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

Abstract

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 (S₁) 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.

Keywords: closo-o-carborane; color change; deboronation; intramolecular charge transfer; nido-o-carborane.

Figure 1

Synthetic routes to the terphenyl-based closo- and nido-o-carboranyl complexes, closo-DT, closo-PT, nido-DT, and nido-PT. Reaction conditions: (i) Ethynyltrimethylsilane, CuI, Pd(PPh₃)₂Cl₂, NEt₃/toluene, r.t., 24 h. (ii) K₂CO₃, methanol, r.t., 2 h. (iii) B₁₀H₁₄, Et₂S, toluene, 110 °C, 72 h. (iv) n-tetrabutylammonium fluoride (TBAF), THF, 60 °C, 2 h.

Figure 2

UV–Vis absorption and photoluminescence (PL) spectra for (a) closo- and nido-DT (λ_ex = 292 nm) and (b) closo- and nido-PT (λ_ex = 345 nm). Black-solid: absorption spectra in THF (30 μM) for closo-species. Black-dash: absorption spectra in THF (30 μM) for nido-species. Blue-solid: PL spectra in THF (30 μM) at 298 K for closo-species. Blue-dash: PL spectra in THF (30 μM) at 298 K for nido-species. Green-solid: PL spectra in THF (30 μM) at 77 K for closo-species. Orange-solid: PL spectra of the films (5 wt% doped on PMMA) at 298 K for closo-species. Inset figures show the emission color in each state of closo-species under irradiation by a hand-held UV lamp (λ_ex = 295 nm for closo-DT and 365 nm for closo-PT).

Figure 3

Frontier molecular orbitals of closo-DT and closo-PT in their ground states (S₀) and first excited singlet states (S₁), and their relative energies calculated by DFT (isovalue = 0.04). The transition energy (in nm) was calculated using the TD-B3LYP/6-31G(d) level of theory.

Figure 4

¹H-NMR spectral changes of (a) closo-DT and (b) closo-PT upon increasing the amount of added fluoride anions and comparison with those of nido-DT and nido-PT (∗ from residual THF in THF-d⁸, † from n-butyl group of excess TBAF, and + from n-butyl group for each nido-compound).

Figure 5

Spectral changes in the emission of (a) closo-DT (3.0 × 10⁻⁵ M, λ_ex = 292 nm) and (b) closo-PT (3.0 × 10⁻⁵ M, λ_ex = 345 nm) in THF in the presence of different amounts of TBAF, upon heating at 60 °C for 2 h. Insets are photographs of each closo- and nido-type (3.0 × 10⁻⁵ M in THF) under a UV lamp (λ_ex = 295 nm for DT derivatives and 365 nm for PT derivatives).