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

Abstract

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 (<5 s). This fast response is due to the electron-withdrawing NO2 and CN, which facilitate cis-to-trans isomerization via thermal relaxation at room temperature without additional stimuli. This work elucidates structure–property relationships in photoswitching BCOs and advances the possibility of developing systems in which ordered nanostructures can be easily optically written and erased.

Synthesis and Self-Assembly of Silicon-Containing Azobenzene Liquid Crystalline Block Copolymers

“…The preferential in-plane orientation of the nanostructure is attributed to the wetting behavior of the low-surface energy PDMS block at the interfaces of the BCP films . However, the orientation can be tuned by applying an external field or by taking advantage of the photo-responsive properties of the PMAAzo LC block. , …”

Multiple Azoarenes Based Systems – Photoswitching, Supramolecular Chemistry and Application Prospects

“…Because of photoinduced change in porosity, these systems show reversible on/off capture of CO 2 from gas phase. Lee et al prepared a block co‐oligomer A‐63 , where two azobenzene units are attached at both ends of a flexible siloxane chain [49] . This triblock system shows rapid and reversible ordering and disordering of lamellar superstructures and smectic mesophases in response to UV light.…”

“…Lee et al prepared a block co-oligomer A-63, where two azobenzene units are attached at both ends of a flexible siloxane chain. [49] This triblock system shows rapid and reversible ordering and disordering of lamellar superstructures and smectic mesophases in response to UV light. Azobenzene crystal, which becomes isotropic upon photoisomerization.…”

Formation of hierarchical platelets with morphological control by self-assembly of an azobenzene-containing liquid crystalline diblock copolymer

“…[20][21][22] Recently, inspired by crystallization-driven self-assembly (CDSA) of BCPs in solution, the study of LC-driven self-assembly (LCDSA) has attracted growing interest, which provides a promising alternative methodology to control the morphologies of micelles. [23][24][25] For instance, Liu et al 26 demonstrated that polygonal cylindrical aggregates bearing sharp bends could be formed from LC poly(perfluorooctylethyl methacrylate) (PFOEMA)-containing triblock polymers. In particular, the seeded growth and selfseeding approach, [27][28][29][30] borrowed from the polymer crystallization and living CDSA, can also be used in the LC system to create hierarchical architectures of controlled dimensions and composition, such as branched, hairy plate, and star-like micelles.…”