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

초록

Two novel heteroleptic cyclometalated iridium(III) complexes, namely (C<<^>>N)(2)Ir(BL) (Ir1 and Ir2) (C<<^>>N= 2-(2,4-difluomphenyepyridine (dfphpy, for Irl) 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 (MLCT)-M-1 (metal-to-ligand charge transfer) absorption bands centered at lambda(ads) = 483 nm, in addition to borane-centered CT absorption bands at similar to 326 nm. The photoluminescence (PL) spectra for these complexes demonstrated phosphorescent emission in the near infrared region (lambda(em) = similar to 690 nm), which was mainly attributed to the (MLCT)-M-3 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, H-1 NMR experiments in THF-d(8) 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 (MLCT)-M-3 transition, reinforcing the dipyrromethene-centered pi-pi* transition, and further inducing the enhancement of the band-gap.

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