Influence of Intermolecular Structural Effects on Radiative Efficiency in Xanthene-Based Carboranyl Luminophores.

Two -carboranes with (i) 9,9-dimethyl-9-xanthene and (ii) spiro[fluorene-9,9'-xanthene] moieties ( and , respectively) were prepared and characterized. Single X-ray crystallography analysis revealed the presence of intermolecular hydrogen bonds in 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 -carborane moiety. The absolute quantum efficiency (Φ) of the ICT-based emission in the film state equaled 49% for and 20% for but was as high as 90% for the crystals of both compounds. The crystal structures of and revealed that the -carboranyl-appended phenyl plane was orthogonal (85-89°) to the carbon-carbon bonding axis in the -carborane, indicating the existence of a strong -π-interaction, which was identified as the structural basis for the ICT-based transition. These results implied that the intermolecular structural effect of 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 -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.