Wavelength-dependent photochemistry of acetaminophen in aqueous solutions

• Photochemistry of acetaminophen in aqueous solution was reinvestigated.
• Competition of photo-Fries reaction and photoionization were observed.
• Nature of both primary and secondary photoproducts was determined.
• Wavelength-dependent mechanism of APAP photolysis was proposed and discussed.

The influence of irradiation wavelength and intensity on photochemistry of acetaminophen (APAP) in aqueous solution was investigated by combination of steady-state and laser flash photolysis as well as HPLC and LC–MS. Steady-state irradiation at 254 nm leads to APAP disappearance with the quantum yield 0.0014 and to formation of 1-(2-amino-5-hydroxyphenyl)ethanone (P1) as a main primary photo-Fries product. In opposite the laser excitation at 266 nm leads predominantly to two-photon ionization of APAP with the quantum yield 0.013 (I = 70 mJ/cm2) and to the formation of one main product of phenoxyl radical reactions – N-(3,4-dihydroxyphenyl)acetamide (P5). Steady-state excitation at 282 nm leads to both P1 and P5 products formation indicating competition of photo-Fries and photoionization processes. The wavelength-dependent mechanism of APAP photolysis is proposed and discussed.

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