FRET-based probe for fluoride based on a phosphorescent iridium(iii) complex containing triarylboron groups

Abstract

An excellent F⁻probe (complex 1) based on carbazole-fluorene-carbazole (CzFCz) as a fluorescent donor and a cationic Ir(III) complex unit containing dimesitylboryl (Mes₂B) groups as a phosphorescent acceptor has been designed and synthesized. Several reference compounds, such as complex 2 which is similar to complex 1 but without Mes₂B groups, fluorescent donor CzFCz, and phosphorescent acceptors A1 and A2, were also synthesized in order to better understand the influence of Mes₂B groups on the excited state properties and fluorescence resonance energy transfer (FRET) in this system. The introduction of Mes₂B groups on the ligands of the Ir(III) complex unit can lead to a red-shifted and more intense absorption, facilitating efficient FRET from the fluorescent donor to the phosphorescent acceptor. Complex 1 displayed highly efficient orange-red phosphorescent emission with an emission peak at 584 nm in CH₂Cl₂ solution at room temperature. The emission wavelength of complex 1 in film is red-shifted to 600 nm with a shoulder at 650 nm, and its quantum efficiency in film was measured to be 0.15 under excitation at 450 nm. Utilizing the specific Lewis acid–base interactions between boron atom and F⁻, the binding of F⁻ to complex 1 can change its excited state and suppress FRET, quenching the phosphorescent emission from the Ir(III) complex and enhancing the fluorescent emission from CzFCz. Thus, a visual change in the emission color from orange-red to blue was observed. Optical responses of complex 1 to F⁻ revealed that it can be used as a highly selective, colorimetric and ratiometric optical probe for F⁻ utilizing the switchable phosphorescence and fluorescence.