Improvement in Radiative Efficiency Via Intramolecular Charge Transfer in ortho -Carboranyl Luminophores Modified with Functionalized Biphenyls.

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.