Paradise Lost—π-Electron Conjugation in Homologs and Derivatives of Perylene

Abstract

Various Kekulé–structure–based models, aimed at describing π-electron conjugation in polycyclic aromatic compounds are briefly described. Our main concern are benzenoid hydrocarbons, π-electron systems in which the Kekulé–structure–based approaches are expected to yield the best results. Although there are numerous examples in which reasonings based on Kekulé structures render correct results, there exist cases in which significant violations are encountered. Perylene, its homologs, and derivatives are characteristic representatives of such “anomalous” conjugated systems. Violations from the predictions of the Kekulé–structure–based models are verified by means of a variety of Kekulé–structure–independent theoretical methods.