Biphenyl- and Fluorene-Based o‑Carboranyl Compounds: Alteration of Photophysical Properties by Distortion of Biphenyl Rings

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-1-yl]­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-1-yl]­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 X-ray 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 (S1) 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 S1 in the optimized structures supports that these phenylenes must be flat for the formation of stable excited states. Comparison of the quantum efficiency (ΦPL) 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.