Photo-mediated formation and evolution of free radicals on quinone aromatic compounds: Effect of molecular structure

Abstract

Molecular structure strongly affects the formation and persistence of persistent free radicals (PFRs) during the photochemical transformation of quinone aromatic compounds, yet the underlying mechanisms remain unclear. In this study, the formation and evolution of PFRs in five quinones aromatic compounds under the irradiation and subsequent dark conditions were investigated using electron paramagnetic resonance (EPR) spectroscopy. Results showed that upon irradiation, all tested quinones rapidly generated PFRs, though the kinetic profiles varied among compounds. After 2 h of irradiation, the relative generation of PFRs followed the order: 2-chloro-p-benzoquinone > 1,4-naphthoquinone > anthraquinone > 2-methyl-p-benzoquinone > 1,4-benzoquinone. However, the light-induced radicals generated from model quinones exhibit low stability, dissipate gradually, and eventually revert to their initial levels. The PFRs decayed rapidly following two-stage kinetics, with the 1/e lifetimes ordered as: 1,4-benzoquinone > 2-methyl-p-benzoquinone > anthraquinone > 1,4-naphthoquinone > 2-chloro-p-benzoquinone. Fourier transform infrared (FTIR) and X-ray photoelectron spectroscopy (XPS) confirmed that PFRs formation involved quinone groups (C=O) conversion to C–O and electron rearrangement within the aromatic rings. Spin-trapping and H₂O₂ measurements further revealed that the involvement of ROS generation and consumption in PFRs evolution. These findings provide insights into the photo-transformation processes of aromatic compounds in environmental matrices and a scientific basis for evaluating their reactivity and ecological risks.

Graphical abstract

Keywords

Persistent free radicals / Quinone / Reactive oxygen species / Evolution / Irradiation

Highlight

	● Light irradiation rapidly generated PFRs from model quinones.
	● The generated free radicals on quinones revert to their initial levels.
	● Substituted and polycyclic quinones exhibited higher PFRs formation.
	● Substituted and polycyclic quinones had shorter PFRs lifetimes.
	● PFRs formation involved C=O to C–O conversion and electron rearrangement.

References

[1] Balakrishna S , Saravia J , Thevenot P , Ahlert T , Lomnicki S , Dellinger B , Cormier S A . (2011). Environmentally persistent free radicals induce airway hyperresponsiveness in neonatal rat lungs. Particle and Fibre Toxicology, 8(1):

[2] Bykov V N , Ukhanev S A , Ushakov I A , Vologzhanina A V , Antsiferov E A , Klimenko L S , Lvov A G . (2024). Activation of anthraquinone’s electrophilicity by light for a dynamic C–O bond. Journal of the American Chemical Society, 146(3): 1799–1805

[3] Chen J , Jin B K . (2015). Investigation on redox mechanism of 1,4-naphthoquinone by in situ FT-IR spectroelectrochemistry. Journal of Electroanalytical Chemistry, 756: 36–42

[4] Chen X , Alvarez P J J , Masiello C A . (2025). Environmentally persistent free radicals in biochar: environmental context and future research needs. Environmental Science & Technology, 59(23): 11440–11454

[5] Cheng Y , Pan Z Z , Lv J T , Cao D , Sun H , Luo L , Liu Z G . (2026). Dissolved black carbon input reshapes molecular phototransformation and fate of dissolved organic matter at the soil-water interface. Water Research, 290: 125099

[6] Chu L G , Yu H R , Wang X H , Sun Z Y , Lu D N , Wang R B , Li H J , Chen Z H , Gao J , Vione D . et al. (2025a). Accelerated photochemical generation of reactive species in microdroplets by humic-like substances: role of the air-water interface and molecular mechanism. Environmental Science & Technology, 59(24): 12221–12231

[7] Chu Z K , Kang Y Q , Qi K M , Xie X Y , Li S T , Wang Z W . (2025b). Molecular-level insights into the spatiotemporal variation of dissolved organic matter: linking of FT-ICR MS with spectroscopy by rank correlation. Journal of Environmental Chemical Engineering, 13(3): 116360

[8] Duan J J , Li Y , Gao J N , Cao R Z , Shang E X , Zhang W . (2022). ROS-mediated photoaging pathways of nano- and micro-plastic particles under UV irradiation. Water Research, 216: 118320

[9] Frontana C , González I . (2005). The role of intramolecular hydrogen bonding in the electrochemical behavior of hydroxy-quinones and in semiquinone stability.. Journal of the Mexican Chemical Society, 49(2): 61–69

[10] Girlando A , Pecile C . (1975). Vibrational analysis of chlorinated para-benzoquinones—II. Infrared and Raman spectra of five chloroparabenzoquinone derivatives. Spectrochimica Acta Part A: Molecular Spectroscopy, 31(9−10): 1187–1200

