Adsorption dynamics of benzene derivatives onto the surface of hydroxylated silica upon photoexcitation: Effect of halogen and methyl substituents

Adsorption of toxic substances is an important research field. In this work, the adsorption dynamics of halogenated and methylated benzenes onto hydroxylated silica upon photoexcitation has been investigated theoretically. The intermolecular interaction between the hydroxy (OH) group and the pi electron density at the center of the benzene ring was also a major subject. In the ground state, the order of calculated adsorption energy was consistent with the OH center dot center dot center dot C bond strengths (SiO2-MeBe > SiO2-DimeBe > SiO2-BrBe > SiO2-ClBe > SiO2FBe > SiO2-Be), implying this bond to be the major contributor to the adsorption. Upon photoexcitation to the first excited (S-1) state by adsorbing shortwave-ultraviolet (UV), the OH center dot center dot center dot C bonds in each complex were strengthened with only the order of the bond strengths of SiO2-DimeBe and SiO2-MeBe reversed relative to the order in the ground state. In contrast to OH center dot center dot center dot C bonds, the OH center dot center dot center dot p bonds were all indicated to weaken upon photoexcitation. The results showed that the order of OH center dot center dot center dot p bond strengths in the S-1 state to be SiO2-FBe > SiO2-BrBe > SiO2-DimeBe > SiO2-Be > SiO2-ClBe > SiO2-MeBe. However, the change in the corresponding adsorption energy was inconsistent with the order of OH center dot center dot center dot C bond strengths in the S1 state, due to the degree of weakening of the OH center dot center dot center dot p bond being greater than the degree of strengthening of the OH center dot center dot center dot C bonds. These changes were concluded to be induced by local excitation for each of the six complexes. This work has presented the photophysical dynamics of an adsorbent before and after photoexcitation.