Yin Xiuping, Li Xinxing, Li Xuyi, Biczysko Malgorzata, Zhu Shourong, Xu Jiaqiang, Bai Yue-Ling
College of Science, Shanghai University 99 Shangda Road Shanghai 200444 China
International Center for Quantum and Molecular Structures, Department of Physics, College of Science, Shanghai University 99 Shangda Road Shanghai 200444 China.
Chem Sci. 2023 Jun 12;14(25):7016-7025. doi: 10.1039/d3sc02051g. eCollection 2023 Jun 28.
The dark-colored viologen radical cations are unstable in air and easily fade, thus greatly limiting their applications. If a suitable substituent is introduced into the structure, it will have the dual function of chromism and luminescence, which will broaden its application field. Here, Vio1·2Cl and Vio2·2Br were synthesized by introducing aromatic acetophenone and naphthophenone substituents into the viologen structure. The keto group (-CHCO-) on the substituents is prone to isomerize into the enol structure (-CH[double bond, length as m-dash]COH-) in organic solvents, especially in DMSO, resulting in a larger conjugated system to stabilize the molecular structure and enhance fluorescence. The time-dependent fluorescence spectrum shows obvious keto-to-enol isomerization-induced fluorescence enhancement. The quantum yield also increased significantly ( = 1 day, = 25.81%, = 41.44%; = 7 days, = 31.48%, and = 54.40%) in DMSO. The NMR and ESI-MS data at different times further confirmed that the fluorescence enhancement was caused by isomerization, and no other fluorescent impurities were produced in solution. DFT calculations show that the enol form is almost coplanar throughout the molecular structure, which is conducive to stabilizing the structure and enhancing fluorescence. The fluorescence emission peaks of the keto and enol structures of Vio1 and Vio2 were at 416-417 nm and 563-582 nm, respectively. The fluorescence relative oscillator strength of Vio1 and Vio2 enol structures is significantly higher than that of keto structures ( value changes from 1.53 to 2.63 for Vio1 and from 1.62 to 2.81 for Vio2), indicating stronger fluorescence emission of the enol structure. The calculated results are in good agreement with the experimental results. Vio1·2Cl and Vio2·2Br are the first examples of isomerization-induced fluorescence enhancement of viologen derivatives, which shows strong solvatofluorochromism under UV light, making up for the disadvantage that it is easy for a viologen radical to fade in air, and providing a new strategy for designing and synthesizing viologen materials with strong fluorescence.
深色的紫精自由基阳离子在空气中不稳定且容易褪色，因此极大地限制了它们的应用。如果在结构中引入合适的取代基，它将具有变色和发光的双重功能，这将拓宽其应用领域。在此，通过将芳香苯乙酮和萘乙酮取代基引入紫精结构中合成了Vio1·2Cl和Vio2·2Br。取代基上的酮基（-CHCO-）在有机溶剂中，尤其是在二甲基亚砜（DMSO）中，容易异构化为烯醇结构（-CH[双键，长度为m破折号]COH-），从而形成更大的共轭体系以稳定分子结构并增强荧光。时间分辨荧光光谱显示出明显的酮到烯醇异构化诱导的荧光增强。在DMSO中，量子产率也显著增加（=1天，=25.81%，=41.44%；=7天，=31.48%，=54.40%）。不同时间的核磁共振（NMR）和电喷雾电离质谱（ESI-MS）数据进一步证实荧光增强是由异构化引起的，并且溶液中没有产生其他荧光杂质。密度泛函理论（DFT）计算表明，烯醇形式在整个分子结构中几乎是共面的，这有利于稳定结构并增强荧光。Vio1和Vio2的酮结构和烯醇结构的荧光发射峰分别在416 - 417 nm和563 - 582 nm处。Vio1和Vio2烯醇结构的荧光相对振子强度明显高于酮结构（Vio1的值从1.53变为2.63，Vio2的值从1.62变为2.81），表明烯醇结构的荧光发射更强。计算结果与实验结果吻合良好。Vio1·2Cl和Vio2·2Br是紫精衍生物异构化诱导荧光增强的首个实例，它们在紫外光下表现出强烈的溶剂化荧光变色现象，弥补了紫精自由基在空气中容易褪色的缺点，并为设计和合成具有强荧光的紫精材料提供了一种新策略。
