The high reactivity of 6π‐electrocyclization in polar solvents has remained one of the important challenges for diarylethenes because of the emergence of a twisted intramolecular charge transfer (TICT) state at the excited state in such polar media, which usually quenches the photocyclization reaction. Herein we report on the preparation and highly efficient photocyclization of 2,3‐diarylbenzo[b]thiophenes with nonsymmetric side‐aryl units in a polar solvent. While the dithiazolylbenzo[b]thiophene showed a suppressed quantum yield of 6π‐electrocyclization of 54 % in methanol, the replacement of a thiazole unit with a thiophene ring led to a photon‐quantitative 6π‐cyclization reaction. The nonsymmetrical modification into the side‐aryl units was considered to enhance the CH/π interactions between side‐aryl units to support a photoreactive conformation in methanol. The stabilization of the photochromic reactive conformation is expected to suppress the formation of the TICT state at the excited state, leading to highly efficient photoreactivity. 相似文献
Fluorescence excitation spectra of 2-aminopurine (2AP) incorporated into single-stranded DNA di- and trinucleotides, as well as into single- and double-stranded pentanucleotides, have been measured as a function of temperature from 5 to 65 °C. Spectral shifts have been precisely quantitated through difference spectroscopy and spectral fits. G(2AP)C and C(2AP)G oligonucleotides have relatively blue-shifted excitation spectra (especially the former) compared to the 2AP free base. The position of the excitation peak of 2AP free base is temperature independent, those of (2AP)T, G(2AP)C, C(2AP)G and TT(2AP)TT shift about 0.4 nm to the blue from 5 to 65 °C, though the spectra of the G-C-containing oligomers also change shape. The temperature dependence of the A(2AP)T spectral position is 2.5-times stronger, and just rises to that of the free base at high temperature. On the other hand, the decrease of yield with increasing temperature is smallest for A(2AP)T, even compared to the free base. The dominant effect when A neighbors 2AP appears to be temperature-dependent stacking with accompanying energy transfer, while in G- and C-containing trinucleotides a temperature-independent interaction keeps the 2AP excitation spectrum blue-shifted. The effect of double strand formation appears to be small compared to stacking interactions. These spectra can be useful in identifying base neighbors and structures of 2AP in unknown 2AP-labeled DNA. 相似文献
The symmetry of low-coverage (0.1 ML) ethylene adsorbed on Pd(110) and the co-adsorption effect of hydrogen have been investigated by high-resolution electron energy-loss spectroscopy. Ethylene adsorbs with π-bond character intact on both clean and H-covered Pd(110) with characteristic C–C stretching vibrational energies at 178 and 186 meV, respectively. The symmetry of the adsorbed ethylene, however, drastically changes upon the co-adsorption of hydrogen: the C–C axis which is tilted to the clean Pd(110) surface (C1 symmetry) is rearranged such that it becomes parallel to the H-covered Pd(110) surface (C2 symmetry). 相似文献
Using the bubbling argument of Sacks and Uhlenbeck, we prove the existence of n-harmonic maps from the n-sphere to Riemannian manifolds. An application is made to a problem concerning manifolds with strongly pth moment stable stochastic dynamical systems. 相似文献
Enzymatic degradation of polylactic acid is studied experimentally and analytically. Gel permeation chromatography profiles obtained before and after the enzymatic degradation of polylactic acid (PLA) were introduced into the analysis based on a mathematical model. Previously developed techniques were successfully adapted to the analysis of an initial value problem consisting of an endogenous depolymerization model and an initial condition, and an inverse problem to determine the degradation rate for which the solution of the initial value problem also satisfies a final condition. Those problems were solved numerically and numerical results are introduced. Degradabilities of PLA and polyvinyl alcohol are compared.
The palladium-catalyzed 1,1-alkynylbromination of terminal alkenes with a silyl-protected alkynyl bromide is reported. The method tolerates a diverse range of alkenes including vinylarenes, acrylates, and even electronically unbiased alkene derivatives to afford propargylic bromides regioselectively. Mechanistic studies and DFT calculations indicate that the 1,1-alkynylbromination reaction proceeds via the migration of the Pd center followed by the formation of a π-allenyl Pd intermediate, leading to the stereoselective reductive elimination of the C(sp3)–Br bond at the propargylic positon.The first Pd-catalyzed 1,1-alkynylbromination of terminal alkenes using alkynyl bromides, which provides direct access to a variety of functionalized propargylic bromides without the need for an external brominating reagent, is reported.相似文献
Molecular design and photoelectrochemically responsive functions of several redox‐tunable building blocks toward conjugated assembling were described. The viologen dimers and pyridylpyridinium dimers through π‐conjugated linkers showed multi‐step redox processes which were demonstrated electrochemically and photochemically. The 4,6‐pyrimidinylene and 3,6‐pyridazinylene linkers played an important role as effective mediators between two attached electrons in the two viologens or pyridinium groups. The latter pyridylpyridinium dimers and 3‐pyridylviologen have potential to coordinate various metal ions to give conjugated self‐assemblies. 相似文献