[11] Görner H , Sonntag V C . (2008). Photoprocesses of chloro-substituted p-benzoquinones. The Journal of Physical Chemistry A, 112(41): 10257–10263

[12] Huang N , Chen Y Y , Yuan X Y , Li Y Y , Lu Y , Jiang Y L , Xu H C , Ren L X , Wang D W . (2025). Photo-transformation of nitrate and fulvic acid driven by guest iron minerals. Frontiers of Environmental Science & Engineering, 19(1): 7

[13] Jia H Y , Wen C Q , Hu Y J , Li H X , Ao X W , Sun W J , Mao T , Li Z F . (2026). From enemy to ally: dissolved organic matter synergizes contaminants removal under light-driven processes. Water Research, 292: 125373

[14] Jia H Z , Nulaji G , Gao H W , Wang F , Zhu Y Q , Wang C Y . (2016). Formation and stabilization of environmentally persistent free radicals induced by the interaction of anthracene with Fe(III)-modified clays. Environmental Science & Technology, 50(12): 6310–6319

[15] Jiang J Q , Zhao H X , Xia D M , Li X T , Qu B C . (2020). Formation of free radicals by direct photolysis of halogenated phenols (HPs) and effects of DOM: a case study on monobromophenols. Journal of Hazardous Materials, 391: 122220

[16] Li F F , Lv J T , He A N , Xu J , Zhao L X , Wang X W , Mao L , Li S S , Wang H L , Wang Y W . et al. (2025). Unexpected generation of singlet oxygen at the air–water interface of aqueous microdroplets. Journal of the American Chemical Society, 147(34): 30574–30581

[17] Li X T , Shen C , Zhao H X , Jiang J Q , Ban Z X , Chen Z Y , Qu B C . (2020). Photoformation of persistent free radicals on a montmorillonite-humic acid complex simulated as particulate organic matter in an aqueous solution. Environmental Science: Processes & Impacts, 22(9): 1842–1851

[18] Lu X Y , Zhu X R , Ma Z Y , Che Z W , Li Y F , Bao J C , Liu Y . (2022). Oxygen-containing surface functional groups, mesoporous structure and photothermal effect co-modulated highly-efficient H₂O₂ production and pollutant degradation. Electrochimica Acta, 426: 140755

[19] Miao Z Y , Zhang H , Zhang J , Liu Z , Xue Z H , Abbas G . (2025). Effects of iron-oxidizing bacteria and humic acid of various molecular weights on iron corrosion in drinking water distribution systems. Frontiers of Environmental Science & Engineering, 19(1): 13

[20] Novotny E H , Martin-Neto L . (2002). Effects of humidity and metal ions on the free radicals analysis of peat humus. Geoderma, 106(3−4): 305–317

[21] Oniki T , Takahama U . (1994). Effects of reaction time, chemical reduction, and oxidation on ESR in aqueous solutions of humic acids. Soil Science, 158(3): 204–210

[22] Pelzer K M , Cheng L , Curtiss L A . (2017). Effects of functional groups in redox-active organic molecules: a high-throughput screening approach. The Journal of Physical Chemistry C, 121(1): 237–245

[23] Pu M R , Zhu Y W , Tan P , Zhu Y L , Lai X , Chen H , Qu J F , He F . (2023). Chlorination and position isomerization to enhance the photovoltaic performance of polymer donors. Macromolecules, 56(15): 5865–5872

[24] Qin L J , Yang L L , Yang J H , Weber R , Ranguelova K , Liu X Y , Lin B C , Li C , Zheng M H , Liu G R . (2021). Photoinduced formation of persistent free radicals, hydrogen radicals, and hydroxyl radicals from catechol on atmospheric particulate matter. iScience, 24(3): 102193

[25] Qin X Y , He Y T , Liu S M , Shi B Y . (2024). Persistent free radicals in natural organic matter activated by iron particles enhanced disinfection byproduct formation. Water Research, 266: 122387

[26] Sarmiento D J, Majestic B J (2025). The photochemistry of quinones and combustion-derived particles in forming hydroxyl radicals and singlet oxygen in the atmosphere. Atmospheric Environment, 351: 121189

[27] Sharpless C M , Aeschbacher M , Page S E , Wenk J , Sander M , McNeill K . (2014). Photooxidation-induced changes in optical, electrochemical, and photochemical properties of humic substances. Environmental Science & Technology, 48(5): 2688–2696

[28] Shi J Q , Long T , Zhou Y X , Wang L , Jiang C P , Pan D G , Zhu X . (2021). Efficiency and quantitative structure-activity relationship of monoaromatics oxidation by quinone-activated persulfate. Frontiers in Chemistry, 9: 580643

