Graduate School of Chemical Sciences and Engineering , Hokkaido University , Sapporo 060-0810 , Japan.
Elements Strategy Initiative for Catalysts and Batteries (ESICB) , Kyoto University , Kyoto 615-8520 , Japan.
Inorg Chem. 2019 Apr 1;58(7):4686-4698. doi: 10.1021/acs.inorgchem.9b00410. Epub 2019 Mar 12.
Excited state intramolecular proton transfer (ESIPT) has been a topic of interest due to its potential to lead to multiple emissions. Although many organic molecules showing ESIPT emission are already known, studies on metal complexes showing ESIPT and their related theoretical understandings are very limited. In this study, we focus on [Zn(hqxc)(DMSO)] (Zn-hqxc: hqxc = 3-hydroxy-2-quinoxalinecarboxylate, DMSO = dimethyl sulfoxide), which shows ESIPT emission in the solid state, even though the hqxcH ligand does not show ESIPT emission. To gain insights into the role of the zinc atom and the emission mechanisms, we examined excited states of free hqxcH and the Zn-hqxc complex using time-dependent density functional theory calculations. From the results, it was shown that the zinc atom triggers a structural change of the hqxcH ligand from the lactam form (3,4-dihydro-3-oxo-2-quinoxalinecarboxylic acid) to the enol form (3-hydroxy-2-quinoxalinecarboxylic acid), where the latter form has several stable excited states. Several stable geometries were found for singlet and triplet excited states, suggesting that emissions for the Zn-hqxc complex can be both phosphorescence and fluorescence caused by the enol-enol, keto-keto, and keto-enol forms of the two hqcx ligands in the complex. We found that the photophysical properties of the Zn-hqxc complex are dominated by the ligand due to the filled d of Zn(II). The presented results suggest that, to design new ESIPT metal complexes, one possible approach is to combine a metal atom showing ligand centered emission and a ligand that has separate ESIPT and coordination sites.
激发态分子内质子转移（ESIPT）因其可能导致多种发射而成为一个研究热点。尽管已经有许多显示 ESIPT 发射的有机分子被发现，但研究显示 ESIPT 的金属配合物及其相关理论理解非常有限。在这项研究中，我们专注于[Zn(hqxc)(DMSO)]（Zn-hqxc：hqxc=3-羟基-2-喹喔啉羧酸，DMSO=二甲基亚砜），它在固态下显示 ESIPT 发射，尽管 hqxcH 配体不显示 ESIPT 发射。为了深入了解锌原子的作用和发射机制，我们使用含时密度泛函理论计算研究了游离 hqxcH 和 Zn-hqxc 配合物的激发态。结果表明，锌原子促使 hqxcH 配体从内酰胺形式（3,4-二氢-3-氧代-2-喹喔啉羧酸）转变为烯醇形式（3-羟基-2-喹喔啉羧酸），后者具有几个稳定的激发态。对于单重态和三重态激发态，发现了几个稳定的几何形状，表明 Zn-hqxc 配合物的发射可以是磷光和荧光，这是由配合物中两个 hqcx 配体的烯醇-烯醇、酮-酮和酮-烯醇形式引起的。我们发现，由于 Zn(II)的填满的 d，Zn-hqxc 配合物的光物理性质主要由配体决定。所提出的结果表明，为了设计新的 ESIPT 金属配合物，一种可能的方法是结合显示配体中心发射的金属原子和具有单独 ESIPT 和配位位点的配体。
