Li Quan, Qian Hai, Shao Baihao, Hughes Russell P, Aprahamian Ivan
Department of Chemistry , Dartmouth College , Hanover , New Hampshire 03755 , United States.
J Am Chem Soc. 2018 Sep 19;140(37):11829-11835. doi: 10.1021/jacs.8b07612. Epub 2018 Sep 5.
Strain has been used as a tool to modulate the reactivity (e.g., mechanochemistry) and thermal isomerization kinetics of photochromic compounds. Macrocyclization is used to build-up strain in such systems, and in general the reactivity and rates increase with the decrease in macrocycle size. To ascertain the effect of strain on recently reported bistable hydrazone photoswitches, we incorporated them into macrocycles having varying aliphatic linker lengths (C3-C7), and studied their switching behavior, and effect of macrocycle size on the thermal isomerization rate. Surprisingly, while the systems with C3-C5 linkers behave as expected (i.e., the rate is faster with smaller linkers), the isomerization rate in the systems with larger aliphatic linkers (C6-C8) is enhanced up to 4 orders of magnitude. NMR spectroscopy, X-ray crystallography and DFT calculations were used to elucidate this unexpected behavior, which on the basis of our analyses results from the buildup of Pitzer (torsional), Prelog (transannular) and Baeyer (large angle) strain in the longer linkers.
应变已被用作调节光致变色化合物反应性（如机械化学）和热异构化动力学的工具。大环化用于在此类体系中积累应变，一般来说，反应性和速率会随着大环尺寸的减小而增加。为了确定应变对最近报道的双稳态腙光开关的影响，我们将它们纳入具有不同脂肪族连接链长度（C3 - C7）的大环中，并研究了它们的开关行为以及大环尺寸对热异构化速率的影响。令人惊讶的是，虽然具有C3 - C5连接链的体系表现符合预期（即连接链越短速率越快），但具有较大脂肪族连接链（C6 - C8）的体系中的异构化速率提高了多达4个数量级。我们使用核磁共振光谱、X射线晶体学和密度泛函理论计算来阐明这种意外行为，根据我们的分析，这是由于较长连接链中皮策（扭转）、普雷洛格（跨环）和拜尔（大角度）应变的积累所致。
