Photoisomeric molecular tagging velocimetry with CCVJ

Abstract

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 9×10−3 and a Péclet number of 1009 is investigated. Furthermore, the addition of methyl-β-cyclodextrin (methyl-β-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-β-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.