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Profiles two experts in the development of the aromaticity concept, Henry Edward Armstrong, professor at Imperial College London, England and Ernest C. Crocker, author of an article about the six aromatic electron structure of benzene, pyridine and five-membered ring heteroaromatics. Works and contributions to the theory of aromaticity; Pre-electronic and electronic theories of aromaticity; Development of a Benzene Model.

Discusses the aromaticity concept as a cornerstone of heterocyclic chemistry. Importance of heterocyclic chemistry; Tautomerism; Biological and technological properties; Extent and classification of heteroaromatic compounds.

Historically, the aromatic sextet originates with Crocker (2), Armit and Robinson (3), and Hückel (4). The inscribed circle for denoting an aromatic sextet was introduced by Armit and Robinson, but it was Clar who defined how the sextet was to be used in polycyclic aromatic compounds. However, in some papers a mistaken form of Clar formulas is found. The present note stipulates that such mistakes should be corrected before their acceptance in respectable scientific journals, including this Journal. [ABSTRACT FROM AUTHOR]

Generalized helicenes are defined. A theoretical analysis is presented for the intramolecular dehydrogenation of helicenes, bridging adjacent rungs of the helix with the formation of 4-, 5-, and/or 6-membered rings, yielding helical tubulenes to which Euler's formula for polyhedra no longer applies as such constructions are derived from Möbius strips and correspond to nonplanar graphs (i.e., have crossing lines when drawn in two dimensions). Intermolecular dehydrogenation of two parallel-superimposed polycyclic aromatic hydrocarbons with formation of similar rings leads to convex polyhedral carbon cages. Carbon helical nanotubes and some carbon cages that are different from fullerenes are also discussed. [ABSTRACT FROM AUTHOR]

Absorption and steady state fluorescence measurements and AM1 semiempirical calculations on some alkyl-substituted 2,5-diphenyloxazole derivatives are reported in the aim of characterizing the properties of their first excited singlet state. The absorption spectra in cyclohexane and methanol present 2 bands in the range of 220-400 nm and are similar to those of the parent compound, 2,5-diphenyloxazole (PPO). The emission bands are slightly bathocromic shifted (approx. 10 nm) and the emission efficiency decreases subsequent to substitution with alkyl fragments. The MO calculations indicate that the properties of the first excited singlet are determined by the PPO fragment. The calculated singlet-triplet gap was correlated with the probability of nonradiative intersystem crossing (ISC) processes. Values of about 14000 cm-1 were found, meaning a low ISC quantum yield. [ABSTRACT FROM AUTHOR]