Synthesis and sensing properties of tricationic imidazolium and guanidinium fluorophore

Abstract

Fluorescent derivatives of triphenylamine containing imidazolium, guanidine, and guanidinium peripheries are efficiently synthesized by Palladium-catalyzed coupling reactions. The target compounds are characterized by 1H- and 13C nuclear magnetic resonance, mass-spectrometry, UV-Vis and fluorescent spectrophotometry. The photophysical investigation of these compounds in 10 mM of HEPES buffer pH 7.0 reveals their maximum absorption wavelengths ranging between 319 to 362 nm, maximum emission wavelengths of around 428 to 456 nm, and quantum efficiencies of 17.97, 7.40, 3.16 and 2.39% for F1 - F4, respectively. A screening for sensing properties indicates that the emission signal of F1, which contains imidazolium groups, can be selectively quenched by hydrogen sulfide ion with a Stern-Volmer quenching constant of 4.91×105 M-1 and a detection limit of 0.093 µM. Mechanistic examination by NMR suggests that the fluorescent signal changes may involve the binding between the proton at the 2-position of the imidazolium ring with hydrogen sulfide. For F2 which contains guanidine receptor, the fluorescent signal is selectively quenched by thiosulfate ion with a Stern-Volmer constant of 1.06×105 M-1 and a detection limit of 1.31 µM.