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

Closo-o-carboranyl compounds bearing the ortho-type perfectly distorted or planar terphenyl rings (closo-DT and closo-PT, respectively) and their nido-derivatives (nido-DT and nido-PT, respectively) were synthesized and fully characterized using multinuclear NMR spectroscopy and elemental analysis. Although the emission spectra of both closo-compounds exhibited intriguing emission patterns in solution at 298 and 77 K, in the film state, closo-DT mainly exhibited a π-π* local excitation (LE)-based emission in the high-energy region, whereas closo-PT produced an intense emission in the low-energy region corresponding to an intramolecular charge transfer (ICT) transition. In particular, the positive solvatochromic effect of closo-PT and theoretical calculation results at the first excited (S(1)) optimized structure of both closo-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 nido-compounds, nido-DT and nido-PT, exhibited the only LE-based emission in solution at 298 K due to the anionic character of the nido-o-carborane cages, which cannot cause the ICT transitions. The specific emissive features of nido-compounds indicate that the emissive color of closo-PT in solution at 298 K is completely different from that of nido-PT. As a result, the deboronation of closo-PT 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 closo-o-carboranyl compounds that exhibit the intense ICT-based emission, as naked-eye-detectable chemodosimeters for fluoride ion sensing.