Novel Dimeric o-Carboranyl Triarylborane: Intriguing Ratiometric Color-Tunable Sensor via Aggregation-Induced Emission by Fluoride Anions

A dimeric o-carboranyl triarylborane compound (2) with a biphenylene bridge group was prepared and characterized. Also, its solid-state structure was determined via X-ray diffraction. Treatment of 2 with an excess amount of KF in the presence of 18-crown-6 formed a dimiertype potassium salt, [2-F-2][K-18-crown-6](2); its structure was fully confirmed by multinuclear NMR spectroscopy. UV-vis,,vis titration experiments carried out in THF showed that 2 binds fluoride ions with a binding constant (K) of 8.5 x 10(5) M-1. The linear decline of the UV/vis absorption of 2 upon titration with fluoride suggested that the triarylborane moieties acted as independent binding sites, which were not affected by each other. Contrary to a single emission (lambda(em) = 376 nm) of 2 assignable to an intramolecular charge transfer (ICT) transition at 298 K in THF, a broad low-energy emission band was additionally observed at 77 K, which is dominant in the film state. The TD-DFT calculation on the first excited singlet state (S-1) of 2 show's that the low-energy emission band originates from the CT nature between carborane and triarylborane groups, Aggregation-induced emission (ATE) of 2 was clearly confirmed by enhanced photoluminescence intensity (lambda(em) = 489 nm) upon increasing the water fraction (f(w)) in the THF solution of 2, and it further accounts for the intense emission in the solid state. Interestingly, am emission spectrum of a film sample of 2 upon addition of two equivalents of fluoride ion was was mostly similar to that of [2-F-2] [K.18-crown-6](2), indicating that the ICT-based AIE nature of 2 could be red-shifted by fluoride binding.