Intriguing ‘Turn-on’ phosphorescent response in the near infrared region upon fluoride binding: Dipyrromethene chelating-based Triarylboryl‒Iridium(III) conjugates

Two novel heteroleptic cyclometalated iridium(III) complexes, namely (C^N)2Ir(BL) (Ir1 and Ir2) (C^N = 2-(2,4-difluorophenyl)pyridine (dfphpy, for Ir1) and 2-phenylpyridine (phpy, for Ir2)), chelated by the triarylboryl dipyrromethene ancillary ligand (BL) were synthesized and characterized. The molecular structures of both complexes in the solid state were fully analyzed by single-crystal X-ray diffraction, and were found to be composed of Ir(III)-centered triarylborane conjugated systems. The UV–vis absorption spectra of both complexes displayed the typical 1MLCT (metal-to-ligand charge transfer) absorption bands centered at λabs = 483 nm, in addition to borane-centered CT absorption bands at ~326 nm. The photoluminescence (PL) spectra for these complexes demonstrated phosphorescent emission in the near infrared region (λem = ~690 nm), which was mainly attributed to the 3MLCT transition (L = BL). In addition, fluoride titration experiments using these complexes showed gradually quenched absorption bands in the region of 300–350 nm, corresponding to borane-centered CT transitions. Furthermore, 1H NMR experiments in THF-d8 for both complexes in the presence of fluoride sources revealed that the fluoride was bound to the dimesitylboryl moieties. Interestingly, upon the addition of fluoride, the PL spectra exhibited a ratiometrically ‘turn-on’ and gradually blue-shifted emissive response. Computational calculations and cyclic voltammetry measurements using the neutral Ir(III)-conjugates and their fluoride adducts strongly suggested that the intriguing emissive response is the result of fluoride binding interrupting the original 3MLCT transition, reinforcing the dipyrromethene-centered π–π* transition, and further inducing the enhancement of the band-gap.