Non-alternant non-benzenoid aromatic compounds: past, present, and future.

The development of the chemistry of non-alternant non-benzenoid hydrocarbons is reviewed in connection with the International Symposium on Novel Aromatic Compounds (ISNA) and the author's personal anecdotes. Past molecules include the classics of this class of compounds, pentalene, azulene, and heptalene, and the structurally integrated congeners such as indacenes formed by benzannelation and benzinterposition, which were extensively studied during the last century. Indenofluorene isomers constructed by benzannelation to indacenes represent present molecules, which have attracted much interest in recent years in view of their potential in optoelectronic applications. The unusual properties of the most exotic isomer, indeno[2,1-b]fluorene, are specifically described to demonstrate that the benzannelation is not simply a modification for stabilization. Finally, as future molecules, those consisting of linear and cyclic arrays of indenofluorene motifs formed by further extension are proposed.