DIRECT OBSERVATION FOR THE REDUCTION OF EXCITON BINDING-ENERGY INDUCED BY PERPENDICULAR ELECTRIC-FIELD

We experimentally study the effect of perpendicular electric field on the exciton binding energy using a specially designed step quantum well. From photoluminescence spectra at the temperature of 77 K, we have directly observed remarkable blueshift of the exciton peak due to the transition from spatially direct to spatially indirect excitons induced by electric field. (C) 1995 American Institute of Physics.

Binding Energy of Biexcitons in GaAs Quantum-Well Wires

The binding energy of a biexciton in GaAs quantum-well wires is calculated variationally by use of a two-parameter trial wavefunction and a one-dimensional equivalent potential model. There is no artificial parameter added in our calculation. Our results agree fairly well with the previous results. It is found that the binding energies are closely correlative to the size of wire. The binding energy of biexcitons is smaller than that of neutral bound excitons in GaAs quantum-well wires when the dopant is located at the centre of the wires.

Energy band and acceptor binding energy of GaN and AlxGa1-xN

The hole effective-mass Hamiltonian for the semiconductors of wurtzite structure is established, and the effective-mass parameters of GaN and AlxGa1-xN are given. Besides the asymmetry in the z and x, y directions, the linear term of the momentum operator in the Hamiltonian is essential in determining the valence band structure, which is different from that of the zinc-blende structure. The binding energies of acceptor states are calculated by solving strictly the effective-mass equations. The binding energies of donor and acceptor for wurtzite GaN are 20 and 131, 97 meV, respectively, which are inconsistent with the recent experimental results. It is proposed that there are two kinds of acceptors in wurtzite GaN. One kind is the general acceptor such as C, substituting N, which satisfies the effective-mass theory, and the other includes Mg, Zn, Cd etc., the binding energy of which deviates from that given by the effective-mass theory. Experimentally, wurtzite GaN was grown by the MBE method, and the PL spectra were measured. Three main peaks are assigned to the DA transitions from the two kinds of acceptor. Some of the transitions were identified as coming from the cubic phase of GaN, which appears randomly within the predominantly hexagonal material. The binding energy of acceptor in ALN is about 239, 158 meV, that in AlxGa1-xN alloys (x approximate to 0.2) is 147, 111 meV, close to that in GaN. (C) 2000 Published by Elsevier Science S.A. All rights reserved.

