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排序方式: 共有614条查询结果,搜索用时 343 毫秒
531.
Natalie A. Pace Brandon K. Rugg Christopher H. Chang Obadiah G. Reid Karl J. Thorley Sean Parkin John E. Anthony Justin C. Johnson 《Chemical science》2020,11(27):7226
In singlet fission (SF) the initially formed correlated triplet pair state, 1(TT), may evolve toward independent triplet excitons or higher spin states of the (TT) species. The latter result is often considered undesirable from a light harvesting perspective but may be attractive for quantum information sciences (QIS) applications, as the final exciton pair can be spin-entangled and magnetically active with relatively long room temperature decoherence times. In this study we use ultrafast transient absorption (TA) and time-resolved electron paramagnetic resonance (TR-EPR) spectroscopy to monitor SF and triplet pair evolution in a series of alkyl silyl-functionalized pentadithiophene (PDT) thin films designed with systematically varying pairwise and long-range molecular interactions between PDT chromophores. The lifetime of the (TT) species varies from 40 ns to 1.5 μs, the latter of which is associated with extremely weak intermolecular coupling, sharp optical spectroscopic features, and complex TR-EPR spectra that are composed of a mixture of triplet and quintet-like features. On the other hand, more tightly coupled films produce broader transient optical spectra but simpler TR-EPR spectra consistent with significant population in 5(TT)0. These distinctions are rationalized through the role of exciton diffusion and predictions of TT state mixing with low exchange coupling J versus pure spin substate population with larger J. The connection between population evolution using electronic and spin spectroscopies enables assignments that provide a more detailed picture of triplet pair evolution than previously presented and provides critical guidance for designing molecular QIS systems based on light-induced spin coherence.Pentadithiophene derivatives produce triplet pairs efficiently with secondary spin state evolution that depends on their unique intermolecular juxtapositions. 相似文献
532.
The hydrazinolysis reaction of 5-perfluoroalkyl-1,2,4-oxadiazoles has been investigated. Nucleophilic addition of the reagent to the C(5)-N(4) double bond of the oxadiazole ring, followed by ring-opening and then ring-closure involving the beta-nitrogen atom of the hydrazino moiety and the C(3) of the oxadiazole ring, explains the formation of 5-perfluoroalkyl-1,2,4-triazoles as final products. Useful applications in synthesis of this uncommon hydrazinolysis can be claimed. 相似文献
533.
Pozzar F Sassi A Pace G Lora S D'Archivio AA Jeràbek K Grassi A Corain B 《Chemistry (Weinheim an der Bergstrasse, Germany)》2005,11(24):7395-7404
Six gel-type functional resins, that is, three poly-DMAA-co-TMPTP (DMAA = N,N-dimethylacrylamide, TMPTP = trimethylolpropyltrimethacrylate) samples with different degrees of cross-linking (0.6, 1.2, 1.7 % mol) and three poly-DMAA-co-MA-co-TMPTP (MA = methacrylic acid, ca. 5.5 % mol) samples with 1.7, 3.5, and 7 % mol cross-linking were investigated with ISEC (inverse steric exclusion chromatography), and ESR and CP-MAS (cross polarization magic angle spinning) 13C NMR spectroscopy after swelling in water and other solvents. This unprecedented combination of conceptually independent physicochemical techniques provides a thorough overall consistent picture of the morphology of the resins on the nanometer scale and of the molecular accessibility of the swollen polymer framework to the paramagnetic probe TEMPONE (2,2,6,6-tetramethyl-4-oxo-1-oxypiperidine) and to selected solvents. 相似文献
534.
535.
Salvatore Andini Giuseppina Castronuovo Vittorio Elia Filomena Velleca 《Journal of solution chemistry》1995,24(5):485-497
Calorimetric, densimetric, and nuclear relaxation time measurements were carried out at 25°C on binary and ternary aqueous solutions containing L and D forms of the following amino acids bearing substituted alkyl chains: lysine, glutamine, arginine, asparagine, serine, and homoserine. For three of them (lysine, glutamine, and arginine), very high differences were found between the values of the homochiral and heterochiral pairwise enthalpic interaction coefficients. Volume and spectroscopic data showed that it is possible to detect chiral recognition by techniques other than calorimetric. The role of the zwitterionic interaction and of the substituted side chain is discussed to explain the enhanced chiral recognition of these -aminoacids in respect to those bearing unsubstituted alkyl chains (alanine, -aminobutyric acid, valines and leucines). 相似文献
536.
