785 research outputs found
Synthesis of Circumpyrene by Alkyne Benzannulation of Brominated Dibenzo[hi,st]ovalene
A transition-metal catalyzed alkyne benzannulation allowed an unprecedented synthesis of circumpyrene, starting from 3,11-dibromo-6,14-dimesityldibenzo[hi,st]ovalene (DBOV). The circumpyrene was characterized by a combination of NMR, mass spectrometry, and single-crystal X-ray diffraction analysis, revealing its multizigzag-edged structure. Two newly introduced C═C bonds in circumpyrene strongly perturbed the electronic structures of DBOV, as evidenced by increased optical and electrochemical energy gaps. This is in good agreement with an increased number of Clar’s sextets as well as a decreased number of π-electrons in the conjugation pathway of circumpyrene, according to anisotropy of the induced current density (ACID) calculations. The present approach opens a new avenue to multizigzag-edged nanographenes and offers insights into their (opto)electronic properties
Tunable Hydrophobicity in DNA Micelles:Design, Synthesis, and Characterization of a New Family of DNA Amphiphiles
This work describes the synthesis and characterization of a new family of DNA amphiphiles containing modified nucleobases. The hydrophobicity was imparted by the introduction of a dodec-1-yne chain at the 5-position of the uracil base, which allowed precise and simple tuning of the hydrophobic properties through solid-phase DNA synthesis. The micelles formed from these modified DNA sequences were characterized by atomic force microscopy, dynamic light scattering, and polyacrylamide gel electrophoresis. These experiments revealed the role of the quantity and location of the hydrophobic units in determining the morphology and stability of the micelles. The effects of hybridization on the physical characteristics of the DNA micelles were also studied; these results showed potential for the sequence-specific noncovalent functionalization of the self-assembled aggregates
The abundances of hydrocarbon functional groups in the interstellar medium inferred from laboratory spectra of hydrogenated and methylated polycyclic aromatic hydrocarbons
Infrared (IR) absorption spectra of individual polycyclic aromatic
hydrocarbons (PAHs) containing methyl (-CH3), methylene (>CH2), or diamond-like
*CH groups and IR spectra of mixtures of methylated and hydrogenated PAHs
prepared by gas phase condensation were measured at room temperature (as grains
in pellets) and at low temperature (isolated in Ne matrices). In addition, the
PAH blends were subjected to an in-depth molecular structure analysis by means
of high-performance liquid chromatography, nuclear magnetic resonance
spectroscopy, and matrix-assisted laser desorption/ionization time-of-flight
mass spectrometry. Supported by calculations at the density functional theory
level, the laboratory results were applied to analyze in detail the aliphatic
absorption complex of the diffuse interstellar medium at 3.4 mu-m and to
determine the abundances of hydrocarbon functional groups. Assuming that the
PAHs are mainly locked in grains, aliphatic CHx groups (x = 1,2,3) would
contribute approximately in equal quantities to the 3.4 mu-m feature (N_{CHx} /
N_{H} approx 10^{-5} - 2 * 10^{-5}). The abundances, however, may be two to
four times lower if a major contribution to the 3.4 mu-m feature comes from
molecules in the gas phase. Aromatic =CH groups seem to be almost absent from
some lines of sight, but can be nearly as abundant as each of the aliphatic
components in other directions (N_{=CH} / N_{H} < 2 * 10^{-5}; upper value for
grains). Due to comparatively low binding energies, astronomical IR emission
sources do not display such heavy excess hydrogenation. At best, especially in
proto-planetary nebulae, >CH2 groups bound to aromatic molecules, i.e., excess
hydrogens on the molecular periphery only, can survive the presence of a nearby
star.Comment: 34 pages, 19 figures, ApJS, 208, 2
Supramolecular Complexes from CdSe Nanocrystals and Organic Fluorophors
Heterosupramolecular structures from CdSe or CdSe/ZnS core/shell nanocrystals and a specially designed organic fluorophore (diazaperylene) have been prepared to investigate the electronic interactions at nanoscopic semiconductor surfaces. To determine the average number of dye molecules per nanocrystal, we monitored the size-dependent molar extinction coefficient of CdSe particles. The NC/dye ratio is depending on the particle size and varies between 3 and 20 dye molecules per nanocrystal. We demonstrate that the fluorescence of the nanocrystals is completely quenched upon the formation of the supramolecular NC/dye complex even if the surface of the CdSe particles is covered with a few monolayers of the high band gap semiconductor ZnS
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