writer：Quan Li, Hai Qian, Baihao Shao, Russell P Hughes, Ivan Aprahamian
keywords：Photochromic, hydrazone, macrocycle, thermal half-life, strain
source：期刊
specific source：JACS, 2018, 140(37), 11829-11835
Issue time：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