·00037 ± 0·00005 for monomer and excimer species, respectively.  相似文献   

2.
Abstract— Fluorescence emission from merocyanine 540 (MC540) dimers was observed in dipalmitoylphosphatidylcholine (DPPC) vesicles. This unusual behavior was observed only for vesicles in the gel-phase state. No dimer fluorescence was observed either in monopalmitoylphosphati-dylcholine (C16PC) micelles or in liquid-crystalline DPPC vesicles, indicating that dimer fluorescence efficiency increases in highly packed interfaces. The excitonic theory of Kasha was used to interpret the spectral features. The overall fluorescence quantum yield (φr) decreases with decreasing lipid: probe ratio, not only because of the presence of a weakly fluorescent dimer that absorbs a high fraction of the total absorbed light but also due to quenching of monomer emission. This suggests the existence of probe domains. The dimer fluorescence quantum yields (φm) were estimated in DPPC large unilamellar vesicles (LUV) and DPPC multilamellar vesicles. The dependence of φr with probe concentration is compatible with values of φm lower than 0.05. The dimerization equilibrium of MC540 in C16PC micelles and DPPC-LUV was also studied. Apparent dimerization equilibrium constants, Kdapp and dimer absorption spectrum were calculated in C16PC micelles for the first time. The dimerization equilibrium constant in DPPC-LUV was calculated and discussed in terms of the fraction of volume occupied by the lipid phase.  相似文献   

3.
The main DNA-3-CPs furan side adduct has been isolated following acidic hydrolysis of photomodified DNA and subsequent reversed-phase high performance liquid chromatography purification. This photoadduct has been identified as a thymine-3-CPs C4-cycloadduct Thy<5 4'6 5', >3-CPs based on its optical spectroscopic features and of its plasma desorption mass spectrometric characteristics. Moreover, its reversed-phase high performance liquid chromatography retention time is in agreement with a cis-syn stereochemistry as compared to the chromatographic properties of the cis-syn model adduct Thy< 5 4'6 5' >3-CPs previously identified. Further support for the cis-syn stereoconfiguration assignment was provided by fluorescence quenching experiments using iodide and silver ions as external and internal quenchers respectively. These data strongly indicate that the Thy< 5 4'6 5' >3-CPs photoadduct is located inside the DNA helix in agreement with its cis-syn stereochemistry.  相似文献   

4.
Abstract Porphyrin-C60 dyads in which the two chromophores are linked by a bicyclic bridge have been synthesized using the Diels-Alder reaction. The porphyin singlet lifetimes of both the zinc (Pzn-C60) and free base (P-C60) dyads, determined by time-resolved fluorescence measurements, are ≦17 ps in toluene. This substantial quenching is due to singlet-singlet energy transfer to C60 The lifetime of Pzn-1C60 is -5 ps in toluene, whereas the singlet lifetime of an appropriate C60 model compound is 1.2 ns. This quenching is attributed to electron transfer to yield Pznbull;+-C60bull;-. In toluene, P-1C60 is unquenched; the lack of electron transfer is due to unfavorable thermodynamics. In this solvent, a transient state with an absorption maximum at 700 ran and a lifetime of-10 μs was detected using transient absorption methods. This state was quenched by oxygen, and is assigned to the C60 triplet. In the more polar benzonitrile, P-1C60 underoes photoinduced electron transfer to give P+-C60bull;-. The electron transfer rate constant is −2 × 1011 s−1.  相似文献   

5.
Abstract— Absorption, fluorescence and phosphorescence spectra as well as fluorescence and phosphorescence quantum yields of 8-X-5-deazaflavins (X = C1, NO2, p -NO2-C6H4, N(CH3)2, NH2, p -NH2-C6H4, p -N(CH3)2-C6H4-N=N) were determined. It was found that all these data are highly influenced by the substituent at position 8 of the 5-deazaisoalloxazine skeleton. Also the photoreduction of 8-X-5-deazaflavins in the presence of electron donors was studied. It was established that the photoreduction leads to the formation of a 5,5'-dimer and/or a 6,7-dihydro compound. Reduction of the C(6)-C(7) bond is promoted by strong electron-donating substituents and bulky electron donors. 5-Deazaftavins with a reducible substituent at position 8 exhibit reduction of the substituent prior to the reduction of the 5-deazaisoalloxazine skeleton.  相似文献   

