Photochemistry of Aqueous Warfarin and 4-Hydroxycoumarin

Warfarin, a common anticoagulant pharmaceutical and rodenticide, has an aqueous-phase absorption spectrum with a strong π → π* band in the UV-B (ε310 ∼ 12,000 M–1 cm–1), making it susceptible to solar photolysis. The photochemistry and photodynamics of aqueous warfarin (WAR) and its core chromophore, 4-hydroxycoumarin (4HC), were characterized with multiple techniques, including photodegradation kinetics using HPLC, liquid chromatography quadrupole time-of-flight mass spectrometry, ultrafast broadband transient absorption spectroscopy, UV/vis absorption and emission spectroscopy, and computational chemistry. Effective UV-B photolysis quantum yields of WAR and 4HC in water (deionized or buffered with pH = 4.5–9) are ϕphot = (2.9 ± 0.6) × 10–3 and (4 ± 1) × 10–2, respectively. Both compounds have reduced degradation rates in deoxygenated solutions. Transient absorption spectroscopy results and structural evidence of photoproducts from high-resolution mass spectrometry presented in this work point to a unimolecular ring-opening step of the coumarin moiety to initiate the photochemistry, potentially stemming from the WAR/4HC radical formed following photoionization. The potential involvement of triplet state WAR/4HC following excitation to the S1 state and intersystem crossing is also investigated. Implications for the environmental fate of WAR are discussed in the context of the above results.