Synthesis of the first large annulene fused to cyclopentadienide. A comparison of the effective aromaticity of cyclopentadienide anion with benzene

Magnetic criteria of aromaticity

“…On the other hand, a bicyclic annulation (8-10) reduces the ring current, in accordance with the bond localization that was found in closely related 11. 72 Other moieties, such as a cyclopentadienyl anion 73 and a cycloheptatrienyl cation, 74 were also annulated to the annulene and found to be aromatic. During these studies, an empirical formula connecting the chemical shift of the central Me groups with the aromaticity of the annulated moiety was proposed.…”

Non-alternant non-benzenoid kekulenes: the birth of a new kekulene family

“…It is surprising that the NICS value of the five-membered ring is much more negative than that of the six-membered ring, in contrast to the 54% effective aromaticity of benzene estimated in the cyclopentadienyl anion on the basis of the dihydropyrene indicator reported by Mitchell and co-workers. 79 The negligible aromaticity in the inner eight-membered ring may suggest the presence of annulene-like conjugation circuits only at the outer periphery in dianion 4 2À to generate a 22p electron circuit, leaving the central ring almost neutral. On the other hand, the corresponding NICS(1) values of dication 4 2+ are À21.6, À15.6 and À8.4, respectively, indicating that all rings are strongly aromatic.…”

Negative Photochromism Based on Molecular Diffusion between Hydrophilic and Hydrophobic Particles in the Solid State

“…Negative photochromic materials are expected to show an effective conversion efficiency thanks to the absence of reabsorption of excitation visible light by the photogenerated colorless isomer, − while that of the normal photochromic molecules is not high because of reabsorption of excitation light by the colored isomers. Negative photochromism of such molecules as dihydropyrenes, stenhouse salts, and spiropyrans in solution has been reported, − while negative photochromism in the solid state has scarcely been reported. − Negative photochromism of the crystals of N -salicylidenepyrenes and biindenylidenediones was reported from the colored isomer (the initial state of negative photochromism), which was stabilized by such intermolecular interactions as hydrogen bond and π–π stacking between adjacent molecules. − Even at the photostationary state, the material was not colorless, which was explained by the dye aggregates, which absorb visible light. Negative photochromism has been observed for a spiropyran (SP) in solid doped in poly­(2-ethylhexyl methacrylate- co -vinylpyridine) and a SP covalently immobilized to silica nanoparticles .…”