Building Strain with Large Macrocycles and Using It to Tune the Thermal Half-Lives of Hydrazone Photochromes
作者：Quan Li, Hai Qian, Baihao Shao, Russell P Hughes, Ivan Aprahamian
關鍵字：Photochromic, hydrazone, macrocycle, thermal half-life, strain
論文來源：期刊
具體來源：JACS, 2018, 140(37), 11829-11835
發表時間：2018年
Strain has been used as a tool to modulate the reactivity (e.g., mechanochemistry) and thermal isomerization kinetics of photochromic compounds. Macrocylcization is used to buildup 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 thermal isomerization rate. Surprisingly, while the systems with C3-C5 linkers behave as expected (i.e., the rate is faster with smaller linker), the isomerization rate in the systems with larger aliphatic linkers (C6-C8) is enhanced up to four orders of magnitude. NMR spectroscopy, X-ray crystallography and DFT calculations were used to elucidate this unexpected behavior, which based on our analyses results from the build-up of Pitzer (torsional), Prelog (transannular) and Baeyer (large angle) strain in the longer linkers