Reversible polarization gratings on thin films of polyoxetanes bearing 4-(N,N-diphenyl)amino-4′-nitroazobenzene chromophores

Polarization gratings were fabricated by the two-beam coupling method on the surface of a series of polyoxetanes containing 4-(N,N-diphenyl)amino-4′-nitroazobenzene pendants with the azobenzene chromophores either side-on attached or end-on attached through short or long spacers to the main chain. The dynamics of formation of the gratings or writings was studied in relation to the diffraction efficiency. The erasing behavior of the gratings was also examined, irradiating a linearly polarized single beam. The temporal stability of the diffraction efficiency was further studied after removing the coupled excitation beam. Both the mode of attachment of the chromophore to the main chain and the length of the side-chain spacer greatly influenced the dynamic properties of the diffraction gratings. The presence of a long side-chain spacer and side-on attachment of the chromophore resulted in faster writing and erasing behavior. A preliminary study by solid-state ¹³C NMR of the relaxational behavior of the main-chain CH₂ groups and the pendant chromophores provided a reasonable picture to help explain the photo-induced reorientation and relaxation of the azo chromophores at the molecular level.