Tailoring the optical and dynamic properties of iminothioindoxyl photoswitches through acidochromism

Multi-responsive functional molecules are key for obtaining user-defined control of the properties and functions of chemical and biological systems. In this respect, pH-responsive photochromes, whose switching can be directed with light and acid–base equilibria, have emerged as highly attractive molecular units. The challenge in their design comes from the need to accommodate application-defined boundary conditions for both light- and protonation-responsivity. Here we combine time-resolved spectroscopic studies, on time scales ranging from femtoseconds to seconds, with density functional theory (DFT) calculations to elucidate and apply the acidochromism of a recently designed iminothioindoxyl (ITI) photoswitch. We show that protonation of the thermally stable <em>Z</em> isomer leads to a strong batochromically-shifted absorption band, allowing for fast isomerization to the metastable <em>E</em> isomer with light in the 500–600 nm region. Theoretical studies of the reaction mechanism reveal the crucial role of the acid–base equilibrium which controls the populations of the protonated and neutral forms of the <em>E</em> isomer. Since the former is thermally stable, while the latter re-isomerizes on a millisecond time scale, we are able to modulate the half-life of ITIs over three orders of magnitude by shifting this equilibrium. Finally, stable bidirectional switching of protonated ITI with green and red light is demonstrated with a half-life in the range of tens of seconds. Altogether, we designed a new type of multi-responsive molecular switch in which protonation red-shifts the activation wavelength by over 100 nm and enables efficient tuning of the half-life in the millisecond–second range. Article information: https://doi.org/10.1039/D0SC07000A多响应功能分子是实现化学与生物体系性质与功能定制化调控的核心要素。就此而言，可通过光与酸碱平衡调控开关行为的pH响应光致变色分子（photochrome），已成为极具应用潜力的分子单元。这类分子的设计难点在于，需同时兼顾光响应与质子响应的应用定制化边界条件。 本研究结合飞秒至秒级时间尺度的时间分辨光谱学研究与密度泛函理论（DFT）计算，阐明并应用了近期设计的亚氨基硫代吲哚（iminothioindoxyl, ITI）光开关的酸致变色效应。研究发现，热稳定的Z型异构体发生质子化后，会产生显著红移的吸收带，可使其在500~600 nm波段的光照下快速异构为亚稳态E型异构体。 反应机理的理论研究揭示了酸碱平衡的关键作用：该平衡可调控E型异构体的质子化态与中性态的占比。由于质子化态E型异构体具有热稳定性，而中性态E型异构体可在毫秒级时间尺度内发生再异构化，因此我们可通过调控该酸碱平衡，将ITI的半衰期调控幅度达三个数量级。 最终，本研究证实质子化ITI可在绿光与红光的调控下实现稳定的双向光开关，其半衰期可达数十秒。 综上，本研究设计了一类新型多响应分子开关：其质子化过程可将激活波长红移超100 nm，并可在毫秒至秒级范围内高效调控开关的半衰期。 文章信息：https://doi.org/10.1039/D0SC07000A