P-Terphenyl-based di-o-carboranyl compounds: Alteration of electronic transition state by terminal phenyl groups

Abstract

Herein, p-terphenyl-based o-carboranyl compounds, 4,4″-bis[2-(p-n-butylphenyl)-1-o-carboran-1-yl]-p-terphenyl (1) and 4,4″-bis(2-n-butyl-1-o-carboran-1-yl)-p-terphenyl (2), were prepared and fully characterized by multinuclear NMR spectroscopy and elemental analysis. The molecular structure of 1 was also analyzed by single-crystal X-ray diffraction method. Both compounds showed a dual emission band (λem ∼ 350 and 450 nm) in the rigid state (e.g., solution at 77 K), in contrast with a single emission peak at ca. 350 nm observed in solution at ambient temperature. Theoretical calculations on the first excited singlet state of each compound suggested that emission in the low-energy region distinctly involved intramolecular charge transfer (ICT) between carborane and terphenylene rings of both compounds; however, the high-energy emission of 1 was correlated with ICT between carborane and terminal phenyl rings, while that of 2 involved ICT between carborane and terphenylene as well as a π → π∗ transition of the terphenylene moiety. Thus, it was concluded that the absence of terminal phenyl rings in o-carboranyl compounds leads to a π → π∗ transition-based emissive band. The quantum efficiency and radiative decay values of 2, which are not drastically enhanced in the rigid state compared to the solution state at ambient temperature, also showed that the emission band of 2 is not perfectly correlated with the charge transfer (CT) transition band, in contrast to the case of 1.