6.
Abstract— The hematoporphyrin-sensitized production of singlet molecular oxygen, O2(1Δg), has been investigated in methanol and in aqueous solution. The quantum yield for formation of O2(1Δg) (ΦΔ) has been measured by both steady-state (oxygen consumption) and time-resolved (near-infrared luminescence) methods. In methanol, both techniques indicate that ΦΔ= 0.76 and the value remains independent of sensitizer concentration over a wide range. This finding is consistent with the dye persisting in a monomelic form in methanol solution. In contrast, ΦΔ decreases markedly with increasing sensitizer concentration in water due to dimerization of the dye. Analysis of the steady-state data indicates ΦΔ values of 0.74 and 0.12, respectively, for monomer and dimer. It is further shown that the efficiency whereby quenching of the triplet state by O2 results in generation of O2(1Δg) is substantially lower for the dimer than for the corresponding monomer. Because monomer and dimer possess quite different absorption spectral profiles, the efficacy for photodynamic action with hematoporphyrin exhibits a pronounced wavelength dependence.  相似文献   

7.
Abstract— The photophysical properties of 2-(2'-hydroxy-4'-diethylaminophenyl) benzothiazole (HABT) have been investigated by steady-state and time-resolved spectroscopies. In n-heptane HABT exhibits both normal and tautomer emissions with ∼equal fluorescence intensity at room temperature, in contrast to a previous report in which negligible tautomer emission was observed. The normal/tautomer (400/500 nm) ratio of emission intensity increases as the temperature decreases. Two possible excited-state intramolecular proton transfer (ESIPT) mechanisms are proposed, which cannot be resolved at the present stage. One proposed mechanism incorporates state mixing between -OH and -N(C2H5)2 charge transfer states, resulting in a significant energy barrier for ESIPT. An alternative mechanism is also proposed in which fast proton tunneling may take place between enol and keto forms, which are in equilibrium in the excited singlet state.  相似文献   

8.
Abstract— The acid-base equilibria of the excited singlet and triplet states of acridine orange (AO) were studied by flash-photolysis and fluorometric methods. The dye is a stronger base in the first excited singlet state (pKs= 13.3) than in the triplet and ground states (pKr= 10.3: pKc, = 10.2); acridine orange follows the trend observed with some other heterocyclic compounds, viz. pKs > pKr= pK,c. At room temperature, an anomalous fluorescence occurs from the dye in basic media: the assignment of this emission is discussed.
The semi-reduced dye was studied as a function of pH. In a large pH range (3–14), only the protolytic equilibrium between the cationic (AOH2+) and the neutral (AOH) radicals was observed; the pK value corresponding to this equilibrium was found to be in the range of pH 5–6.  相似文献   

9.
Abstract— The fluorescence quenching of the indole chromophore by NO2 and the fluorescence depolarization of several luminescence probes in aqueous solutions containing hexadecyltrimethylammonium bromide (HDTBr) were measured as a function of added C2–C4 aliphatic alcohol concentration. The fluorescence decay profiles of pyrene in the micellar solutions were also measured to estimate the aggregation number of the micelles. The addition of n -butyl alcohol significantly reduces the fluorescence quenching rate and the aggregation number and increases the extent of fluorescence depolarization in HDTBr micellar systems. The addition of ethyl alcohol shows a similar but smaller effect.  相似文献   

10.
Abstract— The photochemistry and photophysics of bis-(2-(9-anthryl)ethyl glutarate 2 and 2-(9-anthryl) ethyl pivalate 3 have been examined as monomeric models for poly-2-(9-anthryl)ethyl methacrylate 1. The absorption and excitation spectra of 2 show no ground state interaction, but appreciable monomer (51%) and excimer (49%) emission. Delayed monomer and excimer fluorescence are also observed in fluid solution at room temperature. Photolysis of 2 at low temperature produces a photoproduct (44% yield) which is unstable at room temperature and presumably is the head-to-head (H-H) dimer. Steady state irradiation of 2 at room temperature (Λ 350 nm) cleanly produces a head-to-tail (H-T) dimer (6) derived from the singlet state and an additional unidentified adduct (7). The product distribution is concentration dependent, with 6 predominating at high concentrations (> 10−3 M ). The fluorescence spectra of the broken dimers derived from 6 and 7 indicate that substantial ir overlap exists in the transition state leading to each product.  相似文献   

