Host-guest interactions of catechol and 4-ethylcatechol with surface-immobilized blue-box molecules

Catechol and 4-ethylcatechol are highly toxic, carcinogenic, and mutagenic molecules, often produced in water in small quantities as a result of hydrothermal upgrading processes. In this work, we demonstrate the facile immobilization of cyclobis(paraquat-p-phenylene) macrocycles (known as blue box (BB)) on silica surfaces, and show that BB-coated silica surfaces adsorb large amounts of catechol and 4-ethylcatechol, with a molar ratio of 214 : 1 and 432 : 1, respectively, instead of the expected 1 : 1 ratio. The adsorbates formed large aggregates on the silica surface, with a height range of 2-55 nm dependent on the catechol concentration and reaction time, while uncoated silica powder did not adsorb any catechol from water. The aggregates are shown to be poly(catechol)s, as a result of a BB-induced polymerization process in water under non-alkaline conditions. The process is initiated by the formation of radical cations of the adsorbed catechol and 4-ethylcatechol upon the complexation with assembled BB molecules through charge-transfer interactions. This is the first report of the polymerization of guest molecules catalyzed by surface-immobilized BB macrocycles. A BB-coated silica powder was used to extract up to 95% of 4-ethylcatechol in 3 hours and 60% of catechol in 2 hours from dilute aqueous solutions and did not leave particulates behind. More than 90% of the BB⁴⁺ macrocycles could be recovered from the silica surfaces after the water treatment processes.