Photochemical Behaviour and Toxicity Evolution of Phenylbenzoate Liquid Crystal Monomers in Water

Liquid crystal monomers (LCMs) are a group of emerging pollutants that pose potential environmental risks because of their ubiquitous occurrence and toxicity. Understanding their environmental transformation is essential for assessing the ecological risk. In this study, we investigated the photochemical transformation kinetics, mechanism, and photo-induced toxicity of three phenylbenzoate LCMs in water. Their apparent photolytic rate constants were within (0.023 – 0.058) min-1, and the half-lives were < 30.0min, showing lower persistence in water. Dissolved organic matter significantly inhibited their photolysis because of light-shielding effect and quenching of excited triplet states of LCMs. Their photolysis mainly occurred through excited triplet states, and the reactive oxygen species (i.e., ∙OH, 1O2 and ∙O2-) contributed to their degradation. The main photolysis pathways were ester bond cleavage, ▪OH substitution/addition, and defluorination. Experiments and computational simulation revealed that some ·OH addition/substitution products have similar toxicity with LCMs. Additionally, the ∙OH reaction rate constants (kOH) of LCMs were determined to be >1 × 109M-1 s-1, evidence for their high reactivity toward ∙OH. We have further developed reliable methods to estimate kOH of other phenylbenzoate-like LCMs with quantum chemical calculations. These results are useful for understanding the transformation and fate of LCMs in aquatic environments. Environmental implication Liquid crystal monomers (LCMs) are a group of emerging pollutants, posing potential environmental risks due to their ubiquitous occurrence and toxicity. However, their environmental transformation and toxicity variation in water has rarely been reported. This work investigated the photochemical transformation of three phenylbenzoate LCMs, including their kinetics, mechanism, and photo-induced toxicity, and developed a reliable in silico method to estimate the kOH of phenylbenzoate-like LCMs. These results will be helpful for understanding the fate and risks of LCMs in aquatic environment.