11.
Direct photolyses of pyrazinopsoralen (PzPs) with excess olefins such as dimethyl fumarate (DMFu), dimethyl maleate (DMMa) and dimethyl ethylidenemalonate (DMEM) gave C4-photocycloadducts. The photoproducts were determined to be 1:1 C4-cycloadducts formed through the addition of 4',5'-furan double bond of the excited PzPs to the olefins. The fluorescence of PzPs was quenched by olefins with rate constants on the order of 109–1010 M -1/s. The appearance of the long-lived fluorescence component implies a singlet exciplex mechanism for the photocycloaddition reaction of PzPs with excess olefins.  相似文献   

12.
Abstract— Photolysis at 254 nm of alkyl benzohydroxamates [C, H, CONHOR: R = CH3 H2CH3 CH(CH3)2, CH2C6H5 CH(CH3)C2H5 CH(CH3)- n -C6H13] in acetonitrile or hydrocarbon solvents gives benzamide. These reactions can be sensitized by benzophenone (at ca. 350 nm) and are quenched by cis-piperylene. Racemization occurred when 2-octyl (+)-benzohydroxamate was irradiated in cyclohexane. These results are consistent with a mechanism involving a triplet biradical. Photolysis of phenyl benzohydroxamate [C6H5CONHOC6H5] and benzyl N -methylbenzohydroxamate [C6H5CON-(CH3)OCH2Q6H5] cannot be quenched with ris-piperylene and appear to be singlet reactions.  相似文献   

13.
Abstract Solvent-cleaned Merino wool, which was enzymatically digested with protease K, displayed a fluorescence maximum at 500 nm when excited at 430 nm. The yield of this emission was approximately 15 times greater for a fiber tips digest than for a digest of the mid-length region of the same fibers.
Separation of the components in the wool tip digest by silica gel thin-layer chromatography revealed the presence of several fluorescent species. The chromatographic mobility of these species was similar to the behavior observed in a preparation of authentic 1-methyltetrahydro-P-carboline, 1,3-dicarboxylic acid.
Mass spectroscopy of the fluorescent compounds in the wool tip digest displayed molecular ions, [M + H]+ with m/z = 273.0890 and 257.0560, and molecular weights that define the formulae C14H12N2O4 and C11H8O4N2, respectively, which correspond to β-carboline 1,3-dicarboxylic acids. Mass spectral evidence also indicates the presence of two other β-carbolines.  相似文献   

14.
Abstract— An investigation has been made of the reaction between methylene, formed by the photolysis of ketene, and hydrogen. Ethane, ethylene and methane are the major hydrocarbon products, and it has been shown that the formation of these products may be adequately described by the sequence of processes
CH2CO + hv → CH2+ CO (1)
CH2+ H2→ CH3+H (2)
2CH3→ C2H6 (3)
CH3+ H2+ CH4+ H (4)
CH2+ CH2CO → C2H4+ CO (7)
In particular, the relative rates of ethane and methane formation are consistent with the known rate constants for reactions (3) and (4), and it is not therefore necessary to postulate the participation of an 'insertion' process
CH2+ H2→ CH4 (6) to account for the formation of methane.
Decrease of the energy possessed by the methylene, either by increase of the wavelength of ketene photolysis, or by increase of gas pressure, is shown to result in an increase in the reactivity of the methylene towards ketene relative to its reactivity towards hydrogen (i.e. the ratio k2/k2 increases).  相似文献   

15.
制备了β-萘甲酸十六碳酯的L-B膜。分别用Y型、Z型,以及含有硬脂酸的β-萘甲酸十六碳酯与单纯硬脂酸间隔挂法,将分子膜转移至石英载片上,研究了β-萘甲酸十六碳酯分子膜吸收以及荧光光谱。并与溶液和固态中的荧光光谱作了比较。在氯仿溶液以及在固态,β-萘甲酸十六碳酯只呈现其单体的荧光。然而,在L-B膜中,除了观察到单体的荧光外,还观察到了激发态二聚体以及激基缔合物发射的荧光。通过对β-萘甲酸十六碳酯分子膜进行热和水处理后,发现L-B膜中亲水层的介质环境有所改变,从而造成亲水层中的亲水基团的某些物理和化学性质的改变。根据实验事实,提出了β-萘甲酸十六碳酯二聚体和激基缔合物在L-B膜中形成的机制。  相似文献   

