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151.
Genetically controlled MRI contrast mechanisms and their prospects in systems neuroscience research 总被引:1,自引:0,他引:1
Application of MRI contrast agents to neural systems research is complicated by the need to deliver agents past the blood-brain barrier or into cells, and the difficulty of targeting agents to specific brain structures or cell types. In the future, these barriers may be wholly or partially overcome using genetic methods for producing and directing MRI contrast. Here we review MRI contrast mechanisms that have used gene expression to manipulate MRI signal in cultured cells or in living animals. We discuss both fully genetic systems involving endogenous biosynthesis of contrast agents, and semi-genetic systems in which expressed proteins influence the localization or activity of exogenous contrast agents. We close by considering which contrast-generating mechanisms might be most suitable for applications in neuroscience, and we ask how genetic control machinery could be productively combined with existing molecular agents to enable next-generation neuroimaging experiments. 相似文献
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153.
Ciarán R. S. Mooney Michael A. Parkes Andreas Iskra Helen H. Fielding 《Angewandte Chemie (International ed. in English)》2015,54(19):5646-5649
To understand how photoactive proteins function, it is necessary to understand the photoresponse of the chromophore. Photoactive yellow protein (PYP) is a prototypical signaling protein. Blue light triggers trans–cis isomerization of the chromophore covalently bound within PYP as the first step in a photocycle that results in the host bacterium moving away from potentially harmful light. At higher energies, photoabsorption has the potential to create radicals and free electrons; however, this process is largely unexplored. Here, we use photoelectron spectroscopy and quantum chemistry calculations to show that the molecular structure and conformation of the isolated PYP chromophore can be exploited to control the competition between trans–cis isomerization and radical formation. We also find evidence to suggest that one of the roles of the protein is to impede radical formation in PYP by preventing torsional motion in the electronic ground state of the chromophore. 相似文献
154.
Bright Fluorescence and Host–Guest Sensing with a Nanoscale M4L6 Tetrahedron Accessed by Self‐Assembly of Zinc–Imine Chelate Vertices and Perylene Bisimide Edges 下载免费PDF全文
Dr. Peter D. Frischmann Valentin Kunz Prof. Dr. Frank Würthner 《Angewandte Chemie (International ed. in English)》2015,54(25):7285-7289
A highly luminescent Zn4L6 tetrahedron is reported with 3.8 nm perylene bisimide edges and hexadentate ZnII–imine chelate vertices. Replacing FeII and monoamines commonly utilized in subcomponent self‐assembly with ZnII and tris(2‐aminoethyl)amine provides access to a metallosupramolecular host with the rare combination of structural integrity at concentrations <10?7 mol L?1 and an exceptionally high fluorescence quantum yield of Φem=0.67. Encapsulation of multiple perylene or coronene guest molecules is accompanied by strong luminescence quenching. We anticipate this self‐assembly strategy may be generalized to improve access to brightly fluorescent coordination cages tailored for host–guest light‐harvesting, photocatalysis, and sensing. 相似文献
155.
Eric A. Margulies Dr. Yi‐Lin Wu Przemyslaw Gawel Dr. Stephen A. Miller Dr. Leah E. Shoer Prof. Dr. Richard D. Schaller Prof. Dr. François Diederich Prof. Dr. Michael R. Wasielewski 《Angewandte Chemie (International ed. in English)》2015,54(30):8679-8683
Thin films of 5,11‐dicyano‐6,12‐diphenyltetracene ( TcCN ) have been studied for their ability to undergo singlet exciton fission (SF). Functionalization of tetracene with cyano substituents yields a more stable chromophore with favorable energetics for exoergic SF (2E(T1)?E(S1)=?0.17 eV), where S1 and T1 are singlet and triplet excitons, respectively. As a result of tuning the triplet‐state energy, SF is faster in TcCN relative to the corresponding endoergic process in tetracene. SF proceeds with two time constants in the film samples (τ=0.8±0.2 ps and τ=23±3 ps), which is attributed to structural disorder within the film giving rise to one population with a favorable interchromophore geometry, which undergoes rapid SF, and a second population in which the initially formed singlet exciton must diffuse to a site at which this favorable geometry exists. A triplet yield analysis using transient absorption spectra indicates the formation of 1.6±0.3 triplets per initial excited state. 相似文献
156.
