Coplanarity in the Backbones of Ladder-type Oligo(<i>p</i>-phenylene) Homologues and Derivatives

p-Tolyl-substituted ladder-type oligo(p-phenylene)s containing three, four, and five phenylene rings were readily synthesized. The uniform aryl substitution of these systems allowed us to determine the coplanarity of the π-conjugated backbones crystallographically. The intramolecular annulations eliminate almost all of the conformational disorder and enhance the degree of π-conjugation of the backbones, resulting in significant red shifts in the absorption and emission maxima and lower oxidation potentials in the higher homologues.