16.
Abstract— The molecule (1,1'-dipyrenyl)-methyl ether [dipyme] shows intramolecular excimer fluorescence in competition with fluorescence from the locally excited pyrene chromophore. This intensity ratio Ic/Im is sensitive to solvent viscosity. The molecule is soluble in synthetic phospholipid membranes. 1H NMR studies suggest that the molecule is localized in the hydrocarbon region of the membrane. Fluorescence measurements at various temperatures of dipyme dissolved in these membranes show that Ic/Im is exceedingly sensitive to fluidity changes accompanying both the pretransitions and the gel to liquid crystalline transitions of the membrane. These studies can be carried out at a mole ratio of probe to lipid 102-103 smaller than that necessary to observe bimolecular pyrene excimer formation.  相似文献   

17.
Dimerization of chloroaluminum (III) phthalocyanine tetrasulfonates (AIPCS) has been observed in different aqueous alcoholic solvents at room temperature by absorption and fluorescence spectroscopic methods. Both absorption and fluorescence spectral bands of the dimer are red shifted by ca 550 cm-1 from the monomer Q bands in the corresponding spectra, suggesting that the interaction energy between the two monomer subunits is very weak. The fluorescence lifetime of the dimer is longer ( ca 9.5 ns) than that of the monomer ( ca 7–8 ns). These spectral behaviors of AIPCS dimer contrast with those of transition-metallaloid phthalocyanine dimers, which usually have a nonfluorescent face-to-face stacking conformation. The dimer fluorescence is interpreted to be due to the fact that the lowest excited singlet state of the dimer is lower in energy than a charge-resonance state, based on the excitoncoupling theory applied to the face-to-face slipping conformation. The dimerization constant determined spectrometrically decreases with an increase of water content in the aqueous alcoholic solution. Propanol and ethanol have been observed to be more effective than methanol in promoting dimerization. These results indicate that a specific interaction of water with AIPCS plays an important role in the inhibition of dimerization of AIPCS.  相似文献   

18.
Abstract— The synthesis, absorption and emission properties of erythro (e) and threo (t) Na-acetyl-1-pyrenylalanyl-1-methyltryptophan methylester (APTE) are reported. From the dependency of the exciplex emission maximum on the solvent polarity, the exciplex dipole moment of erythro and threo APTE were calculated. The evolution of the ratio of the quantum yield of exciplex emission and the quantum yield of emission from the locally excited state is correlated with solvent polarity and with the tendency of the solvent to interact with the peptide chain through hydrogen bonding. It is shown that solvents, inert towards the peptide function, shift the equilibrium between the two ground state conformations towards C7, in which an exciplex geometry can be reached. Hydrogen accepting solvents shift the conformational equilibrium towards C5, which cannot form an exciplex directly within the lifetime of excited pyrene.  相似文献   

19.
Abstract— One of the four major photoproducts formed by UV irradiation of TpT in aqueous solution has a number of unusual properties. This compound, which we arbitrarily call TpT4, is produced irreversibly from TpT with single-hit kinetics and seems to represent a novel pathway for TpT photolysis different from the conventional cyclobutane ring dimer formation. This communication describes the preparative isolation of TpT4 by ion-exchange chromatography on DEAE-cellulose and its UV, fluorescence and IR spectra. TpT4 has an absorption maximum at 325 mμ, fluoresces at 405 mμ when excited at 325 mμ, and has an IR spectrum consistent with the assumption that one of the thymine rings has been converted to a pyrimidin-2-one structure. In addition, TpT4 appears to have both pyrimidine rings linked together and does not contain a peroxide group as previously supposed. Spectrophotometric titration shows that TpT4 has a pKâ of 10.75 and is unstable to prolonged treatment at extreme pH. A model structure consistent with the present data is given. This model contains the glycosidic and phosphodiester bonds intact with a 5-methylpyrimidine-2-one ring and a thymine ring joined in ether linkage at the C4 positions, with an OH at the C4 position in the thymine ring.  相似文献   

20.
Abstract— The blue light absorption band of roseoflavin is polarized along the axis roughly connecting N3-C8 positions. A weak, second π→π* transition with a polarization angle of ca. 25° is hidden under the short wavelength side of the blue absorption band. The excited state of roseoflavin is somewhat more basic than the ground state, by a 1.5 p K a unit. The fluorescence quantum yield and lifetime of roseoflavin are substantially lower than those of other flavins, thus making it kinetically less efficient as a blue light photoreceptor.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号