Dr. Sergey P. Laptenok Prof. Andras Lukacs Agnieszka Gil Dr. Richard Brust Dr. Igor V. Sazanovich Dr. Gregory M. Greetham Prof. Peter J. Tonge Prof. Stephen R. Meech 《Angewandte Chemie (International ed. in English)》2015,54(32):9303-9307
Proton transfer is critical in many important biochemical reactions. The unique three‐step excited‐state proton transfer in avGFP allows observations of protein proton transport in real‐time. In this work we exploit femtosecond to microsecond transient IR spectroscopy to record, in D2O, the complete proton transfer photocycle of avGFP, and two mutants (T203V and S205V) which modify the structure of the proton wire. Striking differences and similarities are observed among the three mutants yielding novel information on proton transfer mechanism, rates, isotope effects, H‐bond strength and proton wire stability. These data provide a detailed picture of the dynamics of long‐range proton transfer in a protein against which calculations may be compared. 相似文献
157.
Dipl. Chem. Yannick Baeten Dr. Eduard Fron Dr. Christian Ruzié Prof. Dr. Yves Henri Geerts Prof. Dr. Mark Van Der Auweraer 《Chemphyschem》2015,16(18):3992-3996
Phthalocyanines (Pcs) have attracted a lot of interest as small molecules for organic electronics. However, some excited‐state properties of metal‐free phthalocyanines, as for example, the dynamics of the transition between the nondegenerate Qx and Qy states in a metal‐free phthalocyanine, have not been fully established. This effect results in a blue‐shifted shoulder with low intensity in the Pc fluorescence spectrum. This shoulder was suggested to be related to emission from the more energetic Qy state. By using ultrafast femtosecond transient absorption, we have found a clear equilibrium between the Qx and Qy state of metal‐free phthalocyanines in solution. 相似文献
158.
Herein, we report a novel approach for the synthesis of π-conjugated peptide-based donor-acceptor (D-π-A) chromophores, by reacting electron-rich alkynes with tetracyanoethylene. The desired tetracyanobutadiene-scaffolded peptides were obtained in good yields with various optical properties, λmax: 321-492 nm, ε: 21,000-65,000 mol−1 dm3 cm−1 depending on the substitution pattern of the cyanobutadiene scaffold. 相似文献
159.
160.
Almost all time-dependent density-functional theory (TDDFT) calculations of excited states make use of the adiabatic approximation, which implies a frequency-independent exchange-correlation kernel that limits applications to one-hole/one-particle states. To remedy this problem, Maitra et al. [N.T. Maitra, F. Zhang, R.J. Cave, K. Burke, Double excitations within time-dependent density functional theory linear response theory, J. Chem. Phys. 120 (2004) 5932 ] proposed dressed TDDFT (D-TDDFT), which includes explicit two-hole/two-particle states by adding a frequency-dependent term to adiabatic TDDFT. This paper offers the first extensive test of D-TDDFT, and its ability to represent excitation energies in a general fashion. We present D-TDDFT excited states for 28 chromophores and compare them with the benchmark results of Schreiber et al. [M. Schreiber, M.R. Silva-Junior, S.P.A. Sauer, W. Thiel, Benchmarks for electronically excited states: CASPT2, CC2, CCSD, and CC3, J. Chem. Phys. 128 (2008) 134110]. We find the choice of functional used for the A-TDDFT step to be critical for positioning the 1h1p states with respect to the 2h2p states. We observe that D-TDDFT without HF exchange increases the error in excitations already underestimated by A-TDDFT. This problem is largely remedied by implementation of D-TDDFT including Hartree-Fock exchange. 相似文献