Dynamic dual stage phosphorescence chromatic change in a diborylated iridium phosphor for fluoride ion sensing with concentration discriminating capability

Abstract

By adding a strongly electron-accepting B(Mes)₂ group to the ppy-type ligand of phosphorescent iridium(III) cyclometalated complexes, more stabilized metal-to-ligand charge-transfer (MLCT) states can be obtained by transferring electron density from the pyridyl moiety to the boron atom of the B(Mes)₂ group in the metallophosphors to give red phosphorescence. Taking advantage of the binding effect between boron atom and F⁻ ion, the phosphorescent emission color of the iridium(III) cyclometalated complex can be dynamically changed by the external F⁻ ions sequentially from red to yellow and to green through modulation of the charge-transfer emitting states, representing very unique F⁻ ion sensing behavior. In the first step, destabilization of the MLCT states is caused by the weak binding between boron and F⁻ ion, which is then accompanied by switching of the MLCT process to form high-energy MLCT states as induced by the strong binding between boron and F⁻ ion in the second step. Not only does the dynamic phosphorescence chromatic variation depend significantly on the substitution mode of the B(Mes)₂ moiety on the ppy ligands, but the dynamic emission response also would pave the way to the development of a novel F⁻ ion sensor showing a unique concentration discrimination feature in aqueous solution with good color reversibility and optical response to the naked eye, high selectivity and sensitivity. All of these data provide valuable insight into the molecular design of a new generation of F⁻ ion sensors featuring both concentration discriminating capability and good potential for practical applications.