首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   18篇
  免费   0篇
化学   18篇
  2007年   1篇
  2006年   1篇
  2005年   4篇
  2004年   1篇
  2003年   1篇
  2002年   1篇
  2000年   1篇
  1999年   1篇
  1996年   1篇
  1985年   2篇
  1983年   1篇
  1974年   3篇
排序方式: 共有18条查询结果,搜索用时 15 毫秒
1.
A new procedure for measuring time-resolved emission spectra has been implemented. This technique has subnanosecond time resolution combined with the sensitivity and dynamic range needed to cope with extremely weak luminescence. Using this method the emissions of Cr(NH3)2 (NCS)4? and Cr(NCS)63- in aqueous solution at room temperature have each been analyzed into two components. The fast component has a broad spectrum and is assigned to prompt fluorescence with lifetime below 100 ps. The slow component is dominated by phosphorescence but may include some delayed fluorescence. The phosphorescence lifetime is 5.5 ± 0.5 ns in Cr(NH3)2 (NCS)4? and 1.65 ± 0.1 ns in Cr(NCS)63-. Order of magnitude estimates have been derived for other photophysical parameters.  相似文献   
2.
Colloidal suspensions of 3-aminopropylmethyl(tetraphenyl)silole nanoparticles can be used as selective chemosensors for carcinogenic chromium(VI) analyte. Methylhydrosilole is functionalized by hydrosilation of allylamine, and the colloid is prepared by the rapid addition of water to a THF solution of the silole. The method of detection is through electron-transfer quenching of the fluorescence of the silole colloid (lambda(em) = 485 nm at 360 nm excitation) by the analytes, with hundred parts per billion detection limits. Stern-Volmer plots are linear up to 10 ppm in the case of chromium, but exhibit saturation behavior near 5-10 ppm for arsenic. Dynamic light scattering experiments and AFM measurements show the particle sizes to be around 100 nm in diameter and dependent on solvent composition, with a particle size dispersity of +/-25%. The fluorescence lifetimes of the silole in solution and colloid are approximately 31 ps and approximately 4.3 ns, respectively, while the silole has a lifetime of 6 ns in the bulk solid. A minimum volume fraction of 80% water is necessary to precipitate the colloid from THF, and the luminescence continues to rise with higher water fractions. Colloids in a pH 7 phosphate-buffered suspension show both higher sensitivity and greater selectivity (100-fold) for CrO4(2-) detection than for other oxoanion interferents, NO3-, NO2-, SO4(2-), and ClO4-.  相似文献   
3.
The synthesis, spectroscopic characterization, and fluorescence quenching efficiency of polymers and copolymers containing tetraphenylsilole or tetraphenylgermole with Si-Si, Ge-Ge, and Si-Ge backbones are reported. Poly(tetraphenyl)germole, 2, was synthesized from the reduction of dichloro(tetraphenyl)germole with 2 equivs of Li. Silole-germole alternating copolymer 3 was synthesized by coupling dilithium salts of tetraphenylsilole dianion with dichloro(tetraphenyl)germole. Other tetraphenylmetallole-silane copolymers, 4-12, were synthesized through the Wurtz-type coupling of the dilithium salts of the tetraphenylmetallole dianion and corresponding dichloro(dialkyl)silanes. The molecular weights (M(w)) of these metallole-silane copolymers are in the range of 4000 approximately 6000. Detection of nitroaromatic molecules, such as nitrobenzene (NB), 2,4-dinitrotoluene (DNT), 2,4,6-trinitrotoluene (TNT), and picric acid (PA), has been explored. A linear Stern-Volmer relationship was observed for the first three analytes, but not for picric acid. Fluorescence spectra of polymetalloles or metallole-silane copolymers obtained in either toluene solutions or thin polymer films displayed no shift in the maximum of the emission wavelength. This suggests that the polymetalloles or metallole-silanes exhibit neither pi-stacking of polymer chains nor excimer formation. Fluorescence lifetimes of polymetalloles and metallole-silanes were measured both in the presence and absence of TNT, and tau(o)/tau is invariant. This requires that photoluminescence quenching occurs by a static mechanism.  相似文献   
4.
Abstract— Fluorescence lifetimes of spinach chloroplasts were measured with a modelocked dye laser and time-correlated single photon counting. Information about energy transport and functional organization of the chloroplasts is revealed by such time-resolved fluorescence studies. Quenching experiments using treatment with UV light or the chemical agent dibromothymoquinone are consistent with the notion that there is heterogeneity associated with PS II units and that such heterogeneity is reflected over the entire time range of fluorescence decay, not just in a single component. Phosphorylation experiments were also carried out which permit us to relate these kinetic studies to previous steady state observations.  相似文献   
5.
Thoi VS  Stork JR  Magde D  Cohen SM 《Inorganic chemistry》2006,45(26):10688-10697
Although free dipyrrins (dipyrromethenes) do not strongly luminesce, certain dipyrrinato complexes of BF2 and zinc(II) are known to be intensely luminescent species. Two new dipyrrinato fluorophores, based on complexes with gallium(III) and indium(III), are described. Using a previously described meso-mesityl-substituted dipyrrin, namely 5-mesityldipyrrin (mesdpm), the complexes [Ga(mesdpm)3] and [In(mesdpm)3] were prepared and structurally characterized. The complexes display the expected octahedral geometry about the metal ions. In some solvents, such as hexanes, the complexes emit green light upon excitation with UV light at room temperature, with quantum yields of 2.4% ([Ga(mesdpm)3]) and 7.4% ([In(mesdpm)3]) and lifetimes in the low nanosecond range. Observations are consistent with assignment to ligand-localized transitions, and this interpretation is further confirmed by density functional calculations described herein. The new complexes are important additions to the widely used family of dipyrrin-based fluorescent species and show that dipyrrinato complexes containing metals other than BF2 and zinc(II) may be useful fluorophores.  相似文献   
6.
