Springer Nature is making SARS-CoV-2 and COVID-19 research free. View research | View latest news | Sign up for updates

Fluorescence of Scopoletin Including its Photoacidity and Large Stokes Shift


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.

This is a preview of subscription content, log in to check access.

Scheme 1
Fig. 1
Fig. 2
Fig. 3
Fig. 4
Fig. 5
Fig. 6


  1. 1.

    Moriya T (1983) Excited-state reactions of coumarins in aqueous solutions. I. The phototautomerization of 7-Hydroxycoumarin and its derivative. BCSJ 56:6–14.

  2. 2.

    Simkovitch R, Huppert D (2015) Photoprotolytic processes of Umbelliferone and proposed function in resistance to fungal infection. J Phys Chem B 119:14683–14696.

  3. 3.

    Simkovitch R, Pinto da Silva L, Esteves da Silva JCG, Huppert D (2016) Comparison of the photoprotolytic processes of three 7-Hydroxycoumarins. J Phys Chem B 120:10297–10310.

  4. 4.

    Abu-Eittah RH, El-Tawil BAH (1985) The electronic absorption spectra of some coumarins. A molecular orbital treatment. Can J Chem 63:1173–1179.

  5. 5.

    Crosby DG, Berthold RV (1962) Fluorescence spectra of some simple coumarins. Anal Biochem 4:349–357.

  6. 6.

    Smith GJ, Dunford CL, Roberts PB (2010) The photostability and fluorescence of hydroxycoumarins in aprotic solvents. J Photochem Photobiol A Chem 210:31–35.

  7. 7.

    Smith GJ, Weston RJ, Tang Y et al (2012) Photoproducts of 7-Hydroxycoumarins in aqueous solution. Aust J Chem 65:1451–1456.

  8. 8.

    Nahata A, Dixit VK (2008) Spectrofluorimetric estimation of Scopoletin in Evolvulus alsinoides Linn. And Convulvulus pluricaulis Choisy. Indian J Pharm Sci 70:834–837.

  9. 9.

    Goodwin RH, Kavanagh F (1950) Fluorescence of coumarin derivatives as a function of pH. Arch Biochem 27:152–173

  10. 10.

    Kostova I, Bhatia S, Grigorov P et al (2011) Coumarins as antioxidants. Curr Med Chem 18:3929–3951

  11. 11.

    Vianna DR, Bubols G, Meirelles G et al (2012) Evaluation of the antioxidant capacity of synthesized coumarins. Int J Mol Sci 13:7260–7270.

  12. 12.

    Malik A, Kushnoor A, Saini V et al (2011) In vitro antioxidant properties of Scopoletin. J Chem Pharm Res 3:659–665

  13. 13.

    Capra JC, Cunha MP, Machado DG et al (2010) Antidepressant-like effect of scopoletin, a coumarin isolated from Polygala sabulosa (Polygalaceae) in mice: evidence for the involvement of monoaminergic systems. Eur J Pharmacol 643:232–238.

  14. 14.

    Gnonlonfin GJB, Sanni A, Brimer L (2012) Review Scopoletin - a coumarin phytoalexin with medicinal properties. Crit Rev Plant Sci 31:47–56.

  15. 15.

    Wharton J, Izaguirre I, Surdock A et al (2018) Hands-on demonstration of natural substance fluorescence in simple tree extracts: sycamore. J Chem Educ 95:615–619.

  16. 16.

    Gao Z, Hao Y, Zheng M, Chen Y (2017) A fluorescent dye with large stokes shift and high stability: synthesis and application to live cell imaging. RSC Adv 7:7604–7609.

  17. 17.

    Reijenga J, van Hoof A, van Loon A, Teunissen B (2013) Development of methods for the determination of pKa values. Anal Chem Insights 8:53–71.

  18. 18.

    Fink DW, Koehler WR (1970) pH effects on fluorescence of umbelliferone. Anal Chem 42:990–993.

  19. 19.

    Grzywacz J, Taszner S, Kruszewski J (1978) Further study on the forms of Umbelliferone in excited state. Zeitschrift für Naturforschung A 33:1307–1311.

  20. 20.

    Mattoo BN (1958) Dissociation constants of hydroxy coumarins. Trans Faraday Soc 54:19–24.

  21. 21.

    Pinto da Silva L, Simkovitch R, Huppert D, Esteves da Silva JCG (2017) Combined experimental and theoretical study of the photochemistry of 4- and 3-hydroxycoumarin. J Photochem Photobiol A Chem 338:23–36.

  22. 22.

    Ferrari AM, Sgobba M, Gamberini MC, Rastelli G (2007) Relationship between quantum-chemical descriptors of proton dissociation and experimental acidity constants of various hydroxylated coumarins. Identification of the biologically active species for xanthine oxidase inhibition. European Journal of Medicinal Chemistry 42:1028–1031.

  23. 23.

    Jacquemin D, Perpète EA, Ciofini I, Adamo C (2008) Fast and reliable theoretical determination of pKa* for photoacids. J Phys Chem A 112:794–796.

  24. 24.

    Houari Y, Jacquemin D, Laurent AD (2013) TD-DFT study of the pKa∗ for coumarins. Chem Phys Lett 583:218–221.

  25. 25.

    Romero R, Salgado PR, Soto C et al (2018) An experimental validated computational method for pKa determination of substituted 1,2-Dihydroxybenzenes. Front Chem 6.

  26. 26.

    Lakowicz JR (2006) Principles of fluorescence spectroscopy, 3rd ed. Springer US

  27. 27.

    Pillman HA, Blanchard GJ (2010) Effects of ethanol on the organization of phosphocholine lipid bilayers. J Phys Chem B 114:3840–3846.

  28. 28.

    Frisch MJ, Trucks GW, Schlegel HB, et al Gaussian 16 Revision B.01

  29. 29.

    Schmidt J, Polik W WebMO pro. WebMO, LLC, Holland, MI, USA. Available from

  30. 30.

    Becke AD (1988) Density-functional exchange-energy approximation with correct asymptotic behavior. Phys Rev A 38:3098.

  31. 31.

    Becke AD (1993) Density-functional thermochemistry. III. The role of exact exchange. J Chem Phys 98:5648.

  32. 32.

    Lee C, Yang W, Parr RG (1988) Development of the Colle-Salvetti correlation-energy formula into a functional of the electron density. Phys Rev B 37:785.

  33. 33.

    Grzywacz J, Taszner S (1977) Ionic forms of umbelliferone. B ACAD POL SCI SMAP 25:447–453

  34. 34.

    Grzywacz J, Taszner S (1982) Influence of pH on the absorption and fluorescence spectra of 6,7-Dihydroxycoumarin in aqueous solution. Zeitschrift für Naturforschung A 37:262–265.

  35. 35.

    Zhang J, Liu C, Wei Y (2011) Fluorescence quantum yield and ionization constant of umbelliferone. Chemistry Bulletin / Huaxue Tongbao 74:957–960

  36. 36.

    Nad S, Pal H (2003) Photophysical properties of Coumarin-500 (C500): unusual behavior in nonpolar solvents. J Phys Chem A 107:501–507.

Download references


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.

Author information

Correspondence to Mark A. Muyskens.

Ethics declarations

Competing Interests

The authors declare no competing financial interest.

Additional information

Publisher’s Note

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Electronic supplementary material


(DOCX 919 kb)

Rights and permissions

Reprints and Permissions

About this article

Verify currency and authenticity via CrossMark

Cite this article

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).

Download citation


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