Shiraishi Yasuhiro, Yomo Keiichiro, Hirai Takayuki
Research Center for Solar Energy Chemistry and Division of Chemical Engineering, Graduate School of Engineering Science, Osaka University, Toyonaka560-8531, Japan.
ACS Phys Chem Au. 2023 Jan 31;3(3):290-298. doi: 10.1021/acsphyschemau.2c00067. eCollection 2023 May 24.
Design of spiropyrans showing spontaneous isomerization driven by the polarity of solvents is an important consideration for the synthesis of optical sensory materials. Although some spiropyrans undergo polarity-driven isomerization, they must be heated owing to the high activation energy required for isomerization. In this study, we describe that a spiropyran containing a hydroxynaphthalimide unit () exhibits a polarity-driven isomerization at room temperature. It exists as a colorless spirocyclic (SP) form in less polar solvents but is isomerized to a colored merocyanine (MC) form in polar solvents. The equilibrium amount of the MC form increases with an increase in the polarity of solvents. The MC form involves two resonance structures-the quinoidal and zwitterionic forms. In polar media, the zwitterionic form dominates mainly owing to solvation by polar molecules. Solvation stabilizes the negative charge of the zwitterionic form and decreases its ground state energy, thereby enhancing SP → MC isomerization. The SP ⇌ MC isomerization terminates within barely 30 s even at room temperature because the naphthol moiety with high π-electron density lowers the activation energy for the rate-determining rotational step.
设计受溶剂极性驱动而能自发异构化的螺吡喃，是合成光学传感材料时的一项重要考量。尽管一些螺吡喃会发生极性驱动的异构化，但由于异构化所需的高活化能，它们必须加热。在本研究中，我们描述了一种含有羟基萘二甲酰亚胺单元（）的螺吡喃在室温下表现出极性驱动的异构化。它在极性较小的溶剂中以无色螺环（SP）形式存在，但在极性溶剂中异构化为有色部花青（MC）形式。MC形式的平衡量随溶剂极性的增加而增加。MC形式涉及两种共振结构——醌型和两性离子型。在极性介质中，两性离子型占主导，主要是由于被极性分子溶剂化。溶剂化稳定了两性离子型的负电荷并降低了其基态能量，从而增强了SP→MC异构化。即使在室温下，SP⇌MC异构化在短短30秒内就会终止，因为具有高π电子密度的萘酚部分降低了速率决定旋转步骤的活化能。