[29] Shi Y F , Dai Y C , Liu Z W , Nie X F , Zhao S , Zhang C , Jia H Z . (2020). Light-induced variation in environmentally persistent free radicals and the generation of reactive radical species in humic substances. Frontiers of Environmental Science & Engineering, 14(6): 106

[30] Sommereyns A , Leupold S , Rudlof F , Willeke M , Ghosh A , Ohannessian B , Merwin D , Elahi R , Barcikowski S , Vogel N . et al. (2024). Light matters: quantifiable optical interaction between near-infrared laser radiation and nano-additivated polymers in all states of matter present in laser powder bed fusion. Additive Manufacturing, 86: 104211

[31] Song Y , Buettner G R , Parkin S , Wagner B A , Robertson L W , Lehmler H J . (2008a). Chlorination increases the persistence of semiquinone free radicals derived from polychlorinated biphenyl hydroquinones and quinones. The Journal of Organic Chemistry, 73(21): 8296–8304

[32] Song Y , Wagner B A , Lehmler H J , Buettner G R . (2008b). Semiquinone radicals from oxygenated polychlorinated biphenyls: electron paramagnetic resonance studies. Chemical Research in Toxicology, 21(7): 1359–1367

[33] Sullivan A B , Reynolds G F . (1976). Substituent effects on the rate of decay of p-benzosemiquinone anion radicals. Journal of Physical Chemistry, 80(24): 2671–2674

[34] Tan M X , Hong C , Zheng X S , Chen B L , Chu C H . (2026). Natural organic matter association modulates pyrite surface electron transfer kinetics and path for reactive oxygen species generation. Environmental Science & Technology, 60(4): 3280–3288

[35] Urakawa K , Yatsuoka Y , Kawabata Y , Masu H , Matsuda M , Ishikawa H . (2022). Borocyclic radicals prepared from orthoquinone-containing polycyclic aromatics by photo-irradiation. Journal of Organic Chemistry, 87(5): 3647–3651

[36] Vyas H M , Adeleke B B , Wan J K S . (1976). Chemically induced dynamic magnetic polarization. Part X. The photoreduction of 2-methyl-p-benzoquinone. Spectroscopy Letters, 9(10): 663–672

[37] Wang X Q , Han Y , Cao Y W , Ni Y P , Wang D Z , Luan Y N . (2025). The formation, stabilization mechanism, and environmental impacts of persistent free radicals in soil humic substances. Agronomy, 15(3): 602

[38] Wu P , Xie M H , Clough T J , Yuan D , Wu S H , He X D , Hu C S , Zhou S G , Qin S P . (2023). Biochar-derived persistent free radicals and reactive oxygen species reduce the potential of biochar to mitigate soil N₂O emissions by inhibiting nosZ. Soil Biology and Biochemistry, 178: 108970

[39] Yang X , Yang F , Pang Y L , Du C Y , Wang K , He C , Tong H J , Liu F B . (2025). A critical review of secondary environmentally persistent free radicals: formation, properties, environmental and health impacts. Journal of Hazardous Materials, 498: 139680

[40] Zhang C , Zhang Y , Shi S L , Pan Y W , Qin R , Zhang Y . (2025). The influence of substituent effects of polyphenols on the removal of Cr(VI): the significance of electron-donating and electron-withdrawing groups. Separation and Purification Technology, 364: 132500

[41] Zhang Y Q , Zhou Y S , Wang H C , Xiao K , Luo Z H , Li Y J , Xing R , Jiang K , Fu D L , Liu W X . et al. (2024a). Environmental persistent free radicals in highly polluted soils and the association with polycyclic aromatic compounds. Environmental Research, 262: 119853

[42] Zhang J H , Zhang W L , Wang Y , Jiang S Y , Wang Y , Ding Z Z , Liu X M . (2024b). UV/H₂O₂ produced degradation of 2,4-D and 4-CPA. Journal of Cleaner Production, 457: 142440

[43] Zhao S , Gao P , Miao D , Wu L , Qian Y J , Chen S P , Sharma V K , Jia H Z . (2019). Formation and evolution of solvent-extracted and nonextractable environmentally persistent free radicals in fly ash of municipal solid waste incinerators. Environmental Science & Technology, 53(17): 10120–10130

[44] Zheng X S , Wu B B , Zhou C , Liu T , Wang Y L , Zhao G Q , Chen B L , Chu C H . (2023). Sunlight-driven production of reactive oxygen species from natural iron minerals: quantum yield and wavelength dependence. Environmental Science & Technology, 57(2): 1177–1185

[45] Zhu K C , Jia H Z , Zhao S , Xia T J , Guo X T , Wang T C , Zhu L Y . (2019). Formation of environmentally persistent free radicals on microplastics under light irradiation. Environmental Science & Technology, 53(14): 8177–8186

Rights & permissions

Higher Education Press 2027