Four biphenyl-and fluorene-based o-carboranyl compounds, 4-[2-(p-n-butylphenyl)-1-o-carboran-1-yl]biphenyl (1B), 4,4″-bis[2-(p-n-butylphenyl)-1-o-carboran-1yl]biphenyl (2B), 2-[2-(p-n-butylphenyl)-1-o-carboran-1-yl]fluorene (1F), and 2,7-bis[2-(p-n-butylphenyl)-1-o-carboran-1yl]fluorene (2F), were prepared and fully characterized by multinuclear NMR spectroscopy and elemental analysis. The crystal structures of 1B and 2B, analyzed by single-crystal Xray diffraction, exhibited distinct distortions of the central biphenyl rings with dihedral angles of 44.2 and 33.1°. In photoluminescence measurements, fluorene-based carboranyl compounds in the rigid state (e.g., in solution at 77 K and as films) exhibited a noticeable emission in the low-energy region below 400 nm. 1F displayed a low-energy emissive trace in solution at ambient temperature, whereas biphenyl-based carboranes mainly exhibited high-energy emissions above 400 nm. TD-DFT calculations on the first excited singlet (S 1 ) state of each compound suggested that the emission in the low-energy region involves intramolecular charge transfer (ICT) between the carborane and central phenylene rings, indicating that the radiative decay of these ICT bands could be enhanced by preventing the distortion of the central biphenyl groups. Furthermore, less distortion on the biphenyl rings of the biphenyl-based analogues at S 1 in the optimized structures supports that these phenylenes must be flat for the formation of stable excited states. Comparison of the quantum efficiency (Φ PL ) and radiative decay constants (k r ) of the o-carboranes also reveals definitively that the structural features of the biphenyl groups can have a decisive effect on those photophysical properties.
In this study, we synthesized quinolinol-ligand (2-methylquinolin-8-ol, q)-based aluminum complexes with phenyl (D1), biphenyl (D2), and naphthyl (D3)-bridged triarylboranes, and employed them as organometallic chemosensors for sensing mediated by fluoride anions. In addition, these dyad complexes were fully characterized by multinuclear nuclear magnetic resonance (NMR) spectroscopy and elemental analysis. UV/Vis titration experiments examining the association of D1, D2, and D3 with fluoride demonstrated that the dyad complexes associated in a 1:1 binding stoichiometry in tetrahydrofuran (THF), with binding constants (K) in the range of 2.7-6.6 × 10 4 M −1 . Moreover, these dyad complexes showed a ratiometrically decreasing fluorescence response in photoluminescence titration experiments upon binding of fluoride to the borane moiety, thereby giving rise to a turn-off chemosensor for detection of fluoride anions. It could be postulated that these turn-off properties were caused by the interruption of the intramolecular charge-transfer (ICT) transition between the q 2 Al part and the bridged phenoxy groups. Furthermore, the theoretical calculation results for the dyad complexes and these fluoride adducts clearly indicate that the ICT transition between the q 2 Al and bridged phenoxy groups of the complexes could be interrupted by the binding of fluoride to borane.
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