Lee Changyeon, Ndaya Dennis, Bosire Reuben, Kim Na Kyung, Kasi Rajeswari M, Osuji Chinedum O
Department of Chemical and Biomolecular Engineering, University of Pennsylvania, Philadelphia, Pennsylvania 19104, United States.
Department of Chemistry, University of Connecticut, Storrs, Connecticut 06269, United States.
J Am Chem Soc. 2022 Jan 12;144(1):390-399. doi: 10.1021/jacs.1c10256. Epub 2021 Dec 28.
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 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 microscopy and X-ray measurements for different mesogen end groups (NO 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 NO 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.
在大分子体系中，光驱动的有序转变很少被观察到，这通常是由于动力学限制。在此，我们报道了一系列嵌段共低聚物（BCO），它们在室温下能响应光照及其缺失而快速有序和无序转变。该体系是一种三嵌段结构，刚性偶氮苯（Azo）介晶基团连接在柔性硅氧烷链的两端。紫外线诱导的Azo从反式到顺式异构化，以及在无紫外线的情况下，分别驱动层状超结构和近晶相的无序化和有序化，具体表现为材料的液化和固化。通过显微镜和X射线测量，研究了不同介晶端基（NO或CN）以及硅氧烷与介晶之间不同碳链长度（0C或12C）对BCO自组装和光开关动力学的影响。12C间隔基团的存在导致了分层有序，介晶的近晶层与更大长度尺度的片层共存，而没有间隔基团时只有近晶有序。这些分层有序的BCO显示出高度持久的片层，这与传统嵌段共聚物中看到的曲折、低持久性的“指纹”型结构形成对比。去除紫外线照射后的重新排序动力学极其迅速（<5秒）。这种快速响应归因于吸电子的NO和CN，它们在室温下通过热弛豫促进顺式到反式异构化，无需额外刺激。这项工作阐明了光开关BCO中的结构-性能关系，并推进了开发能够轻松地通过光学方式写入和擦除有序纳米结构的系统的可能性。
