An Investigation of the Photoinduced Changes of Absorption of High-Performance Photoaddressable Polymers

Recently, a novel class of photoaddressable polymers (PAPs) has been reported, which allows for reversible photorecording with values of the photoinduced birefringence in excess of 0.2.1 These polymers are copolymers of methacrylate‐type monomers with a mesogenic and a nonmesogenic photoactive azobenzene in the side chain. This report presents a detailed analysis of the three‐dimensional photoinduced reorientation of side chains within thin layers of PAP materials, utilizing polarized UV/Visible spectroscopy, infrared, and photothermal deflection spectroscopy (PDS). The three‐dimensional orientation distribution before and after illumination with 514 nm polarized light was investigated by angle‐dependent UV/Vis absorption spectroscopy. As‐prepared samples possess an uniaxial orientation with the photoactive units being preferentially aligned within the plane of the substrate layer. These films become biaxial upon photoalignment with polarized light incident normal to the substrate plane. For PAP materials containing mesogenic side chains, the out‐of‐plane absorption stays nearly unchanged, proving that the photoinduced reorientation occurs mainly within the layer plane. For a homopolymer bearing only a nonmesogenic azobenzene in the side chain, photobleaching is observed, associated with a decrease in absorption in all three principle directions. This finding can be understood in terms of an orientational coupling between the photodegradation product and the active azobenzene units. Photodegradation was well established by infrared spectroscopy. Photothermal deflection spectroscopy has been performed to manifest photoinduced changes in the transparent wavelength region of the PAP material, below the absorption edge. In contrast to the pronounced optical changes in the UV/Vis and IR spectra, the absorption in this range remains nearly unchanged. This result provides evidence that that the polymers can be well applied for holographic storage in thick‐layer devices. Over three spectral regions, photoinduced absorption changes were measured for copolymers comprising a nonmesogenic and a mesogenic azobenzene side chains for applications including optical storage. On illumination, these polymers show little out‐of‐plane reorientation and minor changes in transmission below the absorption band.