Diarylethenes belong to the most eminent photoswitches and have been studied for many decades. They are found in virtually every field of application and have become highly valuable molecular tools for instilling light-responsiveness into materials, catalysts, biological systems, or pharmacology. In this work, we present a novel and distinct type of pyrimidine-based aza-diarylethene, which undergoes a highly unusual zwitterion-forming photoreaction. During this fully reversible process, a CN double bond is established under concomitant aromatization and thiophene-ring opening. The metastable zwitterion thus possesses a positively charged extended aromatic structure and an independent conjugated thiolate function. It can further photoisomerize between a more stable Z and a less stable E isomer, resulting in effective four-state photoswitching. Unusual for diarylethenes, the metastable isomers show negative solvatochromism and red-shifted absorption in apolar solvents. With this behavior, aza-diarylethenes effectively bridge the properties of merocyanines and diarylethenes. Thermal stability of the zwitterions can be modulated from very labile to highly stable behavior in response to pH, again in a fully reversible manner. Pyrimidine-based aza-diarylethene thus establishes a unique photoreaction mechanism for diarylethenes, allowing control of charge separation, thermal stability, and color generation in a different way than hitherto possible.

中文翻译：

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可逆 C=N 键形成控制氮杂二芳基乙烯光电开关中的电荷分离

二芳基乙烯属于最杰出的光电开关，并且已经被研究了几十年。它们几乎存在于每个应用领域，并已成为非常有价值的分子工具，可将光响应性注入材料、催化剂、生物系统或药理学中。在这项工作中，我们提出了一种新颖且独特的基于嘧啶的氮杂二芳基乙烯，它经历了非常不寻常的两性离子形成光反应。在这个完全可逆的过程中，CN双键在伴随的芳构化和噻吩环打开的情况下建立。因此，亚稳态两性离子具有带正电荷的扩展芳香族结构和独立的共轭硫醇盐官能团。它可以进一步在更稳定的Z异构体和不太稳定的E异构体之间进行光异构化，从而产生有效的四态光切换。与二芳基乙烯不同的是，亚稳态异构体在非极性溶剂中表现出负溶剂化显色性和红移吸收。通过这种行为，氮杂二芳基乙烯有效地弥合了部花青和二芳基乙烯的特性。两性离子的热稳定性可以响应 pH 值从非常不稳定的行为调节到高度稳定的行为，同样以完全可逆的方式。因此，基于嘧啶的氮杂二芳基乙烯建立了一种独特的二芳基乙烯光反应机制，允许以与迄今为止不同的方式控制电荷分离、热稳定性和颜色生成。