Pure and Applied Chemistry

Abstract: We present the results of our systematic study on the synthesis, structures, and electronic properties of a series of cyclic π-conjugated systems with monocyclic six- to eight-membered rings as well as polycyclic benzenoid aromatics, which are fully surrounded by rigid bicyclic σ-frameworks such as bicyclo[2.2.2]octene (BCO). The study has also been extended to sulfur-containing systems such as thiophene, 1,2- and 1,4-dithiins, and oligothiophene. Such a structural modification using bicyclic frameworks is effective in elevation of the highest occupied molecular orbital (HOMO) levels of neutral π-systems and in remarkable stabilization of the corresponding cationic systems by both σ-π conjugation and kinetic effects. This has led to various new cationic π-conjugated systems such as highly stable radical cations as well as closed-shell mono- and dications, which could have never been attained by any other conventional techniques. The properties of benzene and entirely planar cyclooctatetraene (COT) possessing a marked bond fixation caused by structural modification with a strained bicyclic system (i.e., bicyclo[2.1.1]hexene, BCH) are also described.