Light-Controlled Lyotropic Liquid Crystallinity of Polyaspartates

In nature, proteins like rhodopsin act as transducer for photo-chemical reactions causing biological responses (e.g. enabling vision). The underlying concept – a photo-induced conformational change of the protein as amplifier of the photo-responsive moiety – can also be adopted by synthetical polymers or foldamers that have the propensity to form ordered secondary structures (e.g. polypeptides). An alternative approach to amplify photo-chemical responses is their incorporation into liquid crystals. With only a few exceptions, photo-insensitive liquid crystals are doped with dyes that favour disorder upon irradiation. In theory, photo-responsive polypeptides, capable of forming lyotropic liquid crystals, could exploit both amplification approaches but, in practice, their photo-responsivity is hampered by the reduced mobility of polypeptides in concentrated solutions. Here we show that the E/Z photo-isomerisation of an azobenzene containing polyaspartate initiates a helix-coil backbone transition, which reversibly alters the polypeptide solution from anisotropic to isotropic. In contrast to other photo-responsible polymers14, in which thermal relaxation to the more stable photo-isomer is quite fast, both photo-isomers are thermally stable and interconvertible by visible light in a single solvent. Local irradiation and magnetic fields lead to spatial resolution and unidirectional architectures of the liquid crystal, respectively. Our results demonstrate that photo-isomerisation on a molecular level is amplified in three stages via intra- and intermolecular interactions to yield a unidirectional, chiral liquid crystal. We believe, the morphological changes of the liquid crystal induced by light will facilitate a multitude of applications, like photo-alignment or the photo-control of solution viscosity and anisotropic

diffusion. When incorporated into layer-by-layer architectures the polymer could find application in biomedicine and the spatial and temporal resolution could be exploited in nano-technology.