Fast Photoswitchable Order–Disorder Transitions in Liquid-Crystalline Block Co-oligomers

• Published in: Journal of the American Chemical Society 2022-01, Vol.144 (1), p.390-399
• Main Authors: Lee, Changyeon, Ndaya, Dennis, Bosire, Reuben, Kim, Na Kyung, Kasi, Rajeswari M, Osuji, Chinedum O
• Format: Article
• Language: English
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/jacs.1c10256

Optically driven ordering transitions are rarely observed in macromolecular systems, often because of kinetic limitations. Here, we report a series of block co-oligomers (BCOs) that rapidly order and disorder at room temperature in response to optical illumination, and the absence thereof. The system is a triblock where rigid azobenzene (Azo) mesogens are attached to each end of a flexible siloxane chain. UV-induced trans-to-cis Azo isomerization, and vice versa in the absence of UV light, drive disordering and ordering of lamellar superstructures and smectic mesophases, as manifested by liquefaction and solidification of the material, respectively. The impacts of chemical structure on BCO self-assembly and photoswitching kinetics are explored by in situ microscopy and X-ray measurements for different mesogen end groups (NO2 or CN), and different carbon chain lengths (0C or 12C) between the siloxane and the mesogen. The presence of the 12C spacer leads to hierarchical ordering with smectic layers of mesogens existing alongside larger length-scale lamellae, versus only smectic ordering without the spacer. These hierarchically ordered BCOs display highly persistent lamellar sheets that contrast with the tortuous, low-persistence “fingerprint”-type structures seen in conventional block copolymers. The reordering kinetics upon removal of UV illumination are extremely rapid (