环糊精衍生物的设计、合成及其对环辛烯光异构化研究

环糊精(Cyclodextrins, CDs)经化学修饰后可以得到各种类型的衍生物，不仅可以扩展其原有的键合能力，而且还可以改变其选择性，是当代超分子化学的一个研究热点。环糊精第二面的仲羟基比第一面的伯羟基有着更好的催化性能，第二面的选择性修饰将产生更多有价值的衍生物，可用于催化、酶模拟、手性识别等方面。 取代苯甲酰基修饰环糊精对顺式环辛烯(cis-cyclooctene)光异构化反应有非常重要的影响作用，苯环上取代基的性质和取代位置与产物的%ee值和对映体构型之间存在某种内在联系。有目的地选择适宜取代基，设计、合成新型环糊精光增感剂, 有可能按预定目的得到更高%ee值的反式环辛烯；同时，取代苯甲酰基修饰环糊精对cis-cyclooctene光异构化的增感机理有待于进一步阐明。 本论文工作对环糊精的化学修饰以及超分子体系对cis-cyclooctene不对称光异构化反应方面的进展进行了调研。合成了一系列单-6-位取代苯甲酰基修饰环糊精，用于cis-cyclooctene光异构化增感反应，并用圆二色光谱滴定法研究这些环糊精衍生物与cis-cyclooctene的相互作用，以探索光增感反应机理。在此基础上，探讨了环糊精第二面的选择性修饰方法。内容主要包括： 1. 简要介绍了超分子化学的概况，并对环糊精的选择性修饰方法和超分子体系对cis-cyclooctene不对称光异构化反应的主要成果和最新进展进行了评述。 2. 合成了12种单-6-O-(取代苯甲酰基)-β-环糊精，其中10种为新化合物。采用紫外光谱、红外光谱、核磁共振波谱以及质谱等手段对化合物的结构进行了表征。 3. 探索了直接选择性修饰环糊精第二面的便捷新方法。用取代苯甲酰咪唑酯为酰化试剂，0.2M碳酸盐缓冲溶液(pH=9.9)作催化剂，能够有效地活化2-位仲羟基，对环糊精第二面进行选择性修饰，此方法既简便又经济；同时，发现取代苯甲酰基能够在β-CD第二面的2-位、3-位羟基间相互迁移。 4. 用单-6-O-(取代苯甲酰基)-β-环糊精作光增感剂，对cis-cyclooctene光异构化反应进行研究。实验结果证明：取代苯甲酰基上的取代基性质、位置、长度对反应的对映选择性有很大影响；此外，反应体系溶剂极性对产物的%ee值和对映体构型也有重大影响。用单-6-O-(3-甲氧苯甲酰基)-β-CD作增感剂，cis-cyclooctene光异构化反应产物(R)-trans-cyclooctene的对映选择性为45.8%ee，是到目前为止取得的最好对映选择性。 5. 采用圆二色光谱滴定法研究环糊精衍生物与cis-cyclooctene的相互作用，计算包结物的平衡常数，研究包结物的相对稳定性，为探索光增感反应机理提供基础。我们猜测：电子效应对cis-cyclooctene光异构化反应的影响，可能比取代基位置对反应的影响更大，借助电子效应有希望获得更高的%ee值。 Cyclodextrins can be subjected to diverse modifications to give a wide variety of cyclodextrin derivatives, which could not only extend their original molecular binding ability, but also alter their molecular selectivity. Therefore, cyclodextrin chemistry is currently a significant topic in supramolecular chemistry. The more open secondary hydroxyl side of CDs is stated to be catalytically very important, modifications of this face are believed to produce valuable derivatives for catalysis, enzyme mimic, chiral discrimination, etc. Mono-6-O-(substituted benzoyl)-β-CDs as novel supramolecular photosensitizing hosts have recently excited considerable attention in photochirogenesis. The supramolecular photosenstization of cis-cyclooctene mediated by them gave chiral trans-cyclooctene, enantiomeric excess of which was critically affected by the substituent introduced to the sensitizer moiety. In order to enhance the photoenantiodifferentiating ability, and elucidate the origin mechanisms of substituent-dependent enantioselectivity, in this work a series of mono-6-O-(substituted benzoyl)-β-CDs have been synthesized, and applied for enantiodifferentiating photoisomerization of cis-cyclooctene. The major contents are as follows: 1. The general aspects of supramolecular chemistry were descibed briefly. The new progress and important achievements on methods of selective modification of cyclodextrin and supramolecular enantiodifferentiating photoisomerization of cis-cyclooctene were reviewed. 2. Twelve mono-6-O-(substituted benzoyl)-β-CDs including ten novel compounds have been synthesized. Their structures have been characterized by using UV-vis, IR, NMR and MS methods. 3. A new convenient strategy for direct acylation of β-cyclodextrin on the secondary hydroxyl face was achieved by using the combination of N-benzoylimidazole and carbonate buffer in DMF, and the acyl migration between the C-2 and C-3 hydroxyl groups of β-cyclodextrin was found. 4. Experiments using mono-6-O-(substituted benzoyl)-β-CDs as chiral sensitizing hosts for mediating the enantiodifferentiating photoisomerization of cis-cyclooctene, were carried out. The results indicate that enantiomeric excess was critically affected, or even switched in sign, by the substituent introduced to the sensitizer moiety, and polarity of solvent. Using mono-6-O-(3-methoxybenzoyl)-β-CD as chiral sensitizing host, (R)-trans-cyclooctene was obtained in up to 45.8% enantiomeric excess, which is the highest value ever reported for supramolecular photochirogenesis with analogous hosts. 5. The conformational variation of these modified CDs and their complexation behaviors with cis-cyclooctene were examined by circular dichroism spectroscopy in water-methanol mixed solvents, which reveal that the orientation of chromophore was highly sensitive to the type, position and length of the introduced substituents. In the end, the complex stability constants(Ks) were calculated, and the mechanisms of reaction were discussed. Maybe, electronic effects are more important than positions of substituents for mediating the enantiodifferentiating photoisomerization of cis-cyclooctene.

