Combined experimental and theoretical study of the photochemistry of 4- and 3-hydroxycoumarin

Luís Pinto da Silva, Ron Simkovitch, Dan Huppert, Joaquim C.G. Esteves da Silva*

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

13 Scopus citations


Steady-state and time-resolved optical spectroscopy techniques were employed to study the photophysics and photochemistry of 4- (4HC) and 3-hydroxycoumarin (3HC). We found that the p Ka of 4HC in aqueous solutions is about 7.5 and the p Ka* is only 4.5. We also found that the nonradiative rate of 4-hydroxycoumarin is high and the excited state proton transfer (ESPT) rate to water is rather slow and so an ESPT process is not observed. Quantum mechanical calculations confirm that the ESPT rate should be slow and the intersystem crossing rate S1 → T is large. An ESPT process was also not observed for 3HC, but it might still take place and not being observed as the fluorescence quantum yield of the anion is two orders of magnitude smaller than that of the protonated form. The nonradiative decay of protonated 3HC is explained with an H-atom abstraction mechanism, leading to the formation of a radical species.

Original languageEnglish
Pages (from-to)23-36
Number of pages14
JournalJournal of Photochemistry and Photobiology A: Chemistry
StatePublished - 1 Apr 2017


  • Excited state proton transfer
  • Hydroxycoumarins
  • Nonradiative decay
  • Photoacids
  • TD-DFT
  • Time-resolved spectroscopy


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