Isothiouronium-derived simple fluorescent chemosensors of anions

Fluorescence-active chemosensors comprising a naphthalene moiety covalently attached, via a methylene spacer, to a isothiouronium anion-binding subunit have been proposed and characterized. The corresponding 2-[(S-benzyl-N'-methyl- N-isothiouronio) methyl] naphthalene hexafluorophosphate 1 and dissymmetric 3,3'-bis[(S-benzyl-N'-methyl-N- isothiouronio) methyl]-2,2'-dimethoxy-1,1'-binaphthalene bis(hexafluorophosphate) 2, prepared in multi-synthetic paths via the corresponding thiourea derivatives as key synthons, respectively, showed almost no fluorescence spectra in MeCN based on a photoinduced electron transfer (PET) quenching process (switch off state). However, the binding of anionic guests to the isothiouronium unit could regulate the efficiency of the PET process to induce selective anion-induced fluorescence emission (switch on state), due to the fluorescent enhancement of the PET quenched fluorophore (retrieval) of the naphthalene unit. Notably, in the case of 2, the maximum enhancement (ca. 1600%) in the emission was reached upon addition of 3 equiv. of AcO-, compared to that with no anion bound. In this way, the PET sensors exhibited favorable affinity for oxoanions, which are functional groups of important biological relevance, emitting fluorescence in the presence of the anions. The findings suggest that use of the isothiouronium group would be extremely advantageous for designing new switch on-type chemosensors of oxoanions.