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Fluorescence of Scopoletin Including its Photoacidity and Large Stokes Shift

Abstract

Scopoletin is highly fluorescent in water and acts as a photoacid exhibiting excited-state proton transfer, ESPT, competitive with fluorescence. Its absorbance and emission spectral characteristics yield ground-state and excited-state pKa values of 7.4 ± 0.1 and 1.4 ± 0.1, respectively. The pKa* implies an ESPT rate constant an order of magnitude smaller than that for umbelliferone. This report provides quantum yield measurements in water that are comparable to quinine sulfate, and fluorescence lifetime values that are on a par with other similar coumarins yet provide insight into the ESPT process. The scopoletin anion is observed in tetrahydrofuran by reaction with a strong base. The Stokes shift of aqueous scopoletin is >100 nm in the pH range 3 to 7 due in part to its action as a photoacid. Modeling by density functional theory methods provides reasonable support for the experimental results.

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Acknowledgements

The authors are grateful for the following fellowships that supported people who worked on this project: the Thedford P. Dirkse Summer Research Fellowship (HP, MVB) and the Rollin M. Gerstacker Foundation Student Research Fellowship (JY). We thank Prof. Gary Blanchard (Michigan State University) for his generous provision of access to the TCSPC system. The computational modeling is supported by National Science Foundation (MRI-grant 1726260). The authors thank Drs. Chad Tatko, Michael Barbachyn, and Carolyn Anderson, for their helpful guidance; and Yukun Tu and Daniel Harmon for their assistance in the research lab.

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Correspondence to Mark A. Muyskens.

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Pham, H.T., Yoo, J., VandenBerg, M. et al. Fluorescence of Scopoletin Including its Photoacidity and Large Stokes Shift. J Fluoresc 30, 71–80 (2020). https://doi.org/10.1007/s10895-019-02471-4

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Keywords

  • Stokes shift
  • Photoacid
  • Fluorescence spectroscopy
  • Quantum yield
  • Lifetime