Design and synthesis of stimuli-responsive conjugated polymers having azobenzene units in the main chain

Poly(p-phenyleneethynylene)- and heteroaromatic-containing poly(phenylene)-based conjugated polymers having photoisomerizable azobenzene units in the main chain were prepared using palladium-catalyzed cross-coupling reaction of 4,4'-diiodoazobenzenes with 1,4-diethynylbenzenes and bis (trimethylstannyl)heteroaromatics, respectively. Polymers possessing n-hexyl side chains on the aromatic rings were soluble in common organic solvents such as tetrahydrofuran, chloroform, and toluene. The polymers were thermally stable in air (T-0 > 330 degreesC). Poly(p-phenyleneethynylene)-based polymers (3) and thiophene-, furan-, and N-methylpyrrole-containing poly(phenylene)-based polymers (5) showed remarkably red-shifted absorptions in the visible region (lambda (max) > 400 nm) due to the extended main-chain conjugation, while the pyridine-containing poly(phenylene)-based polymer (5d) possesses a low degree of main-chain conjugation (lambda (max) < 380 nm). Polymer 5d with low degree of main-chain conjugation showed reversible photoisomerization of azobenzene units, which was accompanied by the change in its electrochemical property.