Department of Chemistry, Duke University, Durham, North Carolina 27708, United States.
J Am Chem Soc. 2015 May 20;137(19):6148-51. doi: 10.1021/jacs.5b02492. Epub 2015 May 5.
The mechanically accelerated ring-opening reaction of spiropyran to a colored merocyanine provides a useful method by which to image the molecular scale stress/strain distribution within a polymer, but the magnitude of the forces necessary for activation has yet to be quantified. Here, we report single molecule force spectroscopy studies of two spiropyran isomers. Ring opening on the time scale of tens of milliseconds is found to require forces of ∼240 pN, well below that of previously characterized covalent mechanophores. The lower threshold force is a combination of a low force-free activation energy and the fact that the change in rate with force (activation length) of each isomer is greater than that inferred in other systems. Finally, regiochemical effects on mechanochemical coupling are characterized, and increasing force reverses the relative ring opening rates of the two isomers.
螺吡喃的机械加速开环反应向有色甲川菁提供了一种有用的方法，可以在聚合物内成像分子级别的应力/应变分布，但激活所需的力的大小尚未被量化。在这里，我们报告了两种螺吡喃异构体的单分子力谱研究。发现在数十毫秒的时间尺度上打开环需要约 240 pN 的力，远低于先前表征的共价机械试剂。较低的阈值力是低自由能激活能和每个异构体的速率随力（激活长度）变化的事实的组合，这大于在其他系统中推断出的情况。最后，对机械化学偶联的区域化学效应进行了表征，并且增加力会逆转两种异构体的相对开环速率。
