There is growing interest in creating solids that are responsive to various stimuli. Herein we report the first molecular-level mechanistic picture of the thermochromic polymorphic transition of a series of MAN-NI dyad crystals that turn from orange to yellow upon heating with minimal changes to the microscopic morphology following the transition. Detailed structural analyses revealed that the dyads assemble to create an alternating bilayer type structure, with horizontal alternating alkyl and stacked aromatic layers in both the orange and yellow forms. The observed dynamic behavior in the solid state moves as a yellow wavefront through the orange crystal. The overall process is critically dependent on a complex interplay between the layered structure of the starting crystal, the thermodynamics of the two differently colored forms, and similar densities of the two polymorphs. Upon heating, the orange form alkyl chain layers become disordered, allowing for some lateral diffusion of dyads within their own layer. Moving to either adjacent stack in the same layer allows a dyad to exchange a head-to-head stacking geometry (orange) for a head-to-tail stacking geometry (yellow). This transition is unique in that it involves a nucleation and growth mechanism that converts to a faster cooperative wavefront mechanism during the transition. The fastest moving of the wavefronts have an approximately 38° angle with respect to the long axis of the crystal, corresponding to a non-conventional C-H···O hydrogen bond network of dyad molecules in adjacent stacks that enables a transition with cooperative character to proceed within layers of orange crystals. The orange-to-yellow transition is triggered at a temperature that is very close to the temperature at which the orange and yellow forms exchange as the more stable, while being lower than the melting temperature of the original orange, or final yellow, solids.

中文翻译：

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固态比色切换的机理分析：单烷氧基萘-萘二甲酰亚胺供体-受体二元组

人们对创造对各种刺激有反应的固体越来越感兴趣。在此，我们报告了一系列 MAN-NI 二元晶体的热致变色多晶型转变的第一个分子级机械图，这些晶体在加热时从橙色变为黄色，转变后的微观形态变化很小。详细的结构分析表明，二元组组装形成交替的双层类型结构，具有橙色和黄色形式的水平交替烷基和堆叠芳香层。观察到的固态动态行为作为黄色波前穿过橙色晶体。整个过程严重依赖于起始晶体的层状结构、两种不同颜色形式的热力学之间的复杂相互作用，以及两种多晶型物的相似密度。加热后，橙色形式的烷基链层变得无序，允许二元组在它们自己的层内进行一些横向扩散。移动到同一层中的任一相邻堆叠允许二元组将头对头堆叠几何形状（橙色）交换为头对尾堆叠几何形状（黄色）。这种转变是独一无二的，因为它涉及成核和生长机制，在转变过程中转换为更快的协同波前机制。移动最快的波前与晶体的长轴成大约 38° 角，对应于相邻叠层中二元分子的非常规 CH···O 氢键网络，可实现具有协同特性的过渡在橙色晶体层内进行。