Alteration of intramolecular electronic transition via deboronation of carbazole-based o -carboranyl compound and intriguing 'turn-on' emissive variation.

The conversion of closo-o -carborane-containing compounds to the nido-o -species via deboronation causes photophysical changes that could be used for sensing applications. 9-Methyl-9 H -carbazole-based closo - ( closo -Cz) and nido-o -carboranyl ( nido -Cz) compounds were prepared and fully characterised by multinuclear NMR spectroscopy and elemental analysis, and the solid-state molecular structure of closo -Cz was analysed by X-ray crystallography. Although the closo -compound exhibited an emissive pattern centred at λ em = ca . 530 nm in the rigid state only (in THF at 77 K and as a film), nido -Cz demonstrated intense emission in the near-UV region ( λ em = ca . 380 nm) in both solution and film states at 298 K. The positive solvatochromic effect of nido -Cz and the results of theoretical calculations for both the o -carboranyl compounds supported that these emissive features originate from intramolecular charge transfer (ICT) corresponding to the o -carborane. Furthermore, the calculations verified that the electronic role of the o -carboranyl unit changed from acceptor to donor upon deboronation from closo -Cz to nido -Cz. Investigations of the radiative decay mechanisms of closo -Cz and nido -Cz according to their quantum efficiencies ( Φ em ) and decay lifetimes ( τ obs ) suggested that the ICT-based radiative decays of closo -Cz and nido -Cz readily occur in the film (solid) and solution state, respectively. These observations implied that the emission of closo -Cz in the solution state could be drastically enhanced by deboronation to nido -Cz upon exposure to an increasing concentration of fluoride anions. Indeed, turn-on emissive features in an aqueous solution were observed upon deboronation, strongly suggesting the potential of closo -Cz as a turn-on and visually detectable chemodosimeter for fluoride ion sensing.