Building Strain with Large Macrocycles and Using It To Tune the Thermal Half-Lives of Hydrazone Photochromes

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.

张力（strain）常被用作调控光致变色化合物反应活性（如机械力化学）与热异构化动力学的工具。大环化反应可在这类体系中引入张力，且通常而言，反应活性与速率随大环尺寸减小而提升。为探究张力对近期报道的双稳态腙光开关的影响，我们将这类光开关嵌入具有不同脂肪族连接臂长度（C3–C7）的大环结构中，研究了其开关行为以及大环尺寸对热异构化速率的影响。令人意外的是，尽管C3–C5连接臂的体系符合预期（即连接臂越小，速率越快），但具有更长脂肪族连接臂（C6–C8）的体系，其异构化速率提升可达4个数量级。我们通过核磁共振波谱法、X射线晶体学以及密度泛函理论（DFT）计算阐释了这一反常现象，分析结果显示，该现象源于更长连接臂中皮策（扭转）张力、普雷洛格（跨环）张力与拜耳（大角）张力的共同作用。