Absolute fluorescence quantum yields are reported for the rhodamine 6G cation and the fluorescein dianion dyes in nine solvents. This information is combined with previously reported fluorescence lifetimes to deduce radiative and nonradiative decay rates. Along the alcohol series from methanol to octanol, rhodamine 6G displays an increasing radiative rate, in parallel with the square of the refractive index increase, and a slightly decreasing nonradiative rate. Fluorescein is different: the apparent radiative rate actually decreases, suggesting that the emissive species is perturbed in some fashion. For both dyes, fluorescence yields are enhanced in D2O, rising to 0.98, in parallel with a corresponding increase in lifetimes. Protonated solvents invariably give shorter lifetimes and lower quantum yields, contrary to some previous speculation. From this work and an analysis of existing literature values, more precise values have been obtained for two previously proposed absolute quantum yield standards. The yield of fluorescein in 0.1 N NaOH(aq) is 0.925+/-0.015, and for rhodamine 6G in ethanol, it is 0.950+/-0.015. In both cases, the solutions are assumed to be in the limit of low concentration, excited close to their long-wave absorption band and at room temperature but may be either air-saturated or free of oxygen.  相似文献   
7.
The synthesis of caged NADP analogues 18, 19, and 20 has been accomplished by utilizing the transglycosidase activity of solubilized NAD glycohydrolase (porcine brain) to incorporate caged nicotinamides 2, 3, and 4 into NADP. The synthesis of several nicotinamides modified at the carboxamide with o-nitrobenzyl photolabile groups is demonstrated as well as their potential for enzymatic transglycosidation. These results further demonstrate the feasibility of direct enzymatic transglycosidation of sterically hindered substrates into NAD(P), although high nicotinamide analogue water solubility was found to be a necessary trait for yield enhancement with certain analogues. Caged analogues were surveyed under aqueous conditions for net NADP photorelease, while the UV and fluorescent properties of both analogues and their photobyproducts were assessed for compatibility with systems that rely on optical monitoring of enzyme activity. A highly water-soluble alpha-methyl-o-nitrobenzyl group 8 was developed for the synthesis of 20 in order to enhance net NADP photorelease. Compound 20 demonstrated a high 75% net NADP photoreleased without substantial UV optical blackening or fluorescent byproducts. Analogues 18 and 19 were shown by ESI/MALDI-MS to photogenerate primarily adducts of NADP with deleterious UV and fluorescent properties. Our work stresses the superior release properties conferred by alpha-methyl substitution on aqueous carboxamide photorelease from o-nitrobenzyl compounds.  相似文献   
8.
A family of heteroleptic RuII coordination complexes containing substituted 1,10-phenanthroline (phen) ligands with extended conjugation was found to exhibit two simultaneously emissive excited states at room temperature in fluid solution. These systems demonstrate a breakdown of the standard nonradiative decay pathways that normally lead to a single, dominant, lowest energy emissive excited state in RuII complexes and most other chromophores. The structural requirements for dual emission were explored through the synthesis and characterization of isomeric systems. Two features were found to be primarily responsible for resolvable dual emission. Extended conjugation at the 4-position of the 1,10-phenanthroline ligand was identified as an essential feature, and asymmetry in the phenanthroline ligand substitutions appears to greatly facilitate the production of these two nonequilibrated emissive states. Additional complexes were studied which displayed "tunable" emissive characteristics for the two excited states as a function of covalent and noncovalent modification.  相似文献   
9.
Solvent Dependence of the Fluorescence Lifetimes of Xanthene Dyes   总被引:2,自引:0,他引:2  
Fluorescence lifetimes of Ave representative xanthene dye species-the rhodamine B zwitterion (RB=), the rhoda-mine B cation (RB+), the rhodamine 6G cation (R6G+), the rhodamine 101 zwitterion (R101) and the fluorescein dianion (F2-)-were measured in H2O, D2O and in a series of alcohol solvents ranging from methanol to octanol. The lifetimes of both RB= and RB+ increased markedly as the solvent was varied from water to octanol. In contrast, the lifetimes of R6G+ and R101± decreased slightly over the alcohol series and that of F2- increased only slightly in the same series. For all the dyes studied the fluorescence lifetimes observed in D2O were slightly longer than those in H2O. Possible causes for the variations observed are discussed.  相似文献   
10.
From picosecond flash photolysis and spectroscopy, the absorption spectra of the first excited singlet S1 and of the first excited triplet T1 of (OEP)SnCl2 are determined; we also determine triplet quantum yield, φt ≈ 0.80. Similar S1 and T1 phototransient absorption spectra occur in (OEP)Pd. However, two Cu porphyrins show only one phototransient absorption spectrum. Our T1 absorption results are generally consistent with the available earlier spectroscopic studies made with other techniques. Our value for φt lies intermediate between values given by two earlier studies, which were based on two different methods.  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