Colorful clusters : Silver nanoclusters consisting of only a few atoms exhibit large chemical‐environment‐responsive shifts of their optical absorption and emission bands, that is, large solvatochromism (see picture). The photophysical characteristics and electrochemiluminescence of the Ag clusters give them remarkable advantages over larger nanoparticles in applications such as molecular sensing.
The electron ionization mass spectra of 27 cis- and trans-annelated 1,4,4a,5,6,7,8,8a-octahydro-2H-3,1,2-benzoxazaphosphinine 2-oxides were recorded to clarify the effects of the ring heteroatom (O or N), ring annelation, the P configuration and the substituents attached to the ring or to the N and P atoms. For compounds 1-12 different alkyl radical and alkene losses and the cleavage of the P-heteroatom bonds, instead of the P-C bonds, were representative and dependent mainly on the substitution on the N and P atoms. The replacement of Ph and OPh by N(CH2CH2Cl)2 on the P atom had a dramatic influence on the fragmentation process: new fragment ions were obtained and very little M+ (1-3%) was formed. Only slight differences were found between some of the corresponding isomers, but interestingly the compounds formed clear groups on the basis of the differences in their fragmentation, depending on the ring-N and ring-P substituents. 相似文献
The electronic excitation spectra of unsubstituted linear silanes (n-Si(m)H(2m+2), m = 1-6), cyclopentasilane (c-Si5H10), and neopentasilane (neo-Si5H12) have been studied at the coupled-cluster approximate singles and doubles (CC2) level using Dunning's quadruple-zeta basis sets augmented with diffuse functions (aug-cc-pVQZ). Comparisons with measured ultraviolet spectra for Si2H6 and n-Si3H8 show that CC2 calculations using these basis sets yield excitation energies in good agreement with experiment. The calculated excitation thresholds for Si2H6 and n-Si3H8 of 7.61 and 6.68 eV are only 0.05 eV larger than the gas-phase values of 7.56 and 6.63 eV, respectively. For n-Si4H10, n-Si5H12, and neo-Si5H12, the calculated excitation thresholds of 6.51, 6.14, and 6.87 eV for the lowest dipole-allowed transitions are about 0.4 eV larger than the corresponding liquid-phase data of 6.05, 5.77, and 6.53 eV; the discrepancy can mainly be attributed to solvent effects. The obtained excitation thresholds for n-Si6H14 is 5.85 eV, whereas no experimental data are available for its optical gap. Calculations using the Karlsruhe triple-zeta valence basis sets augmented with single and double sets of polarization functions show that very large basis sets augmented with diffuse functions are needed for obtaining accurate excitation energies. The optical gaps for silanes obtained using the triple-zeta polarization basis sets were found to be 0.4 and 0.2 eV larger than those obtained using Dunning's quadruple-zeta basis sets. Excitation thresholds calculated at density functional theory levels using generalized gradient approximation are 0.7-1.0 eV smaller than the experimental values and by employing hybrid functionals they are 0.3-0.4 eV below the experimental thresholds. By adding the present basis-set correction and environmental effects to the previously calculated CC2 value for the excitation threshold of the Si29H36 silicon nanocluster, the extrapolated absorption threshold is 4.0 eV as compared to the recently reported experimental value of 3.7 eV. 相似文献
Time-dependent density functional theory (TDDFT) calculations show that silicon nanoclusters (Si-NC) capped by linear silane groups have large oscillator strengths of the same magnitude as reported in recent experimental studies. We propose a mechanism where linear silanes attached to the Si-NC surface affect the optical properties enhancing the oscillator strengths and thereby accounting for the bright luminescence observed in the blue region of the visible spectrum. The anisotropic emission seen experimentally can also be explained by the presence of the silane groups on the cluster surface. The excitation energies are found to be only slightly affected by the silanes, whereas the oscillator strengths increase with the length of the silane chain and are significantly larger than obtained for unsubstituted Si-NCs. In TDDFT studies of Si-NC dimers interconnected by a linear silane bridge, we obtained large oscillator strengths indicating that such structures could be useful light sources for optical devices. 相似文献
Clustering is a hard combinatorial problem which has many applications in science and practice. Genetic algorithms (GAs) have turned out to be very effective in solving the clustering problem. However, GAs have many parameters, the optimal selection of which depends on the problem instance. We introduce a new self-adaptive GA that finds the parameter setup on-line during the execution of the algorithm. In this way, the algorithm is able to find the most suitable combination of the available components. The method is robust and achieves results comparable to or better than a carefully fine-tuned non-adaptive GA. 相似文献