New York University Abu Dhabi , P.O. Box 129188, Abu Dhabi, United Arab Emirates.
J Am Chem Soc. 2013 Sep 18;135(37):13843-50. doi: 10.1021/ja4056323. Epub 2013 Jul 29.
A paradigm shift from hard to flexible, organic-based optoelectronics requires fast and reversible mechanical response from actuating materials that are used for conversion of heat or light into mechanical motion. As the limits in the response times of polymer-based actuating materials are reached, which are inherent to the less-than-optimal coupling between the light/heat and mechanical energy in them, a conceptually new approach to mechanical actuation is required to leapfrog the performance of organic actuators. Herein, we explore single crystals of 1,2,4,5-tetrabromobenzene (TBB) as actuating elements and establish relations between their kinematic profile and mechanical properties. Centimeter-size acicular crystals of TBB are the only naturally twinned crystals out of about a dozen known materials that exhibit the thermosalient effect-an extremely rare and visually impressive crystal locomotion. When taken over a phase transition, crystals of this material store mechanical strain and are rapidly self-actuated to sudden jumps to release the internal strain, leaping up to several centimeters. To establish the structural basis for this colossal crystal motility, we investigated the mechanical profile of the crystals from macroscale, in response to externally induced deformation under microscope, to nanoscale, by using nanoindentation. Kinematic analysis based on high-speed recordings of over 200 twinned TBB crystals exposed to directional or nondirectional heating unraveled that the crystal locomotion is a kinematically complex phenomenon that includes at least six kinematic effects. The nanoscale tests confirm the highly elastic nature, with an elastic deformation recovery (60%) that is far superior to those of molecular crystals reported earlier. This property appears to be critical for accumulation of stress required for crystal jumping. Twinned crystals of TBB exposed to moderate directional heating behave as all-organic analogue of a bimetallic strip, where the lattice misfit between the two crystal components drives reversible deformation of the crystal.
从硬质到软质、基于有机的光电技术的范式转变需要使用致动材料来实现快速且可逆的机械响应，这些致动材料可将热或光转化为机械运动。由于聚合物基致动材料的响应时间限制已经达到，这是由于它们内部光/热和机械能之间的耦合不理想所致，因此需要一种概念上全新的机械致动方法来超越有机致动器的性能。在此，我们探索了 1,2,4,5-四溴苯（TBB）单晶作为致动元件，并建立了它们运动学轮廓和机械性能之间的关系。TBB 的厘米尺寸针状单晶是已知约 12 种材料中唯一具有热致伸缩效应的天然双晶材料-一种极其罕见且视觉效果令人印象深刻的晶体运动。当经历相变时，这种材料的晶体储存机械应变，并迅速自行启动以突然跳跃释放内部应变，跃升至几厘米。为了确定这种巨大晶体运动的结构基础，我们从宏观尺度研究了晶体的机械轮廓，通过在显微镜下对外力诱导的变形进行研究，从纳米尺度通过纳米压痕进行研究。对超过 200 个暴露于定向或非定向加热的孪晶 TBB 晶体进行高速记录的运动学分析表明，晶体运动是一种运动学上复杂的现象，至少包括六种运动学效应。纳米尺度测试证实了其具有高弹性，弹性变形恢复（60%）远优于以前报道的分子晶体。这种特性似乎对于晶体跳跃所需的应力积累至关重要。适度定向加热下的 TBB 孪晶晶体表现为双金属条的全有机类似物，其中两个晶体组件之间的晶格失配驱动晶体的可逆变形。
