Studies on two- and three-dimensional π-conjugated systems with unique structures and properties

A series of two-dimensional, cyclic pi-conjugated systems with the six-membered to eight-membered rings as well as polycyclic aromatic compounds and sulfur-containing compounds, which were fully surrounded by rigid bicyclic sigma-frameworks, were synthesized and their structures and properties elucidated. This structural modification is characteristic to cause an elevation of the HOMO levels of neutral pi-systems and to stabilize remarkably the corresponding cationic systems by both thermodynamic and kinetic effects. In contrast to annelation with a strain-free bicyclic system, such as bicyclo[2.2.2]octene, annelation with a more strained system, such as bicyclo[2.1.1]hexene, brings about a large bond fixation effect upon the cyclic pi-system. Thus, pi-conjugated systems with quite unusual electronic structures can be prepared by using combination of these structural modifications. As another example of unusual structure, a novel three-dimensional pi-conjugated system, i.e., fullerene encapsulating molecular hydrogen, was synthesized from empty fullerene using organic reactions. This is taken as an entirely new approach to the endohedral fullerenes, of which the production has so far relied only on physical methods.