Subgap Two-Photon States in Polycyclic Aromatic Hydrocarbons: Evidence for Strong Electron Correlations

Strong electron correlation effects in the photophysics of quasi-one-dimensional $\pi$-conjugated organic systems such as polyenes, polyacetylenes, polydiacetylenes, etc., have been extensively studied. Far less is known on correlation effects in two-dimensional $\pi$-conjugated systems. Here we present theoretical and experimental evidence for moderate repulsive electron-electron interactions in a number of finite polycyclic aromatic hydrocarbon molecules with $D_{6h}$ symmetry. We show that the excited state orderings in these molecules are reversed relative to that expected within one-electron and mean-field theories. Our results reflect similarities as well as differences in the role and magnitude of electron correlation effects in these two-dimensional molecules compared to those in polyenes.Comment: 11 pages, 5 figures, 2 table

Charge Delocalization in Self-Assembled Mixed-Valence Aromatic Cation Radicals

The spontaneous assembly of aromatic cation radicals (D+•) with their neutral counterpart (D) affords dimer cation radicals (D2+•). The intermolecular dimeric cation radicals are readily characterized by the appearance of an intervalence charge-resonance transition in the NIR region of their electronic spectra and by ESR spectroscopy. The X-ray crystal structure analysis and DFT calculations of a representative dimer cation radical (i.e., the octamethylbiphenylene dimer cation radical) have established that a hole (or single positive charge) is completely delocalized over both aromatic moieties. The energetics and the geometrical considerations for the formation of dimer cation radicals is deliberated with the aid of a series of cyclophane-like bichromophoric donors with drastically varied interplanar angles between the cofacially arranged aryl moieties. X-ray crystallography of a number of mixed-valence cation radicals derived from monochromophoric benzenoid donors established that they generally assemble in 1D stacks in the solid state. However, the use of polychromophoric intervalence cation radicals, where a single charge is effectively delocalized among all of the chromophores, can lead to higher-order assemblies with potential applications in long-range charge transport. As a proof of concept, we show that a single charge in the cation radical of a triptycene derivative is evenly distributed on all three benzenoid rings and this triptycene cation radical forms a 2D electronically coupled assembly, as established by X-ray crystallography

A mild and selective method for the conversion of nitroalkanes to carbonyl compounds

Nef reaction is one of the most commonly used method for the conversion of nitroalkanes to carbnyl coacpunds. It was found that the salt of primary nitroparaffins are transformed in to aldehydes and those of secondary nitro coapounda into ketones, when they are treated with aqueous acid. Ernate procedures for effecting this transformation have been reported. Cedures appeared in the literature after the work of Sehecter and Williams using potassium penaanganate as the axidant. Recently, Kornblum and Steliou reported useful modif icationa of the original permanganate proeedtlre. A number of modifications and alt In fact most of the modified pro- Although the yield of the carbony1 compounds is generally high i n simple nitroalkanes Using the modi-fied permanganate procedures, the yield drops down considerably in the case of substrates containing other oxidizable functional groups like carbon-carbon double bonds and hydroxyl groups. Hence a need still exists for the development of a methodology which is mild and more selective

Phonons in fcc Strontium

A more realistic model, requiring a few parameters to explain the interactions among distant neighbours and valid to various crystallographic structures, is developed to study the phonon dispersion in fcc metals the model free from usual fitting procedure, is employed to obtain phonon dispersion in fcc stiontium, which has attracted scant attention so far .The computed results showing good agreement with the recent experimental findings lend reliability and credibility to the theory

3-Ethyl-4-methyl-1H-pyrazol-2-ium-5-olate

The title compound, C6H10N2O, is a zwitterionic pyrazole derivative. The crystal packing is predominantly governed by a three-center iminium–amine N+—H⋯O−⋯H—N inter­action, leading to an undulating sheet-like structure lying parallel to (100)

3-(7,8,13,14-Tetra­hydrodi­benzo­[a,i]phen­an­thridin-5-yl)benzene-1,2-diol

In the title compound, C27H21NO2, the half-chair conformation of the alicyclic rings gives rise to a slightly folded structure of the central tricyclic tetra­hydrophenanthridine unit. Tandem intra­molecular O—H⋯N and O—H⋯O hydrogen bonds give rise to adjacent S(6) and S(5) rings, respectively, which dictate the conformation of the 5-aryl substituent. In the crystal structure, an inter­molecular C—H⋯O contact generates chains parallel to [101]. Short O—H⋯π and C—H⋯π contacts are also observed