This work considers excited state intramolecular proton transfers (ESIPT) occurred in multiple hydroxyl-containing compounds with one proton transfer site in the normal form. If several hydroxyl groups are located close to each other in a molecule, then the ESIPT process can lead to the next one. A proton donor site in the first ESIPT will be a proton acceptor during the second reaction. Therefore, a number of consecutive excited state proton transfers can occur. This work deals with the case of two successive proton transfers occurred in the molecular system. Such process is called as a stepwise excited state intramolecular double proton transfer (stepwise ESIDPT). It leads to the formation of two molecular tautomers. Therefore, fluorescence of such compounds can contain different emission bands correspond to emission of normal form and two tautomers. In this work, a rigorous analysis of fluorescence decay kinetics has been made using the model with three species, including a normal molecular form and two tautomers. The work presents theoretical framework of fluorescence decay analysis of ESIDPT process taking into account three species emission. Theoretically, the stepwise proton transfers can be consisted of more than two ESIPT reactions. It depends on molecular structure and number of involved hydroxyl groups. Here, a formal analysis of fluorescence decay kinetics has been made in the case of a stepwise process consisting of two proton transfers. Moreover, the quantum-chemical calculations have been performed in the case of scutellarein. It is a multiple hydroxyl-containing flavone and, therefore, it can be applied as a model molecule to study stepwise intramolecular proton transfers. The hypothetical scheme of ESIDPT has been proposed for this compound.

Graphical abstract

中文翻译：

---

逐步激发态双质子转移和荧光衰减分析

这项工作考虑了在正常形式下具有一个质子转移位点的多种含羟基化合物中发生的激发态分子内质子转移 (ESIPT)。如果一个分子中的几个羟基彼此靠近，则 ESIPT 过程可以导致下一个羟基。第一个 ESIPT 中的质子供体位点将在第二个反应中成为质子受体。因此，可以发生许多连续的激发态质子转移。这项工作涉及分子系统中发生两次连续质子转移的情况。这样的过程称为逐步激发态分子内双质子转移（逐步ESIDPT）。它导致形成两个分子互变异构体。所以，此类化合物的荧光可以包含对应于正常形式和两种互变异构体的发射的不同发射带。在这项工作中，使用具有三个物种的模型对荧光衰减动力学进行了严格的分析，包括正常分子形式和两个互变异构体。该工作提出了考虑三种物质发射的 ESIDPT 过程荧光衰减分析的理论框架。理论上，逐步质子转移可以由两个以上的 ESIPT 反应组成。这取决于分子结构和所涉及的羟基的数量。在这里，在由两个质子转移组成的逐步过程的情况下，对荧光衰减动力学进行了正式分析。此外，已经在灯盏花素的情况下进行了量子化学计算。它是一种含多羟基的黄酮，因此可以用作模型分子来研究逐步分子内质子转移。已经针对该化合物提出了 ESIDPT 假设方案。

图形概要