Impact of the Electronic Environment in Carbazole-Appended o-Carboranyl Compounds on the Intramolecular-Charge-Transfer-Based Radiative Decay Efficiency

초록

The impact of the electronic effect of the carbazole group on the radiative decay process based on the intramolecular charge transfer (ICT) transition of closo-o-carborane was examined herein. Four closo-o-carboranyl compounds (C1-C4) appended to the C1-C4 positions of 9H-carbazole were synthesized and fully characterized by multinuclear NMR spectroscopy and elemental analysis. The molecular structures of C1 and C4 were determined by X-ray diffraction crystallography. All compounds demonstrated their lowest absorption band in the region of lambda(abs) = 330-345 nm, which was attributable to spin-allowed pi-pi* transitions on the carbazole groups, and a weak intramolecular charge transfer transition between the o-carborane and carbazole moieties. These compounds exhibited strong yellow emission (lambda(em) = 550-600 nm) in the rigid state (in THF at 77 K and in the film form), although no emission was observed in THF at 298 K. On the basis of the theoretical calculation results for the optimized first excited state (S-1) structures of the compounds, the emission bands could be assigned to ICT transitions corresponding to the o-carborane units. Photoluminescence experiments of the compounds in THF-water mixtures also confirmed the aggregation induced emission phenomenon. Interestingly, C1 and C3 exhibited significantly higher quantum yields (Phi(em)) and radiative decay constants (k(tau)) in the film state than C2 and C4, indicating more efficient ICT-based radiative decay for C1 and C3, where the o-carborane cage is appended to ortho- and para-positions (C1- and C3-positions) of the carbazole group. In addition, energy barrier calculations for the various compounds revealed that such differences in the emission characteristics were unrelated to the structural variation effect. Consequently, the experimental results suggested that ICT-based radiative decay in these o-carboranyl compounds can be enhanced when the o-carborane cage is linked to the ortho- and para-positions of the carbazole group, which are anionic in character due to electronic resonance.

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