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Zn Coordination and the Identity of the Halide Ancillary Ligand Dramatically Influence the Excited-State Dynamics and Bimolecular Reactions of 2,3-Di(pyridin-2-yl)benzo[g]quinoxaline
Inorganic Chemistry ( IF 4.6 ) Pub Date : 2021-10-18 , DOI: 10.1021/acs.inorgchem.1c02484
Lauren M Loftus 1, 2 , Emma C Olson 1, 3 , David J Stewart 1 , Alexis T Phillips 1, 3 , Kuppuswamy Arumugam 4 , Thomas M Cooper 1 , Joy E Haley 1 , Tod A Grusenmeyer 1
Affiliation  

The optical properties of coordination complexes with ligands containing nitrogen heterocycles have been extensively studied for decades. One subclass of these materials, metal complexes utilizing substituted pyrazines and quinoxalines as ligands, has been employed in a variety of photochemical applications ranging from photodynamic therapy to organic light-emitting diodes. A vast majority of this work focuses on characterization of the metal-to-ligand charge-transfer states in these metal complexes; however, literature reports rarely investigate the photophysics of the parent pyrazine or quinoxaline ligand or perform control experiments utilizing metal complexes that lack low-lying charge-transfer (CT) states in order to determine how metal-atom coordination influences the photophysical properties of the ligand. With this in mind, we examined the steady-state and time-resolved photophysics of 2,3-di(pyridin-2-yl)benzo[g]quinoxaline (dpb) and explored how the coordination of ZnX2 (X = Cl, Br, I) affects the photophysical properties of dpb. In dpb, we find that the dominant mode of deactivation from the singlet excited state is intersystem crossing (ISC). Coordination of ZnX2 perturbs the relative energies of the ππ* and nπ* excited states of dpb, leading to drastically different rates of ISC as well as radiative and nonradiative decay in the [Zn(dpb)X2] complexes compared to dpb. These differences in the rates change the dominant singlet-excited-state decay pathway from ISC in dpb to a mixture of ISC and fluorescence in [Zn(dpb)Cl2] and [Zn(dpb)Br2] and to nonradiative decay in [Zn(dpb)I2]. Coordination of ZnX2 and the choice of the halide ligand also have profound effects on the rate constants for excited-state bimolecular reactions, including triplet–triplet annihilation and oxygen quenching. These results demonstrate that metal coordination, even in complexes lacking low-lying CT states, and the choice of the ancillary ligand can dramatically alter the photophysical properties of chromophores containing nitrogen heterocycles.

中文翻译:

锌配位和卤化物辅助配体的特性显着影响 2,3-二(吡啶-2-基)苯并[g]喹喔啉的激发态动力学和双分子反应

几十年来,人们对含有氮杂环配体的配位配合物的光学性质进行了广泛的研究。这些材料的一个亚类,利用取代吡嗪和喹喔啉作为配体的金属配合物,已被用于从光动力疗法到有机发光二极管的各种光化学应用中。这项工作的绝大部分集中在这些金属配合物中金属到配体的电荷转移状态的表征上;然而,文献报道很少研究母体吡嗪或喹喔啉配体的光物理学,或利用缺乏低电荷转移 (CT) 状态的金属配合物进行控制实验,以确定金属 - 原子配位如何影响配体的光物理性质. 考虑到这一点,g ]喹喔啉 ( dpb ) 并探讨了 ZnX 2 (X = Cl , Br , I ) 的配位如何影响dpb的光物理特性。在dpb 中,我们发现从单重激发态失活的主要模式是系统间交叉(ISC)。ZNX的协调2个扰乱相对能量的ππ的*和Ñ的π*激发态DPB,导致ISC的大幅不同的速率,以及在辐射和非辐射衰变的[锌(DPB)X 2 ]相比,复合物DPB. 这些速率的差异改变了主要的单线态激发态衰变途径,从dpb中的 ISC 到 [Zn(dpb)Cl 2 ] 和 [Zn(dpb)Br 2 ]中 ISC 和荧光的混合物,以及 [Zn(dpb)Br 2 ] 中的非辐射衰变。 Zn(dpb)I 2 ]。ZnX 2的配位和卤化物配体的选择也对激发态双分子反应的速率常数有深远的影响,包括三重态-三重态湮灭和氧猝灭。这些结果表明,即使在缺乏低 CT 状态的配合物中,金属配位和辅助配体的选择也可以显着改变含有氮杂环的生色团的光物理性质。
更新日期:2021-11-01
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