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排序方式: 共有171条查询结果,搜索用时 156 毫秒
101.
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Henriette Gauduchon-Truchot A. Contardi S. Erighian M. I. Knjaginitschew Ch. F. Poe R. T. Dewey C. N. Acharya H. S. Miller J. Speirs W. J. Mitchell A. Henwood R. Carey R. A. Osborn J. B. Wilkie E. J. Schwoegler B. J. Babler L. C. Hurd Z. Zakrzewski H. J. Fuchs R. A. Harte S. Morgulis H. C. Spencer 《Analytical and bioanalytical chemistry》1938,113(9-10):367-370
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Hooi Ling Kee James R. Diers Marcin Ptaszek Chinnasamy Muthiah Dazhong Fan Jonathan S. Lindsey David F. Bocian Dewey Holten 《Photochemistry and photobiology》2009,85(4):909-920
The photophysical properties of two energy‐transfer dyads that are potential candidates for near‐infrared (NIR) imaging probes are investigated as a function of solvent polarity. The dyads ( FbC‐FbB and ZnC‐FbB ) contain either a free base (Fb) or zinc (Zn) chlorin (C) as the energy donor and a free base bacteriochlorin (B) as the energy acceptor. The dyads were studied in toluene, chlorobenzene, 1,2‐dichlorobenzene, acetone, acetonitrile and dimethylsulfoxide (DMSO). In both dyads, energy transfer from the chlorin to bacteriochlorin occurs with a rate constant of ~(5–10 ps)?1 and a yield of >99% in nonpolar and polar media. In toluene, the fluorescence yields (Φ f = 0.19) and singlet excited‐state lifetimes (τ~5.5 ns) are comparable to those of the benchmark bacteriochlorin. The fluorescence yield and excited‐state lifetime decrease as the solvent polarity increases, with quenching by intramolecular electron (or hole) transfer being greater for FbC‐FbB than for ZnC‐FbB in a given solvent. For example, the Φ f and τ values for FbC‐FbB in acetone are 0.055 and 1.5 ns and in DMSO are 0.019 and 0.28 ns, whereas those for ZnC‐FbB in acetone are 0.12 and 4.5 ns and in DMSO are 0.072 and 2.4 ns. The difference in fluorescence properties of the two dyads in a given polar solvent is due to the relative energies of the lowest energy charge‐transfer states, as assessed by ground‐state redox potentials and supported by molecular‐orbital energies derived from density functional theory calculations. Controlling the extent of excited‐state quenching in polar media will allow the favorable photophysical properties of the chlorin–bacteriochlorin dyads to be exploited in vivo. These properties include very large Stokes shifts (85 nm for FbC‐FbB , 110 nm for ZnC‐FbB ) between the red‐region absorption of the chlorin and the NIR fluorescence of the bacteriochlorin (λ f = 760 nm), long bacteriochlorin excited‐state lifetime (~5.5 ns), and narrow (≤20 nm) absorption and fluorescence bands. The latter will facilitate selective excitation/detection and multiprobe applications using both intensity‐ and lifetime‐imaging techniques. 相似文献
106.
Huffman PR Brome CR Butterworth JS Coakley KJ Dewey MS Dzhosyuk SN Golub R Greene GL Habicht K Lamoreaux SK Mattoni CE McKinsey DN Wietfeldt FE Doyle JM 《Nature》2000,403(6765):62-64
Accurate measurement of the lifetime of the neutron (which is unstable to beta decay) is important for understanding the weak nuclear force and the creation of matter during the Big Bang. Previous measurements of the neutron lifetime have mainly been limited by certain systematic errors; however, these could in principle be avoided by performing measurements on neutrons stored in a magnetic trap. Neutral-particle and charged-particle traps are widely used for studying both composite and elementary particles, because they allow long interaction times and isolation of particles from perturbing environments. Here we report the magnetic trapping of neutrons. The trapping region is filled with superfluid 4He, which is used to load neutrons into the trap and as a scintillator to detect their decay. Neutrons in the trap have a lifetime of 750(+330)(-200) seconds, mainly limited by their beta decay rather than trap losses. Our experiment verifies theoretical predictions regarding the loading process and magnetic trapping of neutrons. Further refinement of this method should lead to improved precision in the neutron lifetime measurement. 相似文献
107.
Douglas Magde Maurice W. Windsor Dewey Holten Martin Gouterman 《Chemical physics letters》1974,29(2):183-188
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
108.
Natural products are a rich source of therapeutics; however, artificially reengineering the biosynthetic pathways that generate these compounds could potentially generate "designer" drugs. Last month in Chemistry & Biology, Burkart and coworkers reported their technique to track and better understand the components of these pathways. 相似文献
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Kee HL Kirmaier C Yu L Thamyongkit P Youngblood WJ Calder ME Ramos L Noll BC Bocian DF Scheidt WR Birge RR Lindsey JS Holten D 《The journal of physical chemistry. B》2005,109(43):20433-20443
Boron-dipyrrin chromophores containing a 5-aryl group with or without internal steric hindrance toward aryl rotation have been synthesized and then characterized via X-ray diffraction, static and time-resolved optical spectroscopy, and theory. Compounds with a 5-phenyl or 5-(4-tert-butylphenyl) group show low fluorescence yields (approximately 0.06) and short excited-singlet-state lifetimes (approximately 500 ps), and decay primarily (>90%) by nonradiative internal conversion to the ground state. In contrast, sterically hindered analogues having an o-tolyl or mesityl group at the 5-position exhibit high fluorescence yields (approximately 0.9) and long excited-state lifetimes (approximately 6 ns). The X-ray structures indicate that the phenyl or 4-tert-butylphenyl ring lies at an angle of approximately 60 degrees with respect to the dipyrrin framework whereas the angle is approximately 80 degrees for mesityl or o-tolyl groups. The calculated potential energy surface for the phenyl-substituted complex indicates that the excited state has a second, lower energy minimum in which the nonhindered aryl ring rotates closer to the mean plane of the dipyrrin, which itself undergoes some distortion. This relaxed, distorted excited-state conformation has low radiative probability as well as a reduced energy gap from the ground state supporting a favorable vibrational overlap factor for nonradiative deactivation. Such a distorted conformation is energetically inaccessible in a complex bearing the sterically hindered o-tolyl or mesityl group at the 5-position, leading to a high radiative probability involving conformations at or near the initial Franck-Condon form of the excited state. These combined results demonstrate the critical role of aryl-ring rotation in governing the excited-state dynamics of this class of widely used dyes. 相似文献