Deboronation-Induced Turn-on Phosphorescent Sensing of Fluorides by Iridium(III) Cyclometalates with <i>o</i>-Carborane

Abstract

Heteroleptic tris-cyclometalated Ir(III) complexes bearing an o -carborane at the 4- or 5-position in the phenyl ring of the ppy ligand ( closo - 1 and - 2 ) were prepared and characterized. The X-ray crystal structure of closo - 1 reveals the fac arrangement of the three C ∧ N chelates around the Ir atom. Treatment of closo complexes with fluoride anions led to selective deboronation of the closo -carborane cage, producing the corresponding nido -carborane-substituted complexes ( nido - 1 and -2 ). Whereas closo - 1 and - 2 were almost nonemissive in THF, nido - 1 and - 2 were highly phosphorescent (Φ PL = 0.94–0.95). Theoretical studies suggested that, while the emission quenching in closo - 1 can be ascribed to the substantial involvement of o -carborane in the 3 MLCT excited state, the intraligand charge transfer ( 3 ILCT) state from the nido -carborane to pyridyl moieties is responsible for the efficient phosphorescence in nido - 1 . The addition of fluoride to the buffered THF/H 2 O solution (1/1, v/v, pH 7) of closo - 1 and - 2 under mild heating led to strong emission intensity, allowing the turn-on phosphorescence detection of fluoride in aqueous medium at the ppb level.