State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, People's Republic of China.
School of Material Science & Engineering, Hubei University, Wuhan, Hubei 430062, People's Republic of China.
J Am Chem Soc. 2021 Feb 10;143(5):2232-2238. doi: 10.1021/jacs.0c10183. Epub 2021 Feb 1.
Charge-separated states with a lifetime scale of seconds or longer not only favor studies using various steady-state analysis techniques but are important for light-energy conversion and other applications. Through a steric-hindrance-induced method, unprecedented photoinduced generation of a partially charge separated (PCS) state with a lifetime of days has been detected in the "visual" mode during the decay of excited states to a commonly observed fully charge separated (FCS) state for viologen analogues. One pale yellow 4,4'-bipyridine-based metalloviologen compound, with an interannular dihedral angle of 1.84° in 4,4'-bipyridine, directly decays to the purple FCS state after photoexcitation. The other pale yellow compound, with a similar coordination framework but a larger interannular dihedral angle (33.74°), changes first to a yellow PCS state and then relaxes slowly (in the dark in Ar, ca. 2 days; 70 °C in Ar, ca. 1 h) to the purple FCS state. The two-step coloration phenomenon is unprecedented for viologen compounds and their analogues and also rather rare for other photochromic species. EPR and Raman data reveal that photoinduced charge separation first generates univalent zinc and radicals and then the received electron in Zn(I) slowly distributes further to 4,4'-bipyridine. Reduction of π-conjugation and a direct to indirect change in band gap account for the prolongation of the relaxation process and the capture of the PCS state. These findings help to understand and control decay processes of excited states and provide a potential design strategy for multicolor photochromism, light-energy conversion with high efficiency, or other applications.
具有秒级或更长时间尺度的电荷分离态不仅有利于使用各种稳态分析技术的研究，而且对于光能转换和其他应用也很重要。通过空间位阻诱导的方法，在“可见”模式下，在激发态衰减过程中，检测到前所未有的部分电荷分离（PCS）态的光诱导生成，其寿命为天，对于紫罗碱类似物，这是一种常见的完全电荷分离（FCS）态。一种浅黄色的 4,4'-联吡啶基金属紫罗碱化合物，在 4,4'-联吡啶中具有 1.84°的环间二面角，在光激发后直接衰减到紫色 FCS 态。另一种浅黄色的化合物，具有相似的配位骨架但环间二面角较大（33.74°），首先转变为黄色的 PCS 态，然后缓慢松弛（在 Ar 中暗处，约 2 天；在 Ar 中 70°C，约 1 小时）到紫色 FCS 态。这种两步变色现象对于紫罗碱化合物及其类似物来说是前所未有的，对于其他光致变色物种来说也相当罕见。EPR 和拉曼数据表明，光诱导电荷分离首先生成单价锌和自由基，然后接收电子在 Zn(I) 中缓慢进一步分布到 4,4'-联吡啶。π 共轭的减少和带隙的直接到间接变化解释了弛豫过程的延长和 PCS 态的捕获。这些发现有助于理解和控制激发态的衰减过程，并为高效率的多色光致变色、光能转换或其他应用提供了一种潜在的设计策略。
