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

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 (Mes(2)B) 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 Mes(2)B 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.