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

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