Steric crowding can give rise to unusual molecular structures. This dissertation describes the synthesis and characterization of three types of crowded polycyclic aromatic compounds.

The goal of the first project was to synthesize in-cyclophanes with non-reacting functional groups pressed toward the centers of benzene rings. Attempts to make triptycene-based, C₃-symmetric cyclophanes were unsuccessful. However, a C₂-symmetric, benzophenone-based cyclophane, compound 76, was successfully prepared. As shown by its X-ray structure, the carbonyl group of 76 points directly at its basal benzene ring, and the CO-Ar contact distance is 2.91 Å. Although this distance is longer than the closest known CO-Ar contact, compound 76 is the first in-ketocyclophane.

The second project arose from a computational study of the conformational dynamics of large twisted acenes. The initially calculated barrier for phenyl rotation in compound 102 was unusually high (36.7 kcal/mol), and this necessitated experimental verification. A bis-trifluoromethyl derivative of 102, compound 104, was synthesized, and a dynamic NMR experiment revealed a rotation barrier of only 19.2 kcal/mol. A new round of computational studies identified a low-energy, less obvious reaction pathway which combined both the twist inversion and phenyl rotation processes.

The third project describes the syntheses of large, non-planar, polycyclic aromatic molecules that utilized novel cycloadditions of pyrenequinones and crowded cyclopentadienones. Thermal reaction conditions yielded a double Diels-Alder adduct (215a) that is the largest freestanding molecular arch. A Lewis-acid catalyzed reaction produced a unique chiral molecular tweezer (218a), which resulted from a double hetero-Diels-Alder addition.