Poly(p-phenylene)-based novel conjugated polymers having photoisomerizable azobenzene units in the main chain were prepared by the palladium-catalyzed Suzuki coupling of 1,3-propanediol diesters of 1,4-phenylenediboronic acids with 4,4'-diiodoazobenzenes, and the reversible cis-trans isomerizations of azobenzene units were achieved. Despite their stiff structures, the polymers are soluble in common organic solvents such as chloroform, toluene, and tetrahydrofuran if either or both of the monomers possess n-hexyl side chains on the aromatic rings. The resulting polymers possess high molecular weights (M-n > 7 x 10(3)) and are thermally stable (T-0 > 350 degrees C) and fluorescent. The azobenzene units in the conjugated polymer backbone underwent the photochemical cis-trans reversible isomerizations in both tetrahydrofuran and toluene upon irradiation with UV light of appropriate wavelengths at 20 degrees C. The thermal cis-to-trans isomerization was also readily accomplished in toluene at 80 degrees C. Moreover, the solid-state photochemical trans-to-cis isomerization took place at 20 degrees C. These isomerization processes were accompanied by the change in the three-dimensional hydrodynamic volume of the polymer, which was evidenced by size exclusion chromatography.