Lin Yangju, Kouznetsova Tatiana B, Foret Alex G, Craig Stephen L
Department of Chemistry, Duke University, Durham, North Carolina 27708, United States.
J Am Chem Soc. 2024 Feb 14;146(6):3920-3925. doi: 10.1021/jacs.3c11621. Epub 2024 Feb 3.
The spiropyran mechanophore (SP) is employed as a reporter of molecular tension in a wide range of polymer matrices, but the influence of surrounding environment on the force-coupled kinetics of its ring opening has not been quantified. Here, we report single-molecule force spectroscopy studies of SP ring opening in five solvents that span normalized Reichardt solvent polarity factors () of 0.1-0.59. Individual multimechanophore polymers were activated under increasing tension at constant 300 nm s displacement in an atomic force microscope. The extension results in a plateau in the force-extension curve, whose midpoint occurs at a transition force * that corresponds to the force required to increase the rate constant of SP activation to approximately 30 s. More polar solvents lead to mechanochemical reactions that are easier to trigger; * decreases across the series of solvents, from a high of 415 ± 13 pN in toluene to a low of 234 ± 9 pN in -butanol. The trend in mechanochemical reactivity is consistent with the developing zwitterionic character on going from SP to the ring-opened merocyanine product. The force dependence of the rate constant (Δ) was calculated for all solvent cases and found to increase with , which is interpreted to reflect a shift in the transition state to a later and more productlike position. The inferred shift in the transition state position is consistent with a double-well (two-step) reaction potential energy surface, in which the second step is rate determining, and the intermediate is more polar than the product.
螺吡喃机械力发色团（SP）在多种聚合物基质中用作分子张力的报告基团，但周围环境对其开环的力耦合动力学的影响尚未得到量化。在此，我们报告了在五种溶剂中对SP开环的单分子力谱研究，这些溶剂的归一化赖卡特溶剂极性因子（）范围为0.1 - 0.59。在原子力显微镜中，单个多机械力发色团聚合物在300 nm s恒定位移下，随着张力增加而被激活。伸长导致力 - 伸长曲线出现一个平台，其中点出现在转变力*处，该转变力对应于将SP活化速率常数提高到约30 s所需的力。极性更强的溶剂会导致更容易引发的机械化学反应；*在一系列溶剂中逐渐降低，从甲苯中的415 ± 13 pN的高位降至正丁醇中的234 ± 9 pN的低位。机械化学反应性的趋势与从SP到开环部花青产物过程中发展的两性离子特性一致。计算了所有溶剂情况下速率常数（Δ）的力依赖性，发现其随增加，这被解释为反映了过渡态向更靠后且更像产物位置的转变。过渡态位置的推断转变与双阱（两步）反应势能面一致，其中第二步是速率决定步骤，且中间体比产物极性更强。
