A fluorescence-active 1,3-bis(isothiouronium)-derived naphthalene exhibiting versatile binding modes toward oxoanions in aqueous MeCN solution: new methodology for sensing oxoanions

A unique binding property of the PET (photoinduced electron transfer) sensor 1 is described, in which a naphthalene moiety at the 1,3-positions is attached by isothiouronium units that serve as anion-binding units. Fluorescence emission (λ_max=339 nm) was significantly switched on at an excitation wavelength of 279 nm upon recognition of anion in 6% (v/v) H₂O/MeCN solution at 25 °C. Titration curves with HPO₄²⁻ and AcO⁻ differ greatly; that with HPO₄²⁻ shows a stoichiometric 2:1 host-to-guest binding, whereas that with AcO⁻ shows a biphasic 1:1 and 1:2 host-to-guest binding, reflecting principally the charged shapes of the oxoanions. This finding is attributable to the plural built-in isothiouronium units with a high anion-binding ability, and suggests a new way to develop fluorescent sensors for oxoanion recognition.