Various physical principles can be utilized for molecular tagging velocimetry (MTV). In this article, the capabilities and limitations of photoisomerization in MTV are studied. The molecular rotor 9-(2-carboxy-2-cyanovinyl)julolidine (CCVJ) exists in two isomers. Although the $E$ isomer, which is present in the absence of light, yields a fluorescent behavior, the $Z$ isomer is a photoproduct with no detectable luminescence. This regenerative behavior is utilized for MTV. The two time constants define the experimental limitations of the method for tagging and recovery. It is shown that, whereas the tagging time constant is in the range of milliseconds to seconds, the recovery time is in the range of minutes to hours, enabling measurements of slow flow phenomena. A flow in a rectangular channel with a Reynolds number of $9\times {10}^{-3}$ and a Péclet number of 1009 is investigated. Furthermore, the addition of methyl-$\beta$-cyclodextrin (methyl-$\beta$-CD) to the solution increases the fluorescent quantum yield and the isomeric rate constants, supporting the hypothesis of a complex formation with CCVJ. It is shown that photoisomeric MTV is suitable for analyzing slow flow regimes in the liquid phase. The addition of methyl-$\beta$-CD increases the dynamic range of the results. In comparison with existing MTV principles, photoisomerization has the advantage of an intrinsic regenerative behavior. The cost-efficient and simple setup make it an alternative in biological and environmental flow studies.

中文翻译：

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CCVJ的光异构分子标记测速

各种物理原理可用于分子标记测速（MTV）。本文研究了MTV中光异构化的能力和局限性。分子转子9-（2-羧基-2-氰基乙烯基）芥洛啶（CCVJ）存在两种异构体。虽然$Ë$ 在没有光的情况下存在的异构体会产生荧光行为， $ž$异构体是一种无可检测发光的光产物。这种再生行为被用于MTV。这两个时间常数定义了标记和回收方法的实验局限性。结果表明，虽然标记时间常数在毫秒到秒的范围内，但是恢复时间在几分钟到几小时的范围内，从而可以测量慢流动现象。雷诺数为的矩形通道中的流$9\times {10}^{-3}$并研究了佩克莱（Péclet）数1009。此外，添加甲基$\beta$-环糊精（甲基-$\beta$-CD）溶液增加了荧光量子产率和异构体速率常数，支持了CCVJ形成复合物的假说。结果表明，光异构体MTV适用于分析液相中的慢流态。除甲基$\beta$-CD增加结果的动态范围。与现有的MTV原理相比，光异构化具有固有的再生行为。具有成本效益和简单的设置使其成为生物和环境流量研究的替代方案。