Tilt orientation of p-methoxyazobenzene side chains in liquid crystalline polymer films by irradiation with nonpolarized light

Unambiguous observation of the reorientation of azobenzene chromophores toward the propagation direction of the actinic light has been performed. Prolonged irradiation of a spin-coated film of liquid crystalline poly[6-{4-(4'-methoxyphenyl)azophenyloxy}hexyl methacrylate] (C6MeO) with 436 nm light for n-pi* excitation led to a marked decrease of the pi-pi* absorption band. Spectral analysis in combination with conoscopic observation revealed that the reorientation of the azobenzene chromophores occurs in such a way that the longitudinal molecular axis of the azobenzene aligns in parallel with the propagation of light to form strong H-aggregates. The tilt orientation of the azobenzene was more readily achieved by irradiation with light at high temperatures near or above the glass transition temperature. Concerning the effect of spacer length on the three-dimensional orientation, the azobenzene groups showed a tendency to orient more homeotropically as the alkylene spacer length increases.