Deboronation-Induced Ratiometric Emission Variations of Terphenyl-Based --Carboranyl Compounds: Applications to Fluoride-Sensing.

--carboranyl compounds bearing the -type perfectly distorted or planar terphenyl rings (- and -, respectively) and their -derivatives (- and -, respectively) were synthesized and fully characterized using multinuclear NMR spectroscopy and elemental analysis. Although the emission spectra of both -compounds exhibited intriguing emission patterns in solution at 298 and 77 K, in the film state, - mainly exhibited a π-π* local excitation (LE)-based emission in the high-energy region, whereas - produced an intense emission in the low-energy region corresponding to an intramolecular charge transfer (ICT) transition. In particular, the positive solvatochromic effect of - and theoretical calculation results at the first excited (S) optimized structure of both -compounds strongly suggest that these dual-emissive bands at the high- and low-energy can be assigned to each π-π* LE and ICT transition. Interestingly, both the -compounds, - and -, exhibited the only LE-based emission in solution at 298 K due to the anionic character of the --carborane cages, which cannot cause the ICT transitions. The specific emissive features of -compounds indicate that the emissive color of - in solution at 298 K is completely different from that of -. As a result, the deboronation of - upon exposure to increasing concentrations of fluoride anion exhibits a dramatic ratiometric color change from orange to deep blue via turn-off of the ICT-based emission. Consequently, the color change response of the luminescence by the alternation of the intrinsic electronic transitions via deboronation as well as the structural feature of terphenyl rings indicates the potential of the developed --carboranyl compounds that exhibit the intense ICT-based emission, as naked-eye-detectable chemodosimeters for fluoride ion sensing.