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

초록

In this study, we found that the electronic effects ofthe functionalgroups on aromatic units attached to o-carboranylspecies can enhance the efficiency of intramolecular charge transfer(ICT)-based radiative decay processes. Six o-carboranyl-basedluminophores having attached functionalized biphenyl groups with CF3, F, H, CH3, C(CH3)(3), andOCH(3) substituents were prepared and fully characterizedby multinuclear magnetic resonance spectroscopy. In addition, theirmolecular structures were determined by single-crystal X-ray diffractometry,which revealed that the distortion of the biphenyl rings and the geometriesaround the o-carborane cages were similar. All compoundsexhibited ICT-based emissions in the rigid state (solution at 77 Kand film). Intriguingly, the quantum efficiencies (phi(em)) of five compounds (that of the group with CF3 couldnot be measured because of its extremely weak emissions) in the filmstate increased gradually as the electron-donating power of the terminalfunctional group modifying the biphenyl moiety increased. Furthermore,the nonradiative decay constants (k (nr))for the group with OCH3 were estimated to be one-tenthof those for the group with F, whereas the radiative decay constants(k (r)) for the five compounds were similar.The dipole moments (mu) calculated for the optimized first excitedstate (S-1) structures gradually increased, from that ofthe group with CF3 to that of the group with OCH3, implying that the inhomogeneity of the molecular charge distributionwas enhanced by electron donation. The electron-rich environment formedas a result of electron donation led to efficient charge transferto the excited state. Both experimental and theoretical findings revealedthat the electronic environment of the aromatic moiety in o-carboranyl luminophores can be controlled to accelerateor interrupt the ICT process in the radiative decay of excited states. We found that the electronic effectsof the functional groupson aromatic units attached to o-carboranyl speciescan enhance the efficiency of intramolecular charge transfer (ICT)-basedradiative decay processes. Six o-carboranyl-basedluminophores having attached functionalized biphenyl groups were prepared.The photophysical study for these compounds with theoretical analysisrevealed that the electronic environment in o-carboranylluminophores can be controlled to accelerate or interrupt the ICTprocess in the radiative decay.

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