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Two o -carboranes with (i) 9,9-dimethyl-9 H -xanthene and (ii) spiro[fluorene-9,9'-xanthene] moieties ( XTC and sXTC , respectively) were prepared and characterized. Single X-ray crystallography analysis revealed the presence of intermolecular hydrogen bonds in XTC crystals. Although both compounds did not exhibit emission in tetrahydrofuran solutions at 298 K, intense bluish emission was observed in the solid states and frozen tetrahydrofuran solutions at 77 K. According to the results of theoretical calculations, this emission originated from an intramolecular charge transfer (ICT) transition with the o -carborane moiety. The absolute quantum efficiency (Φ em ) of the ICT-based emission in the film state equaled 49% for XTC and 20% for sXTC but was as high as 90% for the crystals of both compounds. The crystal structures of XTC and sXTC revealed that the o -carboranyl-appended phenyl plane was orthogonal (85-89°) to the carbon-carbon bonding axis in the o -carborane, indicating the existence of a strong exo -π-interaction, which was identified as the structural basis for the ICT-based transition. These results implied that the intermolecular structural effect of XTC in the randomly aggregated solid state (film) helped maintain the above orthogonality and, hence, the high efficiency from the ICT radiative mechanism. Thus, we concluded that the ICT radiative efficiency of o -carboranyl luminophores in the aggregated solid state can be controlled by specific intermolecular interactions and that the molecular geometric design inducing this feature can be important for developing highly efficient carboranyl luminophores.

In this study, we found that the electronic effects of the functional groups on aromatic units attached to o -carboranyl species can enhance the efficiency of intramolecular charge transfer (ICT)-based radiative decay processes. Six o -carboranyl-based luminophores having attached functionalized biphenyl groups with CF 3 , F, H, CH 3 , C(CH 3 ) 3 , and OCH 3 substituents were prepared and fully characterized by multinuclear magnetic resonance spectroscopy. In addition, their molecular structures were determined by single-crystal X-ray diffractometry, which revealed that the distortion of the biphenyl rings and the geometries around the o -carborane cages were similar. All compounds exhibited ICT-based emissions in the rigid state (solution at 77 K and film). Intriguingly, the quantum efficiencies (Φ em ) of five compounds (that of the group with CF 3 could not be measured because of its extremely weak emissions) in the film state increased gradually as the electron-donating power of the terminal functional group modifying the biphenyl moiety increased. Furthermore, the nonradiative decay constants ( k nr ) for the group with OCH 3 were estimated to be one-tenth of those for the group with F, whereas the radiative decay constants ( k r ) for the five compounds were similar. The dipole moments (μ) calculated for the optimized first excited state (S 1 ) structures gradually increased, from that of the group with CF 3 to that of the group with OCH 3 , implying that the inhomogeneity of the molecular charge distribution was enhanced by electron donation. The electron-rich environment formed as a result of electron donation led to efficient charge transfer to the excited state. Both experimental and theoretical findings revealed that the electronic environment of the aromatic moiety in o -carboranyl luminophores can be controlled to accelerate or interrupt the ICT process in the radiative decay of excited states.