New parameters of many-body potentials: studying the thermal and mechanical properties of noble metals

New parameters of nearest-neighbor EAM (1N-EAM), n-th neighbor EAM (NN-EAM), and the second-moment approximation to the tight-binding (TB-SMA) potentials are obtained by fitting experimental data at different temperatures. In comparison with the available many-body potentials, our results suggest that the 1N-EAM potential with the new parameters is the best description of atomic interactions in studying the thermal expansion of noble metals. For mechanical properties, it is suggested that the elastic constants should be calculated in the experimental zero-stress states for all three potentials. Furthermore, for NNEAM and TB-SMA potentials, the calculated results approach the experimental data as the range of the atomic interaction increases from the first-neighbor to the sixth-neighbor distance.

Study of competitive binding of enantiomers to protein by affinity capillary electrochromatography

Affinity capillary electrochromatography (CEC) with zonal elution method was used to probe the competitive interactions of enantiomers with protein. In this approach, a known concentration of a competing agent is continuously applied to a CEC column with bovine serum albumin (BSA) physically adsorbed on SAX packing while injections of a small amount of analyte are made. The binding sites of solutes on the BSA molecule were determined by the changes in the retention factors of the solutes resulted from the addition of competitive agent. By using D- or L-tryptophan as competitive agents and D-, L-tryptophan and benzoin enantiomers as injected analytes showed that BSA molecule has a primary site to strongly bind L-tryptophan, but D-tryptophan dose not bind at this site; D- and L-tryptophan share a weak binding site on the BSA molecule. Benzoin enantiomers do not share any binding sites with either D- or L-tryptophan. Non-chiral compounds of trichloroacetic acid and n-hexanoic acid were applied as the competitive agents to study the binding of warfarin enantiomers to BSA, it was observed that trichloroacetic acid and n-hexanoic acid had a same binding site for warfarin enantiomers binding to BSA molecule. (C) 2002 Elsevier Science B.V. All rights reserved.

Competing binding of metal ions with protein studied by microdialysis

A method has been established to study the competing binding of metal ions with protein by a combined technique of microdialysis with high performance liquid chromatography (HPLC). Ni2+, Cd2+, Zn2+, Cu2+ and human serum albumin (HSA) were chosen as model metal ions and protein. The experimental results show that Ni2+ and Cu2+ share a common primary binding site on HSA, and Zn2+ and Cd2+ share a different common primary binding site from them, but there is a common multi-metal binding site for all of those four metal ions. This method show advantages of fast sampling, easily to be operated and especially to be useful when ideal spectroscopic probes are not available for the study of interaction between protein and metal ions.

Control of cocklebur seed germination by nitrogenous compounds : Nitrite, nitrate, hydroxylamine, thiourea, azide and cyanide

Germination of non-dormant upper cocklebur (Xanthium pinsylvanicum Wallr.) seeds was stimulated by not only CS(NH2)2 but also NH2OH, KCN and NaN3. This stimulation was not via the enhancement of aerobic C2H4 production. NH2OH, KCN and NaN3 in certain concentrations promoted the initial growth of axial and/or cotyledonary parts, but the degree of growth promotion by NH2OH, NaN3 and KCN was slight compared with that by CS(NH2)2. As in the case of CS(NH2)2, however, the germinationstimulating effect of NH2OH disappeared rapidly as the preceding imbibition period was prolonged. In contrast, KCN and NaN3 were still effective in stimulating the germination of aged seeds maintained on a water substratum, as previously seen with anaerobiosis. Anaerobic induction was enhanced not only by NaN3 and KCN but also by NH2OH, KNO3, KNO2 CO(NH2)2 and CS(NH2)2 applied during the anaerobic treatment, but without causing an increase in anaerobic production of C2H4. Furthermore, KCN and NaN3, given prior to the anaerobic treatment acted additively with anaerobic induction. The germination-stimulating actions of nitrogenous compounds are discussed in comparison with those of C2H4 and anaerobiosis.