Hydrogen-Bonded Polymer-Azobenzene Complexes: Enhanced Photoinduced Birefringence with High Temporal Stability through Interplay of Intermolecular Interactions

We study the photoresponsive behavior of thin films of supramolecular 4-nitio-4'-hydroxyazobenzene-poly(4-vinylpyridine) complexes. Hydrogen bonding between the phenol and pyridine moieties allows attaching a chromophore to essentially each repeat unit of the polymer, thereby suppressing macroscopic phase separation and crystallization. Moreover, the noncooperative nature of hydrogen bonding leads to random complexation of the chromophores to the polymer backbone, which enables a systematic study of the effect of chromophore concentration on the photo-orientation of the complexes. Two regimes are observed: Photoinduced birefringence increases linearly with the chromophore concentration until nominally every third polymer repeat unit is occupied. Beyond that concentration, a new regime is observed with a drastically steeper slope of increase. The latter regime is connected to the interplay between the formation of a hydrogen-bonded supramolecular complex and intermolecular interactions between the mesogenic chromophores. Such a behavior significantly enhances the birefringence at high concentrations and also leads to high remnant birefringence. Hence, the supramolecular approach yields a superior optical performance compared to guest-host polymers and even surpasses the properties of many covalently functionalized polymers, while still allowing modular tunability of the materials properties.