Steven A. Fleming Sean M. Carroll Jennifer Hirschi Renmao Liu J. Lee Pace J. Ty Redd 《Tetrahedron letters》2004,45(17):3341-3343
We have used asymmetric dihydroxylation (AD) of allenes in order to synthesize chiral α-hydroxy ketones. This methodology has been applied to several aryl-substituted allenes. We have found that electron donating groups on the aromatic ring increase the efficiency of the reaction. 相似文献
537.
Simonne C. Pace 《Journal of polymer science. Part A, Polymer chemistry》1978,16(6):1391-1399
Unlike the α,ω-dihalogenopolydimethylsiloxanes, the α,ω-dichloropolydimethyl-N-methylsilazanes show a net preference for cyclic species with respect to linear structures at equilibrium. The aim of this study is to evaluate the perturbations in the molecular constitution of these α,ω-dihalogenopolydimethyl-N-methylsilazanes resulting from the substitution of the terminal chlorine atoms by fluorine atoms. This polymeric family was prepared by reacting (CH3)2SiF2 with nonamethylsilazane [(CH3)2SiNCH3]3. The redistribution of the fluorine atoms with the bridging methylimino groups reached an equilibrium after about 5 months' heating at 150°C for all the samples prepared. The relative abundance of the various molecular species and fragments at equilibrium was deduced from the quantitative analysis of the proton nuclear magnetic resonance (NMR) spectra. The molecular constitution at equilibrium is described by two constants. The first, K = [neso] [middles in chains]/[terminal moieties]2 = (2.8 ± 0.8) 10?2, shows that the presence of terminal fluorine atoms is unfavorable to the formation of short chains. On the other hand, the trimeric cyclic species [(CH3)2SiNCH3]3 are found to be highly favored (K°3 = 550 ± 100 mole/liter). These observations further confirm that the equilibrium constants which control the noncyclic part of polymeric families depend little on the functionality of the substituents exchanged [for example, on changing from ? N(CH3)2 to ? NCH3? ] when the reorganization heat order is one. 相似文献
538.
539.
Francesco Dallavalle Giuseppina Folesani Terenzio Bertuzzi Roberto Corradini Rosangela Marchelli 《Helvetica chimica acta》1995,78(7):1785-1792
Copper(II) complexes of the ligands N2-[(R)-2-hydroxypropyl]- and N2-[(S)-2-hydroxypropyl]-(S)-phenylalaninamide performed chiral separation of N-dansyl-protected and unmodified amino acids in HPLC (reversed phase). With the aim of investigating which species are potentially involved in the discrimination mechanism, the two ligands were synthesized and their complexation equilibria with Cu2+ studied by potentiometry and spectrophotometry in aqueous solution up to pH 11.7. The formation constants of the species observed, [CuL]2+, [CuL2]2+, [CuLH–1]+, [CuL2H–1]+, [CuL2H–2], and [CuL2H–3]?, were quite similar for both compounds and were compared to those of (S)-phenylalaninamide. Most probably, in [CuL2H–3]? the ligands behave as terdentate, with the deprotonated OH group occupying an apical position. 相似文献
540.
A detailed analysis of the effect of calcium carbonate nanoparticles on crystallization of isotactic polypropylene (iPP) is reported in this contribution. CaCO3 nanoparticles with different crystal modifications (calcite and aragonite) and particle shape were added in small percentages to iPP. The nanoparticles were coated with two types of compatibilizer (either polypropylene-g-maleic anhydride copolymer, or fatty acids) to improve dispersion and adhesion with the polymer matrix.It was found that the type of coating agent used largely affects the nucleating ability of calcium carbonate towards formation of polypropylene crystals. CaCO3 nanoparticles coated with maleated polypropylene can successfully promote nucleation of iPP crystals, whereas the addition of nanosized calcium carbonate coated with fatty acids delays crystallization of iPP, the effect being mainly ascribed to the physical state of the coating in the investigated temperature range for crystallization of iPP, as well as to possible dissolution by fatty acids of heterogeneities originally present in the polypropylene matrix. 相似文献