Kinetics and quantum yield of photosensitized reactions from substituted phenols via chemical probe approach

Triplet excited states (3C∗) of organic compounds and singlet oxygen (1O2) are important oxidants in the atmospheric aqueous phase, yet their roles in the degradation of phenolic compounds (PhCs) remain poorly characterized. In this study, we investigated the aqueous-phase photochemical reactivity of five substituted phenols-eugenol, 4-ethylphenol (4-EP), 3-methylcatechol (3-MC), 4-nitrocatechol(4-NC) and 2,6-dimethoxy-4-methylphenol (DMMP)-in the presence of four triplet-state photosensitizers: 3,4-dimethoxybenzaldehyde (DMB), benzophenone (BP), vanillin (VA), and 2-acetonaphthone (2AN). The contributions of reactive oxidative species (ROS), including 1O2, •OH, and 3C∗ were systematically examined using quenching and chemical prober technique. The second-order rate constants (kPhCs,3C∗)for reactions between PhCs and 3DMB∗ ranged from (1–6)✕109 M−1 s−1, following the reactivity order: 4-NC > DMMP > EUG>4-EP>3-MC. However, no consistent reactivity trend was observed across different triplet sensitizers, indicating that different triplet-state photosensitizer exhibit varying reactivity toward phenolic compounds. Contribution analysis confirmed that 3C∗ was the dominant oxidant, accounting for 60–89 % of the observed photodegradation. The steady-state concentrations of 3C∗ and 1O2 were estimated as [3C∗]ss=(5.6–9.6) × 10−14 M and [1O2]ss (1.3–3.4) × 10−13 M, respectively. The quantum yield coefficient (fTMP) of 3C∗ ranged from 90 to 3000 M−1, while the 1O2 quantum yield (Φ1O2) varied between 1 % and 50 %. A strong correlation between fTMP and Φ1O2 suggested that 1O2 is primarily produced via energy transfer from 3C∗. Quantitative structure–activity relationship model showed a moderate correlation between kPhCs,3C∗ and oxidation potentials of PhCs (r2 = 0.34–0.72), while Hammett plots revealed stronger correlation with substituent constants(r2 = 0.61–0.93), highlighting the dominant influence of electronic effects of substituents. Increased substituent numbers appeared to suppress the reactivity of PhCs toward 3C∗, suggesting a combination of steric and electronic inhibition. These results provide new insight into the 3C∗-mediated degradation mechanisms of phenolic compounds in atmospheric aqueous-phase.